Recent advances in nanostructured electrode-electrolyte design for safe and next-generation electrochemical energy storage

Journal Pre-proof Recent Advances in Nanostructured Electrode-Electrolyte Design for Safe and NextGeneration Electrochemical Energy Storage Xian-Xiang Zeng, Yu-Ting Xu, Ya-Xia Yin, Xiong-Wei Wu, Junpei Yue, Yu-Guo Guo PII:

S2588-8420(19)30126-9

DOI:

https://doi.org/10.1016/j.mtnano.2019.100057

Reference:

MTNANO 100057

To appear in:

Materials Today Nano

Received Date: 7 August 2019 Revised Date:

17 September 2019

Accepted Date: 21 September 2019

Please cite this article as: Zeng X.-X., Xu Y.-T., Yin Y.-X., Wu X.-W., Yue J. & Guo Y.-G., Recent Advances in Nanostructured Electrode-Electrolyte Design for Safe and Next-Generation Electrochemical Energy Storage, Materials Today Nano, https://doi.org/10.1016/j.mtnano.2019.100057. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Recent Advances in Nanostructured Electrode-Electrolyte Design for Safe and Next-Generation Electrochemical Energy Storage Xian-Xiang Zenga, Yu-Ting Xua, Ya-Xia Yinb, c, Xiong-Wei Wua, d*, Junpei Yueb, *, Yu-Guo Guob, c, * a

College of Science, Hunan Agricultural University, Changsha, Hunan 410128, P. R. China b CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P. R. China c University of Chinese Academy of Sciences, Beijing 100049, P. R. China d XiangYa School of Public Health, Central South University, Changsha, Hunan, 410078, P. R. China *

Correspondence: [email protected]; [email protected]; [email protected]

Abstract The pursuit for high-efficiency energy utilization stimulates for rapid development of electrochemical storage techniques. While the energy density demand is elevated, the safety consideration has stepped onto a new height. Hence, these two aspects gain much attention in the evolution of electrochemical energy storage. Correspondingly, the electrodes and electrolytes need special design to achieve the set target. This review firstly collects recent progress in nanostructured electrode-electrolyte with tailored design, including specific attention on alkali metal batteries, redox flow batteries and supercapacitors. The electrode materials along with advanced electrolytes can endow these electrochemical energy storage apparatuses with high energy or power density, long lifetime and high safety insurance. Moreover, the particular focus is dedicated to practical concerns for these electrodes, electrolytes and their interfaces, including the thermal behavior and charging/discharging changing of an electrode, electrolyte selection and optimization, and electrode-electrolyte interface construction. Eventually, the general guideline and direction of endeavor to break the current limitation are presented for advanced electrochemical energy storage in the future. Keywords: Nano materials, Structure design, Safety, Energy density, Power, Electrochemical energy storage

1. Introduction The goals for safe and next-generation electrochemical energy storage are established in two aspects: high energy density and power capability. Current commercial lithium-ion batteries with graphite as anode and layered oxides as cathode present great advantages in specific energy density compared to lead-acid batteries and nickel-hydrogen batteries, but nearly reach their physicochemical limits recently. Meanwhile, due to the limitation of ion diffusion (not only from their bulky and clumsy characters compared to electrons but also from their surroundings-counter ions, solvent molecules in their solvation sheaths), the power density seems to possess a ceiling as well. Hence seeking for next-generation energy storage and conversion systems is highly demanded. For this point, tremendous electrochemical energy storage systems including alkalis metal/ion batteries, redox flow batteries and supercapacitors exhibit attractive and fruitful achievements. These systems reveal respective characteristics in different application areas, thus requiring niche-targeting architectures considering the general incompatibility of energy density and power capability in one system. On the other side, to successfully commercializing these systems, ingenious design for the electrode-interfaces, especially with nanostructured springboard have burst into interesting promises (Fig. 1).

Fig. 1. Schematic of nanostructured strategies in electrochemical energy storage systems. In this review, we collect recent crucial advances in nanostructured electrode-electrolyte with tailored design rather than all detail progress for electrochemical energy storage, and specific attention is paid to the alkali metal (lithium, sodium) batteries, redox flow batteries and supercapacitors. These systems are divided into two categories: high energy density system (alkali metal batteries) and high power system (redox flow batteries and supercapacitors). The first part discussed alloyed and alkali metal anodes, Ni-rich layered oxide cathodes, anion redox cathodes, chalcogenide and O2-based cathodes. The discussion not only involves general strategies to handle various challenges, but also gives some

suggestions for the practical application of each system. The second part focuses on recent advances in redox flow batteries and supercapacitors, which are adventitious in some special application fields. The redox flow batteries have been applied in some large-scaled energy storage and newly typed flow batteries including organic flow batteries gain more attentions. For supercapacitors, new direction is a hybrid type, so called pseudocapacitors, where it is hard to distinguish the boundary between capacitors and batteries. Corresponding electrolyte-electrode interface design is illustrated. At last, the general guideline and direction of endeavor to get through the close siege are presented for advanced electrochemical energy storage in the future. 2. High-energy-density systems The high-energy density systems, especially the rechargeable alkalis-ion (such as Li and Na+) batteries have witnessed the widespread application from mobile phones to vehicles and grid-scale energy storage. However, the sluggish augment of energy density for commercial batteries lags behind the drastic growing demands. Actually, current lithium-ion batteries based graphite as anode and LiCoO2 as cathode have reached their upper limits considering gravimetric energy density. Such urgent demands and embarrassing current battery technology stimulate vast efforts to explore post-alkalis-ion batteries. According to E = U Q, where the U is the average output voltage (V) and the Q is the capacity (A h), the energy density can be calculated by dividing the weight (kg) or volume (L) of a electrochemical device to obtain gravimetric energy density (W h kg-1) or volumetric energy density (W h L-1). Supposing the amount of inactive components including separator, current collector and packing materials is constant, the desirable protocol to increase energy density is to employ high-voltage cathode and low-potential anode with equally high capacity, and at the same time, the cathode and anode is expected to be as light-weight and thin as possible. Under such circumstances, the energy density of electrochemical devices can be maximally released at a proper electrolyte ratio. The power density stands for the ability of energy release in a short time scale (P = +

ா ௧

), in other words, the higher working voltage or larger current is, the higher the

power density can be obtained (P = U I). Correspondingly, the rapid transit of charge carriers (ion and electron) should be warranted, which are generally dominated by the length (L) of transportation pathway and diffusion coefficient (DLi) of charge carriers, ௅మ

particularity when a thick electrode is utilized. From the equation, ‫( = ݐ‬௤஽

ై౟ )

, of

which q is a dimensional constant ranging from 2, 4 to 6 for 1D, 2D and 3D diffusion, respectively [1]. The direct and efficient way is reducing the material size and increasing the diffusion coefficient with 3D conducting network. To this end, the nanostructured material design testified by numerous works portends a befitting strategy. Hence, novel electrode materials or battery technology attract more and more attention. In the anode part, lithium metal anode and silicon-based anode both can

present ten times more specific capacity than graphite anode. In the cathode part, nickel-rich layered materials, anion redox activity materials, sulfur, oxygen, etc. gain the most efforts. Meanwhile, with the increase of energy density, the uncertainty of safety problem arises. To settle this issue, several basic conceptions are worth to be elaborated to comprehend the causeofruin. 2.1. Critical parameters 2.1.1. Electrode-electrolyte interface The interface between electrode and electrolyte plays a decisive role in ensuring highly reversibly electrochemical reactions. Such an interface is commonly nominated as solid electrolyte interphase (SEI), and usually only selectively permitted cationic ion pass. A stable SEI layer is regarded as a buffer layer for those unstable electrode/electrolytes and essentially important for the safe operation of batteries. However, how to construct homogenous and stable SEI layer determines their practical future in many high-energy systems. Hence, the interfacial layer has been widely studied, but its veil still does not been completely uncovered, and related exploration will continue. 2.1.2. Volume variation One of the reasons for the difficult construction of SEI layer is the volume variation. Compared with the volume change for carbon (10%), the alloyed anode or alkalis anode will experience tremendous variation during charge and discharge, such as 400% and infinite for silicon and lithium, respectively. The giant volume changes will induce large strains and stress, resulting in cracks, pulverization and isolation of active materials. Furthermore, the volume variation will continuously break the SEI layer and result in poor Coulombic efficiency. This indicates the elasticity is also a vital parameter for SEI. How to overcome the drastic volume variation is a key consideration in maintaining efficient energy conversion. 2.1.3. Overpotential As mentioned above, the volume variation will bring about the continuous growth of SEI layer, which will result in a high interfacial resistance with an increase of SEI thickness. The ever-stopping increase of internal resistance would lead to a large overpotential or polarization, inducing more serious side reactions and worsening the performance of batteries. 2.1.4. Dendrite The dendrite is a centuried topic in the fields involving the electrodeposition technique. The dendrite is a reflection of irregular platting, but the root of alkalis metal dendrite formation and growth differs from the electroplating in industrial applications such as copper, zinc, and so on. The latter one is mostly dependent on the “Sand time” model based on the diffusion process and charge transference number. It is a more complicated and self-promoted process due to the dynamic interfacial chemistry (SEI layer) in the plating of alkalis metal. The dendrite prefers to nuclear and grow at the places with lower energy barrier such as the heterogeneous conductive and porous sites, crack triggered by volume expansion, etc. These

undesirable outcomes will apparently deteriorate the interfacial stability and bring about large polarization and even catastrophe results such as short circuit as illustrating below. The efforts to suppress dendrite involve constructing stable hosts, electrolyte design, interface engineering, and so on. 2.1.5. Short circuit The short circuit of a cell is triggered by the direct contact between cathode and anode, which may be caused separator damage or dendrite piercing, and commonly couples with the domino effect including thermal runaway, electrolyte combustion, and even explosion, which is the most disastrous consequence and safety issue that needs be handled. 2.2. Anode 2.2.1. Alloy-type anodes 2.2.1.1. Silicon-based anodes. Silicon-based materials [2], as one of the most competitive alternatives to the graphite anode due to its low working voltage and high theoretical capacity, faces several challenges. One is the huge volume change during the alloy and de-alloy processes, which will cause a series of harmful results as mentioned above. The other issue is poor electrical conductivity, especially for the high mass loading. Moreover, to meet the commercial production qualification, the cost and manufacturing efficiency are also taken into account. Consequently, nearly all the contributions have been made to resolve these issues, and the outcome is to hunt for a suitable host that can not only accommodate vast volume variation but also render steady interfaces and high electricity. The nanotechnology offers more possibility for the development of silicon-based materials. (1) Silicon/carbon anodes (1.1) Compositing A simple but effective way is to design a silicon-carbon structure. On one hand, the introduced carbon materials provide a fine frame structure to tolerate the volume variation. During the compositing process, it customarily needs nanocrystallization of Si particle, and inevitably causes Si nanoparticle agglomeration and insufficient contact if without additional handling. The carbon supplies an admirable matrix to restrain Si nanoparticle agglomeration and can to form a conducting network. Through self-rearrangement of Si nanoparticles in micro carbon framework, the delamination of active materials from the current collector can be avoided, and a high-energy and long-life battery that paired with LiNi0.6Co0.2Mn0.2O2 cathode was obtained [3]. To increase the conductivity of the coating layer, a simple approach was applied to synthesize nitrogen and oxygen dual-doped hierarchical carbon. The obtained materials deliver superior cycle life and reversible capacity with capacitance contribution up to 30.3% [4].

Fig. 2. Compositing strategy for Si-C anode. (a-b) SEM images of spherical Si/C granules at different magnifications, (c) and cross-section of spherical Si/C granules. (d) Schematic illustration of Si-SiOx-C composites and corresponding SEM image and cycling performance. (e) Schematic illustration of preparation process of SiOx@graphene composites. Reproduced with permission from Ref. [5] (a-c), Ref. [6] (d), and Ref. [7] (e). Copyright © 2018 Elsevier (a-c), 2017 American Chemical Society (d), and 2018 Wiley (e). Such effects will be magnified when the electrode mass loading is high [8]. It should be noted that a suitable size and composition are of significance as the pure Si nanoparticle is unfavorable to achieve a high tap density and cell efficiency, which are the main purpose of Si-based anode for its higher volumetric energy density than that of graphite anode. An effective method is to prepared micro-sized secondary configuration from nano-sized building blocks. This conception has been vastly used in Si-based composite negative electrodes. For example, a dense nanostructured Si-C composite assembled by cluster is prepared by mechanical milling and carbon coating, the tap density attained 0.91 g cm-3 and mass loading over 2 mg cm-2 was realized for stable cycling [9]. Other composite structures involving Si-based microscale alloy and Si-containing graphite composites also receives high tap density over 1 g cm-3 [10], and based on these materials, the batteries get a much improved in the Coulombic efficiency due to remarkable decrease in surface area. In addition, the compositing strategy is appropriate for large-scale production. Guo’s group puts forward a three-step facile and low-cost production line, and a hierarchical Si particle with super P was encapsulated by graphene and reveal outstanding electrochemical performances even at a mass loading up to 8.5 mg cm-2 [5]. The hierarchically structured anode materials gain a 90.8% initial Coulombic efficiency, indicating electrode-electrolyte structure achieves an optimal state that maintains the stable interfaces (Fig. 2a-c). The micro-nano structure design addresses the issues of vast volume change and severe side reactions with electrolytes during repeated cycling. Future endeavor should direct to integrate high mass loading with Coulombic efficiency as well as

low-cost raw materials and processing techniques, and minimizing the weight of inactive materials and optimizing the electrode configuration with lightweight but highly conductive are also wistful. For example, the polymeric binders reveal assisted effectiveness in aggrandizing areal capacity. Some newly developed binders with self-healing and conducting properties have been proposed. With the intensive binding interaction and small binder addition [11], the areal capacity attains to a level over 3 mA h cm-2 [12]. (1.2) Surface/interface engineering The SEI formation and pulverization due to the vast volume change of Si, which dominates the cell Coulombic efficiency, are troublesome problems. The hinge is to decrease side reactions and preventing continuous SEI layer growth. For this point, multi-shell encapsulated Si materials display obvious effect unless an appropriate void space is left and structure integrity is maintained. This kind structure design has popularly used as a model system to study, because in an electrode fabrication process to obtain high tap density, it is hard to ensure that a hollow structure stays unbroken under pressure if the thickness is thin enough. Meanwhile, the electronic conductivity of multi-shell structure is also a problem. The misconnection among interlayers cannot be neglected when considering high-rate charging and discharging. Maybe, the surface coating is a better choice compared with multi-shell design, at least in terms of electronic conductivity. As the coating layer is strongly interlinked with the inner core, the conductivity of the coating layer is of priority. Thus, the selection for coating source prefers to be or that can be converted into the materials with high conductivity. For example, the carbon coating is generally used choice due to its facile preparation and high electricity after annealing, and the cladding layer enhances the interface and thermal stability. Other ways of coating include metal oxide, phosphate, and polymers. For instance, an amorphous TiO2 shell with 3 nm was coated on the Si nanoparticles, showing superior buffering properties in maintaining high structural integrity and enable a safer Si anode compared to conventional carbon-coated one [13]. All these three types of coating manners are subjected to the low electronic conductivity and the coating with nanoscale thin layer is elaborated that calls for exquisite synthesis techniques. Furthermore, for the materials with large size (micro size), it is difficult to realize a homogenized coating. It is noteworthy that the inorganic cladding layer reveals much improved thermal stability and reduced interfacial resistance because of the formed conformal layer. SiOx is a choice for high-energy-density anode, but its poor initial Coulombic efficiency (50-60%), without surface modification, cannot satisfy the utilization requirement. Actually, the Si surface is more or less covered by partial SiOx. Taking the advantage of this point, a high-speed spray pyrolysis method is proposed to prepare Si-SiOx-C composite, delivering a high reversible capacity of 1561.9 mA h g-1 at 0.06 C rate and initial Coulombic efficiency of 80.2% [6]. In this context, the SiOx functions as a benign protecting shield (Fig. 2d). However, how to balancing the content or thickness is still an issue that demands an in-depth study. The oxygen content in SiOx is also important for a high-capacity anode [14]. The researcher found that the structure and valance state of Si varies with the thickness of the surface oxide,

and an optimal five nanometers coating endowed the anode materials high capacity and cycling stability. The augmented electrochemical performance is ascribed to a robust SEI protecting layer that creates a stable interface, which permits the materials working at high charging and discharging depth. Although some SiOx/metal hybrid anode materials are also reported [15], the electrochemical performance is not competent with SiOx/C materials. For example, Guo’s group proposed the carbon anchored SiOx particles to form SiOx/C materials, which show 645 mA h g−1 and can retain 90% capacity after 500 cycles [15]. The high tap density and structure integrity of this anode materials offer an excellent choice in ensuring electrode robustness. Besides, the metal in SiOx/metal materials will add the electrode weight, lowering the energy density of batteries. In this viewpoint, the SiOx/C materials weigh more in terms of next-generation Li-ion batteries (Fig. 2e) [7]. (1.3) Active/inactive alloy From above mentioned Si-based anode materials, a carbon matrix is necessary to fully release the performance, and on the basis of this, some active or inactive metals can also be used as partial substituent for the carbon to form active or inactive alloy. Recently, a technique for preparing air-stable and freestanding lithium alloy/graphene foil including the Si, Sn, and Al was developed, greatly pushing the process for safe and high-capacity battery [16]. Further efforts still need to be devoted to cut down the electrode expansion and contraction caused by these alloy materials. (2) Other alloy anodes Besides the Si-based alloy for Li-ion batteries, several other promising types including Mg, Al, Zn, Sn, Sb, Bi. The general strategy is to incorporate these metals into the carbon framework, in which the carbon as a nano-confinement withstands the volume swelling. Taking the Sb as an example, a hollow Sb@C yolk-shell particle was prepared and a stable SEI film on the outside carbon shell enables it for high-rate Li-ion batteries [17], indicating the effectiveness in upgrading the safety of battery based on alloy anode. The surface defect of stabilizer also plays a vital role in alloy-type anode. On the support of defected metal oxide, a PPy-covered SnS2 shows strong chemical interaction with the support, unprecedented cycle stability for this anode is acquired [18]. One challenge for alloy anode is the structure stability at a high current density and low potential. Binary alloys are promising anode materials for Li-ion battery after forming interdigitated eutectic proposed by Manthiram’ group. It is a kind of alloy foil anodes, which is composed of nanosized electrochemically active materials on the scaffold of electrically conducting network to accommodate the volume change and can avoid the usage of the current collector, tendentious Li deposition at high capacity. Further, a Zn-Sn interdigitated eutectic alloy anode using a cost-effective cold rolling processing. After plastic deformation, a nanostructured foil of zinc conductive base with scattered tin domains offers a record-high volumetric capacity 1084 mA h cm-3, and more importantly, eliminates the risk of direct Li plating occurred in graphite anode at high capacity [19]. In addition, one significant research hotspot for alloy reaction applications turns to the metal anode protections, which will be discussed in the following section.

2.2.2. Alkalis anodes 2.2.2.1. Li metal anode. There is no doubt that Li metal is a tantalizing alternative to graphite due to the lowest redox potential and high theoretical specific capacity, and therefore the research enthusiasm towards it has never been obliterated [20]. On the other side, the high chemical activity of Li metal anode combining with the infinite volume change, as mentioned above, induce most challenges such as dendrites, poor Coulombic efficiency. For sure, numerous efforts have been made to solve these issues, illustrating bellow. (1) Host A host with high electronic conductivity and proper volume and affinity can homogenize Li-ion flux, accommodate volume change, and guide Li deposition. Such host generally owns three-dimensional (3D) structure and exhibits superior performance in constraining dendrite formation. For the sake of making the most of host space and avoiding the dendrite formation, a series of model systems have been come up with to investigate the Li nucleation/growth behavior [21-25]. Enlighten by the lightening rod theory, a porous Cu current collector with vertically aligned microchannel was prepared, where Li was electrodeposited onto the microchannel walls at different current density with a high areal capacity and dendrite-free morphology (Fig. 3a-d) [21].

Fig. 3. Porous host for lithium deposition. (a-d) Simulation of current distribution on the surface of porous Cu and preferable Li deposition site. (e) Schematic illustration of C-wood, ZnO coated C-wood and its composite with Li. (f-g) Electric filed distribution and Li+ flow under the condition of electrostatic equilibrium. (h-i) Surface and cross-sectional morphology after first cycling in Li symmetric cell. Reproduced with permission from Ref. [21] (a-d), Ref. [26] (e), and Ref. [23] (f-i). Copyright: 2017 National Academy of Sciences (a), 2017 Wiley (b-e), and 2018 Elsevier (f-i). Furthermore, porous carbon is an ideal candidate due to its large specific surface area, high porosity, high electronic conductivity and low tortuosity for fast plating/stripping (Fig. 3e) [26]. For instance, a periodic patterned voids of Li/Graphene composites were applied to understand the fundamental revolution of Li deposition via simultaneously controlling the nucleation site at the edges and growth direction in horizontal, ameliorating volume fluctuation and cycling stability. The lithiophilic CNT and acetylene black particles were implemented to construct a

dual-carbon porous sphere framework to uptake of lithium. The dual carbon system can offer a maximized utilization of the pore space and further increase the uptake of Li content, beneficial for high areal capacity and alleviation of volume change [24]. Similarly, a micro-fabrication method has been applied to create hexagonal micro-sized holes arrays to suppress the dendrite growth (Fig. 3f-i) [23]. From these achievements, it is easy to find that a three-dimensional host is a superexcellent architecture, and further adjustments are needed for surface structure, wetting ability and pore size and geometry to preserve uniform Li plating and stripping [25]. Another consideration is to improve the affinity between electrolytes and plating site via polar groups. Interestingly, the lithiophilic nature of heteroatom-doped carbon is uncovered and verifies that the electronegativity, local dipole, and charge transfer are critical factors for Li nucleation through theoretical calculations [27], and numerous doped-carbon hosts have been reported, such as N, B, P-doped carbon materials [22]. Halogen elements can provide a nucleation site of Li as well, as reported in Br-doped carbon materials [30]. Aside from created active sites by doping, one can adopt lithiophilic species or introducing the conformal coating layer by various surface modification techniques [28]. Some metal and metal oxides nanoparticles can be used as lithiophilic sites on carbon fibers to help to syphon molten Li, and the obtained composite Li electrode can work without dendrite formation and tolerate volume change in a Li-containing or Li-free cathode [29]. A cost-effective strategy was proposed by adopting a carbon nanotube (CNT) core encapsulated by hollow silica microspheres, then the Li deposition will occur at the electrical CNT core and confined within the silica protecting shell (Fig. 4a-c) [30]. Recently, Cui’s group carefully studied the seed-guided growth to control Li deposition within hollow carbon sphere with high Coulombic efficiency even in a carbonate electrolyte (Fig. 4d-e), and they found that the nucleation overpotential is dependent on the substrates but the nucleation barrier can nearly vanish for metals with definite Li solubility, such as Au, Mg, Zn, Ag etc. [31].

Fig. 4. Tuning the active site in host for Lithium deposition. (a-c) Schematic and bright-STEM image of Li trapped into the SiO2 microcapsule and corresponding electron energy loss spectroscopy spectra. (d-e) Voltage profile and SEM image for carbon with or without Au seed for Li deposition and corresponding schematic illustration (insert). Reproduced with permission from Ref. [30] (a-c) and Ref. [31]

(d-e). Copyright: 2018 American Chemical Society (a-c) and 2016 Nature publishing group (d-e). Aside from the electronic conductive framework, the ionic conductive framework is another choice. Constructing a Li-ion conductive framework by reacting overstoichiometry of Li with SiO can prevent Li from direct contact with electrolytes and ensure a high Coulombic efficiency [32]. Furthermore, the volume changes during charge and discharge can be alleviated and the battery still reveals low polarization and stable cycling under a high power output at 10 mA cm-2. The other thing-elastic property should be considered for the host to adjust to the volume change. A bulk nanostructured design is proposed to resist the fracture of Li metal anode caused by vast volume variation during the high-rate cycling [33]. (2) Interface engineering Except for the function of the host, it is more advisable to create a protecting surface or interface layer to render safe Li anode. Possible routines, such as constructing self-formed or artificial SEI, have exhibited satisfactory results. It has been shown that additives in the liquid electrolytes or increasing the salt concentration can facilitate to achieve a steady SEI. The additives include salt additives and inorganic additives. LiNO3 is classical additive for ether-based electrolytes, which can improve the stability and Coulombic efficiency of lithium plating and stripping. Unfortunately, LiNO3 possesses very poor solubility in commonly-used carbonate-based electrolytes. Zhang et al. reported that some salts can facilitate the LiNO3 to dissolve into these electrolytes [34]. Interestingly, LiNO3 can possess a very good solubility in flame-retarded phosphate-based electrolytes and significantly improve the compatibility of these electrolytes with lithium anode [35]. LiF is another very effective salt to tune plating behavior of lithium, which possesses even worse solubility than LiNO3 in many solvents. However, as one of the most popular components in SEI, LiF can tune lithium growth in the way of dense patterns rather than dendrites. Based on the guide of designing rich-fluoride SEI, numerous artificial SEI layers containing LiF were reported. Cui and co-author delicately designed LiF-protected Li metal reservoir by overlithiation of mesoporous AlF3 framework, a nearly unchanged skeleton and restrain dendrite growth during cycling are achieved, and thus realizing an ultrahigh current density up to 20 mA cm-2 in symmetric cells (Fig. 5a-c) [36]. Other lithium salts, including LiFSI, LiDFOB, LiBOB, were used to adjust the plating behavior as well. The widely-used additive (FEC) for silicon anode can be transplanted into metallic Li anode as well. It has been reported that FEC can significantly improve the Coulombic efficiency of Li plating and striping. It is worth noting that all those additives are used to facilitate the formation of homogeneous and stable SEI.

Fig. 5. Artificial SEI for lithium metal anode. (a) Scheme of plating behavior of skeleton with different conductivity, (b-c) and corresponding SEM images of Li surfaces and composite anode of Li/Al4Li9-LiF after 1 mA h cm-2 at 1 mA cm-2. Li surface and sectional morphology images with (d-f) or without (g-i) reactive polymer composite coating and optical imaging. Reproduced with permission from Ref. [36] (a-c) and Ref. [37] (d-i). Copyright: 2017 AAAS Science (a-c) and 2019 Nature publishing group (d-i). Aside from in-situ formed stable SEI during cycling process via additives, stable artificial SEI can be constructed with a special design and should be adaptive to the dynamic change of Li anode during the plating and stripping process [38]. For example, underneath the Li polyacrylic acid film, the side reactions and Li dendrite have been immensely reduced. Recently, a polymer-inorganic SEI was designed via a reactive polymer composite, it shows superior uniformity and mechanic strength, and can render LiNi0.5Co0.2Mn0.3O2 to work at high capacity and smooth Li plating/stripping with lean electrolyte (Fig. 5d-i) [37], which also shows a universality for other metal anode such as sodium and zinc. A general methodology to wet the interface for compact contact with Li anode is forming an alloy [16, 39, 40]. For example, a Li-rich composite alloy film is synthesized as a fast ion conductor and with surface insulation and shows superior effectiveness in sustaining steady electrodeposition over 1,400 h at a practical current density of 2 mA cm-2 [41]. (3) Functional Separators A robust separator endows Li metal batteries with much safer operation. The widely applied method is coating with metal or metal oxide [42], carbide [43], polymer [44], or their composites [45-48]. Several advantages can be acquired from the functional modifications. Firstly, the mechanical strength can be improved, and this property benefits greatly for Li dendrite suppression. Secondly, the electrolyte wetting ability will be also ameliorated for homogeneous ion flux, which is vital for smooth Li plating and stripping. Thirdly, the thermal stability is anticipated to be enhanced, surpassing the commercial bilayer or trilayer polymer membranes.

Yin and co-author proposed an MXene debris modified eggshell membrane, this separator showed an intensified mechanical strength and thermal stability up to 140
with well-defined dimensional stability. Moreover, the fast electrolyte infiltration ability was superior to that of the commercial separator, retarding polysulfide to crossover to react with Li anode with low overpotential because of good electricity and cycling stability [43]. The electricity of separator is significant in stabilizing metallic Li anode. The surface of polypropylene membrane was decorated by the polydopamine/graphene layer, the improved electricity and outstanding wetting ability facilitated the electrochemical reaction in Li-based cells with low resistance at the interfaces between separator and electrodes even at 0
. In addition, the coating layer provided an accessional site for Li storage without dendrite formation. On the basis of nanosized and microsized glass fiber, a 20 µm thick composite separator exhibited excellent stability up to 200
and superior electrolyte wettability. Meanwhile, the evenly distributed pore induced uniform current, thus validly reinforced the Li anode with a life of 620 hours at 0.65 mA cm-2 and good rate performance for cathode (Fig. 6a) [49].

Fig. 6. Tuning electrodeposition of lithium metal via functional separators. (a) Schematic illustration of separators based on cellulose and glass fiber for homogeneous Li+ flux. (b) SEM images of Nano cellulose, (c) and after PPy coating from top and side view. Reproduced with permission from Ref. [49] (a) and Ref. [46] (b-d). Copyright: 2019 Elsevier (a) and 2018 Wiley (b-d). Besides the mechanical strength in dendrite suppression, the intrinsic nature such as reaction activity and charge state to some extent does favor to defend the Li anode or cathode facing active material drifting. A composite coating layer consisted by nanosized TiO2 in the porous Kynar polymer was covered on the Celgard separator with the improved wetting ability and offered a platform to react with Li dendrite, Together with enhanced electrolyte holding capacity, the separator ensured small SEI and charge-transfer resistance with Ni-rich cathode materials [50]. However, the stability of polymeric separators at low voltage (< 1 V) should consider the trace water seriously when pairing with Li metal anode [47]. A bilayer separator consisted of the insulated nanocellulose fiber and redox-active

polypyrrole-nanocellulose to guarantee normal operation even direct contacting with Li anode and extra capacity nearly one-fold increment (Fig. 6b-d) [46]. Further, a trilayer separator was constructed by symmetric hydrophilic cellulose nanofiber on plasma-treated polyethylene, the cellulose nanofiber is 2.5 µm in thickness with mesoporous pore size (ca. 20 nm). This design unifies the Li+ flux through the well-proportioned channel and high thermal stability up to 200
as well as thermal shutdown ability, thus obtaining a smooth Li deposition with prolonged cycling stability [48]. The by-product generated at Li anode can drift to the cathode side, and deteriorate the cathode performance due to formation of resistive components. A polydopamine layer was sandwiched by the polyethylene to obtain Li+-permeable but detrimental chemical-hindered separator, homogenizing the ion flux for a relieved Li dendrite [44]. The ion sieving membrane should not only exhibit a positive effect on retarding for unwanted side reaction product, but also for selective Li+ permeation. Typically, a redox mediator, 5,10-dihydro-5,10-dimethylphenazine, is hindered by the graphene oxide nanochannels, the lithium-ion transference number was augmented. With proper capacity limitation at 0.75 mA h cm
2, a high energy efficiency of above 80% could be realized. An integrated all fibrous bi-layered separator was constructed by pristine and anionic cellulose nanofibers. This is an architecture with continuous ion and electron conduction pathway, discarding the current collector for Li anode that was accommodated by metal deposited poly(ethylene terephthalate) nonwovens. Meanwhile, the anionic cellulose nanofibers inhibited the polysulfide shuttling benefited from electrostatic interactions for improved kinetic and stability. With this structure design, the Li-S battery offered 457 W h kg-1/565 W h L-1 on the basis of whole cell. Recently, the metal organic framework materials (MOF) show promising effectiveness in upgrading the separator in Li metal protection [51]. A flexible MOF-based membrane (MOF@PVDF-HFP) was prepared by Zhou and co-worker. Profited from the uniform and narrow pore size distribution in MOF, the homogenized Li+ flux endows a high-current-density Li plating and stripping at 10 mA cm−2 and high capacity of 936 mA h g-1 after 200 cycles with 5.8 mg cm−2 sulfur loading for Li-S pouch cell. The Lewis acid-base interaction has also been applied to trap polysulfide by carbon nanotube encapsulated by zeolitic imidazolate [51]. The nanotechnology has revealed a vast potential in separator preparation and post-treatment with functionalization moieties for specific purposes, especially in active materials fixation and dendrite suppression. The burgeoning metal or covalent organic framework materials work as a booster to facilitate their development. (4) Nanostructured electrolytes Various liquid electrolytes have been discussed in previous sections. In this part, the main focus will be paid to the solid-liquid and solid electrolytes to settle the issues including interfacial resistance and compatibility. (4.1) Solid-liquid electrolytes Before achieving real all-solid-state batteries, a transitional stage that is generally defined as hybrid Li metal battery will exist [52]. Correspondingly, the solid-liquid

hybrid electrolytes [53] or quasi-solid electrolytes [54-56] have been developed to satisfy the requirement for safe Li metal batteries. The main aim of these routines is to introduce a wetting layer to reduce the interfacial resistance and improve ionic transportation in electrodes. To this end, an infiltrated quasi-solid electrolyte with transference number of 0.79 was prepared by in-situ polymerization (Fig. 7a-b), with an inner-rigid and outer-ductile structure, the battery impedance was reduced, and a repellent formed after lithiation improves the battery safety, showing good shielding effect in suppressing Li dendrite and cycling by-product drifting [56]. This fabrication strategy is suitable to construct an all-in-one battery with low interfacial resistance and for mass production at an industrial scale and enables both bulk and rapid interfacial ion transportation, and satisfy the requirement for cathode with or without Li source. An ionic liquid-derived nanostructured Li-rich fluoride was applied as a solid electrolyte additive to suppress Li dendrite growth. The asymmetric design with inorganic ceramic conductor and soft polymer coating triggers extensive attention. With the help of garnet conductor, an ion redistributor is obtained between the separator and Li anode, and resolves the problem of anisotropic distribution of Li ions (Fig. 7c-d), which may cause uneven nucleation, and enhances the ion conductivity and strength for dendrite suppression and prolongs the lifespan for cells [57].

Fig. 7. Suppressing lithium dendrites based on nanostructured electrolytes. (a-b) Sketch of i-QSE based Li metal battery and SEM image of i-QSE. (c) SEM image of deposited Li underneath the garnet modified polypropylene separator with liquid electrolyte, insert is the diagram of garnet modified PP separator. (d) Internal Li+ distribution simulation. Reproduced with permission from Ref. [56] (a-b) and Ref. [57] (c-d). Copyright: 2019 Elsevier (a-b) and 2018 AAA Science (c-d). (4.2) Solid electrolytes The intrinsic safety property and ability to restrain Li dendrites spur the research on solid electrolytes that cater for the future high-energy-density batteries. A rational design for solid electrolytes is built on increasing the ionic conductivity and extending interfacial compatibility with cathodes and anodes, as well as improving mechanical

flexibility [58]. In addition, the ion transference number is important for high-rate and stable Li plating/stripping [59]. Single-ion conducting electrolyte was obtained via electrospun nanofibers [60], it shows more satisfactory thermal behavior than that of commercial liquid electrolyte-polyolefin systems, and is equipped to inhibit Li dendrites. Generally speaking, inorganic solid electrolytes, such as Li7La3Zr2O12 (LLZO), possess high ionic conductivity, high shear modulus and unit transference number, which are widely regarded as the enabler for lithium metal anode. One of the challenges for LLZO is high interfacial resistance, which is derived from the surface layer of Li2CO3. A breakthrough was realized by depositing alumina at nanoscale thickness with atomic layer deposition. This conformal layer exceptionally reduces the interfacial resistance from more than one thousand Ohm cm2 to an ignorable value (Fig. 8a-b). The lithiophilic interface allows a high-voltage full cell with steadily cycling performance [61]. Furthermore, Li et al. proposed a facile method to remove Li2CO3 by reacting the Li7La3Zr2O12 with carbon, dramatically reducing the resistance to 28, 92 (65 oC), and 45 Ohm cm2 for the interfaces with Li anode, LiFePO4, and organic liquid electrolyte, respectively [62]. Besides the interfacial engineering for solid electrolyte, the nanostructured design is also significantly important to render a low-impedance host for Li metal. An integrated porous ceramic electrolyte is applied as a host to accommodate a high-capacity Li deposition and the issues of Li anode including volume expansion and interfacial resistance can be solved in one 3D ion-conducting interconnected network [63]. The porous structure can magnify the contact area between electrolytes with electrode materials and reduce the area resistance to ca. 7 Ω cm2 [64]. However, more and more studies have suggested that pure inorganic solid electrolytes are not able to suppress lithium dendrites when applied current reach up to a certain value [65]. Several theories have been proposed to fundamentally understand the mechanism of lithium propagation into inorganic solid electrolytes. One explanation is that lithium dendrites grow through the imperfect surface of solid electrolytes, such as voids, cracks, grain boundaries, and more seriously, the deposited lithium can create new cracks following the existing defects and performs as Griffith-like behavior. The newly-developed suggestion is that the high electronic conductivity of solid electrolytes induces direct formation and growth of lithium inside themselves [66, 67]. All these results suggest that single inorganic solid electrolyte cannot solve the issues of lithium metal anode. Composite solid electrolytes composing of polymer and ceramic electrolytes provide another choice. Inorganic electrolytes can significantly improve the ionic conductivity, transference number of polymer electrolytes, and mechanical properties of polymer electrolytes [68-70]. The anions can also be fixed by the incorporated inorganic fillers [71], leading to a uniform space charge and flat Li plating with stable surface shielding layer [72]. Different from the random mixing of inorganic nanoparticles and polymer electrolytes, the aligned ceramic nanowires as fillers for composite solid electrolytes can significantly boost the ionic conductivity (one order of magnitude higher than that of a random one at 30
) (Fig. 8c) [73]. The research

indicates that the structural control and arrangement is significant in ameliorating the conductivity of the electrolyte and thus improving the performance of Li metal batteries.

Fig. 8. Interfacial engineering for fast ion transportation. (a) Wetting ability of Li7La2.75Ca0.25Zr1.75Nb0.25O12 coated by the Al2O3 nanolayer, (b) and corresponding impedance before and after Al2O3 coating by ALD technique. (c) Schematic illustration of Li+ transportation in solid electrolyte within particle and allied nanowire. (d) Schematic illustration of thin asymmetric LLZO coating polymer electrolyte. Reproduced with permission from Ref. [61] (a-b), Ref. [73] (c), and Ref. [74] (d). Copyright: 2017 Nature publishing group (a-c) and 2018 American Chemical Society (d). Different interfaces possess different characteristics, e.g. the cathodic side should be soft with the anti-oxidation ability and the anodic side should own suitable mechanical strength to suppress dendrite growth [75]. The solid electrolyte with tailored design such as asymmetric characteristics (Fig. 8d) [74] and multilayered heterogeneous structure [76] has proved to reinforce the application of Li metal anode in high-voltage systems. New solid electrolytes with high electrochemical stability and ionic conductivity have been developed [77]. These electrolytes provide a new option for the fast ion transportation at solid electrolyte-Li interface and criteria for solid electrolytes with further improvement, including high shear modulus (10 times higher than the one of lithium), high transference number, high ionic conductivity (>10-4 S cm-1), low electronic conductivity (< 10-10 S cm-1), and good compatibility with lithium. (5) Anode-free designs Anode-free electrodes stand for a workable solution for the batteries with high energy density. Taking the Li battery for example, the avoiding usage of Li host cuts down the weight of cell, enhancing the energy density (per unit weight or unit volume) compared to current Li-ion batteries. Meanwhile, if replacing the graphite by the Li anode, batteries also exhibits an increment in output working voltage. Additionally, the manufacture cost for Li anode and related components will be saved.

The high-efficiency utilization of Li in the anode-free design weighs more heavily than the conventional Li secondary batteries as the cathode is the unique source for Li, and most commercial electrolytes cannot meet the requirement in cyclability (< 50 cycles) and Coulombic efficiency (< 90%) [78-80]. Particularly, in order to increase the energy density, the battery tends to pair high-voltage cathode with metal anodes and anti-oxidative electrolytes. For the anode side, after coating with mechanically flexible thin layer including polymer or alumina [81, 82], the plated Li can be shaped in a flat morphology and corresponding batteries obtain improvements in Coulombic efficiency and cycle life. As to the electrolyte, the electrolytes with high concentration gain tremendous attention. Typically, Zhang et al. proved that the effectiveness of ether with LiFSI in assisting smooth Li deposition on Cu substrate under optimal charging/discharging procedure [83]. However, such electrolyte is disappointing when paired with NMC-Cu “anode-free” cells. Toward this, Xu et al. put forward a bisalt ether electrolyte with good cycling stability up to 4.4 V, verifying that the TFSI- and FSI- decompose into a new interphase to promote dense and conformal Li plating [84]. It is noting that, considering the cost and energy density, the high Li content in electrolyte is not a preferential choice. Recently, a more practical candidate was realized by Dahn et al [78]. With a salt mixture of LiDFOB/LiBF4 (ca. 1.2 M) in FEC:DEC (volume ratio 1:2), the NCM-Cu “anode-free” pouch cell operates normally between 3.6 and 4.5 V with 80% capacity retention after 90 cycles, which is the longest service life with zero Li excess. This enlightening work also reveals that the suitably applied pressure benefits for dendrite-free Li deposition and the slowly-consumed Li salt waits for further handling protocols. 2.2.2.2. Other alkalis metal anodes. Other alkalis metal anodes include sodium (Na) and potassium (K), although revealing satisfactory abundance and low cost, they face more complicated kinetics and severe challenges at the anode side, and the heavy atomic weight lows their energy density. To enlarge the accommodation capacity and shorten the ion diffusion pathway, nanostructured materials endow batteries with excellent rate and power capability. It should be noted that the magnified surface area may result in side reactions that restrain the cell efficiency and lower material density and then the battery volumetric energy density. The efforts concentrate on the balance of pros and cons. However, the nanostructured design advances electrode development, especially for composite or alloy anodes. The following part will focus on the Na metal anode. (1) Na anode The compositing and alloy reaction provide nanomaterials with higher storage ability for Na, but the dendrite-like Na platting and drastic reaction with electrolyte make the implementation of Na anode difficult. Compared with the Li anode, Na is more ductile and reactive, this property is beneficial to composite Na with various framework to form 3D hosts, such as the carbon materials [85, 86], porous metal current collectors [87], alloy [88], and other hybrid anodes [89]. For example, a 3D flexible carbon felt host was prepared for the dendrite-free anode in carbonate electrolytes. When paired with Na0.67Ni0.33Mn0.67O2 cathode, a highly stable full cell was demonstrated (Fig. 9a-c) [85]. The 3D structure can offer

Na metal a vast and free space to relieve inhomogeneous residual stress and homogenize the Na+ flux for smooth anode without dendrite formation and large overpotential [90].

Fig. 9. Conducting host for Na composite anode. (a) Schematic illustration of Na/Carbon felt anode fabrication. (b-c) side-view of carbon felt before and after Na infusion. Reproduced with permission from Ref. [85] (a-c). Copyright: 2018 Wiley (a-c). This conception has also been applied for the Al or Cu current collector design [91]. A Na anode-free batteries were achieved on the nanocarbon nucleation layer formed on Al current collectors [91]. Such a configuration can load Na of 12 mA h cm-2 even at 4 mA cm-2 and provide a full cell with∼400 Wh kg-1 when working with pre-sodiated pyrite cathode. The anode-free design upgrades the energy density for alkalis metal batteries, especially suitable for systems with active materials that need extra supplementation. Combining with the idea of alloy reaction, the lower energy barrier for flat Na deposition is anticipated compared with pure sodium. Typically, a Na-Au alloy interphase was constructed by reacting with Au film on Cu substrate, enabling the Na stripping and plating to efficiently process under small polarization, and it is a universal strategy with other metal like Sn and Sb, showing with average Coulombic efficiency up to 99.9% at 2 mA cm-2 (Fig. 10a-b) [92]. The liquid Na metal anode is a favorable choice to achieve high ion conductivity and low interfacial resistance between with solid electrolyte [93]. The Goodenough and co-worker proposed a room-temperature liquid Na-K anode that was acquired through vacuum infiltration of the alloy in the porous metal membrane. This anode enables safely working without dissolution into the liquid organic electrolyte [88].

Fig. 10. Alloy reaction for stable Na deposition. (a) Schematic illustration of Na deposition and stripping on the Cu decorated with Sn or Sb nanoparticles. (b) Coulombic efficiency of Na cycling on different electrodes. Reproduced with permission from Ref. [92] (a-b). Copyright: 2019 Wiley (a-b). (2) Electrolyte design The electrolyte design for sodium anode is more challenging because of its extreme activity. The critical is to build a robust SEI, and some strategies can refer to that applied to Li metal anode but need modifications. For example, the fluoride-rich SEI does good to hindering continuous reaction between Na metal and electrolyte, Cui and co-author firstly demonstrated an even Na deposition and high Coulombic efficiency (99.9%) in glyme solvents containing sodium hexafluorophosphate [94]. Dating back to the root of dendrite formation, the metal deposition is not proportional to the linear flux of ions, there exists a concentration gradient from the bulk to electrode interface, and the dendrite forms at the location with high ion concentration. The methodology is to uniform the ion distribution or obtain a single-ion conductor. For this aspect, electrolytes with high salt concentration or additives and solid electrolytes gain increasing attention [95, 96]. Zhang and co-author utilized “inert” hydrofluoroether diluent to reduce the electrolyte viscosity while maintaining relative low concentration for sodium bis(fluorosulfonyl)imide (2.1 M) in 1,2-dimethoxyethane-bis(2,2,2-trifluoroethyl)ether (LHCE) (Fig. 11a-b). This electrolyte helped the Na deposition with flat morphology and ultrastable cycling at 20 C rate over 40000 cycles [95]. Wang and co-author firstly introduced potassium bis(trifluoromethylsulfonyl)imide as an additive to stabilize Na anode with the static shielding effect [96], enabled a 10 mA h cm-2 capacity with deeply cycling. Actually, the ionic liquid electrolyte reveals similar effect for its static backbone for pure sodium ion transportation [97]. Typically, Wei et al. adopted the electrochemical impetus to trigger 1,3-diallyl imidazolium perchlorate polymerization, and a sealed membrane covers the Na metal anode, drastically inhibiting the side reactions and dendrite growth (Fig. 11c) [98]. Similarly, thickness-tunable graphene was covered on the Na metal surface as the model system to study the Na plating and stripping behavior [99], discovering that the thickness within 10 nm influenced the rate performance greatly. Exemplarily, a stable Na cycling was firstly realized in carbonate electrolyte without additive at 2 mA cm-2 for 3 mA h cm-2 [99].

Fig. 11. Interface stabilization at anodic surface and cathodic bulk phase. (a-b) Schematic of LHCE electrolyte and SEM image of Na deposited on Cu with 2 mA h cm-2 at 1 mA cm-2. (c) Schematic illustration of 1,3-diallyl imidazolium perchlorate electropolymerization for stabilized Na anode. (d-e) The succinonitrile plastic crystal electrolyte enables stable cycling with Na3V2(PO4)3 cathode. Reproduced with permission from Ref. [95] (a-c), Ref. [98] (c), and Ref. [100] (d-e). Copyright: 2018 American Chemical Society (a-b) and 2017 Wiley (c-e). In addition to the modification liquid electrolytes, solid electrolytes provide higher security coefficient for Na batteries due to the non-flammability and thermal stability, higher ion transference number for homogeneous ion flux, and barrier for cycling by-products drifting [101]. The challenge for solid electrolytes when pairing with sodium metal anode is the interfacial ion transportation and suitable mechanical strength considering the soft characteristic [102]. Although pure inorganic ceramic conductors show high ion mobility and thermal stability and considerable efforts have been devoting to the field, their brittle nature adds the difficulty in processing and rigid surface deteriorates the contact effectiveness. A facile avenue is to introduce a wetting layer or polymer buffer layer, such as the plastic-crystal electrolyte (Fig. 11d-e) [100] and ionic liquid [97]. The mediated layer not only greatly lowers the interfacial impedance for fast cross-boundary ion transportation, but also hinders the electron shuttling for restrained decomposition reaction for Na3SbS4 electrolyte with Na metal. Computational Study verified that the root cause of mixed ionic- and electronic-conducting phase was Na2S and Na3Sb [103]. The mixed conducting interlayer leads to continuous SEI growth and deteriorates the battery lifespan. Li and co-worker designed an interlayer with the graphene-like structure on Na3Zr2Si2PO12 by the chemical vapor deposition technique, the interfacial resistance between with Na anode reduced from 524 Ω cm2 to 46 Ω cm2, enabling the Na symmetric cells to work under 1 mA cm-2 with 1 mA h cm-2 over 1000 hours without dendrite formation [104]. Another consideration is to utilize the polymer electrolyte due to its low mass density and flexibility. A transitional electrolyte, that is gel polymer electrolyte, was in-situ formed on Na anode surface, the highly conductive electrolyte realized a robust interface in Na-S battery with a reversible capacity of 877 mA h g-1 at 0.1 C [105]. A newly designed hybrid polymer electrolyte endowed the Na symmetric cell with over 3500 hours at 0.5 mA cm-2 and firstly found that the interphase migrated

into the polymer electrolyte [102]. Some newly developed solid electrolytes have been applied to Na anode protection [106]. Exemplarily, the concentrated NaY xNH3 (Y = I-, BF4- or BH4-) shows outstanding conductivity for a dendrite-free Na anode with high reversibility. A compromised strategy is to incorporate inorganic conductor into the polymer electrolyte to obtain composite electrolyte, this design integrates the high ion conductivity, flexibility, easy processing ability, and good contact into one electrolyte. For example, the Na3Zr2Si2PO12 was incorporated into the polyvinylidene fluoride matrix to form a composite electrolyte, the interfacial impedance was reduced and the Na deposit was equalized [107]. (3) K anode and electrolytes As a burgeoning field, the study of K anode can refer to Li or Na anode but with some particular characteristics. Firstly, the cell voltage of K+/K is 0.15 V lower referring to Li+/Li in ethylene carbonate/diethyl carbonate [108]. It means that the battery with K anode can deliver higher output voltage when paired with same cathode materials. Secondly, the abundance is competitive to Na but higher than Li in the earth [109]. Meanwhile, the price and distribution is more accessible, which offers sustainable affordability for large-scale energy storage regardless of specific energy. Thirdly, the electrochemical process of K anode in potassium salt-based electrolyte is well defined in a narrow voltage window [110]. Such a redox behavior offers a flat plateau during the battery cycling process. Fourthly, the large ion size of K+ brings in smaller size of solvated K+ compared to solvated Li+ and Na+, showing fast conductivity and high transference number. Therefore, the rate determination step is dominated by the diffusion process in electrode materials. To this end, much efforts have been applied to address this crux in potassium secondary batteries. Due to the highly reactive property, the pure K metal anode is not compatible with the ether and carbonate electrolytes containing KPF6 or KTFSI, but can work steadily in the KFSI/DME with ∼99% Coulombic efficiency at ambient temperature [111]. Substantial works reveal that a robust SEI is crucial to maintain the normal operation K metal-based batteries. Through choosing suitable solvent, electrolyte salt and concentration, the dilemma of uneven K plating/stripping and irreversible cathode reaction can be resolved [112, 113]. A safer methodology is to employ the non-flowable electrolyte to replace the liquid electrolyte for stabilizing the K anode surface. As reported by Goodenough et al., the poly(methyl methacrylate) gel electrolyte containing KPF6 enables the K/polyamine battery with 98% of its initial capacity after 100 cycles compared with 20% capacity fading in liquid electrolyte [114]. Actually, a popular routine is to develop K alloys, such as Na-K [115] or Bi-K [116], as a more acceptable anode materials. Using an aligned CNT membrane to adsorb the liquid Na-K alloy at room temperature, a flexible anode is established, and a stable and dendrite-free anode is acquired due to the immiscibility between Na-K alloy and liquid electrolyte [117]. Corresponding full cell shows reduced volume change and excellent rate capability. Associating with graphite as intercalating host, the Na-K liquid alloy forms a network-like self-healing composite with rapid electron

and mass transporting ability, ensuring a long-term working for 5000 hours at 20 mA cm-2 and 16 mA h cm-2 cycling at 80 mA cm-2 [118]. Thanks to the mixed 3D conducting network and abundant nucleation sites for K affiliating [119], the K plating/stripping realizes a stable and low-overpotential profiles in the traditional carbonate electrolyte, delivering a promising processing method for high-efficiency K utilization. 2.3. Cathode 2.3.1. Transition metal oxide cathodes If the energy density of batteries outperforms the LiCoO2 cathode, nickel-rich (Ni-rich) cathodes, and Li-rich cathodes are the most promising candidates to achieve the goal [120-122]. Generally, the Ni content is anticipated to be enhanced to over 60%, and even higher than 80% and the material capacity correspondingly increases. Unfortunately, the increment of Ni content incurs structural and thermal instability. The issues of phase transformation from layered to rock-salt structure, oxygen evolution, reaction with the electrolyte or Ni dissolution will magnify with the increase of Ni content at the charging state, and all these predicaments increase the risk of thermal runaway and even disastrous battery explosion [123]. For this dilemma, advanced design of active materials and interfacial design between electrode and electrolytes are essentially important and should be carefully screened, and corresponding strategies for cathode material modification includes surface modification, core-shell or concentration gradient structure design, doping and electrolyte designs. 2.3.1.1. Surface modification. Coating is a viable method to relieve the cathode reactivity as a physical blocking layer, and heterogeneous reaction between material or cathode and electrolyte can be restrained. Corresponding coating materials are generally inactive in chemistry or electrochemistry. The commonly exploited species are (Li-containing) oxide [124] and fluorides [125]. Partial of the coating layer can be converted into artificial SEI, including some polymers or ionic conductors. For example, LiNi0.6Mn0.2Co0.2O2 with a nano-thick poly(acrylonitrile-co-butadiene) coated layer can be stabilized and worked efficiently with low interfacial impedance (Fig. 12a-c) [126]. The exothermic peak of NCM after poly(acrylonitrile-co-butadiene) coating reveals a shift of 5 oC to higher temperature (290 oC) and a dramatic suppression (49% decline) in total heat generation compared to pristine NCM cathode. Obviously, the surface coating layer restrains the side reactions between NCM and electrolyte, enhancing the thermal stability of batteries with solid-state electrolyte. As the example shows, the coating is efficient to stabilize the interface and to cope with large interfacial resistance [20], but the compatibility still deserves broad and comprehensive investigations [127]. An in-depth comprehension of the mismatch between Ni-rich cathode and solid electrolyte demonstrates that the interfacial potential drop determines the dynamic of battery, and a nano-thick Li1.4Al0.4Ti1.6(PO4)3 coating layer is implemented onto the LiNi0.6Mn0.2Co0.2O2, releasing the potential drops and improving the rate and cycling performance (Fig. 12d-f) [128].

Fig. 12. Cathode materials coating for low-resistance interfaces. (a) TEM image of poly(acrylonitrile-co-butadiene) coated LiNi0.6Mn0.2Co0.2O2. (b-c) Impedance spectra for cathode with or without polymer coating. (d-e) TEM image of LiNi0.6Mn0.2Co0.2O2 coated by 5 nm Li1.4Al0.4Ti1.6(PO4)3 conductor (NCM@LATP), (f) interfacial potential curve for pristine and NCM@LATP. Reproduced with permission from Ref. [126] (a-c) and Ref. [128] (d-e). Copyright: 2018 Wiley (a-c) and 2018 American Chemical Society (d-e). 2.3.1.2. Core-shell structure. The core-shell structure is proposed by the Sun et al. and built on the homogeneous crystallographic constituent as the core with highly thermal stable shell [129]. Obviously, the thicker of the shell, the longer lifespan and more favorable thermal stability are for the cathode materials, but a small compromise of capacity and sluggish ion/electron transportation kinetic will occur. Thus, to gain the same capacity or rate ability, a slightly higher working voltage is necessary. In addition, the unsynergetic volume variation between core and shell inevitably induces stress change and structure damage, and then capacity and rate capability deteriorate. As a result, the hetero phase boundary is still a hinge that calls for delicate design to ward off the core/shell phase segregation during the synthesis or repeated electrochemical reactions, and at the same time guaranteeing the efficient charge transportation. 2.3.1.3. Concentration gradient structure. Another effective avenue is to construct a concentration gradient structure that can simultaneously sustain the electrochemical performance and thermal stability. The difference between with core-shell structure is that the Ni-rich core is encapsulated by the shell with concentration gradient distribution. In this case, the disconnectivity that exists in the core-shell structure and cracks during battery cycling vanish, the pathway for ion and electron is unblocked, and thus an upgraded electrochemical performance is acquired [130]. Further, a dual-sloped concentration gradient cathode that consists of maximized Ni concentration in the core part and a steep Mn gradient at the surface part is developed (Fig. 13a) [131]. The structural design is attributed to the reduced microcrack formation and crystal axis contortion along the c direction, and the exothermic decomposition is delayed to 200 oC with less heat generation due to the low surface

area contacting with electrolyte even at a high delithiation state, exhibiting pronounced improvements in cycling stability and thermal stability that surpass that of NCA.

Fig. 13. Bulk and surface structural adjustment for prolonged cycle life. (a) Cycling stability and morphology comparison of compositionally graded Li[Ni0.84Co0.06Mn0.09Al0.01]O2 (TSFCG-Al) and pristine NCA. (b-c) SEM image of Li1.214Mn0.530Co0.128Ni0.128O2 and corresponding energy dispersive spectra. (d-e) Li salts with triple anions stabilized Ni-rich NCM cathode and long-term cycling stability between 2.7 V-4.5 V at 2 C rate. Reproduced with permission from Ref. [131] (a), Ref. [124] (b-c) and Ref. [132] (d-e). Copyright: 2016 American Chemical Society (a) and 2018 Wiley (b-e). The aforementioned strategies can be applied equally to the Li-rich or Mn-rich cathode materials [133]. For instance, a controlled oxidation treatment renders the Li-rich cathode with uniform spinel Li4Mn5O12 coating and the coating layer shows a compatibility with the bulk structure and verifies the significantly restraining the escaping of lattice oxygen and maintaining surface stabilization (Fig. 13b-c) [124]. Adopting a simple after-treatment, a Li-rich cathode is obtained with increasing the nickel content, the average discharge voltage is boosted from 3.5 to 3.8 V [120]. Given the two same energy density systems, the one derived from higher operative voltage guarantees a high energy efficiency than the other one given the two same energy density systems. Hence a higher operative voltage demonstrates a higher energy quality, which is a substantially important parameter for the future cathode design. 2.3.1.4. Doping. Besides above strategies, other solutions like doping have also been introduced to protect transitional metal oxide cathode. Through doping, the phase conversion from layer to spinel structure during Li-intercalation and extraction can be restricted to certain extent and the operative voltage decay can be relieved as well. The increase of Ni content in Li-rich cathode can stabilize crystal structure but concomitantly leads to a decreased reversible capacity that is caused by less active Mn4+/3+ redox couple and reversible oxygen anions in charge/discharge process [120]. Further, the relationship between oxygen redox and oxygen release needs to be verified, and the mechanism for voltage fade is unclear [134]. The molybdenum has been utilized as a dopant to improve the oxygen stability via strong interaction with

oxygen and to activate Mn4+/3+ redox couple in the hexavalent state [135]. Other element doping (such as boron) can effectively impede transition metal ion migration by blocking the transit channel as well as improved cycle life. The obtained full cell reveals an energy density up to 472.1 W h kg-1 [136]. With the advanced characterization techniques, recent work discovered that the voltage fade is linked to defect formation, such as large pores, high-density dislocations and cations migration [137]. In addition, the surface reaction is associated with oxygen radicals and phase transitions, but its interaction with bulk oxygen redox is indeterminate. For example, the niobium was used as the surface dopant in the Li-layer to intensively bind with oxygen. The inactivated surface oxygen enhances the structural stability and shows a high initial capacity of 320 mA h g-1 with 94.5% retention ration after 100 cycles [138]. The variation in structure stability also fetch in reduced heat generation from 107.18 J g-1 to 77.53 J g-1, indicating a more favorable thermal stability and safety. Meanwhile, by adjusting the oxygen-centered structure such as creating partially cation-disordered sites and oxygen vacancy in the bulk lattice can help to mitigate voltage decay and stabilize the active oxygen redox dynamic [139]. 2.3.1.5. Electrolyte design. The electrolyte optimization is imperative to guarding stable SEI for stable battery operation at high cut-off voltage [140]. The efforts to elevate the voltage tolerating ability is realized via salt additives or new solvent systems. For example, using the ternary salt in ethylene carbonate and ethyl methyl carbonate as the optimized electrolyte, a stable Li metal battery consisting of Ni-Rich cathode, LiNi0.76Mn0.14Co0.10O2, can operate at high rate condition (2 C) with >80% capacity retention after 1000 cycles (Fig. 13d-e) [132]. The augmented electrochemical performance is ascribed to the intensified structural and interfacial stability at both cathode and anode sides. Furthermore, the electrolyte additive can dramatically enhance the battery safety and cycle life [141]. Related additives (generally less than 5-10% in weight or volume ratio) toward Ni-rich cathode (general Ni ≥ 60%) working at high voltage should meet several criteria. Firstly, the additive encounters oxidation prior to the electrolyte and forms shielding layer on the cathode to stop continuous growth of cathode electrolyte interphase, guaranteeing a low cell impedance [142]. Secondly, the additive is also anticipated to improve the cathode stability at high working temperature [143]. For instance, the tris(trimethylsilyl)borate occurs oxidation to reinforce the CEI at cathode and restrain the electrolyte deterioration as well as electrode disintegration at 60 oC [144]. Besides, the Si and O in silyl-type additives own the ability of scavenging fluoride and hydrogen species derived from the electrolyte salt, which is also efficacious for anode. Another prominent additive is the divinyl sulfone [145]. It is discovered that the vinyl joins into the sulfone-containing interlayer between electrolyte and Ni-rich cathode surface upon being electrochemical oxidation, sustaining 91.9% capacity after 100 cycles at 60 oC. In addition, the combination of two or more additives reveals synergic effect on reducing cell resistance and transition metal dissolution from cathode [80]. Recently, the non-flammable electrolytes bring new breakthrough. Taking the trimethyl phosphate as the example, it will decompose to species that contain phosphorus radical ([P]•). The [P]• can eradicate the combustible radicals (H•, O2*,

HO•), which works as a critical role in sustaining the chain reaction of combustion [146]. Similar analogues include triethyl phosphate [147], diethyl ethylphosphonate [148], tris(trifluoroethyl) phosphate [149], etc. A troublesome issue is the introduced anti-flammable constituent will deteriorate the cell Coulombic efficiency and lead a high-impedance interface layer at both anode and cathode side. A choice is to combine with high salt content [147] or secondary salt [150], these new electrolytes can not only ensure the safety but also improve the compatibility with Li and graphite anodes, enhancing the Coulombic efficiency. A fluorinated electrolyte has been developed to match Li metal anode with 99.2% efficiency and LiNi0.8Mn0.1Co0.1O2 cathode (~99.93%). The study indicates fluorinated interphase with 10 nm plays a significant role in stabilizing the aggressive surface [151], and the tendency of oxygen release for delithiated NMC811 was constrained. Meanwhile, the all-fluorinated electrolyte postpones the temperature of electrolyte oxidation with less heat generation, retaining the structure and thermal stability of Li metal batteries. Tan et al. developed a flame-retardant electrolyte that can nitride Li surface for dendrite-free Li plating and stripping, and when paired with Ni-rich cathode (LiNi0.8Mn0.1Co0.1O2), the Li metal batteries shows superior cycling stability and safety detection such as igniting and needle piercing [152]. It is envisaged that, on the premise of stable anode protected by nanostructured passivating layer, the employment of functional electrolytes will entitle high-voltage cathode materials with new chance in the future. Solid electrolytes are regarded as an effective solution to augment energy density and settle the safety issues of batteries. For the high-voltage/capacity transition metal oxide cathode materials, the primary consideration is the anti-oxidation ability. A general perspective is to employ composite electrolytes considering their low density and facile processing ability. Meanwhile, the rigid-flexible property can reduce interfacial resistance and dendrite-suppressing ability. Matching the polymer backbone and Li1.4Al0.4Ge1.6(PO4)3 conductors, a multilayered solid electrolyte with heterogeneous structure is prepared and an electrochemical stability window up to 5 V is achieved, showing outstanding cycling stability when paired with Ni-rich cathode and Li anode [76]. Apparently, with a reasonable arrangement of the electrolyte layer, the electrolyte stability can be extended. Unfortunately, the interface still requires further wetting modifications to ameliorate contact and reduce resistance, such as adding a small amount of liquid electrolyte, utilizing gel electrolytes, and so on. These strategies suppress the dissolution of transition metal cations and HF attacking, prolonging battery’s lifespan. From a practical viewpoint, the future efforts endeavor for improving the electrode density (such as 3.6 g cm-1) by increasing the active materials ration (94 : 3 : 3) in slurry and stabilized electrode-electrolyte interface under the precondition of high-voltage over 4.5 V (vs. Li+/Li). To reduce the interface within the electrode and contacting with electrolyte, a possible routine is to utilize single crystalline cathode, which can lower the specific area and risk of gas evolution at both room and high temperature. The solid batteries assembled with transition metal oxide cathode and Li metal anode provide after constituents optimization including negative/positive electrode thickness and their ratio, electrolyte content, inactive materials weight, and

cell design [20]. 2.3.2. Chalcogen cathodes 2.3.2.1. Sulfur/Selenium cathodes. Li metal batteries based on chalcogen cathodes (such as oxygen, sulfur and selenium) deliver high specific energy. Among these conversion-type cathodes, Li-S battery with intrinsic advantage of low cost and abundant raw materials, and tremendous ingenious composite electrodes including porous container or nanosized module to resolve the conductivity [153], but the still pending poly(sulfide/selenide) dissolution and sequential shuttle between with Li anode hinder its application process. Sulfur cathodes as the first barrier to restrain the polysulfide to shuttle have encountered so much attention that this contribution will focus on the electrode-electrolyte interface with tailored design at nanoscale level. (1) Electrolyte upgradation The electrolyte is the key to suppress the polysulfide formation [154], compared with the liquid electrolyte after modification with solvents [155], Li salts, or additives [156]. In recent years, more attention has been paid to solid-state electrolytes [157]. Following features for solid-state electrolytes overmatch liquid electrolytes. Firstly, the possibility of polysulfide is eliminated due to the static characteristic. Secondly, solid-state electrolytes display high stability, satisfying the wide voltage ranging from cathode and Li anode. Thirdly, the safety level can be promoted with general noncombustible property, especially for inorganic electrolytes. In this circumstance, even with very few amounts of Li anode, a high-energy density battery can be acquired [158]. The solid polymer electrolyte is a good choice for Li-S battery due to its flexibility and low mass density, but the low ionic conductivity at room temperature hinders its applications. Blending with inactive fillers or active conductors can largely constrain the polymer backbone to crystalize [159, 160], and thus maintain their fast ion conductivity. Further, the interface between polymer and filler provide an additional pathway for ion transportation [161]. Such nanoscale percolation tunnels are also benefited for polysulfide adsorption. If with extra interactions such as electrostatic attraction, the shuttle behavior can be eliminated and an enhanced Li salt dissociation to promote single Li-ion transportation. Inorganic electrolytes generally own high ion conductivity and mechanical strength but face perplexing large interfacial resistance. The SEI on Li surface or S cathode involves the composition of sulfide. Consequently, the introduction of sulfide-containing electrolyte may promote a stable interface formation and match for different cathode materials [162]. Zhang and co-worker proposed an implantable SEI protected Li anode to pair with the sulfur cathode and exhibit superb cycling stability either in ether or ester electrolytes [163]. The sulfide-based solid electrolytes include glass-ceramic and their derivations. With proper treatments, the glassy-state ion conductor can be converted into crystalline electrolyte, thereinto, a class of sulfur-substituted conductor, defined as thio-LISICON, shows faster ion conductivity because of expanded Li diffusion pathway and lower dissociation energy after large-size and low-electronegativity sulfur ions [164]. However, these electrolytes are not stable with the Li anode, deteriorating their usefulness in Li-S batteries. A recently

reported sulfide conductor, Li9.54Si1.74P1.44S11.7Cl0.3, delivers the highest conductivity and greatly improved stability versus Li anode [165]. In addition, NASICON and garnet-type ion conductors are promising candidates to build solid-state Li-S batteries [166]. With the softening by polyethylene oxide (PEO), the Li7La3Zr2O12 nanoparticles were incorporated into this conducting matrix, and compatible interfaces with the sulfur cathode and Li anode were obtained [159]. An LLZO electrolyte with a porous layer for sulfur accommodation and dense layer for blocking polysulfide diffusion was designed [167], this electrolyte provides a fine host for sulfur with mass loading up to 7 mg cm-2 and ca. 99% coulombic efficiency (Fig. 14a-c). Other promising solid electrolytes, such as lithium anti-perovskite and lithium hydrides have also been developed [168, 169]. For example, the LiBH4 as an additive can relieve the surface impedance to obtain a favorable contact, a satisfactory electrochemical performance can be obtained at 120
[169].

Fig. 14. Solid-state electrolyte design for Li-S batteries. (a) Schematic of hybrid Li-S battery consisted by bilayer solid-state electrolyte (SSE) to support S/CNT cathode, (b) Elemental mapping of SSE-supported S/CNT cathode (La: red, S: green), (c) and charging/discharging profiles with ca. 7.5 mg cm-2 at 0.2 mA cm-2. (d) Sketch of the in-situ polymerization inside the battery system. Reproduced with permission from Ref. [167] (a-c) and Ref. [54] (d). Copyright: 2017 Royal Chemical Society (a-c) and 2018 AAAS Science (d). It is noteworthy that there exists a transitional state to realize a solid-state Li-S battery due to the extremely poor electronic conductivity, a compromised choice is to employ the quasi-solid or hybrid electrolytes in the Li-Chalcogen batteries, especially for the Li-S battery [170]. Liu et al. developed a quasi-solid electrolyte that can simultaneously satisfy the requirements of voltage for sulfur, polyanion and transitional metal oxide cathode materials and protective effect on Li anode (Fig. 14d) [54]. The improved battery performance is ascribed to the in-situ gelation process converts the unstable cyclic structure into an anti-oxidation linear structure in advance, and endows the electrolyte with outstanding polysulfide shuttle. (2) Binder optimization The binder is an auxiliary part in electrode functions as a glue to sustain the integrity of electrode materials and well contact with the current collector. In the Li-S battery, another consideration for binder selection is the species that consists of rich

anchoring sites for polysulfide [171]. It is a promising routine, usually choosing the chemicals with strong binding effects toward the polysulfide. Existing researches indicate that the polymer with polar groups such as oxygen and imine show shield function for polysulfide shuttle (Fig. 15a-c) [172, 173]. A natural Gum Arabic (GA) polymer proposed by Xu et al. reveals remarkable performance in maintaining 10.8 mA h cm-2 areal capacity. The formed polysulfide can be restricted within the uniform but nanoscale GA coating. Related DFT calculations verify that the precise oxygen content is determinable to conductivity and polysulfide shuttling [173]. The cathode loading is a significant parameter toward high-energy-density battery. A weaved biopolymer network was employed as a robust binder to fixed an ultrahigh areal capacity up to 26.4 mA h cm-2 through an intermolecular binding effect [174]. This is a cost-effective but friendly approach to hold back polysulfide shuttle.

Fig. 15. Functional binder and separator for suppressed polysulfide shuttling. (a) Simplified molecular structure of PPA binder derived from poly(ethylene glycol) diglycidyl ether (PEGDGE) and polyethylenimine crosslinking. (b-c) The calculated binding energy between PPA binder with Li2S and adsorbing effect for sulfur. (d) The crystal structure of HKUST-1, (e-f) SEM image of MOF@GO separator and its application in Li-S battery for long-term cycling test. Reproduced with permission from Ref. [172] (a-c) and Ref. [175] (d-f). Copyright: 2018 Wiley (a-c) and 2016 Nature publishing group (d-f). (3) Separator modification Besides the function of isolating the cathode and anode, the separator can also be a solution for battery issues such as dendrite suppression, poly(sulfide/selenide), etc. Especially in recent years, the nanotechnology engineering offers enormous possibilities for separator modification. The specific modification with affinity organic group [175] or inorganic blocker offers additional protective umbrella in Li-S batteries [176]. For instance, the matrix of carbon supported metal or metal oxide, or polymer-inorganic composites can not only improve the utilization efficiency of active species but also capture the polysulfide through the complexing interaction or chemical bonding [177-179]. The interfaces are protected by the dual CNT paper at both anode and cathode sides, the conductive interlayers fast the ion and charge transfer with uniform distribution, achieving at a high sulfur loading of 12.6 mA h cm-2 [180].

The metal-organic framework (MOF) with adjustable building blocks offers enormous possibilities for the Li-S battery [181]. Zhou’s group has developed a series of MOF-based separators. MOF works as an ion selective permeation membrane to mitigate polysulfide shuttling and guarantee Li-S battery operate steadily over 1500 cycles with negligible capacity fading (Fig. 15d-f) [175]. Another work exemplifies an ionic sieve to selectively permitted Li+ but blocking polysulfide to crossover to the Li anode side, effectively prolonging the cycle life with 2000 cycles and only 0.015% capacity decay per cycle, and the Li anode has been guarded at 10 mA cm-2 without dendrite growth. The prominent function in constitution and morphology control in pore size and distribution and intrinsic molecule polarity endows the Li-S battery with outstanding stability. 2.3.2.2. Oxygen cathode. The problems and corresponding solutions towards the Li anode can be searched in some recent reviews [182], the barrier for Li-O2 batteries exists in the reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) (Fig. 16). In the following section, the possible reaction mechanism will be briefly discussed, and the emphasis is focused on the electrode and electrolyte design. (1) Brief introduction of reaction mechanism Indisputably, the ideal charging and discharging products are transformation between Li+, O2 and Li2O2 in the nonaqueous electrolyte. These two processes contain two electrons transfer and happen at a triple-phase interface, those are solid, liquid and gas. The mass transportation of Li+ and O2, as well as the conductivity (electronic and ionic) of reaction product, are crucial to rate ability and working overpotential. Meanwhile, the uncontrolled reaction pathway for ORR and OER evokes unwanted intermediates or by-products, disturbing the occurrence of ORR and OER at normal voltage and slowing the reaction kinetics. The discharging process leads to the formation of Li2O2 , as discussed previously, this reaction involves the O2 adsorption and consequential ORR [183]. The transfer and reduction of O2 are largely dependent on the pore structure and defect of cathodes. The carbonaceous materials have been widely studied to comprehend the mechanism. For example, the heteroatom doping with strong electron withdrawing ability reveals favorable sites to attract O2 as the adsorption process is endothermic [184]. Under this condition, the Li2O2 prefers to nucleate in the form of a small cluster or film covering on the electrode. Of note, the size, morphology, crystallization, conductivity and deposit sites are influenced by the conditions such as cathode surface properties, electrolyte, current density, and so on [185].

Fig. 16. Schematic illustration of principle design for the oxygen electrode. (a) Li2O2 formation/decomposition, (b) mass transport, (c) Li2O2 storage, (d) and growth. Reproduced with permission from Ref. Ref. [182]. Copyright: 2018 Royal Chemical Society. The charging process converts the Li2O2 into Li+ and O2, and possible reaction sites have been proposed at the interface between Li2O2 with electrolyte or cathode. The one-electron path (Li2O2 → Li+ + LiO2 + e-, 2LiO2 → Li2O2 + O2) and two-electron routine (Li2O2 → 2Li+ + O2 + 2e-) have been both put forward [186]. Although the first path was observed in some specific electrolyte and catalyst systems , the mainstream of research stands on the two-electron routine [187], the difference is whether the superoxide will produce or not. Till now, however, there is no unified and accurate comprehension for the reaction mechanism [188, 189]. The Li2O2 nature (conductivity, defect) that relates to the discharging condition also affects the OER process, including overpotential and cell efficiency at last. In addition, the side reactions between catalyst and impurity with electrolyte lead to parasitic products such as LiOH and Li2CO3 that hinder the OER proceeding. (2) Stabilized cathodes and electrolytes As mentioned above, the oxygen cathode architecture needs reasonable design, and a suitable catalyst that can accelerate the cathode active species conversion without causing the side reactions between with the electrolyte is necessary. In addition, the stable cathode/electrolyte interface is also important to acquire a highly efficient cathode, which influences on the working overpotential significantly. (2.1) Cathode modification The cathode is the accommodation for Li2O2 formation and decomposition. Considering the reaction process, the following aspects are taken into account when designing the cathode materials. (i) The cathode materials should be porous and benefit for the Li+ and O2 infiltration. (ii) Suitable space and surface structure for Li2O2 deposit, morphology control, and storage. (iii) Rich catalytic sites are necessary to help Li2O2 to form and decompose. Firstly, the diffusion rate of Li+ and O2 is vital for fast electrochemical reaction on

the cathode material, a porous structure is essential and can provide additional space for the Li2O2 deposition [190]. Meanwhile, the electronic conductivity is requisite to high enough for charging or discharging at high current density [191]. Furthermore, the mass density is anticipated to be low enough for high energy density. Carbonaceous materials have been studied as prominent cathode materials due to the adjustable pore size and distribution, electronic conductivity via heteroatoms doping, or as the supporter for various catalysts, such as metal [192], metal oxide [193], metal carbide [194], metal sulfide [195], alloy [196], etc. Secondly, the cathode material structure varies accompanying with the different sites for Li2O2 deposition, morphology and crystallization. On the one hand, uniform nucleation and morphology will lead to smooth Li2O2 covering on the cathode materials surface, and promote its decomposition due to the electronic contact. On the other, if the material exhibits amorphous structure, the disordered atom distribution gives rise to weak conjunction, and then a low overpotential can trigger the oxygen evolution. Thus, the crystal structure of cathode materials also correlates with the stability during the battery operation as the side reactions between cathode and electrolyte have been proved in aqueous systems. In addition, the cathode materials provide an excellent host for the intermediates and final products (Li+, O2, or Li2O2), and in turn, facilitate their reactions [197]. Thirdly, the catalytic sites can be the defect of carbon materials and above-mentioned catalysts [198]. The goal for catalyst development is to decrease the catalyst amount, especially for noble metal without compromising the catalytic efficiency. Researchers turn to the heterogeneous structure to accelerate the electrochemical reaction speed and drag down the working overpotential. At the same time, the transitional metal compounds such as oxide, carbide, sulfide, phosphide display catalytic rule. Some work point out that surface acidity of catalyst determines the catalytic activity. A fitted surface acidity can lessen the charging overpotential by cutting down the energy barrier of the governing step for catalysis. Correspondingly, the adsorption energy for reaction intermediates plays the decisive role in the electrocatalytic process, that is the lower the adsorption energy is, the higher the catalysis is, and thus the lower overpotential. However, whether the catalysts experience the oxidation or reduction during charging and discharging process is not clear. (2.2) Electrolyte improvement The overpotential during charging/discharging process is largely induced by the slow mass transportation and insulated nature of products at the cathode surface. Consequently, the key is to find a suitable electrolyte that coexists stably with cathode materials and catalysts. This is because the electrolyte is liable to degrade in an environment containing superoxide radicals, especially for electrolytes with polymer backbone [199]. Meanwhile, the widely-used carbonate electrolytes are also not compatible with superoxide radical intermediates [200]. The ether-based and sulfone-based electrolytes show better stability [201], and the ionic electrolytes behave the highest stability [202], but their compatibility towards Li anode should be considered. Recently, some other solvents [203] with optimized salt spices [204, 205]

show wide electrochemical stability window and exhibit prominent effectiveness in sustaining the cathode/electrolyte stability and prolonging the battery lifespan. Some battery systems adopt mixed salts to augment the cycling stability. (2.2.1) Redox mediators Recently, redox mediators are applied to catalyze insulated Li2O2 to from or decompose in a homogenous manner on the electrode surface and attain the aim of improving Coulombic efficiency and lowering the charging overpotential [206, 207]. The requirement for charging and discharging differs in respective characteristics. As mentioned previously, the charging process leads to the formation of Li+ and O2, which is assisted by the passing electrons from the redox mediator between electrode and Li2O2. The blurry reaction mechanism is simpler than that of the discharging process [208], and generally, only experiencing the direct transfer from Li2O2 to redox mediator (Fig. 17a). Suitable potential range and solubility are two key factors. Under a proper potential (2.96 V vs Li+/Li), the stability of intrinsic and tolerating the attack from active oxygen species is of priority. The 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is an admirable accelerator for the formation of conformal Li2O2 film, this product endows the battery with a fast OER process. From another angle, a soluble mediator of N-Methylphenothiazine (MPT) was introduced to oxidize the Li2O2 at the cathode, the author discovered that the Li2O2 is decomposed via chemical reaction after the MPT was electrochemically activated [209]. In such a condition, the charge overpotential decreased to 0.67 V and Coulombic efficiency improved to ca. 76%.

Fig. 17. Mechanism illustration for OER/ORR processes with or without redox mediators. (a) Schematic of the ORR/OER reaction without mediators. (b) Sketch of charge/discharge reactions on positive electrode in the existence of 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). (c) Charge/discharge profile on porous carbon electrode with gas diffusion layer in the presence of 0.3 M LiClO4 in DME containing the DBBQ-TEMPO (25 mM, respectively) (solid lines) or not (dashed lines). The O2 pressure is 1 atm and current density is 1 mA cm-2areal. Reproduced with permission from Ref. [208] (a) and Ref. [206] (b-c). Copyright: 2018 American Chemical Society (a) and 2017 Nature publishing group (b-c). As for the discharging process, the crux of the problem is the poor kinetic for the formation of insulated Li2O2, which might passivate the cathode or catalyst and

blocking the transportation pathway for oxygen and electrolyte, and subsequent large overpotential to motivate the discharging process [183]. Stability, as the first priority, guarantees charging and discharging capability at high current density. Another is the shuttling effect that should be eliminated to avoid internal short-circuit. The working potential is also important. Yu et al. developed a kind of phenolic antioxidant, 2,6-di-tert-butyl-hydroxytoluene (BHT), and found that the BHT delivered a potential (~ 3.0 V vs. Li+/Li) approaching to thermodynamic potential for Li-O2 battery (2.96 V vs. Li+/Li). The BHT can simultaneously amelioration for ORR and OER process with about 1 V reduction for charging voltage and 72% increment for discharging capacity compared with the system without BHT [210]. Bruce and co-author put forward the idea of dual mediators, the 2,5-Di-tert-butyl-1,4-benzoquinone (DBBQ) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), which have been brought to respectively promote the discharge and charge process on the carbon surface (Fig. 17b-c). This strategy makes the Li2O2 formation and decomposition in the liquid phase and enables Li-O2 battery to work under 2 mA h cmareal-2 at 1 mA  cmareal-2 with small overpotential, tremendously improving the stability of carbon electrode [206]. Further, adopting a dual-protected strategy, the polyimide-modified carbon electrode and LiNO3/CsI redox mediator was utilized jointly, and the synergic effect of polyimide stabilizer and LiNO3/CsI redox mediator endows the Li-O2 cell low overpotential and favorable working performance [211]. Noteworthy, the parasitic chemistry caused by impurity and active oxygen species will degrade the function of redox mediator. To overcome this problem, the water is implemented as the stabilizer and jointed with the 2,5-di-tert-butyl-1,4-benzoquinone and motivated the discharge process via two-electron reduction for O2 in the solution phase to obtain crystalized Li2O2. At the same time, the side reaction is liable to be constrained and the whole OER and ORR processes benefit a lot for favorable rate and cycling stability performance [212]. Thus, the mechanism comprehension for active oxygen species is the first step to remove or minimize related parasitic chemistry, and tremendous researching efforts have been paid to this domain. (2.2.2) Solidified electrolytes The ultimate choice of electrolyte for Li-O2 batteries is utilizing solidified electrolytes [213], which offer advantages than liquid such as negligible solvent evaporation, non-flammability, free gas permeation, easy scalability, and so on [214]. More importantly, solidified electrolytes supply a fine shield for redox mediator self-shuttling [215], cycling by-products drifting and dendrite growth at Li anode , improving the battery lifespan and safety at harsh working conditions or exterior force . For example, the LIPON protection can extend the cycling stability at smaller overpotential compared with conventional ether electrolytes (Fig. 18a-b) [204]. The working condition correlates with the performance of Li-O2 batteries, a desirable status is in ambient environment, but the water and CO2 in air will definitely deteriorate electrodes and electrolytes. The passivation layer on electrodes or electrolyte decomposition are anticipated to prevent. With the help of super-hydrophobic SiO2 matrix that contained ionic liquid for Li+ conducting, a water-resistant quasi-solid electrolyte was developed with other several commendable

characters such as good mechanical strength, thermal stability and wide electrochemical stability [216], delivering promising suppression effect for side reaction products on Li anode in humid atmosphere. Further, a polymer electrolyte containing I-/I2 redox mediator was designed to facilitate the discharge product over the repeated cycling, and enabled the battery to operate in ambient air with 15% relative humidity over 400 cycles. The embedded redox mediator efficiently reduced the polarizing voltage and improved the battery efficiency by inhibiting the LiOH and Li2CO3 formation at electrode surface (Fig. 18c-d) [217]. Under these protection, the Li anode is safe from the attack by aggressive intermediates originated in cathode and dendrite formation , and the battery safety can be improved even by cutting, owning the potential in flexible electronics [218]. Despite the solidified design, the air permeability will not be hindered while the Li dendrite is suppressed in the flame-resistant gel electrolyte containing polyurethane and aerogel SiO2 [219]. Meanwhile, the moisture-proof ability was enhanced to 40%.

Fig. 18. Solid-state electrolyte protected Li anode for low-overpotential Li-O2 battery. (a) Schematic illustration of LIPON-protected Li anode for Li-Oxygen battery, (b) the hexamethylphosphoramide electrolyte was tested compared with the conventional dimethyl ether and tetraethylene glycol dimethyl ether electrolytes. (c) Photograph and SEM image of gel electrolyte containing 0.05 M LiI (LiI-GPE), (d) and charging and discharging curves for Li-O2 battery based on the LiI-GPE. Reproduced with permission from Ref. [204] (a-b) and Ref. [217] (c-d). Copyright: 2017 Wiley (a-d). The in-situ synthesis is favorable strategy to construct a low-resistance cathode-electrolyte interface. The exterior stimulus to trigger the pre-incorporated monomer to react at the cathodic side, then an integrated architecture can be obtained. This methodology has been widely implemented into the all-in-one cell design [75, 127]. At the same time, if the cathode structure with specific design, the rate performance can be further improved [220]. Combining with the thermal stable Li1.5Al0.5Ge1.5(PO4)3 membrane, the crossover of soluble products was restrained in a eutectic molten salt electrolyte (LiNO3/KNO3). The in-situ obtained Ni-based LixNiO2 supplied robust catalytic sites to accelerate the OER and ORR reaction at 3.5 and 2.6 V, respectively, and highly reversible discharge capacity (11 mA·h cm-2) with oxygen evolution overpotential was achieved, which is

20 folds than that in the liquid electrolyte (Fig. 19a). This work reveals that the Li2O formation needs four electrons [220]. Further, it is more advisable to couple the redox mediator with other mediators or solidified electrolytes for a synergic effect in ameliorating Li2O2 formation/decomposition. A quasi-solid electrolyte with sandwich structure, that is polypropylene carbonate/Li-Nafion/polymethylmethacrylate (PPC/Li-Nafion/PMMA), was introduced to isolate anolyte and catholyte that involved TEMPO (Fig. 19b). This electrolyte endowed the cell with stable Li-electrolyte interface and eradicated redox mediator shuttling, improving the cell safety and lifespan up to 200 cycles within 4 V [221].

Fig. 19. Electrolyte design in Li-O2 batteries. (a) Configuration of the inorganic electrolyte Li-O2 cell and schematic illustration of Li2O formation during discharge. (b) Scheme of PPC/Li-Nafion/PMMA electrolyte and corresponding side-view SEM image. Reproduced with permission from Ref. [220] (a) and Ref. [221] (b). Copyright: 2017 AAA Science (a) and 2017 Wiley (b). 3. High-power-density systems 3.1. Redox flow batteries As an excellent choice for clean energy storage systems, the redox flow battery shows increasingly attractive attention [222, 223]. The primary stimulus for redox flow battery is developing aqueous systems with low cost, high power density and safety for grid-scale energy storage applications. It is a kind of electrochemical energy storage devices with separated electrode and electrolyte by membrane [224]. With the auxiliary impetus, the redox species in the electrolyte reacts on the electrode surface, and during the process, the chemical energy and electrical energy are switched between with each other (Fig. 20). With the occurrence of new technologies such as molecular and materials engineering, the scope of electrode and electrolyte expands from the metal to organic active materials [225]. The boundary of conventional redox flow batteries have been fuzzified due to the advent of hybrid flow systems such as organic redox flow battery, flow Li battery, and so on [226]. For the aim of boosting the performance for clean energy storage in the era of smart automation [227], the discovery of new electrode, electrolyte [228], membrane and flow fields [224, 229] is demanded to be accelerated with special design in nanoscale or molecular level for materials [230].

Fig. 20. Schematic illustration of RFBs. Reproduced with permission from Ref. [230]. Copyright: 2017 Nature publishing group. 3.1.1. Vanadium redox flow batteries (VRFBs) Vanadium redox flow batteries employ acidic electrolytes that contain VO2+/VO2+ and V2+/V3+, which respectively stands for Cm+/C(m+1)+ and An+/C(n-1)+, to react for the energy storage and release. The carbon-based materials are commonly used as electrodes to supply the zone for redox reactions due to their high stability in the corrosive electrolyte [231, 232], and a fast electronic conductivity is a vital parameter for the electrode. Meanwhile, a high catalytic performance for cathode and anode that respectively avoid oxygen and hydrogen evolution is necessary [233], The key is to resolve the sluggish heterogeneous mass and electron transfer . For this, the basic idea is to create reaction sites with high specific surface area and catalytic activity [234, 235]. The surface modification is a facile but effective strategy to improve the catalytic efficiency for the carbon-based electrodes, including acid or alkalis treatments, electrochemical or thermal activations [236], metal or nonmetal compounds decorations [237, 238], etc. Typically, a porous electrode design inspired from biostructure is obtained, and the formed micro/nano scale connected pore benefit for electrolyte infiltration and vanadium ions reaction. Additionally, the nitrogen and oxygen groups function as the promoter for wettability and synergistically catalytic sites, and thus, a high energy efficiency (81.9%) was obtained at 320 mA cm-2 [239]. The catalytic efficiency is largely dependent on the catalyst content and the space utilization provided for maximizing the electrocatalytic activity of electrode in VRFB [235]. Through in-situ polymerization, a hierarchical carbon micro/nanonetwork with rich heteroatoms was developed on the basis of graphite felt. The obtained electrode offers superexcellent cycling stability up to 2000 cycles at 250 mA cm-2 and outstanding charging/discharging capability at 400 mA cm-2 (Fig. 21a-b). The dramatic augmentation for battery performance is ascribed to the multiple electron pathways and ample active sites [240].

Fig. 21. Electrode design and mechanism analysis for stable VRFBs. (a) The schematic illustration of hierarchical carbon micro/nanonetwork by in-situ polymerization and (b) corresponding cycling performance. (c) Energy efficiency of P and F co-doped carbon felt for VRFB cycling. (d) Charge transfer between V2+ and graphene surface. Reproduced with permission from Ref. [240] (a-b), Ref. [234] (c) and Ref. [241] (d). Copyright: 2018 Wiley (a-b), 2018 the Royal Society of Chemistry (c), and 2018 Elsevier (d). It is important to illuminate the catalytic site and improve the surface wetting ability of the electrode. Exemplified with nitrogen as the main doping element, Guo and co-author uncover the catalytic center for nitrogen carbide with the experimental and computational approaches [242]. Meanwhile, through the introduction of phosphorus and fluorine doping (Fig. 21c), a highly wetting graphite felt was obtained and dramatically prolong the lifespan at 120 mA cm-2 [234]. Meanwhile, Zhao’s group investigate the influence of multiple doping elements such as nitrogen (Fig. 21d), boron and phosphorus on graphite electrode through density functional theory calculations, founding that the phosphorus doping showed obvious catalytic effect both for V2+ and V3+ except for the improvement of wetting ability and electronic conductivity [241]. 3.1.2. Organic redox flow batteries (ORFBs) Tremendous attention has been paid to low-cost and highly stable organic electrode materials [225]. The crucial challenge locates at the stability during long-time operation, as the organic materials are susceptible to experience side reactions at both electrode sides as well as the parasitic reactions between with electrolytes. The redox-active materials can be categorized as carbonyl compounds [243], heterocyclic aromatics [244], and radicals [245]. The carbonyls-based and its derivatives occupy an important part in the RFB field. By definition, all the organic molecules that contain a carbonyl group can be classified into carbonyl compounds [225]. To acquire high energy density, many efforts concentrate on the solution ability and stability of electrolyte. Yao and co-worker reported a series of aqueous battery systems that finely matched quinones and cathode materials could work under the condition ignoring the pH, charge carrier, temperature, and atmosphere [246]. This work sheds light on other molecules design with delicate chemical characterization and computational simulation [247]. The urea has been implemented to help hydroquinone to dissolve and demonstrated its popular

effect in solubilization, reaching a 25.3 W h L-1 in the hybrid flow battery containing Li anode. On the basis of carbonyls, heterocyclic aromatics are products derived from partial carbon atoms substitution by heteroatoms such as N, O, S, etc. The delocalized spin and charge enable the heterocyclic aromatics to be a catholyte or anolyte covering a wide potential window (Fig. 22a-b) [248]. Take indigo as an example, the voltage difference attains 1 V for oxidation and reduction. However, its poor solubility is a problem needed to be addressed. Introducing the strategy of hydrogen substitution, the concentration of indigo in water can be boosted while maintaining the electrochemical activity (Fig. 22c-d) [249]. Similarly, quinoxalines will react at ∼2.6 and ∼2.9 V (vs. Li+/Li) with high concentration up to 7 M in carbonate. Meanwhile, the relative narrow working voltage window makes the quinoxalines suitable for aqueous RFBs.

Fig. 22. Heterocyclic aromatics with high concentration as active materials for ORFBs. (a) The schematic illustration of alkaline quinone flow battery with (b) CV curves of corresponding redox couples. (c-d) Charge transfer process between indigo and indigo carmine and corresponding CV curves. Reproduced with permission from Ref. [248] (a-b) and Ref. [249] (c-d). Copyright: 2015 American Association for the Advancement of Science (a-b) and 2016 the Royal Society of Chemistry (c-d). The radical redox species also acquire much attention, especially for the methyl viologen, nitroxide radical, and their derivatives. Based on the methyl viologen (MV, anolyte) and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-HO-TEMPO, catholyte), an organic was developed with NaCl as supporting electrolyte [250]. Further, conjugated with thiazolo[5,4-d]thiazole and pyridinium, a highly-soluble viologen derivative. After pairing with 4-trimethylammonium-TEMPO, a total ORFB could release 70% energy density and 99.97% capacity retention ratio [251]. With the assistance of bis(trifluoromethane) sulfonamide counter anion, a two-electron utilization of methyl viologen anolyte in nonaqueous organic redox flow battery was realized [244]. The radical redox reactions generally contain the process of anions or cations formation and reveal fast kinetics, beneficial to the power density.

Additionally, the dissolution is not as difficult as the abovementioned carbonyls and heterocyclic aromatics [252], this advantage possibly brings the radical-based RFB high energy density as well [253]. 3.1.3. Other redox flow batteries Abovementioned VRFBs and ORFBs have revealed promising application value providing obstacles such as low solubility and stability have been resolved. New systems that offer high capacity and endurable service life are still needed. Integrating with the conception in Li-ion batteries or other metal-based redox flow batteries, a series of novel systems have been developed (Fig. 23a-b) [254]. For instance, combining with chemical characterization and computational simulation, the urea has been demonstrated its popular effect in solubility and implemented to help hydroquinone to dissolve, reaching a 25.3 W h L-1 in the hybrid flow battery containing Li anode [247]. The Li salt can influence the solubility. Adopting the eutectic method, a ca. 6-fold increment in solubility for phthalimide derivatives was acquired, discovering the LiTFSI facilitated the chemical bond breaking and reduced viscosity with urea contribution [255]. The obtained ORFB showed excellent cycling stability (Fig. 23c-d).

Fig. 23. New Li-containing RFB systems. (a-b) The schematic illustration of hybrid redox flow battery containing LiFePO4 and TiO2 and corresponding photograph. (c-d) The phase diagram of LiTFSI in the co-solvent of phthalimide derivative and urea. Reproduced with permission from Ref. [254] (a-b), Ref. [255] (c-d). Copyright: 2015 American Association for the Advancement of Science (a-b) and 2018 Elsevier (c-d). The crossover for active materials is a troublesome problem. Yu et al. proposed a MOF porous membrane with gradient-distributed pore in Celgard that sustained the ion conductivity. The 3D channel embedded separator helps the Li/ferrocene RFB with excellent rate ability up to 12 mA cm-2, prolonging the battery lifespan [256]. Similarly, the polysulfide shuttle is the stumbling block for lithium-sulfur flow batteries. Zhang et al. utilized the 1-methyl-1-propylpiperidinium chloride anchored SiO2 as a mediator to confine polar polysulfide within nonpolar carbon carrier, the polysulfide could be prevented from drifting to Li anode, and made the lithium-sulfur flow battery cycle over 1000 times with high coulombic efficiency (99%) [257]. To increase the energy density, a lithium organic nanocomposite suspension via 10-methylphenothiazine was dissolved, the solubility of active species and the energy density of RFB were both boosted up to 190 W h L-1 with Coulombic efficiency larger

than 98% [258]. Further, a solvent-free redox flow battery was developed with the liquid benzoquinone and naphthoquinone, which tethered with diethylene glycol monomethyl ether moieties in LiBF4/propylene carbonate (Fig. 24a). The redox flow batteries owned an average discharge voltage of 2.57 V and achieved 264 W h L-1 for energy density, offering a new choice for high-capacity redox flow batteries [259]. The voltage matching is a vital principle to obtain high output energy. Stevenson et al. design a cobalt ([CoII/III(P3O9)2]4–/3–) and vanadium ([VIII/II(P3O9)2]3–/4–) complexes as catholyte and anolyte species, respectively to build a cell with voltage of 2.4 V and Coulombic efficiencies larger than 90% nearly 100 cycles [260]. In a similar way, a zinc-vanadium hybrid redox flow battery that utilized zinc bromide as electrolyte with high-voltage and energy-efficient advanced energy storage system [261]. The ferrocene-based metallocenes have been studied with huge attention on account of its high redox potential (ca. 3.1-3.7 V vs. Li+/Li). To achieve high solubility, the introduction of polar groups to reduce the symmetry is resultful [262], and also will increase the working voltage (Fig. 24b) [263]. Typically, a (NH4)3[Fe(CN)6] and (NH4)4[Fe(CN)6] catholyte materials with high solubility was designed, and when paired with 1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridinium anolyte, a stable aqueous redox flow battery with a power density of 72.5 mW/cm2 is obtained without supporting electrolyte [264]. The ferricyanide/ferrocyanide redox couple generally functions best in a neutral or near neutral condition, the rapid deactivation mechanism in the strong alkaline electrolyte has been unveiled, the reason is the chemical decomposition caused by cyano group dissociation [265], indicating the reaction environment impacts on the activity of redox couple significantly.

Fig. 24. Improve the solubility of active materials with wide working voltage. (a) Charge and discharge profile of benzoquinone and naphthoquinone tethering diethylene glycol monomethyl ether in LiBF4/propylene carbonate. (b) CV curves for FcNCl (red), FcNBr2 (purple), FcN (black), and MV (blue). The dashed curve is the cyclic voltammogram of 0.5 M NaCl with labeled onset potentials for HER (-1.00 V) and OER (1.50 V). Reproduced with permission from Ref. [259] (a) and [263] (b). Copyright: 2017 Wiley (a) and 2018 American Chemical Society (b). After tuning the pH of electrolyte, a hybrid Zn-I2 redox flow battery was demonstrated with unprecedented energy density of 330.5 W h L-1 because of the enhanced potential [266]. Coupling with ammonium bromide catholyte and viologen anolyte, a high-voltage (1.51 V) and high-rate (40 mA/cm2) redox flow battery with neutral pH could also be obtained [267]. Newly developed redox flow batteries are

adopting eutectic electrolytes with related coordinating chemistry high-energy-density systems, and has been well-reviewed for reference [268].

for

3.2. Supercapacitors Supercapacitor is an electrochemical energy storage device involving the electrodes, electrolyte and separator and owns faster charging/discharging capability than that of the rechargeable battery and higher energy density than that of the conventional capacitor [269, 270]. With the aid of nanotechnology, the superior rate capability and energy density undergo dramatic augmentation after tailored design in electrode and electrolyte component and structure. Especially for the hybrid capacitator proposed by Amatucci and co-workers, no matter Li- or Na-based systems, it contains a “battery side” and “capacitor side” and can match carbonate electrolytes in battery to deliver 4 to 5 times higher in energy density than that for conventional electrochemical double layer capacitor (EDCL) (Fig. 25) [271, 272].

Fig. 25. Schematic diagram for hybrid supercapacitor. Reproduced with permission from Ref. [272]. Copyright: 2018 American Chemical Society. The charge storage mechanism in hybrid supercapacitor is not totally similar to the EDCL both at surface and in bulk, combining with dual charge storage mechanisms: EDCL and Faradic process to storage charge. There may exist reversible ion adsorption, intercalation, redox reaction, alloy reaction or even plating/stripping of metals. With the increment of current density, the nanostructured electrode goes through almost synchronous reaction speed, and the contribution from the surface and bulk is hard to distinguish. Taking the hetero-NiCo2S4/Co9S8 submicron-spindles as an example, the hierarchical hollow porous architecture exposes vast space to accelerate the charge adsorption and desorption at 15 A g-1, an energy density of 33.5 W h kg-1 can be achieved at 150 W kg-1 in a mesoporous device design. In this situation, the boundary between surface and bulk get blurring at such a short time [273]. Still, several parameters hinder the advancement for supercapacitor, and can be summarized in following aspects: To begin with, the poor electronic conductivity hinders the fast reaction at interfaces, especially for the supercapacitor with Faradic reactions. The general strategy is applying the conducting layer onto the electrode materials to relieve this issue. The carbon coating or supporting is widely used in metal oxide electrode [274]. Ultrafine Co3O4 with abundant oxygen vacancy was formed on the graphene to obtain a controllable conductivity and electrochemical activity due to the formation of

midgap electronic states. The capacitance retention ratio attains 93.7% with 916.5 F g-1 at 10 A g-1 [275]. The Ni was applied to construct 3D porous hetero-NiO/Ni components at mesoscale, the conductivity is obviously improved, showing promise in other metal/metal oxide composites [276]. Of noteworthy, the equilibrium of active materials is vital to unleash the capacitance. The mesoporous nickle cobaltite-graphene was paired with doped carbon to extend the battery lifespan to 10000 cycles with 52.2 W h kg-1, indicating the synergism effect for optimal electrode loading to obtain a high-performance supercapacitor [277]. Secondly, the low specific area still cannot satisfy the requirement for increasing more charge storage for higher capacity. The nanostructure provides an excellent solution to conquer this problem and has been applied to a variety of cathode materials such as carbonaceous materials, metal oxides, polymers. The 3D structure is a favorable selection to elevate the storage space for charge. Hu et al. prepared a porous graphene-like carbon with 3D structure with a specific area of 1500 m2 g-1 and excellent conductivity of ca. 800 S m-1. In addition, the wetting ability was outstanding in both aqueous and ionic liquid electrolytes, corresponding power densities up to 1066.2 and 740.8 kW kg−1, respectively [278]. A nitrogen-doped carbon with hierarchical porous structure was realized through KOH activating silkworm cocoon and got a high specific area up to 3386 m2 g-1 and defect density, the energy density can be boosted to 34.41 W h kg-1 in organic electrolyte and 112.1 W h kg-1 in ionic liquid electrolyte [279]. From these two examples, it can be concluded that electronic conductivity and specific surface area are both significant to achieve high energy density and power density. Alternative strategies can cover the hierarchical structure with nanosized design through in-situ growth and heteroatoms doping [280]. Furthermore, the higher voltage means higher energy density for supercapacitor according to the equation: E=1/2 CV2. Toward different electrolyte systems, the intrinsic voltage limit are different. The electrochemical stability window is 1.23 V for aqueous, and the voltage can attain to 4.5 V (vs. Li+/Li) for organic-based electrolyte but commonly with sluggish ionic conductivity. The over-ranged working voltage depletes the electrolyte, which consumes extra active material, and the insufficient conductivity drags the power density down. For aqueous-based electrolyte, the handicap locates at the water splitting because of overpotential. For example, the sodium ions is introduced as the physical barrier to suppress the reaction possibility for HER. Paired with the Na0.25MnO2 cathode and carbon anode, a 2.7 V asymmetric supercapacitor with 61.1 W h kg-1 in energy density was accomplished at 982 W kg-1 and could maintain 10000 cycles with 93.7% capacitance retention at the power density of 42.9 kW kg-1 [281]. Replacing the carbon anode in conventional symmetric capacitor with insert-type materials, such as Li4Ti5O12, the charging or discharging potential changes and needs impetus under a voltage of about 3.5 V, which can be satisfied for organic electrolytes. As the Li+ is accommodated by the host Li4Ti5O12, the counter anions will move to the counter electrode for storage and charge equilibrium. The carbonate electrolytes are commonly used in Li/Na-ion batteries, but their ion conductivities are not up to standard for high-rate supercapacitor.

Consequently, the selection and pairing for solvent and salt requires comprehensive consideration. Last but not least, other easily overlooked side reactions such as the alloy reaction between active materials and current collector lead to poor cycling stability. Suitable current collectors are necessary to improve the energy density. The problem involves alloy reaction between with current collector such as LixAl alloy below 0.5 V, uncontrolled dendrite growth at a high current density that is the merit for supercapacitor. Moreover, the underlying safety risk caused by dendrite piercing needs consideration. Substituted by the Cu or Ni current collector can avoid this issue, but the energy density will be compromised. Next subsection will give a brief introduction of progress in electrode and electrolyte with nanostructured design to resolve abovementioned issues. 3.2.1. Electrode materials The Li/Na-ion supercapacitor is a high-profile electrochemical energy storage device, which is constructed by the Li-ion battery anode, EDLC cathode, and Li+/Na+-containing electrolyte. As mentioned above, for the charge balancing, the Li+/Na+ will be intercalated/de-intercalated from the Li/Na-ion host materials while the counter ion (anions) will be adsorbed/desorbed from the carbonaceous electrode. Reactions potential and mechanism are different at each side. Starting from nanostructured Li4Ti5O12 (ca. 1.5 V vs. Li+/Li) and activated carbon (ca. 4.0 V vs. Li+/Li), a series of anode materials have been developed to extend the full cell voltage beyond 2.5 V, mainly including insertion-type, conversion-type, and alloy-type materials. While the carbonaceous materials have been regarding as the primary choice for cathode materials, and a patch of new carbon arises [282]. 3.2.1.1. Insertion-type. The insertion-type materials are derived from the Li/Na-ion battery. During the ion insertion and extraction, the host materials will not experience obvious volume change but call for stable interface to guarantee acceptable cycling life. Except for the well-studied graphite in Li-ion batteries, other carbon analogues need to resolve the problem of low Coulombic efficiency especially for the initial cycle as in hard carbon. The main cause is the irreversible reaction between carbon and electrolytes and capacity loss. The pre-lithiation is a good avenue [283]. For example, the graphite was protected by an artificial SEI layer and shows outstanding cycling stability even under 5 V cut-off voltage compared with the pristine graphite, which is also called as dual-ion battery [284]. The author deems that the preformed shielding layer is crucial to stabilize the anion pathway. This point has been also verified by other carbon-based supercapacitor systems with nanostructured carbon [285]. Besides, the supercapacitor’s performance correlates with other factors such as mass loading, working voltage and porosity [286]. Yang et al. studied the electrode thickness matching for lithium-ion capacitors with high volumetric performance and found that the highly dense and porous structure is vital to volumetric energy density. Based on the porous activated carbon/graphene composites, the LIC achieved 98 Wh  L-1 with 68% increment compared with activated carbon/graphite-based LIC [287]. In addition, the pseudocapacitive materials involving Nb2O5 [288], MXene [289] are important candidates. When the Li+ intercalates into the bulk phase of V2O5, there

also exists a capacitive behavior and the vast interlayer space offers the possibility for other ions such as Na+ and K+. Such structure superiority has also been improved in the Vanadate. For example, the Zn2+ acts as the pillar to stabilize the ab plane in V2O5, thus protecting the structure form huge distorting during the Na+ insertion and de-insertion [290]. Similarly, the Nb2O5 owns ordered bulk channel to accommodate active ions and bring in capacitive charge storage with approximate voltage range (1-1.75 V) that is akin to Li4Ti5O12 when paired with graphene. However, the low electronic conductivity and low-voltage output limit the energy density upgradation, generally falling within 76 W h kg-1 at low current density. Future efforts should direct high-voltage output with fast bulk kinetics. MXene is a class of transition metal carbides and carbonitrides with good electrical conductivity and high specific surface area discovered by Gogotsi and his colleagues [289]. This material has been applied to storage ions from monovalent Li+ and Na+ to multivalent Mg2+ and Al3+, and exhibit pseudocapacitive galvanostatic profiles similar to other analogous materials [291]. Coupling with the nitrogen-doped porous carbon, a high power capability up to 67.5 kW kg-1 can be achieved [292]. A bistacked titanium carbide was prepared to construct model NIC with active carbon. Benefiting from the reversible Ti redox site, the Faradic capacitance is simultaneously acquired except for the EDLC contribution. Another kind of star materials is the MOF due to their adjustable porosity, dimensionality, constitution from the molecule level, and their derivatives supply great potential in supercapacitor field [181, 293]. Typically, an all MOF-derived TiO2/C nanocomposite anode and 3D nanoporous carbon cathode have been constructed and benefiting from the hierarchical 3D nanoporous structure and extrinsic pseudocapacitance contribution, the obtained Na-ion supercapacitors shows 142.7 W h kg−1 at 25 kW kg−1 in the voltage range from 1 to 4 V [294]. Other insertion-type materials used in supercapacitor are layered oxides,[295] spinel oxides [296], and olivine [297]. The layered oxides firstly used for supercapacitor date from the LiCoO2-active carbon system, showing a 37 kWh kg-1 at 125 W kg-1 [298], but the power density is not satisfactory. The cause roots in sluggish ion transportation kinetics. Thus, strategies including atom doping to expand the interlayer space have been adopted. As evidenced by the Ti substitution in P2-Na2/3Ni1/3Mn2/3O2, the obtained Na2/3Ni1/3Mn1/3Ti1/3O2 shows obvious capacitance augment from 2.5 to 4.3 V [299]. The representative olivine material, LiFePO4, can release 69 Wh kg-1 in the voltage range from 2 to 4 V at a relatively low rate when paired with the active carbon. In the same way, the NaFePO4 has been tested to give out 50 W h kg-1 at 180 W kg-1 [300]. These two supercapacitor shares one common point that they all behavior excellent cycling stability, which is attributed to the stable polyanion structure and electron-conducting carbon coating layer [301]. The NASICONs, especially for the Vanadium-based materials (Li3V2(PO4)3 and Na3V2(PO4)3), shows interesting multi-electron redox processes accompanying with the conversion from V2+ to V4+, and the charging/discharging platform ranging from 2 V to 3.4 V. Thus, this kind of materials can achieve for cathode or anode in one electrode and shows outstanding rate and cycling performances [302]. When assembled with active carbon, the NIC supercapacitors can also deliver 53 F g-1 at 1

mA cm-2 and 95% capacitance retention after 10000 cycles, outperforming that of other similar systems [303]. 3.2.1.2. Conversion-type. Conversion-type materials provide additional pseudocapacity due to the Faradic reaction and attract increasingly attention in augmenting the energy density. While the question is that the conductivity (electronic and ionic) and cycling stability for conversion-type materials are insufficient [304]. For instance, the MoS2 usually exhibits a capacity more than theoretical value, which comes from two parts: the excess portion is contributed by the Li intercalation and the other originates from the conversion reaction to form Mo and Li2S. To achieve a rapid reaction kinetic, the nanosizing and electron-ion mixing conducting network can be built. The MoS2 was composited with graphene to obtain a LIC with 45.3 W h kg-1 at 40000 W kg-1 [305]. The well-matched graphene conducting phase and expanded interlayer space endows the composite excellent rate and cycling performance in the NIC [306], attaining 43Wh kg-1 at a record-high power density of 103 kW kg-1 with 10000 cycles, which surpass the value achieved in LIC [307]. Similar materials include AxBy (where A stands for the transition metal, such as V, Cr, Mn, Fe, Co, Ni, Cu, Ru, etc. B stands for anions, such as O, S, N, F, etc.), these materials usually exhibit slope-like behavior for their charging-discharging curves and are controlled by the reaction and diffusion if the electronic conductivity has not been resolved properly [308]. The oxygen vacancy is also important to increase the pseudocapacitive charge storage capability [309], the underlying reason is that the oxygen vacancy not only results in larger interlayer spacing but also stabilize the crystal structure, ensuring the rapid charge storage kinetics in MoO3 (Fig. 26a-b).

Fig. 26. Structural regulation of active materials for extra capacitance contribution. (a) The crystal structure of reduced MoO3–x, (b) and CV tests for reduced MoO3–x (solid) and fully oxidized MoO3 (dash) at 10 mV s-1 (tenth cycle). (c-d) The molecular structure of maleic acid (MA) and polyvinylidene fluoride (PVDF) and schematic of the reaction product. (e) CV profile of MA@PVDF electrode composed by total capacitance and surface capacitance (shadow) at 0.1 mV s-1. Reproduced with permission from Ref. [309] (a-b) and Ref. [310] (c-e). Copyright: 2017 Nature publishing group (a-b) and 2018 Wiley (c-e).

Recently, the binary metal oxides or sulfides emerge as important category materials for hybrid supercapacitors. The core reason is the complete utilization of multi electrons redox reaction for all the transition metals. Meanwhile, the second metal can improve the electronic conductivity as well as buffer the volume change after carbon or conducting polymer. The conductivity is also an obstacle for organic electrode materials [310]. Thanks to the self-assembly technique, the maleic acid@polyvinylidene fluoride (MA@PVDF) granules were prepared as a multiscale Li+ holder with Faradic reaction and non-Faradic storage (Fig. 26c-e), and a 4.3 V LIC with MA@PVDF and active carbon delivered an energy density of 70.9 W h kg-1 at 10750 W kg-1, exceeding the LIC or NIC with same type [310]. 3.2.1.3. Alloy-type. The materials that can react with Li or Na to form alloy own advantages of high storage capability and electrical conductivity for low resistance. Sn and Sb are two types of promising candidates but facing serious volume change that deteriorates the cycling stability. In addition, the alloying reaction also regains the concern in metallic Li/Na anode protection. The hot spot deserves a new gazing at the alloyed ion storage [41]. The nanostructured design and embedding into carbonaceous supporting offer a fine host to tolerate volume change, which has been unified to ameliorate vast volume change during supercapacitor operation. For example, the Sn-carbon nanocomposite was synthesized in a limited space to form a nitrogen-doped interconnected carbon framework. The obtained LIC can deliver 195.7 W h kg-1 and 84.6 W h kg-1 at 731.25 W kg-1 and 24375 W kg-1, respectively [311]. Direct growing on the carbon fiber cloth, the Sb2S3 micro-nanospheres can maximize the accommodation ability for volume change and further improve the electronic conductivity and contacting area with electrolytes [312]. Further, combing with a powerful cross-linked binder, a carbon coated TiSb2 with submicrometer size reveals improved rate and cycling stability than that of pure Sb due to the higher volume-tolerating ability [313]. Bi is also a good choice for alloy anode for NIC. Li et al. utilized the porous NaBi as the anode to pair with active carbon in a diglyme solvent containing 1.5 M NaPF6, discovering that the Na+ reacts with the anode to form Na3Bi while the PF6- was adsorbed on the active carbon in the charging process. The PF6- was reversibly released and the Na3Bi eventually converted into Bi with Na+ after discharging. On this basis, the gradually acquired 3D porous structure as a robust host to warrant the rapid reaction and high energy density of 11.1 kW kgtotal-1 at 106.5 W h kgtotal-1 [314]. Apparently, the Li+/Na+ can react with active or inactive metals to form alloys at a proper potential. The alloy-type cathode is designed to pre-alloy with other metal and then avoid the alloy reaction with Li/Na. The mentioned strategies are always integrated to achieve a comprehensive boosting for the alloy-type anode. Table 1 Surface area of different electrodes and performance comparisons. Electrode

specific surface area [m2 g-1]

Potential range [V]

Energy density [W h kg-1]

Power density [W kg-1]

Capacity retention/Lifesp an [cycles]

Ref.

SMG NPHCs NHCN VO-CF AC/G N-NbOC G-ZVO M-TiO2-RGO TiC TiO2/C NMNC-Al Li2MnO3 LiFePO4 Na2/3Ni1/3Mn1/3Ti1/3O2 NaMn1/3Co1/3Ni1/3PO4 LiFePO4 Na3V2O2(PO4)2F @PEDOT C-NVP

2350 14.18 1402 193.1 45.6 63.6 175.5 3738 179.5 1497 5 1282 -

3~5 1~4.7 2~4.5 2.5~3.9 2~4.3 1~3 1~3.8 1~4.0 0~4.5 1~4 2~4.5 0~0.8 2.7~4.1 2.5~4.15 0~3 2.2~3.8 1~4.2

245.7 146 233 120 86.6 88.7 94.7 101.5 142.7 63 35.1 43.3 50 20 158

1626 650 112 200 6090 223 247 67500 25000 6600 495.2 10.7 180 42000 7000

96%/500 91%/40000 63%/500 98.9%/3000 81%/1100 87%/2000 80%/2000 82%/5000 90%/10000 98%/10000 85.1%/6000 68%/1000 83.9%/500 92%/1000 92%/100000 83%/8000

[283] [284] [285] [286] [287] [288] [290] [291] [292] [294] [295] [296] [297] [299] [300] [301] [302]

45

2.5~3.8

118

95

95%/10000

[303]

Fe3O4@C MoS2-C-RGO MoS2-C 3D-IEMoS2@G MoO2/MoSe2 R-MoO3−x MA@PVDF

63.4 24.4 24 36.7

0~4.0 0~4.0 1~3.8 1~4.3 1~3.4 1.5~3.5 0~4.3

110.1 188 111.4 140 71 158.4

250 200 240 630 14316 107.5

95.7%/1000 80%/10000 77.3%/10000 99%/10000 92%/7000 76%/10000 10000

[304] [305] [306] [307] [308] [309] [310]

Sn-C SO/CFC TiSb2-C20h NaBi

389 19

2.0~4.5 1.5~4.3 1.0~4.2 1.5~3.5

195.7 124 132 11.1

731.25 5800 114 106.5

70%/5000 90%/1000 63%/1000 98.6%/ 1000

[311] [312] [313] [314]

3.2.2. Electrolytes Accompanying with the explosive development of electrode materials, the conventional electrolyte in supercapacitor progressively convert into the one that is stable at a high potential to cater to the multiple-electron reactions [315]. For the electrolyte in LIC and NIC, the Li or Na salt is essential, whether in an aqueous or organic solvent. Undoubtedly, electrolytes exert influence on the whole performance of supercapacitors. The anticipated electrolyte shall meet the criteria of high ion conductive, nonflammable as well as the ability to suppress Li/Na dendrites when at a high current density [316]. The ion conductivity and electrochemical stability of the electrolyte are the most significant two indexes. The aqueous electrolyte consists of acid, alkalis, neutral, and shows high ion conductivity compared with the organic electrolytes. However, the highest cut-off voltage window (ca. 2.2 V for neutral) is general narrow than that in

the organic electrolytes, which can reach 4.0 V or higher. Through solvent or salt modification, the aqueous electrolyte can also be stable within 4.5 V [317], but such electrolytes are highly viscous and not cost-effective, deserving improvements. The acetonitrile and carbonate solvents are popular for their high salt dissociation constant and wide operating temperature [315]. It is not difficult to find out polar groups such as nitrile, carbonyl, ether help to dissolve inorganic or organic salts, and the appropriate solvent pairing can improve the ion conductivity, particular for the ionic liquid electrolytes [318]. Recent work also finds that the solvent has a great impact on the arrangement of ions or molecules for charge storage and the carbonate electrolyte will endow Ti3C2 with highest volumetric capacitance [319]. With the asymmetric electrolyte design, the electrochemical stability window will be extended (Fig. 27) [320]. The specific design can satisfy the respective requirement at cathode and anode sides, endowing the supercapacitor with high energy and power densities. As an example, Fu et al. designed an alkaline-neutral electrolyte separated by K+-conducting membrane, the electrochemical stability window is widened to 2.3 V and the LIC delivered an energy density of 50 W h kg-1 at 571 W kg-1 with outstanding capacitance retention ratio (88%) after 5000 cycles [321].

Fig. 27. Effects of salt species and concentration on working voltage for electrodes. Concentration effect of Li2SO4 (left) and LiTFSI (right) on the working voltage for active carbon and MnO2. Reproduced with permission from Ref. [320]. Copyright: 2018 Elsevier. Except for electrolyte salts and solvents optimization, the electrolyte with intrinsic redox activity is another hotspot. Typically, the TEMPO, an ambipolar organic mediator, is introduced into electrolytes to simultaneously trigger oxidation and reduction, delivering 51 W h kg-1, roughly 1.4 times increment in capacitance. Meanwhile, the TEMPO electrolytes also guarantee a long lifespan with 4000 cycles [322]. A dual-active electrolyte has also been testified with 0.05 M FeBr3 and polyvinyl alcohol to prolong the lifespan of supercapacitor with boosted capacitance and improved Coulombic efficiency [323]. Although the generated surface passivating layer may also block surficial active area, we hold the view that partial capacitance may originate from the surface charge storage resulted from the contribution of the formed protective layer, which is a ubiquitous phenomenon in Li+ or Na+ battery as

discussed previously. The desired format for electrolytes is changing from liquid to gel or quasi-solid, and solid at last. Their non-flow state avoids the solvent leakage and impose a force to suppress dendrite and active material drifting. Currently, the research focus is applied onto the gel or quasi-solid considering the insufficient ion conductivity in solid electrolytes. A highly conductive polymer electrolyte (13.8 ± 2.4 mS cm-1) containing lithium polyacrylate acid and polyacrylamide was prepared and shows wide electrochemical stability window up to 1.5 V in the neutral system [324]. The poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) is a good candidate to prepare membrane conducting Li+ and Na+ in supercapacitors [325]. A light-weighted hybrid supercapacitor is designed and enables rapid ion diffusion, thus improving the safety and rate capability of 5 kW kg-1 with 32 and 36 W h kg-1 for LIC and NIC, respectively [325]. Taking advantage of higher thermal stability of quasi-solid electrolyte, a NIC with high volumetric capacitance (28.5 W h L-1 at 9.4 kW L-1) was prepared at room temperature and 50
[326]. Such a favorable performance also resulted from the low interfacial resistance of PVDF-HFP membrane. Besides, the gel electrolytes can also extend the ductility and mechanical strength after crosslinking with other polymer network or compositing with inorganic nanoparticles for the flexible devices [327]. With the assistance of ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate, a gel polymer electrolyte is prepared with superior conductivity (7246 S m-1) and wide electrochemical stability window (3.5 V). The excellent characters ensure the supercapacitor with a high energy density of 75 W h kg-1 on the basis of hierarchical graphene nanocomposites and conspicuous flexibility after repeated bending tests [328]. 4. Conclusions and Perspectives Electrochemical energy storage devices are undergoing profound alteration in the “Nano Age”. The nanotechnology has been demonstrated to be a splendid way in tailoring electrode and electrolyte structures for energy and power density upgradation in next-generation equipment, involving alkali metal batteries, redox flow batteries and supercapacitors, which suit for the deployments in different area. The high-energy-density systems are popular within consumer electronics and electrified transportation vehicles due to the relatively high request or limitation in device size and weight. For instance, the battery with Ni/Li-rich cathode and Si-C/Li composite anode owns high density and thus can be widely used in the size-limited devices such as the electrombile, exhibiting great potential in next-generation batteries. While the high-power-density systems lay more attention on the capability of fast charge and discharge. Typically, the RFB fits for large-scale energy storage and its power density can be many times of that for current Li-ion batteries. Such systems are applicable to storage excess electricity produced by other power generators and work as a backup power to modulate the supply in the power grid. As to the supercapacitor, it can complete charge and discharge in a few seconds, and usually cooperate with other power source to start engines in a wide temperature range. 4.1. Electrodes

The nanostructured electrode will definitely promote the electron/ion transportation and corresponding reactions will be accelerated. For rechargeable alkalis metal batteries, the reserved structure can tolerate the dimensional and stress variations and prevent the active materials detaching from the current collector. On the other side, a higher specific surface area generally brings in aggravated parasitic reactions that lead to the consumption of large amounts of electrolyte and low Coulombic efficiency, and concomitantly, the low tap density drags down the energy density of the battery. Therefore, an optimum ratio of nano and micro is anticipated. Besides, the cathode materials suffer from the drastic alternation in the structure under abuse conditions. Adaptable surface modifications at nano scale have shown splendid efficacy, which is worthy of further investigation for a massive production at low expense. Making the material evolution clear is of great significance in understanding the critical factors that determine structural stability and guiding the exploration of high-voltage and high-capacity cathode. In RFBs, the challenge mainly locates at developing robust electrodes with the abundant catalytic sites and high activity. Due to the outstanding safety preponderance, the low-cost aqueous RFB systems is still the primary choice for grid-scale energy storage. In addition, the RFBs with organic redox couples and hybrid systems that integrating rechargeable alkalis battery and RFBs are gratifying approaches for energy density promotion. Prior to their implementations, their redox potential demands for fine regulation, and the conductivity and solubility are expected to be enhanced. Some particular attention should be paid to the active material crossover and alkalis metal protection if non-aqueous electrolytes have been replaced by water-based solvents. Meanwhile, redox reaction kinetics and mechanism among various electrodes deserve for systematical studies from the aspect of molecular level. Supercapacitors, especially for the systems that involve alkalis ions in the electrolytes, have been experiencing explosive growth with the assistance of nanotechnology and material science. Searching for new materials with further improved electrochemical performance is still indispensable. Although the well-studied EDLC mechanism benefits for exploration of new-type supercapacitors, the fundamental study is urgently expected to embrace the charge storage in the existing and newly appearing materials including MXene, MOF, COF, etc. For instance, the correlation needs to be revisited between charge storage ability and material activity, surface polarity, specific surface area, pore size and distribution, the volume expansion and contraction when alkalis ion storage in host materials arises as a new issue. The aim is to develop voltage-matched and high-capacity electrode materials. 4.2. Electrolytes Different from the electrode, the criteria for electrolyte is consistent: high ion conductivity and electrochemical stability. Salt concentration has been verified to show a great impact on the electrochemical stability window of electrolyte, no matter for aqueous or non-aqueous electrolytes. While the mechanism for ion conducting and stability improvement is not clear for these new concentrated electrolyte systems,

which quests for unambiguous fundamental comprehension. Confining the liquid electrolyte with normal concentration (such as 1 M) into the nonstructural insulating host can meet the criteria for electrolytes and improve the selective permeability for anticipated ions. In this situation, the electrolyte exhibits superiority to that of commercial liquid electrolytes, such as higher ion conductivity and stability, which we think is a good candidate akin to gel or quasi-solid electrolyte at the transitional stage for safe energy storage devices. Apparently, the ultimate choice is the solid-state electrolyte. Currently, a considerable quantity of electrolytes such as the garnet and sulfide electrolytes attain the requirement for ion conductivity, some even surpass the commercial liquid electrolyte. With unique nanostructured design, lightweight inorganic electrolytes are achievable and polymer electrolytes can also turn into highly-conductive species at room temperature. Challenge still exists in the integration of the one with high conductivity into another electrolyte. Correspondingly, more efforts are desirable for elucidating ion transportation pathway, the function of ions and electrons, etc. 4.3. Interfaces On the basis of favorable electrodes and electrolytes, the interface compatibility and stability play a determinable role for electrochemical energy storage devices. Different systems call for tailored interfaces to satisfy the requirements of fast and steady ion and electron flux, especially at the condition of high current density and capacity. The charge carriers from the electrode or electrolyte meet at the boundary phase to react due to the different chemical and electrochemical potential, and simultaneously, a so-called passivation layer (SEI) will be built, which is the core for stable operation of batteries at a wide voltage window. Nanotechnology supplies vast space and opportunity for guiding artificial SEI construction, reducing interfacial resistance, and improving compatibility with electrode and electrolyte. For example, the nano-thick coating on electrode materials can improve their structural stability under the destructive conditions, the conformal passivating layer will dramatically lower the interfacial impedance, artificial SEI can stabilize the alkalis anode for dendrite-free plating and stripping. Under such circumstance, what is the pathway for ions and electrons and how does the electrochemical reaction proceed? The mechanism of electrochemical reactions in the non-static systems with flowing electrolyte or electrode is more difficult to probe, and having a clear idea of interfacial structure and composition evolution and their influence on electrochemical performance or intrinsic correlations deserve in-depth investigations. 4.4. Other considerations It is noteworthy, the single methodology is hard to handle all the issues existing in the electrochemical energy storage devices. Seemingly, the combination with possible strategies such as advanced characterizations and theoretical computations can find the way out. It is advisable to construct in-situ or operando equipment to monitor the the component and transformation of electrode, electrolytes, and their interfaces at

different charge/discharge states for the whole cell during the operation process. Combining with the powerful and high-throughput screening techniques, the research efficacy for materials involving electrode, electrolyte, catalyst, interfacial stabilizer, and other related device components can be greatly boosted. The accurately obtained evolution process and mechanism in material, chemistry, physics, and computational sciences at a micro/nano scale will shed light on the comprehension for related electrochemical energy storage devices.

Conflict of interests The authors declare no competing financial interest. Acknowledgements This work was supported by the Basic Science Center Project of Natural Science Foundation of China under grant No. 51788104, the National Key R&D Program of China (Grant No. 2016YFA0202500), the National Natural Science Foundation of China (Grant No. 21773264, 21805062, 51772093, 51803054), the “Transformational Technologies for Clean Energy and Demonstration”, Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA21070300), the "Double first-class" School Construction Project and Outstanding Youth Fund of Hunan province (Grant No. SYL201802008, 2019JJ20010, 2019JJ50223). J. Yue would like to thank the China Postdoctoral Science Foundation Grant (No. 2019T120135). The authors also would like to thank the help from Dr. Xu-Dong Zhang and Wen-Peng Wang.

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Biography

Xian-Xiang Zeng is an associate Professor at Hunan Agricultural Univeristy. He received his Ph.D. at Institute of Chemistry, Chinese Academy of Sciences (ICCAS). His research interests are solid electrolytes and corresponding interfacial issues with electrode materials in rechargeable batteries.

Yu-Ting Xu is currently a postgraduate student at Hunan Agricultural University. Her main research focuses on safe rechargeable lithium/zinc batteries.

Ya-Xia Yin is a Professor at ICCAS. She received her Ph.D. in Beijing Chemical Technology. Her research focuses on nanostructured electrode materials for advanced Li-ion and Li-S batteries, and sodium-ion batteries.

Xiong-Wei Wu received his Ph.D. at Central South University in 2011 and joined in Hunan Agricultural University at the same year, and worked as a chief scientist of Hunan Yinfeng New Energy Co., LTD. During this period, he worked as a senior visiting scholar of ICCAS from 2015 to 2017. His research focuses on the key materials for vanadium redox flow batteries and other new battery power sources.

Junpei Yue received his Ph.D. in Physical Chemistry from Justus-Liebig-University Giessen (Germany) under the supervision of Prof. Bernd M. Smarsly in 2016. He is working as a postdoctoral research fellow with Prof. Yu-Guo Guo at ICCAS. His research focuses on solid-state batteries and the interface issues in batteries.

Yu-Guo Guo is a professor of Chemistry at ICCAS. He received his Ph.D. in Physical Chemistry from ICCAS in 2004. He worked at the Max Planck Institute for Solid State Research at Stuttgart (Germany) first as a Gust Scientist and then a Staff Scientist from 2004 to 2007. He joined ICCAS as a full professor in 2007. His

research focuses on nanostructured energy materials and electrochemical energy storage devices, such as Li-ion, Li-S and solid lithium batteries.