Lithium metal anodes: Present and future

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Lithium metal anodes: present and future Renheng Wang , Weisheng Cui , Fulu Chu , Feixiang Wu PII: DOI: Reference:

S2095-4956(20)30001-2 https://doi.org/10.1016/j.jechem.2019.12.024 JECHEM 1050

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Journal of Energy Chemistry

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17 November 2019 26 December 2019 27 December 2019

Please cite this article as: Renheng Wang , Weisheng Cui , Fulu Chu , Feixiang Wu , Lithium metal anodes: present and future, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2019.12.024

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Review

Lithium metal anodes: present and future Renheng Wangb,1, Weisheng Cuib,1, Fulu Chua,1, Feixiang Wua,* a

School of Metallurgy and Environment, Central South University, Changsha 410083, Hunan,

China b

College of Physics and Optoelectronic Engineering, Shenzhen

University, Shenzhen 518060, Guangdong, China 1

These authors contributed equally to this work.

* Corresponding author. E-mail address: [email protected] (F. Wu).

Abstract: Commercial lithium-ion (Li-ion) batteries based on graphite anodes are meeting their bottlenecks that are limited energy densities. In order to satisfy the large market demands of smaller

and

lighter

rechargeable

batteries,

high-capacity

metallic

Li

replacing

low-specific-capacity graphite enables the higher energy density in next-generation rechargeable Li metal batteries (LMBs). However, Li metal anode has been suffering from dendritic problems, interfacial side reactions, volume change and low Coulombic efficiency. Therefore, performance enhancements of Li metal anodes are rather important to realize the high energy density characteristic of metallic Li. In this review, the annoying Li dendrite growth, unstable reaction interface and practical application issues of Li metal anodes are summarized and detailedly discussed to understand the current challenges concerning Li metal anodes. For overcoming such remaining challenges, the corresponding strategies and recent advances are covered and categorized. Finally, we discuss future opportunities and perspectives for developing high-performance Li metal anodes. Keywords：Lithium; Anode; Lithium metal battery; Surface protection; Li dendrite

1

Renheng Wang received his Ph.D. degree in Metallurgical Engineering from Central South University (CSU) in 2015. From January 2016 to October 2018, he worked as a postdoctoral follow of Prof. Xiaodong Chen at Nanyang Technological University and Prof. Han Zhang at Shenzhen University, respectively. He is now a researcher in College of Physics and Optoelectronic Engineering, Shenzhen University. His research focuses on the synthesis and application

of

nanomaterials

and

composites

for

clean

energy

storage,

such

as

high-power/high-energy lithium ion batteries.

Weisheng Cui received his Ph.D degree from Beijing Jiaotong University in 2019, Beijing, China. He has the electrical engineering background and the research experience in the field of power electronics. He focuses on the generation and application of plasma under atmospheric pressure and vacuum condition during his Ph.D period. He is currently working as a postdoctoral researcher in Shenzhen University; his research interests include the modification of 2D materials with plasma technology and their applications in energy storage and conversion.

Fulu Chu gained his Bachelor‟s and Master‟s degrees from Xiangtan University in the School of Materials Science and Engineering in 2015 and 2018, respectively. He is currently a Ph.D. candidate in Metallurgical Engineering at Central South University under the supervision of Prof. Feixiang Wu. And during his Master's degree, he was a joint-training student under the supervision of Prof. Chilin Li in Shanghai Institute of Ceramics, Chinese Academy of Sciences. His current research interests mainly focus on the interface between Li and liquid electrolyte, dendrite suppression of Li metal anodes and advanced energy storage/conversion.

Feixiang Wu is a professor of the School of Metallurgy and Environment at the Central South University (CSU). He did his Ph.D. in Metallurgical Physics and Chemistry in 2014 at CSU. From 2012 to 2014, he was a visiting scholar in Prof. Yushin‟s laboratory in Materials Science at the Georgia Institute of Technology (GT). After graduation, he worked as a research associate in Yushin group at GT (2015-2016). From 2016 to 2019, he worked as a Humboldt Fellow in Maier group at the Max Planck Institute for Solid-State Research. His research interests focus on materials and electrolytes for rechargeable batteries.

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Introduction Due to the lowest reduction potential and smallest ionic radii of lithium (Li), Li-based

rechargeable batteries among metal-ion batteries have been viewed as the most promising battery system to achieve high gravimetric/volumetric energy densities and power density [1,2]. Through the research efforts of the past two decades, rechargeable Li-ion batteries have been widely used in our daily life, including electric tool, laptop, digital product, cell phone, electric vehicle and drone. Recently, the sharp growing market demands the lighter, thinner and cheaper rechargeable Li and Li-ion batteries for commercialization. However, the current commercial Li-ion batteries are meeting their theoretical limitations (~250 Wh kg-1 and ~680 Wh L-1). The graphite-based anodes suffer from low theoretical specific capacities (~372 mAh g-1) and volumetric capacities (~735 mAh cm-3), which limit the further increase of energy densities achieved by commercial Li-ion batteries [1,3]. Therefore, the innovation of battery materials including both cathode and anode have been mainly devoted to improve the energy densities of next-generation lithium-based batteries, assembling lighter and thinner batteries for various applications. Among high-capacity anodes, Li metal is a perfect anode material candidate for the design of rechargeable batteries as it has extremely high theoretical specific capacities (3860 mAh g-1 and 2061 mAh cm-3) and the lowest negative electrochemical potential (-3.04 V vs. the standard hydrogen electrode) [4–9]. According to previous assumptions on practically achievable energy densities of future Li and Li-ion batteries, the energy densities of a battery system can reach the maximum when a positive electrode matches the Li anode (can beat the Si and graphite anodes) [1]. Whether it is commercial cathode or conversion type cathode, the gravimetric and volumetric energy densities (maximum values calculated based on theoretical capacity utilization) of produced lithium metal batteries are very competitive, potentially reaching the values of 900~1900 Wh L-1 and 400~1000 Wh kg-1 (Fig. 1) [1,2]. We shall note that these energy densities are largely determined by the practical electrolyte dosage, and capacity utilization in future Li-metal batteries (LMBs). Therefore, Li metal batteries, such as commercial cathode-Li batteries, high-voltage cathode-Li batteries, metal fluoride-Li batteries, Li-O2 and Li-S batteries in Fig. 1 have been viewed as very promising candidates for next-generation high energy rechargeable batteries, which may realize longer driving mileage of at least 400 miles per single charge and 3

solve the current dilemma faced by Li-ion batteries [10–12].

Fig. 1. Estimation of the practically achievable energy densities (maximum values) offered by Li-metal batteries using different cathode materials. The green region composed of several typical Li-metal battery systems roughly presents approximately achievable energy densities of future Li-metal batteries based on multiple cathodes. Because of the high energy density characteristics of lithium metal batteries, metallic Li anodes are being widely studied at present [13]. However, the Li metal anode suffers from knotty issues making the Li metal anode technology too immature to serve in lithium metal batteries. In this article, the remaining challenges from fundamental to practical, and classified strategies of Li metal anodes are reviewed and studied. Finally, the guideline and main trends in the future development of Li metal anodes are discussed. 2

The development history of Li metal batteries LMBs can be classified into primary batteries and secondary batteries according to whether

they are rechargeable or not. Primary LMBs with high energy densities, such as Li-I2, Li-MnO2, Li-CuO, and Li-(CF)n batteries were born in the 1970s and mainly used in watches, calculators and portable medical devices based on a long time discharge. Generally, the energy density of secondary LMBs is less than that of primary LMBs. However, the secondary LMBs have a wider application scope because they can be charged repeatedly. The LMBs in the following descriptions refers to the secondary LMBs. The earliest LMBs, designed by Stanley Whittingham at Exxon in the 1970s, used Li metal as the negative electrode and TiS2 as the positive electrode [3]. However, the research of LMBs has been in a standstill since 1980s due to the serious safety hazards caused by Li dendrites growth. In the late 1970s, Armand proposed the concept of LIBs, also called 4

rocking-chair cells, using intercalation materials for both cathode and anode sides. Based on the charge-discharge theory of LIBs, Li ions were transferred from one side to the other side, that do not need to be reduced into Li atoms during charging and Li dendrites can be completely avoided. In 1991, LIBs began to be commercialized and they also revolutionized the appearance of electronic products at the same time [14,15]. Shortly, the graphite anode was employed as the best choice in conventional LIBs, and has been used till today. However, the energy density of LIBs has basically reached the limit, which cannot meet the rapid expansion of the market. Recently, owing to the demand of high energy densities, lithium metal batteries have regained the plenty of research efforts, which are back in sight.

Fig. 2. Development history of LMBs and LIBs.

3 The remaining challenges of Li metal anodes The aggressive Li metal chemistry has met severe challenges while operates LMBs through the charging/discharging process. Among multiple issues, Li dendrite formation is one of the most pressing challenges causing critical safety risk, which is due to the inherent property viz. high diffusion barrier of the Li atom [16]. To be specific, the Li dendrites tend to form in either pits of the initial stripped Li metal foil or on the surface of the initially plated Li anode [17]. According to previous observations, the repeated lithium plating/stripping (deposition/dissolution) processes result in a large number of Li dendrites on the anode surface and low Coulombic efficiency (CE) [13,17–20]. The continuous growth of Li dendrites can penetrate the separator and cause short circuit in the battery, thus producing high current discharge, inducing a lot of heat and even explosions. The rapid and uneven dissolution of Li dendrite near the active site will also separate the lithium dendrite from the matrix and produce "dead Li" [21]. The "dead Li" here primarily 5

refers to electrically isolated Li metal during repeated volume change, which is shrouded by a thick SEI layer comprising inorganic/organic Li-species. As results, the broken Li needles and particles lose the electron and ion transports (Fig. 3). The formation of "dead Li" causes the loss of active lithium in the electrode and reduces the specific capacity of the battery. For example, Archer et al. found that the fiber-like deposition is the natural characteristic of the Li, and the physical orphaning is the main limitation for the poor reversibility performance of Li [22].

Fig. 3. The remaining challenges (from fundamental to practical) of Li metal anodes. In addition, the cycle performance of LMBs was mainly determined by the consumption of electrolytes, which is mainly related to the interaction between highly reactive Li metal and electrolyte. When the Li metal anode contacts with the electrolytes, a solid electrolyte interphase (SEI) layer will be formed on the surface of Li metal [23]. In the following Li plating/stripping process, the defects in SEI layer caused by infinite volume expansion and shrinkage will further lead to consumption of the electrolyte, growth of SEI, “dead Li" and surface pulverization (Fig. 3). Meanwhile, the cracked SEI layer will expose many defects and in turn accelerate deposition of Li atoms on the defects forming Li dendrites (Fig. 3). Consequently, the Li dendrites, “dead Li”, consumption of electrolyte, and thick SEI will greatly worsen the electrochemical performance of LMBs, such as low CE, short cycle life and safety hazard. Beyond coin cells in the laboratory, Li metal pouch cells have been viewed as a suitable battery system for future applications. However, they would inevitably suffer from even worse 6

cases under larger current density, high areal capacity and no-pressure environment. Predictably, the above-discussed remaining challenges of Li metal anodes would be more complex in pouch cells. For example, in pouch cells, the powdering and the polarization are more critical in LMBs [24]. According to the report in Fig.4 by Zhang and co-workers, powdering of Li metal with the accumulation of “dead Li” in polarization zone leads to the degradation of Li metal anode at low current density, while the short-circuit causing safety hazard is the main challenge at high working current density. In the zone of transition in Fig.4, both formations of “dead Li” and Li dendrites could happen [25]. Considering the practical applications of LMBs, Li metal anodes face some knotty problems so far. For example, similar to risk caused by Li dendrite formation, the high reactivity between Li and air (containing moisture and oxygen) make it too dangerous to use in electric vehicles. The short circuit or traffic accidents may cause the explosion of LMBs, which make the LMBs hard to achieve large-scale product uses in EVs. In addition, owing to the rather poor plasticity of Li metal, bending and extrusion can produce irreversible deformation and microscopic defects on the Li metal surface which are permanent changes occurred within the material itself. These defects caused by deformations would be active sites for growing Li dendrites during cycling. For matching the areal capacity of 4~6 mAh cm-2 in conventional cathodes, the thin Li metal foil with corresponding thickness of 20~30 μm attaching on copper current collector are needed. Therefore, besides the safety risk in applications, the automate mass production of suitable lithium metal negative electrodes are very challenging because of high reactivity, high corrosivity, high air-sensitivity and low plasticity of Li metal (Fig. 3).

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Fig. 4. The remaining issues of Li metal anodes in pouch cells: photos showing the morphology change of Li anode and separator at different current densities and capacities; Failure mechanism map and electrochemical diagrams (bottom schematic figures) obtained from practical Li metal pouch cells under various electrochemical conditions (reproduced from ref. [25] with permission of WILEY-VCH).

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The recent advances of Li metal anodes In order to overcome above challenges, the main direction is to achieve a uniform Li

deposition during Li plating and stripping, protecting lithium metal from Li dendrite formation. So far, multiple strategies have been proposed to achieve high-performance Li metal anodes. The recipe and related method or rationale for enhancing the long-term cycle stability of Li metal anodes is summarized in the Fig. 5. The most popular way is to form a thin and dense protection 8

layer (before or during cycling) which is ionically conductive on Li metal (Fig. 5a). Ideally, targeting for uniform Li deposition, the ion conductor layer should be thin and dense film that can successfully prevent electrolyte penetration and Li dendrite formation. This kind of ion conductor layer can be solid electrolyte interphase (SEI) layer formed in-situ by electrolyte modifications from liquid to solid (Fig. 5d). Beside ion conductor, electron conductor protection layer/matrix on Li metal anode was also employed (Fig. 5b). We shall note that the electron conductor layer/matrix should have well-distributed nucleation sites offering uniform Li deposition. Otherwise, the Li dendrite may form on the surface of electron conductor. However, these surface protections would be destroyed by infinite volume change during repeated cycles, causing ever-increasing thickness of SEI, loss of Li, dendrite formation and consumption of electrolyte. To further enhance the stability of reaction interface and prevent the Li dendrite formation, significant efforts went into the development of lithiophilic hosts with sophisticated architectures in order to confine active Li (Fig. 5c). Such success was obtained progressively by exploiting porous carbons, carbon hollow spheres, carbon nanotubes (CNTs), graphene, 3D current collectors, as well as 3D electron (or ion) conductive frameworks, which can suppress the volume change to drive more stable SEI. In addition, the various binder designs have been reported for improving the mechanical stability of the composite architecture to overcome volume change, demonstrating a more stable reaction interface (Fig. 5e). For preventing the short circuit, the modified separators with good mechanical properties are more rigid to prevent the Li dendrites penetration (Fig. 5f). For electrolyte modifications in Fig. 5(d), effective electrolytes for high-performance Li metal anodes mainly include solids and non-aqueous liquid electrolytes. The design principle should focus on the stable interface between highly reactive Li metal and electrolytes during electrochemical reactions. The well passivated surface layer formed in-situ on Li metal would play significant roles in restraining the continuously undesired side-reactions between fresh Li and electrolytes. Especially, pushing both liquid and solid electrolytes to high oxidation potentials is necessary to the emerging high-voltage cathodes-based LMBs. For solid electrolytes, Li ion conductivity, tolerated physical mechanics and the easy fabrication for large-scale application are very important. In addition, low electronic conductivity and high interface affinity/contact on Li 9

are the expected properties of solid electrolytes. When considering the liquid electrolytes, various approaches such as functional additives, fluorinated solvents, novel salts and considerable high salt concentrations have been widely explored in liquid electrolytes to enhance the overall cycling performance. Moreover, for future applications, operating temperature range, industrial cost and flammability need to be taken into account for large-scale uses. Besides above strategies, simulations combined with experiment results have been largely employed to provide valuable insights to understand the Li-metal battery chemistry under various conditions [26–29].

Fig. 5. Main strategies for high-performance Li metal anode enhancement and their method/rationale: formation of (a) ion conductor layer and (b) electron conductor layer on Li 10

metal anode, (c) design of Li hosts, (d) modification of electrolyte, (e) binder design, (f) modification of separator. Schematic figure in (e) reproduced from ref. [30] with permission of WILEY-VCH, while schematic figure in (f) is reproduced from ref. [31] with permission of The Royal Society of Chemistry.

4.1 Liquid electrolyte modifications The proposal of SEI originates from Peled in 1979 when he and his coworkers focused on the research of batteries using alkali and alkaline earth metals as anodes [32]. They found that a very thin film with complex composition was in-situ formed when the metal anode contacted with the nonaqueous electrolytes. The concept of SEI was extended to the electrodes with different metal and nonmetal materials and it has been extensively investigated in the past two decades [33–37]. As the formation of SEI is mainly determined by electrolytes, the composition of electrolytes is vital for inducing the formation of a thin, stable, dense, and elastic SEI layer. Therefore, in LMBs, the modifications of electrolytes including solvents, salts and additives, have been viewed as the mostly effective strategy to stabilize the Li metal anodes [38–40]. As summarized by in a previous report [9], the major species of SEI on the Li metal vary widely depending on different solvents. For example, the surface compositions of CH3CH–(OCO2Li)CH2OCO2Li, (CH2OCO2Li)2, CH3OLi, HCOOLi, ROLi(CH3(CH2)3OLi) and ROLi(CH3OLi) will be formed in propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), methyl formate (MF), tetrahydrofuran (THF), and 1,2-dimethoxyethane (DME), respectively. Carbonate electrolyte solvents are important for the development of LMBs in the future. A recent work has been done to comprehensively understand the effect of different substituents on cyclic carbonates, which has provided a guidance for the design and selection of cyclic carbonates [41]. The inorganic salts in the SEI layer are usually determined by the salts including organic in electrolytes. In addition, the Li salts also play a role in the formation of SEI because the Li salt anions have different reduction characteristics with the Li metal anode, such as LiTFPFB [42]. To enhance the quality of SEI layer, the functional additives with higher reduction voltages are added to the electrolytes. Typically, the additives are easy to react with the Li metal and form a more stable and denser SEI layer before 11

the one produced by other components in electrolytes. Delnick found that the ionic conductivity of SEI is depending on its imperfection, and the resistance of SEI continues to grow with the increase of its thickness [43]. The composition and structure of SEI change gradually from the inner surface with the Li metal anode to the outer surface with the solution. The main ingredients in the inner surface of the SEI layer are low oxidation materials while the components of outer part of the SEI are mostly in higher oxidation state, which closely related to the solvent [44]. Short ethers-based electrolytes with low viscosity are less reactive with Li metal compared to esters and have shown less dendrite formation [45]. Yamada et al. also demonstrated that just increasing the salt concentration (> 3 M) in dimethyl carbonate (DMC) solvent can enable single solvent systems for high voltage (5.0 V) chemistries under certain circumstance [46]. Meantime, the highly concentrated electrolytes exhibited enhanced oxidative/reductive stability, which offered a solution for expanded electrochemical stable potential window [47–49]. The high concentrated (4~5 M) Li bis(fluorosulfonyl)imide (LiFSI) in 1,2 dimethoxyethane (DME) is able to increase the Li metal cycling efficiency to about 99% and there is not evident dendrite formation [50,51]. However, the high concentration electrolytes (HCEs) have the shortcoming of high cost (which is proportional to the salt concentration), poor ionic conductivity, and poor wetting capability. Note that wettability of non-aqueous electrolyte is a key parameter in practical production of lithium-based batteries, which not only influences production efficiency, but also severely influences the yield of conforming batteries. Poor wetting capability means would consume more time to implement complete electrode invasion in non-aqueous electrolyte, reducing the efficiency of the production line and worsening the initial SEI formation. Zhang et al. developed a localized high-concentration electrolyte (LHCEs) by diluting an HCE with an “inert” diluent, which can greatly lower the total Li salt concentration to a practical level while maintaining (or even enhancing) the unique characteristics of the HCE [52]. Utilizing co-solvent electrolyte is another approach to cope with the drawbacks of HCEs. Dong et al. designed a co-solvent electrolyte with high concentration of 5 mol kg-1 bis(trifluoromethanesulfonyl)imide (LiTFSI)/ethylacetate (EA) electrolyte and dichloromethane (DCM) diluent [53]. Because of the unique structure of co-solvent, a wide range of stable potential window, high ionic conductivity and low viscosity at 12

ultra-low temperature of -70 °C is realized (Fig. 6a), demonstrating high energy and power densities in a rechargeable metallic Li battery [53]. By using an oxidizing co-solvent of ethyl methyl sulfone (EMS) in the electrolyte, Li and Ji achieved a dense and macroscopically smooth surface morphology of the plated Li surface, in spite of the not necessarily high CE efficiency [54]. To cope with the issue of insoluble of LiNO3 in carbonate solvents such as fluoroethylene carbonate (FEC), Zhang et al. designed the carbonate/ether co-solvent electrolytes, using FEC and dimethoxyethane (DME) as solvents and the additive of LiNO3 [55]. The combination of FEC and LiNO3 enhanced the uniformity of the SEI and delivered a much-prolonged cycle life (over 1000 cycles) and high CE (99.96%) even in very rigorous condition (Fig. 6b) [55]. Similarly, the Li2S5 and LiNO3 as co-additives in the LiTFSI-DOL/DME electrolyte induced uniform and sulfurized SEI on Li metal anode with a high ionic conductivity of 3.1×10-7 S cm-1 and attained a CE of 98% during 200 cycles at 1.0 mA cm-2 [20]. In addition to polysulfides, FEC and LiNO3, several additives including lithium bis(oxalate)borate (LiBOB), methyl viologen (MV), Mg(TFSI)2 and vinylene carbonate (VC), which can drive in-situ formation of SEI layer through spontaneous and preferential reaction with the contact of Li, have been proved to stabilize the Li metal anode [56-60]. Based on the mechanism of forming facile thin layers by alloying with Li, a series of inorganic salt additives containing metal ions (e.g., MClx (M=As, In, Zn or Bi), AlCl3, AlI3, In(TFSI)3) stabilize lithium metal with the new SEI formation (Li-rich composite alloy films) that can effectively prevent the Li dendritic growth [61–64]. For example, the synergy of fast lithium ion migration through Li-rich ion conductive alloys coupled with an electronically insulating surface component can stabilize and sustain electrodeposition over 700 cycles (1400 h) of repeated plating/stripping at a practical current density of 2 mA cm−2 (Fig. 6c) [61]. Moreover, Linda F. Nazar and co-workers further came up with an in-situ formed Li+ single-ion-conducting Li3PS4 layer to homogenize the Li+ flux, thus suppressing the uncontrolled Li dendrite formation [65].

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Fig. 6. Liquid electrolyte modifications: (a) Schematical images to illustrate the solvation structures of diluted electrolyte, concentrated electrolyte and co-solvent electrolyte. Reproduced from Ref. [53] with permission of Wiley-VCH. (b) FEC/LiNO3 electrolyte for highly stable lithium metal batteries (Reproduced from ref. [55] with permission of Wiley-VCH) (c) Li-rich composite alloy films formed in situ on lithium by a simple and low-cost methodology (Reproduced from ref. [61] with permission of Springer Nature); (d) non-flammable fluorinated electrolyte enables stable cycling of high-voltage cathodes (LiNi0.8Mn0.1Co0.1O2 and LiCoPO4) in a Li-metal battery (Reproduced from ref. [66] with permission of Springer Nature).

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The morphology of plated Li metal is determined by a variety of factors involving both the salts and solvents. Liu et al. enhanced the stability of non-flammable phosphate electrolytes by increasing the molar ratio of salt to solvent [67]. These non-flammable electrolytes show reduced reactivity toward Li-metal electrodes. Wang et al. increased the LiFSI concentration to ~10 M and proved that the Li metal anode in carbonate electrolytes has an excellent CE performance [68]. In addition, the highest plating efficiency occurs when the LiFSI is the primary salt and it even exceeds LiPF6 and LiAsF6 in carbonate solvents with additives [50]. Alvarado et al. investigated the bisalt effect on the interphasial chemistry of both Li metal and high Ni cathode materials and found that the co-existence of two anions (TFSI- and FSI-) introduces entirely new interphases through preferential decomposition mechanisms [69]. As a result, the plated Li metal has a denser, more conformal morphology. Through fluorination of electrolyte, Li et al. designed a class of high-concentration full-fluoride (HFF) electrolytes with a large donatable fluorine concentration (DFC) for 5 V rechargeable LMBs, achieving a high-performance in the high-voltage cathode-based LMBs [70]. Wang et al. developed a non-flammable fluorinated electrolyte for the Li metal anode and high-voltage cathodes (LiNi0.8Mn0.1Co0.1O2 and LiCoPO4), resulting in F-rich CEI and F-rich SEI (dendrite-free) in LMBs and demonstrating high-energy densities (Fig. 6d) [66]. The challenges of poor Li plating/stripping, Li dendrites growth, poor safety and aggressive high-voltage cathodes have all been solved simultaneously [66].

4.2 Artificial protection layers Besides in-situ formed SEI layer induced by solvents, salts or additives in electrolytes, the construction of artificial protective layers before assembling cells are very effective [61,63,65,71– 74]. The most popular way to fabricate artificial SEI is via a designed chemical reaction or deposition between Li metal and precursors [75–77]. The physical and chemical properties of the artificial SEI layer are important factors to suppress the Li dendrites. According to previous literatures, the artificial SEI layers are better to be ion conductor, which should be thin and chemically stable in electrolytes [19,78]. Zhu et al. have successfully demonstrated a PDMS as an artificial protection film on Cu foil to induce uniform Li deposition and avoid the Li dendrite 15

formation [79]. Gao et al. constructed a chemically and electrochemically active polymer layer viz. poly((N-2,2-dimethyl-1,3-dioxolane-4-methyl)-5-norbornene-exo-2,3-dicarboximide), on the Li metal anode to serve as a grafted polymer skin which not only incorporate ether-based polymeric components into the SEI but also induce Li deposition/dissolution under the skin in a dendrite/moss-free manner (Fig. 7a) [74]. The atomic-layer deposition (ALD) and molecular-layer deposition (MLD) are also important methods in the preparation of artificial protective layers [80– 83]. Cho et al. engineered a zirconia (ZrO2) encapsulation layer, which has a high-κ dielectric property, on Li metal anode with ALD [84]. Dasgupta et al. proved that the deposition of ultrathin ZnO layer on the current collector by ALD can facilitate the initial Li nucleation, which controls the morphology and reversibility of subsequent cycling [85]. This platform based on this method can realize a Li metal anode CE up to 99.5%. Elam et al. deposited a conformal organic/inorganic hybrid coating on the Li metal anode directly for the first time using MLD to cope with the parasitic reactions at the Li-electrolytes interface [86]. Zhao et al. introduced a controllable Alucone layer to protect the Li foil by MLD method [87]. As a result, the formation of mossy-like Li dendrites is suppressed and the lifetime in different electrolytes (carbonate-based and ether-based) is extensively improved. Later, they proposed an ultrathin polymer film of “polyurea” on the Li anode via the MLD method and the abundant polar groups in polyurea can evenly redistribute the Li ion flux and avoid the formation of Li dendrites (Fig. 7b) [88]. Luo et al. realized a uniform Li plating/stripping without any dendrite formation at a current densities up to 5 mA cm-2 with a thin high-polarity β-phase poly(vinylidene difluoride) (β-PVDF) layer on Cu (Fig. 7c) [89]. Yan et al. designed a dual-layered film on Li metal anode and prevented the Li anode from the corrosion of electrolytes [90]. Guo et al. fabricated a Li polyacrylic acid (LiPAA) polymer SEI layer using in-situ reaction between Li metal and polyacrylic acid (PAA) [91]. With the high binding ability and excellent stability of the LiPAA polymer, the smart SEI has a high elasticity property and can accommodate the deformation of Li during plating/stripping processes through adapting and regulating the interface, achieving a stable cycling of 700 h [91].

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Fig. 7. Examples of artificial protection layers for Li metal anodes: (a) a grafted polymer skin on the Li metal anode (Reproduced from ref. [74] with permission of American Chemical Society); (b) Molecular-layer deposition method turning a organic polyuria thin film for Li metal anode (Reproduced from ref. [88] with permission of Wiley-VCH); (c) The high-polarity β-phase poly(vinylidene difluoride) (β-PVDF) as a promising artificial solid-electrolyte interphase coating on both Cu and Li metal anode (Reproduced from ref. [89] with permission of Wiley-VCH); (d) An artificial protective layer consisting of lithium fluoride (LiF) and graphite fluoride (GF) enables a high-performance Li metal anode (Reproduced from ref. [92] with permission of Springer Nature). The lithium fluoride (LiF) component in the SEI layer has been proved to be pivotal to prevent the dendrite growth in LMBs [93,94]. The Li+ has been verified to have a faster diffusion rate through the LiF than Li2CO3, as the energy barrier of the former is 0.09 eV lower than the latter [93]. Peng et al. proposed a transplantable LiF-rich layer (TLL), which comprised cross-linked nanoscale LiF domains and could prevent the side reactions between Li metal and electrolytes, resulting in a long-term cycling of Li metal anode [72]. Hou et al. presented a LiF17

and Li3N- enriched artificial SEI protective layer on Li anode, which can stabilize the Li metal and enhance the interface compatibility on Li metal anode [95]. As a result, the symmetric Li/LAGP-PEO/Li cells with SEI-protected Li anodes cycled with a small voltage hysteresis at a current density of 0.05 mA cm−2 at 50 °C for nearly 400 h [95]. Lang et al. proposed a simple solution method via the in-situ reaction between metallic Li and polyvinylidene fluoride (PVDF)-dimethyl formamide (DMF) solution to fabricate LiF-coated Li metal anode at a relatively low cost [96]. The produced chemically and mechanically stable artificial SEI film suppressed dendrite formation and reduced side reactions between the metallic Li and the carbonate-based electrolyte, providing a stable cycling over 300 plating/striping cycles at 3 mA cm−2 [96]. In addition, NH4HF2 was used to form a LiF-rich host on Li metal, producing a dendrite-free Li metal anode [97]. As shown in Fig. 7(d), the Li metal has a protective layer consisting of LiF and graphite fluoride (GF) via a reaction between GF and molten Li, which enables long-term stability in ambient air and demonstrates a safe and dendrite-free cycling at current densities from 1 to10 mA cm-2 for a long cycle life in the Li stripping/plating experiments [92]. Beside those ionically conductive layers, electronic conductor layer with homogeneous nucleation sites (Lithiophilic Sites) on rough Li metal or Cu foil can also enable uniform Li deposition without dendrite formation for Li metal anode. Zhang and co-workers demonstrated N-doped graphene matrix with uniform lithiophilic sites which can effectively regulate Li nucleation sizes and sites, resulting in dendrite-free Li anodes [98]. Li et al. adopted thin-film Cu3N to react in-situ with initial deposited metallic lithium on commercial Cu current collector, thus forming homogeneous conductive network to effectively enhance the overall electrochemical performance of Li|Cu and LiFePO4|Cu cells such as cyclic stability and reaction kinetics during Li stripping/plating [99]. The improved uniformity of surface electronic conductivity can be detected by the peak-force tunneling atomic force microscopy, further validating the crucial importance of homogenous surface electronic conductivity for dendrite-free Li metal anodes [99]. Beyond that, other interfacial films can also serve as electron conductor layer (or mixed conductor layer) to escort high-performance Li anodes, which derived from Li-Sn or copper acetate additive even or Lixi0.35La0.52[V]0.13TiO3 (LixLLTO) layer [100–102]. Therefore, these uniformly electron conductor 18

layers with uniform distribution of Li nucleation sites are able to remedy original surface disorganization of Li metal or Cu foil and induce a considerably more homogenous electric field and electron flux. Hence, the subsequent Li+ flux could have equal access to capture electron, realizing uniform nucleation and planar growth of metallic Li, then form a SEI on it (Fig. 5b). In addition, the mixed conducting interphase (MCI) enabled transport both electrons and Li ions could synergistically guide uniform Li stripping/plating and protect Li anode from dendric growth. As detailedly discussed by Cheng et al. [23], the mixed conducting interphase would couple with the formation of SEI outside to achieve dendrite-free Li anodes. However, we shall note that mass transport of lithium ions will be in three dimensions rather than simply one-directional, the interface film on Li anode with single electronic conductivity would be not favorable for the sake of reliability. Given that inevitably electrochemical hot spots and easily accessed electrons, Li deposition pattern would be easily altered from flat deposition to directly random plating, leading to the disastrous dendrite growth. Therefore, ionically conductive layer or mixed conducting framework on the interface between Li anode and electrolyte will gain more attention in the solution of artificial protection layers.

4.3 Lithium host Except for the unrestrained Li dendrites growth, the volume expansion is also one of the most negative features that hinder the application of LMBs. As a bulk electrode, Li metal anode performs a significant volume change during the plating/stripping process, resulting in the fracture of SEI, dendrite formation, consumption of electrolytes and low CE. Through a proper Li host matrix with uniform Li nucleation sites, the obvious volume change of Li metal could be mitigated and the Li dendrites could be regulated evenly during cycling [18,98,103]. As the most commonly used anode current collector, Cu foil has been studied extensively to realize the stable cycling of Li metal [104–107]. Because of the “lithiophobic” property of Cu metal, the Li nucleation will be formed in the plating process [108,109]. Shen et al. proposed an 3D Cu electrode by directly growing porous carbon nanotube (CNT) sponge on Cu foil (called C-host@Cu) [110]. The Cu foil works as the current collector and the CNT sponge works as the host for Li metal anode. This 19

structure has been proved to have stronger bonding and better physical contact with Cu foil and lower interfacial resistance than other host material physically attaching to the substrate [110]. Zhang et al. proposed a simple way to produce vertically aligning CuO nanosheets (VA-CuO NSs) on a Cu foil, where Cu acts as both the current collector and the source of Cu for CuO growth. The vertically aligned lithiophilic CuO nanosheets on a Cu collector can effectively stabilize lithium deposition [111]. Yang et al. developed a 3D Cu current collector with a submicron skeleton and high electroactive surface area for Li metal anodes that exhibits low voltage hysteresis and stable cycling over 600 h with Li dendrite formation (Fig. 8a) [104]. The application of alloy has also been investigated by other researchers‟ group. Lu et al. used Al-Zn oxides (AZO) coating to modulate the surface properties of Cu foam and the prepared Li host has excellent stability at high current densities [112]. Shi et al. used a liquid Ga-induced alloying-dealloying strategy to fabricate a self-supported, 3D porous Cu foil and this porous composition has a large surface area and could effectively homogenize the Li plating and alleviate the growth of Li dendrites [113]. Ye et al. fabricated a binary Li-Al alloy layer through in situ electrochemical generation and successfully guided the uniform metallic Li nucleation and growth [114]. As a simple method, Huang et al. proposed an air annealing way to achieve an evenly distributed ZnO coating on a brass (Cu-Zn alloy) mesh [115]. The mesh also serves as the current collector and source of Zn for ZnO growth. This strategy can dramatically reduce the overpotential of Li nucleation and restrain the formation of Li dendrite [115].

20

Fig. 8. Examples of hosts for Li metal anodes: (a) 3D Cu current collector for Li metal anode (Reproduced from ref. [104] with permission of Springer Nature); (b) Stacked reduced graphene 21

oxide layers for Li metal anode (Reproduced from ref. [116,101] with permission of Springer Nature); (c) Hollow carbon spheres embedded Au nanoparticles for Li metal anode (Reproduced from ref. [109] with permission of Springer Nature); (d) a self-smoothing Li-C anode supported by amine-functionalized 3D mesoporous carbon fibers for Li metal anode (Reproduced from ref. [117] with permission of Springer Nature); (e) 3D garnet-type ion-conductive framework for Li metal anode (Reproduced from ref. [118] with permission of National Academy of Sciences). In the view of practical application, the porous, heavy and thick 3D current collector may decrease the energy densities of LMBs. Except porous Cu hosts, other materials such as carbon (C) hosts, ion conductor frameworks and polymer hosts were also be investigated [116,118–121]. Before constructing composites, many hosts need to be treated to form „lithiophilic‟ sites, such as ZnO, metal nanodots/atoms, and functional groups [117,119]. For example, as shown in Fig. 8(b), molten Li was successfully infiltrated into stacked graphene layers resulting in a graphene-Li metal composite anode that demonstrates low content of „lithiophilic‟ layered reduced graphene oxide, low dimension variation (∼20%) during cycling, good mechanical flexibility and excellent electrochemical performance [116]. Zhang et al. designed a coralloid silver-coated carbon fiber. Due to lithiophilic nature of Ag, the molten Li can be easily infused into the carbon fiber framework forming a composite Li anode (CF/Ag-Li) [122]. Zhou et al. applied commonly used low-cost carbon fiber cloth (CFC) as the 3D Li anode scaffold and deposited Li metal into the inner surface of CFC and produced a stable SEI layer [123]. Shi et al. modified the carbon fibers with a thin lithiophilic LiC6 layer that can regulate the Li+ deposition to achieve promising cycle stability in working batteries [124]. Chen et al. constructed a coaxial-interweaved structure that can perform a high Li affinity to guide uniform Li deposition, provide sufficient pathways for ion and electron transport, and improve stability during Li plating and stripping [125]. Liu et al. developed CoNx-doped graphene matrix to achieve a dendrite-free Li anode because of the strong lithiophilicity provided by CoNC sites [126]. Sun et al. introduced carbon paper (CP) as the interlayer to designed a dendrites-free Li anode and realized a high capacity stable performance LMB [127]. Xie et al. proposed that a novel 3D hollow carbon host coated by a thin-layer through atomic layer deposition effectively guides lithium deposition inside the hollow carbon sphere and 22

simultaneously prevents electrolyte infiltration by sealing pinholes on the shell of the hollow carbon sphere [128]. As results, in an ether-based electrolyte, the 3D hollow carbon host demonstrated stable cycling of 500 cycles with a high CE of 99% (cycling rate of 0.5 mA/cm2 and a cycling capacity of 1 mAh/cm2) [128]. In addition, Yan et al. implanted Au seeds (no nucleation barrier) into hollow carbon host to facilitate Li deposition solely inside the nanocapsules, which successfully eliminate dendrite formation and enable enhanced cycling performance, even in corrosive alkyl carbonate electrolytes (Fig. 8c) [109]. Afterwards, the Au seeds were capsuled by wrinkled graphene cage (WGC) for high-performance and dendrite-free Li metal anodes [129].

Cao et al. introduced 3D printing technique using cellulose nanofiber (CNF) into fabrication of Li anode host for the first time [130]. The special porous structure of CNF host equips the anode properties of better ion accessibility and uniformed local current density and therefore the dendrite formation due to uneven Li plating-stripping is suppressed [130]. Niu et al. proposed an amine-functionalized 3D mesoporous carbon fibers as anode scaffold. The functional groups on the carbon host contributes to a uniform Li deposition and a preferred nucleation of Li in the pores and the cavities, which demonstrate a reversible and „self-smoothing‟ Li deposition during cycles (Fig. 8d) [117]. In order to achieve uniform Li deposition, Xue et al. designed a composite porous core-shell carbon fiber with evenly coated by mixed SiO2 and TiO2 hybrid, as a super lithiophilic host materials for lithium anodes. As a result, the addition of amorphous SiO2 and TiO2 offer controllable nucleation and deposition of metal Li inside the porous core–shell fiber even at ultrahigh current densities of 10 mA cm-2 [131]. As an ionic conductor framework (Fig. 8e), Li uniformly deposited in the pores of the 3D garnet host without growing Li dendrites on the outer surface, resulting in the dendrite-free deposition and continuous rise/fall of Li metal during plating/stripping in the 3D ion-conductive host, and showing stable cycling at 0.5 mA cm-2 for 300 h with a small over-potential [118].

4.4 Binder and separator for Li metal anode Binder designs and separator modifications have been employed as effective strategies to overcome volume change and suppress lithium dendrites in LMBs. A suitable binder can prevent 23

the cracks in Li metal anodes during cycling, resulting in a stable interphase. A thermodynamically stable and robust separator can withstand the thermal runaway caused by an internal short circuit and the mechanical penetration induced by Li dendrites. Recently, Yoo et al. proposed a polyrotaxane-incorporated poly(acrylic acid) (PRPAA) binder for CNT networks that can overcome a large stress (volume change) during repeated lithium uptake-release in the host, thereby enhancing the mechanical integrity of the corresponding electrode without any cracks formation during operation of such Li metal anode (Fig. 9a) [30]. Hu et al. developed that a commercial separator covered by thermally conductive boron-nitride (BN) nanosheets solves the safety issues, demonstrating a high CE over 100 cycles at 0.5 mA cm-2 in conventional organic carbonate-based electrolyte [132]. For guiding uniform deposition of Li crystal seeds in the early plating process, magnesium (Mg) nanoparticles were deposited on single side of separator, resulting in dendrite-free lithium metal anode over 400 cycles [133]. Maeyoshi et al. developed a porous and polar separator that effectively enhance the uptake of the concentrated electrolytes and then induce the formation of the anion-derived SEI layer for high-performance lithium metal anode [134]. Wang et al. prepared HAPs/PVA separator with an outstanding Young's modulus and high mechanical flexibility, which efficiently restrain the growth of Li dendrite and induce a homogeneous stable SEI [135]. Liu et al. designed a sandwiched separator with a middle layer of silica nanoparticles inside. Through a chemical lithiation, these SiO2 nanoparticles can consume dangerous Li dendrites that have already penetrated into the separator, which prevents the further growth of Li dendrites and significantly extend the life of LMBs up to approximately five times (Fig. 9b) [136]. Zhao et al. introduced a functional separator coated with a promising solid-state electrolyte Al-doped Li6.75La3Zr1.75Ta0.25O12 (LLZTO), which acts as an ion redistributor to induce uniform Li ion distribution for the dendrite-free Li deposition [137].

24

Fig. 9. Examples of binder and separator for Li metal anodes: (a) A polyrotaxane-incorporated poly(acrylic acid) (PRPAA) binder for mechanically stable Li hosts in LMBs (Reproduced from ref. [30] with permission of Wiley-VCH); (b) Silica Nanoparticle Sandwiched Separator for stable cycling of LMBs (Reproduced from ref. [136] with permission of Wiley-VCH).

4.5 Solid-state LMBs The optimization of liquid electrolytes by adjusting the composition can increase the ionic conductivity, induce the passivation film in-situ formed on electrodes and greatly improve the cycling stability of the Li metal anode [138–141]. However, as the descriptions above, the liquid electrolytes utilizing lots of organic solvents which are flammable cause tremendous safety risks in LMBs. Due to the liquid status of the electrolytes, Li dendrites are able to grow freely in the electrolytes when they break through the SEI layer and finally cause short-circuit. According to the modeling proposed by Monroe and Newman, when the shear modulus of the electrolyte is about twice that of the Li metal anode, the dendrite can be suppressed [142]. Therefore, the solid-state electrolytes (SSEs) may have a blocking impact on the growth of Li dendrites and prevent it from penetrating the separator. However, in recent studies, Li dendrites also formed in SSEs causing the degradation issues [143]. Therefore, the modifications of SSEs are necessary to build high-performance solid-state LMBs. Typically, the SSEs are mainly divided into three groups: solid polymer electrolytes (SPEs), gel polymer electrolytes (GPEs), and inorganic solid electrolytes (SEs). 25

Generally speaking, the SPEs are the electrolytes containing or consisting of macromolecular structure. Therefore, the SPEs can further represent the systems of organic polymer electrolytes and organic-inorganic composite electrolytes. The commonly used organic polymer electrolytes include polyethylene oxide (PEO), polyphenylene oxide (PPO), poly methoxyethoxyethoxy phosphazene (PEEEP), polyacrylonitrile (PAN) etc. Wright et al. [144] first reported the phenomenon of polymer conductivity (PEO) in 1973 and afterwards Armand et al. [145] proposed that the combination of polymer and salts based on PEO can act as the electrolyte and be applied in batteries. PEO-based SPEs have the characteristics of high shear modulus and chemical stability with Li metal anode, and therefore they have been thoroughly investigated in the past years [146– 150]. The ionic conductivity of SPEs is normally low compared with the liquid electrolytes, and increasing the temperature could improve their conductivity. However, the mechanical strength of PEO decreases under high temperature and the constriction of the growth of Li dendrites is unstable [9]. The composite polymer electrolytes are possible solutions to compensate this problem [151–153]. Recently, Wan et al. developed an ultrathin (8.6μm) and lightweight solid polymer electrolyte that is a nanoporous polyimide (PI) film filled with polyethylene oxide/lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI). The produced solid polymer composite (PI/PEO/LiTFSI) electrolyte enabled long-term cycling (over 1000h) of Li metal anode and high safety operation at an elevated temperature (Fig. 10a) [154]. Ma et al. constructed a composite polymer electrolyte CPL (Cellulose acetate/Polyethylene glycol/Li1.4Al0.4Ti1.6P3O12) with a viscoelastic and nonflammable interface to tackle the contact issue and prevent the Li dendrite formation [155]. 2D materials, such as MXene, have high specific surface areas compared to 0D or 1D materials, and therefore are considered to be promising candidates for composite electrolytes. Pan et al. uniformly dispersed small amounts of Ti3C2Tx into a poly (ethylene oxide)/LiTFSI complex (PEO20-LiTFSI) to manufacture the MXene-based nanocomposite polymer electrolytes (CPEs) that enhanced the electrochemical performance of LMBs [156]. A heterogeneous multilayered solid electrolyte (HMSE) is designed by Duan et al. and the interfacial instability problems could be settled by different electrolyte component/electrode interfaces, demonstrating a small voltage hysteresis and dendrite-free Li metal anode upon cycling [157]. 26

Furthermore, they also fabricated an in-situ plasticized SPE with double conducting polymer network (DN-SPE) through facile polymerization of two kinds of liquid polymer monomers with proper chain length [158]. This DN-SPE shows enhanced ion conductivity, high mechanically flexibility, wide electrochemical window (4.7 V vs. Li+/Li), high thermal stability (stable up to 200 °C) and good ability to suppress Li dendrites. In addition, the hybrid electrolytes have also been proposed to compensate the shortcomings of the organic and inorganic electrolytes. Bruce et al. designed a 3D bicontinuous structured hybrid electrolytes with the ionic conductive polymer layer in one side and the ceramic solid electrolyte Li1.4Al0.4Ge1.6(PO4)3 (LAGP) in the other side [159]. This method has increased the mechanical properties of the electrolyte without significantly compromising its ionic conductivity. Similar method has also been reported in the previous report [160]. They proposed an asymmetric solid electrolyte (ASE) with a dense Li7La3Zr2O12 (LLZO) layer modified with 7.5 nm polymer electrolyte on the Li metal side and a soft layer of polymer electrolyte on the other side. The LMBs utilizing ASE deliver high capacity retention reaching 94.5% for more than 200 cycles with an extra-high CE exceeding 99.8% per cycle [160].

Fig. 10. Recent advances concerning high-performance LMBs: (a) an ultrathin and flexible solid polymer composite PI/PEO/LiTFSI for stable cycling of Li Metal anode (Reproduced from ref. [154] with permission of Springer Nature), (b) engineering the garnet solid electrolyte/Li metal interface by constructing an intermediary Li-metal alloy (Reproduced from ref. [161] with permission of American Association for the Advancement of Science). Another method of compensating the drawbacks of SPEs is combing them with the high ionic 27

conductivity property of liquid electrolytes. After absorbing the organic solvents, the polymer electrolytes will convert into GPEs. GPEs exhibit a state of jelly and compose of polymer and liquid electrolytes [162]. With the existence of liquid electrolytes, the ion conductivity of GPEs is high and the cycling performance of batteries is guaranteed [163]. The polymer materials mainly include PEO, PPO, PAN, polyethylene glycol (PEG), polymethyl methacrylate (PMMA), and polyvinyl chloride (PVC), polyvinylidene Fluoride (PVDF) [164–169]. The researches of GPEs have been carried out decades before. Matsui et al. investigated the combination of PEO-PPO and LiClO4 in EC/PC in 1995 [170]. Tatsuma et al. improved the GPE of PAN and LiClO4 in EC/PC in 1999 [171]. Choi and Lee et al. studied GPE with ionic liquid [172]. However, because of the special structure of GPEs, their mechanical property is insufficient to block the growth of Li dendrites and the liquid electrolytes components within it also have the deficiency of thermal stability. SEs are known as inorganic solid-state Li ion conductors, which are excellent materials to prevent Li dendrites from penetrating the separator. Up to now, different kinds of Li compounds have been investigated thoroughly [173–178]. The slower Li ion diffusion rate has been found in the SEs comparing with the liquid electrolytes and the contact between SEs and the Li metal anode is also poor because of the rigidity of the SEs. Furthermore, the critical current density of SEs is much lower than that of the liquid electrolytes, which hindered its application in large capacity battery [179–181]. The interfaces between Li anode and SEs can be divided into three types [182]: thermodynamically stable interfaces, thermodynamically unstable interface with a mixed ionic-electronic conducting interphase, and thermodynamically unstable interfaces with a kinetically stabilized SEI film. Wang et al. introduced an LiF-rich SEI layer between Li3PS4-SEs and the Li metal anode, suppressing the growth of Li dendrites [183]. For uniform thin-film fabrication, atomic layer deposition (ALD) and molecular layer deposition (MLD) are more suitable due to their ability to control the film thickness at atomic level [82,184] and the low environmental temperature of 28~250 °C [185–187]. Song and Sun et al. proposed to utilize the ALD coating on the Li1.3Al0.3Ti1.7 (PO4)3 (LATP) solid electrolytes surface to reduce the side reactions and stabilize the LATP/Li interface [188]. Dasgupta et al. realized the ALD interlayer on 28

Li3BO3-Li2CO3(LBCO) SE, and the film growth exhibits self-limiting and linear over a range of deposition temperatures [189]. The fabricated ALD LBCO electrolyte can be used in Li metal thin-film batteries and exhibit a high ionic conductivity and excellent cycling stability. Sun et al. solved the problem of interfacial instability between Li metal and SEs by developing an inorganic-organic hybrid interlayer through MLD [190]. The side reactions and Li dendrites at the interfacial between Li and SEs are effectively suppressed. Among the inorganic SSEs including oxides, sulfides and nitrides, the garnet type SSEs (e.g. Li7La3Zr2O12) have been viewed as very promising ceramic electrolyte candidates due to their relative higher ionic conductivity from 10-3 to 10-4 S cm-1. In order to obtain a good contact between ceramic solid and Li metal, garnet solid was coated by Al layer to improve the contact (good wettability) with Li via the Li-Al alloy formation at the interface, which additionally stabilized the interfacial resistance upon constant Li plating/stripping process (Fig. 10b) [161]. Recently, ceramic/polymer composite electrolytes have been viewed as a very promising system for high-performance LMBs due to the combined advantages of the ceramic and polymer, performing improved mechanical properties and contact interface wetting [191].

4.6 Self-healing for Li metal anode Various approaches have been used in previous sections and achieved improved performance during Li plating/stripping cycling process. However, the preceding methods usually depend on special measures or the mechanical blocking to suppress the formation or growth of Li dendrites. The growth of Li dendrites is merely slowed down and it will continue if the passive protective measures fails. Another train of thought has been proposed to eliminate the Li dendrites by curing or healing the Li dendrites initiatively. Ding et al. reported a Li dendrites-free deposition via a self-healing electrostatic mechanism, which employed electrolyte additive containing cations (e.g. cesium or rubidium ions) [192]. The additive cations around the initial growth tip of the protuberances repel the incoming Li+ and disrupt the conventional dendrite growth process via a positively charged electrostatic shield, result in a uniform Li deposition (Fig. 11a) [192]. As the same working mechanism of a self-healing electrostatic shield, Cs+ was added into the PEO-based 29

solid polymer electrolytes for Li metal anode, inducing a dendrite-free Li deposition [193]. In the past years, it is generally accepted that the higher current density will accelerate the formation and growth of Li dendrites. But recent reports proved high current caused self-heating of the battery, which led to self-healing of Li dendrites (Fig. 11b) [194,195]. When the plating/stripping current density is raised to more than ~9 mA cm-2, the self-heating of the battery will be launched and the extensive migration of Li anode surface will be triggered. The surface diffusion heals the dendrites and smoothens the Li metal surface. They found that self-heating strategy enables Li metal batteries with high CE, longer cycling life and higher safety [194,195].

Fig. 11. Schematic diagram of the self-healing mechanism during the Li deposition: (a) electrolyte additive containing multivalent metal ions enable cations adsorption on initial Li protuberant tip, preventing the further growth of Li dendrites and smoothing the Li deposition (Reproduced from ref. [192] with permission of American Chemical Society); (b) Self-heating treatment coming from high working current density enables the self-healing of Li dendrites in LMBs (Reproduced from ref. [194] with permission of American Association for the Advancement of Science). 5

Conclusion and perspective 30

In this review, we have discussed the motivation, history, challenging issues and categorized strategies, as well as recent advances of Li metal anodes. In order to guide the Li metal protections for enhancing the electrochemical performance of LMBs, we have outlined and classified the effective strategies for optimizations of Li metal anodes in details, as summarized in Fig.5. Although tremendously valuable progresses have been made in coping with the Li dendrites growth, low CE, volume change, poor cycle stability, poor plasticity and safety issues, there is still a long distance away from the actual commercial application of LMBs. Despite these challenges, scientists in battery field think a massive scale-up of the Li metal anode is imminent. For achieving a new level of energy density (500 Wh kg-1), Li metal anode has been viewed as one of the most important candidates for next-generation Li metal batteries. Therefore, the future strategies are necessary to realize the long-term stable cycling and dendrite–free Li metal anodes with high safety. Since the reaction interface are rather important, the protection SEI with high stability is required. The in-situ formation of protection SEI on Li metal via electrolyte modifications (from liquid to solid) are immediately effective, low cost, and uniform. The developments of novel electrolyte components including additives, Li salts and solvents should move forward, inducing formation of dense, thin (10~50 nm), stable, homogeneous and ion conductive surface protections on Li metal anode during initial cycles. We should be soberly aware of the chemical activity difference between graphite and lithium metal, while the finite amount of electrolyte additive will be consumed very soon during the SEI formation in LMBs. However, the unstable reaction interface undoubtedly limits effect of such electrolyte additive as the cycling proceeds. Owing to the large volume change and further pulverization upon plating/stripping process, a suitable Li host or modified collector may be employed to eliminate the side effects of volume change. But the excessive specific surface of 3D structured host or current collector would lead to more parasitic side reactions and more electrolyte consumption, resulting in the dry up and a sudden failure of cell during operation. Consequently, we should balance the high porosity in hosts or substrate, otherwise it would also reduce volumetric energy density of Li metal anode. For overcoming the Li dendrite growth and unstable interface (caused by volume change), we should employ electrolyte modification and Li host design as a whole. Considering the future applications of Li metal anode, Li metal anodes based on Li powder instead 31

of metal foil should be developed to change the situation of Li metal anode (metallic belt) and offer high performance and high safety. Among electrolyte modifications, more research efforts should be paid on solid-state electrolytes to achieve high-performance solid-state LMBs. However, many technical problems of solid-state electrolyte have yet to be resolved before the large-scale commercial application. The Li ion conductivity of SSE at room temperature is still much lower than that of organic liquid electrolytes, which limits the rate capability of the working batteries. Additionally, the electronic conductivity in bulk phase of SEs is also the key issue in the practical application of solid-state LMBs, which cause the growth of lithium filaments in the grain boundary of SSEs. Therefore, new type of SSEs with higher Li ion conductivity and rather low electron conductivity (insulator) need to be developed. Unfortunately, reductive reactions (e.g. sulfide converted to Li2S and LixPy, reduction of doping ions in SEs) driven by Li metal seem to be inevitable due to the lowest redox potential of Li, altering the interfacial chemistry and exacerbating the interface impedance. Besides side chemical reactions, the physical contact of Li-metal/solid electrolyte interface should be a priority issue to be resolved, while organic polymer wetting materials acting as buffer layer have been proved to be an effective way. Hence, thin, dense and highly conductive solid electrolytes composed of polymer-ceramic composites should be suitable candidates. Other strategies like artificial interface engineering to enhance interfacial contact/stability, are also important to achieve long-term cycle stability of solid-state LMBs without Li dendritic problem and solid-solid interface issues. Slightly gratifying, in the process of moving towards actual commercial application, LMBs have entered a new stage as the 300 Wh kg-1 pouch cells emerged. Li metal pouch cells, as an excellent model system for future applications, deserve more attention to be deeply investigated so as to provide more insightful and easily overlooked issues. When the researches focus on pouch cells instead of coin cells, the four issues will be the main challenges in the future: i) reducing anode redundancy of Li metal to increase the Li utilization at utmost; ii) regulating the interfacial chemistry and the reasonable amount of electrolyte without drying up prematurely; iii) the challenge of cell swelling; iv) harsh impact on ultrathin lithium metal under high current density. We believe that the breakthrough of lithium metal commercialization in the future will be driven 32

by electrolyte optimizations and designs of Li hosts. Overall, of all the alkali metal negative electrodes, Li metal is the most likely to take the lead in commercial applications. We shall note that we should consider the LMBs as a whole. Both cathode chemistry and Li metal anode should be considered when modifying the battery components. Despite the difficulties, we are still looking forward to it.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments This work was financially supported by the Innovation-Driven Project of Central South University (No.2019CX033), the National Natural Science Foundation of China (No: 51904344), the Natural Science Foundation of Guangdong Province (2019A1515012111), the Science & Technology Innovation Commission of Shenzhen (Grant No. 20180123) and the Shenzhen Science and Technology Program (KQTD20180412181422399). References [1] F. Wu, G. Yushin, Energy Environ. Sci. 10 (2017) 435-459. [2] F. Wu, O. Borodin, G. Yushin, MRS Energy & Sustainability 4 (2017) E9. [3] D. Lin, Y. Liu, Y. Cui, Nat. Nanotechnol. 12 (2017) 194. [4] M. Balaish, E. Peled, D. Golodnitsky, Y. Ein-Eli, Angew. Chem. Int. Ed. 54 (2015) 436-440. [5] Y.-X. Yin, S. Xin, Y.-G. Guo, L.-J. Wan, Angew. Chem. Int. Ed. 52 (2013) 13186-13200. [6] N. Wu, Z.-Z. Yang, H.-R. Yao, Y.-X. Yin, L. Gu, Y.-G. Guo, Angew. Chem. Int. Ed. 54 (2015) 5757-5761. [7] Y. Hu, T. Zhang, F. Cheng, Q. Zhao, X. Han, J. Chen, Angew. Chem. Int. Ed. 54 (2015) 4338-4343. [8] J.M. Tarascon, M. Armand, Nature 414 (2001) 359-367. [9] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.-G. Zhang, Energy Environ. Sci. 7 (2014) 513-537. [10] J.-G. Zhang, W. Xu, W.A. Henderson, Lithium Metal Batteries and Rechargeable Lithium Metal Batteries, Springer Nature, Switzerland, 2017. [11] F. Wu, H. Lv, S. Chen, S. Lorger, V. Srot, M. Oschatz, P.A. van Aken, X. Wu, J. Maier, Y. Yu, 33

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Graphical abstract

Lithium metal anode boosts both volumetric and gravimetric energy densities of next-generation lithium metal batteries. In this review article, the remaining challenges and future solutions of Li metal anodes are discussed from various aspects.

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