The recent research status quo and the prospect of electrolytes for lithium sulfur batteries

Chemical Engineering Journal 369 (2019) 874–897

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Review

The recent research status quo and the prospect of electrolytes for lithium sulfur batteries ⁎

Fan Lanlana,b, Deng Nanpinga,c, Yan Jinga,c, Li Zhenhuana,b, , Kang Weimina,c, Cheng Bowena,b,

T ⁎

a

State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes, Tianjin Polytechnic University, Tianjin 300387, China b School of Materials, Science and Engineering, Tianjin Polytechnic University, 300387 Tianjin, China c School of Textiles, Tianjin Polytechnic University, Tianjin 300387, PR China

H I GH L IG H T S

ions transfer mechanism and electrode–electrolyte interfaces architecture. • Discuss the design principles of modifying liquid and solid electrolyte. • Review current trends in materials selection for LiSBs electrolyte. • Discuss • Give a outlook on the future research and development on reliable electrolyte.

A R T I C LE I N FO

A B S T R A C T

Keywords: Lithium sulfur batteries Ion transfer Liquid electrolytes Solid electrolytes

Soaring demand for efficient and economic electric energy storage system has intensively promoted the development of rechargeable batteries. Lithium sulfur battery may be one of the most promising candidates in the frontier of modern electrochemistry owing to its high theoretical specific capacity (1672 mAh g−1), high energy density (2600 Wh kg−1), low cost, and environmentally friendliness. However, the deactivation of active substances and polarization of electrodes caused by “shuttle effect” hampered commercial applications of lithium sulfur batteries. The explosiveness of research on lithium-sulfur batteries in recent years acquired outstanding breakthroughs and achievements in solving the aforementioned problems. Particularly, standing the perspective of electrolyte, as a transmission medium of all ions in the redox reaction, it is important to inhibit the dissolution and diffusion of polysulfide. Here, a comprehensive overview of the recent advance of electrolyte including liquid electrolyte and a solid electrolyte in lithium sulfur battery is presented. For the liquid electrolyte, it mainly concentrates on modifying electrolyte to improve the interfacial architectures and properties, including the changing of solvent, selecting of salts or additives and matching anion and cation of the ionic liquid. In order to ameliorate the high interface impedance and poor ion conductivity under an atmospheric temperature of the solid electrolyte, various methods were proposed, which contains building novel structures of the gel electrolyte, introducing inorganic particles into the polymer, modifying the inorganic solid electrolyte and constructing composite electrolyte. Finally, some perspectives on the future research and development of electrolyte in lithium sulfur battery are provided.

1. Introduction The explosive growth of electric vehicles and portable electronic devices has intensively promoted a strong demand for high energy storage systems. Amid existing large spectrum energy storage devices, lithium ion batteries (LiBs), basing on lithium metal oxides (e.g., LiMn2O4 [1], LiCoO2 [2,3]and LiCoxNiyMnzO2 [4]) or lithium

phosphates (e.g., Li3V2 (PO4)3 [5] and LiFePO4 [6]) as the cathode and carbon as the anode, have dominated as key-enabling technologies in the market. However, the existing practical LiBs fall far below the constantly updated requirements of the electronic devices with a highenergy density, and beyond the theoretical value of LiBs. Hence, it is necessary to break through the ceiling of energy density to satisfy the severe target of modern markets. Lithium sulfur batteries (LiSBs), with

⁎ Corresponding authors at: State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes, Tianjin Polytechnic University, Tianjin 300387, China. E-mail addresses: [email protected] (Z. Li), [email protected] (B. Cheng).

https://doi.org/10.1016/j.cej.2019.03.145 Received 19 November 2018; Received in revised form 24 February 2019; Accepted 16 March 2019 Available online 18 March 2019 1385-8947/ © 2019 Published by Elsevier B.V.

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Fig. 1. (a) Schematic diagram of LiSBs with 1 M LiTFSI in the mixture of DOL and DME (1:1 v/v) during charge/discharge; (b) A typical charge–discharge profile in a traditional liquid Li-S cell.

the theoretical capacity of l672 mAh g−1, have a six-fold higher specific energy density than that of conventional lithium-ion batteries due to the multielectron conversion electrochemistry between elemental sulfur and lithium (Fig. 1a) [7–9]. During the discharge of cell, high voltage plateau (≥2.4 V) and low voltage plateau (≥2.0 V) are corresponding to the reduction from S8 to the long chain lithium polysulfides (Li2Sx, 4 ≤ x) and reduction from short chain lithium polysulfides (Li2Sx, x ≤ 4) to the Li2S, respectively. In the charge curves, only one long plateau region at approximately 2.3 V is apparent, followed by an obvious potential rise at the end of charging, which is coherent with the oxidation from Li2S to S8 (Fig. 1b) [10]. Additionally, sulfur is abundant, cheap, nontoxic and environmental benignancy in nature, promoting LiSBs to become one of the most promising rechargeable batteries currently being developed [11]. However, the commercial application of LiSBs, in light of the above beneficial features, is still hindered by several intrinsic problems resulting from the sulfur and complex chemistry reaction of LiSBs: 1) insoluble and electronically insulating of elemental sulfur and discharge product (Li2S2 and Li2S); 2) formation of lithium dendrites during the cycle process; 3) the 80% volume expansion/contraction of active materials during discharge/ charge process; 4) The loss of active substances and consumption of electrolytes due to the dissolution of polysulfide intermediates. It was noteworthy that the problems of LiSBs, such as shorting cycle, low coulombic efficiency, poor safety, and high self-discharge rate, are attributed to the dissolution, diffusion, and side reactions of soluble lithium polysulfides in the electrolytes. Therefore, to address abovementioned dilemmas, various strategies have been used to anchor lithium polysulfides inside or around the cathode: 1) physical interception by nanostructured host materials, including porous carbons nanofibers [12,13], nanotubes [14], spheres [15] and metal organic framework (MOF) [16]), grapheme [17] and conductive polymers [18], can provide conductive paths and vessel for redox reactions. The polysulfide intermediates are constrained within the framework and effectively restrained their dissolution; 2) the chemical adsorption with heteroatom doping materials, containing nitrides [19,20], oxides [21,22], carbides [23,24] and sulfides [25], is applied to inhibit polysulfide intermediates migration and protect them from shuttling to the anode and reacting with lithium metal; 3) replacing the sulfur element with a cathode material with non-dissolving mechanisms, such as sulfur-poly(acrylonitrile) (SPAN) [26] and MoS3 [27], to hinder polysulfides from diffusing to the anode. Nevertheless, in consideration of the reaction mechanism of LiSBs, complete confinement is difficult to achieve because the conventional liquid electrolyte is inevitably contacted with active materials in the cycling, resulting in the dissolution of polysulfides and considerable increase in viscosity of electrolyte,

while the polysulfides chemical disproportionation produces Li2S2 or Li2S precipitates, leading to reduction of porosity in both the cathode and separator. Electrolyte, which not only directly links the cathode and anode but also is the medium for lithium ion transport in the redox reaction, is an important part of the battery, affecting and controlling interface reaction and safety of cell. In the past few years, two basic types of electrolyte systems have been studied. One system is the conventional organic liquid electrolytes. Because of the reaction of nucleophilic addition and substitution between reduced sulfur species and carbonate esters, conventional carbonate solvent in lithium ion battery usually is not suitable for LiSBs [28]. Linear and cyclic ethers, such as dimethyl ether (DME) and 1,3-dioxolane (DOL), are suitable solvents for LiSBs. However, the deterioration of battery performance caused by the high solubility of polysulfides in the solvent also followed. From the perspective of electrolytes, including the change of solvent and selection of salts or additives, several functionalized design principles aiming to improve battery cycle stability have been presented: 1) facilitating the formation of surface protective layer on the sulfur cathode to control the dissolution of polysulfides; 2) controlling the migration of lithium ions and inhibiting the diffusion of polysulfides via regulating electrolyte composition; 3) creating a protective layer on the anode surface to suppress the side reactions between polysulfides and lithium metal. For example, LiI [29] and LiNO3 [30] added into the electrolyte can form the SEI film on the cathode and anode surface, respectively. Another system is a solid-state electrolyte. Unlike liquid electrolyte, the design principle of solid state for the battery mainly focuses on ameliorating the high interface impedance and poor ion conductivity under atmospheric temperature. For instance, the additions of anion receptors with Lewis acidity as a plasticizer into the polymer electrolytes are expected to interact with Lewis basic anions, which can achieve the high transference number for lithium ions in the polymer electrolytes. Additionally, benefit from the synergistic effect between the liquid and solid electrolyte, the poor interface contact of super ion conductors is improved, simultaneously, the serious shuttle effect of liquid electrolyte also is effectively restrained. Explosive research on LiSBs in recent years has promoted the vigorous development of electrolytes. Herein, we focus on the comprehensive review of electrolyte recent research. Firstly, we highlight the ions transfer mechanism and electrode–electrolyte interface architecture. Secondly, the synergy effect between and among the solvents, lithium salts and additives in the liquid electrolyte are summarized. Thirdly, the modification of polymer electrolyte, inorganic electrolyte and the design principles of composite electrolytes are discussed (Fig. 2). Finally, a conclusion on performances of different electrolytes 875

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Fig. 2. (a) Electrolyte composition and design purpose. (b) Illustration of the mechanism of LiSBs on different electrolytes (The black and red arrows indicate the direction of movement of lithium ions and PS during discharge, respectively, and the length of the arrow indicates the speed of movement). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

result in sulfur (Li2S) not only provides the excellent specific capacity but also paves the way to LiSBs operated successfully at room temperature. However, the dissolution and diffusion of polysulfides intermediates across the separator and react with electrodes (both sulfur cathode and lithium anode), which engenders several technical obstacles. The mainstream of LiSBs research is dedicated to addressing the sulfur and polysulfides intermediates by infiltrating molten sulfur into porous conductive carbon materials in the past few years. Whereas the interfacial behavior between electrodes and electrolyte is essential to

and a prospection on the future research and the development on reliable electrolyte systems of commercially viable LiBSs are gave. 2. Lithium ion transfer mechanism and electrode–electrolyte interfaces 2.1. Lithium ion transfer mechanism The multi-step electrochemical reduction of elemental sulfur (S8) to 876

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diffusion of lithium ions. In the system of the polymer electrolyte, lithium ion transport is composed of three principal mechanisms, containing interchain hopping, intrachain hopping, and codiffusion with short polymer chains (Fig. 3c). Generally, interchain hopping is a rare event because it occurs on the 100 ns timescale and requires a wellconnected network of viable solvation sites, and the short polymer chains transmission exists in most polymers, hence, most polymer electrolytes possess low ionic conductivity at room temperature due to slow molecular chain kinetics [37]. Webb et al. [38] simulated the lithium ion transmission in the polyester (PEO)-based polymer electrolytes and founded a transition along an edge in the solvation-site networks, in which PEO segments motion together with hopping of intersegmental lithium ion from one PEO segment to another jointly promoted the movement of lithium ions. They also pointed out that lithium intersegmental hops were central to implement lithium ion transport because the lithium diffusion coefficient was equal to that of PEO chains without them. In the field of inorganic solid electrolytes, which are mostly based on super-ionic conductors, lithium ion diffusion occurs through concerted migrations of multiple ions with low energy barriers rather than through isolated ion hopping in typical solids [39–41]. The crystal structural framework determined the energy landscape of the ion migration, for example, face-centered cubic had a higher lithium ion migration barrier than body-center cubic, and an energy barrier of ionic diffusion was determined by highest energy of the energy landscape along diffusion path [42]. He and his group [43] clearly explained the reason for the exceptionally higher ionic conductivity of several super-ionic conductors than typical solids by using ab initio modelling. During the migration of multiple ions, the energy is released when migrating downhill of the ions at the high energy sites can cancel out a part of the energy barrier from the other uphill-

suppress polysulfides intermediates diffusion, but it is relatively less emphasized. The physical and chemical structure of the interphase is dependent on the ion transport changes, which occur during battery cycling and concomitant changes. Therefore, it is great scientific interests to understand transfer mechanism of ions in the electrolytes before analyzing the interface behavior. In the organic liquid electrolytes, due to the strong Lewis acidity of the ether bond, lithium ions diffuse in a solvated form, that is, the electrolyte is centered in a lithium ion and small ether molecules is a shell layer. Generally, the coordination number of lithium ion with –C–O–C– in the first solvation shell is 4 [31,32], and the type of solvent determines the amount of lithium-coordinated ether [33,34]. When a single molecule provides insufficient oxygen atoms to insufficiently meet the requirements of its lithium ion tetradentate, bimolecular or trimolecular of ethers are required. Fu and his coworkers [35] analyzed the effect of a series of ethers (dimethoxyethane (G1), diglyme (G2), triglyme (G3), tetraglyme (G4), and 15-crown-5) on the solvation and transmission of lithium ions by AIMD (ab initio molecular dynamics) simulations. Low ion conductivity and self-diffusion coefficient are unambiguously attributed to the high molecular weight of solvents. In addition, the high binding energy between lithium ion and coordinating ethers also leaded to the high sulfur lithiation overpotential and the low concentration of lithiated sulfur (Fig. 3a). When the two solvents were blended, the original balance was broken and rebalanced. For example, adding DME to the solution quickly changed the DOL coordination (a coordination number of 4), and the coordination of DME and DOL equalize (about 1.8) while the ratio of DME/DOL achieved 20/80 [36] (Fig. 3b). It was noted that the tight counterion binds of the additive contained in the organic electrolyte also acted on the lithium ion, such as LiNO3, and rebuilded the structure of the shell layer and changed the

Fig. 3. (a) Binding energy of the solvated Li+ in 0.5 M LiTFSI ether electrolytes [35]. (b) Simulation snapshot of molecules surrounding Li+ (gray spheres) in their first solvation shell in system Iva [36]. (c) Scheme illustrating the three cation transport mechanisms in PEO-salt electrolytes [37]. (d) Li+ diffusion in super-ionic conductors. Crystal structures of LGPS, LLZO and LATP marked with Li sites, the probability density of Li+ spatial occupancy during AIMD simulations, The zoom-in subsets show the elongation feature of probability density along the migration channel (Li: green; O/S: yellow), Van Hove correlation functions of Li+ dynamics on distinctive Li+, diffusion model for concerted migration [43]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 877

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electrolytes, including LiFSI/DME, LiTFSI/DME, LiDFOB/DME, LiBOB/ DME, LiBF4/DME, LiPF6/DME, LiClO4/DME, and LiTFSI/DME/DOL electrolyte. Although the stability of LiDFOB and LiBOB was better than that of other DME electrolytes, reacting with the dissolved polysulfides to deteriorate the stability of the SEI layer. Hence, none of them can not satisfy LiSBs requirements, which contain that SEI films are strong enough to prevent the interaction of lithium with sulfur and polysulfides, and that the salts are stable in the electrolyte containing dissolved sulfure and polysulfides species. Besides the issues of interfacial between the liquid electrolytes with lithium metal anodes, LiSBs still confront various interfacial challenges between cathode surface and electrolyte. Apparently, to understand the reactions between electrolytes and cathodes, including the precipitation/dissolution and the kinetic processes of the sulfides, is essential for electrolytes design. The polysulfide dissolved in the electrolyte loosed the contact points of sulfur particles with the carbon matrix and impeded further solid-state reduction of sulfur. Therefore, there may be another proper contact remain in sulfur particles and carbon host. Peter and his group [50] investigated the reaction processes of two different positive electrode structures and elucidated specific LiSBs mechanistic concepts. Dissolved molecular S8 diffused to reduction site at the current collector, and then reduced to a highly soluble polysulfide (Fig. 4c). As the consumption of dissolved sulfur species, the powerful driving force promoted the fast replenishment of sulfur species in solution to enable the electrochemical reactions at the current collector. Chemical bonding between the metal compounds with oxygen group element also acts as a pivotal part in confining the diffusion of lithium polysulfide intermediates and facilitating fast electrochemical conversion on the electrode surface at the cathode/electrolyte interface. Metal phosphide and chalcogenide materials, including Ni2P [51], FeP [52], MoP [53], CoS and CoSe2 [54] nanoparticles, rely on a surface oxidation layer to achieve the chemical bonding with polysulfides. Highly electronegative O created high-valence metal (M) sites, which can form MeS bonding by reacting with short chain polysulfides and then strongly adsorb polysulfides [55]. The polysulfide shuttle produced by high-loading sulfur in liquid electrolyte systems is fatal for the high electrochemical performance. The solid-state electrolytes have been recognized as a fundamental approach, but the formation of a highresistance space charge layer greatly reduces the lithium ion migration kinetics on the interface and thus increases the interfacial resistance [56]. To improve the ionic conductivity at the interface between the sulfur cathode and the solid electrolyte, Nagata et al. [57] investigated a positive composite electrode, which was prepared by using P2S5 and a solid electrolyte (Li1.5PS3.3 and LiI), exhibiting relatively low activation energy and high ionic conductivity. The used Li3PS4 with excellent ionic conductivity, which was transformed by reaction between the Li2S with P2S5 on the cathode surface in situ during discharge, could remarkably contact with the active material. From the anode interfacial modification point of view, Eshetu et al. [58] demonstrated lithium azide as an electrolyte additive for all-solid-state LiSBs, and a thin, compact and highly conductive passivation layer was formed on the lithium anode. The 8e- reduction of LiN3 leaded to conductive Li3N as the only product, so the SEI layer obtained in the presence LiN3 additive was richer in Li3N in the short time, and exhibited lower overpotential and higher surface coverage. Summary: The transmission of lithium ions in a single medium is clear, but the impact of the complex ion environment in the actual operation of the battery on ion transport is elusive. Understanding the transport mechanism of lithium ions in different electrolyte systems is of incredible importance for the design of electrolytes. For the LiSBs, the SEI layer on cathode and anode, containing composition, structure, formation mechanism and property, is a pivotal role in achieving the excellent electrochemical performance. In addition, the formation kinetics of SEI and lithium ion transport process in the SEI layer needs to be investigated. Especially, for the all-solid-state LiSBs, how to build a favorable ionic interface between the electrodes and the solid

climbing ions, so concerted migration of multiple ions possess a significantly lower energy barrier than the typical crystal structural framework (Fig. 3d). Hence, concerted ion migration with lower barriers can be activated by inserting mobile ions into high-energy sites. Although the transport mechanisms of independent lithium ions in the electrolyte have been reported, the complex ion transport mechanism caused by different lithium ion environments still needs further exploration and analysis. 2.2. Electrode-electrolyte solid electrolyte interphase behavior Although lithium ion transmission process in LiSBs is similar to the traditional lithium ion batteries (LiBs), the redox intermediates, electrolyte composition, and additives are different, thus the interface theory of the traditional lithium ion battery cannot be applied to the LiSBs. Introducing soluble polysulfides into the system of battery promoted the interface reaction process of nonconductive nature of sulfur and Li2S, on the other side, the electrode–electrolyte interfacial structure is changed dramatically due to enormous volume change and repeated dissolution and deposition of sulfur, followed by capacity fading, low efficiency, and self-discharge. Thus, interface properties play a vital role in the cycling performance of LiSBs. Factors of the electrode–electrolyte interfacial properties embrace electrochemical and chemical reactions on the interface, formation mechanism of interfacial layers, and composition of the interface, thermodynamic and kinetic behaviors, and ionic transport across the interface. Grasping aforementioned elements is uppermost for the design of strategies to improve the performance of LiSBs. Solid electrolyte interphase (SEI) on the lithium surface, which resulted from the spontaneous reaction between lithium metal and electrolytes, always continuously regenerates and strips during repeated cycling. Aurbach group firstly researched the formation of SEI film by a redox reaction between the common electrolyte (DOL/LiTFSI/LiNO3) and lithium metal in LiSBs via Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) [44]. LiNO3 as a strong oxidative agent was reduced to form various insoluble LixNOy species on the lithium anode surface. Actually, it is more complicated to form stable SEI film on the lithium electrode cycling in the electrolyte solution with mixed lithium salts. At the beginning of the cycling, LixNOy and polysulfides simultaneously deposited on the lithium anode, and then the polysulfides were oxidized to Li2S by LiNO3 which deposited above the previous layer (Fig. 4a) [45]. Although the deposited L2S contributed to the formation of SEI film, the consumption of LiNO3 and active material slashed long-lifespan cycle ability of LiSBs. Liu et al. [46] grafted tween-20 on active lithium metal to obtain high performance and high-safety LiSBs. Alkyl chains on lithium surface sufficiently suppressed the contact and reaction between polysulfides and lithium metal, and the result of the etching depth showed the only a minute amount of L2S near metallic lithium surface. On the other side, the long-term and stable interface of the grafted-Li anode can evidently prevent the decomposition reactions of electrolyte (Fig. 4b). Wang et al. [47] prepared a smooth and uniform SEI layer by electropolymerization of DOL under the controlled current densities. When the cycling current was not above a threshold, DOL could form cross-linked polymeric SEI layer on the lithium anodes surface to accommodate the volume change and suppress dendrite growth during electrochemical cycling. However, high current density and long cycles of the electrochemical pretreatment of the lithium anode leaded to the excessive cross-linking of DOL and then impeded the diffusion of lithium ion, and fast decomposition of DOL and lithium salts under complex electrochemical reactions. According to the composition of the SEI layer on the lithium anode, the utilization of lithium salts plays an important role in forming the stable SEI film in the liquid electrolyte. Xu et al. [48] found that the surface chemical species on the graphitic anode were different in the same carbonate mixture by using LiBOB and LiPF6, respectively. In addition, Zheng et al. [49] analyzed the stability of the SEI film between the lithium anode and eight 878

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Fig. 4. (a) Schematic presentation of the SEI behavior on Li-metal anode cycling in different electrolytes; Depth profile for the lithium anode surface cycling in 0.1 M LiNO3/0.1 M Li2S6/DIOX/DME (1:1, v/v) [45]. (b) Schematic diagram of tween-grafted lithium metal; DFT calculations on the affinity between the alkyl chain and Li2S8, as well as Li2S6, to evaluate the resistance of alkyl chains to polysulfides [46]. (c) Schematic representation of the dissolution of sulfur in the electrolyte and its subsequent diffusion to, and reduction at, the carbon current collector, in a conventional LiSBs with a mixed C/S electrode and in the battery designed for this research [50].

exhibited low resistance values, excellent stability, and high capacity, and DEGDME has a lower overvoltage. Therefore, a single solvent is difficult to satisfy the basic requirements of LiSBs liquid electrolyte, and the optimization of electrolyte based on the mixture of solvents is of practical importance. For example, DME with linear structure can provide the high polysulfide solubility and fast polysulfide reaction kinetics, while ring-shaped DOL easily forms a stable SEI film on the surface of the lithium anode, and the combination of the two has a synergistic effect on improving the specific capacity and capacity retention of active materials. Additionally, other solvents in binary or ternary mixtures, including DME/DEGDME [64], TEGDME/DIOX [65], DIOX/DME [66], DOL/TEGDME [67,68], and DME/DOL/TEGDME [69], have also been studied as LiSBs electrolytes. Based on dissolution mechanism of LiSBs and transport mechanisms of lithium ions in the liquid electrolyte, these mixtures need to satisfy the requirements, containing chemically stable for polysulfide and lithium anode, high polysulfide solubility and low viscosity for polysulfide solution. Hence, the optimization of the mixture, including the choice of solvents and proportions, is critical to achieving the excellent electrochemical performance of LiSBs. Wang et al. [70] investigated the electrochemical performance of LiSBs with LiClO4 in DOL/DME by altering the content of DOL in DME and found the optimal composition ratio of the DME/ DOL was 2/1. Barchasz et al. [65,71] reported that the solvation ability of the electrolyte was the key factor for high electrochemical performances, and the ether chain length was correlated to the solvent solvation ability. The high DIOX contents have the insufficient solvation ability, leading to the poor performances of the TEGDME/DIOX binary electrolyte. In addition, an increasing amount of oxygen atom in the solvent molecule can enhance the solvation ability of lithium ions in ether solvents, and the number of oxygen atoms in the different electrolyte solvents shown in Fig. 5b. The first discharge capacity of LiSBs was closed to 1100 mAh g−1 due to the existence of the PEGDME in the electrolyte. Continuous redox between non-conductive bulk sulfur (or oxidation insoluble Li2S) and polysulfides contributes to the high utilization of active materials during discharge and charge. Moreover, the

electrolyte is still one of the most important issues. 3. Liquid electrolyte 3.1. Organic liquid electrolyte The most common liquid electrolyte concept of LiSBs bears much resemblance to lithium ion battery, containing solvents with small organic molecules, lithium salts, and additives. Due to the unique of the reaction process and the solubility of polysulfides intermediate in LiSBs, a large number of researches about organic liquid electrolyte were dedicated to restrain the shuttle effect and then improve the capacities and cycle stability of LiSBs in the past decade. Thus, the LiSBs electrolyte not only premeditate the volatility, thermal stability and interface compatibility with the electrode material, but also consider the appropriate polysulfides intermediate solubility and viscosity. 3.1.1. Structure and properties of ether solvents Chemical stability between polysulfides intermediate and electrolyte is considered to be crucial to achieving the excellent electrochemical performance of LiSBs. However, polysulfides intermediate react with carbonate solvent easily, and trigger solvent decomposition due to nuclephilic addition or substitution reaction (Fig. 5a) [28]. So that the suitable solvents for LiSBs electrolytes are limited within the linear and cyclic ethers, for example, dimethyl ether (DME) [59], tetra (ethylene glycol) dimethyl ether (TEGDME) [60], poly (ethylene glycol) dimethyl ether (PEGDME) [61] and fluorinated ether [62]. The ether solvents contain higher dielectric constant and lower viscosity, which facilitated to the transport of lithium ions, on the other hand, the ether bond (–C–O–C–) of the ether can form super molecular chelating structures by its oxygen, and lead to much stronger solvation capability for cations. Lorenzo et al. [63] investigated the characteristics of electrolytes based on various ether-based solvents and founded that increasing the ether-chain length facilitated to the improvement of thermal stability. It was noted that the TEGDME-based solutions 879

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Fig. 5. (a) Plausible mechanism for EC and EMC decomposition by polysulfide [28]. (b) Effect of chemical reactivity of polysulfide toward carbonate-based electrolyte on the electrochemical performance of LiSBs [65]. (c) Coulombic efficiency of LiSBs with 1.0MLiTFSI DOL/DME (5/5) and 1.0 M LiTFSI DOL/TTE (5/5) electrolyte at 0.1 C rate [79]. (d) Schematic diagram of traditional and fluorinated ether based electrolyte, inset, voltage profiles for the 50th cycle (C/10), relative amounts of each of the four sulfur containing compounds detected during the first discharge [84]. (e) Schematic illustration of the fabrication process for sulfur/ microporous carbon composites, influence of various carbon sources on the pore volumes of the activated carbons prepared from alkali-complexes, Cycling performance of KC/S, NC/S, and KNC/S electrodes at 1 C [101].

shuttle-based self-discharge, thereby self-discharge of batteries with low-loading was more serious than high-loading batteries. Gordin et al. [80] contrasted the effect of electrolyte on the self-discharge of different sulfur-loading batteries in the bis(2,2,2trifluoroethyl) ether (BTFE) electrolyte. The results showed self-discharge was decreased from 30% to 4% and 25% in low and high loading cells, respectively, because protective SEI layer on the anodes was in conjunction with improving protection of cathode. Nevertheless, enormous soluble polysulfides in the high loading cell should increase its viscosity and cell impedance after the dissolve into the electrolyte, but the excessively high electrolyte/sulfur (E/S) ratio will reduce the energy density of the battery [81–83]. Therefore, achieving the tradeoff between the two is one of the key technologies to improve the performance of LiSBs. Sara et al. [84] pioneered a fluorinated ether (TFEE) based electrolyte with excellent coulombic efficiencies and high-energy at low electrolyte loading (E/S = 6 μl/mg) and high loading sulfur cathodes (4 mg cm−2). The solubility of polysulfides in TFEE-based electrolyte was less than traditional ether due to worse stabilization of lithium ion in the fluorinated ethers. These works remarkably exhibited the effectiveness of focusing on the reduced polysulfide solubility and then mitigated the polysulfide shuttling (Fig. 5d). Chung et al. [78] systematically investigated superior electrochemical characteristics of high loading LiSBs (46 mg cm−2) with an electrolyte/sulfur ratio of as low as only 5. Notwithstanding six months after storage, the rested cathode still showed good polysulfide retention and limited electrode degradation.

introduction of sulphide co-solvents, not only contributed extra capacity to the cell, but also significantly improved redox kinetics and better reversibility due to subsequent reduction of soluble dimethyl polysulfide (DMPS) specie to the lithium organosulfides [72,73]. Although electrolyte with high solvation ability can reduce the polysulfide precipitation and delay the cathode passivation, shuttle effect is one of the culprits for capacity fading [74]. Taking a cue from the experience of fluoroethylene carbonte as a film-improving agent for lithium ion battery [75,76], the organo-fluorine compounds, including low melting points, viscosities and flammabilities, are also reported to restrict the self-discharge and parasitic reaction with the lithium anode of LiSBs in the late years, the structure and electrochemical performance of fluorine-based electrolytes are listed in Table 1. F-doping plays a role in strengthening and stabilizing SEI while the hierarchical chemical compositions of LiF-enriched interfacial layers form through initial electrochemical lithiation, and self-limiting layer consists of sulfate/sulfite/sulfide species is favorable for suppressing parasitic reactions on the lithium-metal anode [77,78]. On the other hand, donorability of fluorinated ethers is greatly weakened by steric hindrance of fluorine, which exhibited poor solubility for polysulfide. In the reaction process, fluorinated compounds show poor thermal stability in contact with lithium metal and tend to chemically react with it, for example, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) reacted with lithium to form a conjugated C]C bond and LiF [79]. The composite layer on the lithium anode surface played a role as a physical barrier, inhibiting parasitic reaction of polysulfides with the lithium anode and eliminating the self-discharge of the LiSBs (Fig. 5c). In addition, self-discharge in low-loading may be caused by irreversible active material loss, and cells with high-loading suffered primarily from

3.1.2. Carbonate-based electrolyte with a novel cathode Actually, the low boiling and flash points of the ether-based 880

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Table1 the species and electrochemical performance of fluorine-based electrolytes. Fluoride structure

Component ratio

Function

Capacity retention (CR) or selfdischarge rate (SR)

First discharge specific capacity

Ref

ETFE/DOL (1:1, v/v)

Reducing polarization and accelerate transfer kinetics

∼65% after 100 cycles (CR)

1000 mAh g−1 at 0.1 C

[78]

BTFE/DOL/DME (1:1:2, v/ v/v)

Decreasing self-discharge

TTE/DOL (5:5, v/v)

Facilitate to form SEI layer on the cathode and anode Decreasing solubility of polysulfides

TTE/DOL (2:1, v/v)

[80]

∼95% after 50 cycles (CR)

1100 mAh g−1 at 0.1 C

∼78.6% after 50 cycles (CR)

[85] [86]

∼50.8% after 150 cycles (CR)

1100 mAh g−1 at 0.2 C

[87]

TTE/DOL (1:1, v/v) TFEE/DOL (1:2, v/v)

Controlling the formation of the Li2S particles Mitigating the self-discharge Decreasing lithium ion solvation ability

∼4% (SR) ∼62.6% after 100 cycles (CR)

990 mAhg−1 at 0.1 C

[88] [89]

FDE/DOL-DME (4:1, v/v)

Decreasing solubility of polysulfides

∼58.4% after 200 cycles (CR)

1200 mAhg−1 at 0.5 C

[90]

DME/DOL/TETFE (1:1:1, v/v/v)

Limiting the dissolution of polysulfides

∼77.8% after 100 cycles (CR)

900 mAhg−1 at 0.2 C

[91]

TTE/DOL (1:1, v/v)

capacity decay of 0.0015% per cycle. In addition, several studies reported that an ultra-microporous material (d < 0.7 nm) effectively trapped polysulfide intermediates, and occurring “quasi-solidstate” reaction when lithium ions migrate into the pores. Hu and his coworkers [101] fabricated microporous carbon as a cathode from alkali-complexes by combining an immediate carbonization process and self-activation strategy, and the cell exerted high specific capacity 616.3 mAh g−1 after 600 cycles at 1 C (Fig. 5e). They noted irreversible electrochemical reactions were happening only at slightly larger space (> 0.7 nm) specific. So it is necessary that increasing the ultra-micropores (< 0.7 nm) volume ratio in carbon and avoiding producing larger sub-nanometer space (0.7–2 nm).

electrolyte may bring about great safety concerns when operated at elevated temperature. Although carbonate-based electrolytes are chemically incompatible with the polysulfide (Li2Sn, n > 4), the lower vapor pressure of them still attracts good graces of the scholars. In order to avoid the nucleophilic reactions, recent studies suggest the microporous carbon [92] or sulfur-rich cathode material with the non-dissolving mechanisms, such as SexS1−x [93], MoS2 [94] and S@pPAN [95], can effectively suppress the direct contact between the solvent molecule in carbonate-based electrolytes and polysulfide. S@pPAN with a high specific capacity, good cycling stability, and brilliant compatibility with the commercial carbonate-based electrolyte serves as the matrix to hold sulfur to suppress the dissolution of polysulfides in the electrolyte [96]. Xu et al. [97] noted the synergistic effect between S@pPAN cathode and carbonate-based electrolytes facilitated the formation of a compact and thin SEI layer on the electrodes. Surface modifications suppressed the side-reactions and improved the electrode kinetics. On the other side, the SEI layer on the lithium surface prevented further electrolyte decomposition and lithium corrosion. Combining with the merits of the organo-fluorine compound, FEC with a relatively high melting point has been extensively studied to contribute to the formation of a stable SEI layer on lithium metal [98,99]. Chen et al. [100] added EMC and 1 M LiFSI into the FEC and fabricated a new carbonate electrolyte and found LiF rich SEI layer forming on lithium anode by a synergistic effect between LiFSI and FEC promoted reduction of side reactions and decoration of the lithium deposit morphology. The cell exhibited glorious electrochemical performance with high capacity of 1270 mAh g−1 after 1000 cycles at 2 C and an inappreciable

3.1.3. Type and concentration of lithium salt In the system of lithium secondary batteries, conductive lithium salt plays a role in affording ions and increasing the conductivity of the electrolyte. So it should possess certain properties to be a component of a suitable electrolyte and to satisfy demands of battery operation, containing: 1) lithium salts should have low dissociation energy between anions and lithium ion and high solubility to promote their dissolution in organic solvents; 2) the anions in lithium salts should exhibit optimum Lewis bases. It is noteworthy that the higher ability of donating electrons can reduce the stability of the anions, in contrast, lower Lewis bases of the anion will adsorb nucleophilic polysulfides and increase the dissolution of polysulfides; 3) lithium salts should assist the formation of SEI film properties to ensure that the electrolyte will not be continuously consumed in the subsequent cycle; 4) lithium salts 881

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Table 2 The physical properties of several common lithium salts, containing a solvent, ionic conductivity, donor number, and initial decomposition temperature. Salt

Solvent

Ionic conductivity (mS/cm)

Donor number

Initial decomposition Temperature (°C)

LiPF6 LiBF4 LiClO4 LiBOB LiFSI LiTFSI LiODFB

EC/DMC EC/DMC EC/DMC DME/DOL TEGDME/DOL DME DBE EC/DMC DME/DOL TEGDME/DOL EC/DMC/FEC

10.8 4.9 10.1 7 5 14.9 12 9 11 7 –

2.5 6 8.4 – – 5.4 –

125 175 – 275 200 360 200

exfoliation and also suppress the polysulfides shuttle (Fig. 6b). Additionally, in concentrated electrolytes, a large amount of lithium-ion flux not only enhanced the uniformity of lithium deposition and dissolution in charge/discharge process but also increased the pressure from the electrolyte to restrain the growth of dendrites [110–112]. In higher concentration electrolytes, such as 12 M LiFSI/DME and 12 M LiFSI/DOL/TBA, the solid–liquid-solid reaction was replaced by a quasi-solid-state reaction mechanism [113]. Hardly free solvent molecules restricted the possible dissolution of polysulfides and improved the security of cell, and reduction of FSI- on the anode dramatically increased the lithium metal cycling stability and the Coulombic efficiency. Nevertheless, the high cost of lithium salt becomes a barrier to its commercial development. Zheng and his coworkers added the HFE with low solubility of LiFSI into LiFSI/DME to form pseudo-high-concentration electrolyte to reduce the cost of the salt. Low donor ability and permittivity of HFE resulted in a minimized solubility of polysulfide and displayed a much high capacity of 1449 mAh g−1. In addition, adding a large anionic size of lithium salt into concentrated electrolyte help to reduce the amount of lithium salt in small molecules. Synergistic action of the overlapping of the electron cloud and the steric hindrance in the electrolyte helps to form a denser barrier to inhibit the dissolution of polysulfide [114,115]. Combing high initial capacity of LiFSI and surpassing long cycle stability of LiTFSI, high capacity, stable cycle life and high Coulombic efficiency of the cell are obtained. Moreover, homogeneous lithium anode surface also achieves by the stable electrostatic shield [110].

should possess thermal and electrochemical stability. The physical properties of several common lithium salts are shown in Table 2. Unlike the behavior of lithium salts in lithium ion battery, the lithium salts in LiSBs system not only should exhibit excellent ion transport ability but also not be reacted with sulfur and polysulfides [102]. For example, the trace amount of hydrogen fluoride remaining in the LiPF6 easily reacts with the cyclic ether solvent to engender a series of acid-catalyzed polymerization reactions [103]. Imine-based salts, such as LiTFSI, LiBETI, and LiFS, owning outstanding ionic conductivity, excellent thermal stability, and high oxidation potential, are a real contender to LiPF6. Park et al. [104] investigated the influence of the various anionic species in the same solvent on the discharge capacities, containing TF−, TFSI−, EBTI− and PF6−. The result showed that TFSI- and EBTI- formed the stable SEI film on lithium surface and exhibited better cycle life performance than others. Actually, the addition of TFSI− in the electrolyte weakened the Li–S networks in sulfides by interacting with lithium ions and accelerating the surface reaction [105]. Simultaneously, lithium ions coordinated with O and F atoms from the solvents and salts and then weaken Li–S networks in LiFSIbased electrolyte [106]. Lang et al. [107] indicated the properties of the anions in lithium salts influenced the structures and processes of sulfide deposition/decomposition during discharge/charge process. During the deposition process, the higher viscosity and lower conductivity of LiTFSI-based electrolyte even covalent Li–C bonds between Li2S and graphene together promoted the deposition and the planar growth of lamellar Li2S in LiTFSI-based electrolyte. The stable surface structure of Li2S in LiFSI-based electrolyte was in favor of the 3D growth of spherical Li2S. During the decomposition process, because the sites of charge transfer occur at the direct contact points between the bottom Li2S and the cathode, lamellar Li2S tends to decompose layer by layer and spherical Li2S take center-to-edge mode to generate hollow spheres (Fig. 6a). The concentration of lithium salt in traditional liquid electrolytes is limited to 1–2 M, however, based on solvation of lithium ions, a large shell leads to relatively lower mobility of solvated lithium cations in low-salt concentration electrolytes. High concentration electrolyte, in which the number of solvated lithium cations was decreased, alleviated the property of sluggish ionic conductivity and promoted electrochemical reactions by providing sufficient lithium ion transport. Suo et al. [108] exhibited an electrolyte (LiTFSI/DOL/DME) with ultrahigh salt concentration (7 M) and high lithium-ion transference number (0.73). Nearly saturated salt concentration, which owned the high viscosity, was a barrier to intermediate (Li2Sn) dissolution. For the low concentrated electrolyte and graphene cathode, the intercalation of solvent molecules conjunction with lithium salt between graphene planes could result in graphite exfoliation and the failure of graphite electrode, and electrochemical lithium ion intercalation and de-intercalation of graphite electrodes cannot take place. The super-concentrated ether electrolyte (LiTFSI/DOL/DME, 5 M) with the distinctive networking structure of lithium ion and TFSI- was introduced by Pan et al. [109]. The high concentration of TFSI anions has a unique networking structure of lithium cations and TFSI anions with lithium ionsolvating DME/DOL solvents, which was reduced to form a stable surface film on a graphite electrode to inhibit sustainable graphite

3.1.4. Functional additive for forming SEI In the LiSBs system, although several strategies were used to ameliorate the electrochemical performance and safety of the cells, a small quantity of electrolyte additive may be one of the most inexpensive and effective methods. Ordinarily, the functions of additive in the liquid electrolyte contain 1) to assist the constructions of stable SEI films and protect lithium anode; 2) in conjunction with solvent and lithium salts to inhibit the dissolution of polysulfides. Currently, common additives include nitrate salts, iodized salts, organic sulfides, and a conductive agent. Thereinto, lithium nitrate (LiNO3) is one of the most studied and widely used as additives in an organic electrolyte. It is favorable to form an in situ stability protective surface film on the lithium surface and prohibit reacting with the dissolved polysulfides [116–118]. On the other hand, LiNO3 is also charged with catalyzing the transformation of soluble long-chain polysulfides to elemental sulfur near the end of the charging process. Cui group studied the synergetic effect of both lithium polysulfide (Li2S8) and lithium nitrates in the ether-based electrolyte [119]. The reaction between Li2S8 and lithium anode reduced the formation of the nucleation sites on the lithium surface, forming the stable and uniform SEI to restrain the growth of the lithium dendritic. After 100 cycles at 2 mA cm−2, the lithium surface still displayed relatively uniform morphology, by contrast, the inhomogeneous growth of the massive lithium dendrites was clearly exposed in the electrolyte with the only LiNO3 (Fig. 6c). Our group also researched the influence of the presence of the LiNO3 on the electrochemical performance of the cell [120]. The results indicated the battery with LiNO3 manifested a high 882

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Fig. 6. (a) Schematic illustration of the sulfide reactions at the cathode–electrolyte interfaces in LiTFSI–LiFSI binary-salt electrolyte [107]. (b) A possible mechanism of reversible lithium intercalation and de-intercalation at a graphite electrode in superconcentrated LiTFSI-DME/DOL electrolytes [109].(c) Schematic of the morphologies of lithium deposited on the substrate in without lithium polysulfide and containing lithium polysulfide electrolytes [119]. (d) A summary of the deposition/stripping process of Li and the corresponding SEI composition in electrolytes with and without LiNO3 [120]. (e) Schematic illustration of the formed protective layer on the surface of the sulfur cathode by adding Py into the electrolyte; Long term cycling performance of LSB with the basic electrolyte and 5 wt% Py added electrolyte at the rate of 1C, inset photograph of the Li2S6 adsorption by PPy in DOL/DME solution [130].

initial discharge capacity > 1300 mAh g−1, and excellent cycling stability with the 809.1 mAh g−1 after 300 cycles, owing to suppress the migration of soluble polysulfide to the Li anode surface by LiNO3-derived surface film. The component of SEI film on the lithium surface in the LiSBs with the LiNO3 contained Li2S2O3, Li2SO4, Li2S, Li2S2, and LiNxOy, and it required to continuously consume a large amount of LiNO3 during long-term cycling. Adams et al. [121] did the research about the long term stability of LiSBs by using high concentration LiNO3 electrolytes. The existence of high concentration LiNO3 in electrolytes played the part of a co-salt to improve the ionic conductivity and promoted the occurrence of the cycling process in the liquid phase via increasing the solubility of long-chain polysufides. Furthermore, due to the strong oxidizability of LiNO3, Li2O formed on the lithium surface to stabilize lithium anode. With the successive plating/stripping of the lithium, porous residual SEI layer was continuously built during each cycle forms, and the cell demonstrated the excellent electrochemical performance, that the initial capacity was 1349 mAh g−1 and the capacity retention achieved 90% after 100 cycles (Fig. 6d). Nonetheless, the trace consumption of lithium nitrate still existed during the cycle, and it was evidenced by the small losses of the coulombic efficiency per cycle. In addition to the concentration of LiNO3, the binders containing oxygen functional groups were also reported to improve the reduction of LiNO3 [122]. Carboxylic groups (–COO–) presented in binders, for example, sodium alginate (NaAlg) and sodium carboxymethyl cellulose (NaCMC), probably served as a chemical trap which can be a hurdle for leakage of polysulfides. And the oxygen-containing functional group also promoted the LiNO3 decomposition at potentials lower than 1.8 V. Therefore, it is necessary to consider the presence of oxygen-containing functional groups on the choice of the binders in order to avoid rapidly capacity decay. The SEI film on the surface of lithium anode can greatly suppress the reaction between the polysufides and lithium, but the shuttle effect is still present due to the

concentration difference. In recent years, another role of LiNO3 in the LiSBs was discovered by Zhang et al. [123]. In the cells cycling system, the NO3 radicals were apt to be reduced by polysulfide anions (Sn2−) anions because of the higher potential than the oxidation of the Sn2−, and the highly soluble polysulfide also was instantly oxidized to elemental sulfur, as described by Equation:

2NO3− − 2e → 2NO3 2NO3 − Sn2 − → 2NO3− +

n S8 8

Based on the positive role of nitrate anion on improving shuttle effect of LiSBs, the other nitrate salts, such as cesium nitrate (CsNO3) [124], lanthanum nitrate (La(NO3)3) [125] and potassium nitrate (KNO3) [126], were also carried out for LiSBs as the electrolyte additives. On the other hand, metal cations (La+, Cs+, and K+) can form metal or alloy layers on the lithium surface to improve the stability of lithium anode and restrain the consumption of nitrate anions due to the destruction of the passivation film. For example, the La3+ ions with Lewis acidity are reduced by metallic lithium to metallic La, and then was oxidized by the Lewis basic polysulfides to form La2S3 and deposited on the lithium metal surface. Compared to the nitrate salts, the iodide salts, for example, lithium iodide (LiI) [29] and indium triiodide (InI3) [127], possessed the more effective inhibition of shuttle effect due to the bidirectional protection of positive and negative electrodes. Liu et al. reported a kind of facile and feasible Li2S-based LiSBs system using InI3 as a bifunctional electrolyte additive. During electrochemical cycling, InI3 not only acted as a cathodic redox mediator, which efficiently transformed the outmost surface of Li2S particles into soluble polysulfides, reducing the activation potential of Li2S cathode but also generated a passivation layer containing indium to prevent the anode from corrosion causing by shuttle effect. Inorganic metal salts as additives can effectively stem the reaction 883

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Fig. 7. (a) Molecular structural of the anions and cations in the commonly used ionic electrolytes. (b) Schematic illustration of the function mechanism of polysulfide dissolution and diffusion in ether (left) or PP13TFSI-based (right) solvent; Cyclic performance of LiSBs presenting specific discharge capacity, after an uninterrupted 10 cycles, the cells were rested for 24 h [143]. (c) LiFSI salt solubility of various ionic liquid based hybrid electrolytes [150]. (e) Cycling performance of the LiSBs at 0.1 C and 1 C, (inset) burning tests of LiTFSI-P1, 2O1TFSI and schematic illustration of the electrolyte in LiSBs [152].

electrochemical performance of LiSBs with the addition of an appropriate amount of Py. The conductive pyrrole was slightly oxidized by sulfur cathode to form a protective layer on the sulfur cathode. And it provided effective electron conduction path and the strong polysulfides trapping to suppress the diffusion of polysulfide intermediates and result in the excellent electrochemical performance (Fig. 6e). Combing the advantages of halogen elements and sulfur-containing compounds, Li and his coworkers [131] designed a kind of bifunctional additivethionyl chloride (SOCl2). Before the reactions between lithium metal and electrolyte, a solid LiCl-rich film, which was formed by reacting with lithium metal, could significantly stop the decomposition of electrolyte and cast off the formation of dendrites. Simultaneously, the reduction product (S) from SOCl2 can offer the extra capacity to LiSBs. It was worth to be noted that the high SOCl2 content in electrolyte exhibited a poor electrochemical performance because more sulfur or polysulfides seriously destroy the electrolyte properties and electrode structure. Summary: In the system of organic liquid electrolyte, synergistic effect among with solvent, lithium salts and additives used to ensure the operation of LiSBs must be capable of limiting the polysulfide shuttle and allowing the lithium ion transport and forming the stable SEI film on the electrode. However, the mutual constraints among the three, such as limited lithium salt solubility, the reaction between the lithium salt and the additive, still hinder the most rational design of the

of polysulfides with lithium metal, and have some limitations in inhibiting the dissolution of polysulfides. Due to S-nucleophilicity of thiolate anions, the redox active organyl sulphides can be used as the electrolyte additive to suppress the formation of soluble polysulfide. Wu and his group [128] reported the biphenyl-4,4-dithiol(BPD) electrolyte additive possessed the function which can effectively enhance capacity retention of LiSBs by controlling polysulfide dissolution. During cycling, creating the BPD-short chain polysulfide (Sn2−, 1 ≤ n ≤ 4) complexes due to the delocalization of the S-S bond prohibited the formation of short chain polysulfides and changed the kinetics of formation of short chain polysulfides and decreased polysulfide dissolution. Additionally, the other thiol-based molecules also were evaluated the effect on LiSBs, such as thiophenol (TP), benzene-1,4-dithiol (BD), biphenyl-4,4-dithiol (BPD), 1,4-bis(4-mercaptophenyl)benzene (MPB) and pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), but BPD displayed the optimal additive balances solubility and polysulfide-additive stabilization. Phadke et al. [129] also demonstrated the thiophilicity of thiolates catalyzed the reduction of elemental sulfur to short chain polysulfide while completely bypassing the formation of soluble long chain polysulphides. The cells exhibited excellent electrochemical stability with a capacity above 900 mAh g−1 at a high current rate of 1 C after 500 cycles. In addition, the protective layer on the surface of cathode formed by the reaction of sulfur and organic additives can also block the diffusion of polysulfide. Yang et al. [130] discussed the 884

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composition and structure of SEI film, containing more sulfide related species on the SEI surface, and cells showed higher resistance and low stability. The LiNO3, which can oxidize LixSOy species formed by LiTFSI reduction to LixSOy+1 as well as oxidize sulfur upto sulfate, played a pivotal role in forming a stability surface film on the anode surface in the organic electrolytes. Wang et al. [143] added 0.2 M LiNO3 into the TFSI-based ILs and obtained a uniform surface film with indistinctive deposits, and the LiSBs achieved the excellent electrochemical stability with the zero self-discharge in the 10th cycle (Fig. 7b). In addition to the traditional ILs, Kaoru and his group [144,145] found the mixing the lithium salts and glyme formed the stable [Li(glyme)]+, exhibiting similar properties to ILs due to the strong coordination between the glyme and lithium ion. With the decrease in the molar ratio of glyme, the donor ability of the [Li(glyme)n]TFSA was significantly weakened and then obtained low solubility of Li2Sx. The operation of cell was stable at 800 cycles in the [Li(glyme)3]TFSA and the Coulombic efficiency was > 97.5% in all cycles. In addition, it is feasible to tune physicochemical properties by properly designing the ILs architecture, introducing suitable foreign atoms in the cation/anion structure. Appetecchi et al. [146] found the sulfur functionalizing pyrrolidinium cation exhibited excellent thermal stability, appreciable ion transport at low temperatures and highly reversible stripping/plating process of lithium. Although the neat ILs possess the superior physicochemical behavior as we describe in the previous paragraph, relatively high viscosity and strong complexation between the lithium ion and the anion results in the slow lithium ion diffusion and low lithium ion transference numbers. Based on the solvation ability of the organic electrolytes, incorporating organic electrolyte into the ILs is helpful to change the interaction between lithium ion and anions, and results in weaker and more dynamic clusters, finally improve lithium ion transport [147,148]. Xiong and his coworker [149] found that the addition of 10% DIOX into the PYR14TFSI not only improved the thermal stability of the mixed electrolytes, containing the increase of the flash point and decrease of the glass temperature but also reduced the viscosity of ion electrolyte. Herein, the capacity of the mixture electrolyte doubles the neat ILs, even six folds higher at the highest rate. Paul and his group [150] indicated the addition of DME with two oxygen centers, which facilitated to create more coordination options for the lithium ion, changed the environment of lithium ion and dominated its coordination and transport, finally increasing the LiFSI solubility. Combining with the advantages of concentrated electrolyte, hybrid electrolytes showed the excellent polysulfide insolubility (Fig. 7c). Significantly, although the combination of ILs and organic electrolyte advantages is beneficial to the cells, for example, low solubility of the polysulfide and high ion conductivity, the corresponding sacrifice in specific capacity or coulomb efficiency also exists because of the larger polarization and anode corrosion. Hence, optimizing the ratio of the electrolyte is important for weighing the effect on the electrochemical performance of the battery. The effects of different levels of methylisopropylsulfone (MIPS) on the physical and electrochemical properties of ILs were systematically analyzed by Liao et al., including crystallization behavior, solubility, and diffusion of the polysulfide and interfacial resistance [151]. In a mixture of IL/sulfone (60:40, v/v), the low coordination ability of the IL significantly suppressed the lithium polysulfide formation, simultaneously, the low viscosity of the sulfone reduced the impedance of the electrolyte. Wu et al. [152] further explored the coordination of organic electrolytes and ILs in battery operation and given possible mechanisms. In the region close to the positive electrode, the organic electrolyte helps to dissolve the polysulfide and thus activates the sulfur element to offer the good conductivity for electrolyte system, in another region, the ILs inhibits shuttle suppression due to the high viscosity (Fig. 7d). In addition, the optimized electrolyte also exhibited excellent thermal stability and non-flammable. Similarly, utilizing the complementary and synergistic actions between ILs and fluorinated ether, Hu et al. selected

electrolyte. Focusing on current research results, whether facilitating the formation of the lithium anode SEI film or changing the reaction path to inhibit the polysulfide diffusion, and the fuzzy reaction mechanism still exist. Understanding the effect of solvent and lithium salts structure and composition on battery performance can help to counterbalance the interaction effect. In addition, strategies to enable practical, long cycling of LiSBs should concentrate on the seek the new electrolyte additives, which not only can take complete reversible reaction with other components in the electrolyte or lithium metal, but also can control the morphology of lithium metal deposits. To be honest, during the design for the more appropriate electrolyte, many technical obstacles should be overcome. 3.2. Ionic liquid electrolyte Ionic liquids (ILs) are wholly composed of charge-delocalized bulky ions, which possess a relatively large radius and a relatively loose structure, resulting in the weak force between the anions and cations. Thus, they can exist in a liquid state at room temperature. The ILs, which possess several excellent properties, including negligible volatility, low flammability, high thermal stability (> 200 °C), reasonably high ionic conductivity (0.1–10 mS cm−1) and a wide electrochemical potential window (> 4 V), attracted the eyes of the researchers in electrochemical field, especially, in lithium secondary batteries [132,133]. In organic electrolyte system of LiSBs, solvents with high donor number more effectively coordinate with the Lewis acidic cation, and then lead to the dissociate lithium from polysulfide and consequently augment the polysulfide dissolution, finally resulting in a decrease of open circuit voltage (OCV) and discharge capacity [134]. The unique structure and properties of ILs are excellent at inhibiting polysulfides, and the mechanisms of inhibition mainly are based on the hard and soft acids and bases (HSAB) theory, in which the weak interaction between the soft Lewis base anions and Lewis acidic cations are effective for suppressing the polysulfide dissolution and diffusion. The anions and cations in the commonly used ionic electrolytes are shown in Fig. 7a. Park and his group [135] investigated the saturation solubility of polysulfide in the kinds of the binary mixtures of ILs and lithium salts and found the difference in the solubility could be rationalized in terms of the donor ability of the IL solvent. Actually, the lithium ion dissolved in the ILs easily form a new lithium salt with the anion of the ILs to reduce the anionic donor capacity, for example, Li(FSA)3 [136]and Li (TFSA)2 [137]. In addition, if the size of the ionic solutes and ions of ILs matched well, site exchange may occur easily, so the similarity of IL anion size and polysulfide anion is deemed to influence polysulfide solubility [138,139]. The solvent properties of ILs is dictated by the interplay of cations and anions, thus, the combination of the same anion (cation) and different cation (anion) results in different saturation solubility of the polysulfide. Yan et al. [140] also systematically analyzed the effect of the cations in TFSI-based ILs on the electrochemical performance of LiSBs. Although the P13 cations with the low ionic conductivity were benefited for inhibiting the dissolution of the polysulfides and reducing the shuttling problems of LiSBs during the discharge/charge process, the high overpotential still deteriorated the stability of the cells owing to the slow lithium ion diffusion. With the reduction of lithium salt concentration in the ILs, the LiSBs showed a high specific capacity of 1580 mAh g−1 in the initial cycle and impressive cycling stability because of the low viscosity. However, improper choice of anions leads to the disappointing charge/discharge behavior of LiSBs, for example, both side reaction of [FSA]− and BF4− with nucleophilic polysulfide anions [141]. Like the organic electrolytes, a small amount of dissolved polysulfide interacts with the ionic electrolyte and lithium salt to form a barrier on the anode surface and to restrain the corrosion of lithium metal. For the TFSI-based ILs, SEI is formed by decomposition products from both cations and anions of the electrolyte [142]. The introduction of polysulfide changed the chemical 885

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crosslink density compared with other monomers. Thus, the high ion conductivity of PETEA-based gel electrolyte was propitious to reduce the polarization of LiSBs, and the high-strength PETEA-based gel matrix also pre-covered on the cathode surface and formed a flexible passivation layer to against the volumetric change of sulfur particles during the charge/discharge process, and then restrained the continuous interfacial reaction due to the polysulfide dissolution (Fig. 8b). Moreover, the immobilization of polysulfides can also be strengthened by the strong interaction between lithium sulfide and the oxygen donor atoms in ester (C]O) groups [172]. To shorten the lithium ion transmission channel and increase the energy density of the battery, Chen et al. [173] combined PEO with LiTFSI to form a swelled amorphous gel polymer as the combination of a binder and a separator. The similarity between structure (-EO-) of PEO and the ether-based liquid electrolyte was favorable for capturing the electrolyte and decreasing the electrolyte evaporation. Additionally, the strong lithium ion solvability of swelled PEO/LiTFSI on cathode surface facilitated to the formation of conducting gel network across the whole sulfur-based cathode. Even if under low E/S ratio, the cell still could deliver a capacity with 1200 mAh g−1 and good cycle life (Fig. 8c). The hydrogen bond was expected to form between the hydroxyl groups and anions in lithium salts, which increased the ion transference number and decreased polarization. Song et al. [174] exploited an environmental benignity gel electrolyte using natural biomass lignocellulose (LC). The N, O and F elements in the lithium salt could form the hydrogen bond with the –OH in the DOL, DME and LC. The action of the kinds of molecules amplified the anion and decreased the rate of migration, and thus exhibited the large ion transference number. In addition, the strong interaction among the hydroxyl, carboxyl and carbonyl in the LC with the anions of polysulfides also suppressed the polysulfides diffusion (Fig. 8d). The chemisorption of polysulfides by inorganic oxides has been mentioned in the strategy of optimizing the modification of cathode materials, that a negatively charged site is introduced on the surface of the carbon material to adsorb polysulfide anions. Likewise, inorganic oxides also can be used to modify gel electrolyte, which reduces the crystallinity of the polymer and inhibits the diffusion of polysulfides. Zhang et al. [175,176] systematically analyzed the application of SiO2 on the surface of the separator and cathode, respectively. The main functions of SiO2 containing: 1) providing more desirable passageways for the transfer of lithium ion; 2) strong chemisorptions of SiO2 significantly restricting the diffusion of polysulfides. PEO/SiO2 gel electrolyte coating significantly enhanced the wettability of liquid electrolyte and promoted tight contact between the electrode and separator. On the other hand, the 50% SiO2 filler in the gel polymer on the cathode surface served as a reservoir to absorb polysulfides. According to the coordinated function between the PVDF (F) and PMMA (eC]O), PVDF/PMMA/SiO2 and PVDF/PMMA/MMT were prepared by Zhang and his group [177]. Both composites possessed high porosity and ion conductivity. Especially, PVDF/PMMA/MMT exhibited splendid electrochemical stability with the high coulombic efficiency (about 100%) over 100 cycles (Fig. 8e). Other composite gel electrolytes with SiO2 also were reported [178,179]. Nevertheless, due to the high viscosity of gel polymer and strong adsorption of SiO2, polysulfides were confined and then reduced the utilization of sulfur active material. Consequently, Zhang [180] further replaced SiO2 with elemental sulfur(S) to improve the specific capacity of LiSBs. Elemental S not only served as the poremaking agent to promote the adsorption of the liquid electrolyte but also increased the capacity of the cell compared to SiO2. In addition to SiO2, the high specific surface area and strong adsorption properties of nano metal oxides not only enlarged the contact area between the electrode and electrolyte, but also possessed certain Lewis acidic groups, which reacted with Lewis basic groups in polymer segment and lithium salt anions, finally effectively improved the electrochemical performance of polymer electrolyte and inhibited the “shuttle effect” of polysulfides. For example, Angul et al. [168] discovered the increase

the TFTFE as the support solvent of [Li(glyme)]TFSI [153]. They found the polarization of ILs was ameliorated due to the ionic conductionenhancing, accompanying by the coulombic efficiency near 100% and quite a stable cycle capability. In addition, the availability of proper DOL, DME and DOL/DME to improve the stable cycling of ILs also were proved [154,155]. Summary: Strategies of ILs electrolytes focus on meeting the requirements of the cell by properly designing the cations and anions of the ILs or mixing the appropriate organic electrolytes, but the coulombic efficiency of the battery still cannot achieve 100%, and long cycle over 1000 cycles have not been mentioned in current literature. The connatural contradiction between electrochemical performance and physical property of ILs, for example, the high viscosity and low specific capacity, as well as the high solubility of polysulfide and poor coulombic efficiency, still are an insurmountable gap for the high performance LiSBs, and it appeals for more attention and efforts. 3.3. Gel electrolyte As we all know, the morphology of polymer is determined by the ratio of the crystalline and amorphous regions of its molecular segments. Addition of the plasticizer, for example, an inorganic particle and low molecular weight organic matter, will interfere with the crystallization of polymer [156–158]. According to the principle, the gel polymer electrolyte combining polymer matrix and the liquid electrolyte to form a new system, which not only has its high ionic conductivity close to the liquid electrolyte but also is found to be able to decrease the solubility of lithium polysulfides for LiSBs. The quasi solid state polymer acted as a physical barrier to restrict lithium polysulfides diffusion from the cathode. On the other side, the numerous of amorphous regions in the polymer was used as a reservoir for liquid electrolytes to provide good ion conductivity [159]. The mechanical properties imparted by the polymer matrix can support it as a separator, which can reduce electrolyte leakage and flammability and thus ameliorate the safety of LiSBs. In addition, in order to fulfil the requirements of lithium batteries, gel electrolytes should meet the following requirements: 1) ionic conductivity is close to 1 × 10−3 S/cm; 2) the lithium ion transference number is near to 1; 3) good mechanical properties and thermal stability; 4) wide electrochemical window (> 3 V); 5) interaction among the components of the electrolyte ensures the stability of the physicochemical properties of the electrolyte. Reviewing the reports in recent years, the polymer matrix of common gel electrolytes, mainly based on PEO, PVDF and their derivatives, in which the fluorine/oxygen atom promoted the dissociation of lithium salt. On the other hand, these polymers not only possess the high dielectric constant and electron but also have favourable plasticity and processability. The physicochemical and electrochemical properties of the kinds of gel electrolytes were shown in Table 3. Jin and his coworkers [160] prepared the PVDF-HFP membrane with 3–5 μm pores by the phase separation, and the micropores and interconnected skeleton structure not only provided the membrane the ability to store enough electrolytes, but also acted as a framework to transport lithium ions (Fig. 8a). Simultaneously, the interaction among salt, ionic liquid, and PVDF-HFP suppressed the recrystallization process of the membrane to hold battery stability. To build the electrochemical and structurally stable gel electrolytes, the similarity structure between polymer and electrolyte should be firstly considered. Liu et al. [161] and Du et al. [162] both reported a novel PETEA-based gel electrolyte showed the excellent ion conductivity and high ion transference number via in-situ synthesis, respectively. The ether-bond similarity between the PETEA and ether-based liquid electrolyte ensured good compatibility betwixt the polymer matrix and liquid electrolyte, and the special symmetrical star structure with four C]C bonds of PETEA monomer provides the polymerized PETEA with higher 886

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Table 3 The physicochemical and electrochemical properties of the kinds of gel electrolytes. Polymer matrix

Preparation method

Liquid electrolyte

Ionic conductivity (mS cm−1)

Electrolyte uptake (%)

Electrochemical stable window (V)

Decompose temperature (°C)

Ion transference number

Ref

PVDF-HFP PAN/PMMA

phase separation electrospinning

0.254 3.8

– 450

>5 –

350

– –

[160] [163]

PEO/SiO2 PVDF-HFP/ PMMA/SiO2 PVDF-HFP/ PMMA/MMT PVDF/PVP/LTMP PETEA PEO

phase inversion phase inversion

PP14TFSI PP14TFSI/ PEGDME DOL/DME TGDE

4.2 3.12

190 71

– 4.5

– –

– –

[71] [164]

phase inversion

EMD

3.06

67

3.5

–

–

[165]

phase inversion in-situ gelation solution casting

2.6 11.3 1

– –

5.4 4.73 4

166 – 300

– 0.47 0.78

[166] [161] [167]

PEO/MgAl2O4 PVDF/PEO/ZrO2 PPC/SiO2 LC

solution casting electrospinning solution casting –

DOL/DME DOL/DME DIOX/ TEGDME TEGDME/DOL DOL/DME DOL/DME DOL/DME

0.34 0.525 0.164 4.52

300

– 5 4.66 5.3

–

– 0.71 0.83 0.79

[168] [169] [170] [171]

428

120

base interaction mainly occurred on the gel electrolyte, thus lowered the ionic coupling and promoted the salt dissolution. It was noteworthy to mention MgAl2O4 particles also could enhance the electrolyte uphold and retard dissolution of polysulfides during cycling. In the PVDF/PEO/ ZrO2 system, the polymer segment of PEO could disrupt the short term

thermal stability of gel electrolyte was ascribed to the restriction of segmental movement of PEO-chains by interactions between the Lewis acidic site of MgAl2O4 and the oxygen of ethylene oxide (-EO-) moiety. Additionally, the specific polar surface groups of MgAl2O4 filler modified the structure of the filler surfaces and resulted in the Lewis acid-

Fig. 8. (a) Photograph and SEM image of P(VDF-HFP) membrane [160]. (b) The immobilization mechanism for polysulfides by capitalizing on PETEA-based GPE as electrolyte and electrochemial test, the FESEM image of the PETEA-based GPE (the optical images of PETEA-based GPE and its precursor solution are shown in inset) and Surface morphologies of the sulfur electrodes in the S/LE/Li cell after 50 cycles [161]. (c) Up: solid binder powder before adding the electrolyte, bottom: swelling and electrolyte uptake test of binders in the 1 M LiTFIS DME/DOL electrolyte; schematic diagram of the electrode with PEO10LiTFSI nanofilm coating and the confinement of polysulfides by the swelled PEO10LiTFSI gel [172]. (d) The immobilization mechanism for polysulfides and the hydrogen bonding between H–O and part of polysulfides (–SO4, –SO3 and –SO2) on the surface of G2 contact with sulfur cathode [174]. (e) Schematics of the preparation process; cyclability and charge–discharge profiles of the PVDF-HFP/PMMA/MMT [177]. 887

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electrode and ASSE not only aggrandizes energy barrier for lithium ion transport but also increases the polarization of battery [189]. So far, the researches on the preparation of ASSE mostly utilized high-energy ball milling and subsequent heat treatment [190]. On the one hand, solid electrolyte with smaller particle size can smooth the contact surface and increase contact sites. On the other hand, the crystal unit cell or crystallinity of polymer can convert, thereby improving the ionic conductivity of the solid electrolyte during the ball milling process and heat treatment. A composite cathode, which was consisted of the active material, conductive agent and ASSE to reduce the interface impedance and shorten the transmission distance of lithium ions, had been used in the most of batteries [191,192]. In addition, several reports also indicated the combination of solid electrolyte and liquid or gel electrolytes not only compensated for the defects caused by the polysulfide shuttle but also improved the interface properties.

ordered arrangement of the PVDF long chains to decrease the crystallinity. Similar to MgAl2O4, the interactions between the lithium ion and the oxygen atom in PEO/ZrO2 as the Lewis-base resulted in more lithium ion hopping sites and increased the lithium ion transference number, and the cell could still deliver a capacity of 847.2 mA h g−1 after 500 cycles at 1 C and the mass loading of sulfur on the cathode plate was 1.7 mg cm−2 [169]. To utilize gel electrolytes and exert their advantages efficiently, it becomes straightforward to build novel structures that are composed of kinds of polymers. Yang and his group [181] pioneered a sandwiched gel electrolyte (PVDF/PMMA/PVDF) through layer by layer, in which, the PVDF layers with porous structure mainly absorbed ether-based electrolyte and then ameliorate ion conductivity, simultaneously, the PMMA layer with a smooth surface can be utilized to trap the dissolved polysulfides. Therefore, the cell exhibited outstanding electrochemical property with the excellent initial discharge capacity (1711.8 mAh g−1) and the high electrochemical reversibility. To gain more excellent adsorption toward polysulfides, Qu et al. [182] also proposed a sandwich-structured carbon/cellulose nonwoven/poly(ethylene glycol)-blockpoly(propylene glycol)-blockpoly(ethylene glycol) (PEG-PPGPEG) gel polymer electrolyte (NCPCPE) via solution-casting method. The introduction of cellulose extremely improved the cycle life and rate capability by the following mechanisms: 1) the unique chemical structure of cellulose and the porous structure of nonwoven could facilitate the full filling of liquid electrolyte in the polymer to ameliorate ion conductivity; 2) the sulfophilic cellulose backbone could capture the dissolved polysulfides, which could get more opportunities to receive electrons from the current collector and then improve the utilization of active material; 3) cellulose backbone could promote uniform lithium-ion stripping/ plating to restrain the growth of dendrite. Summary: In recent years, the development of gel electrolyte has made great progress in both the actual electrochemical of the battery and the mechanism of operation. However, the decrease in mechanical strength caused by the increase in the amorphous region of the polymer is the potential hazard, leading to internal short circuits, especially as a separator. The swollen polymer also increases the lithium ion transport distance while blocking the polysulfide shuttle, thus reducing the number of lithium ion transfer. Since the reversible transformation of polysulfides trapped in the gel polymer is difficult to carry out, the utilization of the active material is also affected. Therefore, shortening the transmission channel of lithium ions or constructing more jumping sites still requires further efforts and researches

4.1.1. All state-solid polymer electrolytes Due to the absence of liquid electrolyte, the composition of all-state solid polymer electrolytes (ASSPE) is usually included a polymer with a high dielectric constant and a lithium salt with low lattice energy. Among of them, PEO-based ASSPE received extensive attention. Segmental motion and entanglement of ethylene oxide units in PEO are propitious to facilitate ion conduction. On the other side, PEO also has a strong solvating ability towards various lithium salts since the relatively high donicity of ethylene oxide units (EO, donor number = 22) and favorable configurational entropy term. However, the higher crystallinity of PEO at low temperature hinders ion conduction, causing poor electrochemical performance. Therefore, most of the researches are focused on obtaining the satisfied conductivity of PEO-based electrolytes in gradually approaching room temperature. Previous studies about the gel electrolytes, containing barely PEO/Li-Salt and PEO/ XnOm(X = Zr, Al, Si, Ti), have been a review in 4.1. Actually, they have the similar mechanism of action in the ASSPE: 1) the positive effects of ether bonds contained in PEO on polysulfide inhibition and lithium ion transport; 2) chemical bonding formed between the lone pair electrons on the oxygen atom in inorganic oxide and the lithium ion in lithium polysulfide. Similar to the preparation of PVDF/PMMA/MMT gel electrolyte, Zhang and his coworker [193] successfully prepared PEO/ MMT ASSPE as the electrolyte for lithium sulfur battery. The introduction of MMT not only enlarged the amorphous region of PEO but also could interact with oxygen in PEO (Lewis base) to weaken the interaction between oxygen and lithium ions by the Lewis acid site on the surface of MMT anion promoting ion transport. ASSPE delivered a reversible discharge capacity of 634 mAh g−1 after 100 cycles at 0.1 C under 60 °C. Zhang et al. [194] utilized the double effects of the cathode (PANI) and electrolyte (PEO-LiTFSI-MIL-53(Al)–CH3CN) to inhibit polysulfide shuttle. Chemical adsorption of amine groups in PANI cathode restrained polysulfide diffusion, simultaneously, the electronegativity of MIL-53(Al)-TFSI− effectively hindered polysulfide dissolution by the electrostatic interaction. Based on the above reasons, the cell obtained the highest discharge capacity of 640 mAh g−1 and still retained 558 mAh g−1 after 1000 cycles at 60 °C and 0.5 C. They indicated more lithium ions accumulated and infiltrated into the interior of the cathode in a further cycle. Finally, inactive sulfur on the internal surfaces could be reutilized. Nevertheless, the slow migration of lithium ions may result in incomplete electrochemical reaction, and weak interface compatibility between solid and solid also bring about the extension of the activation cycles. Zhu et al. [195] designed an interlayer membrane using PEO, super P, LiTFSI and MIL-53(Al) between the cathode and PASSE. Super P nanoparticles in electron concentration strongly attracted lithium ions and repelled TFSI-, thus formed a path for the ion transport and facilitated the dissociation of lithium salt. Moreover, gradient change formation of electrons and lithium ions, and construction of structural similarity between electrode and electrolyte were propitious to obviously improve the electrochemical performance and interfacial compatibility of the battery. Even

4. Solid-state electrolyte 4.1. All solid-state electrolyte All solid-state electrolyte (ASSE), by definition, which was composed of non-liquid components, such as a polymer (PEO) and inorganic materials (LISICON, NASICON, and perovskite), and was a valid alternative for developing lithium-secondary batteries with high safety and long cycle life [183–185]. When compared with the gel electrolyte and liquid electrolyte, the ASSE possesses more excellent physical and chemical performance containing: 1) outstanding thermal stability and nonflammability to improve the safety of the battery; 2) higher mechanical strength to inhibit the growth of lithium dendrites and ameliorate electrochemical stability; 3) as the physical barriers to restrain the transfer of polysulfides; 4) higher lithium ion transfer number to promote the kinetics of redox reactions. Different from the transport of lithium ion in liquid, which involves the vehicular motion of an entire solvation shell structure surrounding lithium ion, moving mechanism of lithium ion in solids is hopping or exchanging of the naked lithium cation [186–188]. However, the contact between the electrode and the solid electrolyte is different form the solid and liquid, in which the nondirectional penetration of the liquid electrolyte can accelerate electrode and electrolyting contact, and thus the smaller contact area between 888

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Fig. 9. (a) Preparation of HNT modified flexible electrolyte and mechanism of HNT addition for enhanced ionic conductivity; Stress–strain curves of the PEO + LiTFSI + HNT and PEO + LiTFSI electrolytes; Cycling performance of the battery at 25 °C and 0.1 C [198]. (b) Preparation of starch hosted electrolyte films and molecular structure characterization of the starch host; cycling performance of the starch hosted electrolyte based all-solid-state lithium sulfur battery [199]. (c) Schematic of the solid-state electrolyte framework; SEM images of cross-section microstructure and after H+ corroding, 9 M; coulombic efficiency, charge and discharge capacities of the battery [204]. (d) Schematic of the garnet bilayer framework after sintering, the inset is a photo of the punched bilayer disk before entering; schematic of the CNT coated bilayer garnet framework (SEM of the CNT coated porous layer in set) and hybrid solid-state bilayer LiSBs; Voltage profile of the hybrid bilayer LiSBs with a high sulfur mass loading of approximately 7.5 mg cm−2 at 0.2 mA cm−2 [205].

near 100% after 94 cycles at room temperature. This group also prepared the starch polymer electrolyte using grade starch [199]. At the low current rate (0.1 C), the battery exhibited superb electrochemical stability with averaged value of 864 ± 16 mAh g−1 and 93% retention after the 100 cycles, even if the current rate was increased (0.5 C), the discharge capacity still contained 562 ± 118 mAh g−1 at 0.5 C for 1000 cycles at room temperature (Fig. 9b).

if pure sulfur was used as the cathode material, the battery still exhibited the excellent discharge specific capacity with 792.8 mAh g−1 after 50 cycles at 0.5 C and 80 °C. The formation of ionic solubility gradient could also be obtained by the opposite electrical properties in inside and outside of HNT [196]. Lithium ions were absorbed on the negatively charged outer tube silica surface, on the contrary, TFSI− may be accommodated on the positively charged inner tube aluminol surface. The Lewis acid-base interactions among HNT, LiTFSI, and PEO effectively shortened the free lithium ion transfer distance and lowered ionic coupling, finally paved the way for lithium ion transport. In addition, the interactions have proved to enhance both electrochemical and mechanical stability of electrolyte (Fig. 9a). In addition to the PEO, natural materials, such as sucrose and starch, which composed of carbon and oxygen and contained –C–O–C– a functional group similar to the PEO, also can provide active sites for reversible lithium ion conduction for lithium battery solid electrolytes. Liu’s group [197,198] developed a B-sucrose/PEO/LiTFSI solid polymer. Adding PEO ameliorated the mechanical properties of electrolytes, and boron linkages in place of sucrose crystal formation decreased in crystallinity and reduced bulk resistance. Increasing ether sites and adding boron centers also significantly enhanced ion conduction properties. The Coulombic efficiency for each cycle remains

4.1.2. All solid-state inorganic electrolyte All solid-state inorganic electrolytes (ASSIE), including crystalline electrolyte(thio-LISICON, NASICON-type, and garent-type) and glassy electrolytes (Li2S-XmSn, X = P, Si and B), are good ion conductors and electron insulators, and have been successfully applied to lithium sulfur battery due to the high ion conductivity and transfer number, low conductance activation energy and elevated temperature resistance [200]. In addition, micro-thin ASSIE is able to provide more space for active material to achieve higher energy density. In the system of the crystalline super ion conductors, the lattice formed by the non-moving skeleton ions provides a channel for ion migration, and the migration ions constitute a sub-lattice with relatively low activation energy. To achieve high ion conductivity, the number of vacancies is much larger than that of movable ions [201]. Hence, ionic transport generally relies 889

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on the concentration and distribution of vacancy. With a high conductivity (> 1 mS cm−1), the stable Li1+xTi2−xAlx(PO4)3 in the air was prepared by Wang and coworker [202]. Contacting with lithium metal, the reduction of Ti4+ caused deformation of the octahedra and resulted in the anisotropic expansion of the unit cell, which may weaken the connection of crystals and grow to small cracks in the ceramic. Replacing Ti by Ge could get a proper and stable electrochemical window, and eliminate the reduction issue. The effect of LAGP modified separator on polysulfide was analyzed by Wu et al. [203]. LAGP not only ameliorated the wettability of the separator, but also decreased the pore diameter of the separator. Compared with PP separator, the LAGP modified separator showed the more satisfied initial discharge capacity (1128.2 mAh g−1) and reversible discharge capacity (770 mAh g−1) after 50 cycles. However, the high cost of Ge also restricted practical application. The lithium-stuffed garnet oxide (LLZO) with high ionic conductivity (1 × 10−3 S cm−1), chemical stability and wide electrochemical window also catch the eyes as a candidate for LiSBs. However, the large size grain formation during sintering process not only deteriorated the strength of the electrolyte, but also impaired the ion conductivity. Huang and his group [204] successfully introduced MgO into the Li6.4La3Zr1.4Ta0.6O12 to fabricate the high strength solid electrolyte, which could effectively push back the lithium dendrites. The MgO additive effectively promoted intergranular bonding and inhibited grain growth. Cubic garnet-type LLZTO-MgO ISSE showed the high fracture strength with 135 MPa, and the excellent capability with 685 mAhg−1 at 0.2 C after 200 cycles (Fig. 9c). Besides inhibiting lithium dendrites by high strength, the solid electrolytes should possess sufficient space to accommodate the 80% volume change of sulfur cathode. Fu et al. [205] successfully designed 3D bilayer garnet solid electrolyte by animating dense and porous tapes into a bilayer tape and then co-sintering to remove the organic binders and polymers. The porous could provide sufficient space for high loading of active material and intermediate, and also increase the number of reaction sites for sulfur and promote the rapid reaction. Simultaneously, the thin dense support layer efficiently restrained the formation of lithium dendrites. Furthermore, the unboundedness between dense and porous layers provides a continuous transport path for lithium ions. Even if the sulfur mass loaded up to 7.5 mg/cm2, the cell still contained a total cell energy density with 248.2 Wh kg−1 (Fig. 9d). Due to the sulfur ion radius larger than oxygen ion, substituting of sulfur to oxygen in the oxide solid electrolyte may provide the satisfactory electrochemical performance by expanding the ion transportation channel. Nagao et al. and Suzuki et al. [206–208] utilized the mechanical milling to fabricate the Li3.25Ge1−yPyS4 as a solid electrolyte. Combining composite cathode, the cells exhibited relatively high reversibility. Suzuki noted sulfur was a good binder and provided a better solid–solid interface between each component in the composite cathode (S/AB/thio-LISICON) by the high temperature milling. For the glassy solid electrolyte, although channels diameter for ion is not as uniform as the crystal structure due to the disordered threedimensional structure, short- and medium-range order still exists in the amorphous structure. Thence, similar to the crystal structure, ions at local sites are excited to neighbor vacancy and then collectively diffuse on a macroscopic scale. The Li2S-P2S5, which contains outstanding compatibility with the electrode and good flexibility by amorphous structure, has been mainly studied. Nagao et al. [209] found that 80Li2S–20P2S5 SE had the flexibility to accommodate the periodic volume variation of lithium and formed homogeneous lithium deposition on the surface of the solid electrolyte, finally inhibited the growth of lithium dendritic. However, the grain boundaries and inhomogeneous composition of the compressed powder sample can engender the high activation energies in the process for the ion transportation, thence a dense solid electrolyte may be a key for making a high performance solid state battery. Yamada et al. [210] prepared the 75Li2S-25P2S5 solid electrolyte by high energy ball milling. Li/SE/Li still contain the lower impedance (< 50 Ω) after seven days of storage, specially, the Rct

of solid electrolyte after cycling was much smaller than that of liquid electrolyte because the transport path of lithium ions on the SEI film had been changed. It was noted that the full specific capacity of sulfur (1600 mAh g−1) at 0.05 C was achieved and Coulombic efficiency was 100%. In addition, metal sulfide doping in solid electrolytes not only created vacancy defects but also broadened the channels of lithium transmission. Xu and co-workers [211] obtained Li7P2.9S10.85Mo0.01 by doping the MoS2 into Li2S-P2S5, which exhibited the high ionic conductivity of 4.8 mS cm−1 at room temperature and enlarged the electrochemical window up to 5 V. The Li7P2.9S10.85Mo0.01 exhibited the faster reaction kinetics and lower resistance than Li7P3S11 due to the appearance of more defects and wide ion transmission channel by the substitution of P with Mo atoms. LiSBs cells with Li7P2.9S10.85Mo0.01 electrolyte showed a high discharge capacity (1020 mAh g−1), which was higher than that of Li7P3S11 based electrolyte (775 mAh g−1). Lithium borohydride (LiBH4), which exhibits the low grainboundary resistance, chemical stable with lithium metal, good mechanical strength, and flexibility, has already been used as solid electrolyte [212]. Compared to aforementioned super ion conductors, borohydrides may have the stabilized hexagonal polymorph due to the partial substitution of BH4 by halogen atoms (LiBH4-LiX, X = Cl, Br, I), decreasing the operating temperature [213,214]. To examine the effect of Cl- in crystal lattice on the performance as the solid-state electrolyte in LiSBs, the doping of LiCl into LiBH4 was researched by Unemoto et al. [215]. A highly deformable feature of LiBH4-LiCl electrolyte was favorable for establishment of lithium ion transport paths across the interface between the cathode and electrolyte layers, in addition, the existence of Cl atom has no effect on the reaction of LiBH4 and S. However, the possibility of H2S formation may be a safety issue. Supti et al. [216] prepared nano-confined LiBH4 in mesoporous silica as solid electrolytes to improve the BH4- rotational diffusivity and lithium mobility at room temperature. The high ion transport number (0.96) of LiBH4 close to a purely cationic conductor. Another borohydride (LiM (BH4)3Cl, M = La, Ce, and Gd) with good ionic conductivities exceeding 0.1 mS cm−1 at room temperature and around 1 mS cm−1 at 70 °C was analyzed by Nguyen [217]. Combining ionic conductivity, polarization stability, and electrochemical window, LiCe(BH4)3Cl was the most electrochemically stable and suitable as a solid electrolyte for LiSBs, and the first discharge reached 1186 mAh g−1 at 70 °C. 4.2. Composite electrolytes The composite electrolytes, which mainly introduce liquid, polymer and another electrolyte into the inorganic solid, aim to ameliorate the LiSBs electrochemical performances. It is worth to be noted that solid electrolytes are mostly used in the form of composite solid electrolytes (polymer, ceramic, liquid electrolyte) in the LiSBs. Firstly, the activation energy for lithium ions to pass through the solid–solid interface is much larger than that of the solid–liquid interface. The introduction of liquid electrolytes can build a bridge for ion transport between solid and solid, reducing the interface impedance and speeding up the reaction kinetics. Secondly, as a buffer layer, polymer solid electrolyte prevents the crack on the inorganic solid electrolytes causing by electrode volume change. Finally, solid electrolytes serve as separator or shield to restrain the polysulfide diffusion and lithium dendrites growth in the electrochemical reaction. It is noted that the dissolution of polysulfide plays a positive role in efficiently using the non-conductive sulfur cathode. Wang and his group [218] indicated the soluble polysulfide not only were critical for activing Li2S (oxidize the solid Li2S through a direct chemical reaction) but also played the role in redox mediators. So they designed the composite electrolyte with the no self-discharge, containing two kinds organic electrolytes (providing the stable interfacial impedance and dissolving polysulfide) and a LATP solid electrolyte (transferring lithium ion and restraining polysulfide shuttle effect) (Fig. 10a). Hence, the polysulfides intermediates can be constrained in the cathode 890

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Fig. 10. (a) Schematic illustration of activation process of Li2S cathode over initial charge and the architecture for LiSBs composed of Cu foil/Li metal/organic electrolyte/ceramic separator−1 (LATP)/organic electrolyte−2/Li2S cathode/Super P carbon/Ti foil (+) from top to bottom; representative galvanostatic charge restdischarge curves as a function of time; the corresponding galvanostatic profiles as a function of specific capacity [218]. (b) Facile Li-ion transfer interlayer to accommodate the roughness at the Li anode/LiSICON membrane interface, voltage profiles of the Li || LiSICON || polysulfide batteries prepared with the MEAs, schematic of a hybrid Li IILYZPII Li2S6 cell with a Li-metal anode, liquid/LYZP hybrid electrolyte, and a dissolved lithium-polysulfide cathode, discharge capacities as a function of cycle number of the hybrid Li IILYZPII Li2S6 cell and the Li IICelgardII Li2S6 cell without the LiNO3 additive [219]. (c) Schematic diagram of the interface morphology between the garnet electrolyte and Li metal in LiSBs; the electrochemical impedance spectrum (EIS) test at the frequency of 0.1e1 MHz for the IL-garnet/Li interface evaluation [220]. (d) Sketch of the cell with a bilayer electrolyte configuration (3 vol% Al2O3- and LICGC-based CPEs); Li plating and stripping test with a current density of 0.1 mA cm−2 holding for 2 h; galvanostatic cycling performance of the bilayer cell at 70 °C [224].

sulfur, but also decreases the interfacial resistance between LAGP ceramics and electrodes. In addition, with the increase of FDE content, the ion transport number increased and ion conductivity decreased of FDE–LAGP hybrid electrolyte. With the 80% content of FDE, the cell exhibited outstanding cycling stability within the initial discharge capacity of 915.0 mAh g−1 and reversible capacity of 668 mAh g−1 after 1200 cycles. Although the interface resistance between the solid electrolyte and solid electrodes has been improved by introducing the liquid electrolyte, the crack of electrolyte has not been mitigated due to the press of electrode expansion. Blanga et al. [222] designed a novel polymer-in ceramic Li10+xIxSnP2S12-P(EO)-LiI composite film with high ion-conduction properties via properties electrophoretic deposition. In this system, PEO/LiI electrolyte filled the spatial gaps between the ceramic particles and offseted the deformation pressure of cathode, finally reduced the possibility of lithium dendrite growing in the gap. In addition, PEO also promoted the dissociation of lithium salt and formation of amorphous sulfide. Tao and his coworkers [223] synthesized solid-state composite electrolyte consisting of Li7La3Zr2O12 (LLZO) and PEO, which can operate at human body temperature. The ratio of LLZO/PEO has an obvious influence on the ion conductivity, and more LLZO (> 20%) in the composite can reduce the conductivity because of the percolation behavior. On the contrary, with the decreased of LLZO content (< 15%), the ion conductivity increased due to the crystallinity degree reduction of PEO, hence the dispersed LLZO nanoparticles act as the interfacial stabilizer. Based on the carbon foam/S/LLZO composite cathode, the capacity retention of cell can reach as high as 98.7% after

apartment to increase L2S utilization. Yu and his coworkers [219] fabricated a soft inter layer between the lithium anode and solid electrolyte by inducing the DME/DOL mixture with 1 M LiCF3SO3 and 0.1 M LiNO3 into the Li1.3Al0.3Ti1.7(PO4)3. The PP film soaking with liquid electrolyte not only alleviated the roughness between the electrolyte and the surface of the lithium metal but also built a facile ionicpath at the metal/ceramic interface. All the same, they also found LATP may be instable under the liquid polysulfide environment, so they tried to substitute NaSICON-type for LATP. In this system, LYTP mainly played three roles, including ion conductor, separator and barrier of polysulfides shuttle. Notably, owing to the diffusion inhibition of the dissolved polysulfide by LYZP membrane, the LiNO3 additive may be excluded from electrolyte (Fig. 10b). Wu et al. [201] prepared the modified PP film separator by coating LAGP powder on the surface. Due to the decrease of pores size and an increase of wettability of PP, the modified separator provided the satisfactory inhibition of polysulfide and excellent initial discharge capacity of 1128.2 mAh g−1. Insulating gaps between the solid electrolyte and lithium anode hinder lithium ion transportation and decrease the utilization of sulfur. To eliminate the above-mentioned dilemmas, the flow able and seamless interfacial thin liquid electrolyte layers were used for ameliorating the stability of electrochemical and physical in solid state LiSBs during the cycling (Fig. 10c) [220]. The positive role of fluorine-containing electrolytes have explained in LiSBs (in 3.1), Gu and his group [221] analyzed the effect of FDE content for the long life LiSBs. FDE electrolyte containing fluorinated structure not only effectively avoided the loss of active 891

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In the system of LiSBs, the design of electrolyte is more complicated than traditional lithium-ion batteries due to the unique of active material reaction. Firstly, the electrolyte should form a stable interface with the cathode and anode simultaneously, facilitating the reaction. Secondly, the electrolyte has a fast transport channel for lithium ion transfer while inhibiting the transport of polysulfide anions. Thirdly, the composition of the electrolyte cannot react malignantly with the active material or lithium metal. The strong nucleophilic reaction between liquid electrolyte and sulfur, low ion transfer number, high solubility of ethers for polysulfide, weak strength of gel electrolyte and high interface resistance of solid electrolyte still are barriers for high energy density in LiSBs. Hence, some efforts have been implemented, for example, the adding additive increases liquid electrolyte viscosity and improve interface stability, doping inorganic particles ameliorate ionic conductivity of gel electrolyte and designing reasonable composite electrolyte establish stable interface. To some extent, the cycle stability, active material utilization and cycle life of cells have been improved. However, in the liquid electrolyte, the contradiction between the loss of the active substance caused by the shuttle, the electrode polarization and the utilization of the active substance by the activated reduction product is still unavoidable. In all-solid electrolytes, polysulfide can be trapped on the positive side by solid electrolytes, but barren reaction condition (high interface impedance) limits polysulfide multi-step reactions, specifically, most solid electrolytes exhibit the excellent initial specific capacity and rapidly decline in a further cycle. To sum up, the gel and solid electrolytes may be a more suitable candidate electrolyte for the next generation of high performance LiSBs. Standing in the market position, in addition to the stable electrochemical performance, safety and cost are also crucial. Flammability, pyrolysis and leakage of liquid electrolyte and short circuit of gel electrolyte are potential safety hazards. For solid electrolytes and composite electrolytes, the expensive raw materials and harsh preparation conditions of inorganic ceramics are lead to the high cost. Notably, when the battery is expanded from the laboratory to the production line, charge/ion transfer, interface stability and inherent

90 cycles. In order to further improve the ion conductivity of solid electrolyte, Judez and his coworkers [224] obtained flexible and freestanding bilayers CPEs with LiCGC and Al2O3 fillers by the classic hotpressing technique. Although the presence of LiCGC fillers caused the unstable interface between lithium metal and electrolyte, Al2O3-based layer could significantly improve the lithium anode/electrolyte interfacial properties and obtain ameliorative cycling ability with a high CE (> 99%) after 50 cycles. Furthermore, LICGC-based layer with NASICON crystal structure possessed high ionic conductivity (∼0.1 mS cm−1) at room temperature and improved the utilization rate of sulfur (Fig. 10d). Summary: In the foregoing section, the system of LiSBs based on polymer and inorganic solid electrolytes achieved the great progress on the electrochemical performances and the composite electrolytes have been proved as efficient design to fully exert the advantages of individual components. However, there are some issues still exist: 1) the strength requirement of cathode and anode for the solid electrolyte may be contradictory; 2) the interface resistance still hinders the operation of the battery under the long cycle and high current density; 3) the preparation and application conditions of sulfur-containing inorganic electrolytes hinder the commercialization process. 5. Conclusion and prospection The research of LiSBs with the high energy density and specific capacity, containing the development of positive electrode materials, the design of electrolytes and protection of negative electrodes, achieved the great progress in the past several years. Notably, the electrolyte is an indispensable component of LiSBs, and wide research has been organized into several parts in this review. The physchemical properties, electrochemical performance, insufficient and prospect of three kinds of electrolytes have been summarized in the sections, respectively. In order to more intuitively display the respective characteristics of these electrolytes, the diagrams about the performance of different electrolytes are shown in Fig. 11.

Fig. 11. Advantages (green) and disadvantages (red) of different electrolytes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 892

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strength of the electrolyte might be remarkably altered. Furthermore, in addition to the inherent properties of electrolytes, commercialization of LiSBs also is decided by the synergy between electrolytes and electrodes. At the end of this review, we attempt to propose several potential solutions to address the aforementioned problems: 1) modifying polymer to increase the movable segments (grafting and doping) and establishing support layer on the gel surface to improve the strength (coating and compounding); 2) designing and establishing an intelligent lithium-ion selective high-speed transmission solid interface; 3) developing new inorganic ceramic materials (easy processing, good flexibility and low temperature operation) by ion exchange. Although some issues in LiSBs are still unknown or contradictory, further study and research will gradually reveal its mysterious veil.

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