Towards safe lithium‒sulfur batteries from liquid-state electrolyte to solid-state electrolyte

Abstract

Lithium–sulfur (Li‒S) battery has been considered as one of the most promising future batteries owing to the high theoretical energy density (2600 W·h·kg⁻¹) and the usage of the inexpensive active materials (elemental sulfur). The recent progress in fundamental research and engineering of the Li‒S battery, involved in electrode, electrolyte, membrane, binder, and current collector, has greatly promoted the performance of Li‒S batteries from the laboratory level to the approaching practical level. However, the safety concerns still deserve attention in the following application stage. This review focuses on the development of the electrolyte for Li‒S batteries from liquid state to solid state. Some problems and the corresponding solutions are emphasized, such as the soluble lithium polysulfides migration, ionic conductivity of electrolyte, the interface contact between electrolyte and electrode, and the reaction kinetics. Moreover, future perspectives of the safe and high-performance Li‒S batteries are also introduced.

Key words： lithium–sulfur battery; liquid electrolyte; polymer electrolyte; solid-state electrolyte; battery safety

Contents

Introduction

Liquid electrolyte for Li–S batteries

Flame retardant

Ionic liquid

Functional additives

The polymer electrolyte

Cathode design

Design of polymer electrolyte

Stabilizing lithium anode

Inorganic solid electrolytes

Cathode design

Carbon materials

Metal compounds

Ameliorating the interact between sulfur and inorganic solid electrolyte

Inorganic solid-state electrolyte

Modification

Polymer/inorganic solid composite electrolyte

Anode optimization

Summary and future perspectives

Acknowledgements

References

1 Introduction

The growth of global energy demand is driving the development of renewable energy and energy storage technologies [1]. Sustainable energy is often intermittent and dispersive, which requires the stable, high-performance and reliable energy storage systems for better application. Energy storage technologies set up a bridge between the sustainable energy conversion and employment [2], and the high-performance energy storage systems with high-energy density and long-life are urgently required [3‒4]. Rechargeable batteries as energy storage devices are facing unprecedented opportunities for rapid development [5].

Among the existing energy storage systems in Fig.1(a), lithium-ion batteries (LIBs) have become the popular star and brought revolutionary changes in human society, since Sony has commercialized the LIBs using LiCoO₂ as cathode and petroleum coke as anode in 1990 [6–8], and are widely used in portable electronic devices and electric vehicles owing to their light weight, high open-circuit voltage, large capacity and no memory effect [9]. The energy density of LIBs has been reported to reach ~450 W·h·kg⁻¹ in the anode-free system [10], whereas the average value of commercial LIBs is about 300 W·h·kg⁻¹, which is hard to achieve a great enhancement due to the limited theoretic capacity of the employed active materials [11‒12]. Meanwhile, the cost and eco-friendliness of LIBs also impede their development (Fig.1(b)) [13‒14]. Thus, new battery systems with high-energy density are expected to solve these existing dilemmas.

Fig.1 (a) The energy densities of several energy storage systems. (b) The ranking of properties of LIBs.

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Among the various high-energy-density battery systems in theory, lithium–sulfur (Li–S) battery has attracted widespread attention due to its high theoretical energy density (2600 W·h·kg⁻¹) with high specific capacity cathode (S, 1675 mA·h·g⁻¹) and anode (Li, 3860 mA·h·g⁻¹) [15]. Meanwhile, the active material S, as one of the abundant elements in earth, endows the Li–S battery low cost and environment friendly. As shown in Fig.2 [16–23], the investigation on Li–S battery has started from the 1960s [16‒17], accompanied by the boomed research on the Li metal batteries, but the advance is slow in the following 30 years owing to the issues of mental Li and poor cycle performance. The S‒C composites with low sulfur content [18] and S@pPAN [19] have demonstrated the feasible cycle in ester-based electrolyte in 2002. The shuttle effect of sulfur cathode is found [20], and LiNO₃ as the additives [21] is proved to inhibit it in ether-based electrolyte. The breakthrough in the great promotion of Li–S battery has achieved by Prof. Nazar in 2009 [22], which stirs up the research enthusiasm all over the world. Sulfur encapsulated in microporous carbon has shown the stable performance in long-term cycling [23], which further verifies the availability of Li–S battery in ether electrolyte. In 2010s, massive research on Li–S battery is spread out, including the optimization on cathode, separator, electrolyte and anode. On cathode side, various methods have adopted in preparing high-performance and stable cathode with functional binders and conductive agents [24‒25]. Numerous host materials ranged from inorganics to organics with the adjustable constitutes and structures [26–28] have applied in enhancing the performance of Li–S battery, which are proven to effectively confine the free diffusion and facilitate the electrochemical conversion of lithium polysulfides (LiPSs), and attract the main attention of the researchers. Designed electrode structures, such as three-dimensional (3D) structure [29], laminated construction [30‒31], and hierarchical structure [32‒33] could also be available to inhibit the volume effect and construct the stable electrode, especially the high sulfur loading electrode. Meanwhile, the mechanism in the electrochemical processes, involving the active materials, host materials and other components, is carefully studied through the advanced and systemic technologies, including the in-situ and ex-situ characterization means, such as in-situ XRD, in-situ SEM, in-situ TEM, in-situ FTIR, in-situ UV-Vis, NMR, synchrotron radiation, and cryoelectron microscopy [34‒35]. Theoretical calculation, such as density functional theory (DFT) [36] and molecular dynamics simulation [37] is also applied to explore the electrochemical nature, and guide the materials design. Massive efforts have also been exerted on separator by modifying the commercial separator or fabricating the novel membrane to endow it more functions from a sole physical barrier to inhibiting the free diffusion of soluble LiPSs, improving the electrochemical conversion of LiPSs and the even deposition of Li anode [38]. The appropriate electrolyte for Li–S battery is constantly optimized including the composition and proportion in liquid electrolyte, gel electrolyte and solid electrolyte [39]. On anode side, surface engineering on Li metal could enhance the interfacial stability to some extent [40]. Meanwhile, the deposition skeletons have been proved to tailor the growth behavior of Li and reduce the anode failure [41]. Besides, novel anodes are attempted to replace metallic Li, such as alloy anodes, lithiated carbon and silicon [42]. In this stage, the practical capacity of Li–S battery could range from 3 to 39 A·h, corresponding to the obvious promotion in energy density from > 300 to ~600 W·h·kg⁻¹, and endow its application in energy storage power stations and unmanned aerial vehicles [43]. In the next stage (2020s), the emphasis is mainly focused on how to motivate the practical process of Li–S battery. Some essential parameters near the practical level, such as high sulfur loading (> 4.0 mg·cm⁻²), low electrolyte usage (3‒3.5 μL·mg⁻¹), appropriate negative/positive ratio, and high stability and safety, are particularly considered in the related investigations [44]. High gravimetric and volumetric energy densities of Li–S battery under long-term cycling and high depth of discharge (DOD) are anticipated to achieve to compete with the next generation high-energy secondary battery systems.

Fig.2 A brief roadmap containing the important development of the Li–S battery.

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In principle, the total electrochemical reaction of the Li–S battery is about the mutual transformation between sulfur and lithium sulfides with the participation of Li ions. However, the specific reaction paths are obvious different in different liquid electrolyte systems. In the ether electrolyte (Fig.3(a)‒Fig.3(c)), high-order LiPSs are generated accompanied by the beginning of the reduction reaction, corresponding to the high potential platform around 2.3 V (vs. Li/Li⁺), then long-chain LiPSs gradually convert into short-chain LiPSs, and final solid Li₂S₂/Li₂S, related to the low potential platform at ~2.1 V (vs. Li/Li⁺) [45–47]. The oxidation procedure involves the reverse conversion from the solid state to the liquid state, even to the solid S₈ [47]. In the ester electrolyte system, solid transformation tends to occur in the sulfur penetrated into the microporous hosts (Fig.3(d)) [48], small molecule sulfur (Fig.3(e)) [49] and sulfide polyacrylonitrile (Fig.3(f)) [50], along with the typical single reduction platform below 2.0 V (vs. Li/Li⁺) (Fig.3(g)) and the formation of solid Li₂S during the electrochemical reaction (Fig.3(h)) [51]. In the mixed electrolyte employing ether/ester co-solvent, the redox reactions are proven to be similar with the sulfur cathode in the ester electrolyte. The dissolution‒deposition mechanism has changed into solid conversion mechanism by in-situ forming robust solid electrolyte interface (SEI) layer on sulfur surface, and further avoiding the dissolution and diffusion of LiPSs with one discharge platform (Fig.3(i)‒Fig.3(k)) [52].

Fig.3 The reaction mechanism of the Li–S battery in different liquid electrolytes. (a) Schematic of the reactions in Li–S cell. Reproduced with permission from Ref. [45]. (b) The typical discharge and charge profile of the Li–S cell. Reproduced with permission from Ref. [46]. (c) Sulfur K-edge XANES upon charge and discharge. Reproduced with permission from Ref. [47]. (d) Schematic illustration of the reaction mechanism of the nanoporous carbon–sulfur composite in carbonate electrolyte. Reproduced with permission from Ref. [48]. (e) Schematic of the lithiation/delithiation processes of S chains in a Li–S battery. Reproduced with permission from Ref. [49]. (f) Overall reaction of Li/SPAN cell. Reproduced from Ref. [50]. (g)(h) A voltage profiles of cells at C/20 and the corresponding ex-situ XPS spectra of S 2p core-level at the final discharged state. Reproduced from Ref. [51]. (i) A schematic illustration of the charge/discharge mechanism of the sulfur cathode in the carbonate/ether co-solvent electrolyte with the corresponding (j) charge/discharge profiles and (k) TEM image of the cycled cathode. Reproduced with permission from Ref. [52].

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The matched sulfur-containing active materials in the ether electrolyte are various, which could range from solid to liquid, involved in elemental sulfur, lithium sulfide, liquid LiPSs and organic sulfur with high or low sulfur loadings. The soluble LiPSs would be generated during the redox reactions, which are essential to guarantee the normal electrochemical conversion, but could lead to their free diffusion, accompanied by the severe shuttle effect, low active materials utilization and serious anode corrosion [53]. Therefore, effectively anchoring the migration and facilitating the conversion of LiPSs are vital to enhance the electrochemical performance of Li–S battery. The choice on the active materials for sulfur cathode in ester electrolyte is often restricted, which are mainly concentrated on the permeated sulfur in micropores with the limited sulfur content (< 60%) and sulfide polyacrylonitrile [54]. The crystalline sulfur is generally undesired to be existed to avoid the adverse reaction with the electrolyte [55].

2 Liquid electrolyte for Li–S batteries

Liquid electrolyte is generally made up of high-purity organic solvent, lithium salts, and functional additives in a certain proportion. Lithium salts could provide the required Li ions and exert certain influence on the electrode surface, which could be selected in LiTFSI, LiBOB, LiPF₆, LiBF₄, LiFSI, LiClO₄, LiTf, LiDFOB, LiAsF₆, LiPF₂O₂, and LiNO₃ (Tab.1), according to the applicable battery systems [56–69]. Their variable concentrations in different solvents would endow the obvious discrepancy in the ion conductivity. LiTFSI is the frequently-used lithium salt with good stability and high ion conductivity for Li–S batteries. Meanwhile, binary-salt and multi-salt systems could be adopted to meet more demands [56].

Tab.1 The employed lithium salts in Li–S batteries [56–69]

Chemical name (Abbreviation)	σi/(mS·cm−1)	tmic/°C	Solvent	c/(mol·L−1)
Lithium bistrifluoromethanesulfonimide (LiTFSI)	9.67	25	DOL/DME	1.0
	5.1	25	PC	1.0
	9.0	25	EC/DMC	1.0
Lithium trifluoromethanesulfonate (LiTf)	~3.3	25	DOL/TEGDME	1.5
	2.22	20	EC/PC	1.0
	7.41	20	EC/DME	1.0
Lithium bis(fluorosulfonyl)imide (LiFSI)	11.99	25	DOL/DME	1.0
Lithium hexafluorophosphate (LiPF6)	10.7	25	EC/DMC	1.0
	5.8	25	PC	1.0
Lithium perchlorate (LiClO4)	8.4	25	EC/DMC	1.0
Lithium tetrafluoroborate (LiBF4)	4.9	25	EC/DMC	1.0
Lithium hexafluorarsenate (LiAsF6)	14.52	20	DME/EC	1.0
	5.94	20	EC/PC	1.0
Lithium dioxalate borate (LiBOB)	4.14	25	PC	0.5
	8.9	25	DME	0.5
Lithium difluoroxalate borate (LiDFOB)	‒	‒	DOL/DME	‒
Lithium difluorophosphate (LiPF2O2)	‒	‒	DOL/DME	‒
Lithium nitrate (LiNO3)	~5.5	30	Tetraglyme/DMSO	1.0

Notes: σi, ion conductivity; tmic, temperature at which the ion conductivity is measured; c, concentration.

Solvent, including aqueous phase and non-aqueous phase, is employed to dissolve lithium salts, which could profoundly impact the Li⁺ conduction behavior by the solvation and desolvation with Li⁺. Ethers, sulfones and esters are common organic solvents in liquid-state Li–S batteries, with single solvent or multicomponent mixed solvents to meet the requirements of physical and chemical properties. The LiPSs have been proven well dissolved in the ether solvent, especially in chain ethers, such as dimethyl ether, 1,2-dimethoxyethane (DME), and tetraethylene glycol dimethyl ether (TEGDME), which is crucial to the subsequent redox reactions. Cyclic ethers could aid to stabilize the Li anode by regulating the SEI layer. Ester-based solvents are also demonstrated to effectively fabricate stable Li–S batteries with the particular active materials and the typical solid-state reaction. However, the melting points and boiling points of the common ester and ether solvents listed in Tab.2 are generally lower than 250 °C, which could possess some potential safety hazards in practical application with the flammable property [70–73]. Thus, it is very important to improve the thermal stability of electrolyte.

Tab.2 Melting points and boiling points of ester and ether electrolytes [70–73]

Electrolyte	Solvent	tm/°C	tb/°C
Ester	Ethylene carbonate (EC)	36.4	238
	Propylene carbonate (PC)	−49	242
	Butene carbonate (BC)	−53	240
	Dimethyl carbonate (DMC)	2‒4	90
	Diethyl carbonate (DEC)	−74	127
	Ethyl methyl carbonate (EMC)	−55	107
	Methylpropyl carbonate (MPC)	−43	130
	γ-Butyrolactone (BL)	−43.5	204
	Vinylene carbonate (VC)	~20	165
	Methyl formate (MF)	−99	31.75
	Ethyl formate (EF)	−79.6	54.3
	Methyl acetate (MA)	−98.1	56.3
	Ethyl acetate (EA)	−84	77.1
	Ethyl propionate (EP)	−73.8	99
	Ethyl butyrate (EB)	−98	121.6
	Ethylene sulfite (ES)	−11	159
	Propane sultone (PS)	−14	54
	Dimethyl sulfite (DMS)	−141	126
	Diethyl sulfite (DES)	−112	159
Ether	1,3-Dioxolane (DOL)	−95	74
	1,2-Dimethoxyethane (DME)	−58	85
	Tetraethylene glycol dimethyl ether (TEGDME)	−45	216
	Polyethylene glycol dimethyl ether (PEGDME)	55	84.5
	Diethylene glycol dimethyl ether	−64	160
	Triethylene glycol dimethyl	−44	249
	Tetrahydrofuran (THF)	−108.5	66
	2-methyltetrahydrofuran (2Me-THF)	−137.2	79.9
	Tetrahydropyrane (THP)	−45.2	87.9
Others	Acetonitrile (AN)	−48.8	81.6
	Dimethylsulfoxide (DMSO)	18	189
	Sulfolane (SL)	28.45	287.3
	Acetone	−94.7	56.3
	N,N-Dimethylformamide (DMF)	−60.4	153

Notes: tm, melting point temperature; tb, boiling point temperature.

Meanwhile, the instable lithium anode in liquid-state electrolyte is still needed to be solved [74]. Lithium dendrites are easy to generate during the deposition and growth process on the surface of lithium anode, which could pass through the separator and contact with the cathode, and lead to short circuit and potential safety hazards. In addition, the uneven deposition of lithium may result in “dead lithium” with high chemical reaction activity, and aggravate the consumption of electrolyte [75], which could reduce the cycle life of Li–S batteries, and put the battery at risk in the charge/discharge processes. Moreover, metal lithium anode also faces the danger of corrosion by the freely diffused LiPS, accompanied by the by-products and the further damage on the electrochemical performance of the Li–S battery [76].

2.1 Flame retardant

Li–S battery with high energy density has been widely studied and developed to achieve the practical application, and its safety has become an important prerequisite for the further commercialization. The current organic electrolyte for Li–S batteries is prone to volatilize into gas and accumulate, even in explosion and combustion as the working temperature increases. Introducing some additives to the electrolyte solution could make the combustible electrolyte difficult to burn or nonflammable, reduce the heat release and self-heat release rate of the battery, and improve the thermal stability of the electrolyte, so as to avoid combustion or explosion of the battery under overheating conditions. This kind of additives is known as flame retardant. When the battery reaches the critical temperature, the integrated flame retardant will be released to stop the combustion of the battery in a short time. In addition, the utilization of integrated flame retardant would not reduce the performance of the battery [77]. Therefore, adding a certain amount of flame retardant to electrolyte is one of the important ways to improve the safety performance of battery.

At present, most of the flame retardant additives for Li–S battery are organic phosphates and fluorides [78]. Phosphorus flame retardants have certain mutual solubility with non-aqueous electrolyte, but endow high viscosity, which could reduce the conductivity of electrolyte and poor electrochemical stability. Fluoride flame retardants possess good stability, high lightning and dielectric constant, corresponding to the certain flame retardant effect and the optimization on the properties of SEI film with the improved compatibility between electrolyte and negative electrode materials [79]. Some fluoride flame retardants have good high temperature stability, low viscosity, low melting point and good low temperature performance.

Yang and co-workers have reported triethyl phosphate as flame retardant in liquid-state Li–S battery (Fig.4(a)) [80]. Flame retardant electrolyte could improve the oxygen index of electrolyte, reduce the flammability of electrolyte, have little adverse effect on the specific discharge capacity of battery, and improve the safety performance of battery. When the battery is in thermal runaway, phosphorus containing molecules are thermally decomposed to produce phosphorus containing free radicals, which could remove hydrogen and hydrogen oxygen active free radicals produced by side reactions, so as to reduce the risk of fire and explosion. Wang and co-workers used tris(2,2,2-trifluoroethyl) phosphite (TTFP) as flame retardant electrolyte. Adding TTFP to the electrolyte could help shorten the combustion time of the battery (Fig.4(b)) [81]. The optimum addition amount of TTFP is explained by self-extinguishing time (Fig.4(c)). Addition amount of flame retardant electrolyte in liquid-state Li–S battery influences the thermal stability of liquid-state Li–S battery (Fig.4(d)). Yu and co-workers have demonstrated that the high concentration of fluoroethylene carbonate (FEC) is conducive to improve the thermal stability of liquid-state Li–S battery [82]. Yang et al. used triethyl phosphate (TEP) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl (TTE) as flame retardant electrolyte (Fig.4(e)), which could effectively reduce the decomposition of electrolyte and alleviate the generation of lithium dendrite in the negative electrode at 60 °C [80].

Fig.4 (a) Schematic illustration of safe Li–S battery with intrinsic flame-retardant organic electrolyte. Reproduced with permission from Ref. [80]. (b) Flame tests with the effect of TTFP, and (c) SET and conductivity of the electrolytes with different TTFP contents. Reproduced with permission from Ref. [81]. (d) 6.5 mol·L⁻¹ LiTFSI/FEC and 1 mol·L⁻¹ LiTFSI/FEC added in liquid-state electrolyte. Reproduced with permission from Ref. [82]. (e) Schematic illustration of Li–S batteries using standard carbonate (STD) electrolyte and TEP/TTE (IFR) electrolyte. Reproduced with permission from Ref. [80].

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2.2 Ionic liquid

Hundreds of ionic liquids with different properties can be formed by the orderly combination of different anions and cations. Ionic liquid is tasteless and nonflammable with extremely low vapor pressure, accompanied by the reduced environmental pollution from volatilization. Ionic liquids have good solubility in both organic and inorganic substances, by properly adjusting cations and anions, which could make the reaction proceed under homogeneous conditions. Ionic liquid with wide operating temperature range (from −40 to 300 °C), good thermal and chemical stability, is easy to separate from other substances and could be recycled. Ionic liquid endows the advantages of wide electrochemical window, high ionic conductivity, nonvolatility and incombustibility. Ionic liquid could be used in electrolyte for Li–S battery to solve the disadvantages of traditional additives, such as poor thermal stability, poor high-pressure resistance and single function.

Ionic liquid has relatively limited anion selection, usually including fluorine coordination anions (PF₆⁻ and BF₄⁻) and sulfonamide anions (TFSA⁻ and FSA⁻), which have lower melting point and higher conductivity compared with traditional organic solvents. The melting point of ionic liquid is also related to the volume of cations. The introduction of larger or longer alkyl chains on cations could reduce the melting point, but would correspondingly increase the viscosity and sacrifice a certain ionic conductivity. Therefore, asymmetric cations with long alkyl chains are generally used to achieve a wide liquid range. In addition, ionic liquid based on inorganic cations (such as K⁺ and Li⁺) endows high melting points and is suitable for high-temperature applications.

Ionic liquid electrolyte could improve the safety, high-temperature and cycle performance of the battery. Ionic liquid plays an active role in inhibiting the shuttle process of polysulfides, facilitating the sufficient cathode reaction, homogenizing the Li deposition at the solid electrolyte interface and inhibiting the growth of lithium dendrites. Josef and co-workers have reported ionic liquids and their polymers in the electrolyte of Li–S battery to solve some existing problems [83]. Ionic liquid-based electrolyte may reduce the solubility of polysulfides, and its direct polymerization on the Li surface could protect Li anode. Watanabe’s group has reported [Li(G₄)_x][TFSA]/HFE (G₄: tetraglyme) in Li–S battery (Fig.5(a)) [84]. The ionic conductivity and the Li⁺ intercalation/deintercalation properties could be regulated by adjusting the proportion of G₄ in ionic liquid, which significantly affect the battery performance. Cai and co-workers have used 1-ethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide ([EMIM]TFSI) in Li–S battery (Fig.5(b)) [85]. Ionic liquid could act as the “soft contact” to adjust the Li⁺ transfer process of cathode, anode and electrolyte, and alleviate the damage to the battery during charging/discharging processes, which endows Li–S battery better performance and safety. Sun et al. [86] have also employed [EMIM]TFSI in Li–S battery to adjust the interface between cathode/electrolyte and anode/electrolyte (Fig.5(c) and Fig.5(d)). The Li–S battery using ionic liquid is more difficult to burn through the combustion experiment (Fig.5(e) and Fig.5(f)). Dai’s group has introduced methylpropylpyrrolidinium bis(trifluoromethane sulfonyl)imide ([MPPY][TFSI]) in the electrolyte of Li–S battery [87], which could strongly inhibit the polysulfide shuttle, and promote the lithium transport due to the synergistic electrostatic interaction between anchored cations and anions. Dai’s group has also proposed the strategy of mixing ionic liquid and sulfones, and took advantage of the synergistic effects to reduce viscosity, increase ionic conductivity, reduce polysulfide dissolution and improve safety [88]. The mixture of ionic liquid and sulfones also exhibits significantly different physicochemical properties, including thermal properties and crystallization behavior, contributing to the excellent cycle stability. This strategy provides a way to alleviate polysulfide dissolution and shuttle, and improve ionic conductivity. Compared with traditional organic solvents, non-volatile and non-flammable ionic liquids show the potential to develop safer electrolytes.

Fig.5 (a) [Li(G₄)_x][TFSA]/HFE electrolyte used in Li–S battery. Reproduced with permission from Ref. [84]. (b) Preparation route of polymer/LiTFSI/ionic liquid electrolyte. Reproduced with permission from Ref. [85]. (c) Li–S battery without ionic liquid, (d) Li–S battery with ionic liquid, (e) Li–S battery approach the flame without ionic liquid, and (f) Li–S battery approach the flame with ionic liquid. Reproduced with permission from Ref. [86].

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2.3 Functional additives

The stability of lithium anode also affects the safety of Li–S batteries. The electrolyte could be continuously consumed due to the continuous decomposition on the lithium anode in the repeated charge/discharge processes. The formed passivation film on the lithium surface may also make some lithium inactive and unable to participate in the reaction, corresponding to the poor cycle performance of lithium anode. Meanwhile, lithium dendrites may grow on the lithium anode during the cycle, which could potentially lead to short circuit, or even spontaneous combustion and explosion in serious cases [89]. Many ways have been tried to protect lithium anode, such as electrolyte additive, lithium surface modification, solid electrolyte, microstructure design, and alloy anode [90]. Among them, electrolyte additive is considered to be the simple, low cost and easy way to realize industrial application, which could have a decisive impact on the impedance, cycle stability, dendrite formation and coulomb efficiency of lithium metal, especially generating robust SEI film [91]. As a passive film, the SEI film is generally ion conduction and electrical insulation, which could hinder the further consumption of electrolyte and improve the service life of battery. The thermal runaway of the battery is firstly along with the thermal decomposition of SEI film, and then the electrolyte reacts with the exposed charged negative electrode. As the temperature further increases, the membrane shrinks and melts, resulting in a large-scale short circuit, the decomposition of electrolyte and positive electrode, and finally explosive combustion. Thus, people have intentionally or unintentionally added a variety of additives to the electrolyte, which could greatly change the performance of the battery, and become one of the most effective ways by rationally using electrolyte additives.

Difluoro ethylene carbonate (DFEC), vinyl carbonate (VC), 1,3-propanesulfonate (PS), prop-1-ene-1,3-sultone (PES), ethylene sulfite (ES), 1,3,2-dioxathiolane 2,2-dioxide (DTD) or LiNO₃ could decompose on the lithium surface in advance through their active properties to help form a stable SEI film [92]. The reaction of LiPS in the electrolyte of Li–S battery could be improved, accompanied by the reduced the content of LiPS and the alleviated reaction between LiPS and lithium anode, which could effectively protect the lithium anode. Fu’s group has reported benzenedithiols (BDTs) as an electrolyte additive (Fig.6(a)) [93]. Among the three isomers of BDT, 1,4-BDT has an effect on the reaction of Li–S battery by binding with sulfur atoms through oligomerization reaction, which could change the original redox sulfur path and inhibit the shuttle effect of LiPS. In addition, 1,4-BDT could facilitate the formation of the smooth and stable SEI in repeated electroplating and stripping, improve the reaction kinetics, reduce the overpotential of Li–S battery and significantly enhance the stability of the battery. Wang’s group has developed a new electrolyte with dimethyl trisulfide (DMT) [94]. DMTs could react with sulfur to generate dimethyl polysulfide (DMPS), change the cathode electrochemical reaction path and improve the reaction kinetics. Meanwhile, the formed DMPS could also cooperate with the lithium nitrate in the electrolyte to enhance the performance of lithium anode.

Fig.6 (a) Schematic illustration of safe Li‒S battery with 1,4-BDT additives. Reproduced with permission from Ref. [93]. (b) The schematic of reaction between SOCl₂ and lithium anode, and the cycled Li (c) without and (d) with the protection by SOCl₂. Reproduced with permission from Ref. [95]. (e) SEM image of lithium anode surface, (f) SEM image of lithium anode surface without protection, and (g) SEM image of lithium anode protection by TBAI. Reproduced with permission from Ref. [96].

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Sulfonyl chloride (SO₂Cl₂), as an electrolyte additive, aids to the construction of dense and LiCl-rich SEI films, which could accelerate the Li⁺ diffusion and inhibit the growth of lithium dendrites. Li et al. have employed thionyl chloride (SOCl₂) as electrolyte additive, which has been proved to enhance the interfacial stability of metal lithium anode and improve the long-term cycle performance of Li–S battery (Fig.6(b)‒Fig.6(d)) [95]. Adding moderate SOCl₂ (2 vol.%) into the traditional carbonate electrolyte could simultaneously solve the problems on both cathode and anode. The generated insoluble inorganic substances, such as LiCl and Li₂SO₃, could be deposited at the interface of the lithium anode to form a uniform and dense artificial SEI film.

Lithium iodide (LiI) could catalyze the polymerization of DOL on the surface of lithium metal in the LiTFSI/DOL-DME electrolyte [96], and also exists in the SEI film of lithium metal, which could effectively reduce the diffusion barrier and promote the transfer of Li⁺ to obtain the uniform lithium deposition morphology. Liao’s group have used tetrabutylammonium iodide (TBAI; TBA⁺ = [Bu₄N]⁺) as electrolyte additive in (CH₃CN)₂ LiTFSI/TTE [96]. TBAI is oxidized to I₂ during charging, and then I₂ is reduced and deposited on Li surface to form LiI and Li₂S_n. Soluble TBA⁺ could port these substances to the cathode for reoxidation. Electrochemical and SEM/EDS results show that once TBAI is activated, it would avoid the S deposition on the surface of Li negative electrode, but it is irreversibly consumed in the repeated cyclings (Fig.6(e)‒Fig.6(g)). Dai et al. have used cationic surfactant (hexadecyl trimethyl ammonium chloride (CTAC)) as electrolyte additives to inhibit the dendritic growth of Li by lithiophobic repulsion mechanisms [97]. The cationic surfactant molecules could aggregate around protuberances to form the nonpolar lithiophobic protective outer layer via electrostatic attraction, and drive lithium ions to deposit in the adjacent regions to produce dendrite-free uniform Li under large current and long service hour. The corresponding Li–S battery exhibits the excellent electrochemical performance even with high current density.

Therefore, the safety of Li–S batteries could be improved to some extent by using flame retardant, ionic liquid and electrolyte additives in liquid electrolyte. Meanwhile, polymer electrolyte and inorganic solid-state electrolyte also show huge potential in solving the security issues of Li–S batteries, which are the hot research topics in recent years.

3 The polymer electrolyte

Flame retardant additives could inhibit the combustion of electrolyte, but the commonly used small molecular organic phosphate compounds are unstable in the process of charging and discharging, and tend to decompose on the negative surface, resulting in the reduction of the battery performance. In order to solve the contradiction between safety and electrochemical performance caused by the addition of small molecular flame retardants, polymer electrolyte with high safety and high performance could effectively solve these problems. The study of polymer electrolyte could be traced back to 1973. Fenton et al. have found that ion-conductive electrolyte could be formed by combining polyethylene oxide (PEO) with alkali metal sodium salts [98]. In 1970s, Armand et al. have formally proposed the use of polymer electrolyte as solid-state electrolyte for LIB [99]. Since then, the polymer electrolyte has attracted extensive research, mainly on the exploration of ion transfer mechanism and new polymer electrolyte systems [100].

The polymer electrolyte consists of polymer matrix and lithium salts [101], corresponding to the dissolved lithium salts in the matrix to obtain a solid solution. The atoms in polymer electrolyte coordinate with Li⁺ to form a network complex after the dissolution of lithium salts [102]. The polymer possesses ionic conductivity with the movement of Li⁺ in the network [103]. In the case of PEO, amorphous and crystalline regions are concurrent (Fig.7(a)), and ion transport is commonly conducted in the amorphous phases of PEO. Ions or ion clusters could coordinate with the oxygen atoms on PEO chain through complexation. The PEO chains could vibrate under the action of the electric fields, and then the ions or ion clusters are in dissociation with the primary oxygen, and the intrachain or the interchain hopping occurs to form new coordination with other oxygens (Fig.7(b)). Ions seem to flow in the local region by the segmental motion of PEO [104]. Therefore, the polymer matrix for polymer electrolyte generally contains polar functional groups that could coordinate with Li⁺, such as −C−N, C=O, −O, −N, and −S. Generally speaking, the Li⁺ first coordinates with the polar group on the polymer chain, and then the Li⁺ departs from the coordination group, and bonds with other segments under the electric fields. In this way, the Li⁺ is constantly coordinated and departed from the coordination groups to realize the ion flow. The polymer substrates of the polymer electrolyte mainly include poly(vinylidene-fluoride) (PVDF) [105], poly(vinylidene-fluoride)-hexafluoropropylene copolymer (PVDF-HFP) [106], polymethyl methacrylate (PMMA) [107], poly(ethylene glycol) diacrylate (PEGDA) [108], and PEO [109] (Tab.3 [101,108,110–127]). The corresponding Li–S batteries could deliver relatively excellent electrochemical performance, especially in PVDF and PVDF‒HFP systems. However, fluorine in these polymers is easily replaced by sulfur and polysulfides to form thiol and unsaturated polymers [110]. Hence, more stable polymers such as PEO and its derivatives have been the researchers’ focus. PEO-based solid polymer electrolyte possesses good dissolving ability for lithium salts and mobility ability for PEO chains [128]. But their room temperature ion conductivity (10⁻⁶ to 10⁻⁹ S·cm⁻¹) is relatively low, which has been a great obstacle in the practical application of Li–S batteries [129].

Tab.3 The Li–S battery with gel polymer-based electrolyte [101,108,110–127]

Electrolyte	Co/(mA·h·g−1)	σi/(S·cm−1)	Ref.
PEO	‒	2.3×10−4	[111]
	1457 a)	6.89×10−4	[112]
	1350	1.11×10−4	[113]
	1210	9.5×10−6 b)	[114]
		1.1×10−4 c)
	562	1.69×10−4	[115]
	833	4.7×10−4	[116]
PVDF	1160	~10−4	[117]
	1383.1	6.72×10−4	[118]
	843	7.08×10−4	[119]
	1245.9	1.45×10−4	[101]
PMMA/PVDF	486	0.9×10−3 d)	[120]
PVDF-HFP	1029	1.1×10−3	[121]
	895	1.3×10−3	[122]
	543	9.64×10−4	[110]
	704.5	1.1×10−3	[123]
	1200	2.27×10−3	[124]
PEGDE-polyethylenimine (PEI)	720	0.75×10−3 e)	[125]
PEGDA-P(BA-co-[EVIm]TFSI)	1179	5.4×10−3	[108]
P(BA-co-PEGDA)	1033	2.04×10−2	[126]
Poly(epichlorohydrin) rubber	~750	1×10−4‒2×10−4	[127]

a) At 80 °C. b) At 20 °C. c) At 40 °C. d) At 60 °C. e) At 30 °C.Notes: Co, initial capacity; σi, ion conductivity.

Fig.7 (a) The morphologies of semi-crystalline PEO and (b) the related mechanism of ion transport. Reproduced with permission from Ref. [104].

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Although stable polymer electrolyte could provide a guarantee for the high safety Li–S battery, some problems in polymer electrolyte-based Li–S batteries still exist. The soluble LiPS could also be generated in polymer electrolyte during redox reaction, and fixing LiPS to the sulfur cathode to inhibit their shuttle effect is the research focus [130]. The ionic conductivity of polymer electrolyte is essential to be enhanced to well meet the need of Li–S battery, which could be realized by increasing the number of charged particles and their migration speed. The metal lithium anode is easy to produce dendrites and react with LiPS, resulting in serious safety risk and the limited application of practical Li–S battery. Therefore, limiting the dissolution and migration of LiPS in polymer-based electrolyte, and the appropriate modification on polymer electrolyte with high ion conductivity and stabilizing effect on lithium anode are important research content to improve the safety and performance of polymer-based Li–S batteries.

3.1 Cathode design

In sulfur cathode, many literatures have reported many methods to improve the sulfur fixation ability of sulfur cathode [126], such as porous carbon materials and catalytic mechanism. These ways are first employed in liquid-state Li–S battery, and are gradually applied in polymer electrolyte-based Li–S battery [117]. Except for reasonable design of sulfur cathode, the interaction between polymer electrolyte and sulfur cathode is also investigated [116]. In traditional polymer electrolyte-based Li–S battery, conductive skeleton materials are frequently used to fix the sulfur on the cathode, and then assembled with polymer electrolyte and lithium anode into Li–S battery (Fig.8(a)) [131]. LiPS is dissolved in the electrolyte of liquid Li–S battery, while some polymer electrolytes, such as pentaerythritol tetraacrylate (PETEA), could utilize their unique properties to effectively restrain the free diffusion of LiPS (Fig.8(b)) [132]. The introduction of interlayer in fabricating polymer electrolyte-based Li–S battery is also a common design to reduce the shuttle effect of LiPS (Fig.8(c)) [112].

Fig.8 (a) Schematic illustration of polymer electrolyte-based quasi-solid-state Li–S battery. Reproduced with permission from Ref. [131]. (b) The immobilization mechanism for polymer electrolyte reduces the lithium polysulfide dissolve in electrolyte. Reproduced with permission from Ref. [132]. (c) Interlayer used in polymer electrolyte-based quasi-solid-state Li–S battery. Reproduced with permission from Ref. [112].

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3.2 Design of polymer electrolyte

In polymer electrolytes, the migration of ions occurs mainly in the amorphous phase regions of polymer [113], and Li⁺ is easy to transfer with the motion of the molecular chain segments. However, most of the polymer substrates have high crystallization and are difficult to achieve molecular chain movement [124]. Therefore, the glass transition temperature of the polymer matrix should be reduced, and the amorphous phase proportion needs to be increased in order to obtain polymer electrolyte with high ion conductivity. Copolymerization, crosslinking, blending and grafting could change the glass transition temperature and crystallinity of the polymer electrolyte [133]. Crosslinking is a common modification method, including physical and chemical crosslinking [119]. Yang’s group [125] has reported a new super-high ionic conductive polymer. Grafting CH₂CH₂O units in the PEO (Fig.9(a) and Fig.9(b)) strengthens the interaction between Li⁺ and polymer, and facilitates its transport in polymer electrolyte. Compared with tradition PEO-based electrolyte, the corresponding Li–S battery processes high specific capacity (950 mA·h·g⁻¹ at 0.2 C, Fig.9(c)) and benign stability. Wang’s group [108] has proved that polymer is easy to be modified (Fig.9(d)), and introducing functional groups (such as ester groups) into the polymer could limit the dissolution of LiPS, stabilize the structure of the polymer and improve the electronic conductivity of Li⁺ (Fig.9(e)). Choudhury et al. [127] have also found that the oxygen in polymer electrolyte could coordinate with Li⁺ and support its transportation (Fig.9(f)).

Fig.9 (a) Principle of gel electrolyte with high ionic conductive, (b) synthesis of novel gel electrolyte, and (c) discharge/charge profiles of Li–S battery with gel electrolyte at 0.2 C. Reproduced with permission from Ref. [125]. (d) Synthesis route of multifunction gel polymer, and (e) Li⁺-ion transportation in gel polymer electrolyte. Reproduced with permission from Ref. [108]. (f) Possible mechanism of Li⁺-ion transportation. Reproduced with permission from Ref. [127].

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Blending is a popular and simple modification method. The polymer with low glass transition temperature would reduce the glass transition temperature of the whole system, if the polymer constituents endow large difference in glass transition temperature [134]. The increase of the amorphous area is more conducive to the migration of lithium ions, corresponding to the improved conductivity of polymer electrolyte [123]. In addition, blending could also increase the electrolyte adsorption, and optimize the thermal stability and oxidation stability of the polymer electrolyte.

Lithium salts are also important in polymer electrolyte [135]. The formed lithium salts with small cations and large anions normally endow small dissociation energy, accompanied by the easy dissociation of Li⁺, which are quite requisite in polymer electrolyte to obtain high Li⁺ migration number. The ideal situation is that the anion is possibly fixed, while the movement of charge is mainly concentrated on the flowing of Li⁺ [136]. Han et al. [118] have designed a kind of MOF(Mg-MOF-74)-modified polymer electrolyte for Li–S battery (Fig.10(a)). TFSI⁻ anions could be fixed in MOF by the steric hindrance and the Lewis acid-base effect, which could promote the more orderly flow of Li⁺ and its migration number (Fig.10(b)). Thus, the redox reactions of cathode and anode are stable, corresponding to the enhanced electrochemical performance and stability.

Fig.10 (a) MOF-PVDF gel polymer electrolyte for the Li–S battery, and (b) Li⁺-ion transport in MOF. Reproduced with permission from Ref. [118].

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Inorganic fillers, such as Al₂O₃ [137], TiO₂ [137], SiO₂ [138], ZnO [139], MgO [140], ZrO₂ [141], CuO [142], BaTiO₃ [143], LiAlO₂ [138], montmorillonite clay [144], carbide [145], nitride [145], boron compounds [146], and cellulose nanocrystals [147], have been added to obtain the composite polymer electrolytes and improve their properties. The crystallinity of the polymer could be reduced with the enhanced mechanical properties. Pathways for the ion conduction in the interphase could be formed to improve the ionic conductivity. Meanwhile, the surface properties of the fillers could affect the ionic dissociation, and the mobility of polymer and Li salts. Moreover, the interfacial stability and electrochemical stability of the polymer electrolyte could also be enhanced to some extent.

3.3 Stabilizing lithium anode

Challenges still occur in the practical application of lithium anode, such as lithium dendrites, unstable SEI layer, dead lithium, volume expansion and side reactions with the free LiPS. Compared with liquid electrolyte, the designed polymer electrolyte could act as a physical barrier to prevent the migration of LiPS to the lithium anode. Tu’s group [148] has employed PVDF-HFP/PETT-ester as polymer electrolyte in quasi-solid-state Li–S battery (Fig.11(a)). The surface of lithium anode could be visibly ameliorated with smooth and intact morphology (Fig.11(b) and Fig.11(c)), due to the low and flat over-potential in Li/GPE/Li cell (Fig.11(d)). In addition, polymer electrolytes prepared by chemical methods possess good bending performance, good wettability, high ionic conductivity and safety, which could effectively prevent the growth of lithium dendrites. The −N, −S and −O groups in polymer electrolyte could effectively adsorb LiPS and prevent their migrating to the lithium anode through the polymer electrolyte. Lee’s group [149] have used poly(methacrylic acid)-co-PCL-co-poly(methacrylic acid) and poly(DMAEMA)-co-PCL-co-poly(DMAEMA) to synthesize cross-linked polymers as the polymer electrolyte for the Li–S battery (Fig.11(e)). The rich −N and −O in polymer electrolytes are proved to effectively anchor the LiPS in the cathode side and enhance the electrochemical performance and stability (Fig.11(f)). Meanwhile, the interfacial engineering between Li anode and polymer electrolyte could improve the stability of anode and the battery. Yan’s group [150] has reported a compatible polymer interface layer by in-situ polymerization on the surface of lithium anode. The formed tween polymer layer is demonstrated to enhance the compatibility and stability of the Li/electrolyte interface, accompanied by the even dissolution/deposition of Li without lithium dendrites and adverse reactions with LiPS, thus the related Li–S battery shows the improved structural and electrochemical stabilization (Fig.11(g)).

Fig.11 (a) Optic photograph of GPE membrane and Cellgard 2300 separator; SEM images of the surface morphology of lithium electrodes after stripping/deposition cycles for (b) Li/LE/Li cell and (c) Li/GPE/Li cell; (d) the evolution of polarization in lithium symmetric cells along with the lithium stripping/deposition cycles. Reproduced with permission from Ref. [148]. (e) Synthesis of cross-linked polymers, and (f) Li−S battery with polymer electrolyte. Reproduced with permission from Ref. [149]. (g) The mechanisms of bare Li and polymer modified-Li during cycling. Reproduced with permission from Ref. [150].

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Compared with liquid electrolyte, polymer-based electrolyte has certain advantages, but the dissolution of LiPS in polymer-based electrolyte and their shuttle and side-reaction with lithium anode also impair the electrochemical stability and bring the potential safety hazards. Thus all-solid-state Li–S battery seems to solve these problems.

4 Inorganic solid electrolytes

Solid-state electrolytes, especially the inorganic solid electrolyte, possess higher melting point and boiling point in contrast with liquid-state electrolyte, which is safe in practical application, and the assembled all-solid-state Li–S battery endows high reliability with the reduced high-temperature flatulence, electrolyte corrosion and leakage [151‒152]. Meanwhile, the electrochemical process of sulfur cathode is different in inorganic solid electrolyte, corresponding to the transformation from elemental sulfur to lithium sulfides [153], which could avoid the generation of LiPS and the consequent capacity decay by their diffusion [154]. Moreover, the conductivity of inorganic solid electrolyte as single ion conductors could reduce side reactions and prolong the service life [155].

The term of “solid state ionics” was formally defined in 1967 by Prof. Takehiko Takahashi and his co-worker [156], while the transport phenomenon of ion in solids (such as Ag₂S and PbF₂) could go back to 1838 by Faraday [157]. Then the breakthroughs constantly spring up from theoretical perfection to more findings in the solid-state conductors (Fig.12(a)) [158]. One kind of these ion conductors with light and small Li⁺, called “lithium-ion conductor” or even “lithium super ionic conductor”, has attracted the research focus and achieved many milestones, from Li₃N to LISICON, NASICON, thio-LISICON, Garnet and antiperovskite-type solid electrolyte [159–164]. The inorganic solid-state electrolyte mainly contains LISICON, Argyrodite, Garnet, NASICON, Li-nitride, Li-hydride, perovskite, Li-halide, thio-LISICON, and antiperovskite-type compounds [165] (Fig.12(b)), which possesses different ion conductivities for the reasonable choice. Some solid electrolytes endow the high ion conductivity equal or near the value of liquid electrolyte. In the application process, the stability window is also considered in priority to adapt the reaction system [166] (Fig.12(c)). Thus, the assembled solid-state Li–S battery is composed of cathode, solid electrolyte and Li metal anode [167] (Fig.12(d)). Inorganic solid-state electrolyte is still one of the key factors to determine the electrochemical properties of battery [168]. High energy all solid-state Li–S battery requires the optimized solid electrolyte with high ionic conductivity [169]. It is necessary to enhance the ion conductivity and chemical stability of the inorganic solid electrolyte [170]. For most of the studied inorganic solid electrolytes, the mechanism of ion migration is still a controversial topic due to the lack of clear structure property correlation. Ion migration in inorganic solid electrolyte is usually realized by the transfer of mobile ions through various defects in the crystal structure. Among different types of defects, point defects determine the type and concentration of carriers, which directly affect the ionic conductivity of ion migration.

Fig.12 (a) Historical development of solid electrolytes. Reproduced with permission from Ref. [158]. (b) Reported total ionic conductivity of solid-state lithium-ion conductors at room temperature. Reproduced with permission from Ref. [165]. (c) Electrochemical stability ranges of various electrolyte materials grouped by anion, with corresponding binary for comparison. Reproduced with permission from Ref. [166]. (d) The schematic of the solid-state Li–S battery. Reproduced with permission from Ref. [167].

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In the numerous inorganic solid-state electrolytes, sulfur-based solid-state electrolytes are currently reported to have high ion conductivity [171], which are considered to be industrialized in the future [172]. The high ion conductivity is attributed to the non-bridged sulfur in the materials [173]. During the transmission process of Li⁺-ion, non-bridged sulfur is conducive to continuously interact with lithium-ion. Meanwhile, these electrolytes possess wide composition variation range, high thermal stability, good safety performance and wide electrochemical stability window. The improvements on the sulfur-based inorganic solid-state electrolyte include improving the Li⁺-ion conductivity, air stability, and electrochemical stability [174]. Synthesizing fast ion conductor structure with 3D Li⁺-ion transmission channel and doping have been the hot research topics to greatly improve the ion conductivity and reduce the interface resistance caused by the crystalline boundaries in sulfur-based inorganic solid-state electrolyte [175–177]. Tab.4 summarizes the relevant data of sulfur-containing inorganic solid electrolytes in Li–S batteries [173–175,178–194]. The battery employing the solid electrolyte with higher ion conductivity possesses the superior electrochemical performance, indicating the importance of high-performance inorganic solid electrolyte.

Tab.4 The reported solid-state electrolytes in Li–S batteries [173–175,178–194]

Electrolyte	Co/(mA·h·g−1)	σi/(S·cm−1)	Ref.
Li3PS4	1216 a)	1×10−4	[178]
Li3PS4	1270	2×10−4	[179]
0.67Li3PS4‒0.33LiI	~1600	~9.3×10−4	[180]
Li2S‒P2S5	837	1×10−3	[181]
78Li2S‒22P2S5	671 b)	6.3×10−4	[182]
70Li2S·29P2S5·1SeS2	658	5.28×10−3	[173]
Li7P3S11	995	1.7×10−3	[183]
Li7P3S11	1482	‒	[184]
Li6PS5Cl	1850	3.15×10−3	[174]
Li6PS5Cl	932	1×10−3	[185]
Li10SnP2S12	1601.7	3.2×10−3	[186]
Li7P2.9Sb0.1S10.75O0.25	~1300	1.61×10−3	[187]
Li7Ni0.2P3.1S11	614	2.22×10−3	[188]
Li10GeP2S12	1173.1	1.2×10−2	[189]
Li10GeP2S12	930	8.27×10−3	[175]
Li10GeP2S12	1139	4.33×10−3	[190]
Li10GeP2S12	716	1.2×10−2	[191]
Li10GeP2S12	840	1.2 ×10−2	[192]
Li10GeP2S12	< 800	8.27×10−3	[193]
75%Li2S‒24%P2S5‒1%P2O5		8×10−4
Li10GeP2S12	703.2 a)	4.08×10−8	[194]
75%Li2S‒24%P2S5‒1%P2O5		7.2×10−8

a) At 60 °C. b) At 50 °C.Notes: Co, initial capacity; σi, ion conductivity.

However, some issues are still urgent to be solved to enhance the performance of solid-state Li–S batteries. The composition and structure of the electrode/electrolyte interface are quite different from the bulk phase of the materials. Interface instability consequently occurs, leading to the adverse impact on conductivity. The understanding on the essence of the interface would guide people to choose the appropriate materials to develop the high-performance solid-state batteries. The transport of ions strongly depends on the dense contact of solid particles in solid-state battery system. These point contacts are very sensitive to the generated stress in the process of redox reactions, which may result in cracks and poor physical contact. Lithium dendrites could also generate and grow during the repeated dissolution/deposition, which may pierce the solid electrolyte and cause the battery failure. The poor ion and electronic conductivity of elemental sulfur is still a knotty problem to limit the electrochemical performance [195]. In order to achieve high performance of all-solid-state Li–S battery with inorganic solid electrolyte, more efficient methods have been used in developing the key materials (cathode, solid-state electrolyte and anode) [196].

4.1 Cathode design

The conductivity of sulfur cathode has a great influence on the electrochemical performance of all-solid-state Li–S battery [197]. A rich electronic and lithium-ion transport channel is necessarily established inside the cathode to ensure that more sulfur could participate in the battery reaction during the charge/discharge process [182]. Thus, the electrochemical properties of sulfur cathode are mainly improved from the following aspects: (i) The three-phase interface between sulfur, carbon additives, and inorganic solid-state electrolyte should be carefully designed to ensure long-term cycle stability of all-solid-state Li–S battery. Inorganic solid-state electrolyte does not have the good mobility and immersion as liquid-state electrolyte [198], thus the interface resistance caused by solid interface contact between sulfur cathode and inorganic solid-state electrolyte is an inevitable problem for all-solid-state Li–S battery [199]. Interface contact is one of the biggest challenges [181] due to the limited contact area and the inherent immovability of solid-state electrolyte [192], which may astrict the transportation of charges at the interface and lead to large interface resistance [200], further affecting the stability of the electrochemical cycle of the battery and the energy density of the battery [201]. Side-reactions on the electrode/electrolyte interface could also happen, along with the reduction on the interfacial stability. (ii) Increasing the interface contact area and improving interface stability are the keys to achieve excellent electrochemical properties of solid-state Li–S battery. Meanwhile, the volumetric change of the sulfur cathode during the charge/discharge process may result in stress, which could cause the material pulverization, destroy the ion diffusion channels, increase the interface impedance, and reduce the electrochemical stability of the battery [202‒203]. Therefore, building an effective and stable electrolyte/electrode interface and the abundant electronic/ion channels in sulfur cathode is critical to improve the electrochemical performance of Li–S batteries, by selecting appropriate materials and developing the composite process and electrode preparation technology of active materials, solid electrolyte and conductive agents.

4.1.1 Carbon materials

Carbon materials are the ideal sulfur storage materials by virtue of several advantages, such as high conductivity, adjustable size and physicochemical properties [204]. Researchers have reported the design of zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and 3D carbon materials in all-solid-state Li–S battery with solid-state electrolyte [184]. Carbon nanofibers and nanotubes have low surface area, and only provide a relatively small active surface for electrochemical reactions of sulfur [205]. Some 3D carbon endows high surface area and abundant pore structures, which could provide sufficient active surfaces for the redox conversion of sulfur [183]. Finely regulating pore structure and surface group always needs complicate methods [206]. Therefore, the development of carbon materials with high mechanical flexibility, excellent electronic conductivity and high surface area is of great significance [190]. Carbon materials in sulfur cathode have been proved to improve the electrochemical performance of inorganic solid electrolyte-based all-solid-state Li–S battery in the literature reports.

Han et al. [207] have used core‒shell S@BP2000 as cathode in all-solid-state Li–S battery (Fig.13(a)). Carbon material (BP2000) could improve the electronic conductivity of sulfur cathode, and more sulfur could be involved in the charge and discharge process of the battery. TEM images illustrate the benign contact between sulfur and porous carbon (Fig.13(b)‒Fig.13(e)). This all-solid-state Li–S battery displays the outstanding performance with a reversible capacity of 985.3 mA·h·g⁻¹ at 3 C after 1200 cycles at room temperature (Fig.13(f)), owing to the improved Li⁺-ion and electronics of Li–S battery. The optimization of cathode structure could improve the electrochemical performance and cycle stability of Li–S battery. Zhang’s group [208] has reported the improved sulfur utilization by enhancing interfacial electronic contact. Increasing contact area between sulfur and CNT makes more sulfur participate in the reaction (Fig.13(g)) and avoids the production of dead sulfur and Li₂S in Li–S battery (Fig.13(h)). Sulfur utilization could be improved by increasing the electron transportation in sulfur cathode. Wang’s group [185] has synthesized in-situ growing carbon on Li₂S (Fig.13(i)), which could enhance the performance of solid electrolyte-based all-solid-state Li–S battery. Yamamoto et al. [191] have reported unprecedented nanoporous graphene in fabricating sulfur cathode for all-solid-state Li–S battery. The obtained materials with pore volume (2.79 cm³·g⁻¹) could achieve full contact with sulfur, and the ideal electrical conductivity (up to 18 S·cm⁻¹) could reduce the interfacial resistance of cathode. Yao’s group [194] has reported Li₂S/carbon nanotube cathode for all-solid-state Li–S battery. Carbon nanotubes could significantly improve the electrochemical performances of Li₂S by building the benign electronic channels. The related battery endows a reversible capability of 651.4 mA·h·g⁻¹ after 300 cycles at 1.0 C and 60 °C.

Fig.13 (a) The preparation schematic of S@BP2000 for all-solid-state Li–S battery; high-resolution TEM images of (b)(c) BP2000 and (d)(e) S@BP2000, and (f) cycle performance of S@BP2000 cathode in solid-state electrolyte-based all-solid-state Li–S battery at 3 C. Reproduced with permission from Ref. [207]. CNT and sulfur with (g) uniform and (h) nonuniform electronic pathway. Reproduced with permission from Ref. [208]. (i) Synthesis of Li₂S/C nanocomposite. Reproduced with permission from Ref. [185].

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4.1.2 Metal compounds

Metal compounds with good electronic conductivity and Li⁺-ion diffusion could also be used in sulfur cathode to match inorganic solid electrolyte, and improve the electrochemical performance of Li–S battery [209]. Nazar’s group [210] has reported layered VS₂ for all-solid-state Li–S battery. The typical lamellar structure of VS₂ without impurities endows the S/VS₂ in moderate sulfur content with the enhanced Li⁺-ion and electronic conductivity (Fig.14(a)‒Fig.14(f)). The unhindered transportation of Li⁺-ion and electrons promotes the electrochemical performance of Li–S/VS₂ battery with a high utilization of sulfur, which is close to 85%, and the satisfying cycle stability, especially in high sulfur loading (Fig.14(g)‒Fig.14(i)). The areal capacity could reach 7.8 mA·h·cm⁻² at the high active material loading of 15.5 mg·cm⁻². This work confirms that metal compounds reserve the enormous application value in sulfur cathode for all-solid-state Li–S battery. Yao’s group [193] have fabricated 10% rGO‒VS₄ and utilized the conversion reaction to in-situ generate the active materials and construct the 3D electronic/ionic conductive framework to improve the electrochemical transformation in all-solid-state Li–S battery (Fig.14(j)). In addition, Yao and co-workers [211] have synthesized the dual cathode (FeS₂@S) for all-solid-state Li–S batteries (Fig.14(k)). Active FeS₂ hosts for sulfur possess the smooth fast electron and Li⁺-ion conduction, which contributes to the enhancement on the electrochemical performance. These reported results in literatures demonstrate that metal compounds have an important effect on improving the Li⁺-ion/electronic conductivity of cathode materials for all-solid-state Li–S batteries.

Fig.14 (a)(b) SEM images and (c) XRD pattern of VS₂; (d)(e) SEM images and (f) TGA curve of VS₂/S composite (sulfur content ≈ 33 wt.%); (g) the proposed microstructure and discharge mechanism for the Li–S/VS₂ battery; (h) electrochemical profiles of Li–S/VS₂ battery at a cathode loading of 7.7 mg·cm⁻² and (i) the capacity of cells with an active material loading of 15.5 mg·cm⁻². Reproduced with permission from Ref. [210]. (j) Synthesis of 10% rGO-VS₄@Li₇P₃S₁₁ used as sulfur carrying material in all-solid-state Li–S battery. Reproduced with permission from Ref. [193]. (k) Synthesis of FeS₂@S nanoparticles as cathode used in all-solid-state Li–S battery. Reproduced with permission from Ref. [211].

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4.1.3 Ameliorating the interact between sulfur and inorganic solid electrolyte

Increasing the contact between sulfur and solid-state electrolyte could improve the utilization rate of sulfur. The conventional method is to mix sulfur with solid-state electrolyte to assemble Li–S battery (Fig.15(a)) [186]. Reducing the particle size of sulfur-based inorganic solid-state electrolyte and in-situ growing solid electrolyte upon the loaded electrode materials are also effective ways to solve the large interface resistance caused by the poor electrode/electrolyte solid contact [212]. Kanno’s group [213] has used high-temperature mechanical milling to simultaneously decrease the size of sulfur and solid electrolyte to achieve the good contact between sulfur and solid-state electrolyte in the synthesized sulfur cathode (Fig.15(b)). Lin et al. [178] have synthesized core‒shell structured Li₂S@Li₃PS₄ (LSS) nanoparticles as cathode materials (Fig.15(c)‒Fig.15(e)). The Li₃PS₄ encapsulated nano Li₂S could dramatically reduce the interfacial resistance, and enhance the ionic conductivity of Li₂S from 10⁻¹³ to 10⁻⁷ S·cm⁻¹ at 25 °C. The corresponding solid Li–S battery shows more excellent rate performance and cycle stability than Li₂S nanoparticles (Fig.15(f) and Fig.15(g)). These studies prove that reducing the particle size and increasing the contact area between the active materials and electrolyte could prompt more sulfur to participate in the reaction process of the all-solid-state Li–S battery.

Fig.15 (a) The S–C|Li₁₀SnP₂S₁₂|Li–In all-solid-state Li–S battery assembled by conventional methods. Reproduced with permission from Ref. [186]. (b) High-temperature mechanical milling synthesis sulfur cathode. Reproduced with permission from Ref. [213]. (c) Synthesis of Li₂S@Li₃PS₄ nanoparticle; SEM images of (d) nano Li₂S and (e) Li₂S@Li₃PS₄ (LSS); (f)(g) Electrochemical characteristic of nano Li₂S and LSS as the cathode used in Li–S battery. Reproduced with permission from Ref. [178].

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The cathode material is designed to improve the Li⁺-ion and electronic conductivity, thus enhance the performance of Li–S battery [214]. Recently, researchers have made some progress in Li⁺-ion and electronic conductivity [180], and developed some promising carbon materials [208], metal compounds [215], and optimized their chemical states and physical properties, to obtain the superior battery performance.

Meanwhile, the role of various materials is required to be better understood [189], and more experimental research on materials is needed to guide the design of new multifunctional cathode materials, and finally improve the cyclic stability and rate capability of Li–S battery [216].

4.2 Inorganic solid-state electrolyte

4.2.1 Modification

Many sulfur-based solid-state electrolytes with high ion conductivity and fast ion transferring structure are constantly reported through the suitable modification [217], which increase the researchers’ confidence in the future development and practical applications of all-solid-state Li–S battery [218]. At present, the main research is concentrated on doping heteroatoms, such as Sb, Ce, Nb, Zr, Si, Sn, Al, F, Cl, Br, I, and O, as listed in Tab.5 [171,173,177,187,219–238]. Gao and co-workers [187] have adopted Sb and O to moderately replace P and S of Li₇P₃S₁₁ solid-state electrolyte (Fig.16(a)). The corresponding solid Li–S battery with Li₇P_2.9Sb_0.1S_10.75O_0.25 electrolyte shows good rate capability and cycle performance (Fig.16(b) and Fig.16(c)). The reversible capacity could maintain at 1374.4 mA·h·g⁻¹ at 0.05 C after 50 cycles, which is higher than that of the Li–S battery using the Li₇P₃S₁₁ electrolyte (614.1 mA·h·g⁻¹). Meanwhile, the ionic conductivity and air stability could be enhanced at the same time. Zhou et al. [220] have synthesized Li₇P_2.9Ce_0.2S_10.9Cl_0.3 solid-state electrolyte by introducing Ce and Cl (Fig.16(d)), which endows the low resistance (25 Ω, Fig.16(e)), and high Li⁺-ionic conductivity (3.2 mS·cm⁻¹, Fig.16(f)). This indicates that heteroatom doping is beneficial to enhance the electrochemical properties of the solid electrolyte. These studies demonstrate that heteroatomic doping is a promising preparation method to manufacture high performance solid-state electrolyte and improve electrochemical performance of solid-state Li–S battery. Moreover, more elements are urgently needed to introduce and develop the novel electrolyte to enrich the sulfur-based solid-state electrolyte material system, and simple and environmental preparation methods are also called for developing in the future [239].

Tab.5 The properties of modified inorganic solid-state electrolytes [171,173,177,187,219–238]

Element or substance	Electrolyte	Co/(mA·h·g−1)	σi/(S·cm−1)	Ref.
Sb, I	Li7Sb0.05P2.95S10.5I0.5	622.3	2.55×10−3	[177]
Si, Cl	Li9.54Si1.74P1.44S11.7Cl0.3	‒	2.5×10−2	[219]
Si, Cl	Li9.54Si1.74P1.44S11.7Cl0.3	969	1.6×10−2	[171]
Sb, O	Li7P2.9Sb0.1S10.75O0.25	1309.7	1.61×10−3	[187]
Ce, Cl	Li7P2.9Ce0.2S10.9Cl0.3	617	3.2×10−3	[220]
Se	Li6PS5−xSexI	‒	2.8×10−4	[221]
Nb, O	Li6.988P2.994Nb0.2S10.934O0.6	472.7	2.82×10−3	[222]
Nb, O	Li7P2.88Nb0.12S10.7O0.3	773	3.59×10−3	[223]
F	Li9.95SnP2S11.95F0.05	‒	6.4×10−3	[224]
Si	Li1.3Al0.3Ti1.7P3O12‒0.05Si	‒	1×10−3	[225]
I	Li9.54Si1.74P1.44S11.7I0.3	570.5	1.35×10−3	[226]
Sr, Mo	Li6.65La2.95Sr0.05Zr1.8Mo0.2O12	909 a)	6.43×10−4	[227]
W, Ta	Li6.5La3Zr1.5Ta0.5O12‒2Li2WO4	1012.6 b)	‒	[228]
Br	Li5.4PS4.4Cl1.2Br0.4	‒	8.17×10−3	[229]
Sn	Li1.3Al0.3Sn0.35Ti1.35(PO4)3	‒	4.71×10−4	[230]
Cl	Li7P2S8I0.5Cl0.5	1051	3.08×10-3	[231]
Cl	Li9.9SnP2S11.9Cl0.1	‒	2.62×10−3	[232]
Zr	Li2.6Er0.6Zr0.4Cl6	‒	1.13×10−3	[233]
Ta	Li6.4La3Zr1.4Ta0.6O12	‒	1.4×10−4	[234]
Al, Ta	Li6.25La3Zr1.55Al0.1Ta0.45O12	‒	6.7×10−4	[235]
Al, Nb	Li6.25Al0.2La3Zr1.85Nb0.15O12	‒	3.04×10−4	[236]
SeS2	70Li2S·29P2S5·1SeS2	658	5.28×10−3	[173]
LiF	Li1.3Al0.3Ti1.7(PO4)3‒0.15LiF	‒	1.767×10−4	[237]
LiNf	Li6.05Ga0.25La3Zr2O11.8F0.2	‒	5.6×10−4	[238]

a) Measured at the 4th cycle. b) Measured at the 50th cycle. Notes: Co, initial capacity; σi, ion conductivity; LiNf, lithium Nafion.

Fig.16 (a) Preparation of all-solid-state Li–S battery with the Li₇P_2.9Sb_0.1S_10.75O_0.25 electrolyte, and (b) rate performance and (c) cycling stability of Li–S battery with the Li₇P_2.9Sb_0.1S_10.75O_0.25 solid-state electrolyte. Reproduced with permission from Ref. [187]. (d) Li–S battery with the Li₇P_2.9Ce_0.2S_10.9Cl_0.3 electrolyte, and (e) impedance spectra and (f) ionic conductivities of the solid-state electrolytes of Li₇P₃S₁₁, Li₇P_2.9Ce_0.2S_11.1, and Li₇P_2.9Ce_0.2S_10.9Cl_0.3. Reproduced with permission from Ref. [220].

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4.2.2 Polymer/inorganic solid composite electrolyte

Ionic/electronic insulation of active material sulfur/inorganic solid electrolyte, low sulfur loading, thick solid electrolyte, and high interface impedance are the major problems in the cathode side. The point-to-point solid contact on the interface of lithium metal/inorganic solid electrolyte often leads to large interface contact resistance and uneven current distribution. Thus, the interface issue including sulfur/inorganic solid electrolyte and lithium metal/inorganic solid electrolyte is pressing. Compared with single electrolyte, composite electrolyte takes into account of the respective advantages of solid electrolyte and polymer electrolyte to alleviate the existing problems. Adding polymer into inorganic solid-state electrolyte could relieve the interface problems.

The intrinsic ionic conductivity of polymer electrolyte could be improved by radically altering the structure of polymer and introducing inorganic nanoparticles [240‒241], mainly due to the interaction between nanoparticles and Li⁺. The dispersed nanoparticles could hinder the regular arrangement of polymer segments [242], further decrease the glass transition temperature, increase the proportion of amorphous phase, and facilitate the migration of Li⁺ [243]. In addition, the working temperature and mechanical properties of polymer electrolyte could also be enhanced [244]. Thus, the composite electrolytes with evenly dispersed inorganic solid electrolyte particles in polymer electrolyte could possess the benign mechanical properties and conductivity, as well as the improved interface stability [245]. Their porous structure could also be obtained with the increased porosity and liquid absorption. Tu’s group [246] have reported a novel composite electrolyte based on PVDF‒HFP and Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) nanoparticles. PVDF‒HFP with low crystallinity and abundant functional groups endows polymer electrolyte with high dielectric constant and ionic conductivity. The added LATP could improve the mechanical stability and electrolyte adsorption of the composite electrolyte. The synergistic effect though reasonable structural design could promote the electrochemical performance of the battery. Tao et al. [247] have developed nano Li₇La₃Zr₂O₁₂ (LLZO)-filled PEO electrolyte for Li–S battery. The LLZO nanofillers in PEO electrolyte could act as the Li⁺-ion conductor and the interfacial stabilizer (Fig.17(a)). The formed protective layer and electrostatic interaction between Li⁺ and PEO could guide the uniform deposition of Li⁺ on lithium surface. Zhu et al. [248] have prepared a novel bilayer structure with (CNF/S) cathode and Li_0.33La_0.557TiO₃ (LLTO)/nanofiber-PEO solid-state composite electrolyte. CNF/S and PEO/LiTFSI could reduce the interfacial resistance and enhance the electrode/electrolyte interfacial stability (Fig.17(b)). LLTO nanofibers in PEO endow the solid-state composite electrolyte with fast and continuous electron/ion transportation pathways. Wang’s group [249] has employed Li_6.5La₃Zr_1.5Ta_0.5O₁₂/poly-(ethylene glycol) diacrylate (LLZTO/PEGDA) composite solid electrolyte in Li–S battery through the thermal initiation, demoulding, and cutting process (Fig.17(c)). The Li⁺-ion conductivity could be distinctly improved by adding LLZTO, and the shuttle effect is effectively alleviated in the composite electrolyte (Fig.17(d)). Bieker’s group [250] has also demonstrated that common solid-state electrolyte could solve the shuttle effect compared to liquid-state electrolyte, but would cause the insufficient interfacial wetting and chemo-mechanical separation, while the composite electrolyte containing inorganic solid-state electrolyte and polymer electrolyte could simultaneously solve the shuttle effect, interface contact between sulfur cathode and electrolyte, and improve the reaction kinetics (Fig.17(e)). Therefore, the synergy optimization on cathode, electrolyte and anode could be more conducive to improve the performance of the Li–S battery by using polymer/inorganic solid-state composite electrolyte [251‒252].

Fig.17 (a) LLZO filled in PEO assemble of Li–S battery. Reproduced with permission from Ref. [247]. (b) Design of cathode/electrolyte (CNF/S-PEO/LLTO) bilayer structure for Li–S battery. Reproduced with permission from Ref. [248]. (c) Synthesis of LLZTO/PEGDA composite solid electrolyte, and (d) possible mechanism for LLZTO/PEGDA electrolyte suppression of shuttle effect. Reproduced with permission from Ref. [249]. (e) The difference between liquid-state solid-state and solid/polymer composite electrolyte used in Li–S battery. Reproduced with permission from Ref. [250].

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4.3 Anode optimization

Lithium metal is considered to be an anode material for high-energy-density battery in the future, owing to its high theoretical capacity and low voltage platform. The interfacial issues between metal Li and solid-state electrolyte impede the reliability of the all-solid-state Li–S battery, such as poor physical contact, undesired chemical side-reactions, and the formation of dendritic Li and holes [253]. Exerting high pressure and appropriate heating could improve the physical contact to some extent [254‒255]. The rational elemental doping on solid-state electrolyte, the optimized surface properties with benign flatness and purity, and the few defects have also been proven to reduce the interfacial impedance [256‒257]. Lithiophilic substances, such as Al₂O₃ [258], ZnO [259], Al [260], Ge [261], Si [262], MoS₂ [263], and graphite [264], could serve as the interfacial layer to transfer ions. Inorganic and organic buffer layers could effectively reduce the direct contact between lithium metal and solid-state electrolyte, and lower adverse reactions and Li uneven plating/deplating [265–267]. In terms of anode, the alloy of lithium and indium is the conventional anode for all-solid-state Li–S battery. Li alloy is an effective substitute for Li anode in all-solid-state Li–S battery, but the suitable anode should have the advantages of high conductivity, large capacity, medium potential, and low volume expansion. Pan et al. [268] have reported Li‒Al alloy as carbon-free and binder-free anode for all-solid-state Li–S battery (Fig.18(a)). The working potential of the optimized Li‒Al alloy anode is located in the actual stability window of Li₁₀GeP₂S₁₂ (LGPS1), which could effectively avoid the reduction and decomposition of the electrolyte (Fig.18(b)). The electrochemical test and structural characterization show that the alloy anode could stably operate for more than 2500 h at 0.5 mA·cm⁻², with the enhanced interfacial physical/chemical stability close to LGPS1 (Fig.18(c)‒Fig.18(g)). Meanwhile, except for the high stability of anode, the energy density value of the selected anode for Li–S battery is also emphatically considered (Fig.18(h)) [269]. High-energy-density anodes have been the focus of pursuit. Thus, the research on the anode of solid-state Li–S battery still needs to be further carried out.

Fig.18 (a) Schematic of Li‒S battery with Li‒Al alloy anode and its reaction mechanism; (b) practical stability window of the LGPS1 electrolyte and the chemical potential of different anode; images of (c) pristine LGPS1, and that (d) after contacting with Li_0.8Al for 8 h, (e) after the Li_0.8Al-LGPS-Li_0.8Al cell cycling for 100 h, and (f) after contacting with Li for 8 h; (g) galvanostatic Li plating/stripping profiles of the Li-LGPS1-Li cell at 0.5 mA·h·cm⁻² (blue) and 0.1 mA·h·cm⁻² (gray), and the Li_0.8Al-LGPS1-Li_0.8Al cell at 0.5 mA·h·cm⁻² (red). Reproduced from Ref. [268]. (h) Theoretical energy densities of Li‒S battery with different anode materials. Reproduced from Ref. [269].

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All-solid-state Li–S battery has attracted the widespread attentions owing to the benign safety performance and high energy density. The existing optimization on cathode, electrolyte and anode has vastly improved the electrochemical performance of solid-state Li–S battery, but the research from theory to practice is required to carry on to solve the knotty issues and promote the industrialization of all-solid-state Li–S battery.

Therefore, based on the above description, different kinds of electrolyte all possess the distinct advantages and disadvantages employed in Li–S battery. As shown in Fig.19, liquid electrolyte endows the high ion conductivity and benign processibility, which could extremely deliver the electrochemical capacity of Li–S battery and facilitate the large scaled production. But the corresponding cost and safety issues restrict the popularization and application. The polymer electrolyte shows smaller reduction on ion conductivity and processiblity, but possesses higher ion transference number, safety and mechanical property compared with liquid electrolyte. Inorganic solid electrolyte has the high ion transference number (~1), safety and mechanical property, as well as benign eco-friendliness and cost, showing the huge potential in fabricating high-performance Li–S battery, except for the continuous improvement on ion transport and manufacturing technique.

Fig.19 The Radar map of the basic properties of the electrolyte for Li–S batteries.

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5 Summary and future perspectives

In summary, high-energy-density Li–S batteries have achieved many breakthroughs through the continuous research, and are gradually marching towards the application stage. The safety of Li–S batteries is crucial and non-negligible. Electrolyte, as one important part of the battery, could affect the electrochemical performance of battery, and the security properties. The solvent in liquid-state Li–S battery is flammable and explosive, along with the diffusion of soluble LiPS and the growing of lithium dendrites. Adding flame retardant and utilizing ionic liquid could reduce the risk of fire and explosion. Functional additives could improve the stability of lithium anode, and thus enhance the availability of Li–S battery. Polymer electrolyte and inorganic solid-state electrolyte could prevent the burning and explosion of electrolyte and lithium dendrites to some extent, and reduce the possibility of electrolyte leakage and side-reaction even at high temperature. These mentioned findings have significantly promoted the security properties of Li–S batteries, but further research is still required to accelerate the practical process of Li–S batteries.

In the future, high safety liquid electrolyte systems are anticipated by using solvents with high boiling temperature. Novel multifunctional additives are expected to optimize the electrolyte by regulating the composites and structures, and exert their roles with the adequate usage. The liquid electrolyte could preferably guarantee the normal electrochemical reactions of sulfur cathode, especially in the condition of high sulfur content, high sulfur loading and trace electrolyte usage, and stabilize Li and Li-based anode by ameliorating the SEI layer from structure to component and facilitating the uniform dissolution/deposition. Meanwhile, the electrolyte with wide temperature range is urgently needed to accommodate the harsh working conditions. Moreover, green, low-cost and high-performance solvents, lithium salts and additives are required to commercialize the Li–S battery. On the aspect of polymer electrolyte, more testing techniques should be employed to understand the nature of ion conduction in the polymer matrix, and further guiding the preparation of novel polymer electrolyte with high ionic conductivity from composition to structure. Eco-friendly polymer electrolyte made by simple method and cheap raw materials could contribute to the practical process of Li–S battery. Besides, confining the “shuttle effect” of LiPS could be a typical character of the polymer electrolyte in the future to achieve the electrochemical stability of Li–S battery (Fig.20). As for the inorganic solid electrolyte, high ion conductivity by reasonably regulating the composition, structure and morphology of the solid electrolyte from theory to practice could also be desired to improve the electrochemical properties of Li–S batteries (Fig.20). Easy and energy-saving preparing methods are essential to accelerate the application of inorganic solid electrolyte. Stable and thin solid-state electrolyte layer is needed to fabricate the high-energy-density Li–S battery. Strength and toughness of inorganic solid electrolyte could achieve the balance to improve the processibility and stability in the application. It is required to resolve interfacial issues by optimizing the interface area and adopting the multiple electrolytes. The future researches about the safety and performance of Li–S batteries based on the electrolyte may accelerate the proceedings of the upcoming commercialization.

Fig.20 The main research directions of polymer electrolyte-based Li–S battery and all-solid-state Li–S battery.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 21905289 and 22005217), the Tianjin Sci. & Tech. Program (Grant No. 20JCQNJC00440), the Young Elite Scientists Sponsorship Program by Tianjin (Grant No. TJSQNTJ-2017-05), and the Incubation Program of Youth Innovation in Shandong Province, China.

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