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Recent Progress in All-Solid-State Lithium−Sulfur Batteries Using High Li-

Ion Conductive Solid Electrolytes

Article · February 2019

DOI: 10.1007/s41918-019-00029-3

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Electrochemical Energy Reviews https://doi.org/10.1007/s41918-019-00029-3

REVIEW ARTICLE

Recent Progress in All‑Solid‑State Lithium−Sulfur Batteries Using High Li‑Ion Conductive Solid Electrolytes

Ediga Umeshbabu1 · Bizhu Zheng1 · Yong Yang1

Received: 24 October 2018 / Revised: 26 December 2018 / Accepted: 4 February 2019 © Shanghai University and Periodicals Agency of Shanghai University 2019

AbstractRechargeable lithium−sulfur (Li−S) batteries are one of the most promising next-generation energy storage systems due to their extremely high energy densities and low cost compared with state-of-the-art lithium-ion batteries. However, the main obstacles of conventional Li−S batteries arise from the dissolution of lithium polysulfides in organic liquid electrolytes and corresponding safety issues. To address these issues, an effective approach is to replace conventional liquid electro-lytes with solid-state electrolytes. In this review, recent progress in the development of solid electrolytes, including solid polymer electrolytes and inorganic glass/ceramic solid electrolytes, along with corresponding all-solid-state Li−S batter-ies (ASSLSBs) and related interfacial issues at the electrode/electrolyte interface, will be systematically summarized. In addition, the importance of various solid-state lithium ion conductors in ASSLSBs will be discussed followed by detailed presentations on the development of various forms of sulfur-based positive electrode materials (e.g., elemental sulfur, lithium sulfide, metal sulfides, lithium thiophosphates, and lithium polysulfidophosphates) along with key interfacial challenges at the electrode/solid electrolyte interface (cathode/SE and anode/SE). Finally, this review will provide a brief outlook on the future research of ASSLSBs.

Keywords All-solid-state lithium−sulfur batteries · Ionic conductivity · Interfacial impedance · Solid electrolytes · Sulfur-based composites

1 Introduction

Li−S batteries have been investigated since the 1960s and have drawn great attention in recent years. This is because sulfur cathodes and lithium metal anodes can deliver exceptionally high theoretical specific capacities (i.e., Li metal ~ 3800 mAh g−1 and sulfur ~ 1675 mAh g−1) and a high specific energy (2600 Wh kg−1, based on batteries using sulfur cathodes and Li metal anodes) which are supe-rior to contemporary lithium-ion battery cathodes (e.g., 387 Wh kg−1 for a LiCoO2/graphite cell) [1–6]. Moreover, sulfur is naturally abundant, inexpensive and eco-friendly. Because of this, sulfur has been recognized as an optimal cathode material for future rechargeable Li batteries. However, the

commercialization of rechargeable Li−S batteries using con-ventional organic electrolytes has been severely hampered by several formidable challenges [3, 7–9]. One challenge is that both sulfur and its discharge product Li2S are electronically insulating, resulting in the low utilization of active materi-als and inferior cycling performances. Another challenge is the significant volume change of sulfur (~ 80%) during charge–discharge cycling caused by the different density of sulfur and Li2S, resulting in stringent microstructural changes, reduction of interfacial contact and low active material utilization [8]. Furthermore, intermediate lithium polysulfides (Li2Sx, x = 4–8) formed during the sulfur con-version reaction are highly soluble in liquid electrolytes and are prone to react with Li metal to produce lower-order polysulfides and cause serious shuttling effects that can lead to low coulombic efficiencies, high self-discharge rates, and poor cycling performances [3, 7, 10–12]. To address these challenges, significant research has been conducted in recent years on Li−S batteries such as the encapsulation of sulfur in conductive multi-porous hosts and the use of unique separators, interlayers, electrolyte additives, hybrid

* Yong Yang yyang@xmu.edu.cn

1 Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory for Physical Chemistry of Solid Surface, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

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anode structures, and novel binders [8–17]. However, these approaches have only partially resolved the issues. Until now, a number of high-quality review articles have been systematically conducted on those potential solutions [2, 3, 10, 18, 19].

Alternatively, solid-state lithium ion conductors, also referred to as solid electrolytes, have attracted great attention in recent years as promising alternatives to conventional liq-uid aprotic electrolytes because the use of solid electrolytes in Li−S batteries is able to address several key challenges caused by liquid electrolytes [20–37]. First, solid electrolytes are much safer than volatile and flammable liquid electro-lytes in current Li−S batteries. Second, solid electrolytes can inherently circumvent the shuttle effect because solu-ble polysulfides cannot permeate into the solid electrolytes in which direct electrochemical conversion between sulfur and Li2S occurs instead of the formation of polysulfides in ASSLSBs. Third, solid electrolytes exhibit high Li-ion trans-ference numbers ( t

Li+≈1, because only Li ions can migrate)

at room temperature, which is crucial to the uniform depo-sition of Li and the suppression of lithium dendrite forma-tion. And lastly, some solid electrolytes such as Li2S–P2S5 (Li3PS4) [36], garnet-type Li7La3Zr2O12 [38, 39], and solid polymer electrolytes [40–42] are compatible with Li metal anodes, and if combined (e.g., NASICON-type LATP and LAGP) show desirable stability at high voltages and can significantly boost energy density as well as overall cell performance.

Aside from the liquid electrolyte and separator being replaced by solid electrolytes (Fig. 1), the working prin-ciples in conventional and ASSLSBs are similar [3, 25], in which during the discharge process, Li ions liberated from the oxidation of the Li anode are transferred through the electrode–electrolyte interface into the sulfur cathode and electrons are transferred from the cathode to the anode through an external circuit. As for the charging process, the migration of Li ions and electrons are reversed and the overall redox reaction can be represented as: S8 + 16Li ↔

8Li2S and lies around 2.15 V (vs. Li/Li+) [8]. Here, the foremost working functions of SEs are similar to liquid aprotic electrolytes and separators but only enable Li+ ions to traverse between the positive and negative electrodes, impeding electron conduction, polysulfide dissolution, and dendrite growth as well as short circuiting. Therefore, to develop high-performance ASSLSBs, solid electrolytes should possess the following features [22–26]: (1) high Li+ ionic conductivity(�

Li+ ), low activation energy at room

temperature (RT) and a tLi

+ close to unity, (2) negligible electronic conductivity, (3) good mechanical strength to prevent dendrite deposition on Li metal, (4) large poten-tial stability window with respect to contiguous electrode materials, (5) good chemical compatibility with both anodes and cathodes, (6) excellent thermal stability, and (7) low interfacial resistances at the electrode/SE interface. Other requirements of optimal solid electrolytes include suitable mechanical properties, low costs, reliable safety, stress-free fabrication, and environmental benignity. The ionic conductivity of different electrolytes, including organic liquid electrolytes, solid polymers, and inorganic glass/ceramic solid electrolytes are depicted in Fig. 2 [27].

In recent years, many reviews have summarized the development and characterization of solid electrolytes, including solid polymer electrolytes and inorganic glass/ceramic solid electrolytes for all-solid-state Li-ion batter-ies (ASSLBs) [21–24, 28–30]. However, little attention has been paid to the application of solid electrolytes in ASSLSBs [25, 31–35]. Therefore, this review will provide a comprehensive and current look into state-of-the-art sul-fur-based positive electrodes, including elemental sulfur, lithium sulfide and metal sulfides as well as sulfide solid electrolyte active materials in ASSLSBs utilizing various solid electrolytes. Moreover, this review will provide a focus on key interfacial challenges at the electrode/SE interface and a promising proposal for the future develop-ment of ASSLSBs.

Fig. 1 Schematic illustrations of a a conventional Li−S battery and b an all-solid-state Li−S battery

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2 Solid Electrolytes

Solid electrolytes (SEs) are pivotal components in the development of high-performance ASSLBs because they

can potentially endow batteries with greater safety, higher energy density, longer life span, less packaging and those which meet state-of-charge monitoring circuit require-ments [26, 27, 36–42]. Currently, two main types of SEs have attracted the attention of academia and industry in which one type is organic solid polymer electrolytes (e.g., Li salts complexed with high molecular weight poly-mers such as poly(ethylene oxide), poly(acrylonitrile), poly(vinylidene fluoride), and polyphenylene oxide) [41–50] and the other type is inorganic glassy/ceramics, in particular sulfides (e.g., glass/glass–ceramic Li2S–P2S5, thio-LISICON Li3.25Ge0.25P0.75S4, Li10MeP2S12 (M = Ge, Sn, Si), Li9.54Si1.74P1.44S11.7Cl0.3, and Li6PS5X (X = Cl, Br, I)) and oxides (e.g., garnet-type Li7La3Zr2O12, NASICON-typeTi/Ge-based lithium phosphate, and perovskites) [36, 37, 51–61]. And numerous reviews have thoroughly dis-cussed SEs development and classification, Li+ ion con-ductivity improvements, and working functions. Table 1 presents an overview of various SEs and their properties. [20–28, 62] In this review, we will focus mainly on the breakthroughs of various sulfur-based cathodes utilizing divergent SEs in ASSLSBs with discussions on critical interfacial issues at the electrode/electrolyte interface. Table 2 presents the ionic conductivities of various SEs reported in literature.

Typically, inorganic solid electrolytes (ISEs) including oxide and sulfide SEs possess unique beneficial features compared with solid polymer electrolytes (SPEs) such as

Fig. 2 Ionic conductivities of various solid electrolytes are shown in comparison with those of organic liquid electrolytes and polymer electrolytes. Reprinted with permission from Ref. [27], copyright 2016, Nature Publishing Group

Table 1 Summary of solid-state electrolyte (SEs) materials and their advantages and disadvantages in ASSLSBs

Type Material �Li

+ (S cm−1) Advantages Disadvantages

Sulfide solid electrolytes Glass/glass–ceramics:Li2S–P2S5Crystalline thio-LISICONs:Li10MP2S12 (M = Ge, Sn, Si)Li9.54Si1.74P1.44S11.7Cll0.3Argyrodites:Li6PS5X (X = Cl, Br, I)

10−2–10−3 (RT) Low grain boundary resistanceExtremely high conductivity

at RTExcellent mechanical strength

and flexibilityEasy to cold-pressingAble to suppress the poly-

sulfide shuttle effect

High cost of raw materials (e.g., Ge element)

Poor compatibility with Li metal and oxide cathode materials

Huge solid–solid interfacial resistance

Sensitive to moisture and airLow oxidation stability

Oxide solid electrolytes Li garnet: Li7La3Zr2O12NASICON type:

Li1+xAlxTi2−x(PO4)3, Li1+xAlxGe2−x(PO4)3

Perovskites

10−3–10−4 (RT) Good thermal and chemical stability

High conductivity at RTEnvironmental benignityLow cost;Atmospheric stabilityExcellent mechanical strengthAble to suppress the poly-

sulfide shuttle effect

High grain boundary resistance;Huge solid–solid interfacial

resistanceExpensive in large-scale produc-

tionPoor wettability against Li metal

Solid polymer electrolytes PEO–LiX(X = ClO4, PF6, BF4

N(SO2CF3)2, etc.)

10−4 (60–80 °C) Flexible, able to tolerate vol-ume change

Good compatibility with electrodes

Low shear modulus

Limited thermal stabilityLow oxidation voltageLow Li+ transfer number t

Li+

Low ionic conductivity at room temperature

Not able to suppress polysulfide shuttle effect completely

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higher ionic conductivities at RT, improved chemical/ther-mal stability, and unity of tLi+ [22, 24, 27]. And of these ISEs, sulfide SEs have been more extensively studied in ASSLSBs compared with oxide SEs because sulfide SEs possess extremely high conductivities (10−2–10−3 S cm−1 at RT) and sound mechanical properties which can pro-vide favorable interfacial formations between the electrode and the SE. Another advantage of sulfide SEs is that their grain boundaries and voids can be easily reduced from the conventional cold-pressing of sulfide SE powders [35–37, 60, 63]. However, despite these promising attributes in ASSLSBs, the application of ISEs face challenges that need to be resolved. For example, the high mechanical strength of ISEs can significantly increase the stress/strain at the elec-trode/electrolyte interface and cannot accommodate vol-ume expansion during cycling, leading to high interfacial

resistances. In addition, the poor wettability/stability of some ISEs against Li metal can impede the utilization of Li metal for application in bulk-type ASSLSBs [35, 62].

Alternatively, SPEs also possess notable benefits over ISEs and liquid aprotic electrolytes such as ease of process-ing and fabrication, as well as greater flexibility and the conservation of advantages of SEs in ASSLBs, including improved safety, broad working temperature ranges, dimen-sional stability, and the ability to suppress lithium dendrite formation [20, 43–45]. However, in the case of lithium den-drite suppression, this greatly depends on the nature of the SPEs such as the type of solid polymer, polymer compo-sition, and associated properties (e.g., ionic conductivity, lithium salt concentration, shear modulus) [41, 46, 48, 64]. In general, SPEs are composed of alkali metal salts (e.g., LiClO4, LiPF6, LiN(SO2CF3)2, etc.) dissolved in a high

Table 2 Ionic conductivities of different solid electrolytes employing in all-solid-state Li−S batteries

Data taken from the reported references [20, 29, 32]Li10GeP2S12 and Li7P3S11 SEs (in bold) show high room temperature conductivities comparable to liquid electrolytes

Electrolyte composition Type/classification Ionic conductivity (S cm−1)

80Li2S·20P2S5 Glass 5.4 × 10−5 (25 °C)75Li2S·25P2S5 Glass 2.0 × 10−4 (25 °C)80Li2S·20P2S5 Glass 2.0 × 10−4 (25 °C)80(0.7Li2S·0.3P2S5)·20LiI Glass 5.6 × 10−4 (25 °C)95(0.8Li2S·0.2P2S5)·5LiI Glass 2.7 × 10−3 (25 °C)Li3.25P0.95S4 Glass–ceramic 1.3 × 10−3 (25 °C)Li3.25Ge0.25P0.75S4 Crystalline 2.2 × 10−3 (25 °C)Li10GeP2S12 Crystalline 1.2 × 10−2 (25 °C)Li9.54Si1.74P1.44S11.7Cl0.3 Crystalline 25 × 10−3 (25 °C)Li10SnP2S12 Crystalline 4.0 × 10−3 (25 °C)Li10Si2P2S12 Crystalline >1.2 × 10−2 (25 °C)γ-Li3PS4 Crystalline 3.0 × 10−7 (25 °C)β-Li3PS4 Crystalline 1.6 × 10−4 (25 °C)Li7P3S11 Crystalline 1.7 × 10−2 (25 °C)Li7P2S8I Crystalline 6.3 × 10−4 (25 °C)80Li2S·20P2S5 Crystalline 7.4 × 10−4 (25 °C)70Li2S·30P2S5 Glass–ceramic 3.2 × 10−3 (25 °C)Li6PS5Cl Crystal (argyrodite) 1.3 × 10−3 (25 °C)Li7La3Zr2O12 Crystal (garnet type) 3 × 10−4 (25 °C)Li1.3Al0.3Ti1.7(PO4)3 Crystal (NASICON) 7 × 10−4 (25 °C)Li1.5Al0.5Ge1.5(PO4)3 Glass–ceramic (NASICON) 4.0 × 10−4 (25 °C)La0.51Li0.34TiO2.94 Crystal (perovskite) 1.4 × 10−3 (25 °C)PEO–LiCF3SO3 Solid polymer 1.0 × 10−7 (25 °C)PEO–LiTFSI Solid polymer 1.8 × 10−4 (30 °C)PEO–LiTFSI–Al2O3 Composite solid polymer 1 × 10−5 (25 °C)PEO–LiTFSI–10% TiO2 Composite solid polymer 4.9 × 10−5 (30 °C)PEO–LiTFSI–10% HNT Composite solid polymer 2.14 × 10−3 (100 °C)PEO–LiTFSI–10% MMT Composite solid polymer 3.22 × 10−4 (60 °C)PEO–LiTFSI–1% LGPS Composite solid polymer 1.21 × 10−3 (80 °C)PEO–LiClO4–LAGP Composite solid polymer 1 × 10−5 (25 °C)

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molecular weight polymer host such as polyethylene oxide (PEO) [43, 46] in which lithium ion conduction is achieved through the local segmental motion of the polymer chain. And since being reported in 1983 by Armand et al. [45], these PEO-based SPEs have attracted growing attention from researchers for application in ASSLSBs in which the most striking features include their high salvation power, better complexation ability, excellent stability with both positive and negative electrodes, good film formation ability, enhanced flame resistance, and reduced costs. Furthermore, the addition of inorganic particles, including passive fillers (e.g., micro- or nanoscale oxides, MOF, zeolites, etc.) and active fillers (e.g., Li7La3Zr2O12, NASICON-type LAGP/LATP, Li10GeP2S12, etc.) into these PEO-based SPEs have been reported to be able to combine the benefits of organic polymers with inorganic materials [40–42, 47–49]. Dispersal of inorganic fillers into SPEs has been shown to increase polymer electrolyte ionic conductivity, mechanical proper-ties as well as interfacial stability [40, 42, 48]. Despite these promising findings however, the literature has also reported that compared with ISEs, SPEs applied in ASSLSBs dem-onstrate relatively low t

Li+ and are unable to suppress poly-

sulfide dissolution completely, resulting in the shuttle effect [50, 65]. Furthermore, the low ionic conductivity at RT is another severe issue for SPEs.

3 Recent Progress in Cathodes Materials of ASSLSBs

Unlike conventional Li−S batteries, ASSLSBs possess only a single charge–discharge plateau due to its direct electro-chemical conversion reaction: S + 2Li

++ 2e

−↔ Li

2S [66,

67] in which polysulfide intermediates are not triggered, leading to enhanced cell performances including long life span, high energy densities and increased efficiencies [68]. However, the poor electric and ionic conductivities of sul-fur and/or Li2S cathodes as well as the insufficient contact between active electrode materials, electronically conductive carbons, and ionically conducting solid electrolyte particles can lead to increased interfacial resistances as well as capac-ity fading in solid-state cells [36, 39, 69]. Here, one effec-tive approach to address these issues is to hybridize active electrode materials (i.e., sulfur or sulfur-based constituents) with both solid electrolytes (Li+ ion conduction additives) and nanocarbons (electron conduction additives), which can improve ion–electron pathways and optimize ion–electron transfer in ASSLSBs [39, 68–71]. Typically, high-energy ball-milling is used by researchers to produce these compos-ite positive electrodes because this method can ensure the adequate mixing of all three components, achieving better ionic and electronic conductivity in the resulting compos-ite electrodes [66, 70]. In addition, this method can allow

for more intimate contact between composite cathodes and SE particles. Figure 3 presents a summary of various solid electrolyte-based ASSLSBs reported in the literature, and Table 3 presents a summary of the electrochemical perfor-mance of various sulfur-based composite electrodes with different solid electrolytes reported in literature.

3.1 Composite Positive Electrodes with Sulfur

3.1.1 All‑Solid‑State Batteries with Sulfide Solid Electrolytes

Recently, there have been many studies investigating all-solid-state batteries with sulfide solid electrolytes using different preparation methods. For example, Tatsumisago et al. [66] prepared a composite of sulfur–acetylene black, AB (BET surface area ~ 68  m2 g−1) and 80Li2S·20P2S5 glass–ceramic SE using mortar grinding as well as planetary ball-milling and reported that all-solid-state Li−S battery obtained using ball-milling method demonstrated high dis-charge capacities (~ 1500 mAh g−1) and long-range cycling stabilities with a retention of 850 mAh g−1 for 200 cycles. The improved performance is attributed to the homogeneous mixing of the components using ball-milling, which reduced particles sizes. In another study, Nagao et al. [72] fabricated a composite of S–AB–80Li2S·20P2S5 SE using high-tem-perature mechanical milling at 155 °C and reported that the assembled S–AB–SE/80Li2S–20P2S5/Li−In cell deliv-ered a discharge capacity of 1087 mAh g−1 with a capacity retention of 97% after 50 cycles. Interestingly, Nagada and Chikusa [67] reported that phosphorus/sulfur (P/S) ratios in glass–ceramic Li2S–P2S5 correlated with the reactivity of sulfur rather than the conductivity of Li+ ions in which cells assembled using a sulfur–AB composite cathode with an optimal SE P/S ratio of Li1.5PS3.3 (60Li2S–40P2S5) displayed

Fig. 3 Year-wise publication trend (from 2003 to 2018) of all-solid-state Li−S battery research reported in the literature. The data were collected from web of Science up to September 20, 2018

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Table 3 The electrochemical chemical performance of different sulfur-based composites for all-solid-state Li−S batteries

Conductive additive

Active material Preparation method

Sulfur (wt%) Solid electrolyte (SE) type

Applied current density

aCapacity (mAh g−1)

bCapacity@ nth cycle

References

Inorganic solid electrolytes-based Li–S batteriesGraphite

(MCMB)Sulfur Ball-milling 23 Li2S–P2S5 84 mA g−1 400 400@18 36

Acetylene black Sulfur Ball-milling 25 80Li2S–20P2S5 0.14 mA cm−2 1200 996@200 66Acetylene black Sulfur Gas phase 30 Li3.25Ge0.25P0.75S4 13 μA cm−2 1500 900 @10 74Super P carbon Sulfur Ball-milling 20 Li6PS5Br 167 mA g−1 1400 1000@50 57CMK3 Sulfur Gap phase 30 Li3.25Ge0.25P0.75S4 0.13 mA cm−2 1600 700@30 73Super-P carbon Sulfur Ball-milling 20 Li6PS5Cl 64 μA cm−2 1400 400@20 77VGCF Sulfur Ball-milling 30 Li3PS4 0.1 mA cm−2 1300 1200@50 76Carbon replica Sulfur Mechanical and

liquid-phase mixing

30 Li3.25Ge0.25P0.75S4 0.5C 2000 1500@50 71

Activated carbon Sulfur Ball-milling 35 70Li2S–30P2S5 0.13 mA cm−2 300 250 @6 114Graphene Sulfur Infiltration 15 Li9.54Si1.74P1.44S11.7Cl0.3 80 mA g−1 969 827@60 81Carbon black Sulfur Infiltration 45 Li7P2.9Mn0.1S10.7I0.3 80 mA g−1 796 800@60 79Acetylene black Sulfur Ball-milling at

155 °C30 Li3.25Ge0.25P0.75S4 0.05C 1200 800@40 115

Polyacrylonitrile Sulfur Thermal anneal-ing

37.7 Li2S–P2S5 26.5 mA g−1 722 605@50 167

Ketjen black Sulfur Hot-pressing 50 Li7P3S11 0.1C 1370 117Carbon nanofib-

ersSulfur Ball-milling 30 75Li2S–25P2S5 25 μA cm−2 1600 1400 @10 161

VGCF, Sulfur Ball-milling 29.9 Li3PS4 0.1 mA cm−2 1270 1230@50 164Acetylene black Sulfur Ball-milling 25 Li3.25Ge0.25P0.75S4 0.05C 1100 500@30 165Ketjen black Sulfur Ball-milling 50 Li3PS4 0.64 mA cm−2 942 – 166Graphene Sulfur Conformal

coating12–15 Li10GeP2S12@75Li2S–

24P2S5–P2O5

0.05C, 60 °C 1629 1500@30 68

MWCNT Sulfur Evaporation – NASICON-type LAGP 20 mA g−1 1510 1400@30 82PEDOT/PSS Sulfur Solution process 60 Li10GeP2S12 0.64 mA cm−2 1100 800@10 169FeS2-AC Sulfur Ball-milling 30 Li3PS4–LiI 83.5 mA g−1 825 1200@20 105Cu Sulfur Ball-milling 27 60Li2S–40SiS2 64 μA cm−2 980 – 163Cu Sulfur Ball-milling 23 80Li2S–20P2S5 64 μA cm−2 660 650@20 116Cu Li2S Ball-milling 18 80Li2S–20P2S5 64 μA cm−2 500 340@20 162Super P carbon Li2S Ball-milling 30 70Li2S·30P2S5 0.018 mA cm−2 1055 603@10 168Acetylene black Li2S Ball-milling 25 75Li2S–25P2S5 64 μA cm−2 600 600@10 92LiI Li2S Ball-milling 20 75Li2S–25P2S5 0.13 mA cm−2 930 930@50 96LiI-VGCF Li2S Ball-milling 80 75Li2S–25P2S5 2C 900 900@2000 97Acetylene black Li2S Ball-milling 18 80Li2S–20P2S5 64 μA cm−2 830 700@10 70Carbon black Li3PS4+5 Solution process 28 β-Li3PS4 0.015 mA cm−2 1400 1200@300 118WVA-1500

carbonLi2S@Li3PS4 Wet chemical 44 β-Li3PS4 0.02 mA cm−2 1216 852 @100 121

Cu Li2S–P2S5–Cu Ball-milling – 80Li2S–20P2S5 64 l μA cm−2 400/Li2S 109@50 110Solid polymer electrolyte-based Li–S batteriesKetjen black Sulfur Solvent casting 40 PEO + LiFSI 0.1C 900 600@50 42Divinylbenzene Sulfur Polymerization 80 LiFSI + PEO 0.1C 110 650@50 84Carbon black Sulfur Solvent casting 50 LiTFSI + PEO 0.05C 722 270@10 170Ketjen carbon Sulfur Solvent casting – LiFTFSI + PEO 0.5C 1394 800@60 85OMC Sulfur Heating process 40 PEO + LiTFSI + 10 wt%

SiO2

0.1C 1266 823@25 83

Super P carbon Sulfur Glass ball-milling 40 PEO + LTF +10 mol% ZrO2

30 mA g−1 400 500@30 48

N-CNS Sulfur Solution stirring – IL@PEO + LITFSI + ZrO2

– 1437 986@40 87

Super P carbon Sulfur Solvent casting 30 PEO + LiTFSI + 10 wt% HNT

0.1C 800 745@100 40

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a near-theoretical capacity of up to 1600 mAh g−1 (with respect to S weight) and a remarkable cycle life beyond 50 cycles (75% of the initial capacity) under high working cur-rents of 6.4 mA cm−2.

Compared with mechanical milling, gas-phase mixing generates internal heating and provides dense microstruc-tures and intimate contact between SEs and electroactive material particles, which can improve the cyclability and rate capability of solid cells [73, 74]. For example, Kob-ayashi et al. [73] obtained a composite electrode by deposit-ing sulfur over conductive AB using a gas-phase treatment (mechanical mixing of sulfur and AB through mortar grind-ing and subsequent heat treatment in a tube sealed under vacuum in which sulfur is evaporated and mixture homoge-neity is improved) and subsequent mechanical milling with thio-LISICON Li3.25Ge0.25P0.75S4SE (thio-LISICON) and reported that the resulting composite electrode possessed reduced sulfur particle sizes (1–10 nm) and improved cell performances with a reversible capacity over 10 cycles of 590 mAh g−1 at a working current of 0.13 mA cm−2. Fur-thermore, researchers have reported that along with elec-tric conductivity, the surface area and porosity of carbon materials are also vital to achieve optimal particle disper-sion and better contact between electrodes and SE particles, in which mesoporous carbon materials such as CMK-3 are promising because they possess 2D ordered mesoporous structures consisting of a hexagonal arrangement of cylin-drical rods [75]. In one example, Nagao et al. [74] prepared a S@CMK-3 composite using gas-phase mixing and sub-sequently incorporated thio-LISICONSE using solid-phase mechanical mixing through high-energy ball-milling to

obtain a composite positive electrode and reported that the corresponding cold-pressed S@CMK-3/thio-LISICON/Li-Al cell possessed improved cyclability with a reversible capacity of > 1300 mAh g−1 over 30 cycles under a current density of 13 μA cm−2. Here, the researchers suggested that the unique mesoporous framework of CMK-3 ensured large surface areas, adequate pore structures and better electronic conduction to sulfur in the resulting sulfur/carbon compos-ite. In addition, other studies have revealed that sulfur par-ticles in mesoporous structure of CMK-3 can interact with the edges of graphene sheets to allow for highly reversible reactions [8].

In another study, Kanno et al. [71] investigated the effects of three different mixing procedures including mechanical mixing, liquid-phase mixing, and combined mechanical and liquid-phase mixing on composite electrodes of sulfur–car-bon replica (S/CRs) and thio-LISICONSE (Fig. 4a). They attributed that the composite electrode prepared through the combined mechanical and liquid-phase mixing process resulted in cells that displayed extremely high capacities of 2000 mAh g−1, favorable cycling performances of 1500 mAh g−1 at 50 cycles, and good coulombic efficiencies close to 100% under 0.5C rate and 213 MPa applied pressure (Fig. 4b, c). Recently, Kinoshita et al. [76] prepared compos-ite electrodes consisting of sulfur, carbon fibers (VGCF), and amorphous Li3PS4SE using high-energy mechanical mill-ing and reported that the performance of the resulting cells was largely influenced by mixing conditions in which desir-able electrochemical results can be obtained if ball-milling was conducted for at least 20 h on the composite (Fig. 4d). Here, the as-assembled solid cells provide optimal discharge

Table 3 (continued)

Conductive additive

Active material Preparation method

Sulfur (wt%) Solid electrolyte (SE) type

Applied current density

aCapacity (mAh g−1)

bCapacity@ nth cycle

References

PANI@Super p Sulfur In situ polymeri-zation

60 PEO + LiTFSI + porous MOF (10 wt% MIL-53(Al))

0.5C 640 558@1000 49

LLZO@carbon Sulfur Pechini sol–gel method

64% PEO + LITFSI + LLZO 0.05C 900 800@200 86

Ketjen black Sulfur Solvent mixing – Al2O3–CPE + LICGC 0.1C 518 512 @50 42Carbon black Sulfur Solvent evapora-

tion42 PEO + 10 wt% montmo-

rillonite0.1C 998 634@100 88

Activated carbon Sulfur Ball-milling – PEO + LiBF4 +10 wt%Al2O3

0.07C 1600 40@10 47

Graphene Sulfur One pot reaction – PEO + LiTFSI + MIL-53(Al)

0.2C 1225 1060@100 171

Super P Sulfur Solvent casting 43 PEO + LiTFSI + MIL-35(Al)

0.5C 1457 793@50 172

Ketjen black Sulfur Solvent casting 40 PEO + LiFSI 0.1C 900 600@50 173Acetylene black Sulfur Solvent evapora-

tion24 PEO + LiTFSI + LiAlO2 0.1 mA cm−2 452 184@50 174

PEDOT/PSS Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate), PANI polyaniline, OMC order mesoporous carbon, LLZO LiLa7Zr2O12a Initial discharge capacity (mAh g−1) at a respective current densityb Retained capacity after nth number of cycles

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capacities of 1300 mAh g−1 at the first cycle and 1200 mAh g−1 at 50th cycle under 0.1 mA cm−2 currents (Fig. 4e). Additionally, Yu et al. [77] reported that the mechanical milling of sulfur, conductive super-P, and Li6PS5Cl SE can produce composite electrode that can deliver a maximum initial capacity of 1400 mAh g−1 at first cycle, but quickly degrades to 400 mAh g−1 over 20th cycle.

In a further study by Xu et  al. [78], the researchers reported that MoS2-doped Li2S–P2S5 (Li7P2.9S10.85Mo0.01) glass–ceramic SEs can provide ionic conductivities as high as 4.8 mS cm−1 and a good stability of 5 V (vs. Li/Li+) in which Li−S batteries using the Li7P2.9S10.85Mo0.01 SE can produce significantly higher capacities of 1020 mAh g−1 and increased cycling stabilities than batteries using Li7P3S11 SEs (775 mAh g−1) (Fig. 5a–c). In addition, the researchers

reported that the shapes of the discharge and charge curves of the cells with Li7P2.9S10.85Mo0.01 and Li7P3S11 SEs were completely different from that of liquid electrolyte Li–S batteries, in which a single characteristic discharge–charge plateau was observed, indicating no polysulfide dissolution. Here, the researchers suggested that the improved perfor-mance stemmed from the high ionic conductivity and good potential stability of Li7P2.9S10.85Mo0.01 SEs against Li metal, which can effectively suppress adverse side reactions and enable better interfacial contact between the electrode and SE particles. In another study by Xu et al. [79], a novel lithium superionic conductor was used, Li7P2.9Mn0.1S10.7I0.3 SE, with a high Li+ ion conductivity of 5.6 × 10−3 S cm−1 and a wide electrochemical stability up to 5 V versus Li/Li+ to obtain all-solid-state S@C/Li batteries, and it was

Fig. 4 a Processes for the fabricating sulfur–carbon replica-thio-LISI-CON solid electrolyte (S/CR/SE) composites: the process schematics depict: (i) mechanical mixing, (ii) liquid-phase mixing, and (iii) com-bination of mechanical and liquid-phase mixing; b charge–discharge curves for all-solid-state battery using composite electrode prepared by combination mixing; c discharge capacity and coulombic effi-ciency of battery as functions of cycle number. The battery was oper-ated at a 0.5 °C rate, and the battery cell was compressed at 213 MPa

during charge–discharge measurements. Reprinted with permission from Ref. [71], copyright 2018, American Chemical Society. d The relation between the first discharge capacity and milling time of the composite electrode; step-B represents milling of S/VGCNF precur-sor with Li3PS4 SE. e The discharge–charge cycle curves of the all-solid-state cells with the composite was prepared with the milling time of 40  h. Reprinted with permission from Ref. [76], copyright 2014, Elsevier

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reported that the resulting battery demonstrated a larger dis-charge capacity (~ 796 mAh g−1), better rate capability, and longer-range cycling stability compared with liquid electro-lyte Li−S batteries (Fig. 5d, e).

Researchers have also reported that graphene (reduced graphene oxide, rGO), with its ultrahigh conductivity,

large surface area, good thermal/chemical stability, excel-lent mechanical properties, and efficient electron transport pathways of S/rGO composites, can alleviate the negative impacts of volume change during cycling [80]. For example, Xu et al. [81] designed an ASSLSB using a composite elec-trode composed of sulfur, rGO, and Li9.54Si1.74P1.44S11.7Cll0.3

Fig. 5 a Schematic diagram of the configuration for all-solid-state Li−S batteries; b galvanostatic discharge/charge curves, and c cyclic performance (capacity vs. cycle number) of all-solid-state Li−S bat-teries based on Li7P2.9S10.85Mo0.01 and Li7P3S11 solid electrolytes. Reprinted with permission from Ref. [78], copyright 2017, The Royal

Society of Chemistry. d Charge–discharge curves and e cycling per-formances of Li−S battery with Li7P2.9Mn0.1S10.7I0.3 solid electrolyte and liquid electrolyte at 0.05C rate. Reprinted with permission from Ref. [79], copyright 2017, The Royal Society of Chemistry

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(LSPSC) SE (Fig. 6a) and reported that the LSPSC SE pos-sessed good compatibility with the S/rGO composite and formed a novel sulfur-based composite electrode through ball-milling. And as a result, the obtained S/rGO/LSPSC/

Li cell produced a discharge capacity of 969 mAh g−1 (first cycle) and good cyclability (~ 85.3% retention) at 80 mA g−1 over 60 cycles (Fig. 6b, c). Here, the researchers attributed these results to the intimate contact between the S/rGO

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electrode and the LSPSC SE, which mainly originated from the high conductivity of the LSPSCSE and the unique prop-erties of the rGO. Similar to rGO, 1DMCNTs (multi-walled carbon nanotubes) also possess desirable physical properties and can be used for Li−S batteries as a cathode comple-ment [19]. For example, Zhou et al. [82] obtained Li−S cells using a sulfur-coated MCNT (S-MWCNT) cathode and a NASICON-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP) SE deposited with an evaporated lithium metal electrode (Fig. 6d). Here, the researchers reported that the as-assembled solid-state S-MWCNTs/LAGP/Li cell demonstrated favorable electro-chemical performances including a high discharge capacity of 1510 mAh g−1at the first cycle with 90% sulfur utilization and 1400 mAh g−1at the end of 30th cycle (Fig. 6e–g) with a nearly 100% coulombic efficiency during all cycles, dem-onstrating that the shuttle effect was completely suppressed.

3.1.2 All‑Solid‑State Batteries with Solid Polymer Electrolytes

The incorporation of inorganic ceramic particles (including conductive and nonconductive fillers) along with polymer matrixes is an effective strategy to improve ionic conduc-tivity and reduce interfacial resistance between electrodes and SEs, leading to improved cell performances [48–50, 83–86]. For example, Liang et al. [83] introduced 10 wt% SiO2 to PEO–LiTFSI(Li(CF3SO2)2N) SPE and reported an ionic conductivity of ~ 5 × 10−4 S cm−1at 70 °C. Here, the researchers assembled an all-solid-state sulfur-OMC (ordered mesoporous carbon spheres)/Li battery using the obtained PEO–LiTFSI–10 wt% SiO2 SPE and reported exceptional durability over 25 cycles, a reversible capac-ity of > 800 mAh g−1, and current density of 0.1 mA cm−2. In another study, Armand et al. [84] attempted to allevi-ate the polysulfide shuttle effect and improve Li−S battery performance by taking on a novel approach to fabricate sulfur cathodes through inverse vulcanization using the

radical polymerization of sulfur and 3,5 divinylbenzene (p(S-DVB)). Here, the researchers reported that Li−S bat-teries using an optimal p(S-DVB) cathode (80 : 20, w/w to S : DVB) and a LiFSI (Li(FSO2)2)N) complex with PEO elec-trolyte can achieve better electrochemical performances at 70 °C and a working current of 0.1C, providing a discharge capacity of 1100 mAh g−1 at the first cycle and an excellent cyclability of 650 mAh g−1 at the end of 50 cycles, along with nearly 100% efficiency at all cycles. In another study, Eshetu et al. [85] utilized a sulfur/ketjen black (S/KB) com-posite electrode and a SPE composed of PEO complexed with a highly conductive Li salt such as LiFSI, LiTFSI, or LiFTFSI (LiCF3(SO2)2NF) in a Li−S battery and reported that the cell with the LiFTFSI/PEO SPE coupling demon-strated better performances at 0.5C, higher areal capacities (1.2 mAh cm−2), and larger specific capacities (1394 mAh g−1

sulfur) as well as longer cyclability (800 mAh g−1sulfur at 60

cycles).MIL53(Al) is a type of metal–organic framework (MOF)

and can serve as a 3D nanofiller for SPEs. In addition, this material exhibits both inorganic and organic hybrid proper-ties such as a large surface area and an ordered microporous structure. And as a result of these properties, Liu et al. [49] fabricated MIL53(Al)-modified PEO–LiTFSI composite SPEs and reported a remarkably improved lithium ionic conductivity of 2.41 × 10−4 S cm−1 at 80 °C compared with the unmodified SPE (9.24 × 10−5 S cm−1). In addition, the researchers reported that the 3D MIL-53(Al) additive can impede shuttle effect and maintain intimate contact between the electrolyte and electrode during charge/discharge cycles. And as a result, cells with a PANI@C/S cathode and the PEO–LiTFSI–MIL53(Al) hybrid SPE reportedly delivered have reversible capacity of ~ 1520 mAh g−1 and an extended durability of over 1000 cycles under a high working C-rate of 4C at 70 °C. Furthermore, Lin et al. [40] reported that a Li−S cell using a halloysite nanotube-modified PEO (PEO–LiTFSI–HNT) composite SPE can provide a dis-charge capacity of 800 mAh g−1(1st cycle) and good cycling stability (93% retention) over 100 cycles at 0.1C. In another study, Tao et al. [86] integrated a single Li-ion conductor (garnet-type Li7La3Zr2O12(LLZO)) with PEO composite SPE for ASSLSBs operating at 37 °C (Fig. 7a) and reported that the resulting cell using the S@LLZO@C composite electrode and the PEO + 15 wt% of LLZOSPE achieved a capacity of 900 mAh g−1 and better stability (800 mAh g−1 retained at 200 cycles) compared with the simple C@S elec-trode (Fig. 7b, c).

More recently, Sheng et  al. [87] investigated Li−S cells utilizing sulfur/N-doped carbon nanosheet compos-ite cathodes and SPEs (PEO-based) modified with ionic liquid-grafted oxide nanoparticles (IL@NPs) such as Si, Zr, and Ti oxides and reported that among the composite SPEs, PEO–IL@ZrO2 provided optimal conductivities at

Fig. 6 a Scheme for the preparation of rGO/S–C–SE composite elec-trode via ball-milling manner; b cycling performance of S–C–SE and S/rGO–C–SE composite cathodes obtained at a current density 80 mA g−1 and corresponding coulombic efficiencies at RT and c rate capability of an ASSLSB with S/rGO–C–SE composite. Reprinted with permission from Ref. [81], copyright 2017, Wiley–VCH. d Schematic diagram of LAGP solid electrolyte-based ASSLSBs (where a LAGP plate, b Li anode was evaporated on one side of LAGP plate, c S-MWCNT cathode was casted on the other side of LAGP plate, d, small amount of ionic liquid was added on the cath-ode electrode, e the as-prepared electrode materials were encapsu-lated with aluminum plastic, and f cross-sectional view of this Li–S cell). e The comparison on cycling performance and coulombic effi-ciency of ASSLSBs with ceramic solid electrolyte and organic liquid Li−S battery. f Discharge/charge curves of solid-state Li−S battery at 25 and 55 °C, g rate capability of solid-state Li−S battery at 55 °C. Reprinted with permission from Ref. [82], copyright 2017, American Chemical Society

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50 °C (4.95 × 10−4 S cm−1) and 37 °C (2.32 × 10−4 S cm−1). Moreover, the researchers in this study reported that the use of PEO–IL@ZrO2 SPE in Li−S cells resulted in excel-lent electrochemical performances and stabilities, in which at 50 °C, a maximum discharge capacity of 986 mAh g−1 was observed, whereas at 37 °C, a maximum discharge capacity of ~ 600 mAh g−1 was observed and maintained after 80 cycles. In another study, Judez et al. [42] devel-oped polymer-rich PEO incorporated with Li-ion conduct-ing glass–ceramic (LICGC) or inactive inorganic Al2O3 as hybrid polymer electrolytes for Li−S batteries (Fig. 7d) and reported that in the comparison with the two composite pol-ymer electrolytes (CPEs), the S/Al2O3–CPE/LICGC–CPE/Li cell presented a higher specific capacity of ~ 518 mAh g−1, larger areal capacity of ~ 0.53 mAh cm−2, and a better charge/discharge efficiency of 99% for 50 cycles at 70 °C (Fig. 7e, f). Zhang et al. [88] also reported that an ASSLSB using a PEO/10 wt% MMT (montmorillonite) composite polymer electrolyte exhibited an average discharge capacity of ~ 634 mAh g−1 for 100 cycles at 0.1 C and 60 °C.

3.2 Composite Positive Electrodes with Lithium Sulfide

Lithium sulfide (Li2S), a discharge product of Li−S batter-ies, is a promising cathode material because it can deliver

an acceptable theoretical capacity of 1166 mAh g−1 and can be paired with different types of Li metal-free anodes such as silicon and tin [89, 90]. However, significant issues of Li2S include its electronically and ionically insulating nature similar to sulfur as well as its high sensitivity to air and moisture, which complicates cathode fabrication pro-cesses. Despite this, many attempts have been made in recent years to improve the conductivity of Li2S in ASSLSBs [70, 91–97]. In one example, Nagao et al. [70] prepared Li2S par-ticles by combining conductive AB with glass–ceramic Li2S. P2S5 SE using high-energy ball-milling and reported that compared with mortar grinding, mechanically ball-milled Li2S–AB–Li2S.P2S5 composite electrodes can provide attractive cell performances such as a large discharge capac-ity of 700 mAh g−1 and superior charge/discharge cycling stabilities. Here, the researchers attributed the improved per-formances to the intimate contact of the triple phase.

The enhancement of Li+ ion conductivity for Li2S can also effectively increase electroactive material utilization such as Li2S utilization upon charge–discharge. For exam-ple, Shin et al. [93] uniformly coated Li2S with carbon using thermal evaporation from a polyacrylonitrile source at 600 °C and reported that the electronic conductivity of the resulting Li2S was greatly improved from 9.21 × 10−9 to 2.39 × 10−2 S cm−1 upon carbon coating. And as a result, a high initial discharge capacity of 585 mAh g−1 (g of Li2S)

Fig. 7 a Schematic illustration of an all-solid-state Li−S battery based on PEO–LiClO4–LLZO composite SPE, b charge/discharge curves of solid-state batteries based on S@LLZO@C and S@C cathodes at a current density of 0.1 mA cm−2 at 50 °C, and c cycling performance and coulombic efficiency of S@LLZO@C cathode at 0.05  mA  cm−2 and operated at 37  °C. Reprinted with permis-sion from Ref. [86], copyright 2017, American Chemical Society. d

Sketch of the battery with a bilayer electrolyte configuration (3 vol% Al2O3-based and LICGC-based CPEs). e Galvanostatic discharge/charge profiles at the third cycle and f cycling performance of Li−S cells with LiFSI/PEO, 3 vol% Al2O3, and 3 vol% LICGC-based SPEs, and bilayer cell at 70 °C. Reprinted with permission from Ref. [42], copyright 2017, American Chemical Society

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was obtained at the first cycle and further increased to 730 mAh g−1 (g of carbon-coated Li2S) by the 10th cycle, with the cell maintaining this high capacity even after 25 cycles. Takeuchi et al. [95] also successfully applied Li2S–carbon composites to ASSLSBs and achieved a cell discharge capacity of 750 mAh g−1 and a maximum energy density of 1490 mWh g−1.

In a further study, Hakari et al. [96] developed Li2S–LiI solid solutions using mechanical milling and reported that the addition of 20 mol% LiI resulted in a twofold improve-ment in Li2S ionic conductivity with the cathode composite comprised of an active material of 80Li2S–20LiI, Li3PS4 SE and conductive VGCF prepared through high-energy ball-milling. Here, the researchers reported that their all-solid-state cell based on the resulting composite cathode exhibited an extremely high performance with a retained capacity of 930 mAh g−1 (based on Li2S) over 50 cycles and a Li2S utilization improvement from 50% to 80%. In another study, Hakari et al. [97] developed Li−S solid-state cells using Li2S-based solid solutions by combining Li2S with a lithium halide (LiX, X = LiCl, LiBr, and LiI) as the active material, 75Li2S·25P2S5 (mol%) glass as the SE, and Li−In as the anode. Here, the researchers reported that among all the composite cells, the cell with a Li2S·LiI (80 : 20 mol%) cathode produced a maximum capacity of > 1100 mAh g−1 (95% of theoretical capacity) at 0.5C. Furthermore, this cell provided outstanding stability (almost 100% retention; 980 mAh g−1 capacity) for 2000 cycles at a current rate of 2C.

3.3 Composite Positive Electrodes with Metal Sulfides

Several transition metal sulfide systems such as NiS [98], Co9S8 [99], FeSx [100], MoS3 [101, 102], TiS [103], and Cu2ZnSnS4 [104] have been employed as cathodes in ASSLSBs due to their large theoretical capacity, promising electrochemical activity, relatively high conductivity, and better interfacial compatibility with sulfide electrolytes. Tat-sumisago et al. investigated several metal sulfides, includ-ing Ni [98]-, Mo [101]-, and Ti [102]-based sulfide positive electrodes for ASSLSBs and reported that the composite composed of NiS and glass–ceramic 80Li2S–20P2S5SE, which was fabricated using high-energy mechanical milling, and mixing with conductive AB though manual grinding in a mortar resulted in an outstanding reversible capacity of 360 mAh g−1 at 50 cycles [98]. In their other studies, Tatsumisago et al. mixed amorphous TiS3 and MoS3 with 80Li2S–20P2S5 SE through high-energy milling to obtain composite electrodes for ASSLBs and reported that both MoS3- and TiS3-based composite electrodes demonstrated maximum reversible capacities of 510 and 670 mAh g−1, respectively, over 10 cycles.

In a further study, Jung et al. [103] studied the effects of mechanochemical reactions between TiS2 cathodes and Li3PS4 SEs and reported remarkable enhancements in Li storage in ASSLBs in which increased ball-milling times led to amplified Li-ion capacity for the resulting com-posite (Fig. 8a–f) and that after 9 min of ball-milling, the TiS2-based composite electrode presented a discharge capac-ity above 837 mAh (g of TiS2)−1 at 50 mA g−1 with excellent cycling life span at different current rates and better coulom-bic efficiencies of more than 100%. Here, the researchers attributed these improved performances to the amorphous Li–Ti–P–S phase which was obtained through the partial reaction between the TiS2 and SE during the high-energy ball-milling process (Fig. 8g). In a related study, Passerini et al. [105] prepared a composite positive electrode through a straightforward ball-milling route using activated carbon, FeS2 (pyrite), and sulfur (C–FeS2–S) components and found that the crystalline structure, morphology, and electrochemi-cal performance of their resulting composite were greatly influenced by the ratio FeS2/S. Here, the researchers reported that their Li/LiI–Li3PS4/C–FeS2–S battery with an optimal C/FeS2/S (40 : 30 : 30 wt%) ratio for the composite cathode demonstrated a high capacity of > 1200 mAh g−1(1.2 mAh cm−2) at an intermediate loading of 1 mg cm−2 and main-tained a capacity of ~ 710 mAh g−1(3.55 mAh cm−2) even at a higher active material loading of 5 mg cm−2 for 20 cycles. The researchers in this study attributed these findings to the combination of sulfur with active transition metal sulfides in the all-solid-state configuration and suggested that this was a promising approach to achieve higher capacities and rate capabilities for next-generation, safer Li batteries.

Recently, the report of new Li superionic conductor Li10GeP2S12 (LGPS) SEs has led to significant break-throughs in high-energy density ASSLBs and was found to provide unique advantages such as increased safety and longer cycling life span [37]. However, the instabil-ity of LGPSSEs against Li metal anodes limits appli-cation in large-scale ASSLBs. To address this, Wang et  al. [104] proposed a novel bilayer solid electrolyte (Li10GeP2S12@70Li2S–29P2S5–1P2O5) concept to prevent the reaction between metallic Li and Li10GeP2S12, in which an ASSLB was assembled using a composite of Cu2ZnSnS4/graphene (CZTS/graphene) combined with LGPS and AB as the cathode, lithium metal as the anode, and a sulfide electrolyte bilayer as the solid electrolyte (Fig. 8h). Here, the researchers reported that in liquid electrolyte batteries, the CZTS/graphene-21(denotes 2 : 1 weight ratio of CZTS to graphene) can maintain a discharge capacity of 556 mAh g−1 for 100 cycles at 50 mA g−1 (Fig. 8i), whereas their assembled ASSLSB using the CZTS/graphene-21 cathode and the bilayer SE (Li10GeP2S12 and 70Li2S–29P2S5–1P2O5) provided a larger discharge capacity of 760 mAh g−1 at the second cycle and retained 545 mAh g−1 over 50 cycles at

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50 mA g−1. In addition, the researchers also reported that at high currents of 250 and 1000 mA g−1, their assembled cell exhibited excellent stability up to 100 and 300 cycles, respectively (Fig. 8j). These enhanced performances of the assembled ASSLSB can be attributed to the fact that in the Li10GeP2S12SE, Ge4+ can be easily reduced by Li metal to form Li-Ge alloys during electrochemical reactions, which can form a new mixed conducting layer that can deterio-rate the interface, whereas no metal ions are present in the 70Li2S–29P2S5–1P2O5 which can form stable solid elec-trolyte interphase (SEI) layers and be more compatible against Li metal [51]. And because of this, a combination of LGPS and 70% Li2S–29% P2S5–1% P2O5SE bilayers is an efficient method to resolve the instability issues between ultrafast lithium ion conducting LGPS and metallic lithium in ASSLBs.

3.4 Solid Electrolytes as Active Materials

High Li-ion conductive solid electrolytes, mainly sulfide SEs, can function as active materials through hybridiza-tion with electronic conductors such as carbons or metals [106–108]. For example, glass/glass–ceramic Li2S–P2S5SEs mixed with conductive nanocarbons through mechanical milling can yield Li2S–P2S5–C composites which can act as active materials. Based on this, Hakari et al. [106] assem-bled ASSLSBs utilizing 75Li2S–25P2S5 glass (Li3PS4) as the SE and active material. They reported that cells with pristine Li3PS4 glass and/or mixtures prepared through the mortar grinding of Li3PS4 glass and AB as the cathode were not operational, whereas cells assembled using ball-milled Li3PS4–AB composite electrodes were reversibly oxidized and reduced as an active electrode material in ASSLSBs. Furthermore, the researchers reported that the cells assem-bled with the Li3PS4–AB composite cathode delivered an improved capacity of 220 mAh g−1 and an improved stability of up to 50 cycles.

These researchers also took another approach [107] in which composite electrodes comprised of Li3PS4 SE and different conductive carbon additives (AB, VGCF, KB, and AC, possessing different physical properties such as morphology, conductivity, and surface area) were prepared through mechanical ball-milling (Fig. 9a) and reported that the performances of the solid-state Li−In/Li3PS4/Li3PS4–C cells were largely influenced by the effective contact area and ionic conductivity of the composites. The average discharge potential of the Li3PS4–carbon compos-ite cells was 2.0 V versus (Li/Li+), corresponding to the oxidation/reduction potentials (Fig. 9a) [109]. Here, the Li3PS4–AC composite-based solid-state cell demonstrated relatively better performances in terms of its large ini-tial discharge capacity and excellent cycling reversibility over 300 cycles. Furthermore, Zhang et al. [108] compos-ited 78Li2S–22P2S5 glass–ceramic with multiple carbons (MC) and used this as active electrode materials in ASS-LBs (Fig. 9c) and reported a maximum reversible capacity of ~ 654.5 mAh g−1(at 44 μA cm−2), outstanding cycling stabilities (480 mAh g−1 at 0.176 mA cm−2 for 60 cycles), and an efficiency of > 99% with an ultrahigh active mate-rial loading of 7 mg cm−2.

The addition of Cu metal powder to SE is also highly beneficial to activate SEs as an active electrode mate-rial. Based on this, Tatsumisago et al. [110] developed a Li2S–P2S5–Cu composite positive electrode through the simple mixing of Li2S–P2S5 (80 : 20  mol ratio) glass–ceramic SE and Cu powder in an agate mortar and reported that the corresponding In/80Li2S·20P2S5SE/80Li2S·20P2S5–Cu battery, possessing an optimal 48/52 molar Li2S/Cu cathode, produced a large reversible capacity of 110 mAh g−1 with respect to 80Li2S·20P2S5–Cu weight and 400 mAh g−1 with respect to Li2S weight. Here, the researchers suggested that the formation of LixCuyS active domains on the initial charge process was crucial to improve the capacity of the resulting cell.

Compared with S and Li2S cathodes, sulfide SEs such as glass/glass–ceramic Li2S–P2S5 possess advantages such as high ionic conductivity and favorable mechanical proper-ties. However, sulfide solid electrolytes demonstrate poor reversibility as cathode materials during charge and dis-charge, suggesting that sulfide solid electrolytes are more suitable for primary batteries. Furthermore, SEs should be thick enough to prevent short circuiting during long-term cycling, which sacrifices battery energy density, meaning that more research is required.

Fig. 8 a–f Initial two charge–discharge profiles of hand mixed (HMe) and ball-milled (BMe) composite electrodes of TiS2-based nanocom-posites at current density of 50 mA g−1. The inset in c shows the first-cycle voltage profiles when charge (delithiation) is carried out first. g Schematic diagram of BMe showing the nanostructures and the roles of each component. Reprinted with permission from Ref. [103], copyright 2014, Nature Publishing Group. h Schematic diagram of all-solid-state lithium battery with Cu2ZnSnS4/graphene compos-ite cathode, bilayer Li10GeP2S12@70%Li2S–29P2S5–1%P2O5 solid electrolyte and Li metal anode. i Cyclic performances of CZTS and CZTS/graphene composites at a current density of 50 mA g−1 in tra-ditional lithium-ion batteries using 1 M LiPF6 in EC and DMC as the liquid electrolyte. j Cycling stability for the CZTS/graphene-21 under the current density of 100 mA g−1 and 1000 mA g−1 in all-solid-state lithium batteries. Reprinted with permission from Ref. [104], copy-right 2016, Elsevier

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4 Recent Progress in Electrode/Electrolyte Interfaces of ASSLSBs

4.1 Cathode/Solid Electrolyte Interfaces

The interface in ASSLBs mainly involves solid-to-solid con-tact (electrode–electrolyte), resulting in large resistances and limited Li-ion transport. Furthermore, the stress/strain at the electrode–electrolyte interface is greatly increased due to the volume change of active electrode materials such as S/

Li2S during repeated charge–discharge cycling, leading to loss of contact or interfacial separation [25, 68, 111–114]. As a result, ASSLSBs experience poor cycling stability and segregation; therefore, a crucial task in the develop-ment of ASSLSBs is to alleviate interfacial resistances at the electrode–electrolyte interface. Based on this, various methods have been applied by researchers recently to resolve these interfacial issues and improve battery performances with high durability and rate capability in charge–discharge reactions.

Fig. 9 a Schematic of illustration of Li3PS4 (LPS)-carbon composite electrodes was prepared by ball-milling. Properties of LPS-carbon electrode were characterized using compressed pellets of the elec-trode powders and all-solid-state capacitors with LPS-carbon elec-trodes as both electrodes. Electrochemical performances of LPS were evaluated using all-solid-state batteries based on LPS-carbon com-posite electrode. Factors affecting the electrochemical performances were investigated from relationship between the electrode proper-ties and the electrochemical performances. Reprinted with permis-

sion from Ref. [107], copyright 2017, The Electrochemical Society. b Schematic configuration of the monolithic cell 78Li2S–22P2S5–MC|78Li2S–22P2S5|Li−In. Yellow polyhedrons represent the 78Li2S–22P2S5 particles, and dark dots and rods denote the MC par-ticles. c The corresponding discharge–charge voltage profiles of the cells using 70 wt% 78Li2S–22P2S5–30 wt% MC as the electrodes at 2nd, 10th, and 60th cycles. Reprinted with permission from Ref. [108], copyright 2018, American Chemical Society

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One potential method to resolve interfacial issues is to reduce active material particle size and enhance the contact area of electrode components such as the active material and the SE particles through mechanical ball-milling, potentially improving contact between electrodes and SEs and alleviat-ing interfacial resistances in ASSLSBs. For example, Choi et al. [114] fabricated composite cathodes comprised of sul-fur, conductive AB, and 70Li2S : 30P2S5 glass–ceramic SE through simple grinding in a mortar, ball-milling, and sec-ondary high-energy ball-milling and compared the resulting Li−S battery performances. Here, the researchers reported that the secondary ball-milling method allowed for the ade-quate mixing of all three components with reduced particle sizes and favorable intimate contact, resulting in the high utilization of sulfur during charge–discharge propagation. In another example, Kanno et al. [115] have fabricated com-posite electrode comprised of sulfur, AB, and thio-LISICON SE through mechanical milling at an elevated temperature of 155 °C and reported that at this temperature, the melted sulfur possessed the lowest viscosity, allowing for favora-ble contact between the mixed constituents of melted sulfur, SE, and conductive carbon. Here, the researchers suggested that this approach resulted in a unique cathode structure with reduced particle sizes along with minimal interfacial impedance, allowing for a corresponding ASSLSB using the composite electrode to produce a large discharge capac-ity of 1200 mAh g−1 and long cycling stabilities with an unchanged capacity even up to 40 cycles.

Additionally, metal additives such as Cu can greatly increase the electrode/electrolyte interfacial contact in ASSLSBs because Cu metal/metal powders readily com-bine with sulfur to form electrochemically active CuS. And based on this, Hayashi et al. [116] reported that the ball-milling of sulfur and Cu crystals to form sulfur-based com-posites can provide enhanced interfacial contact between S–Cu electrodes and SEs to mitigate interfacial resistances at the electrode/SE interface, in which their all-solid-state S–Cu/80Li2S·20P2S5 glass–ceramic/Li−In cell demonstrated an excellent reversible capacity of ~ 650 mAh g−1 (on the basis of S and Cu weights) at 64 μA cm−2 for up to 20 cycles.

Yao et al. [68] reported another important strategy to resolve interfacial issues in which they uniformly coated a nanosized amorphous sulfur layer (~ 2 nm) onto rGO sheets to obtain S/rGO hybrid and subsequently homo-geneously distributed this hybrid into a LGPS solid electrolyte to achieve a composite cathode [68]. The researchers suggested that a bilayer sulfide material LGPS/75%Li2S–24%P2S5–1%P2O5, if used as the SE, can prevent undesirable side reactions between Li metal and LGPS and that their as-assembled ASSLBs (Fig.  10a) using this approach effectively minimized the large inter-facial resistance between the active electrode and the SE particles, increasing electronic conduction and buffering

volume expansion of active materials due to stress/strain. As a result, their cell with the nanosized sulfur-coated SE pro-duced excellent performances with extremely high reversible capacity and cyclability at 60 °C (Fig. 10b; ~ 1629 mAh g−1 at the first cycle and ~ 1500 mAh g−1 at the end of the 30th cycle at 0.05C).

More recently, a novel hot-press setup was developed by Busche et al. [117] to reduce interfacial resistances in ASSLSBs, in which grain boundaries and interfacial resist-ances were minimized and fast ion transport through the interface and bulk was accommodated, resulting in a high discharge specific capacity of 1370 mAh g−1 (82% sulfur utilization) and superior cycling stability under 0.1C rate. And because of these performances, this study provides an effective method to alleviate interfacial resistances and enhance contact areas between sulfur electrodes and SEs for the construction of high-performance ASSLSBs.

In another study, Liang et al. [118] investigated sulfur-rich lithium polysulfidophosphate (LPSP, Li3PS4+n), a novel electrode material, with high Li ionic conductivity and remarkable electrochemical performances in ASSLSBs. This sulfur-rich LPSP possessed a long sulfur chain, which was obtained through the wet-chemical reaction of sulfur and Li3PS4 in a tetrahydrofuran solvent at RT. The pro-posed reaction mechanism for the formation of the LPSP electrode and its electrochemical reactivity with Li metal anodes are shown in Fig. 10d, and the ionic conductivity of this sulfur-rich Li3PS4+5 compound was 3 × 10−5 S cm−1 at RT (Fig. 10e), which was larger than that of pristine S/Li2S cathodes. The researchers reported that a Li3PS4+5 cathode-based ASLSB delivered good cycling and rate performances at RT with an initial capacity at 0.1C of 1272 mAh g−1 (by incorporated sulfur weight), a charge–discharge efficiency of 100%, and a capacity retention of 700 mAh g−1 after 300 cycles. In addition, the researchers reported that by increas-ing the temperature to 60 °C, the durability of the cell was significantly improved (Fig. 10f), in which a large capacity of 1400 mAh g−1 and an extended cycling stability with 86% retention over 300 cycles were achieved, suggesting that the improved conductivity of the sulfur-rich LPSP and the reduced interfacial resistance at the cathode/SE interface were essential in the enhancement of ASSLB performance.

In another important study to mitigate large interfa-cial resistances in ASSLBs, Wang et al. [119] developed a single-material battery concept in which an ASSLB was assembled utilizing only LGPS materials. Here, the ASSLB comprised of a superimposed LGPS–C composite cath-ode, a LGPS electrolyte, and a LGPS–C composite anode (Fig. 10c) in which the Li−S (similar to those observed in Li2S) and Ge–S (similar to those observed in GeS2) in the LGPS can serve as active centers for both the positive and negative electrodes in association with conductive carbon black. And as a result, the researchers reported that the

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Fig. 10 a Schematic diagram of all-solid-state Li−S battery based on rGO@S–Li10GeP2S12 composite cathode, Li10GeP2S12 solid electro-lyte, and Li metal anode and b discharge–charge profiles of all-solid-state Li−S battery with amorphous rGO@S-40 composite electrode at 60  °C under 0.05C. Reprinted with permission from Ref. [68], copyright 2017, WILEY–VCH. c Schematic representation of battery made by using single material of Li10GeP2S12. Reprinted with per-mission from Ref. [119], copyright 2015, WILEY–VCH. d Reaction of sulfur with Li3PS4 yields Li3PS4+n at RT in tetrahydrofuran (THF)

solvent, and the electrochemical reaction mechanism for charge–dis-charge of Li3PS4+n (reversible scission and formation of S–S bonds). e Temperature dependency of ionic conductivity of Li3PS4+5 (open square, Ea = 0.37  eV) and Li2S (filled circle, Ea = 0.74  eV), and f cycling performance of Li3PS4+5 cathode with a mass loading of about 0.25–0.60 mg cm−2 at the rate of 0.1C at RT and 60 °C (pink/red: charge; black/blue: discharge). Reprinted with permission from Ref. [118], copyright 2013, WILEY–VCH

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solid-state LGPS–C/LGPS/LGPS–C cell delivered a large reversible capacity and high-rate performances, along with extremely low interfacial resistances resulting from modified interfacial interactions, enhanced interfacial contacts, and effective mitigation of interfacial stress/strain, suggesting that this novel approach is a potential direction to resolve interfacial issues in ASSLBs.

Recently, Han et al. [120] developed a novel bottom-up procedure to resolve interfacial issues by compositing Li2S, Li6PS5Cl SE and polyvinylpyrrolidone (PVP) conductive framework precursors into a hybrid (Fig. 11a), and reported that the key benefits of this strategy included the even con-finement of the active material and SE into the carbon frame-work and the simultaneous enhancement of electron and ion transport (Fig. 11b). And based on this, a corresponding ASSLSB with the Li2S–Li6PS5Cl–carbon composite elec-trode produced enhanced performances with a retained reversible capacity of 830 mAh g−1 after 60 cycles as com-pare with the counterpart Li2S–carbon (Fig. 11c). In another study, Lin et al. [121] developed a novel and highly active core–shell composite structure of Li2S@Li3PS4 to improve the electrode–electrolyte interface (inset of Fig. 11d) and reported that the as-prepared Li2S@Li3PS4 core–shell mate-rial performed similar to lithium superionic sulfide (LSS), in which LSS and its Li+ ion conductivity experienced a 6 orders of magnitude improvement compared with bulk Li2S. Here, the ASSLSB assembled utilizing the LSS cath-ode (Fig. 11d, e) produced a discharge capacity as high as 848 mAh g−1 with respect to Li2S and a great cycling sta-bility of 71% retention over 100 cycles at 60 °C under 0.1C rate. Furthermore, Eom et al. [122] obtained a Li2S–VGCF nanocomposite using a liquid-phase approach to enable the uniform growth of Li2S nanocrystals with highly controlled sizes over a conductive VGCF matrix and reported that the highly dispersed Li2S nanocrystals with uniform size facili-tated better contact with SE particles and VGCF in the corre-sponding ASSLB and that the interfacial properties between the electrode components were improved by optimizing the composition of Li2S and VGCF. And as a result, the result-ing battery presented a large capacity of 600 mAh g−1 and a better coulombic efficiency of up to 100% with better stabil-ity over 20 cycles.

In 2016, Yao et al. [99] developed an ultrafine interfa-cial structure involving sulfide SEs with Li7P3S11 particles (∼ 10 nm) being anchored onto Co9S8 nanosheet surfaces through a bottom-up approach. The researchers suggested that the unique interfacial structure of the Co9S8/Li7P3S11 can provide intimate interfacial contact between SE and electroactive material and can also preserve uniform volume changes in the electroactive material of Co9S8 nanosheets, resulting in favorable conduction pathways at the interface with minimized interfacial impedance. And as a result, an assembled solid-state battery using the Co9S8/Li7P3S11

composite cathode demonstrated an ultrastable performance (~ 501 and 421 mAh g−1 capacities at the first and 1000th cycles under a working current of 1.27 mA cm−2) with high energy and power densities. Similarly, Xu et al. [123] reported that the uniform surface coating of a Li7P3S11 SE layer onto MoS2 particles can also form intimate interfacial contact between SE particles and active materials, leading to severe reductions in interfacial resistance, with solid-state batteries using MoS2/Li7P3S11 electrodes displaying a high capacity of 547 mAh g−1 and better cyclability than that of uncoated MoS2. In a further study, Aso et al. [113] demonstrated that conductive sulfide glass–ceramic SE (Li2S–P2S5(80 : 20 mol%)) coatings over NiS-VGCNF com-posites can also enable intimate solid–solid contact between the NiS-VGCNF electrode and the glass–ceramic SE and that the surface coating can provide favorable Li conduc-tion pathways in the electrode composite, allowing batter-ies assembled with the composite to possess high capacities (~ 300 mAh g−1), better rate capabilities and improved cycla-bility compared with batteries assembled with uncoated composite electrodes.

Studies have also reported that nanostructured materials with small particles sizes can reduce interfacial impedance and alleviate large volume change during electrochemical conversion processes. For example, Long et al. [124] fab-ricated NiS nanorods (20–50 nm in diameter and 2–3 mm in length) using a facile solvothermal method and assem-bled an ASSLSB based on this NiS-LGPS-AB composite cathode together with a Li metal anode and a bilayer solid electrolyte (LGPS and 70%Li2S–29%P2O5–1%P2O5) located between the cathode and anode and reported that the bilayer SE endowed good compatibility with metallic Li and lim-ited the parasitic reactions between the LGPSSE and the Li metal. And as a result, the cell possessed intimate contact between the electrode and SE and was able to suppress the huge volume change during the conversion reaction, produc-ing a high capacity of 401 mAh g−1 and excellent durability with 84% retention at 100 cycles.

4.2 Anode/Solid Electrolyte Interfaces

In addition to cathode/SE interfaces, anode/SE (especially in cases in which Li metal is utilized) interfaces are also important to enhance battery performance in terms of energy density, durability as well as safety [125–129]. Here, the main challenges of various Li/SE interfaces include the poor chemical stability of SEs against Li metal, large interfacial resistances, and the formation and growth of Li dendrites [130–132]. And based on the nature and ability of the SEs, several methods such as the application of high pressure and temperature, the modification of SE surfaces, and the passivation of Li metal electrodes as well as the alloying of bulk/thin-film anodes such as Li−M (M = Si, Sn, Al, In, Ge,

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Fig. 11 a Schematic of composite cathodes based on solution pro-cess. b High-resolution TEM image of Li2S–Li6PS5Cl–C nanocom-posite. c Cycling performances of Li2S–C and Li2S–Li6PS5Cl–C electrodes at current density of 50  mA  g−1. Reprinted with permis-sion from Ref. [120], copyright 2016, American Chemical Society. d Representative voltage profiles of nano-Li2S and lithium superionic

sulfide (LSS) cathode; the inset is the scheme of the forming pro-cess of Li3PS4 coating. e The cycling performances of nano-Li2S and LSS cathode materials for all-solid Li−S cells at 60 °C at 0.1C rate. Reprinted with permission from Ref. [121], copyright 2013, Ameri-can Chemical Society

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etc.) have widely been studied to resolve the aforementioned challenges [67, 73, 114, 132, 133].

4.2.1 Sulfide SEs/Li Interfaces

Most sulfide SEs are not stable against Li metal electrodes and will decompose to form SEI layers with massive inter-facial impedances [129, 134, 135]. In addition, surface con-tact is inadequate if Li metal electrodes are directly attached onto sulfide solid electrolyte surfaces and inhomogeneous interfaces can cause uneven lithium dissolution/deposition and reduce Li metal utilization, leading to serious obstacles such as large interfacial resistances and inferior durability

in ASSLSBs [136, 137]. Recently, the interfacial stabil-ity between Li metal and divergent SE materials such as Li10GeP2S12, Li7P3S11, Li2S–P2S5, argyrodite Li6PS5X (X = Cl, Br, I), garnet-type LLZO and NASICON-type LAGP SEs has been investigated using in situ techniques including X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning electron micros-copy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy measurements [134–140]. For exam-ple, Wenzel et al. [134] systematically studied the chemical reactions at the Li/LGPS SE interface through in situ XPS combined with time-resolved impedance spectroscopy stud-ies, revealing that the decomposition of LGPS SE will lead

Fig. 12 a Schematic of the in situ XPS method to monitor the interac-tions between Li and Li10GeP2S12 (LGPS) as well as the interphase formation between them, b XPS spectra recorded during deposition of 31  nm Li metal on LGPS. S 2p, Ge 3d, and P 2p/Ge 3p detail spectra are shown for different deposition states. With increasing Li deposition time, LGPS decomposes. The identified species are marked and labeled in the spectra. Reprinted with permission from Ref. [134], copyright 2016, American Chemical Society. c SEM image of the cross section of a solid electrolyte (SE) layer with a Li thin film; Li dissolution/deposition curves of d Li foil/SE/Li foil and e Li foil/Li thin film/SE/Li thin film/Li foil cells at a current den-

sity of 0.064 mA cm−2. Reprinted with permission from Ref. [141], copyright 2012, Elsevier. Schematics of the interface between Li and 80Li2–20P2S5 SE are shown in (f, g); f indium was evaporated on the SSE layer, and then Li foil was attached to the indium thin film, and g indium was evaporated on Li foil, and the side of indium thin film was attached on SSE layer. h Charge–discharge curves of all-solid-state cells Li/In thin film/80Li2S–20P2S5/Li4Ti5O12, in which indium was evaporated on the SSE layer (i) or on the Li foil (ii). Reprinted with permission from Ref. [143], copyright 2012, The Electrochemi-cal Society of Japan

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to the formation of an SEI layer that is mainly comprised of Li3P, Li2S, and Li-Ge alloys, increasing interfacial resist-ances (Fig. 12a, b). Similar degradations were also observed by researchers at the interfaces of Li7P3S11/Li and Li6PS5X/Li due to the decomposition of Li7P3S11 (Li2S and Li3P) and Li6PS5X SEs(Li2S, Li3P, and LiX) [135, 139]. In a fur-ther study, Nagao et al. [140] investigated the interfacial behaviors of Li/Li2S–P2S5 glass–ceramic SE using in situ SEM observations and reported that the Li metal electrode tends to grow along the grain boundaries and voids of the SE during Li dissolution/deposition in bulk-type Li metal solid cells, suggesting that the lack of interconnected pores in SEs is critical to prevent Li dendrite formation.

In order to address the aforementioned impediments, two methodologies such as modification of SE surfaces or the passivation of lithium metal electrodes and the alloying of thin-film/bulk-type Li metal have been routinely used to minimize interfacial resistances and enhance contact at the electrode/electrolyte interface [129, 132, 136, 141–144]. For example, Nagao et al. [141] reported that the insertion of vacuum-evaporation thin Li film layers at the interface between a glass–ceramic Li2S–P2S5 SE and Li metal resulted in minimal interfacial resistances and an excellent reversibil-ity of Li dissolution/deposition compared with a Li2S–P2S5 SE with a Li metal electrode directly attached (Fig. 12d, e). Similarly, Nagao et al. [143] also reported that the forma-tion of an In–Li alloy through the embedding of indium thin layer between a Li metal and a glass–ceramic Li2S–P2S5 SE can result in a high Li-ion diffusion coefficient and the preservation of intimate interfacial contact during dissolu-tion/deposition cycling. Here, the researchers also reported that if the indium thin layer directly evaporates at the surface of Li2S–P2S5 glass–ceramic SE surface, the resulting cell can exhibit better performances with higher lithium utiliza-tion and lower overpotential than that of indium thin layers evaporating at the Li metal electrode (Fig. 12f, g), suggest-ing that as indium evaporates, the SE makes contact with the lithium metal and an alloying process occurs between the indium and the Li metal which impetuously “seals” the interface of the indium/lithium. And as a result, trivial

interfacial impedances can be observed at both the indium/SE and indium/Li interfaces. In contrast, if indium directly evaporates at the surface of Li metal, although indium/Li can preserve superior contact through the formation of an alloy, there are no interactions between the SE and indium (the surface-modified lithium metal), meaning that large resist-ances remain at the indium/SE interface, resulting in inferior electrochemical activity.

Inspired by the above results, Kato et al. [138] replaced indium with gold by inserting Au thin-film layers at the Li/Li3PS4SE interface and reported enhanced reversibility of Li utilization(~ 25% after 5 Li dissolution/deposition cycles), decreased interfacial resistances and increased uniform-ity of Li metal surface morphology. Here, the researchers attributed these positive effects to the alloying of the Au (which is highly reactive toward lithium) thin film at the Li/Li3PS4 interface, which inhibits void generation at the electrode/electrolyte interface during lithium dissolution. In addition, the researchers reported that the increased uniformity of the Li metal morphology also enhanced the reversibility of Li utilization [144]. In another study, Zhang et al. [136] suggested an ingenious interfacial re-engineering strategy to in situ fabricate a LiH2PO4(LHP) protective layer between a LGPS SE and Li metal electrode (Fig. 13a). Here, the combined results of EIS and galvano-static Li stripping/plating of the resulting Li–LHP/LGPS/LHP–Li symmetric cell revealed that a protective layer of LHP can significantly stabilize the LGPS/Li interface with smaller interfacial resistances and highly reversible Li utili-zation in bulk–type Li metal solid-state batteries (Fig. 13b, c). And more recently, Zheng et al. [129] reported that trivial amounts of ionic liquid encompassed with lithium salt (LiTFSI/Pyr13TFSI (N-propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide)) can greatly increase Li metal wettability and interfacial stability between LSPS (Li10SnP2S12) SEs and Li metal electrodes through the in situ formation of SEI layers on Li metal. Here, Li strip-ing/plating results of the resulting Li–LSPS–Li cell with an optimal 1.5 M LiTFSI/Pyr13TFSI ionic liquid demon-strated stable cycling performances with a flat voltage pla-teau over a period of 1000 h at 0.038 mA cm−2 and 350 h at 0.115 mA cm−2 (Fig. 13d–f). Moreover, the EIS results of the resulting cell further presented a drastic reduction in interfacial resistance from 1960 to 250 Ω with the addition of the LiTFSI/Pyr13TFSI ionic liquid (Fig. 13g).

4.2.2 Oxide SEs/Li interfaces

Oxide-based garnet-type SEs (Li7La3Zr2O12 and analogs) are believed to be exceptionally stable against Li metal and possess high RT ionic conductivities. However, the seri-ous challenge of garnet-based SEs is the large interfacial resistance in the order of 102 to 103 Ω cm2, which is mainly

Fig. 13 a Schematic of the preparation process of in  situ LiH2PO4 (LHP) protective layer on Li metal electrode; b time evolution of total impedance of LHP–Li/LGPS/LHP symmetric cells with differ-ent electrodes, and c galvanostatic Li dissolution/deposition cycling of LHP–Li/LGPS/LHP–Li symmetric cells at a current density of 0.1 mA cm−2. Reprinted with permission from Ref. [136], copyright 2018, American Chemical Society. d Li stripping/plating curves of Li/LSPS/Li symmetric cells with and without 1.5 M LiTFSI/IL at a current density of 0.038 mA cm−2 and the cycle performance of Li/LSPS/Li symmetric cell with 1.5 M LiTFSI/IL e at 0.115 mA cm−2 and f at different current densities. g Nyquist plots measured with Li/LSPS/Li symmetric cells with and without 1.5 M LiTFSI/IL at room temperature. Reprinted with permission from Ref. [129], copyright 2018, American Chemical Society

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caused by poor wettability (contact) between LLZO SEs and Li metal anodes [145–147]. In addition, intimate contact between LLZO and Li metal is difficult to achieve due to the rigid ceramic nature of LLZO and surface impurities such as Li2CO3. Furthermore, researchers have reported that the large interfacial impedance at the Li/SEs interface pre-dominantly arises from the physical and/or chemical insta-bility between Li metal and divergent SEs [148]. Therefore,

to address these interfacial impedance issues, suitable approaches are necessary but challenging. And although several methods have already been reported in the literature to resolve these interfacial resistance and dissolution/depo-sition issues, this review will focus on recently established novel and promising strategies.

One extensively investigated method is to apply external pressure and/or thermal heating to provide intimate contact

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between Li metal electrode and LLZOSEs. For example, the sintering of Li/LLZO materials from RT to 175 °C can reduce interfacial resistances from 5822 to 514 Ω [145]. However, this approach cannot resolve interfacial issues completely because the high rigidity of LLZOSEs and poor wettability of Li metal will result in the formation of inter-facial microscopic gaps and massive interfacial impedances as well as uneven interfacial current distributions.

An alternative method is the introduction of inorganic buffer layers such as metal/metal oxide (e.g., Si, Au, Al, Ge, Al2O3, ZnO, etc.) layers between the Li metal and the garnet SEs, effectively densifying the ceramic SE and closes the pores/voids on the garnet surface, leading to drastic decreases in interfacial resistance [147–152]. Here, the auxiliary buffer layers should be thin and ionically conductive, which can contribute to high-energy densi-ties on the cell level. For example, Fu et al. [149] reported that by depositing a 20-nm-thin Al layer onto garnet-type Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZN)SE to form an ioni-cally conducting Li-Al alloy, an interfacial layer is activated between the LLCZN and Li metal electrode (Fig. 14a). In addition, the wettability of the LLCZN surface becomes lithiophilic, endowing good physical contact between the LLCZN and the Li electrode. And as a result, a huge reduc-tion in interfacial resistance from 950 to 75 Ω cm2 and stable cycling performances with consistent voltage profiles during

Li stripping/plating in the symmetric L/Al–LLCZN–Al/Li cell was reported (Fig. 14b). In another study, Luo et al. [147] reduced the interfacial resistance between Li metal and LLZO SE by depositing an ultrathin amorphous Si layer (~ 10 nm thick) onto the surface of the LLZO SE through plasma-enhanced chemical vapor deposition and reported that due to the reaction between Si and Li, the surface wettability of LLZO SE transformed from “super-lithiophobic” to “superlithiophilic” (Fig. 14c, d). And as result, asymmetric cell using the modified Si-coated LLZO presented a sevenfold reduction in interfacial resistance (~ 127 Ω cm2) and stable Li striping/plating performances compared with pristine LLZO (~ 925 Ω cm2) (Fig. 14e). More recently, Hu et al. [148] introduced an Al2O3 layer (5 nm thick) using atomic layer deposition onto the surface of Li6.85La2.9Ca0.1Zr1.75Nb0.25O12 (LLCZN) garnet elec-trolytes and reported significant reductions in interfacial resistance from 1710 to 1 Ω cm2 and significant increases in Li metal wettability and galvanostatic Li plating/stripping behaviors(Fig. 14f, g). Similarly, other coatings such as Ge, Mg, and ZnO have also been reported to significantly miti-gate interfacial resistances and enhance Li metal wettability during Li dissolution/deposition [150–152].

And recently, our group [153] also proposed a simple and cost-effective strategy to address interfacial issues between Li metal and Li5.9Al0.2La3Zr1.75W0.25O12 (LAL-ZWO) involving the use of a pencil to apply a graphite-based layer onto the surface of a LALZWO SE. Here, the results indicate that the graphite-modified garnet-type LALZWO symmetric (Li–C/LALZWO/C–Li) cell expe-rienced a significant reduction in interfacial resistances (~ 105 Ω cm2) as compare with unmodified garnet symmet-ric cells (~ 1350 Ω cm2) (Fig. 14h). Furthermore, Li plat-ing/stripping results in our study indicated that the graph-ite-modified garnet-based symmetric cell also possessed outstanding stability with uniform Li plating/stripping for over 1000 h compared with the counterpart unmodified garnet cells (Fig. 14i, j). Overall, the approach used in our study provides a number of benefits such as its simplicity and cost-effectiveness as well as its adequate enhancement of electrode/SE interfacial contact and stability.

As for the instability between NASICON-type electro-lytes such as Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li metal electrode, Lin et al. [154] suggested that the introduction of a stable artificial Li3PO4 SEI layer (~ 200 nm thick) between the Li metal and the LAGP SE can shield the con-tact between them. And as a result, the Li3PO4 layer can effectively impede Li dendrite growth and alleviate unfa-vorable side reactions between the LATP SE and Li metal, resulting in the corresponding symmetric cell exhibiting small interfacial resistances and superior Li dissolution/deposition performances over 200 h.

Fig. 14 a Schematic of engineered garnet SSE/Li interface using Li metal alloy. The pristine garnet SSE has poor contact with Li. Al-coated garnet SSE exhibits good contact with Li due to the Li-Al alloy that forms between the SSE and the Li metal. b Comparison of Nyquist plots of Li/garnet SSE/Li and Li/Al-garnet SSE–Al/Li in the frequency of 1 MHz to 100 mHz at 20 °C. Reprinted with permission from Ref. [149], copyright 2017, Science. c Lithiophobic garnet has poor contact with Li metal, which leads to a high interfacial resist-ance. d The wettability of garnet is significantly improved due to the reaction between Li and Si, and the in  situ formed, lithiated Si can act as a Li-ion conducting layer. e The wettability transition and the in situ formed, lithiated Si lead to the dramatic decrease in garnet/Li metal interfacial resistance from 925 Ω cm2 (bare garnet) to 127 Ω cm2 (Si-coated garnet). Reprinted with permission from Ref. [147], copyright 2016, American Chemical Society. f Comparison of EIS profiles of the symmetric Li non-blocking garnet cells. Inset shows the enlarged impedance curve of ALD-treated garnet cell. g Com-parison of cycling for symmetric cells of Li/bare garnet/Li (black curve) and Li/ALD-treated garnet/Li (red curve) at a current den-sity of 0.1 mA cm−2. The inset is the magnified curve of the ALD-treated cell. Reprinted with permission from Ref. [148], copyright 2016, Nature Publishing Group. Characterizations of graphite-based interface layer in symmetric Li cells; h comparison of EIS profiles of the symmetric Li garnet cells with and without graphite-based inter-face. The inset shows the enlarged impedance curve of the former, i comparison of galvanostatic cycling performance of symmetric cells including Li/bare garnet/Li and Li/modified garnet/Li at a current density of 50 μA cm−2, and j galvanostatic cycling of the Li/graphite-interface-treated garnet/Li cell at a current density of 300 μA cm−2 and insets showing the magnified curve from 0 to 10 h, 495 to 505 h, and 993 to 1003 h. Reprinted with permission from Ref. [153], copy-right 2018, American Chemical Society

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4.2.3 Polymer Layer‑Modified SEs/Li Interfaces

The introduction of conductive polymer interlayers between Li metal and SEs can potentially decrease inter-facial resistances and increase cell ionic performances [155–158]. Furthermore, polymer interlayers can physi-cally impede dendrite formation due to uniform Li-ion flux across polymer/Li interfaces. In addition, polymer interlayers exhibit better wettability toward Li metal and inorganic ceramic layers can be protected from contact with Li metal.

By combining the advantages of polymers and ISEs, Fu et al. [159] composited LLZO garnet with a PEO-based polymer to form a hybrid composite electrolyte (Fig. 15a, b) and reported that the flexible composite SPE possessed excellent mechanical strength and low interfacial impedance at the Li/SE interface. Moreover, the resulting galvanostatic cycling Li/hybrid composite SPE/Li cell demonstrated dendrite-free stability during Li plating/striping for 500 h at 0.2 mA cm−2 and 300 h at 0.5 mA cm−2(Fig. 15c). Fu et al. [157] also made another breakthrough by coating PEO-based polymers onto a dense 3D garnet SE which compensated for interfacial roughness and enabled homogeneous Li-ion flux through the interface. The researchers have reported that the gal-vanostatic cycling behavior of the resulting Li/polymer/garnet/polymer/Li cell revealed less interfacial resistances and an excellent reversibility over 160 h at 0.3 mA cm−2. In another study, Goodenough et  al. [160] developed ASSLSBs with extremely small interfacial resistances through the introduction of cross-linked PEO (CPEO) polymer buffer layers between the Li metal and the Li6.5La3Zr1.5Ta0.5O12garnet SE, reporting that the polymer layer adequately increased Li wettability and suppressed Li dendrite growth in bulk-type Li metal batteries. Simi-larly, Zhang et al. [158] addressed the large interfacial issues between Li metal and LAGPSE by introducing a

conductive polymer in which a cohesive PEO-based poly-mer interlayer was inserted between the LAGP SE and Li metal which provided a 3D ion conductive network and superior interfacial contact between the ISE and the electrode (Fig. 15d–f).

5 Summary and Outlook

In summary, recent progress in current sulfur-based cath-ode materials and key interfacial issues at the electrode/electrolyte interface of ASSLSBs utilizing different solid electrolytes have been systematically reviewed. Here, dif-ferent electroactive materials such as various forms of sulfur-based cathodes ranging from sulfur to lithium poly-sulfidophosphates and their additives with variable forms of carbons as well as their preparation routes can greatly affect the performance of Li−S batteries. Nevertheless, the electrochemical performance of solid-state Li−S bat-teries utilizing SEs are inadequate, including cell design, the understanding of interfacial reactions, and cell per-formances such as specific capacity, cyclability as well as energy density. Based on this, important issues must be resolved in the future to enhance the performance of ASSLSBs, such as the effective use of carbon materials as electronic conductors and catalytic sites for S-conversion reactions or as “confined spaces” for dissolved S-species; the preparation of efficient composite cathodes; and the preparation of highly active and flexible electrode/elec-trolyte interfaces.

And despite the significant progress so far in ASSLSBs, many challenges remain in terms of the solid electrolyte, electrode materials as well as their interfaces. These chal-lenges include: (1) a suitable selection criteria for elec-trode material compositions and synthesis procedures to tailor optimal interfacial structures of electrode compo-nents such as sulfur-based electroactive materials, solid electrolytes, and conductive additives; (2) the addition of suitable elastic additives to positive electrodes to reduce strain/stress effects as a result of electroactive material vol-ume changes such as sulfur/sulfur-based materials during repeated charge/discharge cycles; (3) the in-depth under-standing of the electrode/electrolyte interface (mainly the cathode/SE interface), which requires more in-depth theoretical and experimental analysis; (4) the proper use of techniques such as in situ X-ray diffraction, in situ/ex situ solid-state NMR, photoelectron spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy, and in situ electrochemical techniques to obtain in-depth under-standings of the bulk and interfacial structural changes in solid-state lithium−sulfur batteries; and (5) the increase in active material loading and reduction in SE layer thickness

Fig. 15 a Schematic of bare dense garnet layer surface and b poly-mer-coated dense garnet layer surface and cross-sectional SEM image of polymer-coated dense garnet. The polymer layer compensates for interfacial roughness and enables a homogeneous Li-ion flux through the interface. c Voltage profiles of Li plating/stripping cycling at a current density of 0.3 mA cm−2. The red line is the Li/garnet/Li cell, which was prepared by attaching molten Li directly onto a dense gar-net surface. The Li/garnet/Li cell shows an increased voltage curve with large impedance. The black line is the symmetric hybrid elec-trolyte cell. The inset profiles show the detailed voltage plateau of Li stripping/plating in the first few hours and 140 h. Reprinted with permission from Ref. [159], copyright 2017, The Royal Society of Chemistry. Time evolution of the interfacial resistance of symmetric cells d Li/LAGP/Li and e Li/SPE/LAGP/SPE/Li after different stor-age times at 60 °C. f Voltage profiles versus cycling time of symmet-ric cells Li/LAGP/Li and Li/SPE/LAGP/SPE/Li with a current den-sity of 0.1 mA cm−2 at 60 °C. Reprinted with permission from Ref. [158], copyright 2017, The Royal Society of Chemistry

◂

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to achieve higher-energy densities and excellent stability in ASSLSBs.

Acknowledgements We would like to acknowledge the financial sup-port from the National Key Research and Development Program of China (Grant No. 2018YFB0905400) and the National Natural Science Foundation of China (Grant Nos. 21473148, 21621091, 21761132030).

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Dr. Ediga Umeshbabu received his PhD degree from Indian Institute of Technology (IIT) Madras, India, in 2016. Later on, he joined Prof. Yang’s research group as a post-doctoral research associate at Xiamen University. His main research interests are the development of high-capac-ity and high-energy density cath-ode materials for solid-state lithium batteries, including lith-ium–sulfur and lithium-ion batteries.

Bizhu Zheng is currently a PhD candidate in Collaborative Inno-vation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engi-neering, Xiamen University, under the supervision of Prof. Yong Yang. She received her Bachelor’s degree in Chemistry from Xiamen University in 2015. Her research interests focus on the interfacial modification in all-solid-state lithium batteries.

Prof. Dr. Yong Yang is now work-ing as a distinguished professor in Chemistry in Department of Chemistry, Xiamen University. He obtained PhD degree in Chemistry from Xiamen Univer-sity in 1992. His major research interests are new electrode/elec-trolyte materials for Li/Na-ion batteries, in situ spectroscopic techniques, and interfacial reac-tion mechanism study in electro-chemical energy storage systems. He has published over 220 papers in many referred interna-tional journals such as Nature

Energy, Energy and Environmental Science and Advanced Materials. He has obtained several national/international research awards, e.g., Excellent Contribution Award given by Chinese Electrochemical Soci-ety in 2017, Technology Award presented by IBA (International Bat-tery Materials Association) in 2014.

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