Muhammad Khurram Tufail, Niaz Ahmad, Le Yang, Lei Zhou, Muhammad Adnan Naseer,Renjie Chen,3,*, Wen Yang,*

1 Key Laboratory of Cluster Science of Ministry of Education Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China

2 School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

3 Institute of Advanced Technology, Beijing Institute of Technology, Jinan 250300, China

Keywords:Li7P3S11 solid electrolyte 30Li2S-70P2S5 glass ceramics Chemical stability Electrolyte/electrode interphase High energy density all-solid-state lithium batteries

A B S T R A C T The development of an inorganic electrochemical stable solid-state electrolyte is essentially responsible for future state-of-the-art all-solid-state lithium batteries(ASSLBs).Because of their advantages in safety,working temperature, high energy density, and packaging, ASSLBs can develop an ideal energy storage system for modern electric vehicles (EVs). A solid electrolyte (SE) model must have an economical synthesis approach,exhibit electrochemical and chemical stability,high ionic conductivity,and low interfacial resistance. Owing to its highest conductivity of 17 mS·cm-1, and deformability, the sulfide-based Li7P3S11 solid electrolyte is a promising contender for the high-performance bulk type of ASSLBs.Herein,we present a current glimpse of the progress of synthetic procedures,structural aspects,and ionic conductivity improvement strategies. Structural elucidation and mechanistic approaches have been extensively discussed by using various characterization techniques. The chemical stability of Li7P3S11 could be enhanced via oxide doping, and hard and soft acid/base (HSAB) concepts are also discussed.The issues to be undertaken for designing the ideal solid electrolytes, interfacial challenges, and high energy density have been discoursed. This review aims to provide a bird’s eye view of the recent development of Li7P3S11-based solid-state electrolyte applications and explore the strategies for designing new solid electrolytes with a target-oriented approach to enhance the efficiency of high energy density allsolid-state lithium batteries.

1. Introduction

The most recent (modern) power storage gadgets with more safety, maximum energy storage, and low price have confronted auspicious starts and extraordinary challenges. Typically, lithiumion batteries(LIBs)are broadly used in domestic and daily necessities, which control many aspects of our electronic lifecycle, comprising movable devices and rechargeable automobiles. With growing demands and dependency on portable electric devices,cost-efficient, safe, and high energy density batteries have gained significant consideration [1-3]. Currently, lithium-ion batteries have almost attained their limit regarding energy density in liquid-based lithium-ion batteries. Simultaneously, the intercalation/deintercalation phenomena in electrode materials in liquid electrolytes are strongly induced by the high specific capacity and high current rate. Even now, in compound liquid electrolytes,various diluters,constituents, and supplementary effects infinitely produce numerous challenges related to their unstable components. The high flammability of solvents, interfacial resistance,and electrode corrosion are significant examples of these challenges. These challenging factors deter the development of highenergy storage applications with safe production low cost and fast charging [4,5]. Since liquid electrolyte-based LIBs have exposed many issues, this is the time to discover other than LIBs system and prioritize solid electrolytes (SEs) with incombustible, costefficient, and high energy density batteries for the coming era[6-10].

In this scenario, all-solid-state lithium batteries (ASSLBs)expose their emergent position as an up-to-date technology to expand current lithium-ion batteries far beyond all critical issues,such as safety, cost, and power density [11,12]. Typically, ASSLBs are designed by three electrolytes:ceramics,polymers,and hybrid SEs to strengthen the lithium-ion conduction and chemical reaction in the batteries. ASSLBs rely on SEs, for which scientists are paying extra attention to discover solid electrolytes with novel properties to increase the performance of solid-state batteries[13].Therefore,for the development of ASSLBs,researchers are trying to find out apposite SEs with attractive electrochemical properties and high stability with electrodes.

In ASSLBs, the flawless necessities for scheming an appropriate SEs are significant ionic conductivity [2], large voltage windows,mechanically strong, and remarkable electrochemical properties[1,14]. The top portion of the stated literature on electrolytes can be omitted to encounter the obligation of four standards of electrolytes with good properties. The inorganic class has various materials,i.e., oxides,sulfides, and oxynitrides exhibited relatively low ionic conductivity, high interfacial impedance among electrodes-electrolytes, subsequently low voltage windows, inappropriate for batteries at commercial scale.Whereas in the organic class of polymer-based SEs, a limited variety of polymers is employed in the SEs to boost ionic conductivity and mechanical properties. The synergistic effect of these properties exposes the high performance, and a small Li-ion transfer number is often neglected in the maximum reported literature [15]. Researchers stated extensive studies for solid electrolytes progress, so this is rather complicated to choose the perfect solid electrolyte with good properties for ASSLBs. In sulfides, Li10GeP2S12(LGPS) solid electrolyte exhibited attractive ionic conductivity ～10-2S·cm-1,but unstable against lithium electrode and highly sensitive to moisture [16]. The Li3xLa0.67-xTiO3(LLTO), Garnet oxide-based solid electrolyte exposed significant ionic conductivity ～10-3S·cm-1and also showed unstable behavior against lithium electrode [17-19]. However, lithium phosphorus oxynitride (LiPON)has substantial stability against Li anode governed by the intervening layer among electrode/electrolyte but showed poor ionic conductivity ～10-6S·cm-1[20]. The lithium argyrodites Li6PS5X(where X = halides; iodide, bromide, chloride), SEs were formed by the substitution of halides, displayed high ionic conductivity～10-2-10-3S·cm-1, but highly unstable against lithium electrode and developed an interfacial layer on interaction[21].Moreover,it is quite challenging to pick solid material with improved conditions to assemble solid-state batteries.So,the selection of apt electrolytes for research to emphasize a sequence of constant investigations becomes essential.

Li7P3S11phase is a promising future-generation solid electrolyte among all sulfide-based solid electrolytes[22].Because of its bodycentered cubic anionic type of aggregation composition,Li7P3S11is said to have a little diffusion barrier (0.30 eV) [23]. On the other side, Li7P3S11exhibited low electronic conductivity [24] due to its extensive bandgap of 3.50 eV [25]. Furthermore, Li7P3S11has strong lithium metal compatibility and a stable electrochemical window[26].As a result,when Li7P3S11solid electrolyte is applied to lithium-ion batteries (LIBs), various electrochemical and chemical properties have been pragmatic[22,27].Nevertheless,it is still a long road from satisfying the growing needs for EVs and energy storage devices.We provide a recent glance at advancing synthetic techniques and ionic conductivity enhancement strategies in this review article. Various characterization methods have been discussed thoroughly to explore structural elucidation and mechanical approaches. The chemical stability of Li7P3S11in the air could be improvedviaoxide doping and the hard and soft acid/base(HSAB) principle. We discussed the challenges to Li7P3S11solid electrolytes, such as low ionic conductivity, air stability, electrode/electrolyte interfacial,and their solutions.We hope that this review will provide useful guidance and strategies for the development of Li7P3S11-based solid electrolytes and that it will be applicable to the design of new air stable and Li metal stable solid electrolytes for ASSBs.

2. Synthesis Routes for Li7P3S11 Glass

It is critical to synthesize the superionic conductor Li7P3S11solid-state electrolyte (SSE) for all-solid-state batteries using simple, cost-effective, and safe methods. The synthesis routes for Li7-P3S11solid-state electrolyte generally involve high energy ballmilling (BM), liquid phase method followed by sintering treatments and microwave synthesis.

2.1. Mechano-chemical routes

Mechanical milling is a typical procedure for synthesizing Li7P3-S11SSE.The precursor stoichiometric composition is ball-milled at a particular speed to mix the raw material equally and followed by high-speed milling for a lengthy time to attain the product. The high speed ball milling (370-510 r·min-1) can break the parent chemical bonds of the raw material and mix homogeneously at the atomic level, developing the solid-state electrolyte after a specific period of rotation speed. Most important, the highenergy ball milling procedure needs an equilibrium between the weight of raw material and milling balls. The high-energy ball milling procedure requires a balance between the weight of raw material and milling balls. The variation outcomes are linked with the grinding routes’energy,depending on the quality of precursor,milling/break time, and the size/number of the balls, and a comparative study on the ball’s nature,size of the milling ball.Two different apparatus ball materials,i.e.,alumina(10 balls with 10 mm,370 r·min-1for 20 h) and zirconia (500 balls with 4 mm, 510 r·min-1for 8-12 h), were used with different conditions. The glass-ceramics achieved from zirconia media are exhibited higher conductivity than their counterpart [28]. Previous research illustrated that glass-ceramics Li7P3S11solid-state electrolyte could be attained by grinding at high speed after post-annealing at different temperatures and explored the Li-ion conductivity at room temperature(RT).Nevertheless,a certain quantity of amorphous phase in the glass-ceramics electrolytes may positively impact the Li-ion conductivity. Hayashiet al. [29] were first described the application of the ball-milling for the development of the LPS solid-state electrolytes family. Seinoet al. [30] informed the Li7P3S11SSE was synthesized from the raw materials Li2S and P2S5viathe ball-milling process. The BM functioned at a revolving speed of 370 r·min-1for 20 h milling time. Seinoet al. used ten alumina grinding balls (10 mm in diameter) in a 45 ml alumina vessel.Moreover, the precursor-balls weight ratio and the revolution speed for high-energy BM are also essential parameters that influence the final product’s purity. The conductive Li7P3S11glass-ceramic was attained from its glassy state after the milling processviaannealing above the crystallization temperature(Tc).As a result of heat treatment, the Li7P3S11metastable phase was precipitated.

2.2. Solid-state reaction routes

The essential requirement of the traditional solid-state reaction routes is that the virtuous solid-solid particle contacts and reactants(s) or products(s) are molten at high temperatures. The high-temperature conditions for kinetic of the solid-state reaction enhance the menace of impurities because of the interaction of vessel material and precursors. Besides, the evaporation of the reagent could decrease the yield of the products. Li4P2S6was the first material belongs to the LPS family synthesizedviasolid-state reaction routes [31]. Similarly, the Li7P3S11glass was prepared from precursors, sealed in the quartz tube, and heated at 700 °C.The molten product was quickly quenched. The resultant glass powder was heated at 280-300 °C for 2 hours to achieve the densified glass-ceramics sample [32]. The parameters for achieving high-energy-density batteries were highlighted in a general overview of the wet and solid routes in Fig. 1.

2.3. Liquid-media synthesis routes

The liquid-state synthesis strategy has various benefits over the solid-state reaction routes, but it is not as suitable as the ballmilling procedures for Li7P3S11solid-state electrolyte synthesis.The liquid-phase reaction approach is an efficient route to develop the SSEs with controllable grain size (nm-μm) and their morphology. The modification of solid electrolyte-related properties has become applicable in an advanced energy storage system.To select an organic solvent that is insoluble to the raw material,i.e., P2S5and Li2S as suspensions. After that, evaporate the solvent until the dry powder is attained. The Li7P3S11SSE was preparedvialiquid-phase routes,a couple of controllable factors are influenced,such as additives,nature of the solvents,and solvent-to-powdered ratio. Ideally, the solvent selection is aprotic to minimize the protonation and formed the sulfide (SH-, HxPS3-x4) ions and toxic hydrogen sulfide gas.Furthermore,the generally polar aprotic type of solvents are utilized for the preparation of the Li7P3S11SSE such as acetonitrile (ACN) [33,34], 1,2-dimethoxyethane [35], tetrahydrofuran(THF)[36]ACN&THF[37]R.Maniwaet al.[38]published a recent method for producing sulfide-based solid electrolytes using anisole and microwave irradiation at 200-300 °C was also developed. Despite the current progress in liquid-media routes,the reaction mechanism between P2S5and Li2S precursors is not yet well explained. However, the mixing of raw material (molar ratio is 70% Li2S:30% P2S5) and solvent firstly led to the creation of Li3PS4+solvent and Li2S:P2S5+solvent.The formation of the Li7-P3S11solid-state electrolyte was achieved during the solvent drying procedure. The enhanced Li-ion conductivity of the final products was driven by variations in the initial lithium sulfide molar contents. These results highlight the importance and clear the concept of reaction mechanism to create Li7P3S11solid electrolyte with significant Li-ion conductivity.Meanwhile,the particle size of Li7P3S11solid electrolyte generated by liquid-media synthesis routes is still in the micrometer range and must be reduced to the Nano-scale. The liquid phase routes appear to be a promising method for producing Nano-scale stable electrolyte particles.Based on liquid-media techniques, the use of Li7P3S11solid electrolyte with decreased particle size allows for the development of high-energy, high-cycling robust all-solid-state lithium batteries.From precursors to solid batteries,the Li7P3S11solid electrolyte needs to be prepared, annealed and densified, Fig. 2 depicted all the steps involved in the synthesis of solid electrolytesvialiquid and solid routes.

Fig. 1. The advantages and disadvantages of the liquid/wet routes and Solid routes for Li7P3S11 solid electrolytes are represented.

2.4. Recent routes for the Li 7P3S11 SSE synthesis

Glass and glass-ceramics Li7P3S11solid-state electrolyte were controllably synthesizedviathe fast microwave-assisted procedure in 30 min. The micro-assisted sample represented comparable conductivity with traditional preparation methods such as melt and quenching glass and glass-ceramics material. The ionic conductivity of the Li7P3S11glass sample is 0.12 mS·cm-1which was prepared by the microwave method [39]. Tadanaga and coworkers published that the ultrasonic irradiation permitted the fast creation of PS3-4 structural units for the design of Li7P3S11with 1.0×10-3S·cm-1using acetonitrile[40].Maniwaet al.proposed a novel approach to synthesize the sulfide-based solid electrolyte,including Li7P3S11using microwave irradiation and anisole in 30 min [38]. All proposed parameters are listed in Fig. 3.

Fig. 2. Flowsheets of liquid/wet and solid routes for the processing of Li7P3S11 solid electrolytes.

Fig. 3. Parameters consideration for the High-energy Ball Milling, liquid media, solid-state reaction, and microwave-assisted routes.

3. Phase Studied of Li7P3S11 Solid Electrolyte

3.1. Glasses

Inorganic glass materials are believed to have better ionic conductivity than crystalline materials due to their open structure and the existence of a large free volume. The binary Li2S-P2S5glasses are used as sulfide based solid electrolyte is the most

well-studied sulfide glass. The activation energy of 70Li2S:30P2S5glass was 43.5 kJ·mol-1, and its ionic conductivity was 3.7 × 10-5S·cm-1[41]. In the sulfide-based glass system, the Liion conductivity could be improved by enhancing the concentration of the charge-containing ions [42,43]. Doping strategy for the glass system with Li-based salts is an effective approach to improving the glass Li-ion concentration and ionic conductivity.Ujiieet al. mechanically ball-milled the 70Li2S:30P2S5glass with lithium halide (LiX, X = I, Br, Cl, F) to achieve higher ionic conductivity [44].

Fig.4. SEM photograph of 70%Li2S:30%P2S5(molar ratio)glass-ceramics(a)cold-pressed;(b)simple densification.Impedance graph(c)cold-pressed material;(d)densified sample at 280°C.Reproduced from Ref.[32]with permission of the Royal Society of Chemistry.(e)DSC curves of the 70%Li2S:30%P2S5(molar ratio)glass Reproduced with permission [45]. Copyright 2016, the American Chemical Society. (f) XRD pattern of the 70% Li2S:30% P2S5 (molar ratio) glass and after heat treatment at 280 °C. Reprinted from Ref. [46] with permission of Elsevier.

3.2. Glass-Ceramics

Glass-ceramic solid electrolytes are generated by the crystallization process of respective glass electrolytes. The precipitation of thermodynamically stable crystalline phases from a parental glass is beneficial for lowering grain-boundary resistance. The glassy Li7P3S11is annealed above the crystallization temperature,and nucleation of the Li7P3S11happens to achieve high ionic conductivity.The heat treatment over the glass transition temperature(Tg)unites the glassy crystals into a denser state in order to obtain the glass powdered into the glass-ceramics as shown in Fig. 4(e).

In summary,the heat treatment temperature was optimized to 250-300°C.There is a significant influence of heating on the storage stability and ion conduction of the Li7P3S11glass-ceramics electrolyte.Seinoet al.reported Li2S-P2S5glass-ceramics exhibited the highest Li-ion conductivity of 17 mS·cm-1and an activation energy(Ea)of 17 kJ·mol-1.To achieve an annealing temperature of 280°C,it enhanced the ionic conductivity due to reducing the grain boundaries shown in Fig.4(a)-(d),(f)[32].Weiet al.described that the Li-ion conductivity is improved from 1 to 1.6 mS-1and reduced the interfacial resistance, increasing the heat treatment temperature up to 250 °C. Much higher heat treatment temperature persuaded the creation of the less conductive phase,i.e., Li4P2S6[47].

3.3. Crystalline

The Li7P3S11crystalline solid electrolyte was obtained by crystallizing the parent glass rather than through a typical solid-state reaction.The crystallization development from the parental glassy liquid, the Li7P3S11crystal solid electrolyte, would be formulated directly by cooling the melted (molar ratio is 70% Li2S:30% P2S5)composition. Tatsumisagoet al. prepared the Li7P3S11crystal by crystallizing the (molar ratio is 70% Li2S:30% P2S5) glass. Fig. 5(a)represented the XRD of the glass and the crystallized sample formulated at various temperatures and annealing times. There was an exothermic variation because of glass-transition behavior at 220°C;two exothermic curves at 270°C and 430°C were observed.The first exothermic curve is credited to the crystallization of the Li7P3S11crystal, and the other peak is ascribed to the Li4P2S6and Li3PS4crystals. The Li7P3S11crystal phase is precipitatedviacrystallization of the parental glass at 280 °C to 360 °C. A single Li7P3-S11crystal phase with maximum crystallinity is precipitated by annealing at 360°C for one hour.It exhibited the highest ionic conductivity of 4.1 mS·cm-1at RT and the lowestEaof 14 kJ·mol-1shown in Fig. 5(b).

The single Li7P3S11crystal phase was achieved by crystallizing from the molten state at different annealing temperatures for a long time and quenching the yielded crystal in ice water. Fig. 5(c) presented the X-ray diffraction pattern of the crystal samples which were collected at a different annealing temperature of precursor melting,i.e., 550 °C, 650 °C, 690 °C, 700 °C, and 750 °C for 48 hours. A single Li7P3S11crystal phase is effectively attained by dropping the annealing temperature from 750-700°C.Moreover,by decreasing the annealing temperature to 690 °C, the lass conductive Li4P2S6phase is formed. Higher ionic conductivity is related to the precipitation of the single Li7P3S11crystal phase.The post-annealed sample attained by heating temperature 700 °C for 48 h also exhibited ionic conductivity of 0.41 mS·cm-1at RT andEaof 32 kJ·mol-1Fig. 5(d). The phase diagram of the 70% Li2S-30% P2S5(molar ratio) composition Fig. 5(e). The Li7P3S11crystal phase is high-temperature because it was precipitated in the supercooled liquid [48].

4. Structural Aspects of Li7P3S11 Solid Electrolyte

Li7P3S11phase crystallized in triclinic geometry with P-1 space group. Two types of structural units, such as P2S4-7di-tetrahedra(corner-sharing),PStetrahedral,surround the Li-ions.The lattice constants of the Li7P3S11phase are α = 102.845°, β = 113.2024°,γ=74.467°;a=1.25009 nm,b=0.60316 nm;c=1.25303 nm with 82.935 nm3lattice volume.The P—S bond length and S—P—S bond angles in P2S4-7structural unit,1.978(3)-2.091(3)?(1?=0.1 nm)and 94.3(3) - 116.8(3)° are calculated correspondingly. Similarly,the bond lengths 1.927(8) - 2.115(8) ?, and bond angles 101.3(5)-113.4(4)° are also calculated for PSa tetrahedral structural unit of Li7P3S11. Li-ions are positioned at the sites around thestructural units and surrounded by three to five sulfur atoms [47]. The disorder sites, interstitial sites, and diffusion mechanism of Li-ions were not cleared yet for the Li7P3S11glassceramics electrolytes. The crystal structure of the Li7P3S11phase and its structural units are displayed in Fig. 6(a).

Fig. 5. X-ray diffraction pattern of the crystallize material synthesized from (a) glass; (b) melt via various heat treatment parameters. The ionic conductivities at different temperatures for the crystalize material from (c) glass; (d) melt. (e) Phase diagram of the 70% Li2S:30% P2S5 (molar ratio) composition based on XRD findings [48].

5. Conduction Mechanism

The deep understanding of the lithium-ion conduction mechanism is confined due to the material complex amorphous nature.The Ab Initio molecular dynamics computational study provides a general consideration of the polarizable structural properties and their dynamics of the Li7P3S11solid electrolyte [49,50]. Takahashiet al. studied zone-based analysis of the lithium-ion conduction and accentuated the possible path. The effective lithium-ion conduction was highlighted concerning the polarizability and dynamics of the sulfur atoms adjacent to the lithium-ion conduction path.The highest flexibility and polarizability are key factors for the development of an effective lithium conduction path. To further explore lithium-ion conduction,color mapping is beneficial to recognize the Li sites with effective migration. Three types of Li are classified based on the period of a more extended stay in the respective zone (Lic, four atoms; Lih, six atoms; Lim, 18 atoms)shown in Fig.6(b).Notably,lithium-ion exchange among the zones Vh(higher spaces), Vm(medium spaces), and Vc(confined spaces){shape of zones; polyhedral S atom locations as vertices} was not frequently detected. The maps of probability density for Lih, Lim,and Lichave been seen from lithium trajectories. The Li-ion conduction pathway originated along theb-axis shown in Fig. 6(c)[50]. The lithium in the sandwich space between the two P2S4-7dimer have been low mobility. Ohkuboet al. studied the Lithium-ion conduction in Li7P3S11crystal and 70Li2S-30P2S5glass by AIMD simulation.The differences in S-S and Li-S local structure between the Li7P3S11crystal and 70Li2S-30P2S5glass are the same,but the P-S-P bond angle distribution is not identical. However,there is an irregular relocation of sulfur in PS3-4 anion as the output of the rotational motion of the glassy phase.This rotational motion of PS3-4 anion in the glass phase decreases the effective pore volume compared to the crystal phase. In this regard, a portion of the pore volume of the glass phase cannot contribute to Lithiumion conduction[49].The homogeneously anisotropic polarizability of sulfur (S) in the 70Li2S-30P2S5glass is a notable feature, as it does not allow fast Lithium-ion conduction pathways like the Li7-P3S11crystal.

6. Structural Characterization Techniques

6.1. MAS-NMR

Magic angle spinning nuclear magnetic resonance (MAS-NMR)is an effective tool for a deep understanding of sulfide-based solid electrolyte’s structure and ion conduction mechanism.31P MAS NMR chemical shift values are sensitive to the phosphorus bonded environment, and this bonding distribution range has consequences for peak width expansion. Two major peaks have been observed in the Li7P3S11solid electrolyte system at 89.8 and 86.3 due to the di-tetrahedra P2S4-7 unit with bridging sulfur(P-S-P) and tetrahedra PS3-4 unit with non-bridging sulfur (P-S),respectively. The superionic crystalline Li7P3S11phase contains two highly conductive PS3-4 and P2S4-7anionic units, whereas the amorphous Li7P3S11phase also contains P2S4-7, PS3-4, (higher conductive) and P2S4-6(less conductive) anionic units [51,52]shown in Fig. 7(a). Since the line shape of the31P MAS NMR spectra did not shrink with increased temperature, the rotational motion of anionic units is likely responsible for the irregular relaxation at the high temperature rather than translation motion. All major peaks line-shapes remained temperatureindependent up to 27 °C when a motional contraction was experienced. The line shape of7Li MAS NMR shows that the motional contraction of the peaks is happened with enhancing temperature. The analysis of the Li7P3S11glass-ceramics sample additional peak is shown in Fig. 7(b). It also showed the occurrence of three peaks, two narrow and one wide. The narrow peaks were assigned to transportable lithium-ions in a crystalline structure,while the smaller broader peak was set to less transportable lithium-ions in the conductive structure.

Furthermore,31P-31P radio-frequency-driven recoupling(RFDR)experiments were carried out to investigate the line widths represented in Fig. 7(c). At higher temperatures, off-diagonal peaks in the two-dimensional spectra fused into the P2S4-7 peak. This phenomenon indicated that the di-tetrahedral dynamical motions provided a channel for lithium-ions, which conduct at lowEa, leading to high ionic conductivity [52]. The Li-ion diffusion across the electrolyte-electrolyte interface as well as electrolyte-electrode played a dynamic role in the performance of ASSLBs and was investigated using solid-state7Li NMR.7Li-7Li NMR experiments were also executed to investigate the movement of lithium ions over the Li7P3S11solid electrolyte and nano-Li2S interface, offering the novel insight into the lithium ions transport restriction as well as the source of the internal resistance in the Li-In/Li7P3S11/Li2S allsolid-state battery system [55].

6.2. XAS

X-ray absorption spectroscopy (XAS) is commonly used to determine local and element-specific electronic structures in material science, chemistry, and physics. The XAS spectra of various Li2S-P2S5glasses were obtained incomplete electron yield mode at the phosphorus (P) and sulfur (S) K-edges to determine the structural units and their evolution with alteration of the precursor composition.The peak limit of the primary absorption function in the S K-edge spectra of the Li2S-P2S5glasses shown in Fig.7(d) is located between 2471.0 eV for 75Li2S:30P2S5and 2472.3 eV for 50Li2S:50P2S5according to their photoelectron transition from occupied S 1s(orbital)to unoccupied S 3p σ*antibonding[56].The second most noticeable prominent peak centered at 2477 eV,which fluctuated slightly with the Li2S-P2S5compositions due to multiple scattering.

The phosphorus (P) K-edge spectrum of Li2S-P2S5glasses was also obtained to investigate the development of the P environment represented in Fig. 7(e). All spectra of XAS have a single dominant peak centered at 2148.3 eV, most likely the result of an electron transfer from a P 1s (orbital) core level tot*2antibonding higher energy state [56,57]. Additionally, no particular spectral variation,neither connectivity (bridging sulfur) nor chain length [58,59].Another factor at lower energy is observed at the 2147 eV for the Li2S-P2S5glass composition of 67Li2S:33P2S5to 50Li2S:50P2S5,which are credited to the presence of Pδ+—Sδ-bond. The expressively shorter Pδ+—Sδ-bond produces distorted PS3-4 tetrahedra unit withC3vsymmetry.There is no such type of spectral distortion observed in the 70Li2S:30P2S5(Li7P3S11) composition [56].

6.3. XPS

Fig.6. (a)The crystal structure of the Li7P3S11 phase and structural units.(b)Typical lithium trajectories of Lih,Lim,and Lic.(c)Probability density mapping of Lih,Lim,and Lic Reproduced with permission [49]. Copyright 2020, the American Chemical Society.

To further investigate the Li7P3S11solid electrolytes, core-line X-ray photoelectron spectrums (XPS) were obtained at the phosphorus and sulfur L-edges of glasses and glass-ceramics for their atomic bonding types and elemental environment. The phosphorus, sulfur, and lithium bonding were discovered by X-ray photoelectron analysis of Li7P3S11solid electrolyte. Both forms of conductive structure units contained three types of bonded sulfur:S 2p3/2with S= P bond (162.1 eV), non-bridging sulfur bond P-S-Li(161.5 eV), and bridging sulfur bond P—S—P (162.8 eV) [60] are shown in Fig. 7(f). Deconvolution of the spectra are represented in Fig. 7(g), the receptive signal of P 2p3/2 at 134.1 eV by thetetrahedral unit and 132.8 eV equivalent theunit,revealed that developed glass-ceramics belonged to the LPS family.

Fig.7. (a) 31P MAS NMR spectrums of Li7P3S11 glass-ceramics at different temperature conditions.(b) 7Li MAS NMR spectra of Li7P3S11 glass-ceramics at different temperature conditions (I) 360 K (II) 270 K (III) no deconvolution NMR spectra at 270 K. (c) Dipolar-correlation RFDR 31P-31P spectrum of Li7P3S11 glass-ceramics at room temperature Reproduced with permission[52].Copyright 2015,the American Chemical Society.(d)XANES spectra of the Li2S-P2S5 glasses at S(sulfur)K-edge and their magnified image of the edge(inset x=70(dark red)is 70Li2S:30P2S5(Li7P3S11)composition.(e)XANES spectra of the Li2S-P2S5 glasses at phosphorus(P)K-edge Reproduced from Ref.[53]with permission of Royal Society of Chemistry. (f) XPS spectrum of sulfur L-edge of Li7P3S11 glass-ceramics. (g) XPS spectrum of phosphorus L-edge of Li7P3S11 glass-ceramics Reprinted from Ref. [54] with permission of Elsevier. (h) Raman spectra of Li7P3S11 glass-ceramics prepared via dissolution-evaporation technique in different solvents Reprinted from Ref. [37] with permission of Elsevier.

6.4. XRD

X-ray diffraction is a very prevailing technique used to analyze the crystal structure,phase,and lattice parameters.Fig.4(f)is represented the XRD pattern of the Li7P3S11solid electrolyte.The sample is well crystallized with a sharp diffraction pattern. The major characteristic XRD pattern at 2θ=19.67°(0 1 2),21.7°(0 1 3),23.6°(0 3 0), 25.8° (0 2 2), 28.8° (2 1 1) and 29.6° are credited to the superionic conductor Li7P3S11phase. Li-ions occur in void spaces surrounding the di-tetrahedral P2S4-7and tetrahedral PS3-4structural units, which tend to move from one position to another if the SE contains enough voids and defects [46,47].

6.5. Raman spectroscopy

Raman spectroscopy is an optical scattering tool used to identify the type of functional bonding,distinguishing and characterizing the materials.Raman spectroscopy is a low-cost, fundamental,and invaluable analytical technique for structural fingerprinting and tracking changes in the structural units of the Li7P3S11solid electrolyte.The Raman spectra of the Li7P3S11solid electrolyte contain two major peaks at 420 cm-1and 405 cm-1and one minor peak at 385 cm-1. The Raman Peaks at 420 cm-1, 407 cm-1, and 386 cm-1are attributed to the stretching vibration of the P-S bond in ortho-thiophosphate (P, tetrahedral), pyro-thiodiphosphate(P2, di-tetrahedral), and hypo-thiophosphate (P2, structural unit ethane like structure with a P-P bond) correspondingly in Fig.7(h)[37].Although the Raman spectroscopic dispersion coefficient data was connected to polyhedral PSx, the existence of the superionic Li7P3S11phase was confirmed [53,61].

Fig. 8. (a) AIMD computation of the new structural unit Mo. (b) A charge density distribution of Li7P3S11, and Mo doped Li7P3S11 solid electrolytes. (c) Calculated and experimental Li-ion conductivities of Li7P3S11, and Mo doped Li7P3S11 solid electrolytes Reprinted from Ref. [69] with permission of Elsevier.

7. Critical Issues and Improvement Strategies

7.1. Ionic conductivity

The superionic conductive phase is distinguished from the normal conductor phase by specific properties such as higher ionic conductivity, low activation barrier, ionic conductive supported structure,and sufficient vacant sites for ionic conduction.The high ionic conductivity of the SSEs(10-2S·cm-1)compared to the liquid electrolyte is needed to apply high-energy all-solid-state batteries at RT. Although Li7P3S11solid electrolyte shows outstanding ionic conductivity and can compete with the liquid electrolyte,it is moderately expensive compared to other compositions. It should be noted that the cost of different compositions decreased with the molar ratio of the Li2S (most expensive precursor) increased are represented in Table 1. A well-known approach for the new material design and modification of the Li7P3S11phase can be concise as a fellow.

Table 1 A comparative study of the ionic conductivities and approximately precursors cost of different compositions (molar ratio) reported in the literature

1. Large in size and extra polarizable ions are desirable for attaining high ionic conductivity

2. Iso-valent substitution of the sulfur (S) by oxygen (O) tend to enhance the ionic conductivity

3. Production of the vacancies and defectsviaaliovalent substitution can enhance the ionic conductivity

4. Design new structural unitsviametal cations doping strategy to increase Li-ion conductivity,i.e., MoS4-4 shown in Fig. 8(a)-(c).

Tu group designed a novel Li7P2.9S10.85Mo0.01glass-ceramics electrolyte by doping MoS2viahigh-energy BM followed the heat treatment method.The obtained solid electrolyte sample exhibited the significant ionic conductivity of 4.8 × 10-3S·cm-1at RT with more stability with lithium metal than the Li7P3S11solid electrolyte[70].Moet al.developed a 95(0.7Li2S-0.3P2S5)-5Li3PO4(molar ratio)electrolyte with high ionic conductivity of 2.5 mS·cm-1at 100°C than the Li7P3S11solid electrolyte[71].Huanget al.explored the substitution of the Li3PO4for P2S5and showed the maximum ionic conductivity of 1.87 mS·cm-170Li2S-29P2S5-1Li3PO4(molar ratio) glass-ceramics electrolyte [72]. Minamiet al. also proposed a substitution of P2S5by P2S3in the 70Li2S·(30-x)P2S5·xP2S3(molar ratio) glass-ceramics and glasses. It also explored that the substitution of the P2S3also promoted creating the P2S4-6anion in the glasses. The Li-ion conductivity of the prepared glass improved with an increment of P2S3contents up to 5 mol%,and the ionic conductivity of achieved glass is 1 × 10-4S·cm-1at RT. The annealed glass-ceramics above the Tcshowed the maximum Li-ion conductivity of 3.9 mS·cm-1at RT [73].

This new phase Li7P3S11-zwould have higher ionic conductivity than the Li7P3S11crystal with no substitution [28]. Our group proposed a strategy to enhance the conductivity of the Li7P3S11glassceramics by the addition the metal phosphide. The metal phosphides doped Li7Ni0.2P3.1S11exhibited the highest ionic conductivity of 2.22 mS·cm-1and itsEaof 21.1 kJ·mol-1.The doped sample’s ionic conductivity was 1.6 times higher than the Li7P3S11pristine sample [74]. The glass-ceramic Li7P3S11was densified in 2013 using a heat press process at 280 °C, dropping the grain boundary(gb) resistance and improving the lithium-ion conductivity of 17 mS·cm-1[32]. A novel Li7P2.9Mn0.1S10.7I0.3glass-ceramic belongs to the Li7P3S11class was recently designed with an increased Li+conductivity of 5.6 mS·cm-1without hot-press in 2017. Mn and I ions doping raises the defects and vacant sites in a solid electrolyte conductive framework, decreasing theEaand improving the ionic conductivity [75]. Minamiet al. reported the incorporation of the oxygen into the 70Li2S-30P2S5solid electrolyteviaa particular substitution of the P2O5for P2S5. The P2O5doped sample offered the ionic conductivity 3 mS·cm-1andEaof 16 kJ·mol-1. The new oxysulfide-based structural P2OS4-6(bridging) unit in the doped crystal of Li7P3S11would enhance the ion conduction [76] due to weakening the interaction among the doped electrolyte and Liion [77]. Junget al. proposed a new 90Li7P3S11-10Li2OHBr electrolyte system by high energy BM followed by heat treatment method and showed the highest Li-ion conductivity of 4.4 × 10-4S cm-1at RT [78]. The Li-ion conductivities of 2.53 mS·cm-1[69],and 2.22 mS·cm-1[51], have been improvedviametal cations MoS2, and FeS2doping strategy for the Li7P3S11system, respectively. The impacts of doping and substitution of foreign dopants in order to achieve high Li-ion conductivities of Li7P3S11solidstate electrolyte are also presented in the Table 2.

Table 2 A comparison between the solid-state routes and liquid-media routes to synthesize the Li7P3S11 solid electrolyte

7.2. Chemical stability in air

Sulfide-based solid electrolytes have a substantial disadvantage in that they have poor chemical stability in the air.Sulfides decompose through hydrolysis and releasing poisonous H2S gas. The use of sulfide material is needed to avoid the hydrolysis process,i.e.,Li7P3S11. The chemical stability of Li7P3S11solid electrolytes was inspected by exposing them to controlled humid air conditions.Li7P3S11solid electrolytes have poor air stability and must be dealt with in an Ar-filled environment and significantly increase the complexity and manufacturing cost. A chemical reaction occurs in which Li7P3S11solid electrolytes are hydrolyzed to generate toxic H2S gas under the action of H2O molecules in the air. The electrolyte structure will collapse during the hydrolysis process,and the ion conductivity will drop sharply. Therefore, the Li7P3S11solid electrolytes chemical stability is a significant challenge that is not ignored in all-solid-state battery commercialization.

Several studies were carried out to explore the air degradation mechanism of sulfide-based electrolytes. Muramatsuet al. [86]investigated the air stability mechanism using the Raman characterization of a sulfide-based sample before and after exposure to the humid air. The quantity of H2S produced is highly dependent on the Li2S-P2S5(molar ratio) composition and distribution of the structural units. The Li+and ortho-thiophosphate (PS3-4 , tetrahedral) structural species of the Li2S-P2S5glass and glass-ceramics materials did not show any variation on exposure to the humid air. After exposure to the moisture, Pyro-thiodiphosphate (P2S4-7,di-tetrahedral) decomposed. The P2S4-7anion is a major structural species of the 67Li2S-33P2S5glass [87].

The-OH and-SH groups are formed by interacting with water molecules in moist air. Furthermore, the -SH moieties react with H2O to form H2S gas and-OH group shown in Fig.9(a).The Raman techniques were carried out before and after the air test.The peak(Li-S) location in the Raman spectrum changed after the Li2S crystal was exposed to humid air.As Li2S crystal is exposed to moist air,it interacts with H2O molecules to decompose into LiOH and LiSH groups, and it is further hydrolyzed to form LiOH and toxic H2S gas Fig. 9(b). In the air-degradation of 67Li2S-33P2S5, the peak intensity of the P2structural units in the 67Li2S-33P2S5glass steadily decreased, and new peaks of (-OH and -SH) emerged.The structural degradation of the glass sample in the air is depicted in Fig. 9(c).As P2S4-7reaction with the H2O molecules in the moisture air decomposed into the OH and SH groups.The-SH group is further hydrolyzed to form -OH and H2S poisonous gas [86]. The P2S4-7also a chief structural conductive unit of the 70Li2S-30P2S5(Li7P3S11) glass and crystal [61].

Fig. 9. (a) Structural variations of the Li2S-P2S5 solid electrolyte in the humid air, Before and after exposure to humid air. Raman spectra (b) 67Li2S-33P2S5 solid electrolyte;(c)Li2S crystal Reprinted from Ref.[86]with permission of Elsevier.XPS S 2p and P 2p spectra of 70Li2S-30P2S5 solid electrolyte(d)-(e)Before exposure;(f)-(g)after exposure. XPS S 2p and P 2p spectra of Li7Sb0.05P2.95S10.5I0.5 solid electrolyte (h)-(i) Before exposure; (j)-(k) after exposure. (l) The amount of H2S generated from 70Li2S-30P2S5 and Li7Sb0.05P2.95S10.5I0.5 solid electrolytes. (m) Raman spectra of 70Li2S-30P2S5 and Li7Sb0.05P2.95S10.5I0.5 solid electrolytes before and after exposure to moist air Reprinted from Ref.[54],with permission of Elsevier.(n)The amount of H2S generated from 70Li2S-30P2S5 and Li6.988P2.994Nb0.2S10.934O0.6 solid electrolytes.(o)Xray diffraction pattern of 70Li2S-30P2S5 and Li6.988P2.994Nb0.2S10.934O0.6 solid electrolyte before and after exposure to humid air Reproduced with permission[61].Copyright 2020, the American Chemical Society.

There are two strategies for improving the air stability of the 70Li2S-30P2S5(Li7P3S11) glass and glass-ceramics. The chemical stability of the Li7P3S11solid electrolytes and other sulfide-based SSEs chiefly follows the principle of soft, hard acid-base (HSAB).According to the HSAB theory, a hard acid preferentially reacts with a hard base, while a soft acid is more likely to respond with a soft base [88]. The most sulfide-based superionic conductor is not chemically stable in humid air due to the oxygen is a hard base and preferably reacts with the hard acid,i.e.,phosphorus,and substitutes the attached sulfur(soft base)[89].It results from the formation of H2S gas, which reduces the chemical stability of the 70Li2S-30P2S5(Li7P3S11) glass and glass-ceramics. The HSAB theorem is a promising strategy for creating new solid electrolyte systems with improved air stability by selecting the central atom that bonds strongly with the sulfur atoms [90]. Many researchers formulated a strategy that includes searching for the Group VA elements in the periodic table for suitable soft acids that bind closely to sulfur to solve this constraint. In the light of the HSAB concept,our group designed an improved air-stable superionic Li7-P3S11-based conductorviaan aliovalent dual doping strategy.Antimony Sb with a large size and a soft acid are chosen for doping at the P site.The new as-prepared sulfide composite is stable enough against O, a considerably harder base than sulfur. Ex-situ XPS was used for the first time to demonstrate the air stability of thiophosphate-based SSEs Fig. 9(d)-(k). The produced quantity of H2S toxic gas,the inherent dynamics of air stability in the Li7P3S11,and dual doped Li7Sb0.05P2.95S10.5I0.5have been further explained using Raman spectroscopy Fig. 9(l)-(m) [54].

The second modification method is oxide doping, which increases the sulfide-based electrolyte’s chemical stability and reduces H2S content in the air. Ohtomoet al. and Tsukasakiet al.examined that hydrogen (H2S) production could be successfully minimized by a combination of Li2O into sulfide-based Li2S-P2S5glass[91,92].Ahmadet al.reported that the oxide was dynamically doping inside of Li7P3S11to improve the chemical stability in the air due to the presence of small fractions of oxysulfide types of structural units along with the conductive unitsandand confirmed by the X-ray diffraction Fig. 9(n)-(o) [61]. Recently Tufailet al.have investigated the air stability of the Li7P3S11solid electrolyte using a couple of techniques as well as mechanistically improved air stabilityviaoxide doping strategy[81] represented in the Fig. 10. The rate of H2S production from Li2S-P2S5glass exposed to humid air was reduced when P2O5was partially replaced with P2S5. H2S gas production was also further decreased by adding ZnO to the Li2S-P2S5-P2O5glasses as hydrogen gas absorbent [93].

Fig. 10. The air stability mechanism of the Li7P3S11 SEs and Li6.95Zr0.05P2.9S10.8O0.1I0.4 SEs. Reprinted from Ref.[81] with permission of Elsevier.

Fig.11. (a)XPS graphs of P 2p and S 2p peaks for the Li deposition.(b)The Li7P3S11 solid electrolyte and SEI resistance changes examine by the time-dependent impedance.(c)Li 1s peaks for deposited sample and formation of Li metal film Reprinted from Ref.[94]with permission of Elsevier.(d)Graph of CCD of doped Li7P3S11 and resistivity for all dopants, the values of CCD of all doped Li7P3S11 solid electrolyte with different doping elements in the table. (e) Graph of CCD and Li-ion conductivity and the values of resistivity in the table for Sn, Fe, Mo, Zn, and Si. (f) Galvanostatic cycling profile of all Zn, Fe, Sn, Si-doped Li7P3S11 solid electrolyte symmetric cells at step increase CCD Reprinted from Ref. [69] with permission of Elsevier. (g) Schematic diagram of Li/solid electrolyte and stable interfacial products. (h) Galvanostatic charging/discharging voltage profile Li/solid electrolyte/Li symmetric batteries with doped and pristine solid electrolyte at 0.2 mA·cm-1 current density. XPS detailed spectra of Li7P2.88Nb0.12-S10.7O0.3 based cell after five cycles(i)O 1s;(j)Nb 3d Reproduced with permission[82].Copyright 2020,the American Chemical Society.(k)Schematic flowsheet formation of GO@LPS.(l)Schematic flowsheet of GO coating to stop the interfacial reaction between the electrolyte and lithium anode.(m)The battery performance of the assembled LCO/SSEs/Li cell at 0.1 C. (n) Charging-discharging curves of LCO/SSEs/Li cells Reprinted from Ref. [95] with permission of Elsevier.

7.3.Interfacial issues between Li7P3S11 solid electrolyte and electrodes

The interfacial aspects between the solid-state electrolyte and electrode are vitally crucial for the practical application of allsolid-state batteries. The chemical reactions between solid electrolytes and alkali metals could lead to a foundation of either stop or facilitate ion movements and kinetics. Janeket al. studied that sulfide-based glass-ceramics are stable with the Lithium(Li)metal and which interfacial reaction products are formed.The stability of the Li7P3S11solid electrolyte with Li metal is examined using the XPS and aging of the interface screen by the impedance spectroscopy shown in Fig.11(a)-(c).The outcomes showed the development of an interphase (reaction zone) composed of the degradation products Li3P and Li2S. The ionic,as well as electronic conductivities of Li2S is very limited. These results, confirmed by time-dependent impedance measurements, and have substantial effects on the established interphase’s efficiency in the assembled cell. Meanwhile, the ionic conductivity of Li3P is also lower than Li7P3S11solid electrolyte. Both impedance spectroscopy andinsituXPS findings illustrated the growth of a slowly expanding SEI with a few nano-meters (nm) of thickness [94]. Wanget al.reported that metal cation doping,i.e., Mo could expressively decrease the SEI interfacial energy and movement rate of the Li+at the interphase, therefore increasing the interphase layer thickness and production of lithium dendrites. The doping outcomes of various elements,including ZnS,FeS2,SiS2,SnS2,and MoS2,illustrated that the critical current density (CCD) rises with the dopants’resistivity.In addition,Si-doped Li7P3S11has a lesser ionic conductivity than Li7P3S11,but it showed significantly higher critical current density (CCD) than Li7P3S11, implying that Si doping aids in the deposition of lithium metal represented in Fig. 11(d)-(f) [73]. Furthermore, since the bond energy of the metal bond is smaller than non-metal bond (covalent), the interface energy between the metal cation doped solid electrolyte and Li anode is also decreased, which is not favorable to Li diffusion at the Li/Li7-P3S11interface and also deposition of lithium metal, subsequently interface impedance increased and the formation of lithium dendrites. Jianget al., reported a new Li7P2.88Nb0.12S10.7O0.3lithium stable electrolyte exhibited ionic conductivity of 3.59 mS·cm-1,enhanced CCD of 1.16 mA·cm-1, and also represented remarkable stability with Li2S cathode material. The enhanced lithium metal stability is credited to the creation of the Li2O and Nb at the Li/Li7-P2.88Nb0.12S10.7O0.3interface,which can avoid further side reactions shown in Fig. 11(g)-(j) [82] Xuet al. proposed a new strategy for Li7P3S11solid electrolyte using boron nitride (BN). The boron nitride (BN) nanoflakes can partially separate the Li alloy and Li7-P3S11solid electrolyte to enhance the electrode-electrolyte interfacial compatibility [96]. Since lithium enters the bulk solid-state electrolyte, triggering the side reaction and dendrite formation,the shield from the lithium anode side is inadequate.The introduction of the lithium halide LiX(F,Cl,Br,and I)into the Li7P3S11solid electrolyte facilitates the ion conduction and prevents lithium dendrite formation. The structural characterization and the outcomes of the LiBr-added Li7P3S11solid electrolyte indicated that the lithium bromide (LiBr) is present in the lattice gap but does not enter the crystal lattice. The extremely electronegative bromide atom reduces the electronic cloud density on the structural units P2S4-7and PS3-4surface and drops their binding to Lithium-ion,thus increasing ionic conductivity. Evaluationsviadensity functional theory (DFT) illustrated that the incorporation of lithium bromide can enhance the interface energy of SEI to the lithium, which is a valuable approach in order to suppress lithium dendrite growth[97].

The graphene oxide-coated Li7P3S11solid electrolyte particles are robust and effective for ASSLB operation. The graphene oxide was able to partly separate the solid electrolyte from the lithium anode. The graphene oxide (GO) and reduced graphene oxide(rGO) reduced by the lithium and facilitated the cyclic stability.Fig. 11(k)-(n) [95,98]. This is a very simple and cost-effective method for completely protecting the Li7P3S11solid electrolyte from side reactions with the lithium metal in the following ways,such as high stable battery performance and eliminating the interfacial issues between the Li7P3S11solid electrolyte and a lithium anode.

Apart from Li/Li7P3S11interfacial issues, Li7P3S11/cathode interface and their corresponding tremendous resistance is also a big challenge that needs to be solved. Despite high Li+conductivity(～10-2S·cm-1@RT) the dreamed performance and energy/power density cannot be achieved without cutting down the Li7P3S11/-cathode interfacial resistance,which could reduce the bulk interfacial resistance of the full device.In this context,different strategies have been adopted as:

(1) Pertinent doping into Li7P3S11SSE;

(2) A thin artificial layer between electrolyte and cathode.

Bingxinet al. designed a composite 70Li2S-29P2S5-1Li3PO4,wherein oxygen atoms potentially suppressed the electrode-solid electrolyte interface and contributed to achieving a high initial discharging capacity of 108 mA·h·g-1[72]. Luet al. reported that the incorporation of Li2ZrO3into Li7P3S11solid electrolyte reduced the electrolyte-electrode interfacial resistance and increased the initial discharge capacity as 113.0 mA·h·g-1compared to pristine counterpart [99]. The combination of sulfide-based 70Li2S-30P2S5(Li7P3S11) solid electrolyte and LiCoO2(LCO) cathodes has been known to be thermodynamically unstable as interionic diffusion between Co and P at the interface is energetically favorable[100].The artificial thin buffer layer of Li-ion conductive and electronic insulators,e.g., Li4Ti5O12[101], LiTaO3[102], and LiNbO3[103] not only suppressed solid electrolyte-cathodeasses was also obtained to investiga interface resistance but also improved the charge transfer kinetics in all-solid-state batteries,as these coating layers have voltage window from the reduction potential of 0.7-1.7 V to the oxidation potential of 3.7-4.2 V.A Li7P3S11/MoS2composite electrode for ASSLBs is designed by the coating of Li7P3S11solid-state electrolyte on the MoS2. The assembled ASSLBs with the Li7P3S11/MoS2composite electrode presented a reversible capacity of 547.1 mA·h·g-1at the current density of 0.1 C and excellent cycling reliability than the untreated counterpart MoS2shown in Fig. 12(a)-(b) [36]. Co3S4@ Li7P3S11[106] Fe3S4@Li7P3S11[107]nano-composite used as a cathode for ASSLBs to enhance the energy density and cost-effectiveness. Fe3S4@Li7P3S11exhibited good performance and presented a discharge capacity of 1001 mA·h·g-1at the 0.1 A·g-1current density. Zhanget al.reported the exclusive structure with improved electronic/ionic conduction achievedviaVS4anchored rGO nano-composite[108]. A recent strategy for designing the high-performance cathode material involves anin-situcoating of Li7P3S11sulfide electrolyte on the nano-composite of CuCo2S4/Graphene. It is a remarkable approach to design the intimate contact between the electrode material and solid electrolyte effectively. The assembled ASSLB using Li7P3S11on CuCo2S4/Graphene-based cathode composite offered the highest initial discharge capacity of 1102 mA·h·g-1at the 50 mA·g-1current density presented in Fig. 12(c)-(d)[8,104].Pyrite(FeS2)cathode material is used as a dopant for Li7P3-S11solid electrolyte for the first time shown in Fig. 12(e), which may increase Li-ion conductivity while lessening the interfacial resistance between the Pyrite (FeS2) cathode material cathode and solid electrolyte [51]. Lithium-free cathode material such as oxide and chalcogenides are also offering good specific capacities(600 mA·h·g-1- 1500 mA·h·g-1) with operational voltages (1.4-2.5 V) Fig. 12(f).

Gaoet al.,optimized a Li7P2.9Sb0.1S10.75O0.25solid electrolyteviapartial O/S and Sb/P dual substitution. The newly designed SEs exhibited good air stability as well as lithium stability.The assembled all-solid-state lithium-sulfur battery with a Li7P2.9Sb0.1S10.75-O0.25solid electrolyte offered a high discharge capacity of 1374 mA·h·g-1at 0.05 C and rate performance of 1158.3 mA·h·g-1at 1 C with 99.8% columbic efficiency [109]. M. K. Tufail and coworker synthesized the Li6.95Zr0.05P2.9S10.8O0.1I0.4(LZPSOI) solid electrolyte, which exhibited high ionic conductivity of 3.01 mS·cm-1and better air-stability. The all-solid-state lithiumsulfur battery assembled with Li6.95Zr0.05P2.9S10.8O0.1I0.4solid electrolyte demonstrated a tremendous initial discharge capacity of 932 mA·h·g-1with 99.2% coulombic efficiency at 0.064 mA·cm-2.Higher ionic & electronic conductivities with lithium iodide (LiI)additive and extra reaction sites contributed to the remarkable cyclic performance as well as jointly improved the utilization of lithium sulfide (Li2S) active material [81]. Sulfurized polyacrylonitrile(S@pPAN),which efficiently reduces sulfur volume expansion,is a good contender for the sulfur cathode. The Te0.05S0.95@-pPAN@Li7P3S11composite has dramatically improved reaction kinetics and great interfacial compatibility with the solid electrolyte due to the coating of Li7P3S11, and the influence of Tedoping are depicted in Fig. 13(a)-(e). The assembled all-solidstate Li/S battery offered good cycle efficiency, outstanding rate performance with reversible capacity of 1173 mA·h·g-1at room temperature over 500th cyclic stability [110]. Hanet al., designed a conversion type battery system with electrode materials (S and Li2S) dissolved (partially) in the liquid electrolyte strategy to achieve the high specific capacity redox kinetics. In order to prevent the mass transfer between the anode and cathode and eliminate shuttling effects, a Li3PS4coated SE Li7P3S11separator is applied. A stabilized interface between the liquid electrolyte and solid electrolyte is vital for exceptional hybrid Li-S battery performance with a discharge capacity of 1047 mA·h·g-1[111].This discovery opens the door to a novel approach for the stability of the liquid/solid electrolyte interface and may pave the way for future research to enhance the durability and stability of high-energydensity batteries. The evolution direction of the Li/S battery from a liquid-state to a solid-state is represented in Fig. 13(f) [80].Despite the fact that these techniques should be readily transferable to Li7P3S11solid-state electrolyte, to the best of our knowledge, only a few methods have been established explicitly for resolving electrode/solid-state electrolyte interfacial issues.

Fig. 12. (a) Schematic flowsheet for the formation of MoS2/Li7P3S11electrode. (b) Cycle performance of pristine MoS2 and Li7P3S11/MoS2 electrodes of 1.0 C current rate and according to MoS2/Li7P3S11 composite coulombic efficiencies. Reprinted from Ref. [36] with permission of Elsevier. (c) Schematic flowsheet for the formation procedure of CuCo2S4/Graphene@LPS nano-composite.(d)Charging and discharging curves CuCo2S4/graphene@10%LPS Reproduced with permission[104].Copyright 2020,the American Chemical Society. (e) Schematic presentation of the typical and novel cathode doped electrolyte-based ASSLBs Reprinted from Ref. [51] with permission of Elsevier. (f)Theoretical specific capacity for Li-free cathode material such as oxide and chalcogenides Reprinted from Ref. [105] with permission of Elsevier.

Fig. 13. Galvanostatic charge/discharge of various cyclic curves at 0.2C (a) Te0.05S0.95@pPAN; (b) Te0.05S0.95@pPAN@Li7P3S11 (c) Comparison of Te0.05S0.95@pPAN@Li7P3S11,Te0.05S0.95@pPAN, S@pPAN cyclic performance at 0.2C. (d) Rate capability of Te0.05S0.95@pPAN@Li7P3S11, Te0.05S0.95@pPAN, S@pPAN at different current density. (e) Cyclic performance of Te0.05S0.95@pPAN@Li7P3S11 at 0.3C Reprinted from Ref.[110]with permission of Elsevier.(f)Development of the all-solid-state Li/S battery Reproduced with permission [80]. Copyright 2020, the American Chemical Society.

Fig. 14. Schematic representation of the design of thin layer sulfide electrolyte. (b) Full solid-state Li-Li2S cell with solid-state electrolyte thickness of 100 μm. (c) Energydensity of all solid state battery using sulfide-based electrolyte Reproduced with permission [114]. Copyright 2019, the American Chemical Society.

The redox behavior of Li7P3S11solid-state electrolytes in the cathode composite permeates the discharge-charge process of ASSLBs. The SEs tolerate severe electrochemical reactions during charge-discharge phenomena and facilitate the Li-ion movements,especially the reversible capacity influence caused by the redox reaction of the SEs contacting the conductive additives. The redox behaviors of the Li7P3S11solid-state electrolytes are determined precisely under various electrochemical windows and their effects on the battery performances. In general, the electrochemical window during the charge-discharge process governs the depth electrochemical reactivity of SEs. Lithium metal/Li4Ti5O12(LTO)battery permit a quick charging-discharging process (6C) in the ten minutes, battery life (600 cycles) at current density (1C) with capacity retention (85%) by optimizing the electrochemical redox behavior of Li7P3S11solid-state electrolytesviascanning voltage window[112].This review paper aims to study the progress of Li7-P3S11solid-state electrolytes and their associated impacts on the efficiency and performance of ASSLBs.Also presented are effective strategies for improving the cyclic stability of Li7P3S11solid-state electrolyte-based all-solid-state lithium batteries.

7.4. Li7P3S11 solid electrolytes for design high energy density ASSLBs

Li7P3S11solid-state electrolytes have been widely regarded as aviable choice for high energy density ASSLBs,which may boost the application of Li metal anode.Nevertheless,the intrinsic properties of Li7P3S11solid electrolytes and their interface issues with electrodes remain the most difficult challenges in developing allsolid-state high-energy-density batteries. The energy density of all-solid-state batteries is generally claimed to be greater than that of liquid lithium-based batteries in the literature.The overall active material of the anode/cathode, their additives, electrolytes (solid and liquid), current collector, and packaging materials must all be considered when calculating the practical energy density of the batteries. Because of its light packing materials, the typical pouch cell has a greater energy density than the cylindrical cell.To attain high energy density for lithium-based batteries is not a primary challenge;instead,it achieves a balance between the other battery evaluation parameters,such as long cycle life,rapid charging, safety, outstanding performance over an extended range of operating temperature and low cost. Randauet al., calculated the energy density at the cell level and also reported standards to attain the high energy density ASSLBs such as the areal capacity of ＞5 mA·h·cm-2,the internal resistance of ＜40 Ω·cm2,and cathode specific energy of ＞500 W·h·kg-1(theoretical) [113]. A 100 μm thick Li3PS4solid electrolyte was effectively integrated in the allsolid-state lithium/sulfur batteries using a Kevlar non-woven scaffold as support.The assembled solid cell showed outstanding performance with Li2S active mass loading of 7.64 mg·cm-2and also achieved a high energy density of 370.6 Wh·kg-1and represented in Fig. 14(a)-(c) [114]. In the preceding section of this review, we concentrated on the synthesis of the Li7P3S11type of superionic conductor, its interface stabilization and applications in order to develop the high energy density ASSLBs rather than all sulfidebased electrolytes.Finally,we highlight the incoming insights into the development of Li7P3S11electrolyte-based systems as well as some of the difficulties encountered in the development of practical ASSLBs.

8. Future Perspectives

All-solid-state batteries(ASSBs)are expected to play an important role in developing a highly efficient battery system for future needs,such as 3-wheelers and 4-wheelers automobiles and useful smart gadgets.Nevertheless,there is a technical and practical need to replace the currently used liquid electrolytes (LEs) with riskfree, solid electrolytes (SEs) without any disruption. Sulfide glass and glass-ceramics electrolytes have excellent formability and Young’s modulus for shaping exemplary electrode/solid electrolyte interfaces in bulk-type ASSBs, resulting in fast charge transfer. In this section, we present several perspectives based on our knowledge.

(1) Theoretical estimation,simulation prediction would provide observations and guidelines for designing new sulfide SEs with good conductivity. Simulation prediction will deliver explanations and directions for ensuring stable electrochemical/chemical compatibility.

(2) Preparation of nanoscale compositesvialiquid phase is an effective method for Li2S and sulfur-based active materials with insulative properties. Meanwhile, the liquid phase routes propose unique prospects for developing the interface and composite cathode, but low ionic conductivity of materials and toxicity of the solvents applied in the synthesis method need to be taken into account. Consequently,advancements in recent synthetic procedures explore the new synthesis method with good quality, scalability, and cost-effectiveness in order to obtain the Li7P3S11solid electrolyte.

(3) Although Li7P3S11solid electrolytes exhibited ionic conductivity of (10-2- 10-3S·cm-1), their chemical stability to moisture air is a tremendous challenge. The synthesis and treatment of Li7P3S11solid electrolyte needs an inert environment, which mightily limits its application to ASSLBs on a commercial scale. The hydration reaction between the humid air and the Li7P3S11solid electrolyte produced the hydrogen gas.The computational and experimental findings presented that replacing sulfur with oxygen is an effective strategy to enhance chemical stability and decrease the generation of H2S gas.Another strategy to overcome the hygroscopic behavior of the Li7P3S11solid electrolyte is the addition of hydrogen sulfide adsorbents such as metal oxides (Bi2O3, ZnO). The introduction of metal oxide presents a nagetive Gibbs energy change (ΔG) with hydrogen sulfide for its spontaneous reaction and is a valuable method for lowering H2S production. Based on the hard-soft acids and bases(HSAB)rules,the phosphorous atom could be replaced by different aliovalent substituents such as Sb, Sn, and Si could also improve the chemical stability.

(4) To achieve low interfacial resistance, intimate physical contact is essential. Numerous approaches, especially those focused on nanotechnology,surface modification,and artificial SEI layer,have successfully addressed the contact issues between Li metal and Li7P3S11solid electrolyte.The application of a soft polymer interlayer will have an ideal interaction between solid electrolyte and Li metal.Inappropriately, however, the polymer-based electrolytes have a low conductivity at RT.

(5) One of the important steps to achieving the fast charging is the application of a stable solid-state superionic conductor.The solid electrolyte could be designvia ex-situorin-situcoating on the lithium metal or various anodes [4]. More research is needed in order to improve the interface properties between the electrode and the sulfide-based solid electrolyte.

(6) In order to attain a high energy density of the Li7P3S11solid electrolyte at the cell level,the narrow thickness of the solidstate electrolyte,lithium metal anode,and higher loading of active material are commonly needed.If the solid-state electrolyte thickness is reduced to 10-30 μm, solid-state electrolyte content in the cathode composite could be 30%-20% (mass) to attain the 400 W·h·g-1, which is feasible and promising for the future ASSLB system. In some cases, templates and support could help to improve the mechanical strength of the solid electrolyte,and roll-to-roll manufacturing operations can also be used on an industrial scale. The key factors and challenges regarding the development of high energy density all-solid-state lithium batteries are summarized in Fig. 15.

Fig. 15. The flowsheet representation of all coherent parameters to develop high energy density all-solid-state lithium batteries (ASSLBs).

Acknowledgements

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

Acknowledgements

This work was supported by the National Natural Science Foundation of China ( 51772030, 21203008, 21975025), the Natural Science Foundation of Beijing (2172051), Beijing Outstanding Young Scientists Program (BJJWZYJH01201910007023), and the State Key Laboratory funding by the project for Modification of Chemical Fibers and Polymer Materials, Donghou University.