Introduction

Energy innovation plays an important role in the reduction of carbon emission in the atmosphere and global warming. Progresses are necessary in energy storage technology for the integration of intermittent renewable energy resources on electricity markets and grids [1]. Lithium-ion batteries (LIBs) have become essential in most portable electronic products because of their high energies and power densities since they were first commercialized by Sony in the 1990s [2, 3]. It is well known that high-performance electrolyte materials have great influences on the batteries’ performance. Compared with solid electrolytes, liquid electrolytes have some intrinsic risks such as explosion, flammability, and volatilization. For example, LiBF4, LiPF6, and LiCF3SO3 dissolved in organic solvents such as ethylene carbonate, propylene carbonate, polyethylene oxide, and dimethyl carbonate, are flammable and have poor electrochemical stability and a limited temperature range of operation [4,5,6,7,8]. Because of the intrinsic solid character, solid electrolytes are much safer than liquid ones. The main advantages of solid electrolytes are that they do not corrode, combust, leak, or cause internal shorting like their liquid counterparts [9]. Although several technological issues still need to be addressed, including the development of a processing technology using sheeting and multiple stacking, the all-solid-state batteries can be considered a future category of electrochemical devices [10, 11]. Moreover, solid electrolytes are inert toward metallic Li and act as a separator, helping resist to dendrite growth. Solid electrolytes need to possess a relatively high ionic conductivity (σ > 10−3 S/cm). However, most existing solid electrolytes have a long way to achieve this goal [12, 13].

Compared with liquid and polymer ones (around 1 × 10−2 S/cm), solid electrolytes (SEs) have relatively much lower ionic conductivities (10−5–10−3 S/cm) under room temperature. This shortcoming restricts the practical utilization of solid batteries in view of the less effective electrode–electrolyte interface and insufficient understanding of interfacial reaction mechanism [14, 15]. Sulfide-type SEs also have their own drawbacks should be solved, such as poor thermal stabilities, easily to be moisture absorption, expensive raw materials like Li2S, and easy to react with lithium metal generated impedance layer, which makes the solid-state batteries gradually deteriorated during cycling. These disadvantages restrict the further applications of sulfide-type SEs. In recent years, to alleviate or overcome the above shortcomings, various doping, coating, and nanostructure modifications of sulfide SEs have been reported [4, 5]. SEs need to possess the following characteristics for their large-scale applications in Li batteries in the future [16, 17]:

  1. 1.

    High ionic conductivity to reduce the resistance polarization effects in a solid battery at room temperature;

  2. 2.

    Interfacial impedance and grain boundary resistance, negligible electronic conductivity;

  3. 3.

    Wide electrochemical stability window;

  4. 4.

    Chemical stability in the presence of electrodes, especially metallic Li anodes;

  5. 5.

    Matching thermal expansion coefficients with both electrodes;

  6. 6.

    More easily polarizable;

  7. 7.

    Low cost, high throughput, easy synthesis, and environmental benignity.

SEs for LIBs should be Li-ion conductors, which require high ionic conductivities at room temperature and low activation energies (E a) for their use over a broad range of operating temperatures [18, 19]. Table 1 shows known inorganic solid electrolytes. Compared with oxide-type ones, sulfide-type electrolytes have smaller electronegativity and binding force to Li+ ion. On the other hand, the bigger ionic radius of S compared with O induces a larger ion channels in the structure and benefits to ionic mobility [20, 21]. Compared with other types of inorganic solid electrolytes, sulfide ones have the highest ionic conductivities in the present, which might be 10−4–10−3 S/cm and have high electrochemical stabilities up to ~ 5 V versus Li+ ion at room temperature [22,23,24].

Table 1 Classification of inorganic solid electrolytes
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In this review, known sulfide-type solid electrolytes are summarized and classified into several parts according to their chemical compositions, including thiophosphates, halide thiophosphates, sulfides without phosphorus, and glassy sulfides electrolytes (Fig. 1). We try to provide researchers new insights into exploring ionic conductors with high conductivities and wide electrochemical windows.

Figure 1
figure 1

Reported ionic conductivities of solid electrolyte materials at room temperature, including thiophosphates, halide thiophosphates, sulfides without phosphorus, and glassy sulfides. Only materials with conductivities above 1 × 10−5 S/cm included

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Sulfide-type solid electrolytes

According to their chemical compositions, sulfide-type solid electrolytes are classified into thiophosphates, halide thiophosphates, sulfides without phosphorus, and glassy sulfide electrolytes. Thiophosphates, halide thiophosphates, and sulfide without phosphorus are all crystalline. Glassy sulfide electrolytes contain glass–ceramic and glass ones.

Thiophosphates

To date, many thiophosphates have been extensively investigated as solid electrolytes for batteries (Table 2). According to their chemical compositions, they can be classified into the following subgroups.

Table 2 Ionic conductivities of thiophosphate SEs
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Li–P–S

Tachez et al. discovered a new sulfide solid electrolyte Li3PS4 with a conductivity of 3 × 10−7 S/cm and an activation energy of 0.46 eV, which is a pioneering work on crystalline sulfide-type SEs containing P. The electrochemical window of Li3PS4 ranges from 0.6 to 3.7 V [35]. Later, many investigations on Li3PS4 and Li7P3S11 were carried out [38,39,40,41]. Phuc et al. [38] prepared plate-like 3 × 0.5 × 0.1 ~ 0.2 μm3 Li3PS4 (Fig. 2). The ionic conductivity of the prepared Li3PS4 was about 2.0 × 10−4 S/cm at room temperature. This work pointed out that the usage of nanosized-solid electrolyte could result in a high loading of active materials, which then enhanced the specific energy of the all-solid-state cells.

Figure 2
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@ 2017, Springer

SEM and HR-TEM (small inset) images of Li3PS4 [38].

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In 2007, Yamane et al. [33] obtained Li7P3S11 with a high conductivity of 3.2 × 10−3 S/cm and an activation energy of 12 kJ/mol. It crystallizes in the triclinic space group P \( \overline{1} \). Its structure can be described as a PS4 tetrahedron- and P2S7 di-tetrahedron-built framework, and Li+ ions locate at the polyhedral cavities (Fig. 3). Highly crystalline Li7P3S11 was obtained by a precipitation reaction from the glasses P2S3–P2S5 [32]. The as-prepared Li7P3S11 has a higher conductivity than of that of Li7P3S11 prepared using Al2O3 media [35]. In 2015, Huang et al. reported Li3PO4-doped Li7P3S11 glass–ceramic electrolytes with the highest total conductivity of 1.87 × 10−3 S/cm at 25 °C and the lowest activation energy of 18 kJ/mol. The triclinic Li7P3S11 prepared from 70Li2S to 30P2S5 (mol%) exhibits the highest Li+ ion conductivity among the xLi2S − (100 − x)P2S5 (mol%) systems. Compared with Li3PS4, Li7P3S11 seems have higher conductivity and lower activation energy. Much smaller size and more symmetrical structure may improve the electrolyte’s conductivity [42].

Figure 3
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@ 2007, Elsevier

Crystal structure of Li7P3S11 [33].

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Li–X–P–S

In 2013, Wang et al. obtained thio-LISICON Li3.25Ge0.25P0.75S4 using a low-temperature solution method. The Li3.25Ge0.25P0.75S4 film shows a Li+ ion conductivity of 1.82 × 10−4 S/cm at 30 °C and an activation energy of 0.42 eV [28]. Inoue et al. synthesized Li7Ge3PS12 by slow cooling from the ternary Li2S–GeS2–P2S5 system, which adopts a cubic argyrodite-type structure (Fig. 5b), and exhibits a high ionic conductivity of 1.1 × 10−4 S/cm at 25 °C and an activation energy of 25 kJ/mol [29]. Kamaya et al. reported a Li+ superionic conductor Li10GeP2S12 (LGPS), which has a new 3D framework structure consisting of (Ge0.5P0.5)S4, PS4 and LiS4 tetrahedra, and LiS6 octahedra [30]. It exhibits an extremely high Li+ ionic conductivity of 12 mS/cm at room temperature (Fig. 4). This new solid battery electrolyte has many advantages in terms of device fabrication (facile shaping, patterning, and integration), stability (non-volatile), safety (non-explosive), and excellent electrochemical properties [high conductivity and wide electrochemical window (0.0–5.0 V)]. Later, LGPS intrigued much interest. Mo et al. found a metastable phase LGPS from the calculated phase diagram in 2012 [30]. They also found that LGPS was not stable against reduction by Li at a low voltage or extraction of Li+ ions with decomposition at a high voltage. The ionic conduction in LGPS happens in three dimensions, rather than in one dimension. In 2014, Liang et al. studied two distinct Li+ ion diffusion processes in the tetragonal LGPS along 1D tunnel (E a = 0.16 eV) and in a plane perpendicular to the 1D tunnel (E a = 0.26 eV), respectively, which were separately verified by Li and P multiple solid-state NMR methods, Wang et al. studied the influences of the anion-host matrixes on the ionic conductivities of Li+ ion conductors. A new descriptor emerges from their discoveries: anion sublattices with bcc-like frameworks are superior for Li-ion diffusion as it leads to a lower activation barrier than in other close packed frameworks. The bcc anion framework allows the Li+ ions to migrate within a network of interconnected tetrahedral sites possessing equivalent energies. This feature is also found in recently synthesized superionic Li-ion conductors, such as Li10GeP2S12 and Li7P3S11 [30]. These new conclusions can be employed to explore fast ion conductors with improved properties [32].

Figure 4
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© 2011, Nature Publish Group

Impedance plots of the conductivity data from low to high temperatures and Arrhenius conductivity plots of Li10GeP2S12 [43].

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Encouraged by the success of LGPS, many scientists also paid their attention to LXPS (X = Sn, Al, Y, Si et al.). Li10SnP2S12 crystallizes in the tetragonal space group P42 /nmc [31]. It has a total conductivity of 4 mS/cm at 27 °C. The replacement of Ge by Sn can largely reduce the cost. Zhou et al. synthesized Li11AlP2S12 for the first time by sintering at 500 °C in 2016. Li11AlP2S12 has a conductivity of 8.02 × 10−4 S/cm at 25 °C, a low E a of 25.4 kJ/mol, and a wide electrochemical voltage window of above 5.0 V (vs. Li+/Li), indicating its promising application in solid-state LIBs [37, 44,45,46]. Zhu et al. found Li−M−P−S (M: non-redox-active element Ge, Al, Y et al.) [47, 48]. The phase and electrochemical stability of Li3Y(PS4)2 (Fig. 5) is expected to be better than current state-of-the-art Li+ superionic conductors [36].

Figure 5
figure 5

@ 2017, American Chemical Society; 2017, Elsevier; 2015, Nature Publish Group; 2013, American Chemical Society; 2016, Royal Society of Chemistry

Crystal structures of a Li3Y(PS4)2 [36], Li7Ge3PS12 (yellow: S; blue: Li/Ge; purple tetrahedron: b (Ge/P)S4) [29], c Li10GeP2S12 [49], d Li10SnP2S12 [31], and e Li11AlP2S12 [37].

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In general, thiophosphate SEs have been widely researched. People focused on improving electrolytes’ conductivity and reducing active energy. Some effective methods can reach this goal, such as controlling electrolytes’ size and structure, mingling some non-redox-active element, and optimizing synthesis methods.

Halide thiophosphates (argyrodite)

Apart from ionic conductivity (Table 3), the electrochemical stability window for solid electrolyte is also one of the most important criteria for battery applications. Most nonmetal electrolytes with phosphorus have the electrochemical stability up to ~ 5 V versus Li at room temperature. In 2012, Boulineau et al. reported Li-argyrodite Li6PS5X (X = Cl, Br, I). The chemical formulae of Li6PS5X (X = Cl, Br, I) are based on a substitution pattern from the corresponding Ag and Cu compounds, in which S atoms is replaced by X atoms, and S and X atoms ordered on separate crystallographic sites [53].

Table 3 Ionic conductivities of halide thiophosphates
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Li6PS5Cl obtained via ball milling has a conductivity of 1.33 × 10−3 S/cm with an electrochemical stability up to 7 V versus Li+ [52]. It crystallizes in the cubic space group F \( \overline{4} \) 3 m (Fig. 6). The first preliminary results showed that the In/Li6PS5Cl/LiCoO2 solid electrolyte battery has encouraging electrochemical performance. In 2016, Kato et al. reported Li+ superionic conductor Li9.54Si1.74P1.44S11.7Cl0.3 with an extra high conductivity of 25 mS/cm, as well as high stability (~ 0 V vs. Li metal for Li9.6P3S12). A fabricated all-solid-state cell based on this lithium conductor was found to have very small internal resistance, especially at 100 °C.

Figure 6
figure 6

@ 2016, American Chemical Society

Crystal structure of Li6PS5Cl determined from DFT calculations. Li+: green, Cl−: red, S2−: yellow, and (PS4)3− tetrahedron: purple [57].

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Rangasamy et al. obtained Li7P2S8I by the reaction of β-Li3PS4 and LiI, which shows an electrochemical stability up to 10 V versus Li/Li+ (Fig. 7) [58]. The oxidation instability of I is subverted via its incorporation into the structure. The inclusion of I also creates stability with the metallic Li anode while simultaneously enhancing the interfacial kinetics and ionic conductivity. Low-temperature membrane processability enables facile fabrication of dense membranes, making this conductor suitable for industrial application.

Figure 7
figure 7

@ 2015, American Chemical Society

Cyclic–voltammogram curve for a Li/Li7P2S8I/Pt cell at 1 mV/s, showing that Li7P2S8I is stable up to 10 V versus Li/Li+ [51].

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Sulfide electrolytes with halogen elements may have higher electrochemical windows. Current investigations are underway to identify the crystal structures and mechanisms of Li-ion conduction in the newly formed phases along with polymeric reinforcement for flexible solid electrolyte membranes. This opens new avenues for the development of inherently safe all-solid Li batteries.

Nonmetal sulfide SEs without phosphorus

The ionic conductivities of SEs without phosphorus are simply summarized in Table 4. Among them, isostructural Li0.6[Li0.2Sn0.8S2] and Li[Li0.33Sn0.67S2] crystallize in the monoclinic space group C2/c, belonging to the Na2IrO3 structure-type, an ordered variant of the layered α-NaFeO2 structure. The structural difference between Na2IrO3 and α-NaFeO2 is that the latter contains alternating layers of Na and Fe fourfold-coordinated with O atoms featuring a mixed Na/Ir layer ordered in a honeycomb framework [60]. Li0.6[Li0.2Sn0.8S2] shows a conductivity of 1.5 × 10−2 S/cm. The activation energy of Li[Li0.33Sn0.67S2] is 0.17 eV, which is comparable to values for the currently best LISICONs such as Li10GeP2S12 (0.21 eV) and Li11Si2PS12 (0.19 eV).

Table 4 Ionic conductivities of SEs without phosphorus
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Li4SnS4 crystallizes in the orthorhombic space group Pnma with Z = 4 (Fig. 8). Its structure features isolated SnS4 tetrahedra. It shows a very promising Li+ ion conductivity of 7 × 10−5 S/cm at 20 °C and 3 × 10−3 S/cm at 100 °C, which is exceptionally high for a ternary compound. An activation energy for Li+ ion transport measured via impedance spectroscopy (0.41 eV) correlates reasonably well with the values (0.29–0.33 eV) deduced from ionic mobility studies by Li solid-state NMR spectroscopy [61]. Li2GeS3 crystallizes in the orthorhombic space group Pnma, and with conductivity of 9.7 × 10−9 S/cm [62].

Figure 8
figure 8

© 2012, American Chemical Society

Crystal structure of Li4SnS4 [58].

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The matrix of Li4−x S n−x As x S4 (x = 0–0.250) is Li4SnS4 (space group Pnma). Li3.875Sn0.875As0.125S4 has a high ionic conductivity of 1.39 mS/cm at 25 °C. The lowest activation energy was observed in the composition of Li3.833Sn0.833As0.166S4 that had the highest ionic conductivity among the Li4–x Sn1–x As x S4 (x = 0–0.250) compounds. The activation energy was as low as 0.21 eV as compared to other well-known solid electrolytes. All XRD patterns show that the structure of Li4–x Sn1–x As x S4 is the Li4SnS4-type (space group Pnma) [63].

Glass electrolytes

Ionic conductivities of glass and glass–ceramic SEs are summarized in Table 5. It can be noticed that there are many chemical compositions can be tuned among these materials.

Table 5 Ionic conductivities of glass and glass–ceramic SEs
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Glass–ceramic

Glass–ceramic electrolytes can be prepared by the crystallization of precursor glasses. The crystallization usually would decrease the ionic conductivities, but the precipitation of a superionic conducting crystal from a precursor glass can enhance the ionic conductivity. The grain boundaries around crystal domains in glass–ceramics are filled with amorphous phases. Thus, glass–ceramic electrolytes usually have lower grain boundary resistances than polycrystalline systems [66]. A heat-treated Li2S–P2S5 glass–ceramic conductor has a high ionic conductivity of 1.7 × 10−2 S/cm [63]. The 75Li2S–25P2S5 (mol%) amorphous powders obtained by mechanical milling have a conductivity of 2 × 10−4 S/cm at room temperature and an activation energy of 34 kJ/mol [65]. Glass–ceramic electrolyte has a high ionic conductivity at room temperature and a negligible electronic conductivity. Advantages of the glass–ceramic SEs over the crystalline ones might include their relatively lower heat treatment temperatures.

Glass

The glassy electrolytes have attracted much attention mainly due to their several advantages over the crystalline materials: isotropic ionic conduction, no grain boundary resistance, ease to be fabricated into film, a wide range of compositions, etc. [71].

Generally, Li+ ion conducting glasses can be divided into oxides and sulfides. For most of the oxide ones, their conductivities at room temperature are too low to be practical for high-energy batteries, normally on the order of 10−6–10−8 S/cm [16], while for sulfide ones, the conductivities of 10−3–10−5 S/cm at room temperature can be achieved in view of high polarizability of S2− ions, such as Li2S–SiS2 and Li2S–P2S5 [71].

Glassy sulfide-type ion conductive materials from Li2S–P2S5 with doped LiI have been investigated extensively. Kennedy and Zhang reported a class of Li2S–SiS2 glasses doped with LiX with an ionic conductivity higher than 10−4 S/cm [69]. Numerous glass sulfide electrolytes based on xLi2S–(1 − x)P2S5 and (x)Li2S–(1 − x)SiS2 have been synthesized, mostly using mechanical milling technique. Shin et al. prepared amorphous and crystalline Li2S–(25 − x)P2S5−x P2Se5, and the crystalline one has a conductivity of about 6 × 10−4 S/cm.

Kennedy firstly synthesized the Li2S–SiS2 SE using a melting-quench method in 1986 [70]. It had an ion conductivity from 10−6 to 10−3 S/cm. The composition of 0.6(0.4SiS2–0.6Li2S)–0.4LiI showed the highest conductivity of 1.8 × 10−3 S/cm and an activation energy of 0.28 eV. Since then, Li2S–SiS2-based glass electrolytes have been widely investigated aiming to improve the ion conductivities and electrochemical stabilities. Besides, high-energy ball-milled amorphous Li2S–SiS2 showed a conductivity of 1.5 × 10−4 S/cm, and Li2S–SiS2 system doped with Li3PO4 had a conductivity of 6.9 × 10−4 S/cm with high electrochemical reduction stability [69]. The utilization of combining sulfide and oxide anions and the precipitation of superionic metastable crystals from glasses are effective ways to improve the Li+ conductivity of glass–based solid electrolytes [71]. Activation energy of 63Li2S·36SiS2·1Li3PO4 was 0.30 eV, and conductivity (298 K) was 1.6 × 10−3 S/cm [56]. The conductivity of a mechanochemically prepared 60Li2S·40SiS2 (mol%) was around 10−4 S/cm at room temperature [69]. The conductivities of the Li2S–P2S5 system could be further improved with the formation of crystalline phases by glass crystallization.

Conclusion and outlook

In summary, solid electrolytes are potential materials for batteries because of their safety and high performance. Therefore, sulfide-type solid electrolytes have higher ionic conductivities and wide electrochemical stability windows than other solid electrolytes. We discussed several types of sulfide electrolytes and make conclusions as follows:

  1. 1.

    Nonmetal electrolytes have been widely studied, which can obtain high ionic conductivity via controlling size and crystal structure.

  2. 2.

    Halide electrolytes have been reported with very wide electrochemical stability window (~ 10 V vs. Li).

  3. 3.

    Glassy electrolytes have several advantages over the crystalline materials: isotropic ionic conduction, no grain boundary resistance, ease to be fabricated into film, a wide range of compositions, etc.

  4. 4.

    Sulfide halide-type SEs containing phosphorus may become a potential material with higher ionic conductivity.

  5. 5.

    The mechanical properties (particularly the malleability and the ductility) of the SE and the electrode materials have a great impact on the contact condition of electrolyte and electrode materials. To improve the solid–solid contact between the electrolyte and the electrode materials, it is necessary to improve the mechanical properties of the SE.

  6. 6.

    A body-centered cubic (bcc) anion sublattice allows the lowest activation barrier and highest ionic conductivity. The bcc anion framework allows the Li ions to migrate within a network of interconnected tetrahedral sites possessing equivalent energies.

  7. 7.

    One of the future studies should be focused on elucidation of the phase relationships in the low- and high-temperature phases of materials.