Effective Approaches of Improving the Performance of Chalcogenide Solid Electrolytes for All-Solid-State Sodium-Ion Batteries

Introduction

Recently, due to the rapidly amplified demand for electric vehicles and power storage systems based on renewable energy, the development of large-scale energy storage systems (ESSs) has aroused the attention of the whole world (Zhong et al., 2014; Dai et al., 2020; Yao et al., 2020). Considering safety and cost of large-scale ESSs, all-solid-state sodium-ion batteries (SIBs) could be an alternative to all-solid-state lithium ion batteries (LIB) (Luo et al., 2016). Additionally, compared with liquid electrolytes, solid electrolytes possess the leak-free state, wide electrochemical windows, and high thermal stability over a broad temperature range (Wang et al., 2019a). Therefore, the development of solid electrolytes is a very meaningful task.

Generally, solid electrolytes can be divided into three main categories: organic solid electrolytes, composite solid electrolytes, and inorganic solid electrolytes (Hou et al., 2018; Krauskopf et al., 2018a; Zhao et al., 2018; Kobayashi et al., 2020). Moreover, inorganic solid electrolytes include Na-β-Al₂O₃, NASICON (Na super ionic conductor), borohydride, and chalcogenide. At present, chalcogenide solid electrolytes (CSEs) are mainly divided into four categories: Na₃MS₄ (M = P, Sb) series (Tanibata et al., 2017; Krauskopf et al., 2018a; Noi et al., 2018; Takeuchi et al., 2018; Zhao et al., 2018; Zhang et al., 2020), Na₃MSe₄ (M = P, Sb) series (Bo et al., 2016; Mahmoud et al., 2019), Na_mM_xP_yX₁₂ (M = Si, Ge, Sn; X = S, Se; 10 ≤ m ≤ 11; x/y = 1/2 or 2) series (Kato et al., 2016; Richards et al., 2016; Wan et al., 2020), and Na₇P₃X₁₁ (X = O, S, Se) series (Wang et al., 2017). The most common in the Na₃MS₄ (M = P, Sb) series is Na₃PS₄, which has two crystal forms: tetragonal (P −4 21 c; a = b = 0.69520 nm, c = 0.70757 nm) and cubic (I −4 3 m; a = b = c = 0.70699 nm) (Tanibata et al., 2017; Krauskopf et al., 2018a; Takeuchi et al., 2018; Zhao et al., 2018). The room-temperature (RT) conductivity of glass ceramic Na₃PS₄ (10⁻⁴ S cm⁻¹, RT) is higher than that of glass (10⁻⁵ S cm⁻¹, RT), and that of tetragonal Na₃PS₄ (t-Na₃PS₄, 10⁻³ S cm⁻¹, RT) is higher than that of cubic Na₃PS₄ (c-Na₃PS₄, 10⁻⁵ S cm⁻¹, RT). Moreover, first-principle calculations (FPCs) show that the conduction of sodium ions in c-Na₃PS₄ and t-Na₃PS₄ is similar (Krauskopf et al., 2018a). For the Na₃MSe₄ (M = P, Sb) series, FPCs indicate that defects are the key to the conduction of sodium ions in cubic Na₃MSe₄ (Bo et al., 2016), including Na vacancy, interstitial Na, and Frenkel defect. Among these defects, the Na vacancy formation energy is the lowest, which is the main mechanism to improve the conduction of sodium ions. As for the Na_mM_xP_yX₁₂ (M = Si, Ge, Sn; X = S, Se; 10 ≤ m ≤ 11; x/y = 1/2 or 2) series (Kato et al., 2016; Rao et al., 2017), their crystal structure is similar to Li₁₀GeP₂S₁₂. Generally, NaM₆ octahedron forms a 3D transmission network via coplanarity, which enables sodium ion to conduct rapidly (Kato et al., 2016). In terms of the Na₇P₃X₁₁ (X = S, Se) series, FPCs demonstrates that the RT ionic conductivities of Na₇P₃S₁₁ and Na₇P₃Se₁₁ are higher than 10 mS cm⁻¹, respectively (Wang et al., 2017). However, the polarizability of oxygen is weaker than that of sulfur and selenium, and the distance between Na⁺ and P⁵⁺ ions of Na₇P₃O₁₁ is shorter than that of Na₇P₃S₁₁ and Na₇P₃Se₁₁. Therefore, the stronger interaction between cations results in the larger change of Na site energy and the higher activation energy required for Na⁺ migration in Na₇P₃O₁₁. Therefore, the RT ionic conductivity of Na₇P₃O₁₁ is only 0.003 mS cm⁻¹.

Since the electronegativity of sulfur is lower than that of oxygen, sulfide electrolytes usually exhibit faster sodium ion conduction than oxide electrolytes because the electrostatic force of sulfur and sodium ion is less than that of oxygen and sodium ion. Compared with oxide solid electrolyte, chalcogenide solid electrolytes (CSEs) could be synthesized at low temperature, which reduces the production cost. Due to the deformability of CSEs, it can achieve good interface contact with electrode interface via simple cold pressing to further reduce the actual production cost. Nay, CSEs have high RT ionic conductivity and low activation energy. Among all kinds of solid electrolytes, CSEs with good formability are a good choice for the preparation of bulk-type all-solid-state SIBs. Therefore, CSEs have an important application prospect in the field of bulk-type all-solid-state SIBs for large-scale energy storage. Despite these advantages, there are still some challenges before CSEs are widely used. Initially, the composite electrodes of bulk-type all-solid-state SIBs are in poor contact. Although CSEs have deformability, investigation results demonstrate cold pressing alone is not enough to form compact components without voids. Secondly, CSEs are unstable in the air, which are easy to deliquesce, oxidize, and generate toxic gases, resulting in a sharp drop of conductivity. Therefore, it is very important to improve the performance of CSEs in bulk-type all-solid-state SIBs.

This review will introduce the CSEs from the following aspects: the modification methods and the implementation technologies, the structure and electrochemical performance, and the auxiliary function of FPCs for modification. Ultimately, we describe the challenges of the CSEs and propose some strategic suggestions.

Improving Approaches of the CSEs

For the CSEs, especially chalcogenide glass-ceramic electrolytes, they not only have high room-temperature ionic conductivity (10⁻³> σ >10⁻⁵ S cm⁻¹) but also have a wide electrochemical window (V_EW, 6 V> V_EW > 4 V) and suitable mechanical properties (Hayashi et al., 2012; Kim et al., 2014; Tanibata et al., 2014a). In recent years, to more effectively enhance the electrochemical properties and mechanical properties of the CSEs, the following effective methods are used in the developments: ion doping and substitution, composite method, crystal transformation method, ceramization, and vitrification, etc. In this section, the effect of modification of different methods on the CSEs is compared vertically, mainly from the aspects of electrochemical window, ionic conductivity, and electrical stability as well as structural stability.

Ion Doping and Substitution

Ion doping and substitution are common methods of material modification (Bo et al., 2016; Kato et al., 2016; Richards et al., 2016; Krauskopf et al., 2017; Rao et al., 2017; Wang et al., 2017; Mahmoud et al., 2019; Wan et al., 2020; Zhang et al., 2020). Cation ion doping into the P site or halogen doping into the S site can introduce vacancies to improve the ionic conductivity of the CSEs (Zhang et al., 2015; Bo et al., 2016; Wang et al., 2016; Krauskopf et al., 2017, 2018a,b; Yu et al., 2017). The bond energy between ions and adjacent atoms could be changed by ion doping and substitution to improve the ionic conductivity of the electrolyte (Krauskopf et al., 2018a). Furthermore, the diffusion activation energy of ions can be further changed by this method (Bo et al., 2016; Krauskopf et al., 2017). Additionally, ion doping and substitution not only introduce vacancy but also play an important role in micro diffusion (Yu et al., 2017; Krauskopf et al., 2018b). After doping and substitution, the free volume caused by phonon is large, and the transition probability of sodium ion from one site to the adjacent site is increased. For example, the ionic conductivity of Sb⁵⁺ instead of P⁵⁺ or Se²⁻ instead of S²⁻ in Na₃PS₄ has been improved markedly (Zhang et al., 2015; Bo et al., 2016; Wang et al., 2016). Similarly, Na₃P_0.62As_0.38S₄ with the replacement of P⁵⁺ to As⁵⁺ was developed with an exceptionally high room-temperature ionic conductivity of 1.46 × 10⁻³ S cm⁻¹ and enhanced moisture stability (Yu et al., 2017). Peradventure, Cl⁻¹ instead of S²⁻ in Na₃PS₄ was obtained by a liquid-phase (suspension) method, such as Na_2.9375PS_3.9375Cl_0.0625 with high room-temperature ionic conductivity of 4.3 × 10⁻⁴ S cm⁻¹ (Uematsu et al., 2019).

Here, we choose one of the most representative examples to exhibit the effect of ion doping and substitution at the same time. The cubic Na₃PS_4−xSe_x (c-Na₃PS_4−xSe_x) materials were developed by the mechanochemical (BM) approach and high temperature (HT), respectively (Krauskopf et al., 2017). The FPCs illustrate that the replacement S with Se could not only expand the diffusion path but also enhance the ionic conductivity. Recently, Yao et al. synthesized the Na₃SbS_3.75Se_0.25 solid electrolyte with less grain-boundary resistance by a liquid/solid fusion technology, as shown in Figures 1a–g. The Na₃SbS_3.75Se_0.25 solid electrolyte shows a high RT ionic conductivity of 4.03 × 10⁻³ S cm⁻¹ owing to the significantly decreased amorphous phase in the electrolyte, and the small particle size of the solid electrolytes enhances the contact between solid electrolyte and electrode, reducing the interfacial contact resistance (Wan et al., 2019).

FIGURE 1

Figure 1. SEM images of Na₃SbS_3.75Se_0.25 electrolytes synthesized by (a) liquid-phase method, (b) solid-phase method, and (c) liquid/solid fusion technology (the inset is the statistical particle size distribution of the electrolytes). HRTEM images of Na₃SbS_3.75Se_0.25 electrolytes synthesized by (d) liquid-phase method, (e) solid-phase method, and (f) liquid/solid fusion technology. (g) STEM images of Na₃SbS_3.75Se_0.25 electrolytes synthesized liquid/solid fusion technology (source Wan et al., 2019). Reproduced by permission of Elsevier B.V. https://doi.org/10.1016/j.nanoen.2019.104109. Correlation between measured enthalpy of migration E_A and prefatory σ₀ and (h) the measured Debye frequency, (i) the measured (total) phonon band center, and (j) the computed total and Na-projected band center (source: Krauskopf et al., 2018a). Reproduced by permission of American Chemical Society https://doi.org/10.1021/jacs.8b09340 Crystal structure of (k) c-Na₃PS₄ and (l) t-Na₃PS₄. (m) Experimental G(r) data for HT-t-Na₃PS₄ and BM-c-Na₃PS₄. (n) Rietveld refinements of XRD for (BM)-c-Na₃PS₄ and HT-t-Na₃PS₄. (o) Nyquist plots of t-Na₃PS₄ and c-Na₃PS₄ at different synthetic approaches. (p) Arrhenius plots of all compounds. (q) Activation energy and room-temperature ionic conductivity for different types of Na₃PS₄ (source Krauskopf et al., 2018a). Reproduced by permission of American Chemical Society https://doi.org/10.1021/acs.inorgchem.8b00458.

Additionally, Krauskopf et al. had investigated the lattice dynamics of the superionic conductor Na₃PS_4−xSe_x by probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering, as shown in Figures 1h–j (Krauskopf et al., 2018b). The experimental results and FPCs have proved the influence of lattice dynamics and the softness of lattice to ion transport. When studying the change of the phonon spectrum or the density of state of the total phonon, they observed the activation barrier of migration and the decrease in migration entropy and emphasized that the paradigm of “the softer the lattice, the better” is not always true. Additionally, ion doping and substitution can also improve the specific capacity of the battery. Although this change enhances the ionic conductivity and charge/discharge-specific capacity, it does not significantly change the electrochemical window of the electrolyte. In addition, owing to the excessive embedment of sodium ions in the electrolyte, this method will lead to the destruction of the electrolyte structure.

Composite Method

In terms of inorganic–organic composite, due to the strong ability of dissolving sodium salt in the amorphous region of polyethylene oxide (PEO), the PEO-based composite solid electrolyte is considered as an ideal solid electrolyte. The addition of inorganic materials reduces the crystallinity of the polymer matrix and improves the activity of the polymer chain. The addition of inorganic compounds to the polymer acts as a plasticizer, which is conducive to the transition of the polymer to an amorphous structure that is easy to transport ions. Moreover, the existence of active crystal and polymer can make the composite solid electrolyte have relatively high ionic conductivity, good flexibility, stability, and interface compatibility with sodium anode.

Recently, a typical inorganic–organic composite CSE of Na₃PS₄-PEO for all-solid-state SIB was obtained by the solution-phase reaction method with room-temperature ionic conductivity of 9.4 × 10⁻⁵ S cm⁻¹ and electrochemical window of ~4.5 V (Xu et al., 2020). Moreover, the PEO coating on the surface of Na₃PS₄ particles effectively hinders the side reaction between electrolyte and sodium metal electrode. After 40 cycles, the specific capacity of all-solid-state SIB is 230 mAh g⁻¹. Additionally, the Na_3−xPS_4−xCl_x-PEO-NaClO₄ solid electrolyte with room-temperature ionic conductivity of 8.75 × 10⁻⁴ S cm⁻¹ was obtained by the heat treatment (Wang et al., 2019b).

However, inorganic–inorganic composite materials are rare, mainly related to glass ceramic electrolytes. For example, Na₃Zr₂Si₂PO₁₂-Na₃PS₄ glass ceramics exhibited high total conductivities of 1.1 × 10⁻³ at 100°C; the results suggest that the composite materials make the contact between particles closer, which is conducive to the improvement of ionic conductivity (Noi et al., 2018). Similarly, for the sulfide-based solid electrolyte Na₃SbS₄-Na₂WS₄ with RT ionic conductivity of 4.28 mS cm⁻¹ prepared by the liquid phase method, they point out that the increase in ionic conductivity is caused by sodium vacancies (Yubuchi et al., 2020).

Crystal Transformation Method

High-temperature heat treatment, ion doping, and substitution are widely applied in crystal transformation. The essence of crystal phase transition is that the crystal structure changes at a temperature higher than its phase transition temperature, while the reason for the phase transition caused by doping and substitution is that its process includes high-temperature treatment steps. The experimental results indicate that the tetragonal structure is stable at low temperature, while the cubic structure is more stable at high temperature (Hayashi et al., 2012). As we know, Na₃PS₄ crystallizes in one of two different polymorphs (i.e., cubic or tetragonal). FPCs of the cubic and tetragonal Na₃PS₄ (c-Na₃PS₄ and t-Na₃PS₄) are illustrated in Figure 1 (Zhao et al., 2018).

The results reveal that both c-Na₃PS₄ and t-Na₃PS₄ are highly conductive solid electrolytes (De Klerk and Wagemaker, 2016). Although t-Na₃PS₄ exhibits higher ionic conductivity than c-Na₃PS₄ (Zhao et al., 2018), the ionic conductivity of c-Na₃PS₄ formed by ion doping and substitution method is higher than that of t-Na₃PS₄ (Zhu et al., 2015). Moreover, for the Na₃PS_4−xSe_x materials, it is found that ion doping could be adopted to modify the crystal type due to its process which includes high-temperature treatment steps. Analogously, although RT ionic conductivity of tetragonal Na₃SbS₄ is high at 3 mS cm⁻¹, both experiments and theoretical calculations illustrate that the ionic conductivity of cubic Na₃SbS₄ is higher than that of tetragonal Na₃SbS₄ (Zhang et al., 2016, 2018).

Ceramization and Vitrification

Ceramization and vitrification is also a common approach to modify the solid electrolyte (Tanibata et al., 2014b; Park et al., 2018; Tsuji et al., 2018; Ma et al., 2020). Vitrification is the process of transforming a substance into the glassy state, in which there is no crystal structure. However, there are crystal phases in the ceramics after ceramization. Glass ceramic is a mixture of crystalline phase and residual glass phase, which is formed by crystallization of glass under the action of catalyst or nucleating agent. Ceramization and vitrification could be mainly used to enhance the stability of electrolyte to better resist the effects of harsh environments and simplify the battery packaging process.

In recent years, chalcogenide glass-ceramic electrolytes have made great progress, such as Na₁₀GeP₂S₁₂ glass ceramic (1.2 × 10⁻⁵ S cm⁻¹, RT) (Tsuji et al., 2018), 94Na₃PS₄·6Na₄SiS₄ glass ceramic (7.4 × 10⁻⁴ S cm⁻¹, RT) (Tanibata et al., 2014b), and AgX (X = S/I)-GeS₂-Sb₂S₃ glass (3.53 × 10⁻⁴ S cm⁻¹, RT) (Ma et al., 2020). Recently, Kim et al. reported a layered chalcogenide compound of composition Na₂(Ga_0.1Ge_0.9)₂Se_4.95 with high room-temperature ionic conductivity (>10⁻⁵ S cm⁻¹), synthesized as a glass-ceramic composite in the ternary system Na₂Se-Ga₂Se₃-GeSe₂ (Kim et al., 2014), as shown in Figure 2. Alkali ions containing crystalline chalcogenides are usually hygroscopic and unstable in the environment. The crystalline phase could be stabilized by inserting the glass matrix and forming a three-dimensional continuous percolation network for rapid sodium ion conduction. The results illustrate that the layered chalcogenides with the planar alkali ion diffusion path can be used as a feasible substitute for traditional solid electrolytes.

FIGURE 2

Figure 2. (A) Ternary phase diagram of glass and glass ceramics. (B) Arrhenius plots of both the bulk and the total conductivities of the glass-ceramic and the glass materials. (C) Curves of the glass-ceramic material. (D) Bode plots of AC impedance spectra of the glass-ceramic and the glass materials at 20°C. The arrows reveal the frequency hopping signal of Na⁺ in the two samples (source: Kim et al., 2014). Reproduced by permission of American Chemical Society https://doi.org/10.1021/cm502542p.

Conclusions and Perspective

In conclusion, ion doping and substitution can raise ionic conductivity of the CSEs. However, its effect on the structure stability needs to be further analyzed. Generally, the FPCs can be used to calculate the phonon spectrum, binding energy, and infrared spectrum of the modified materials, so as to determine the structural stability of the material, and then the experiments of ion doping and substitution could be carried out. The composite method can be widely used to reinforce interface contact. In addition, ceramization and vitrification are also common methods to modify the CSEs, which can improve the ionic conductivity and stability and better resist the influence of a harsh environment. In short, for different electrolytes, it is still necessary to choose appropriate modification methods according to the actual requirement, so as to better augment the performance of electrolytes.

Ultimately, the development of solid-state electrolytes of chalcogenides still faces some challenges, which are summarized as follows:

(1) The CSEs are sensitive to air. In a high-humidity environment, they are easy to deliquesce and release toxic H₂S gas, and they are easy to be oxidized in dry air. Hence, it brings difficulties to the packaging and application of the CSEs.

(2) S or Se in the CSEs is easy to exchange anion with O in oxide anode, and P/Sb/Sn/Ge is easy to be reduced by Na; hence, the interface stability between the electrolyte and the electrode needs to be enhanced.

(3) Although the conductivity of Na₁₀GeP₂S₁₂ and Na₇P₃M₁₁ (M = O, S, Se) electrolytes is high, it is difficult to synthesize them in the pure phase.

In terms of the above problems, we put forward some suggestions as follows:

(1) The theoretical calculation of element doping and surface coating is carried out by the FPCs to improve the efficiency of the experiments.

(2) Coating the CSEs with an oxide-protective layer or compounding with polymer enhances air stability.

(3) The core–shell structure is designed to limit the decomposition of sulfide during volume expansion.

(4) Combined with the phase diagram, the cooling rate is precisely controlled in the process of melting quenching to obtain the electrolyte with excellent performance.

Author Contributions

HD conceived the idea and designed this review. WX collected the related literature resources (with assistance from ZH, YC, XW, BY, ZC, JG, and DY). HD and WX contributed to data analysis and interpretation. WW (with assistance from FX, WZ, RG, and GZ) revised the manuscript. HD wrote the paper. All authors provided feedback.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Financial support from the National Natural Science Foundation of China (No. 61675049) and outstanding doctoral research promotion program of Fudan University (SSH6281011/003) is gratefully acknowledged.

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