Mechanochemical Synthesis and Characterization of XLi4SnS4·(1−x)Li3PS4 Solid Electrolytes

Abstract

Sulfide all-solid-state batteries are of great interest because some sulfide solid electrolytes (SEs) show the equal ionic conductivity of organic liquid electrolyte. However, sulfide SEs have an issue of H2S gas generation when they are exposed to air. Therefore, the development of SEs with high ionic conductivity and high air-stability is required. The 75Li2S·25P2S5 (Li3PS4) glass has been used in all-solid-state cells as a typical SE due to the relatively high ionic conductivity of 3–4 × 10−4 S cm−1 at 25°C. Regarding its air-stability, Muramatsu et al reported that Li3PS4 exhibited the highest air-stability in the Li2S–P2S5 system.[1] In addition, Sahu et al proposed that stability of SEs against moisture in air depended on the hard and soft acids and bases theory.[2] Specifically, Sn-based SEs such as Li4SnS4 show higher air-stability than P-based SEs because Sn is softer acid than P leading to less replacement of the soft base S by the hard base O.Based on the above reports, we focused on the Li3PS4–Li4SnS4 system as a candidate for achieving high ionic conductivity and high air-stability. Our strategy is summarized following two points:1) Increasing the ionic conductivity of Li3PS4 by increasing the amount of Li carrier2) Improving the air-stability of Li3PS4 by adding SnRecently, Zhao et al reported that Sn-substituted Li3PS4 (Li3.2P0.8Sn0.2S4) glass-ceramic shows high ionic conductivity of 1.21 × 10−3 S cm−1 at 25°C.[3] For the further material research, it is important to investigate a relationship between the structure, ionic conductivity and air-stability of xLi4SnS4·(1−x)Li3PS4 systematically. In this study, we prepared xLi4SnS4·(1−x)Li3PS4 glasses/glass-ceramics and investigated their structure, ionic conductivity and air-stability.X-ray diffraction (XRD) patterns of x = 0–0.5 showed halo patterns, while x = 0.6–1 exhibited the diffraction peaks of hexagonal Li4SnS4. As increasing the amount of Li4SnS4, the intensity of the Raman bands attributable to PS4 3− and SnS4 4− units decreased and increased, respectively. Ionic conductivities of as-milled xLi4SnS4·(1−x)Li3PS4 increased with decreasing x value, and the sample of x = 0.1 showed the highest ionic conductivity of 5.5 × 10−4 S cm−1 at 25°C. xLi4SnS4·(1−x)Li3PS4 (x = 0–0.5) glass-ceramics were prepared by heat-treatment of corresponding glasses. XRD patterns of x = 0.1–0.4 glass-ceramics were similar to that of x = 0 (β-Li3PS4), while x = 0.5 showed a different XRD pattern from x = 0.1–0.4 due to the higher amount of Li4SnS4 compared with x = 0.1–0.4. Ionic conductivities of x = 0.1–0.3 glass-ceramics increased to 8–9 × 10−4 S cm−1 compared with corresponding glasses.The air-stability test revealed that air-stability of SEs improved, as increasing the amount of Li4SnS4. Furthermore, H2S amount of x = 0.3–1 unchanged before and after the test. Therefore, x = 0.3 glass-ceramic is a well-balanced SE with high ionic conductivity and high air-stability in xLi4SnS4·(1−x)Li3PS4.