All-solid-state Li-ion batteries are a next generation electrochemical energy storage technology and an emerging alternative to conventional Li-ion batteries owing to higher energy density, wider operation temperature range and better safety properties¹. Solid state ionic conductor as the solid electrolyte (SEs) is the key component in all-solid-state Li-ion batteries. Among existing SEs, sulfides SEs attract extensive attentions due to high room temperature ionic conductivity and favorable mechanical properties. However, currently known sulfide SEs only present a few limited types of crystal structures, which limits the materials design and hinders our understanding of ionic conduction mechanisms. Exploring new compound and new crystal structures beyond known groups is much desired in order to expand the ionic conductor database and advances the knowledge about ionic conduction.

Recently, our group successfully synthesized and identified a new lithium chloro-thiophosphate in Li₂S-P₂S₅-LiCl system, namely Li₁₅P₄S₁₆Cl₃². This compound shows a low ionic conductivity of ~10^-7 S/cm at room temperature. However, this crystal structure provides abundant choices for composition and unit cell size tuning, which may trigger high ionic conductivity. We successfully substituted P in Li₁₅P₄S₁₆Cl₃ with aliovalent cations to tune the Li content and the interstitial site occupancy. As a result, high room temperature conductivity of ~10-5 S/cm was achieved. The structure of the doped compounds was examined with using synchrotron X-ray and neutron diffractions. The Li⁺ diffusion pathway was also theoretically investigated with using ab initio molecular dynamics (AIMD) method and experimentally examined with using ⁷Li solid state nuclear magnetic resonance (NMR) techniques.

1. Auvergniot, J.; Cassel, A.; Ledeuil, J.-B.; Viallet, V.; Seznec, V.; Dedryvère, R., Interface Stability of Argyrodite Li6PS5Cl toward LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂, and LiMn₂O₄ in Bulk All-Solid-State Batteries. Chemistry of Materials 2017, 29 (9), 3883-3890.
2. Liu, ZT.; Zinkevich, T.; He, XF.; Indris, S.; Xiong, S.; Liu, J.; Xu, W.; Bai, J.; Mo, Yifei; Chen, HL., Li₁₅P₄S₁₆Cl₃, a new lithium chloro-thiophosphate as a solid state ionic conductor. Submitted.