Zn substituted Li₄P₂S₆ as a solid lithium-ion electrolyte for all-solid-state lithium batteries

Li-ion conductors are pivotal materials for all-solid-state Li batteries requiring high energy densities and operational safety. PS₄-based thio-phosphate materials have been intensively investigated as solid electrolytes; however, studies on more stable P₂S₆-based materials are scarce. We herein report on the application of Zn-substituted Li₄P₂S₆, Li_4−2xZn_xP₂S₆, as a Li-ion conductor. Owing to the slightly smaller ionic radius of Zn²⁺ than Li⁺, the unit cell volume decreases gradually upon Zn substitution without introducing significant structural changes. However, the ionic conductivity of the substitution phase was improved by 10⁴ times (3.8 ​× ​10⁻⁶ ​S ​cm⁻¹) at x ​= ​0.75 compared to unsubstituted Li₄P₂S₆, which was achieved by generating deficiency on the Li sites via substitution. Such Li-ion deficient site enables Li ions to hop from one site to another in the crystal structure. The 3D diffusion pathway analysis using bond-valence-landscape-energy calculations suggests that the Li diffusion pathways are mostly two-dimensional in the ab plane in this structure. This study shows that an appropriate Li defect concentration within a given structure is critical to improving ionic conductivity.