Structure and Property Changes in Sulfide Solid Electrolytes with Lithiation: A First-Principles Study

Abstract

Sulfide-based lithium-ion conductors such as Li3PS4 (LPS) and Li10GeP2S12 (LGPS) have received considerable attention for use as solid electrolytes for rechargeable batteries because of their high ionic conductivity, wide electrochemical window, and appropriate mechanical properties. However, their wide application in all-solid-state lithium batteries is hindered by spontaneous decomposition and solid electrolyte interphase layer formation when in contact with Li metal. To better understand the structure and property changes of LPS and LGPS with lithiation, we have systematically analyzed lithiated LPS/LGPS with varying Li contents by calculating their structural, electronic, and mechanical properties with density functional theory. The variation of Li-ion conductivity in these materials with the lithiation degree is also evaluated using ab initio molecular dynamic simulations. Our results unequivocally show that Li incorporation leads to decomposition of PS4 3- and GeS4 4- tetrahedra, resulting in the formation of small P and GeP clusters, along with Li2S production. The P and GeP clusters are eventually converted to Li3P and Li15Ge4 when LPS and LGPS are fully lithiated. The structural evolution gives rise to volume expansion and band gap narrowing. Over-lithiated LPS/LGPS tends to show metallic character, which could be partly responsible for the electrolyte failure. Despite the significant structural changes, the Li ion diffusivity and conductivity in both LPS and LGPS remain at the same order of the magnitude. This work provides a better understanding of the reaction behavior of LPS and LGPS with Li metal and also insight into how to analyze the reaction at the Li metal/sulfide electrolyte interface leading to an inorganic interphase layer.