First-Principles Study of Li Ion Diffusion in Lithium Oxythioborate Halide Glass Solid Electrolyte

Abstract

Due to their absence of grain boundaries that limit Li ion transport and provoke dendritic growth, glass materials are considered promising solid electrolytes for all-solid-state lithium batteries. However, the understanding of ion transport in glassy solid electrolytes is limited because of their disordered structure. In this study, the Li ion diffusion mechanism in lithium oxythioborate halide (Li2S–B2S3–LiI–SiO2) quaternary glasses with different SiO2 contents is reported. Oxygen in the glass can increase and decrease Li ion conductivity by disrupting local LiI crystals and forming strong bonds with Li ions. This conductivity is determined by the competition of the two effects of oxygen at each SiO2 content, causing a maximum conductivity of 14.6 mS cm−1 in the 30Li2S∙25B2S3∙45LiI∙25SiO2 composition, comparable to that of a liquid electrolyte of about 10 mS cm−1. Li ion hopping easily occurs in cation-rich environments, as the cations facilitate the breaking of the bonds of Li with anions, especially oxygen, by attracting the anions around Li, which is suggested to be the cation-assisted Li ion diffusion mechanism. This study suggests that controlling the oxygen:sulfur ratio in glassy solid electrolytes is key to promoting Li ion diffusion while minimizing immobilized Li ions and improving moisture stability.