Cation-assisted lithium ion diffusion in a lithium oxythioborate halide glass solid electrolyte

• Diffusion mechanism in lithium oxythioborate halide glasses was investigated. • Cation-assisted Li ion diffusion mechanism in the glass was suggested. • Oxygen increases mobile Li ions and inhibits H 2 S formation but limits Li diffusion. • Sulfur relatively promotes Li ion diffusion but causes H 2 S formation. • Controlling the oxygen:sulfur ratio in glassy solid electrolytes is important. 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, understanding of ion transport in glassy solid electrolytes is limited on account of their disordered structure. This study reports the Li ion diffusion mechanism in lithium oxythioborate halide (Li 2 S–B 2 S 3 –LiI–SiO 2 ) quaternary glasses with different SiO 2 contents. Oxygen in the glass can increase and decrease Li ion conductivity by collapsing local LiI crystals and forming strong bonds with Li ions, respectively. This conductivity is determined by the competition between the two effects of oxygen at each SiO 2 content, resulting in a maximum conductivity of 14.6 mS cm −1 in the 30Li 2 S∙25B 2 S 3 ∙45LiI∙25SiO 2 composition, which is comparable to about 10 mS cm −1 for liquid electrolytes. Li ion hopping readily 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 work suggests that precise control of the oxygen:sulfur ratio in glassy solid electrolytes is key to promoting Li ion diffusion while minimizing immobilized Li ions and improving moisture stability.