Enhanced Ionic Conductivity toward Air-Stable Li4SnS4 Solid Electrolytes Achieved by Soft Acid Bi3+ Doping

Sulfide solid electrolytes have emerged as highly promising candidates for an all-solid-state battery, while phosphorus-free compounds with air stability have not gotten enough attention. In this study, we focus on an air-stable compound, Li4SnS4, whose structure can be adjusted by Bi3+ doping through the combination of a two-step ball milling method and thermal annealing. The successful substitution of Sn4+ by Bi3+ with lower valence and larger ionic radius results in the enhancement of Li+ concentration in electrolytes and lattice volume, benefiting the Li+ migration. Therefore, the modified glass-ceramic Li4.025Sn0.975Bi0.025S4 exhibits an ionic conductivity of 1.35 × 10–4 S cm–1 at room temperature, which is twice as high as that of pure Li4SnS4. Moreover, the soft acid characteristic of Bi3+ can further ensure the air stability of Li4SnS4 based on the hard and soft acid and base theory. The findings revealed from this research provide a potential avenue for improving the performance of air-stable Li4SnS4 solid-state electrolytes for future large-scale applications.