Enhanced Ionic Conductivity toward Air-Stable Li<sub>4</sub>SnS<sub>4</sub> Solid Electrolytes Achieved by Soft Acid Bi<sup>3+</sup> Doping
Lihao Zheng, Juntao Shi, Gaoya Ren, Tiantian Tang, Yefeng Yang, Shenghui Shen, Zhujun Yao
Abstract
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, Li 4 SnS 4, whose structure can be adjusted by Bi 3+ doping through the combination of a two-step ball milling method and thermal annealing. The successful substitution of Sn 4+ by Bi 3+ 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 Li 4.025 Sn 0.975 Bi 0.025 S 4 exhibits an ionic conductivity of 1.35 × 10 –4 S cm –1 at room temperature, which is twice as high as that of pure Li 4 SnS 4 . Moreover, the soft acid characteristic of Bi 3+ can further ensure the air stability of Li 4 SnS 4 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 Li 4 SnS 4 solid-state electrolytes for future large-scale applications.