Anion-sublattice engineering of Li₃PS₄:Br and I incorporation enhances ionic conductivity and Li-metal compatibility

All-solid-state batteries (ASSBs) are promising to enhance the safety and energy density of rechargeable batteries. Li₃PS₄ remains one of the most viable solid electrolytes (SEs) for all-solid-state lithium batteries. However, its low ionic conductivity and poor stability limit its commercial use. In this work, we introduce lithium bromide (LiBr) and lithium iodide (LiI) into the anionic sublattice of Li₃PS₄ to induce local disorder. An eightfold increase in ionic conductivity to 4.36 mS cm^-1 at 25 °C is achieved with the 2Li₃PS₄:LiBr:LiI composition. 2Li₃PS₄:LiBr:LiI also exhibits a high room temperature critical current density of 0.92 mA cm^-2 and improved electrochemical stability against lithium metal. The half-cell fabricated with 2Li₃PS₄:LiBr:LiI as the separator and TiS₂ as the active cathode material shows significantly better rate and long-term cycling performance compared to cells based on Li₃PS₄. Solid-state NMR and Raman spectroscopy indicate the formation of bridging PS₄ ^3- tetrahedra facilitated by the incorporation of I^- and Br^-. This study further highlights the advantages of strategic halide incorporation in thiophosphate electrolytes to enhance the performance of Li₃PS₄-based SEs and, in turn, ASSBs.