Boosting Solid‐State Diffusivity and Conductivity in Lithium Superionic Argyrodites by Halide Substitution

Developing high-performance all-solid-state batteries is contingent on finding solid electrolyte materials with high ionic conductivity and ductility. Here we report new halide-rich solid solution phases in the argyrodite Li₆ PS₅ Cl family, Li_6-x PS_5-x Cl_1+x , and combine electrochemical impedance spectroscopy, neutron diffraction, and ⁷ Li NMR MAS and PFG spectroscopy to show that increasing the Cl^- /S^2- ratio has a systematic, and remarkable impact on Li-ion diffusivity in the lattice. The phase at the limit of the solid solution regime, Li_5.5 PS_4.5 Cl_1.5 , exhibits a cold-pressed conductivity of 9.4±0.1 mS cm^-1 at 298 K (and 12.0±0.2 mS cm^-1 on sintering)-almost four-fold greater than Li₆ PS₅ Cl under identical processing conditions and comparable to metastable superionic Li₇ P₃ S₁₁ . Weakened interactions between the mobile Li-ions and surrounding framework anions incurred by substitution of divalent S^2- for monovalent Cl^- play a major role in enhancing Li⁺ -ion diffusivity, along with increased site disorder and a higher lithium vacancy population.