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 Li ₆ PS ₅ 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 Li ₆ PS ₅ Cl under identical processing conditions and comparable to metastable superionic Li ₇ P ₃ S ₁₁ . 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.
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