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 Li6PS5Cl family, Li6-xPS5-xCl1+x, and combine electrochemical impedance spectroscopy, neutron diffraction, and Li-7 NMR MAS and PFG spectroscopy to show that increasing the Cl-/S2- ratio has a systematic, and remarkable impact on Li-ion diffusivity in the lattice. The phase at the limit of the solid solution regime, Li5.5PS4.5Cl1.5, exhibits a cold-pressed conductivity of 9.4 +/- 0.1mScm(-1) at 298K (and 12.0 +/- 0.2mScm(-1) on sintering)-almost four-fold greater than Li6PS5Cl under identical processing conditions and comparable to metastable superionic Li7P3S11. Weakened interactions between the mobile Li-ions and surrounding framework anions incurred by substitution of divalent S2- for monovalent Cl- play a major role in enhancing Li+-ion diffusivity, along with increased site disorder and a higher lithium vacancy population.