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 7Li 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±0.1 mS cm−1 at 298 K (and 12.0±0.2 mS cm−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.