Halide-enriched lithium argyrodite superionic conductors are considered as promising candidates for all-solid-state batteries due to their soft structure and high ionic conductivity. Challenges remain, including chemical instability and incompatibility with anode materials, and in addition a deeper understanding of the fundamental aspects of ionic transport and performance is required. In this study, we investigated two argyrodite mixed-halide series of compositions, Li_6−xPS_5−xBrCl_x and Li_5.5PS_4.5Br_1.5−xCl_x. By employing a range of techniques including X-ray diffraction (XRD), neutron diffraction, nuclear magnetic resonance (NMR) spectroscopy, electrochemical impedance spectroscopy and machine learning based molecular dynamics, we found that increasing the halide substitution enhances ionic conductivity. Notably, the Li_5.4PS_4.4BrCl_0.6 composition achieves an ionic conductivity of 10 mS/cm, demonstrates superior air stability compared to conventional lithium argyrodites and allows for the fabrication of well-performing all solid-state batteries. Our results reveal that in lithium-poor compositions the lithium environments in the 4a and 4d cages become more alike, facilitating fast long-range lithium-ion transport. This work paves the way for the development of air-stable, high-conductivity sulfide electrolytes, advancing the practical implementation of solid-state batteries.