The increased safety associated with all-solid-state batteries using inorganic ceramic electrolytes make it a promising technology, with potential to replace current commercial battery systems. The key challenges to realize this technology are the development of new solid electrolytes with high ionic conductivity and optimization of the ionic transport pathways across the multiple phases of the battery. In this study an optimal composition of the argyrodite i.e. Li6PS5Cl0.5Br0.5 is synthesized via the mechanical milling method. This material possesses a higher bulk ionic conductivity and reduced activation energy than the single halogen doped argyrodites i.e. Li6PS5X (X = Cl and Br), assessed by temperature-dependent impedance spectroscopy and Nuclear Magnetic Resonance (NMR) relaxometry. A combined X-ray and neutron diffraction analysis reveals an influence of the composition and distribution of halogen atoms on the Li-ion conductivity. All-solid-state batteries fabricated using Li2S as cathode show a high reversible capacity of 820 mAh g−1 for up to 30 cycles. In addition, the Li-ion diffusion across the interface between the Li2S cathode and Li6PS5Cl0.5Br0.5 electrolyte is probed by exchange NMR spectroscopy. It reveals that Li-ion diffusion across this interface was the main factor limiting the performance of Li6PS5Cl0.5Br0.5 in the battery, despite its high bulk ionic conductivity.