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. Li₆PS₅Cl_0.5Br_0.5 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 Li₆PS₅Cl_0.5Br_0.5 electrolyte is probed by exchange NMR spectroscopy. It reveals that Li-ion diffusion across this interface was the main factor limiting the performance of Li₆PS₅Cl_0.5Br_0.5 in the battery, despite its high bulk ionic conductivity.

Solid-state batteries potentially offer increased lithium-ion battery energy density and safety as required for large-scale production of electrical vehicles. One of the key challenges toward high-performance solid-state batteries is the large impedance posed by the electrode-electrolyte interface. However, direct assessment of the lithium-ion transport across realistic electrode-electrolyte interfaces is tedious. Here we report two-dimensional lithium-ion exchange NMR accessing the spontaneous lithium-ion transport, providing insight on the influence of electrode preparation and battery cycling on the lithium-ion transport over the interface between an argyrodite solid-electrolyte and a sulfide electrode. Interfacial conductivity is shown to depend strongly on the preparation method and demonstrated to drop dramatically after a few electrochemical (dis)charge cycles due to both losses in interfacial contact and increased diffusional barriers. The reported exchange NMR facilitates non-invasive and selective measurement of lithium-ion interfacial transport, providing insight that can guide the electrolyte-electrode interface design for future all-solid-state batteries.