Elucidating Li5NCl2: A fully-reduced, highly-disordered nitride- halide electrolyte for solid state batteries with lithium metal anodes

Solid electrolytes that are thermodynamically stable against lithium metal may be key to stabilizing lithium metal/solid electrolyte interfaces which is crucial for realizing all solid state batteries that outperform conventional lithium ion batteries. In this study we investigate Li5NCl2 (LNCl) a fully-reduced solid electrolyte that is thermodynamically stable against lithium metal. Combining experiments and simulations we investigate the lithium diffusion mechanism, different synthetic routes and the electrochemical stability window of LNCl. Li nuclear magnetic resonance (NMR) experiments show that fast Li motion (σRT > 0.1 mS cm-1) is present in LNCl which is however not accessible in macroscopic LNCl pellets. Our ab-initio calculations indicate a disorder-induced wide spread of different lithium jumps in LNCl with certain jumps being much more diffusion limiting than others providing an explanation for the discrepancies in the NMR and electrochemical impedance spectroscopy (EIS) conductivity measurements. The fundamental understanding of the diffusion mechanism we develop herein will guide future synthesis optimizations for LNCl to further improve its conductivity (currently σRT = 0.015 mS cm-1) and may be applied to other highly-disordered fully-reduced antifluorite electrolytes. We further find that the previously reported anodic limit (>2V vs Li+/Li) is an overestimate and find the true anodic limit at 0.6 V which is in close agreement with our first-principles calculations. Because of LNCl’s stability against lithium metal we identify LNCl as a prospective artificial protection layer between highly-conducting solid electrolytes and strongly-reducing lithium metal anodes; Thus we also provide an investigation of the chemical compatibility of LNCl with common highly-conducting solid electrolytes.