Investigating diffusion pathways, mechanochemical milling and the electrochemical stability window for the Li5NCl2/Li9N2Cl3 solid electrolyte

Solid electrolytes which 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 terms of energy density. In this study we investigated LNCl (Li5NCl2 also often reported as Li9N2Cl3), a solid electrolyte that is thermodynamically stable against lithium metal and which features a partially occupied lithium sub-lattice. Combining experiments and simulations we investigated the lithium diffusion mechanism, the effects of mechanochemical treatment and the electrochemical stability window of LNCl. We found that a wide spread of different lithium jumps can occur in LNCl with certain jumps being much more diffusion limiting than others. Additionally, 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 (σRT~1·10-3 mS cm-1). Ball milling LNCl improves the macroscopic conductivity by one order of magnitude (σRT = 0.015 mS cm-1) and we show for the first time that LNCl can be synthesized mechanochemically without any heat treatment step. Previously, the anodic limit of LNCl was reported to be >2V vs Li+/Li but this study shows that the true anodic limit of LNCl is ~0.6 V (vs Li+/Li) which is in close agreement with our first-principles calculations. Based on our results we conclude that the anodic limit of LNCl confines its applicability to an artificial buffer layer between the lithium metal anode and other highly-conducting solid electrolytes and we computationally investigate the chemical compatibility of LNCl with common highly-conducting solid electrolytes.