Insights into the Compatibility and Interphases of Li₆PS₅Cl and Ga-Doped Li₂ZrCl₆ Halide Electrolytes by Solid-State NMR

Li2.1Ga0.1Zr0.9Cl6 (LGZC) halides with high ionic conductivity (>0.4 mS/cm at 25 °C) are considered promising solid electrolytes for beyond Li-ion batteries. However, they suffer from low stability against the Li metal, forming a solid electrolyte interphase (SEI) that causes continuous degradation and limits their long-term cyclability. Incorporating Li6PS5Cl (LPSCl) argyrodites as an interlayer between LGZC and Li metal is a common approach to addressing this issue and improving cyclability. Nevertheless, very few studies assessed the compatibility of halides and sulfides, reporting contradicting results. Thus, this work used solid-state NMR and impedance spectroscopy to investigate interfacial reactions and compatibility between these phases. The results show that LPSCl and LGZC are chemically reactive and decompose to form LiCl and likely Li2S or long-chain polysulfides (Li2S x ), decreasing the total ionic conductivity. Although their interface reaches stability with time, the decomposition reactions are accelerated at elevated temperatures. 6Li-6Li EXSY experiments demonstrate a spontaneous Li-ion exchange between LGZC and LPSCl despite their apparent reactivity. Interestingly, the decomposed products actively participate in this exchange, explaining why these bilayer systems can successfully enhance cell performance. Our study sheds light on the complex interfacial interaction between halides and sulfides, providing insights into the optimal design of solid electrolytes for the next generation of electrochemical devices.