An Iodide-Chloride Solid Electrolyte Compatible with Lithium Metal for All-Solid-State Lithium Batteries

Halide superionic conductors have garnered considerable attention due to their high ionic conductivity, mechanical deformability, and excellent oxidative stability. However, their incompatibility with lithium metal results in a thermodynamically unstable interface that increases interfacial impedance, thereby limiting the performance of halide-based all-solid-state lithium-metal batteries (ASSLBs). In this study, we report the synthesis of a series of iodide-chloride solid electrolytes, Li₂ZrCl_6-xI_x (x = 0-3), designed to enhance the reduction stability of the electrolyte through the high polarizability of I^-. The substitution of I^- promotes covalent bonding with the central cation, thereby reducing its reduction tendency. The Li/Li₂ZrCl₄I₂/Li symmetric cell exhibits stable cycling for over 6000 h at 0.2 mA cm^-2 and withstands high critical current densities up to 6 mA cm^-2. Full cells incorporating Li₂ZrCl₄I₂ as the solid electrolyte exhibit enhanced cycling stability and capacity retention. Furthermore, the characterization by XPS and ToF-SIMS revealed the formation of an interfacial passivation layer composed of LiI and LiCl, which effectively stabilized the lithium-metal electrode and inhibited further electrolyte decomposition. These findings highlight the potential of iodide-substituted halide electrolytes in addressing interfacial challenges associated with lithium metal anodes, providing a promising pathway for the practical implementation of high-energy-density ASSLBs.