An Iodide-ChlorideSolid Electrolyte Compatible withLithium 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‑<i>x</i>I_<i>x</i> (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.