Interlayer Design for Halide Electrolytes in All‐Solid‐State Lithium Metal Batteries

All-solid-state lithium-metal batteries (ASSLMBs) are promising for transportation electrification due to their superior safety and high energy density. Lithium halide electrolytes provide excellent processing flexibility, high ionic conductivity, and anodic stability (>4.1 V), making them highly compatible with high-voltage cathodes, surpassing sulfide electrolytes (<2.1 V). Nevertheless, halide electrolytes suffer from low cathodic stability and form an electronically conductive interphase with lithium, resulting in a critical current density (CCD) of nearly zero. Herein, Li₃YbCl₆ electrolytes are synthesized that are kinetically stable with lithium by forming an electronic insulating solid electrolyte interphase. Guided by critical overpotential criteria, a PI₃ interlayer is designed that transforms into Li₆PI₃ upon contact with lithium, substantially reducing the interfacial resistance of Li₃YbCl₆ against lithium to 34 Ω and achieving a high critical overpotential of 114 mV. By substituting Yb with Lu, Li₃LuCl₆ electrolytes with Li₆PI₃ interlayers reach a CCD of 1.0 mA cm^-2 at a capacity of 1.0 mAh cm^-2, comparable to sulfide electrolytes but with higher oxidation stability. Additionally, Li₆PI₃ enables stable cycling of Li//Li cells with Li₃LuCl₆ electrolytes at 0.5 mA cm^-2 for 400 cycles and maintains 86.5% capacity in Li//LiCoO₂ cells after 220 cycles at 30 °C, paving the way for high-performance ASSLMBs.