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

Abstract 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.