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, LiYbCl electrolytes are synthesized that are kinetically stable with lithium by forming an electronic insulating solid electrolyte interphase. Guided by critical overpotential criteria, a PI interlayer is designed that transforms into LiPI upon contact with lithium, substantially reducing the interfacial resistance of LiYbCl against lithium to 34 Ω and achieving a high critical overpotential of 114 mV. By substituting Yb with Lu, LiLuCl electrolytes with LiPI interlayers reach a CCD of 1.0 mA cm at a capacity of 1.0 mAh cm, comparable to sulfide electrolytes but with higher oxidation stability. Additionally, LiPI enables stable cycling of Li//Li cells with LiLuCl electrolytes at 0.5 mA cm for 400 cycles and maintains 86.5% capacity in Li//LiCoO cells after 220 cycles at 30 °C, paving the way for high-performance ASSLMBs.