Anion Engineering of Ionic Liquid-Based Polymer Electrolytes for Interfacial Stabilization in Lithium Metal Batteries

Polymer solid-state electrolytes promise safer Li-metal batteries but face slow Li+ transport and unstable Li–electrolyte interfaces. The anion is an underused handle despite governing both bulk ion dynamics and interphase chemistry. A unified cation platform was constructed to isolate the roles of Br–, I–, and BF4–, revealing that bromide delivers a dual benefit. This mild, self-limiting halide reaction seeds a uniform LiBr-rich interphase that blocks electrons yet permits Li+, while bromide weakens Li+ coordination in the polymer and accelerates segmental relaxation. This synergy increases interfacial ion flux and enhances bulk conductivity, yielding smooth plating and durable cycling. The bromide electrolyte enables 158 mAh g–1 at 0.2 C in full cells, 92.5% retention after 200 cycles at 0.5 C, and Coulombic efficiency exceeding 99.3%. The results establish anion-focused design as a practical route to polymer electrolytes that are simultaneously stable and fast-ion conducting.