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.