A Cyclosiloxane‐Copolymerized Polyether Electrolyte for High‐Voltage Solid‐State Lithium Metal Batteries

Polyether‐based solid polymer electrolytes (SPEs) hold great promise for solid‐state batteries, yet they suffer from a fundamental trade‐off among ionic conductivity, oxidative stability, and mechanical robustness. Herein, a cyclosiloxane‐copolymerized polyether electrolyte (CS‐PDOX) is developed via in situ ring‐opening copolymerization between 1,3‐dioxane (DOX) monomers and a cyclosiloxane cross‐linker. The incorporation of cyclosiloxane redistributes the local electrostatic environment of the polyether electrolyte, weakens Li + ‐polymer coordination, and lowers the Li + migration barrier, leading to a high room‐temperature ionic conductivity of 8.45 × 10 −4 S cm −1 . Simultaneously, the electron‐deficient Si─O─Si downshifts the highest occupied molecular orbital (HOMO) energy level, expanding the oxidative stability to ∼4.76 V vs. Li + /Li and suppressing side reactions upon pairing with high‐voltage cathodes. Moreover, the engineered weak‐solvation, featuring a weakened Li + ‐polymer coordination environment, drives FSI − anions into the solvation sheath, which favors the formation of an inorganic‐rich solid electrolyte interphase. This interface provides uniform Li deposition and robust mechanical protection, enabling >2400 h lifespans in Li||Li cells. Consequently, the Li|CS‐PDOX|NCM811 cell delivers exceptional cycling stability with 85.0% capacity retention over 400 cycles, and the ∼3.5 Ah Li─Cu||NCM811 pouch cell delivers a remarkable energy density of 445 Wh kg −1 , demonstrating the potential of this strategy for next‐generation high‐voltage solid‐state batteries.