Balancing Oxidative Stability and Ion Transport in Quasi‐Solid Polymer Electrolytes via Chlorine‐Driven Halogenation Engineering

Polyether-based solid electrolytes offer significant promise for lithium metal batteries (LMBs), yet their practical application is limited by poor room-temperature ion transport efficiency and severe oxidative breakdown at high voltages. A key challenge arises from the inherent trade-off between boosting ionic conductivity and widening the electrochemical stability window, rendering simultaneous optimization difficult. To address these challenges, we propose a novel halogenation strategy that precisely modulates the electronic environment of ether oxygen atoms. This approach effectively reduces the nucleophilicity of terminal ─OH groups and inhibits dehydrogenation reactions initiated by main-chain ether oxygens, while simultaneously optimizing Li⁺ coordination dynamics for efficient transport. Our systematic investigation of fluorine (─F), chlorine (─Cl), and bromine (─Br) reveals that Cl exhibits the most balanced electron-withdrawing effect. This leads to an exceptional electrochemical stability window of up to 5.0 V and an ionic conductivity of 1.14 mS cm^-1. The chlorinated polyether electrolyte (PCMO) demonstrates superior performance, including 2000 cycles at 2 C in LFP cells and robust high-voltage resilience with 85.3% capacity retention after 200 cycles at 4.5 V in LCO cells. Additionally, PCMO exhibits reliable performance at -10 °C in NCM622 cells and shows scalable performance in a 2 Ah NCM613/graphite pouch cell, retaining 70.1% capacity after 1000 cycles at 4.4 V.