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.