Deep Eutectic Solvent‐Dominant Crosslinked Polymer Electrolytes Enabling Efficient Chloride‐Ion Transport via Competitive Coordination

Solid polymer electrolytes (SPEs) offer promising solutions to mitigate electrode dissolution and severe volume change in chloride ion batteries (CIBs), yet achieving high ionic conductivity and electrochemical stability remains challenging. Herein, a synergistic strategy combining a tailored deep eutectic solvent (DES) with a crosslinked ethoxylated trimethylolpropane triacrylate (ETPTA) polymer network is proposed to overcome these limitations. By tailoring the cation structure of chloride salts, tributylmethylammonium chloride exhibits weakened cation‐anion interactions and enhanced solvation with succinonitrile, forming a stable, non‐flammable DES with superior Cl − dissociation. Incorporating this DES into the crosslinked polymer network yields flexible, self‐standing SPE films. The optimized ETPTA 3 ‐DES 7 SPE achieves a record‐high room‐temperature ionic conductivity of 6.54 × 10 −4 S cm −1 and markedly improved electrochemical stability over the pristine DES. When applied in symmetric cells and full CIBs, the SPE enables high reversible capacities, excellent rate capability, and stable cycling even under sub‐ambient conditions down to 0 °C. Prototype pouch cells maintain reliable operation under bending and cutting, demonstrating the practical viability of this design.