Decoupling the Trade-Off between Mechanical Properties and Ionic Conductivity in Hydrogel Polymer Electrolytes by Anomalous Water-Induced Microphase Separation

Achieving simultaneous high modulus and high ionic conductivity in hydrogel electrolytes remains a fundamental challenge for durable aqueous zinc-ion batteries. Herein, we overcome this trade-off by introducing an anomalous water-induced microphase separation in a water-stiffening polymer, which comprises poly(benzyl methacrylate) (PBzMA) backbone and poly(ethylene glycol) (PEG) side chains. Simply hydrating this polymer with aqueous zinc salts triggers spontaneous formation of bicontinuous phase structures, driven by the favored hydrophilic PEG–water interactions and unfavored hydrophobic PBzMA–water interactions. This self-organized phase-separated architecture establishes stress-bearing PBzMA domains and interconnected ion transport PEG/electrolyte channels. Crucially, the hydrogel electrolyte achieves significant water-induced stiffening (214-fold increase), yielding a Young’s modulus of 134.6 MPa while maintaining an excellent ionic conductivity of 1.46 mS cm^–1 at 25 °C, surpassing most reported counterparts. Furthermore, the evolution of bicontinuous microphase-separated structures is experimentally elucidated and theoretically validated. When employed in an aqueous Zn||V₂ O₅ battery, the electrolyte enables a high capacity (278 mAh g^–1 at 0.2 C) and exceptional cyclability (∼100% capacity retention after 1000 cycles at 5.0 C). The rigid yet conductive microstructure simultaneously suppresses Zn dendrites via mechanical constraint and rapid homogeneous ion flux. Our strategy of water-induced microphase separation offers a general principle to break the stiffness-conductivity trade-off in next-generation aqueous electrolytes.