CMC–Zn hydrogel electrolyte with self-releasing Zn²⁺ achieves long cycle stability in aqueous zinc ion batteries

Zinc-ion batteries (ZIBs) have garnered significant interest owing to their intrinsic safety, environmental compatibility, and low cost. However, nonuniform Zn deposition and parasitic side reactions during cycling lead to rapid capacity decay and potential short-circuiting. To address these challenges, we developed a carboxymethyl cellulose–zinc (CMC–Zn) hydrogel electrolyte with self-release capability using a metal–ion crosslinking approach. The dynamically reversible CMC–Zn network continuously supplies active Zn²⁺ during cycling, compensating for electrode consumption in real time. Abundant carboxylate and hydroxyl groups regulate uniform zinc nucleation and growth, while the hydrogen-bonding network synergistically suppresses side reactions, as reflected by a low hydrogen-evolution potential (−0.281 V) and reduced corrosion current density (0.03 mA cm⁻²). With these advantages, Zn||Zn symmetric cells achieve an ultralong lifespan of 6,400 h at 0.5 mA cm⁻², and Zn||Cu half-cells deliver a stable coulombic efficiency of 99.1% over 4,200 cycles. In full-cell testing, self-released Zn²⁺ contributes 29% of the overall capacity, enabling Zn||PANI cells to retain 75% capacity after 2,000 cycles and exhibit a rate-performance recovery of 97.4%. A corresponding flexible ZIB maintains stable operation under various deformation conditions, highlighting the strong potential of CMC–Zn hydrogel electrolytes for next-generation flexible energy-storage devices.