Ca ²⁺ /Zn ²⁺ alginate hydrogel electrolyte for high-performance zinc–ion batteries

The growing energy crisis has intensified the focus on green energy, sparking widespread interest in aqueous zinc-ion batteries. However, their development has been hindered by issues in the zinc anode. Here, Ca²⁺/Zn²⁺ alginate hydrogel electrolyte was designed to effectively suppress dendritic growth and parasitic side reactions. The Ca²⁺ primary cross-linking provides a regular "egg-box" network framework for fast ion transport, whereas secondary cross-linking with Zn²⁺ creates a denser, interpenetrating network with calcium, thereby enhancing the hydrogel's mechanical strength. Furthermore, the abundant -OH and -COO^- groups on the alginate chains formed hydrogen bonds with H₂O, which reduced water activity. Meanwhile, the abundant -OH and -COOH groups on the alginate chains formed hydrogen bonds/coordination with H₂O/Zn²⁺, reducing the activity of H₂O and strengthening the ion confinement effect. Therefore, the Zn/SCZ/Zn symmetric cell achieved stable cycling for over 900 hours at 2 mA cm^-2 and 2 mAh cm^-2, while the Zn/SCZ/MnO₂ battery retained 62.03% of its capacity after 700 cycles. This Ca²⁺/Zn²⁺ dual-ion crosslinking strategy for the alginate hydrogel electrolyte offers a novel approach to address the limitations of conventional aqueous electrolytes.