Abstract: Hydrogel electrolytes based on natural polymers have attracted increasing attention in zinc-ion batteries (ZIBs) powering wearable and implantable electronics, but designing natural polymer hydrogels with high ionic conductivity, excellent transference performance, and inhibited Zn dendrites is still challenging. Herein, two natural biocompatible polymers (sodium alginate (SA) and agarose (AG)) are used to prepare composite hydrogel electrolytes ensuring electrostatic interaction between -COO^- groups in SA and Zn²⁺ and coordination between C-O-C groups in AG and Zn²⁺. The as-obtained hydrogels exhibit an elevated ionic conductivity (25.05 mS cm^-1) with a high transference number (0.75), useful for facilitated efficient Zn²⁺ transport. The theoretical calculations combined with experimental results reveal C-O-C groups endowing the as-prepared hydrogels with improved desolvation kinetics and capture ability of Zn²⁺ for achieving dendrite-free Zn deposition. In this way, the assembled Zn symmetric cell shows a long cycle life reaching 700 h at 0.2 mA cm^-2. The exceptional biocompatibility of the hydrogels also results in cell viability assay with a survival rate above 93.5 %. Overall, the proposed hydrogel electrolytes endow solid-state ZIBs with high discharge capacity, outstanding rate performance, long cycle life, good antifreeze capability, and impressive flexibility, useful features for future design and development of advanced ZIBs.

Key words: Natural biopolymer, Hydrogel, Zinc-ion battery, Ether group, Inhibited Zn dendrite, Biocompatibility