Advanced hydrogel-based electrolytes for stable aqueous zinc batteries

In recent years, aqueous Zinc-ion batteries (AZIBs) have emerged as promising next-generation energy conversion devices due to their low flammability, high theoretical gravimetric capacity (approximately 820 mAh g–1), and robust volumetric density (5851 mAh cm–3). Nonetheless, challenges like dendrites growth, hydrogen evolution (HER), and surface passivation on the Zn anode, combined with irreversible material loss at the cathode, hinder their practical implementation. To enhance the stabilities of both electrodes, we focus on designing advanced hydrogel electrolytes to prolong the lifespan of the AIBs. We first applied a polyanionic hydrogel film, assisted by a saline coupling agent, to protect the Zn metal anode. The zincophilic –SO3– on hydrogel aids in regulating zinc ion flux, significantly reducing dendrite formation by guiding 3D Zn accumulation. The synergistic effect between saline coupling agent and hydrogel enhances the crosslink degree at the vicinity of Zn surface, further reducing the water decomposition tendency. For the full cell, we developed a hetero-polyionic hydrogel electrolyte that synergizes the merits of both polyanionic and polycationic hydrogels for a typical conversion type battery based on iodine cathode. The active iodine species are stabilized by the polycationic hydrogel and physically confined by carbon nanotubes (CNT), facilitating polyiodides conversion at carbon-PCH interfaces. To eliminate the dendrites growth from the source, we further reduce the anion concentration polarization by introducing a cation-only conductive hydrogel based on an anion (–SO3–)-tethering strategy. The polyanionic backbone refines the Zn ion solvation structure and reconstructs the hydrogen bond networks of water, promoting rapid Zn2+ desolvation and guides a thin and uniform HER byproducts (Zn(OH)2) formation. Thanks to the solvation effect of amide group (–CONH–) on the branch, an accelerated decoupling of Zn2+ and –SO3– can be achieved, resulting in a comparable ionic conductivity around 10 mS cm–1. Consequently, enhanced Zn cell stability, redox reversibility, and capacity retention were achieved. We posit that our investigation into hydrogel-based electrolytes can guide the evolution of quasi-solid aqueous energy storage solutions.