Gradient Hydrogel Electrolyte Enables High Ionic Conductivity and Robust Mechanical Properties for Dendrite-Free...

Rechargeable aqueous Zinc-ion batteries (AZIBs) hold great promise for sustainable storage, yet their practical deployment is impeded by dendrite growth and hydrogen evolution reaction (HER). Hydrogel electrolytes offer a potential solution to stabilization but suffer from a trade-off in ionic conductivity and mechanical robustness. Herein, by leveraging the Hofmeister effect, the way ions influence the solubility, stability, and structure of polymers in aqueous solutions, a concentration gradient hydrogel electrolyte (CGHE) is designed to reconcile these challenges. By integrating two hydrogels with high (1.5 m OAc^-) and low (0.3 m) acetate concentrations, the CGHE achieves a high Zn²⁺ transference number ( $t_{Z{n}^{2+}}$ = 0.88) and excellent mechanical strength (σ = 1.7 MPa, ɛ_max = 310%). The quasi-solid gradient architecture regulates Zn²⁺ transport and cation selectivity, promoting uniform Zn (002) deposition while suppressing HER through reduced water activity in the networks. Consequently, symmetric Zn//Zn cells exhibit ultrastable cycling over 2,500 h at 1 mA cm^-2, and Zn//Cu asymmetric cells deliver a coulombic efficiency of 99.1%. The Zn//hydrogel//V₂O₅ full batteries retain 91% of capacity after 500 cycles at 2 A g^-1, while the quasi-solid electrolyte offers flexibility and flame resistance, enabling potential safe operation in wearable devices. The gradient electrolyte design provides  a general strategy for constructing advanced electrolytes in metal-based energy systems.