Advanced high-voltage and super-stable sodium–zinc hybrid ion batteries enabled by a hydrogel electrolyte

Aqueous secondary batteries are emerging as promising candidates for next-generation large-scale energy storage systems, considering their high safety and economic viability. However, their commercialization is significantly hindered by the narrow electrochemical stability window and limited energy density. Here, a rationally designed hydrogel electrolyte with unique polymer network and low free water content is developed via a UV-curing strategy, which achieves a high ionic conductivity of 43 mS cm^-1, high mechanical strength and combines with an expanded electrochemical stability window up to 2.5 V (vs Zn/Zn²⁺). The hybrid electrolyte delivers high viability and versatility, and can be paired with different cathode materials to apply to both aqueous Na-Zn hybrid batteries and Zn ion batteries. Significantly, when coupled with Prussian blue cathode, the assembled hybrid batteries exhibit excellent cyclability over 6000 cycles with a much lower capacity decay of per cycle 0.0096% at high current density of 25 C. Furthermore, the Zn||Na₂MnFe(CN)₆ full battery with the synthesized hydrogel electrolyte achieves high energy density of ~220 Wh kg^-1 and excellent rate performance up to 5 C. This research offers valuable insights for the design of aqueous hybrid electrolytes that possess both high ionic conductivity and a wide electrochemical stability window.