Bilayer Hydrogel Electrolytes Regulate Electric-Field-Induced Deformation in Zinc-Sodium Hybrid Batteries

Hydrogels are widely used as quasi-solid-state electrolytes (QSEs) in batteries due to their favorable mechanical properties. However, a critical fundamental phenomenon has remained largely overlooked: the electro-chemo-mechanical evolution of the hydrogel under an electric field. A real-time electric-field-induced deformation in a polyacrylamide (PAM) hydrogel electrolyte is observed, mainly driven by the Zn^…O and Zn^…N adhesion between PAM and Zn under an electric field. This irreversible deformation can induce interfacial separation at the cathode-hydrogel interface, reducing battery life. To solve this problem, a double-layer electrolyte (DLE) is constructed to couple the PAM with a sodium polyacrylate (PANa) hydrogel. The PANa layer undergoes expansive deformation aligned with the electric field, effectively counteracting the contraction of the PAM hydrogel and actively maintaining interfacial contact during cycling. In addition, the redox electrolyte in PANa provides an additional energy density and forms a zinc ferricyanide (ZnHCF) coating on the Zn anode that improves the Zn deposition kinetics. As a result, the zinc-sodium hybrid battery (ZSHB) assembled using DLE exhibits an initial discharge capacity of 117 mAh g^-1 at 0.2 A g^-1, approximately 20% higher than that of the PAM cell, and retains 81.6% of its capacity after 1000 cycles.