Engineering of hydrogel electrolyte for aqueous Zn||LiFePO₄ battery on subzero-temperature adaptability, long cycles and mechanical safety

Aqueous Zn||LiFePO₄ (Zn||LFP) hybrid batteries are gaining increasing interest because of their stable charging/discharging, non-flammability, low cost, and environmental friendliness. However, aqueous hybrid batteries still face the challenges of sluggish charge transfer and ion diffusion processes due to electrolyte freezing at subzero temperatures. Herein, we designed an antifreeze hydrogel electrolyte for low-temperature Zn||LFP battery. This hydrogel electrolyte is comprised of double-network κ-carrageenan/PAAm containing moderately concentrated electrolyte (1 M ZnCl₂ and 7 M LiCl), possesses high elasticity with a fracture strain of 834% and high ionic conductivity of 14.8 mS cm⁻¹ at −40 °C. It also enables a stable Zn plating/stripping efficiency with reduced interface resistance and alleviates undesired H₂ evolution. The corresponding flexible Zn||LFP battery with hydrogel electrolyte displays outstanding cycle stability with capacity retentions of 95.2% and 80.5% after 2000 cycles at −20 °C and −40 °C, respectively. In addition, the battery exhibits low self-discharge rate and retains good electrochemical stability under different destructive mechanical testing including bending, folding, and puncturing. Owing to the ion-orientation effect, cooperative solvation effect, and interface wettability, the hydrogel electrolyte endows Zn||LFP battery with prolonged cyclability, subzero-temperature operation, and mechanical safety as compared to the liquid electrolyte counterpart.