An Anti‐Freezing Hydrogel Electrolyte for Flexible Zinc‐Ion Batteries Operating at −70 °C

Abstract Aqueous zinc‐ion batteries (ZIBs) are considered as the promising candidate of flexible energy storage devices due to their high safety, eco‐friendliness, and low‐cost. However, conventional flexible aqueous ZIBs undergo severe capacity loss at bending and subzero temperature states. Herein, an anti‐freezing hydrogel electrolyte based on zinc tetrafluoroborate (Zn(BF 4 ) 2 ) and polyacrylamide (PAM) is developed. Owing to the strong electronegativity of F atoms, BF 4 − anions interact with water molecules by OH···F to replace the OH···O between water molecules in Zn(BF 4 ) 2 ‐PAM hydrogel electrolyte, inhibiting the formation of ice crystal lattice in hydrogel electrolyte at low‐temperature. Therefore, the Zn(BF 4 ) 2 ‐PAM hydrogel remains unfrozen state, good flexibility, and high ionic conductivity even at −70 °C, which ensures that they can act as the electrolytes of anti‐freezing flexible ZIBs. As a proof of concept, anti‐freezing flexible ZIBs with all‐in‐one integrated configuration are assembled based on Zn(BF 4 ) 2 ‐PAM hydrogel. They display enhanced electrochemical performance at −70 °C, such as high capacity retention and remarkable cyclic stability. Furthermore, they also maintain stable electrochemical performance under different bending states even at −70 °C. This work broadens the practical application of flexible aqueous ZIBs at ultralow temperature.