A Low‐Concentration and High Ionic Conductivity Aqueous Electrolyte toward Ultralow‐Temperature Zinc‐Ion Hybrid Capacitors

Aqueous electrochemical energy storage devices have attracted tremendous attention because of its high safety, low cost, and environmental friendliness. However, their low‐temperature operation is plagued by the freeze of electrolytes. Herein, a 3 mol kg−1 Zn(ClO4)2 electrolyte without adding any organic solvents or antifreezing additives is proposed, which yields a high ionic conductivity of 9.4 mS cm−1 even at ultralow temperatures of −60 °C. The strong electrostatic interaction between Zn2+ ion and water molecules and the structure breaking effect of ClO4− ions to destroy the hydrogen bond network between water molecules in Zn(ClO4)2 electrolyte is revealed by spectroscopic characterization and theoretical simulation. This low‐temperature electrolyte renders the zinc‐ion hybrid capacitor to exhibit a high energy density of 40.91 Wh kg−1 at −60 °C and a long‐cycle life (over 200 days) at −30 °C. This study provides a new path to develop low‐concentration antifreezing electrolytes for aqueous electrochemical energy storage devices.