Anion Chemistry in Regulating Water Structure for Aqueous Supercapacitors Operating at −60 °C

Abstract Aqueous electrolytes featuring intrinsic safety and high ionic conductivity have been envisioned as a competitive and promising electrolyte candidate for application in supercapacitors, but water solidification poses limitations on achieving stable low‐temperature operation of aqueous supercapacitors. Herein, an anion chemistry strategy is proposed and developed to modify water structures to formulate temperature‐adaptability aqueous electrolytes for low‐temperature supercapacitors. Multiple characterization techniques reveal a fundamental correlation between various anions and alteration of water structure. It is noteworthy that ClO₄⁻ with weak hydration can act as a high‐efficiency water structure breaker, inducing a disordered and discontinuous distribution of water molecules. Resultantly, an aqueous electrolyte composed of Mg(ClO₄)₂ features outstanding temperature resistance (solidification temperature below −60 °C), which supports a symmetrical supercapacitor to achieve stable operation, excellent cycle stability, and low capacitance loss (12% at 1 A g⁻¹, 31% at 20 A g⁻¹) from 25 to −60 °C. Furthermore, at −60 °C, a 2.4 V high working voltage is achieved with superior rate capability and high capacitance retention of 81.5% after continuous 30 000 cycles. Determining and mastering specific ion effects can provide a promising approach to designing and developing antifreezing aqueous electrolytes for energy storage at low temperatures.