Microscopic‐Level Anion & Diluent Chemistry in Electrolyte for Aqueous Supercapacitors Operating at High Voltage and Low Temperature

Abstract Aqueous supercapacitors, serving as safe and green high‐power energy storage devices, hold significant potential in various applications. Exploring advanced electrolytes beyond traditional electrolytes is essential for achieving stable high‐voltage and low‐temperature operations. Herein, a hybrid electrolyte with a high electrochemical stability window (3.29 V) and sufficient ionic conductivity (1.5 mS cm −1 at −50 °C) is developed via hybridizing 8 m Ca(ClO 4 ) 2 /H 2 O with acetonitrile (AN) diluent. The charged Ca 2+ cations anchor the oxygen atoms in H 2 O molecules, preventing them from being hydrogen acceptors. Meanwhile, the ClO 4 − anions weakly interact with hydrogen atoms, which ensures strong intramolecular O─H bonds in 8 m Ca(ClO 4 ) 2 /H 2 O. The used AN can contribute to decreased salt dosage, without any compromise to electrochemical stability and safety. Furthermore, the AN with a higher donor number than ClO 4 − can replace the ClO 4 − in Ca 2+ solvation sheaths, which reduces Ca 2+ −ClO 4 − clusters, accordingly suppressing salt precipitation even at −50 °C. Resultantly, a symmetric supercapacitor assembled with 4.2 m Ca(ClO 4 ) 2 /H 2 O‐AN electrolyte presents high‐voltage and temperature‐adaptability features, with excellent rate capability and high long‐term cycling stability from 25 to −50 °C at 2.3 V.