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

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(ClO₄)₂/H₂O with acetonitrile (AN) diluent. The charged Ca²⁺ cations anchor the oxygen atoms in H₂O molecules, preventing them from being hydrogen acceptors. Meanwhile, the ClO₄ ^- anions weakly interact with hydrogen atoms, which ensures strong intramolecular O─H bonds in 8 m Ca(ClO₄)₂/H₂O. 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 ClO₄ ^- can replace the ClO₄ ^- in Ca²⁺ solvation sheaths, which reduces Ca²⁺-ClO₄ ^- clusters, accordingly suppressing salt precipitation even at -50 °C. Resultantly, a symmetric supercapacitor assembled with 4.2 m Ca(ClO₄)₂/H₂O-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.