Dilute Aqueous-Aprotic Hybrid Electrolyte Enabling a Wide Electrochemical Window through Solvation Structure Engineering

The application of superconcentrated aqueous electrolytes has shown great potential in developing high-voltage electrochemical double-layer capacitors (EDLCs). However, the broadening of the electrochemical window of such superconcentrated electrolytes is at the expense of their high cost, low ionic conductivity, high density, and narrow operating temperature range. Herein, the electrochemical window of water (>3 V) at low salt concentration (3 m) is expanded by using an aprotic solvent, i.e., trimethyl phosphate (TMP), to regulate the solvation structure of the electrolyte. Benefiting from the low salt concentration, such electrolyte is simultaneously featured with high ionic conductivity, low density, and wide temperature compatibility. Based on the dilute hybrid electrolyte, EDLCs constructed by using porous graphene electrodes are able to operate within an enlarged voltage range of 0–2.4 V at a wide range of temperatures from −20 to 60 °C. They also present excellent rate capability and cycle stability, i.e., 83% capacitance retention after 100 000 cycles. Density functional theory calculations verify that TMP induces a significant electronic modulation for the bonding environment of the electrolyte. This enables the stronger binding of Na⁺–H₂O with freely migrating TMP to expand the voltage window to exceed the potential limitation of aqueous electrolytes.