Mechanistic Insights into the Intermolecular Interaction and Li⁺ Solvation Structure in Small-Molecule Crowding Electrolytes for High-Voltage Aqueous Supercapacitors

The introduction of a small-molecule crowding agent with low viscosity to expand the operating voltage of aqueous electrolytes is an effective strategy to achieving low-cost and high-voltage aqueous carbon-based supercapacitors (SCs). Herein, the electrochemical stable window (ESW) of a lithium nitrate electrolyte (2 M LiNO3) is expanded to 3.65 V after adding the water-miscible dimethyl sulfoxide (DMSO) with high Gutmann donor number (29.8 kcal mol–1) as the crowding agent. The small-molecule crowding electrolyte (SMCE) has the advantages of low viscosity (3.87 mPa·s), wide temperature flexibility (−40 to 80 °C), better wettability to activated carbon (AC), and nonflammability. The reorganization of the Li+ solvation structure and the regulation of hydrogen bonds (H-bonds) of water in a DMSO-induced highly crowded environment suppress the water decomposition on the charged electrode. In situ differential electrochemical mass spectrometry (DEMS) shows that the detrimental hydrogen and oxygen evolution reactions (HER and OER) in SMCE are substantially inhibited. Molecular dynamics simulations (MDSs) indicate that the formation of a 1H2O-2DMSO molecular aggregate in SMCE destroys the H-bond network of water molecules. The SMCE enables symmetric SCs to deliver a high operating voltage of 2.4 V and an energy density of 44 Wh kg–1 at 1.2 kW kg–1.