Expanding the electrochemical stable window of aqueous-based electrolytes via competitive solvation induced by aprotic solvents

Aqueous electrolytes offer a promising alternative for safe, cost-effective, and scalable energy storage. However, the narrow electrochemical window of water limits their widespread application. Using super-concentrated electrolytes can effectively expand the aqueous electrolytes' electrochemical window, but the use of excessive salts also compromises electrolytes' cost, ionic conductivity, density, wettability, and temperature compatibility. In this study, we propose a competitive solvation strategy to expand the electrochemical window of aqueous KCF₃SO₃ electrolyte up to 3.2 V with low salt concentration (i.e., 1–2 m), avoiding the use of excessive salts and tackling the challenges of super-concentrated electrolytes. Our findings indicate that various aprotic solvents have different capability in modulating the water content in the primary solvation sheath and expanding the electrolytes' electrochemical stable window, which is determined by the aprotic solvents’ negative charge distribution and binding energy with K⁺ cations. The supercapacitor prototype using a 2 m KCF₃SO₃-trimethyl phosphate/H₂O electrolyte achieves an operating voltage of 2.6 V, with a 70 % improvement in energy density than that of aqueous KCF₃SO₃ electrolyte (2.0 V). Additionally, the supercapacitor demonstrated excellent cyclic stability, with 81 % capacitance retention after 100, 000 cycles, along with wide temperature compatibility.