Acidic “Water-in-Salt”\nElectrolyte Enables\na High-Energy Symmetric Supercapacitor Based on Titanium Carbide MXene

Titanium carbide MXene, Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub>, exhibits ultrahigh capacitance in acidic\nelectrolytes\nat negative potentials yet poor stability at positive potentials,\nresulting in low-energy densities for Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub>-based symmetric supercapacitors. Utilizing\n“water-in-salt” electrolytes has successfully expanded\nthe stable operation potential window of MXenes. However, this advancement\ncomes at the cost of sacrificing their high capacitance in acidic\nelectrolytes. In this work, we report an acidic “water-in-salt”\n(AWIS) electrolyte composed of sulfuric acid and saturated lithium\nhalide, which effectively doubled the energy density of the Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub>-based symmetric\nsupercapacitor compared to those with bare acidic electrolytes. Specifically,\nthe AWIS electrolyte successfully expanded the voltage window of the\nsymmetric device to 1.1 V. A high specific capacitance of 112.34 F\ng<sup>–1</sup> (at 10 mV s<sup>–1</sup>) was obtained\ndue to the presence of proton redox. As a result, the symmetric device\nachieved a high-energy density of 19.1 Wh kg<sup>–1</sup> and\na high capacitance retention of 96.3% after 10,000 cycles. This work\ndemonstrates the importance of designing stable and redox-active electrolytes\nfor high-energy MXene-based symmetric supercapacitors.