MXene//MnO₂ Asymmetric Supercapacitors with High Voltages and High Energy Densities

Abstract Aqueous asymmetric supercapacitors (AASCs) can have high voltages and high energy densities. However, the rational design of AASCs with proper negative and positive electrodes remains a challenge. Herein, we report on an AASC using Mo_1.33CT_z MXene films as the negative electrode, and tetramethylammonium cation intercalated birnessite (TMA⁺‐MnO₂) films as the positive electrode in a 21 mol kg⁻¹ lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) electrolyte. Benefiting from a high, stable voltage of 2.5 V, an energy density of 86.5 Wh L⁻¹ at 2 mV s⁻¹ and a power density of 10.3 kW L⁻¹ at 1 V s⁻¹ were achieved. The cells also exhibit excellent cycling stability (>98 % after 1,0000 cycles at 100 mV s⁻¹) and a 51.1 % voltage retention after 10 h. This good performance is attributed to the high stable potential window and high volumetric capacitances of both Mo_1.33CT_z and TMA⁺‐MnO₂ electrodes in highly concentrated electrolytes. This work provides a roadmap for developing high performance AASCs with high voltages and high energy/power densities, with relatively slow self‐discharge rates.