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 1.33 CT z MXene films as the negative electrode, and tetramethylammonium cation intercalated birnessite (TMA + -MnO 2 ) films as the positive electrode in a 21 mol kg À 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 À 1 at 2 mV s À 1 and a power density of 10.3 kW L À 1 at 1 V s À 1 were achieved. The cells also exhibit excellent cycling stability (> 98 % after 1,0000 cycles at 100 mV s À 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 1.33 CT z and TMA + -MnO 2 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.

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 1.33 CT z MXene films as the negative electrode, and tetramethylammonium cation intercalated birnessite (TMA + -MnO 2 ) films as the positive electrode in a 21 mol kg À 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 À 1 at 2 mV s À 1 and a power density of 10.3 kW L À 1 at 1 V s À 1 were achieved. The cells also exhibit excellent cycling stability (> 98 % after 1,0000 cycles at 100 mV s À 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 1.33 CT z and TMA + -MnO 2 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.