We demonstrate an asymmetric supercapacitor in a potassium acetate-based water-in-salt electrolyte, where 2-D titanium carbide MXene and manganese oxide were used as negative and positive electrode materials, respectively. Use of water-in-salt electrolyte enables the assembled asymmetric device to be operated up to a cell voltage of 2.2 V, which overcomes the limited cell voltage issue in aqueous pseudocapacitors (1.2 - 1.4 V). This cell shows excellent rate capability (~48%) between 5 and 100 mV s-1 and good stability (~93%) throughout 10,000 charge-discharge cycles (at 1 A g-1) and 25 h voltage-hold at 2.2 V, which is competitive when compared with the performance of known asymmetric supercapacitors designed with activated carbon electrodes and fluorinated-imide based water-in-salt electrolytes. Moreover, our device shows slower self-discharge and ~32% higher volumetric energy density than activated carbon-based supercapacitors and is promising for applications where volumetric energy density is critical.