MXene—manganese oxides aqueous asymmetric supercapacitors with high mass loadings, high cell voltages and slow self-discharge

How to achieve high mass loadings while maintaining high energy and power densities together with slow self-discharge rates for aqueous asymmetric supercapacitors (AASCs) remains a great challenge. Herein, we tested an AASC using Ti3C2Tz MXene as the negative electrode, a mixture of manganese oxides, Mn3O4 and MnOOH, as the positive electrode with a saturated lithium chloride (14 M LiCl) electrolyte. This device, with electrode thicknesses of > 100 µm, and a mass loading of ~ 10 mg cm−2, resulted in an energy density of ≈ 30 Wh kg−1 at 0.5 A g − 1, a power density of ≈ 23 kW kg−1 at 20 A g − 1, an open cell voltage of 2.3 V, excellent rate capability and cycling stability. When allowed to self-discharge for 54 h at room temperature, ~66% of the voltage was retained. Crucially, after that time the cell voltage was > 1.5 V. This work opens a new opportunity for high performance, environmentally friendly AASCs, where high energy and power densities are combined with slow self-discharge rates at commercial mass loadings.