A cheap asymmetric supercapacitor with high energy at high power: Activated carbon//K_0.27MnO₂·0.6H₂O

Studies of the electrochemical behavior of K_0.27MnO₂·0.6H₂O in K₂SO₄ show the reversible intercalation/deintercalation of K⁺-ions in the lattice. An asymmetric supercapacitor activated carbon (AC)/0.5 mol l^-1 K₂SO₄/K_0.27MnO₂·0.6H₂O was assembled and tested successfully. It shows an energy density of 25.3 Wh kg^-1 at a power density of 140 W kg^-1; at the same time it keeps a very good rate behavior with an energy density of 17.6 Wh kg^-1 at a power density of 2 kW kg^-1 based on the total mass of the active electrode materials, which is higher than that of AC/0.5 mol l^-1 Li₂SO₄/LiMn₂O₄. In addition, this asymmetric supercapacitor shows excellent cycling behavior without the need to remove oxygen from the electrolyte solution. This can be ascribed in part to the stability of the lamellar structure of K_0.27MnO₂·0.6H₂O. This asymmetric aqueous capacitor has great promise for practical applications due to high energy density at high power density.