Nickel-Doped Ultrathin K-Birnessite Manganese Oxide Nanosheet As Pseudocapacitor Electrode with Excellent Cycling Stability for High-Power Pesudocapacitors

We herein report a kind of nickel-doped ultrathin δ-MnO 2 nanosheets prepared using a facile chemical bath deposition method. The obtained δ-MnO 2 materials have 2D ultrathin nanosheet structures with a few atomic layers. Electrochemical measurements indicate that an appropriate amount of nickel doping can remarkably improve the specific capacitance of the δ-MnO 2 and that 1.0 mol % nickel-doped δ-MnO 2 nanosheets display the best specific capacitance of 337.9 F g –1 at 1 A g –1 . The specific capacitance can maintain at 158 F g –1 even as the current density increases to 20 A g –1, demonstrating that the electrode material possesses good rate performance. In addition, the discharge capacity fading from 160.9 to 158.8 F g –1 is slight after 4000 cycles, and the corresponding capacitance retention is as high as 98.6%. The good rate capacity and stability of the δ-MnO 2 nanosheets can be attributed to the ultrathin structure of a few atomic layers which provides large surface areas and lots of reactive active sites. Moreover, the appropriate amount of nickel ion doping at atomic level improves the conductivity of the δ-MnO 2 material.