Doping-driven electronic structure and conductivity modification of nickel sulfide.

The lack of electrical conductivity limits the electrochemical kinetic rate of the electrode material, resulting in the inability to reach its theoretical capacity. A facile method is adopted to improve the intrinsic conductivity of binary NiS 2 /Ni 3 S 4 hybrid nickel sulfide, with the doping of transition metal atoms Co, Mn and Ag. Through the introduction of heteroatoms, the electronic structure of the electrode material is modified and the electrical conductivity is significantly improved, thus enhancing its electrochemical performance. The improvement of conductivity is attributed to the formation of intermediate bands of transition metals and the redistribution of electrons, and the result is demonstrated by experimental and density functional theory (DFT) calculations. As a result, the NiS 2 /Ni 3 S 4  hybrid nickel sulfide after 0.5% amount of Co-doping reaches the highest specific capacitance of 2874 F g -1 at 1 A g -1 , increasing specific capacitance of 653 F g -1 as 29.4% of the specific capacitance of non-doped nickel sulfide. The Co doped nickel sulfide also exhibits remarkable cycling stability compared with non-doped nickel sulfide. The assembled 2% Co-doped nickel sulfide//rGO, 0.5% Mn-doped nickel sulfide//rGO and 0.5% Ag-doped nickel sulfide//rGO asymmetric supercapacitors show a specific energy density of 36.6, 36.1 and 36.0 W h kg -1 at a power density of 800 W kg -1 . This study provides a useful insight into the fabrication of high performance pseudocapacitive materials.