Reduced CoNi2S4 nanosheets with enhanced conductivity for high-performance supercapacitors

Defect engineering on transition metal dichalcogenides has been regarded as an effective method to improve electrochemical properties in terms of generating active sites and enhancing the intrinsic conductivity. This study reports a new high-performance electrochemical supercapacitor made of reduced CoNi2S4 (r-CoNi2S4) nanosheets, which are synthesized via a facile moderate-reduction process. The sulfur-deficient r-CoNi2S4 nanosheets exhibit significantly enhanced conductivity which is induced by abundant sulfur vacancies formed in the reduction reaction. Compared with the pristine CoNi2S4 nanosheets, the r-CoNi2S4 nanosheets are characterized with a higher specific capacity (1117C g−1 at current density of 2 A g−1) as well as excellent rate capability and stable cycling performance. First-principle analysis confirms that the sulfur vacancies originating from the reduction lead to improve hybridization between the Ni and Co d states and the S p states close to the fermi level, and consequently enhance conductivity with the CoNi2S4 nanostructure. Moreover, an ultrahigh energy density of 55.4 Wh kg−1 at the power density of 8 kW kg−1 is obtained in an asymmetric supercapacitor configuration, and 80% capacitance of the supercapacitor remains even after 10000 cycles.