Recent advancements in redox-active transition metal sulfides as battery-grade electrode materials for hybrid supercapacitors

Supercapacitors (SCs) and secondary batteries (SBs) have been widely used for energy storage purposes, but each has its own limitation such as low energy density (Es) and power density (Ps), respectively. These limitations can be overcome by new emerging technology known as hybrid supercapacitor, which involves the merging of both these devices, yielding optimum Ps and Es in a single setup. The performance of hybrid supercapacitor devices relies on the selection of electrodes employed. As a result, numerous electrode materials have been experimented with thus far to enhance their effectiveness. Among which, transition metal sulfides (TMSs) have been found to be a promising battery grade materials due to their high electrical conductivity, enhanced specific capacity, reversible redox-activity, and availability. This review focuses on TMSs recently exploited as positive electrode materials in hybrid supercapacitor devices. Furthermore, binary composites of TMSs have shown better electrochemical performance compared to single and ternary sulfides. Various employed synthesis techniques such as sol gel, hydrothermal, electrodeposition, growth, sputtering, and precipitation methods have been discussed. Moreover, electrochemical assessment techniques particularly CV, GCD, and EIS of reported assembled devices have been explored. The energy and power densities of the hybrid supercapacitor devices which are summarized and compared through Ragone plots have also been analyzed. Lastly, the future prospective of TMSs as electrode materials have been emphasized.