Stoichiometry‐Tuned Ni ₁₇ S ₁₈ for High‐Performance Energy Storage Applications

Transitional metal sulfides (TMSs) show good performance in energy storage systems, where nickel sulfide nanomaterials have exhibited high theoretical capacitance and good redox mechanism with numerous crystalline phases. One of the strategies used for further increasing the electrode performance is stoichiometry tuning, which directly controls the atomic structure, defect chemistry, and electrochemical properties of the material. Our work reports on the synthesis of the nickel sulfide phase Ni 17 S 18 using a simple hydrothermal route, designed by fine‐tuning the Ni: S stoichiometry with three compositional variants, namely, NiS‐1.95, NiS‐2.00, and NiS‐2.05. The samples were characterised using X‐ray diffraction (XRD), Raman spectrometer, field emission scanning electron microscope (FESEM), high‐resolution transmission electron microscope (HR‐TEM), and UV–vis‐NIR spectrophotometer. The variation in sulfur leads to vacancies that create lattice distortions affecting their electrochemical properties. NiS‐1.95 electrode, displayed remarkable charge‐storage performance with a specific capacitance of 868.36 F/g at 10 mV/s, an energy density of 16.95 Wh/kg, a power density of 2.6 kW/kg, and 92.4% capacitance retention for 10 000 cycles. The electrochemical impedance and kinetic study indicated a reduced charge–transfer resistance and a dominant diffusive, battery‐type response. NiS‐1.95 with a reduced sulfur ratio in the nickel‐deficient Ni 17 S 18 phase creates ambient conditions for increased energy storage performance, identifying as a promising electrode material for future energy‐storage devices.