Phase-Controlled Synthesis of Nickel Sulfides: Toward Pure NiS2 with Enhanced Electrochemical Performance in Flexible Supercapacitor

Flexible supercapacitors have attracted considerable attention as next-generation energy storage devices for wearable and portable electronics owing to their high power density and mechanical flexibility.Among various electrode materials, nickel sulfides are promising candidates due to their rich redox activity and relatively good electrical conductivity, which results in their high theoretical capacitance.However, the electrochemical performance of nickel sulfides strongly depends on their phase composition, crystallinity, and microstructure, which are highly sensitive to synthesis conditions.In this study, nickel sulfide was directly grown on carbon cloth (CC) via a one-step solvothermal process, and the effects of synthesis parameters on phase formation and electrochemical performance were systematically investigated.The as-prepared materials were characterized using field-emission scanning electron microscopy (FESEM), energy-dispersive Xray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and the Brunauer-Emmett-Teller (BET) method.By optimizing the precursor ratio (Ni:S = 1:3), solvothermal temperature (160 C), and reaction time (15 hours), a phase-pure NiS2 (NS3-160-15) with high crystallinity and a well-balanced crystalline size is achieved.The optimized NS3-160-15 electrode exhibits a notable specific capacitance of 806 F g -1 at a current density of 1 A g -1 , along with an impressive rate retention of 76.3% at 5 A g -1 .An asymmetrical supercapacitor was fabricated using the assynthesized material as the positive electrode and activated carbon as the negative electrode, achieving a high energy density of 64 mWh cm -2 at a power density of 2281 mW cm -2 and retaining 100% of its initial capacitance after 5000 charge/dicharge cycles.These findings indicate that the low-cost, facile-to-synthesize NiS2/CC electrodes hold great promise as electrode materials for high-performance flexible supercapacitors.