Expanded Electrochemical Stability Window (∼3.0 V) of Ni‐Doped FeS 2 /WS 2 Hybrid Nanocomposite‐Based Supercapacitor Electrode Using Water‐In‐Salt Electrolyte

The impact of two aqueous electrolytes (1 M Na 2 SO 4 and 1 M KOH) and one water‐in‐salt electrolyte (WiSE) (17m NaClO 4 ) on the electrochemical performance is analyzed in conjunction with Ni‐doped FeS 2 /WS 2 hybrid nanocomposite electrodes. The Ni‐doped FeS 2 /WS 2 nanocomposite reveals pseudocapacitive behavior with a high specific capacitance of 474.19 F/g in the 17 m NaClO 4 electrolyte at a low discharge current density of 3.5 mA/g, which is much higher than in 1 M Na 2 SO 4 and 1 M KOH electrolytes. Furthermore, cyclic voltammetry tests show a large potential window of ∼3.0 V for 17 m NaClO 4 electrolyte, which is much wider as compared to only 1.4 V and 1.1 V for the 1 M Na 2 SO 4 and 1 M KOH electrolytes, respectively. Additionally, these high capacitances combine with the large voltage window, leading to the highest energy density of 129.08 Wh/Kg and power density of 680.56 W/Kg, along with the better capacity retention of 87% even following 5000 constructive charge–discharge cycles. The low cost and excellent electrochemical performances suggest that the Ni‐doped FeS 2 /WS 2 nanocomposite employing with WiSE electrolyte, that is, 17 m NaClO 4 may be the best combined effective pair to improve the electrochemical performances of supercapacitors as well as other advanced energy storage and portable energy devices.