High-Energy Lithium-Ion Capacitors Using the NiS₂@MXene Electrode: Evidence of Dual Faradaic Reactions Driven by the Choice of Electrolytes

Transition metal sulfides (TMSs) are emerging as promising supercapacitor electrodes due to their excellent pseudocapacitive properties and substantial theoretical charge density. The combination of TMS and layered MXene synergistically enhances the energy and power, making them ideal for advanced energy storage applications. Here, NiS2@MXene composites were successfully synthesized via a simple probe sonication, and their formation, morphology, and composition were scrutinized using materials characterization techniques. Electrochemical analyses of the NiS2@MXene electrode in the KOH electrolyte revealed the occurrence of a Faradaic reaction involving (Ni3+/Ni2+) redox. Consequently, lithium-ion capacitors (LICs) using the NiS2@MXene and reduced graphene oxide (rGO) as respective positive and negative electrodes in aqueous and different nonaqueous electrolytes were fabricated, and their performances were analyzed in the form of coin cells. It turned out that the aqueous LIC (NiS2@MXene|1 M LiClO4(aq.)|rGO) delivered a specific energy of 48.4 W h kg–1 at a specific power of 800 W kg–1. Notably, the nonaqueous LIC (NiS2@MXene|1 M LiClO4(EC/DMC)|rGO) achieved an increased specific energy of 64 W h kg–1 at a specific power of 1500 W kg–1 and exhibited excellent capacity retention of 80% over 20,000 long cycles. Excitingly, the nonaqueous LIC (NiS2@MXene|1 M LiPF6(EC/DEC)|rGO) delivered a remarkable specific energy of 117.3 W h kg–1 at a specific power of 2100 W kg–1 and successfully powered a commercial LED bulb and a stopwatch, showcasing the practicality of the electrode. The electrochemical measurements and the ex situ X-ray diffraction analyses confirmed the simultaneous occurrence of the conversion/reconversion and the intercalation/deintercalation reactions, facilitated on the electrode that led to such a high Li+ charge storage performance. The conductive MXene matrix contributed to enhancing the charge transport by reducing the diffusion path and led to more active sites by providing a large contact area with the electrolytes. A pouch cell (NiS2@MXene|1 M LiPF6(EC/DEC)|rGO) was fabricated that demonstrated passable specific energy, underscoring the potential of the electrode for practical Li+ charge storage.