Transition metal sulfides such as MoS2 and VS2 have emerged as promising candidates for energy storage applications in recent years. Among the reported transition metal sulfides, the patronite (VS4) form of vanadium sulfide is less explored and understanding its charge storage mechanisms is unusual in the literature. Here, we report the pseudocapacitive energy storage performance of patronite (VS4) nano-bundles synthesized via template-free hydrothermal approach by fabricating a solid-state asymmetric supercapacitor device (SSAD) with MXene (Ti3C2Tx) as the negative electrode. The fabricated VS4//MXene SSAD displayed a remarkable supercapacitive behavior in an expanded working potential window of 1.3 V. The device demonstrated a record high performance with the areal capacitance of 70.9 mF/cm2 at the scan rate of 5 mV/s and demonstrated ~ 4.5 and ~ 6 times superior areal capacitance as compared to the bare VS4 nano-bundles and MXene-based symmetric devices. Further, the device displayed an excellent energy density of 5.65 mW h/cm2 with a remarkable power density of 290.9 mW/cm2 and good cyclic stability ~ 75% after 3000 cycles in neutral electrolyte. Moreover, we have performed extensive density functional theory simulations to present electronic properties of VS4 and MXene supporting pseudocapacitive behavior for VS4 through reversible Faradic redox reactions and pre-dominant electrical double-layer capacitive behavior for MXene due to intercalation/de-intercalation of ions. The diffusion energy barrier for electrolytic ions is lower in VS4 (only 0.17 eV) satisfying better and rapid transport of charge carriers generated in VS4 justifying high charge storage performance. Populated electronic states for V 3d orbital in both the valence band and conduction band justify superior redox-type behavior for VS4 supporting experimental observations.
Graphical abstractEnhanced energy storage performance of asymmetric supercapacitors based on VS4 and MXene.
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