Transition metal sulfides grown on graphene fibers for wearable asymmetric supercapacitors with high volumetric capacitance and high energy density

Fiber shaped supercapacitors are promising candidates for wearable electronics because they are flexible and light-weight. However, a critical challenge of the widespread application of these energy storage devices is their low cell voltages and low energy densities, resulting in limited run-time of the electronics. Here, we demonstrate a 1.5 V high cell voltage and high volumetric energy density asymmetric fiber supercapacitor in aqueous electrolyte. The lightweight (0.24 g cm^-3), highly conductive (39 S cm^-1), and mechanically robust (221 MPa) graphene fibers were firstly fabricated and then coated by NiCo₂S₄ nanoparticles (GF/NiCo₂S₄) via the solvothermal deposition method. The GF/NiCo₂S₄ display high volumetric capacitance up to 388 F cm^-3 at 2 mV s^-1 in a three-electrode cell and 300 F cm^-3 at 175.7 mA cm^-3 (568 mF cm^-2 at 0.5 mA cm^-2) in a two-electrode cell. The electrochemical characterizations show 1000% higher capacitance of the GF/NiCo₂S₄ as compared to that of neat graphene fibers. The fabricated device achieves high energy density up to 12.3 mWh cm^-3 with a maximum power density of 1600 mW cm^-3, outperforming the thin-film lithium battery. Therefore, these supercapacitors are promising for the next generation flexible and wearable electronic devices.