Hierarchically Interconnected Ni₃S₂ Nanofibers as Binder-Free Electrodes for High-Performance Sodium-Ion Energy-Storage Devices

Direct growth of hierarchically interconnected Ni3S2 nanofibers as binder-free electrodes for high-performance sodium (Na+)-ion batteries was demonstrated by a facile one-step hydrothermal method on a nickel (Ni) foam. The hierarchically interconnected Ni3S2 nanofibers can effectively relieve volume expansion of Ni3S2 and shorten diffusion paths of Na+ ions that can enhance the high electronic conductivity during the electrochemical reaction, yielding high specific capacitance, excellent cycling stability, and outstanding rate capability. As a result, a high discharge specific capacity of 584.2 mAh g–1 at a current density of 0.2 A g–1 in the first sodiation process with a capacity retention of 91.9% after 100 cycles (retains 536.9 mAh g–1) can be achieved. The asymmetric Na+-ion capacitor based on the hierarchically interconnected Ni3S2 nanofibers as binder-free anode materials and the activated carbon as a cathode material exhibits high energy and power density. Interestingly, the conversion reaction mechanism of Na+-ion storage in the hierarchically interconnected Ni3S2 nanofibers was also investigated by the ex situ X-ray diffraction studies, suggesting a promising material as the binder-free electrode for advanced Na+-ion energy storage