Nickel cobalt oxide nanowires-modified hollow carbon tubular bundles for high-performance sodium-ion hybrid capacitors

Sodium-ion hybrid capacitors are a prospective energy storage device candidate that couples the superiorities of battery-type and capacitive storage mechanisms. In this study, we fabricate a composite of NiCo₂O₄ nanowires with carbon tubular bundles (CTBs) via a facile hydrothermal and annealing procedure. The density functional theory (DFT) calculations are performed to evaluate the bonding strength between the two components of the composite, the binding energy of the NiCo₂O₄ is calculated to be −0.952 J m⁻², indicating that the NiCo₂O₄ nanowires can be stabilized on both sides of the carbon tubular bundles, which leads to a good cycling performance. Moreover, the composite in this work exhibits a metallic property because of the introduction of carbon material. As expected, when used for sodium storage, the NiCo₂O₄/CTBs shows a high capacity of 298 mA h g⁻¹ at 1 A g⁻¹ and high capacity retention of 92% after 500 cycles, which are superior than the bare NiCo₂O₄ electrode. Consequently, the sodium-ion hybrid capacitor is also assembled with NiCo₂O₄/carbon tubular bundles and commercial activated carbon, which achieves high energy density of 99 Wh kg⁻¹ in a wide potential range from 0.5 V to 4.0 V.