As a promising transition metal oxide, Co₃O₄ is considered a low-cost anode material in lithium-ion capacitors (LICs) due to its sizeable theoretical capacity and excellent electrochemical reversibility. However, its inherently inferior electrical conductivity and huge volume expansion upon long-term operation often cause reduced energy storage and slow rate capability during lithiation/delithiation in LICs. To overcome these limitations, we present a simple annealing approach that leverages both vacancy chemistry and surface engineering to refine the physiochemical structure of Co₃O₄ nanoboxes with adjustable thickness of CoO layers on the surface, enabling precise microstructural control over lithium storage performance. Our analysis shows that electrochemical lithiation of Co₃O₄ leads to an increased generation of oxygen vacancies at octahedral Co²⁺ sites, facilitated by Co²⁺–ligand interactions. Theoretical calculations confirm that these vacancies induce a new electronic density of state in the bandgap and create localized charge imbalances, considerably enhancing electrical conductivity and accelerating faradaic reactions. With this vacancy-engineered structure, Co₃O₄ nanoboxes demonstrate an impressive reversible specific capacity of 917 mAh/g after 100 cycles. Furthermore, LICs based on the Co₃O₄ anode achieve an exceptional power density up to 33.6 kW/kg together with an energy density of 124.1 Wh/kg. This study provides a robust strategy for vacancy engineering in electrode materials to boost the electrochemical energy storage performances.

Carbon nanofiber (CNF) was widely utilized in the field of electrochemical energy storage due to its superiority of conductivity and mechanics. However, CNF was generally prepared at relatively high temperature. Herein, nitrogen-doped hard carbon nanofibers (NHCNFs) were prepared by a low-temperature carbonization treatment assisted with electrospinning technology. Density functional theory analysis elucidates the incorporation of nitrogen heteroatoms with various chemical states into carbon matrix would significantly alter the total electronic configurations, leading to the robust adsorption and efficient diffusion of Na atoms on electrode interface. The obtained material carbonized at 600 °C (NHCNF-600) presented a reversible specific capacity of 191.0 mAh g⁻¹ and no capacity decay after 200 cycles at 1 A g⁻¹. It was found that the sodium-intercalated degree had a correlation with the electrochemical impedance. A sodium-intercalated potential of 0.2 V was adopted to lower the electrochemical impedance. The constructed sodium-ion capacitor with activated carbon cathode and presodiated NHCNF-600 anode can present an energy power density of 82.1 Wh kg⁻¹ and a power density of 7.0 kW kg⁻¹.