Vacancy Chemistry Regulated Cobalt Oxide Nanostructures with Fast Kinetics for High-Performance Lithium-Ion Capacitors

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.