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

Abstract

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.