Oxygen vacancies confined in porous Co3V2O8 sheets for durable and high-energy aqueous sodium-ion capacitors

Sodium-ion capacitors have the potential to deliver high energy, power density, and excellent cycling stability. In this study, ultrathin Co3V2O8 nanosheets are successfully synthesized through an one-pot hydrothermal reaction and a subsequent doping reconfiguration-induced vacancy-forming process. Abundant oxygen vacancies and high porosity are observed in the Co3V2O8 electrode and result in excellent electrochemical performance in 1 M NaOH and Na2SO4 electrolytes. The cathode has a large specific capacity (@NaOH), high-rate capability (@NaOH), wide voltage window (@Na2SO4), and favorable long-cycle stability. Ex-situ X-ray diffraction and X-ray photoelectron spectroscopy show that the Co3V2O8 electrode displays a battery-like behavior related to OH− ions in the alkaline NaOH electrolyte. By contrast, in the neutral Na2SO4 electrolyte, Co3V2O8 mainly shows an intercalation/extraction behavior with Na+ ions. Density functional theory calculation suggests that oxygen vacancy leads to a new state located in the bandgap, which greatly improves the electron transfer efficiency and reduces the sodiation energy barrier of Co3V2O8 in the neutral Na2SO4 electrolyte. Moreover, when paired with a high-voltage activated carbon (AC) anode, full-cell Co3V2O8//Na2SO4//AC delivers high energy/power densities (89.6 Wh·kg−1/330 W·kg−1).