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).