Oxygen vacancies confined in porous Co_(3)V_(2)O_(8)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 Co_(3)V_(2)O_(8)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 Co_(3)V_(2)O_(8)electrode and result in excellent electrochemical performance in 1 M NaOH and Na_(2)SO_(4)electrolytes.The cathode has a large specific capacity(@NaOH),high-rate capability(@NaOH),wide voltage window(@Na_(2)SO_(4)),and favorable long-cycle stability.Ex-situ X-ray diffraction and X-ray photoelectron spectroscopy show that the Co_(3)V_(2)O_(8)electrode displays a battery-like behavior related to OH−ions in the alkaline NaOH electrolyte.By contrast,in the neutral Na_(2)SO_(4)electrolyte,Co_(3)V_(2)O_(8)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 Co_(3)V_(2)O_(8)in the neutral Na_(2)SO_(4)electrolyte.Moreover,when paired with a high-voltage activated carbon(AC)anode,full-cell Co_(3)V_(2)O_(8)//Na_(2)SO_(4)//AC delivers high energy/power densities(89.6 Wh·kg^(−1)/330 W·kg^(−1)).