Stability and kinetics enhancement of hydrated vanadium oxide via sodium-ion pre-intercalation

The bilayered vanadium pentoxide (δ-V2O5·nH2O, VOH) is the most promising cathode material for aqueous zinc ion batteries due to its high theoretical capacity, low-cost, and easy preparation. However, the capacity decay arising from the structure instability remains a significant challenge for the application of VOH. The present study focuses on stabilizing the VOH structure via pre-intercalation of Na+ cations into the bilayers, which occurs by replacing the partial interlayer water. Though has a smaller interlayer spacing, the Na+ intercalated vanadium oxide (δ-Na0.27V2O5·0.7H2O, NaVOH) delivers a large specific capacity of 420 mA h/g (at 0.05 A/g), with improved energy efficiency, reversibility, and cycling stability (88% capacity retention after 2000 cycles at 4 A/g), in comparison to those of V2O5·1.0H2O. The results reveal that Na+ions are favorably intercalated within the VOH layers by the spontaneous ion-exchange with structural water. The introduction of sodium cations, an increased amount of V4+, as well as a reduced interlayer water content in NaVOH collectively result in the fast transport and reaction kinetics as well as the improved structural and electrochemical stability.