Oxygen Vacancy Engineering in Na₃V₂(PO₄)₃ for Boosting Sodium Storage Kinetics

Improving Na-ion diffusion kinetics is an effective strategy to boost the sodium storage performance of electrode materials for sodium ion batteries (SIBs). Herein, an oxygen vacancy engineering is reported to evidently enhance Na-ion diffusion kinetics of Na₃V₂(PO₄)₃ and accordingly boost sodium storage performance. Na₃V₂(PO₄)₃/C with different molar contents of Cu doping (0%, 2.5%, 4%, 5%, and 6%) are synthesized using a simple sol–gel method followed by an annealing treatment. The experimental results show that Cu²⁺ successfully replaces the V³⁺ sites of Na₃V₂(PO₄)₃ and that does not change its phase composition. The introduction of Cu²⁺ not only results in the formation of V⁴⁺ to maintain charge balance, leading to a shorter V-O bond, but also promotes the generation of oxygen vacancies and accordingly facilitates Na-ion diffusion kinetics. As expected, the optimal sample displays a stable capacity of 111.4 mA h g⁻¹ with capacity retention of 90.4% over 300 cycles at 1 C and a high rate capacity of 83.8 mA h g⁻¹ at 20 C. The studies demonstrate that the Cu doping is favorable for the electrochemical enhancement of Na₃V₂(PO₄)₃, which provides a promising prospect for Na₃V₂(PO₄)₃ as a cathode for SIBs.