N-Doped VO _<i>x</i> @g-C ₃ N ₄ Based on In Situ Growth for High-Performance Aqueous Zinc Ion Battery Cathode Materials

Vanadium oxides are regarded as one of the most promising cathode materials for aqueous zinc-ion batteries (AZIB). Among them, vanadium dioxide (VO₂) has garnered significant attention due to its unique tunnel structure and superior electrochemical performance. However, VO₂ also exhibits certain limitations, such as insufficient stability and poor tolerance to high currents. To enhance the rate capability of VO₂, a mixed-valence form of VO₂ with improved rate performance was synthesized by adjusting the amount of the reducing agent (i.e., H₂C₂O₄·2H₂O). Subsequently, a VO₂/g-C₃N₄ composite material was fabricated using an in situ growth method. Research findings indicate that the rate capability and stability of VO₂ are markedly enhanced upon combination with g-C₃N₄. This improvement is attributed to the protective effect of g-C₃N₄ on VO₂. Moreover, the g-C₃N₄ synthesized via the solid-state method features a porous structure, which facilitates the normal intercalation and deintercalation of Zn²⁺ ions, providing numerous pathways for Zn²⁺ movement. Additionally, EDS-mapping images reveal that nitrogen (N) elements from g-C₃N₄ are successfully doped into VO₂ nanorods. The incorporation of N elements significantly boosts the conductivity and ionic diffusion of the composite. The resulting material achieves an exceptionally high specific capacity of 500.18 mAh/g at the current density of 0.2 A/g, and it maintains a specific capacity of 282.54 mAh/g even at a high current density of 10 A/g. After 10,000 charge-discharge cycles, the capacity retention rate remains as high as 87.33% at 10 A/g, demonstrating a substantial improvement in both rate capability and cycling stability. This study provides compelling evidence for the application of mixed-valence vanadium oxides as cathode materials for AZIB and offers valuable insights into the development of vanadium oxide composite materials.