N-Doped VOx@g-C3N4 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.