Effect of Zr/Ti Cosubstitution of Partial V on the Structure and Electrochemical Performance of Na₃V₂(PO₄)₂F₂O for Sodium Batteries

Na₃V₂(PO₄)₂F₂O (NVPFO) is a practically promising cathode material for sodium-ion batteries (SIBs) due to its high crystallographic stability and high theoretical capacity but is severely hampered by low electrical conductivity. According to our previous findings on enhancing electrical conductivity by individual Zr substitution or Ti substitution, Zr and Ti cosubstitution on V sites of NVPFO is designed and implemented in this work to further stabilize the crystalline structure by Zr and expedite the Na⁺ diffusivity together with Ti. The substitution amount of Ti is optimized by investigating the structure and performance of the Zr/Ti cosubstituted NVPFO, and the optimal composition is verified as Na₃V_1.93Zr_0.04Ti_0.03(PO₄)₂F₂O, which exhibits significantly improved electrochemical performance. It delivers specific discharge capacities of 114 and 73 mA h g^–1 at 1 and 20 C, respectively. The capacity retention reaches 78% after 400 cycles at 1 C. The performance enhancement is attributed to the slightly decreased particle size, better-organized surface carbon coating, smaller polarization between charging/discharging, and the faster Na⁺ diffusion coefficient during sodium insertion/extraction when an appropriate amount of Zr⁴⁺/Ti^3+/4+ cosubstitution is used. This work demonstrates an effective strategy to regulate the structure and performance of battery materials via multielement cosubstitution, highlighting the potential of NVPFO as a cathode material for SIBs.