Modulation of the Crystal Structure and Ultralong Life Span of a Na₃V₂(PO₄)₃-Based Cathode for a High-Performance Sodium-Ion Battery by Niobium-Vanadium Substitution

Building better batteries with low cost, long life, and safety can effectively meet the diverse energy demands. Na3V2(PO4)3 (NVP) is a potential cathode in energy storage systems due to its stable crystal structure, high-voltage platform, and rapid migration rate of Na+. Nevertheless, its poor conductivity results in inferior electrochemical properties. Herein, the high-valence niobium (Nb5+) as a dopant can regulate the crystal structure of NVP and act as an activator to catalyze the formation of the graphitization carbon layer, which shortens the electron/ion diffusion pathway and enhances the electrochemical kinetics. Density functional calculations show that Nb5+ doping decreases the band gap energy and promotes electron transport. Physical and chemical characterizations prove that Nb5+ doping induces the lattice distortion of NVP. Cyclic voltammetry and electrochemical impedance tests show that Nb5+ doping promotes Na+ diffusion. Finally, the optimal NVP/Nb-0.3 delivers an excellent performance of 103.8 mAh g-1 with a capacity retention of 92.3% at 1 C for 200 cycles, a rate performance of 99.6 mAh g-1 at 20 C, and cycling stability at 50 C for 6000 cycles with a capacity retention of 72.7%. This modification strategy of cathode materials provides an important reference for optimizing battery performance.