Tm ³ ⁺ -Doped Na ₃ V ₂ (PO ₄ ) ₃ Cathode: Accelerating Charge Transfer and Enhancing Lattice Stability for High-Performance Sodium-Ion Batteries

Sodium vanadium phosphate (Na₃V₂(PO₄)₃) has emerged as a promising cathode candidate for advanced sodium-ion batteries due to its advantageous characteristics, including elevated operating voltage, rapid ionic transport, and exceptional structural stability. However, its practical application is hindered by its low electronic conductivity. To address this limitation, we prepared a Na₃V_1.95Tm_0.05(PO₄)₃@C sample (Tm0.05-NVP@C) using a Thulium³⁺ (Tm³⁺) doping strategy to improve both high-rate performance and long-term cyclability. Electrochemical kinetic analyses combined with theoretical calculations confirm that the incorporation of Tm³⁺ into Tm0.05-NVP@C effectively narrows the electronic bandgap and reduces Na⁺ migration energy barriers, thereby accelerating charge transfer processes. Furthermore, the more negative integrated crystal orbital Hamilton population values of V-O and Tm-O bonds manifest improved lattice stability. Accordingly, the prepared Tm0.05-NVP@C exhibits significant electrochemical performances: it achieves a high reversible capacity of 88.65 mAh g^-1 at 40 C while maintaining exceptional capacity retention of 79.64% after 2500 cycles at 10 C. In situ X-ray diffraction analysis further elucidates the reversible biphasic transformation between Na₃V_1.95Tm_0.05(PO₄)₃ and NaV_1.95Tm_0.05(PO₄)₃. This comprehensive investigation not only demonstrates the efficacy of Tm³⁺ doping in optimizing NASICON-typed cathodes but also provides valuable insights for developing next-generation Na₃V₂(PO₄)₃-based cathodes with improved rate capability and cycle life.