Mn^x+ Substitution to Improve Na₃V₂(PO₄)₂F₃-Based Electrodes for Sodium-Ion Battery Cathode

Na₃V₂(PO₄)₂F₃ (NVPF) is an extremely promising sodium storage cathode material for sodium-ion batteries because of its stable structure, wide electrochemical window, and excellent electrochemical properties. Nevertheless, the low ionic and electronic conductivity resulting from the insulated PO₄^3- structure limits its further development. In this work, the different valence states of Mn^x+ ions (x = 2, 3, 4) doped NVPF were synthesized by the hydrothermal method. A series of tests and characterizations reveals that the doping of Mn ions (Mn²⁺, Mn³⁺, Mn⁴⁺) changes the crystal structure and also affects the residual carbon content, which further influences the electrochemical properties of NVPF-based materials. The sodiation/desodiation mechanism was also investigated. Among them, the as-prepared NVPF doped with Mn²⁺ delivers a high reversible discharge capacity (116.2 mAh g^-1 at 0.2 C), and the capacity retention of 67.7% after 400 cycles at 1 C was obtained. Such excellent performance and facile modified methods will provide new design ideas for the development of secondary batteries.