Nanostructured Sn4+ and Mn2+ Co-Doped Na3V2(PO4)3 Cathode for High-Rate and Ultra-Stable Sodium–Ion Batteries

Abstract

Na3V2(PO4)3 represents a competitive cathode candidate for advanced sodium–ion batteries owing to its high operating voltage, rapid ion diffusion, and stable crystal framework. Nevertheless, the low electronic conductivity remains a critical obstacle for its practical large-scale applications. In this work, we rationally designed a carbon-coated, Sn4+- and Mn2+-codoped Na3V2(PO4)3 nanocomposite with nanoscale primary grains aiming to synergistically enhance the electronic conduction and Na+ transport of Na3V2(PO4)3. On the one hand, the nanoscale architecture, combined with a conformal carbon coating, reduces Na+ diffusion distances and facilitates electron transport, which are key to achieving superior high rate performance and durable cyclability. On the other hand, density functional theory calculations reveal that Sn4+ and Mn2+ codoping generates hybrid orbitals near the Fermi level, which narrows the band gap of Na3V2(PO4)3 and thereby greatly boosts its intrinsic electronic conductivity. Meanwhile, Sn4+ and Mn2+ codoping induces abundant sodium vacancies, which effectively lowers the Na+ diffusion energy barrier and accelerates Na+ transport kinetics. Additionally, the local charge density maps and integrated crystal orbital Hamilton population analyses jointly corroborate the improved structural stability upon Sn4+ and Mn2+ codoping. Owing to the synergistic effects of the nanoscale architecture and electron/vacancy modulation, the optimized Na3V1.9Sn0.05Mn0.05(PO4)3@C delivers impressive reversible capacities of 110.95 and 87.72 mAh g–1 at 1 and 20 C, respectively. Additionally, a high-capacity retention of 90.82% is also achieved at 10 C after 2300 cycles. In situ X-ray diffraction characterization elucidates the reversible two-phase transition mechanism between Na3V1.9Sn0.05Mn0.05(PO4)3 and NaV1.9Sn0.05Mn0.05(PO4)3. This work demonstrates a synergetic nanostructural and heterovalent codoping strategy, providing valuable insights for designing high-performance NASICON-type cathodes.