Synergistic Effect of Ternary Substitution in Na₃V₂(PO₄)₃ for High-Rate and Long-Life Anode-Free SIBs

Doping with metal ions can significantly enhance the electrochemical performance of Na₃V₂(PO₄)₃ (NVP) as a cathode material for sodium-ion batteries (SIBs). Despite its high reversible capacity and high voltage, practical application of NVP is limited by its poor intrinsic conductivity. Herein, we shed light on a facile sol-gel synthesis method to prepare NVP with ternary doping of potassium K⁺, Al³⁺, and SO₄^2- ions, which accelerates the migration of Na⁺ in the crystal structure, as confirmed by theoretical calculations. The new cathode materials exhibit a notable reversible capacity of 115.3 mA·h g^-1 at 0.5C and retain a substantial initial capacity of 79.1 mA·h g^-1 even at 50C. Additionally, we synthesized NVP with the doping of SO₄^2- ion on the PO₄^3- site, which shows 90.6 and 85 mA·h g^-1 at 0.5 and 10C, respectively. These results perfectly demonstrate the advantages of the superiority of ternary substitution on the NVP cathode. Moreover, sodium anode-free full cells of the cathode paired with Cu/SP foil showed impressive long-term cycling stability. This achievement lays a strong foundation for the use of NASICON cathodes with high charge-discharge rates and high energy density, making them a promising alternative to the next-generation lithium-ion batteries.