Synergistic Effect of Ternary Substitution in Na ₃ V ₂ (PO ₄ ) ₃ for High-Rate and Long-Life Anode-Free SIBs

Doping with metal ions can significantly enhance the electrochemical performance of Na 3 V 2 (PO 4 ) 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 +, Al 3+, and SO 4 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 SO 4 2– ion on the PO 4 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.