Synergistic Effect of Ternary Substitution in NaV(PO) for High-Rate and Long-Life Anode-Free SIBs.

Doping with metal ions can significantly enhance the electrochemical performance of NaV(PO) (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, and SO 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 at 0.5C and retain a substantial initial capacity of 79.1 mA·h g even at 50C. Additionally, we synthesized NVP with the doping of SO ion on the PO site, which shows 90.6 and 85 mA·h g 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.