Isostructural and Multivalent Anion Substitution toward Improved Phosphate Cathode Materials for Sodium-Ion Batteries.

Polyanion-type phosphate materials are highly promising cathode candidates for next-generation batteries due to their excellent structural stability during cycling; however, their poor conductivity has impeded their development. Isostructural and multivalent anion substitution combined with carbon coating is proposed to greatly improve the electrochemical properties of phosphate cathode in sodium-ion batteries (SIBs). Specifically, multivalent tetrahedral SiO4 4- substitute for PO4 3- in Na3 V2 (PO4 )3 (NVP) lattice, preparing the optimal Na3.1 V2 (PO4 )2.9 (SiO4 )0.1 with high-rate capability (delivering a high capacity of 82.5 mAh g-1 even at 20 C) and outstanding cyclic stability (≈98% capacity retention after 500 cycles at 1 C). Theoretical calculation and experimental analyses reveal that the anion-substituted Na3.1 V2 (PO4 )2.9 (SiO4 )0.1 reduces the bandgap of NVP lattice and enhanced its structural stability, Na+ -diffusion kinetics and electronic conductivity. This strategy of multivalent and isostructural anion substitution chemistry provides a new insight to develop advanced phosphate cathodes.