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 SiO44- substitute for PO43- in Na3V2(PO4)(3) (NVP) lattice, preparing the optimal Na3.1V2(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 (approximate to 98% capacity retention after 500 cycles at 1 C). Theoretical calculation and experimental analyses reveal that the anion-substituted Na3.1V2(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.