Insight into the Electrochemical Sodium Insertion of Vanadium Superstoichiometric NASICON Phosphate

A slight deviation of the stoichiometry has been introduced in Na3-3x,V2+x(PO4)(3) (0 <= x <= 0.1) samples to determine the effect on the structural and electrochemical behavior as a positive electrode in sodium-ion batteries. X-ray diffraction and XPS results provide evidence for the flexibility of the NASICON framework to allow a limited vanadium superstoichiometry. In particular, the Na2.94V2.02(PO4)(3) formula reveals the best electrochemical performance at the highest rate (40C) and capacity retention upon long cycling. It is attributed to the excellent kinetic response and interphase chemical stability upon cycling. The electrochemical performance of this vanadium superstoichiometric sample in a full sodium-ion cell is also described.