Induced Rate Performance Enhancement in Off-Stoichiometric Na V (PO ) with Potential Applicability as the Cathode for Sodium-Ion Batteries.

Off-stoichiometric Na V (PO ) samples have been prepared by a sol-gel route. X-ray diffraction and XPS revealed the flexibility of the NASICON framework to accommodate these deviations of the stoichiometry; at least for low x values. X-ray photoelectron spectra evidenced the presence of Na P O impurities. The synergic combination of the structural deviations and the presence of Na P O impurities induce a significant improvement of the electrochemical performance and cycling stability at high rates, as compared to the stoichiometric Na V (PO ) sample. The fast kinetic response provided by the induced off-stoichiometry involves a decrease of the cell resistance.