Induced Rate Performance Enhancement in Off-Stoichiometric Na3+3 xV2− x(PO4)3 with Potential Applicability as the Cathode for Sodium-Ion Batteries.

Off-stoichiometric Na3+3 xV2− x(PO4)3 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 Na4P2O7 impurities. The synergic combination of the structural deviations and the presence of Na4P2O7 impurities induce a significant improvement of the electrochemical performance and cycling stability at high rates, as compared to the stoichiometric Na3V2(PO4)3 sample. The fast kinetic response provided by the induced off-stoichiometry involves a decrease of the cell resistance. [ABSTRACT FROM AUTHOR]