On the benefits of Cr substitution on Na4MnV(PO4)3 to improve the high voltage performance as cathode for sodium-ion batteries

Several samples of the Na4-xMn1-xCrxV(PO4)3 (0 ≤x ≤ 1) series are prepared by a sol-gel method that favors the deposition of an in-situ carbon conductive phase to improve the electronic conductivity of these electrode materials. XRD diffraction, electron microscopy, and Raman and XPS spectroscopies are applied to determine their structural, chemical, and morphological properties. Apparent diffusion coefficients of sodium are calculated by different techniques such as cyclic voltammetry, galvanostatic intermittent titration, and impedance spectroscopy. The results reveal the benefit of the partial replacement of Mn by Cr to improve sodium diffusivity, which leads to a net gain in the reversible extraction of sodium at either low and high rates and low interface resistance values for samples with x = 0.6 and 0.8.