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Na3V2-xFex(PO4)3/C (0 ≤ x ≤ 0.5) samples were prepared by a simple sol-gel method. X-ray diffraction patterns revealed high purity and crystalline phosphate indexed in the R-3c space group. On increasing the iron content, the cell volume increases due to the slightly higher radius of Fe3+. Cyclic voltammetry showed two signals at ca. 3.5 and 4.0 V, which evidence a linear dependence of potential on the level of substitution. The appearance of the short plateau at ca. 4.0 V is closely related to the higher iron substitution levels. The evaluation of XPS and 57Fe Mössbauer spectra of charged and discharged electrodes allow ascribing the 4.0 V plateau to V4+/V5+ redox reaction and the extraction/insertion of extra sodium ions from octahedral M1 sites. Galvanostatic cycling showed capacity values as high as 115 mA h g−1 for samples with x = 0.1, 0.2, 0.3 and exceptional cyclability.
Aragón et al. (2015) studied this question.