Crystal Structures and Local Environments of NASICON-Type Na₃FeV(PO₄)₃ and Na₄FeV(PO₄)₃ Positive Electrode Materials for Na-Ion Batteries

In this work, we investigate the crystal chemistry of Fe/V-mixed NASICON [sodium (Na) Super Ionic CONductor] compositions Na3FeV(PO4)3 and Na4FeV(PO4)3 that are structurally related to Na3V2(PO4)3, a positive electrode for Na-ion batteries. To synthesize Na4FeV(PO4)3, various synthesis routes (solid-state, sol–gel-assisted, electrochemical syntheses) were investigated. Direct syntheses resulted in formation NASICON-type phase presence NaFePO4 Na3PO4 impurities. The successful preparation pure has been achieved by sodiation Na3FeV(PO4)3. Both synchrotron X-ray absorption Mössbauer spectroscopy allowed probing local V Fe environments their oxidation states Na4FeV(PO4)3. crystallizes space group C2/c (a = 15.1394(2) Å; b 8.72550(12) c 21.6142(3) ? 90.1744(9)°; Z 12), it is isostructural an ordered ?-form Na3M2(PO4)3 (M Fe, V). It presents superstructure due Na+ ordering, as confirmed differential scanning calorimetry situ temperature diffraction. electrochemically sodiated powder R3?c 8.94656(8) Å, 21.3054(3) 6) within which two sodium sites, Na(1) Na(2), almost fully occupied. allows extraction 2.76 per formula unit voltage range 1.3–4.3 versus Na+/Na through FeIII/II, VIV/III, VV/IV redox couples. This identifies interesting material