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, and electrochemical syntheses) were investigated. Direct syntheses resulted in the formation of a NASICON-type phase in the presence of NaFePO4 and Na3PO4 impurities. The successful preparation of pure Na4FeV(PO4)3 has been achieved by the electrochemical sodiation of Na3FeV(PO4)3. Both synchrotron X-ray absorption and Mössbauer spectroscopy allowed probing the local V and Fe environments and their oxidation states in Na3FeV(PO4)3 and Na4FeV(PO4)3. Na3FeV(PO4)3 crystallizes in the space group C2/c (a = 15.1394(2) Å; b = 8.72550(12) Å; c = 21.6142(3) Å; β = 90.1744(9)°; and Z = 12), and it is isostructural to an ordered α-form of Na3M2(PO4)3 (M = Fe, V). It presents a superstructure due to Na+ ordering, as confirmed by differential scanning calorimetry and in situ temperature X-ray diffraction. The electrochemically sodiated Na4FeV(PO4)3 powder crystallizes in the space group R3̅c (a = 8.94656(8) Å, c = 21.3054(3) Å, and Z = 6) within which the two sodium sites, Na(1) and Na(2), are almost fully occupied. Na4FeV(PO4)3 allows the electrochemical extraction of 2.76 Na+ per formula unit within the voltage range of 1.3–4.3 V versus Na+/Na through the FeIII/II, VIV/III, and VV/IV redox couples. This identifies an interesting material for Na-ion batteries.