Crystal Structures and Local Environments of NASICON-Type Na<sub>3</sub>FeV(PO<sub>4</sub>)<sub>3</sub> and Na<sub>4</sub>FeV(PO<sub>4</sub>)<sub>3</sub> Positive Electrode Materials for Na-Ion Batteries
Sunkyu Park, Jean‐Noël Chotard, Dany Carlier, Iona Moog, Matthieu Courty, Mathieu Duttine, François Fauth, Antonella Iadecola, Laurence Croguennec, Christian Masquelier
Abstract
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.