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

Abstract

In this work, we investigate the crystal chemistry of Fe/V-mixed NASICON [sodium (Na) Super Ionic CONductor] compositions Na 3 FeV(PO 4 ) 3 and Na 4 FeV(PO 4 ) 3 that are structurally related to Na 3 V 2 (PO 4 ) 3 , a positive electrode for Na-ion batteries. To synthesize Na 4 FeV(PO 4 ) 3 , various synthesis routes (solidstate, sol−gel-assisted, and electrochemical syntheses) were investigated. Direct syntheses resulted in the formation of a NASICON-type phase in the presence of NaFePO 4 and Na 3 PO 4 impurities. The successful preparation of pure Na 4 FeV(PO 4 ) 3 has been achieved by the electrochemical sodiation of Na 3 FeV(PO 4 ) 3 . Both synchrotron X-ray absorption and Mossbauer spectroscopy allowed probing the local V and Fe environments and their oxidation states in Na 3 FeV(PO 4 ) 3 and Na 4 FeV(PO 4 ) 3 . Na 3 FeV(PO 4 ) 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 αform of Na 3 M 2 (PO 4 ) 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 Na 4 FeV(PO 4 ) 3 powder crystallizes in the space group R3̅ c (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. Na 4 FeV(PO 4 ) 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 Fe III/II , V IV/III , and V V/IV redox couples. This identifies an interesting material for Na-ion batteries.

NaFe2PO4(MoO4)2: A Promising NASICON-Type Electrode Material for Sodium-Ion Batteries

“…The Fe(1)–O distances are in the range of 1.916(5) to 2.077(4) Å with the average value of 2.000 Å, and the Fe(2)–O distances vary from 1.935(4) to 2.046(4) Å with the average value of 1.994 Å (Figure c,d). The average distances of the two kinds of Fe–O are in agreement with that in α-, β-, and γ-Na 3 Fe 2 (PO 4 ) 3 (∼2.001 Å) and Na 4 FeV­(PO 4 ) 3 (1.986 Å) . Na is surrounded by seven O atoms to form a NaO 7 octahedron with Na–O distances varying from 2.370(6) to 3.018(6) Å (Figure e).…”

“…The bond distances of P(4)−O range from 1.537(5) to 1.565(4) Å with the average value of 1.545 Å, which is larger than the P−O mean bond distances in Na 3 V 2 (PO 4 ) 3 (1.527 Å) 32 and α-, β-, and γ-Na 3 Fe 2 (PO 4 ) 3 (1.528, 1.529, and 1.525 15 and Na 4 FeV(PO 4 ) 3 (1.986 Å). 34 Na is surrounded by seven O atoms to form a NaO 7 octahedron with Na−O distances varying from 2.370(6) to 3.018(6) Å (Figure 2e). The average Na−O bond distance is 2.637 Å, which is larger than those of NaFe 2 PO 4 (SO 4 ) 2 (2.491 Å) 21 and α-, β-, and γ-Na 3 Fe 2 (PO 4 ) 3 (∼2.576 Å), 15 indicating that the large volume of the MoO 4 group can indeed enlarge the interstitial space.…”

Reversible Activation of V4+/V5+ Redox Couples in NASICON Phosphate Cathodes

“…The slight Fe oxidation could be ascribed to exposure to air during transfer or measurements, which is similar to other Fe‐based phosphate cathodes reported. [ 13,32 ] Moreover, the element concentration of samples determined by inductively coupled plasma‐optical emission spectroscopy (ICP) also presents an acceptable consistency with their designed chemical composition (Table S8, Supporting Information), verifying high purities of these cathodes.…”

“…Several prominent peaks between 0–4 Å could be assigned to the first TM‐O (TM = V or Fe) coordination shell and TM‐P/Na outer shells. [ 32,43 ] At the first TMO coordination shell, the lowered EXAFS signal intensity and shorted interatomic distance could be observed along with the extraction of Na + from lattice crystal. While the EXAFS signals collected at the pristine and discharged (to 2.0 V) states are almost overlapping, indicating complete reversibility of the redox processes.…”

Peculiarities of Phase Formation in Mn-Based Na SuperIonic Conductor (NaSICon) Systems: The Case of Na1+2xMnxTi2–x(PO4)3 (0.0 ≤ x ≤ 1.5)

“…5), and P 1̅ (no. 2). , For this purpose, the lowest energy configuration from the DFT/CE sampling corresponding to x = 1.0 and identified as belonging to the monoclinic C 2 space group gave a very good initial guess and rapidly converged to the final solution, with the best overall R wp = 9.52%, a goodness of fit (GOF) of 1.65, and χ red 2 = 2.71. In comparison, for C 2/ c , the results were R wp = 11.8%, GOF = 2.04, and χ red 2 = 4.16 and for P 1̅ R wp = 11.297%, GOF = 1.96 and χ red 2 = 3.83.…”