Comprehensive Investigation of the Na₃V₂(PO₄)₂F₃–NaV₂(PO₄)₂F₃ System by Operando High Resolution Synchrotron X-ray Diffraction

Abstract

Na 3 V 2 (PO 4 ) 2 F 3 is a positive electrode material for Na-ion batteries which is attracting strong interest due to its high capacity, rate capability and long-term cycling stability. The sodium extraction mechanism from this material has been always described in the literature as a straightforward solid solution, but several hints point towards a more complicated phase diagram. In this work we performed high angular resolution synchrotron radiation diffraction measurements, realized operando on sodium batteries upon charge. We reveal an extremely interesting phase diagram, created by the successive crystallization of four intermediate phases before the end composition NaV 2 (PO 4 ) 2 F 3 is reached. Only one of these phases undergoes a solid solution reaction, in the interval between 1.8 and 1.3 Na per formula unit. The ability to resolve weak Bragg reflections allowed us to reveal differences in terms of symmetry amongst the phases, to determine their previously unknown space groups and to correlate them with sodium (dis)ordering in the structure. Rietveld refinements enabled us to follow fine structural modifications in great detail. Intermediate identified phases are not simply described by their unit cell parameters but bond-lengths variations can be tracked, as well as polyhedral distortions and site occupancy factors for mobile sodium ions. For NaV 2 (PO 4 ) 2 F 3 a full crystal structure determination was also carried out for the first time directly from operando measurements, assigning it to the Cmc2 1 space group and revealing two vanadium environments: V 3+ and V 5+ . Our study demonstrates that improved angular resolution and high intensity diffraction data are key parameters for direct observation of fine reaction pathways in electrode materials, and that the obtained insight is crucial for the understanding of (de)intercalation mechanisms in Na-ion batteries.

Na1+yVPO4F1+y (0 ≤ y≤ 0.5) as Cathode Materials for Hybrid Na/Li Batteries

“…On the XRD patterns of the sample at the end of the charge, some reflections of the initial phase disappeared, suggesting the occurrence of the reversible P4 2 /mnm↔I4/mmm transition, which was verified by the Rietveld refinement. The same structural transition was observed earlier during cycling of Na 3 V 2 (PO 4 ) 2 F 3 in a Na cell [30]. The lattice parameters of the as-obtained products and the average sodium content per f.u.…”

“…The lattice parameters of the as-obtained products and the average sodium content per f.u. are shown in Table 5 in comparison with those presented in the literature [30]. It was seen that the refined lattice parameters were in a good agreement with those obtained in [30] for incompletely charged sodium vanadium fluorophosphate.…”

“…are shown in Table 5 in comparison with those presented in the literature [30]. It was seen that the refined lattice parameters were in a good agreement with those obtained in [30] for incompletely charged sodium vanadium fluorophosphate. According to the charge capacity and the XRD refinement, the final composition of the charged sample was~Na 1.1 V 2 (PO 4 ) 2 F 3 , i.e.…”

“…; it was not completely charged. Fully charged Na 1 V 2 (PO 4 ) 2 F 3 was obtained by Bianchini et al and indexed in the Cmc2 1 space group [30]. On the contrary, the XRD pattern of the sample, recorded at the end of the 12th discharge, showed the same set of reflections as the XRD pattern of the pristine sample.…”

Electrochemical and Chemical Na+/Li+Ion Exchange in Na-Based Cathode Materials: Na1.56Fe1.22P2O7and Na3V2(PO4)2F3

“…A new design concept of the simultaneously energy storage of multi-metal ions may overcome respective disadvantages of singlemetal ions and bring to a synergy effect. 6,7 Since the operation of hybrid-ion batteries depends on the reactivity of the selected electrodes in respect of Li + and Na + ions, two Na-based cathode materials, differing by crystal structure, triclinic Na 1.56 Fe 1.22 P 2 O 7 and tetragonal Na 3 V 2 (PO 4 ) 2 F 3 , were selected in this work to study electrochemical and chemical Na + /Li + ion exchange and their performance in hybridion cells.Sodium iron pyrophosphates 8,9 with a triclinic symmetry (S.G. P-1) and sodium vanadium fluorophosphates with tetragonal symmetry (S.G. P4 2 /mnm) [10][11][12] were recently examined and showed good electrochemical properties in Na-cells. To increase a rechargeable capacity of Na 2 FeP 2 O 7 , a series of isostructural compositions Na 2−y Fe 1+y/2 P 2 O 7 (where 0 < y < 0.44) similar to the previously reported Na 2−x M 1+x/2 P 2 O 7 (M = Mg, Ni) 13 were recently synthesized z E-mail: kosova@solid.nsc.ru and evaluated as positive electrodes for sodium batteries.…”