Crystal Structure of Na2V2(PO4)3, an Intriguing Phase Spotted in the Na3V2(PO4)3–Na1V2(PO4)3 System
Park, Sunkyu Wang, Ziliang Deng, Zeyu Moog, Iona Canepa, Pieremanuele Fauth, François Carlier, Dany Croguennec, Laurence Masquelier, Christian Chotard, Jean--Noël
Park, Sunkyu
Wang, Ziliang
Moog, Iona
Fauth, François
Carlier, Dany
Croguennec, Laurence
Masquelier, Christian
Chotard, Jean--Noël
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Alternative Title
Abstract
The Na-superionic-conductor (NASICON) Na3V2(PO4)3 is an important positive electrode
material for Na-ion batteries. Here, we investigate the mechanisms of phase transition in
NaxV2(PO4)3 (1 ≤ x ≤ 4) upon a non-equilibrium battery cycling. Unlike the widely believed
two-phase reaction in Na3V2(PO4)3 – Na1V2(PO4)3 system, we determine a new intermediate
Na2V2(PO4)3 phase using operando synchrotron X-ray diffraction. Density functional theory
calculations further support the existence of the Na2V2(PO4)3 phase. We propose for the first
time two possible crystal structures of Na2V2(PO4)3 analyzed by Rietveld refinement. The
two structure models with the space groups P21/c or P2/c for the new intermediate
Na2V2(PO4)3 phase show similar unit cell parameters but different atomic arrangements,
including a vanadium charge ordering. As the appearance of the intermediate Na2V2(PO4)3
phase is accompanied by symmetry reduction, Na(1) and Na(2) sites split into several
positions in Na2V2(PO4)3, in which one of the splitting Na(2) position is found to be a vacancy
whereas the Na(1) positions are almost fully filled. The intermediate Na2V2(PO4)3 phase
reduces the lattice mismatch between Na3V2(PO4)3 and Na1V2(PO4)3 phases facilitating a fast
phase transition. This work paves the way for a better understanding of great rate
capabilities of Na3V2(PO4)3.
Keywords
crystal structure, Na2V2(PO4)3, phase transition, intermediate phase, metastable, non-equilibrium, operando X-ray diffraction, density functional theory calculations
Source Title
Chemistry of Materials
Publisher
American Chemical Society (ACS)
Series/Report No.
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Date
2021-12-29
DOI
10.1021/acs.chemmater.1c04033
Type
Article