A Combined Operando Synchrotron X‑ray Absorption
Spectroscopy and First-Principles Density Functional Theory Study
to Unravel the Vanadium Redox
Paradox in the Na3V2(PO4)2F3–Na3V2(PO4)2FO2 Compositions
posted on 2020-10-20, 19:13authored byLong H.
B. Nguyen, Antonella Iadecola, Stéphanie Belin, Jacob Olchowka, Christian Masquelier, Dany Carlier, Laurence Croguennec
The
redox processes involved in the Na+ deintercalation
from Na3V2(PO4)2F3, Na3V2(PO4)2F2O, and Na3V2(PO4)2F1.5O1.5 are investigated operando by synchrotron X-ray absorption spectroscopy at the vanadium K-edge.
A continuous evolution in the pre-edge and edge regions is observed
for the three compositions, which is in good agreement with a progressive
oxidation of vanadium. In the oxygen-substituted compositions, Na3V2(PO4)2F2O and
Na3V2(PO4)2F1.5O1.5, the F–V3+O4–F
and F–V4+O4O redox centers coexist
and can be simultaneously activated owing to the reverse inductive
effect promoted by the presence of highly covalent vanadyl bonds on
V4+ sites. Principal component analysis of the dataset
collected operando for the three compositions is
performed. It reveals that three independent spectra, corresponding
to different charge orderings on vanadium sites at different states
of charge, are required to describe these three systems. Furthermore,
the extended X-ray absorption fine structure analysis of the principal
components is performed in order to gain insightful information on
the evolution of the vanadium local environment upon cycling. By the
use of first-principles density functional theory calculations, we
demonstrate that the potential step observed in all Na2V2(PO4)2F3–yOy compositions is related to a
charge ordering on vanadium sites with a preferential oxidation of
vanadium ions in the same bioctahedra.