The Effect of oxygen defects on the structural evolution of LiVPO4F1−yoy cathode materials

Daniel Risskov Sørensen*, Andreas Østergaard Drejer*, Martin Aaskov Karlsen, Ulla Gro Nielsen, Michael Heere, Anatoliy Senyshyn, Dorthe Bomholdt Ravnsbæk

*Corresponding author for this work

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Lithium vanadium fluorophosphate, LiVPO4F, is a promising cathode material for Li-ion batteries because of its high intercalation potential (4.24 V vs Li/Li+) and high stability. However, recent studies show that as-synthesized LiVPO4F very often contains oxygen defects on the fluoride site giving rise to a general composition of LiVPO4F1yOy with vanadium in a mixed +III/+IV valence state. The inclusion of oxygen naturally influences the electrochemical properties greatly, and a thorough material characterization is necessary to understand the performance. In this study, we synthesize lithium vanadium fluorophosphate by two common strategies: solid-state and hydrothermal synthesis. We show that solid-state synthesis provides LiVPO4F, while the hydrothermal method, in contrast to previous reports, leads to the inclusion of ca. 35% oxygen on the fluoride site and significant disorder in the material. The different electrochemical properties were probed by operando synchrotron X-ray powder diffraction to investigate the effects of oxygen inclusion on the structural evolution during electrochemical lithiation and delithiation. This reveals that while LiVPO4F exhibits a typical biphasic phase evolution, the sample with oxygen inclusion on the fluoride site displays extended solid-solution behavior. This explains previous observations of improved capacity retention due to defects.

Original languageEnglish
JournalACS Applied Energy Materials
Pages (from-to)9750-9759
Publication statusPublished - Nov 2020
Externally publishedYes


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