In Situ Studies of Fe4+ Stability in β-Li3Fe2(PO4)3 Cathodes for Li Ion Batteries

Ane Sælland Christiansen, Rune E. Johnsen, Poul Norby, Cathrine Frandsen, Steen Mørup, Søren Højgaard Jensen, Kent Kammer Hansen, Peter Holtappels

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Abstract

In commercial Fe-based batteries the Fe2+/Fe3+ oxidation states are used, however by also utilizing the Fe4+ oxidation state, intercalation of up to two Li ions per Fe ion could be possible. In this study, we investigate whether Fe4+ can be formed and stabilized in β-Li3Fe2(PO4)3. The work includes in situ synchrotron X-ray powder diffraction studies (XRPD) during charging of β-Li3Fe2(PO4)3 up to 5.0 V vs. Li/Li+. A novel capillary-based micro battery cell for in situ XRPD has been designed for this. During charge, a plateau at 4.5 V was found and a small contraction in volume was observed, indicating some Li ion extraction. The volume change of the rhombohedral unit cell is anisotropic, with a decrease in the a parameter and an increase in the c parameter during the Li ion extraction. Unfortunately, no increased discharge capacity was observed and Mössbauer spectroscopy showed no evidence of Fe4+ formation. Oxidation of the organic electrolyte is inevitable at 4.5 V but this alone cannot explain the volume change. Instead, a reversible oxygen redox process (O2− → O−) could possibly explain and charge compensate for the reversible extraction of lithium ions from β-Li3Fe2(PO4)3.
Original languageEnglish
JournalJournal of The Electrochemical Society
Volume162
Issue number4
Pages (from-to)A531-A537
ISSN0013-4651
DOIs
Publication statusPublished - 2015

Bibliographical note

Erratum in Electrochemical Society. Journal, 2015, 162(6) p. X11: Throughout this paper, the prefix “β-” has been used incorrectly to indicate the rhombohedral phase of Li3Fe2(PO4)3. The material studied in the paper is exclusively the rhombohedral phase and all instances of “β-Li3Fe2(PO4)3” should be replaced with “Li3Fe2(PO4)3.”

Keywords

  • Batteries and Energy Storage
  • A

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