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

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.