The usage of Fe at high oxidation states might be interesting for increasing the energy density of Li-ion batteries and also for substituting expensive/toxic elements in the cathode materials. The redox activity of Fe³⁺/Fe⁴⁺ in the spinel LiCo_0.8Fe_0.2MnO₄ has been investigated via in situ Mössbauer spectroscopy and in situ X-ray absorption spectroscopy (XAS) during electrochemical cycling in a complete Li battery cell [1]. The initial Mössbauer spectrum consists of a doublet with isomer shift IS = 0.330(1) mm/s and quadrupole splitting QS = 0.722(1) mm/s (Fig. 1). These parameters are characteristic of Fe³⁺ in a high-spin state on the octahedral sites of the spinel structure. In the beginning of the charging process vs. Li⁺|Li, a broadening of this doublet can be observed. This corresponds to an increased electric field gradient at the site of the Fe nuclei, i.e. an increase of structural disorder around the Fe ions, and is probably cause by the initial oxidation of Co.

During further charging, the Fe³⁺ doublet successively disappears and is replaced by a new narrow doublet with isomer shift of -0.134(6) mm/s. This negative value clearly reveals that Fe⁴⁺ is formed. Further cycling between 5.3 V and 4.3 V reveals that this oxidation is highly reversible.

The XAS spectra acquired during charging of LiCo_0.8Fe_0.2MnO₄ vs. Li⁺|Li clearly show a shift of the Fe K absorption edge towards higher binding energies and thus an oxidation of Fe³⁺ to Fe⁴⁺. The XAS spectra acquired during discharging show again that this process is fully reversible.

In summary, the reversible formation Fe⁴⁺ was clearly observed by both methods, hence proving the electrochemical activity of Fe at voltages above 5 V vs. Li⁺/Li. This enables future research on new environmentally benign and inexpensive electrode materials with operating voltages above 5 V and thus an increased energy density.

[1] Observation of Electrochemically Active Fe³⁺/Fe⁴⁺ in LiCo_0.8Fe_0.2MnO₄ by in situ Mössbauer Spectroscopy and X-Ray Absorption Spectroscopy,

C. Dräger, F. Sigel, R. Witte, R. Kruk, L. Pfaffmann, V. Mereacre, M. Knapp, H. Ehrenberg, S. Indris, Phys. Chem. Chem. Phys. 21, 89-95 (2019).