Characterization of novel lithium battery cathode materials by spectroscopic methods: the Li5+xFeO₄ system

The novel, lithium-rich oxide-phase Li₅FeO₄ (LFO) could, in theory, deliver a specific capacity >900 mAh/g when deployed as a cathode or cathode precursor in a battery with a lithium-based anode. However, research results to date on LFO indicate that less than one of the five Li⁺ cations can be reversibly de-intercalated/re-intercalated during repetitive charging and discharging cycles. In the present research, the system Li5+xFeO₄ with x values in the range of 0.0-2.0 was investigated by a combination of Raman and X-ray absorption spectroscopic methods supported by X-ray diffraction (XRD) analysis in order to determine if the Li₅FeO₄ lattice would accommodate additional Li⁺ ions, with concomitant lowering of the valence on the FeIII cations. Both the Raman phonon spectra and the XRD patterns were invariant for all values of x, strongly indicating that additional Li⁺ did not enter the Li₅FeO₄ lattice. Also, Raman spectral results and high-resolution synchrotron XRD data revealed the presence of second-phase Li₂O in all samples with x greater than 0.0. Synchrotron X-ray absorption spectroscopy at the Fe kα edge performed on the sample with a Li-Fe ratio of 7.0 (i.e., x = 2.0) showed no evidence for the presence of FeII. This resistance to accepting more lithium into the Li₅FeO₄ structure is attributed to the exceedingly stable nature of high-spin FeIII in tetrahedral "FeIIIO₄" structural units of Li₅FeO₄. Partial substitution of the FeIII with other cations could provide a path toward increasing the reversible Li⁺ content of Li5xFeO₄-type phases.