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 Fe-III 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 k alpha 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 Fe-II. This resistance to accepting more lithium into the Li5FeO4 structure is attributed to the exceedingly stable nature of high-spin Fe-III in tetrahedral (FeO4)-O-III structural units of Li5FeO4. Partial substitution of the Fern with other cations could provide a path toward increasing the reversible Li+ content of Li5xFeO4-type phases.