The novel, lithium-rich oxide-phase Li<sub>5</sub>FeO<sub>4</sub> (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<sup>+</sup> cations can be reversibly de-intercalated/re-intercalated during repetitive charging and discharging cycles. In the present research, the system Li<sub>5+<i>x</i></sub>FeO<sub>4</sub> with <i>x</i> 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 Li<sub>5</sub>FeO<sub>4</sub> lattice would accommodate additional Li<sup>+</sup> ions, with concomitant lowering of the valence on the Fe<sup>III</sup> cations. Both the Raman phonon spectra and the XRD patterns were invariant for all values of <i>x,</i> strongly indicating that additional Li<sup>+</sup> did not enter the Li<sub>5</sub>FeO<sub>4</sub> lattice. Also, Raman spectral results and high-resolution synchrotron XRD data revealed the presence of second-phase Li<sub>2</sub>O in all samples with <i>x</i> greater than 0.0. Synchrotron X-ray absorption spectroscopy at the Fe k? edge performed on the sample with a Li-Fe ratio of 7.0 (i.e., <i>x</i> = 2.0) showed no evidence for the presence of Fe<sup>II</sup>. This resistance to accepting more lithium into the Li<sub>5</sub>FeO<sub>4</sub> structure is attributed to the exceedingly stable nature of high-spin Fe<sup>III</sup> in tetrahedral “Fe<sup>III</sup>O<sub>4</sub>” structural units of Li<sub>5</sub>FeO<sub>4</sub>. Partial substitution of the Fe<sup>III</sup> with other cations could provide a path toward increasing the reversible Li<sup>+</sup> content of Li<sub>5</sub><i><sub>x</sub></i>FeO<sub>4</sub>-type phases.
Victor A. Maroni, Christopher S. Johnson, Shawn C.M. Rood, A. Jeremy. Kropf, and Dean A. Bass, "Characterization of Novel Lithium Battery Cathode Materials by Spectroscopic Methods: The Li5+xFeO4 System," Appl. Spectrosc. 67, 903-912 (2013)
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