Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mössbauer spectroscopy study

The extraction and insertion of lithium in solid-state synthesized LiFePO4 has been followed by in situ X-ray diffraction and Mössbauer spectroscopy in ‘coffee-bag’ cells of type 〈Li-metal | liq. el. | LiFePO4〉 during the first cycle. Two-phase Rietveld refinement of the X-ray diffractograms gives the triphylite (LiFePO4) to heterosite (FePO4) phase-ratios as charging and discharging of the cell proceeds. The Fe3+/Fe2+ ratios at each step, as measured by Mössbauer spectroscopy and X-ray diffraction, were in good general agreement with the amount of lithium calculated from the charge passed through the cell; there was, however, a slight tendency for the Mössbauer technique to record a higher concentration of the oxidized phase. The possible existence of a thin interface region at the phase boundary is discussed.