Investigation of the structural changes in Li_1-xFePO₄ upon charging by synchrotron radiation techniques

A systematic study on a LiFePO₄ sample with 600 nm particle size was carried out to investigate the electronic and crystal structure changes during charge and their relationships with the charging curve. In situ X-ray diffraction data collected at 0.1C rate during first charge shows that the crystal FePO₄ is observed at x = 0.3 in Li_1-xFePO ₄, with significant delay than the beginning of the charge plateau. On the other hand, in situ X-ray absorption spectra for the Fe K-edge collected at 0.1C during first charge show that the average oxidation of Fe²⁺ to Fe³⁺ almost synchronized with the charging curve. In addition, the ex-situ soft X-ray absorption data for the Fe L_II-III edges using surface sensitive partial electron yield detection mode show that the lithium content changes at the surface of the sample particles synchronized or ahead to the charge curve, while the data collected at the same time using bulk sensitive fluorescence yield detection mode show that the lithium content changes in the bulk of the particles lag behind the charging curve. The contents of FePO ₄ and LiFePO₄ calculated from in situ XRD in relating to the charge curve are quite different than those calculated from in situ XAS and ex-situ soft X-ray absorption data. This tells us that cautious need to be taken when using XRD data alone to track the state of charge of LiFePO₄ samples, significant delay in observing the new crystal phase formation could occur, especially at relatively high charging rate.