XANES Investigation of Dynamic Phase Transition in Olivine Cathode for Li-Ion Batteries

Dynamic phase transformation in olivine LiFePO4 involving formation of one or more intermediate or metastable phases is revealed by an in situ time-resolved X-ray absorption near edge structure (XANES) technique. The XANES spectra measured during relaxation immediately after the application of relatively high overpotentials, where metastable phases are expected, show a continuous shift of the Fe K-edge toward higher energy. Surprisingly, the Fe K-edge relaxes to higher energies after current interrupt regardless of whether the cell is being charged or discharged. This relaxation phenomenon is superimposed upon larger shifts in K-edge due to changes in Fe2+/Fe3+ ratio due to charging and discharging, and implies an intermediate phase of larger Fe-O bond length than any of the known crystalline phases. No intermediate crystalline phases are observed by X-ray diffraction (XRD). A metastable amorphous phase formed during dynamic cycling and which structurally relaxes to the equilibrium crystalline phases over a time scale of about 10 min after cessation of charging/discharging current is consistent with the experimental observations.