Phase Boundary Propagation in Large LiFePO4 Single Crystals on Delithiation

Large single crystals of LiFePO4 have been chemically delithiated. The relevance of chemical oxidation in comparison with electrochemical delithiation is discussed. Analyses of the Li content and profiles were done by electron energy loss spectroscopy and secondary ion mass spectrometry. The propagation of the FePO4 phase growing on the surface of the large single crystal was followed by in situ optical microscopy as a function of time. The kinetics were evaluated in terms of linear irreversible thermodynamics and found to be characterized by an induction period followed by parabolic growth behavior of the FePO4 phase indicating transport control. The growth rate was shown to depend on the crystallographic orientation. Scanning electron microscopy images showed cracks and a high porosity of the FePO4 layer due to the significant changes in the molar volumes. The transport was found to be greatly enhanced by the porosity and crack formation and hence greatly enhanced over pure bulk transport, a result which is supposed to be very relevant for battery research if coarse-grained powder is used.