Study of the LiFePO4/FePO4 Two-Phase System by High-Resolution Electron Energy Loss Spectroscopy

The intriguingly fast electrochemical response of the insulating LiFePO4 insertion electrode toward Li is of both fundamental and practical importance. Here we present a comprehensive study of its deinsertion/insertion mechanism by high-resolution electron energy loss spectroscopy on thin platelet-type particles of LixFePO4 (bPnma axis normal to the surface). We find that the lithium deinsertion/insertion process is not well-described by the classical shrinking core model. Compositions of the same x value obtained by both deinsertion and insertion gave the same results, namely that the LixFePO4 so formed consists of a core of FePO4 surrounded by a shell of LiFePO4 with respective ratios dependent on x. We suggest that lattice mismatch between the two end members may be at the origin of the peculiar microstructure observed. Furthermore, because of the appearance of isosbestic points on the overlaid EELS spectra, we provide direct experimental evidence that the nanometer interface between single-phase areas composed of LiFePO4 or FePO4 is the juxtaposition of the two end members and not a solid solution. One future prospect of such knowledge is to determine strategies on how to control, on a large scale, the synthesis of nanometer-sized thin platelet-type particles to prepare high-rate LiFePO4 electrodes for future energy storage devices.