Surface effects on the physical and electrochemical properties of thin LiFePO4 particles

The structure of LiFePO4 particles prepared by a new milling route has been investigated, with emphasis on surface effects found to be important for such small particles, whose sizes were distributed in the range 30 - 40 nm. The bulk and surface properties of the particles were investigated by a combination of XRD, TEM, FTIR, and magnetic measurements before and after application of a carbon coating intended to optimize the electrochemical performance of the powder used as a cathode element of a new generation of lithium-ion battery. Before the carbon coating was applied, the particles were well-crystallized in the bulk and free from any impurities, but they were surrounded with a disordered, 8 angstrom thick surface layer in which the iron ions were in the Fe3+ low-spin (S=1/2) configuration. Carbon coating at 750 degrees C reduced the disorder at the surface and switched the Fe3+ ions in the surface layer to the high-spin (S=5/2) configuration. These results are discussed with respect to similar effects recently observed for ferrite nanoparticles used in spintronics and biological molecules such as proteins that contain heme Fe.