Surface Effects on the Physical and Electrochemical Properties of Thin LiFePO₄ Particles

The structure of LiFePO 4 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 Å thick surface layer in which the iron ions were in the Fe 3+ low-spin ( S = 1 / 2 ) configuration. Carbon coating at 750 °C reduced the disorder at the surface and switched the Fe 3+ 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.