Effect of surface carbon structure on the electrochemical performance of LiFePO4

The electrochemical performance of LiFePO4 samples synthesized by sol-gel or solid-state routes varies considerably, although their physical characteristics are similar. Raman microprobe spectroscopic analysis indicated that the structure of the residual carbon present on the surfaces of the powders differs significantly and accounts for the performance variation. Higher utilization is associated with a larger ratio of sp(2)-coordinated carbon, which exhibits better electronic properties than disordered or sp(3)-coordinated carbonaceous materials. Incorporation of naphthalenetetracarboxylic dianhydride during synthesis results in a more graphitic carbon coating and improves utilization of LiFePO4 in lithium cells, although the total carbon content is not necessarily higher than that of samples prepared without the additive. This result suggests that practical energy density need not be sacrificed for power density, provided that carbon coatings are optimized by carefully choosing additives. (C) 2003 The Electrochemical Society.