Reducing carbon in LiFePO4/C composite electrodes to maximize specific energy, volumetric energy, and tap density

Efforts were made to synthesize LiFePO4/C composites showing good rate capability and high energy density while attempting to minimize the amount of carbon in the composite. First, three carbon-coated samples, one coated with carbon after the synthesis of pure LiFePO4, one synthesized with sugar added before the heating steps, and one synthesized with sugar added before heating and subsequently coated with carbon, were studied. The resulting carbon contents for these samples are 2.7, 3.5, and 6.2 wt %, respectively. Electrochemical tests showed that the latter two samples had comparable rate capabilities to the LiFePO4/C composite (15 wt % carbon) recently reported by Huang et al. We believe the synthesis of LiFePO4 with sugar added before heating is the best method because it gives particles having uniform small size that are covered by carbon. Further studies of samples made by this method show that a very small percentage of carbon, even less than 1 wt %, causes a significant increase in rate capability, but unfortunately, a dramatic decrease in tap density. To make LiFePO4/C composites having good rate capability, high energy density, and high tap density, the carbon content and method for coating carbon onto the LiFePO4 particles must be given careful attention. However, based on the studies reported here, we are not certain that all desired parameters can be simultaneously achieved, and this may limit the usefulness of LiFePO4 in some practical applications. (C) 2002 The Electrochemical Society.