Citrate-derived carbon nanocoatings for poorly conducting cathode - A detailed study using TiO2 substrate materials

Preparation of conductive coatings is an effective way for improvement of kinetics in Li battery materials. Although carbon-based conductive coatings are most often used, they have been studied less systematically than, for example, phosphides or phosphocarbides. In this study we tried to find the most important correlations between the preparation conditions and the resulting composition, morphology, and electrical properties of selected carbon-coated materials. As a model substrate material we mainly used TiO2 particles while as a carbon precursor we employed citric acid. For comparison, other substrates and precursors were occasionally used. It is shown that in the typical temperature range between 500 and 800 degrees C, the carbon yield varies from 3 to 10 wt %, depending on the initial heating rate and on the duration of heat-treatment. Correlations between the composition, morphology, and the conductivity of final composites are thoroughly discussed. The steep increase in conductivity beyond the percolation threshold (between 1-2 wt % of carbon) can be correlated with the occurrence of a uniform, 1-5 nm thick carbon film that appears to be continuous on a micrometer scale. Finally, we show that the activation energies for conduction in various carbon-coated samples (0.04-0.13 eV) are strikingly similar to those found previously for phosphide- or phosphocarbide-based coatings and even for LiFePO4 doped with supervalent ions. (c) 2006 The Electrochemical Society.