Impact of the carbon coating thickness on the electrochemical performance of LiFePO4/C composites

Porous, well crystalline LiFePO4 /C Composites with different amounts of carbon have been prepared by a sol-gel technique. The thickness of carbon coatings (paintings) has been determined by high-resolution transmission electron microscopy. It is shown that carbon coating thickness can be controlled by the amount of carbon and it has an impact on the obtained reversible capacity. Furthermore, it is shown that atomic ratio of nonactive Fe(III) phase (presumably Fe3P) in as-synthesized LiFePO4 /C composites depends on the amount of carbon in the composite. Using Mossbauer spectroscopy, we have confirmed that the nonactive, Fe(III) remains nearly unchanged in the composite during cycling. The lowest amount of carbon in LiFePO4 /C composites obtained from citrate anion as a gelling agent was 3.2 wt % and this particular amount corresponds to the carbon coating thickness of about 1-2 nm. The reversible capacity obtained from the above-mentioned composite delivers close to 80% of the theoretical capacity at room temperature with a current density of 170 mA/g (C/1 rate). (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1860492] All rights reserved.