C-containing LiFePO4 materials - Part II: Electrochemical characterization

A series of carbon-coated olivine phase (C-LiFePO4) was synthesized under argon by mechano-chemical activation, with two thermal-treatments (slow or fast) and two temperatures (575 degrees C or 800 degrees C). In spite of similar chemical and structural properties. they showed rather good, but very different, electrochemical behaviors in long range cycling or high rate conditions. All the studied C-LiFePO4 materials were characterized by an inhomogeneous agglomerates size distribution with small primary particles around 100 nm in diameter and by specific surface areas around 20 m(2)/g. The electronic properties were shown to be highly dependant on the synthesis conditions: as expected the higher the thermal-treatment temperature and the longer the thermal treatment were, the better the degradation of the carboneous precursor and thus the higher the electronic conductivity of the C-LiFePO4 material. This study suggests that good electrochemical performances at high rate and during a long range cycling at constant rate imply, for a given composite, a good coating with high electronic conductivity and small primary particles (here around 100 nm in diameter). The material obtained at 800 degrees C with the short thermal-treatment synthesis (15 min) satisfies these requirements. (C) 2008 Elsevier B.V. All rights reserved.