C-containing LiFePO4 materials - Part II: Electrochemical characterization
Résumé
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 °C or 800 °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 m2/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 °C with the short thermal-treatment synthesis (15 min) satisfies these requirements.