Synthesis and electrochemical analysis of vapor-deposited carbon-coated LiFePO₄

Carbon-coated LiFePO₄ electrodes (C-LiFePO₄) were prepared by a vapor deposition process. An improvement in the rate performance of C-LiFePO₄ in lithium cells over uncoated LiFePO₄ was observed. The coating process uses a solid/gas-phase reaction which consists of decomposing propylene gas, as carbon source, inside a reactor containing olivine LiFePO₄ material. This process leads not only to the formation of few monolayers of carbon film at the surface of olivine particles but also allows for carbon to be deposited inside the pores of the particles. The amount of carbon deposited on olivine particles was determined by TG analysis to be around 3.4 wt%. The structural and electrochemical properties of an optimized electrode made of carbon-coated olivine without acetylene black additive are presented. Li/(C-LiFePO₄) cells, cycled at RT and 37 °C between 2.0 and 4.3 V at a C/3 rate, do not show any sign of capacity fade during the first 70 cycles. The area-specific impedance (ASI) of a cell fabricated with carbon-coated olivine made by our process is much lower than the ASI's of cells made of conventional olivine and that of an olivine/carbon composite material reported here for the purpose of comparison. These characteristics, combined with the low-cost coating technique, make the carbon-coated olivine an attractive safe cathode for lithium-ion batteries.