Synthesis and electrochemical properties of olivine-type LiFePO₄/C composite cathode material prepared from a poly(vinyl alcohol)-containing precursor

Olivine structure LiFePO₄/C composite powders are synthesized as cathode materials for Li-ion batteries via a conventional solid-state reaction. Improvement in electrochemical performance has been achieved by using poly(vinyl alcohol) as the carbon sources for the as-prepared materials. The influence of the heat treatment on the physical and the electrochemical properties of LiFePO₄/C materials is investigated. To examine the effect of added carbon content on the properties of materials, a one-step heat treatment has been employed with control of the PVA content in the precursor. Six samples were prepared with 0, 1, 3, 5, 10 and 30 wt.% PVA added to the raw materials. The particle size of LiFePO₄ decreases as the carbon content increases. Materials with medium carbon contents have a small charge-transfer resistance and thus exhibit superior electrochemical performance. Interestingly, for a LiFePO₄/C composite with a low PVA content, an unusual plateau at 4.3 V is observed. It is considered that this is due to the Fe³⁺/Fe⁴⁺ redox reaction of Fe³⁺ compounds that are present as an impurity. For samples with a high PVA amount, a thicker carbon coating provides an obstacle to improve the electrochemical properties.