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Novel Co-Precipitation Method for Synthesis of Carbon-Free LiFePO4

Wednesday, May 14, 2014
Grand Foyer, Lobby Level (Hilton Orlando Bonnet Creek)
H. Feng, A. S. Milev, and G. S. K. Kannangara (University of Western Sydney)
Lithium iron phosphate (LiFePO­4) is an important alternative cathode material to LiCoO2, which applications are limited by resource availability and safety issues. However, the full potential of the material is sometimes difficult to achieve due to its limited electrical conductivity and low rate of Li+ ion diffusion that together may limit the battery performance. One strategy is to manufacture LiFePO4 particles coated with a conductive phase, such as carbon, which improves the electronic contact between the LiFePO4 particles. However, the carbon seems to modulate not only the electric conductivity but also the reduce crystallite sizes and thus reduces the distance for Li+ transport in the LiFePO4 structure. Furthermore, carbon may improve the phase purity of the final product because during high temperature synthesis, carbon acts as a reducing agent and thus minimizes the likelihood of oxidation of Fe2+ to Fe3+. Despite these advantages, the role of different forms of carbon on the synthesis of LiFePO4 and the electrochemical performance of the materials is yet to be fully understood. This could be because the synthetic methods reported in the literature employ carbon containing precursors or solvents and therefore the LiFePO4 product always contain some form of residual carbon. This complicates the analysis of the effect of a specific carbon type on the electrochemical performance of the cathode material.

Carbon-free LiFePO4 material has been synthesised by a novel single-step co-precipitation method from aqueous solutions of LiOH∙H2O, NH4H2PO4 and FeSO4.7H2O precursors. The precipitation sequence and methodology, reaction atmosphere and the role of intermediate phases on the purity of LiFePO4 product has been investigated. The mechanism of formation of phase-pure carbon free LiFePO4 is also investigated and the role of synthesis atmosphere on the phase transformations and the effect of evolving gaseous species on the likelihood of oxidation of Fe2+ to Fe3+ are discussed. The lowest temperature to synthesise phase pure carbon-free LiFePO4 is 550 oC which remains phase pure during further heating up to 750 oC. The novel carbon-free method is suitable for investigation of the effect various forms of carbon (carbon black, nanotubes, meso-porous carbons, etc.) on the electrochemical performance of the cathodes containing LiFePO4.