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

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.