Improvement of electrochemical properties of LiFePO4 fine particles synthesized in ethylene glycol solution resulting from heat treatment

Fine crystalline LiFePO4 particles with an average secondary size of about 200 nm were synthesized in ethylene glycol (EG) solution at 393 K. Their molar ratio of Li:Fe:P was about 1.3:1:1, implying the precipitation of an impurity containing extra Li ions. The unit cell volume of crystalline LiFePO4 in heat-treated particles was larger than that in as-synthesized particles. The Fe K absorption edge in the X-ray absorption spectrum was shifted toward a lower energy position by heat treatment. These results suggest that the composition of crystalline LiFePO4 in fine particles is modified by heat treatment because of the reaction with extra Li ions. Additionally, in the radial structural function obtained by Fourier transform of the extended X-ray absorption fine structure oscillation, the Fe–P correlation and Fe–Fe correlation peaks became high due to the heat treatment, implying the rearrangement of FeO6 octahedra and PO4 tetrahedra. In consequence of the above-mentioned structural changes, the electronic capacity of fine particles precipitated in EG solution was enhanced to about 150 mA/h by heat treatment at 673 K.