Hydrothermal synthesis of LiFePO4 with small particle size and its electrochemical properties

Lithium iron phosphate (LiFePO4) powders were prepared by hydrothermal reactions under a nitrogen atmosphere or an air atmosphere, and the microstructure and electrochemical properties of the LiFePO4 powders were investigated. The LiFePO4 powder prepared under the nitrogen atmosphere (LiFePO4–N2) had a small particle size in the range of 300–500 nm, whereas the powder prepared under the air atmosphere (LiFePO4−air) had a large particle size in the range of 1–5 μm. Although the Fe2+/Fe3+ ratio was not significantly different in both LiFePO4 powders, the Fe2+/Fe3+ ratio in the precursor suspension prepared under the nitrogen atmosphere was much higher than that prepared under the air atmosphere, thereby resulting in the small particle size of the LiFePO4–N2 powder. The discharge capacity of a LiFePO4–N2 electrode was 149 mAh g−1 at a low current density of 10 mA g−1, whereas that of a LiFePO4−air electrode was 83 mAh g−1. Impedance analyses indicated that the charge transfer resistances normalized to the surface area of LiFePO4 particles for the LiFePO4–N2 and LiFePO4−air electrodes were 4.6 and 4.8 Ω m2, respectively. These values were not significantly different. This revealed that the factor dominating the electrochemical properties of LiFePO4–N2 and LiFePO4−air powders was particle size and not crystalline lattice or Fe2+ concentration.