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-N-2) 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 mu m. 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-N-2 powder. The discharge capacity of a LiFePO4-N-2 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-N-2 and LiFePO4-air electrodes were 4.6 and 4.8 Omega m(2), respectively. These values were not significantly different. This revealed that the factor dominating the electrochemical properties of LiFePO4-N-2 and LiFePO4-air powders was particle size and not crystalline lattice or Fe2+ concentration.