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

Abstract

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.