Identification of surface impurities on LiFePO4 particles prepared by a hydrothermal process

LiFePO4 was synthesized by a hydrothermal process at 170 degrees C. The particle size of the LiFePO4 was 0.5 mu m. The hydrothermally synthesized LiFePO4 exhibited a discharge capacity of 65 mAh g(-1) at the potential of 3.5 V vs Li/Li+. Transmission electron microscopy revealed the formation of an amorphous layer on the LiFePO4 particle during the hydrothermal process. This amorphous layer was crystallized by a heat-treatment at 400 degrees C in an argon atmosphere. The alpha-Fe2O3 was generated on the particle surface during the annealing process. Two plateaus at 3.5 and 2.5 V were observed in discharge curve for the annealed LiFePO4. The plateau at 2.5 V was correlated with Li+-ion insertion into alpha-Fe2O3. Addition of ascorbic acid as a reducing agent to the precursor was useful in prohibiting the conversion of Fe2+ to Fe3+ during the hydrothermal process and the generation of alpha-Fe2O3 during the annealing process. It was found that a trace amount of carbonaceous material was formed on the LiFePO4 particle due to the decomposition of ascorbic acid. The hydrothermally synthesized LiFePO4 followed by annealing at 400 degrees C exhibited a discharge capacity of 150 mAh g(-1). (c) 2005 The Electrochemical Society.