摘要
The iron precursor of lithium iron phosphate (LiFePO 4 ) is highly prone to oxidation to Fe 3+ during the hydrothermal synthesis. The Fe 3+ impurities in LiFePO 4 restrict the conduction path of Li + ions in LiFePO 4 , which negatively affect the cell performance. In this paper, we report that ferrocenecarboxylic acid possessing an extremely stable Fe 2+ species and as carbon source has been used successfully to suppress Fe 3+ impurities in LiFePO 4 . The X-ray diffraction results reveal that Li 2 CO 3 first reacts with (NH 4 ) 2 HPO 4 to form Li 3 PO 4 at low temperatures, which above 160 °C further reacts with ferrocenecarboxylic acid to give LiFePO 4 . The infrared spectroscopy, nuclear magnetic resonance, mass spectrometry, and elemental analysis results show that the carbon sources during calcination of LiFePO 4 are derived from polymers through sequential [4 + 2] cycloaddition reactions of cyclopentadiene and 1,3-cyclopentadiene-1-carboxylic acid during decomposition of ferrocenecarboxylic acid. The electron paramagnetic resonance results show that the percentage of Fe 3+ in the synthesized LiFePO 4 is as low as 0.5 mol%. A plausible reaction mechanism for the hydrothermal synthesis of LiFePO 4 is also proposed. The as-synthesized LiFePO 4 shows an orthorhombic olivine with a discharge capacity of 158 mAh g −1 at a discharge rate of 0.1C. The cell also shows excellent C-rate and cycle-life performances.