Mechanism of the Fe3+ Reduction at Low Temperature for LiFePO4 Synthesis from a Polymeric Additive

Comparison is made between the use of either a carbon powder or a polymer additive to the precursors in the synthesis of LiFePO 4 from the Fe(III) compound FePO 4 (H 2 O) 2 and Li 2 CO 3 . The evolution of the structural properties and phase purity with temperature and time have been monitored at all length scales by X-ray diffraction, Fourier transformed infrared spectroscopy, and magnetic susceptibility. The reactor temperature was decreased to 300 °C to investigate the early stages of the reaction. Formation of crystalline LiFePO 4 begins in the range 300−400 °C only if the polymer is used as the carbonaceous additive. This LiFePO 4 formation is made possible by the reduction of Fe(III) species by gases such as H 2 or gaseous hydrocarbons evolved during the calcination of the polymer. Moreover, decomposition of the polymer results in a carbonaceous deposit on the surface of the LiFePO 4 particles. An Li 3 Fe 2 (PO 4 ) 3 impurity found after sintering at 400 °C for 4 h was greatly reduced after sintering at 400 °C for 24 h, and phase-pure LiFePO 4 was attained at 700 °C. Where the solid carbon powder was used as the reducing agent, no Fe(II) species could be detected after sintering at 400 °C. Carbothermal reduction of Fe(III) is ruled out in this temperature range.