Reduction Fe3+ of Impurities in LiFePO4 from Pyrolysis of Organic Precursor Used for Carbon Deposition
Résumé
The structural properties of microcrystalline LiFePO4 prepared with and without carbon coating are analyzed with X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and magnetic measurements for comparison. While nanosized ferromagnetic particles (-Fe2O3 clusters) are evidenced from magnetic measurements in samples without carbon coating, such ferromagnetic clusters just do not exist in the carbon-coated sample. Ferromagnetic resonance experiments are a probe of the -Fe2O3 nanoparticles, and magnetization measurements as well, allowing for a quantitative estimate of the amount of Fe3+. While the fraction of iron in the Fe3+ configuration rises to 0.18% (in the form of -Fe2O3 nanoparticles) in the carbon-free sample, this fraction falls to a residual impurity concentration in the carbon-coated sample. Structural properties show that the carbon does not penetrate inside the LiFePO4 particles but has been very efficient in the reduction of Fe3+, preventing the -Fe2O3 clustering thus pointing out a gas phase reduction process. The carbon deposit characterized by Raman spectroscopy is an amorphous graphite deposit hydrogenated with a very small H/C ratio, with the same Raman characteristics as a-C carbon films obtained by pyrolysis technique at pyrolysis temperature 830±30°C. The impact of the carbon coating on the electrochemical properties is also reported.