Reduction Fe[sup 3+] of Impurities in LiFePO[sub 4] from Pyrolysis of Organic Precursor Used for Carbon Deposition

Abstract

The structural properties of microcrystalline LiFePO 4 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 ͑␥-Fe 2 O 3 clusters͒ 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 ␥-Fe 2 O 3 nanoparticles, and magnetization measurements as well, allowing for a quantitative estimate of the amount of Fe 3+ . While the fraction of iron in the Fe 3+ configuration rises to 0.18% ͑in the form of ␥-Fe 2 O 3 nanoparticles͒ 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 LiFePO 4 particles but has been very efficient in the reduction of Fe 3+ , preventing the ␥-Fe 2 O 3 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.

Preparation of LiFe PO4Using Chitosan and its Cathodic Properties for Rechargeable Li-ion Batteries

“…25 Since there is no other secondary phase in XRD patterns as shown in Figure 2, most of the iron ions were reduced to Fe 2+ after the third calcination at 600 o C. Figure 6 implies that the synthesized chitosan added LiFePO4 powder shows an olivine-type LiFePO 4 phase with no impurities because the residual carbon contributes to the reduction atmosphere during calcination which is sufficient to reduce Fe 3+ to Fe 2+ . 19 Since there is no other secondary phase except impurities as shown in XRD patterns shown Figure 2, most of the iron ions were reduced to Fe 2+ after the third calcination at 600 o C. The shift range of the samples agrees with the binding energy of the oxidation state of Fe 2+ . 24 As shown in Figure 6, the valence states of iron shifted from 3+ to 2+ with calcination times.…”

“…Carbon coated LiFePO4 has been prepared using several methods. [16][17][18][19][20] A carbon coating significantly improves the electrochemical performance of this material; sucrose was proposed 16 as a carbon precursor, and it was used on the initial hydrothermal samples. 16 Added carbon increases the electronic contact between the active materials and the electronic conductor, and controls the particle size.…”

Carbon Coating Stability on High-Voltage Cathode Materials in H2O-Free and H2O-Containing Electrolyte

“…While this strong attenuation of the LFP bands by the carbon coating (thickness ≈1 nm, estimated for a homogeneous coating with a density of ≈2 g/cm 3 using the wt% C and the BET area given in Table I) is surprising based on the reported penetration depth of the Raman laser radiation through carbon of ≈30 nm, 28 it is nevertheless consistent with most other Raman spectra on carbon coated LFP. 13,29,30 With this in mind, the low intensity of the LFP band at 950 cm −1 for the carbon coated samples in Figure 2b indicates a uniform and homogeneous distribution of carbon around the LFP particles, since the laser beam is not focused on a single particle, but has a spot diameter of ≈5 μm, which is ≈5 times larger than the average particle diameter (see Figure 1). 1 − √ 12/13 resulting from the difference in atomic mass of 12 C and 13 C. In conclusion, our material characterization by SEM, elemental analysis, powder conductivity measurements, XRD, and Raman spectroscopy proves the successful synthesis of LFP particles with a homogeneous isotopically labeled 13 C carbon coating.…”

Enhanced Electrochemical Properties of LiFePO<sub>4</sub> as Positive Electrode of Li-Ion Batteries for HEV Application

“…Various types of iron-based impurities have been identified in the olivine framework: for examples γ-Fe 2 O 3 , Fe 3 O 4 , Li 3 Fe 2 (PO 4 ) 3 , Fe 2 P 2 O 7 , Fe 2 P, Fe 3 P, Fe 75 P 15 C 10 , etc. Critical quality control of the product is necessary to obtain a complete understanding of synthesis conditions using combination of experiments such as Raman spectroscopy and magnetic measurements [8][9][10][11][12].…”