Showing papers by "Christian M. Julien published in 2006"

Structural Stability and Phase Transitions in WO3 Thin Films

Abstract: Tungsten oxide (WO3) thin films have been produced by KrF excimer laser (λ = 248 nm) ablation of bulk ceramic WO3 targets. The crystal structure, surface morphology, chemical composition, and structural stability of the WO3 thin films have been studied in detail. Characterization of freshly grown WO3 thin films has been performed using X-ray diffraction (XRD), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy (RS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) measurements. The results indicate that the freshly grown WO3 thin films are nearly stoichiometric and well crystallized as monoclinic WO3. The surface morphology of the resulting WO3 thin film has grains of ∼60 nm in size with a root-mean-square (rms) surface roughness of 10 nm. The phase transformations in the WO3 thin films were investigated by annealing in the TEM column at 30−500 °C. The phase transitions in the WO3 thin films occur in sequence as the temperatur...

Structure and insertion properties of disordered and ordered LiNi0.5Mn1.5O4 spinels prepared by wet chemistry

Abstract: We present the synthesis, characterization, and electrode behavior of LiNi0.5Mn1.5O4 spinels prepared by the wet-chemical method via citrate precursors. The phase evolution was studied as a function of nickel substitution and upon intercalation and deintercalation of Li ions. Characterization methods include X-ray diffraction, SEM, Raman, Fourier transform infrared, superconducting quantum interference device, and electron spin resonance. The crystal chemistry of LiNi0.5Mn1.5O4 appears to be strongly dependent on the growth conditions. Both normal-like cubic spinel [Fd3m space group (SG)] and ordered spinel (P4132 SG) structures have been formed using different synthesis routes. Raman scattering and infrared features indicate that the vibrational mode frequencies and relative intensities of the bands are sensitive to the covalency of the (Ni, Mn)-O bonds. Scanning electron microscopy (SEM) micrographs show that the particle size of the LiNi0.5Mn1.5O4 powders ranges in the submicronic domain with a narrow grain-size distribution. The substitution of the 3d8 metal for Mn in LiNi0.5Mn1.5O4 oxides is beneficial for its charge–discharge cycling performance. For a cut-off voltage of 3.5–4.9 V, the electrochemical capacity of the Li//LiNi0.5Mn1.5O4 cell is ca. 133 mAh/g during the first discharge. Differences and similarities between LiMn2O4 and LiNi0.5Mn1.5O4 oxides are discussed.

Reduction Fe3+ of Impurities in LiFePO4 from Pyrolysis of Organic Precursor Used for Carbon Deposition

Abstract: 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.

Characterization of the carbon coating onto LiFePO4 particles used in lithium batteries

Abstract: While nanosized ferromagnetic particles could poison the performance of the Li batteries containing phospho-olivine, the carbon-film coating the LiFePO4 particles has a beneficial effect on cycling life of the cells. In this paper, we present the properties of the carbon layer deposited at the surface of the LiFePO4 grains. Characteristics of the carbon layer are analyzed using scanning electron microscopy, high-resolution transmission scanning electron microscopy, Fourier transform infrared, and Raman scattering (RS) spectroscopy. The carbon deposit characterized by RS spectroscopy is hydrogenated with very small hydrogen/carbon ratio, so that it belongs to the family of the amorphous graphitic carbon. The carbon deposit is similar to that obtained by pyrolysis technique at high temperature. It is expected to have the same properties (small hardness, high electronic conductivity) that favor both the Li diffusion from the LiFePO4 bulk and the charge-discharge rate of the cell. A model for the Li-ion trans...

Nano-sized impurity phases in relation to the mode of preparation of LiFePO4

Abstract: Microcrystalline LiFePO4 samples have been grown using four different techniques. The structural properties are analyzed using X-ray diffraction (XRD) spectroscopy and Fourier transform infrared spectroscopy (FTIR). Depending on which technique of preparation is used, the material is obtained either free of any detectable impurity, or include impurity phases under the form of nano-sized clusters. The magnetic properties are found to be a powerful tool to characterize them. They give evidence of nano-sized ferromagnetic particles, which can be either strongly magnetic ( γ -Fe2 O3 clusters) or weakly ferromagnetic (Fe2 P clusters), depending on the preparation process. The concentration of magnetic clusters also depends on the preparation process and varies from small concentration ( 1.0 × 1 0 − 6 of γ -Fe2 O3 per formula) in which case no collective behavior is observed, to large concentrations ( 1.9 × 1 0 − 4 of Fe2 P clusters per formula) where the dipolar interaction generates superferromagnetism. Ferromagnetic resonance experiments are also reported, and are a probe of the γ -Fe2 O3 nanoparticles. An overall understanding of the different properties is achieved within a model of superferromagnetism induced by interacting Fe2 P nanoparticles, which is also reported.

Magnetic properties of lithium intercalation compounds

Abstract: Magnetic experiments are powerful tools to study fundamental properties and to check the qualities of samples. Temperature, stress, and impurities of materials can all affect magnetic properties and play an important role in the utilization of these materials for engineering applications. The estimation and analysis of the spontaneous magnetization can reveal ferromagnetic particles as impurities in samples. The shape of the temperature dependence of magnetization is indicative of the origin of the magnetic properties. However, it is necessary to correlate the χ m (T) curves and isothermal M(H) plots to achieve a complete analysis of the electronic properties of the materials. Highlights of magnetic properties of lithium intercalation compounds are briefly described. Intrinsic and extrinsic properties are considered as useful parameters to determine the purity of electrode materials for rechargeable Li-ion batteries.

Structural characteristics of lithium nickel phosphate studied using analytical electron microscopy and raman spectroscopy

Abstract: The structural characteristics of lithium nickel phosphate (LiNiPO4) prepared by solid-state chemical reaction have been studied in detail using the analytical electron microscopy and Raman spectroscopy measurements. The high-resolution transmission electron microscopy and selected area electron diffraction measurements indicate that the grown LiNiPO4 is well-crystallized in olivine structure without any indication of crystallographic defects such as dislocations or misfits. The energy-dispersive X-ray spectrometry coupled with the elemental compositional mapping using high-angle angular dark field scanning electron microscopy confirms the chemical quality of the grown LiNiPO4 in terms of homogeneity and uniform elemental distribution characteristics. The local structure and chemical bonding between NiO6 octahedral and (PO4)(3-) tetrahedral groups probed by Raman spectroscopy also indicate the high-quality of LiNiPO4. Structural analysis of the delithiated Li0.09NiPO4 phase indicates lattice contraction and distortion upon lithium extraction. A detailed analysis and comparison of the pristine and delithiated phases is also reported.

Structural and Magnetic Properties of LiFePO4 and Lithium Extraction Effects

Abstract: The characterization of LiFePO4 and of materials synthesized by chemical delithiation of pure LiFePO4 using potassium peroxydisulfate is reported. X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), Electron Paramagnetic Resonance (EPR) spectroscopy and magnetization measurements are analyzed for lithium concentrations x = 0, 0.5, and 1. XRD and EPR data show that the x = 0.5 sample is phase separated into 0.5LiFePO(4) + 0.5FePO(4), in agreement with prior experiments showing that the mixed-valence LixFePO4 compound is unstable. The magnetic properties give evidence of a superparamagnetism associated to iron-based ferromagnetic nanoparticles, superposed to the intrinsic magnetic contribution of the powder which undergoes an antiferromagnetic ordering at T-N(x = 1) = 50 K and T-N(x = 0) = 125 K. The magnetic clustering (amount of iron in the ferromagnetic nanoparticles, mean size distribution) is independent of the lithium concentration. Both the ESR signal of the magnetic clusters and the magnetization curves give evidence of a magnetic interaction between the magnetic clusters and their environment which acts as a friction.

Magnetic studies of the carbothermal effect on LiFePO4

Abstract: The effect of carbon coating on the properties of LiFePO4 particles is studied from magnetic measurements. Magnetization experiments are an excellent tool to detect very low concentrations of iron-based impurities (< 1 ppm) which are poisoning the phospho-olivine used as positive electrode mate- rial in rechargeable Li-ion batteries. The results indicate that I addition of 5% carbon withdraws traces of the Fe(III) phase such as Fe2P and/or Fe2O3. This carbothermal effect appears to be beneficial for long-term application of LiFePO4 materials in Li-ion batteries. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

LiMn2-yCOyO4 (0 <= y <= 1) intercalation compounds synthesized from wet-chemical route

Abstract: We present the synthesis, characterization and electrode behaviour of LiMn2-yCoyO4 (0 <= y <= 1) spinel oxides prepared by the wet-chemical route assisted by carboxylic acid. The phase evolution was studied as a function of the cobalt substitution and the modification on the intercalation and deintercalation of Li ions. Characterization methods include TG-DTA, XRD, SEM, Raman, FTIR, and SQUID. LiMn2-yCoyO samples crystallize with the cubic spinel-like structure (Fd3m S.G.). Raman scattering and FT-infrared spectroscopy indicate that the vibrational mode frequencies and relative intensities of the bands are sensitive to the covalency of the (Co, Mn)-O bonds. MEB micrographs show that the particle dimension of the LiMn2-yCoyO4 powders ranges in the sub-micron-sized domain with a narrow grain-size distribution. The overall electrochemical capacity of LiMn2-yCoyO4 oxides have been reduced due to the 3d(6) metal substitution. However, a more stable charge-discharge cycling performances has been observed when the electrodes are charged up to 4.3 V as compared to the performance of the native oxide. For such a cut-off voltage, the charge capacity of the Li//LiMn1.8CO0.2O4 cell is ca. 101 mA h/g. Differences and similarities between LiMn2O4 and Co-substituted oxides are discussed there from. (c) 2006 Elsevier B.V All rights reserved.

Characterization of the Carbon-Coating of LiFePO4 by Transmission Electron Microscopy and Raman Spectroscopy

Abstract: We present the properties of the carbon layer deposited at the surface of the LiFePO4 particles. Characterizations include scanning electron microscopy, high-resolution transmission electron microscopy, and Raman scattering spectroscopy. Analuysis of Raman spectra reveals that the carbon deposit is hydrogenated with very small hydrogen/carbon ratio, so that it belongs to the family of the amorphous graphitic carbon. It is expected to have the same properties (small hardness, high electronic conductivity) that favor both the Li diffusion from the LiFePO4 bulk and the charge-discharge rate of the cell.

Positive electrode design for advanced rechargeable lithium batteries

Abstract: This work aims at a comparative study on positive electrodes used in advanced rechargeable lithium batteries. The relationship between structural and electrochemical properties is examined with materials currently studied as lithium intercalation compounds for their potential use in Li-ion batteries. Three types of transition-metal oxides are considered, namely, MnO 2 , LiMn 2 O 4 and LiNi 1-y M y O 2 compounds in their various frameworks. Based on the correlation between structural stability and capacity retention of host lattices, the various methods to improve the cycle life of the Li-ion cell are discussed.

Structural, Magnetic and Electrochemical Properties of the Spinel LiMn2-yCoyO4 Nanosized Powders

Abstract: We present the synthesis, structure, magnetic properties and electrode behaviour of LiMn 2-y Co y O 4 (0≤y≤0) spinel oxides prepared by the wet-chemitry via the carboxylic acid route. LiMn 2-y Co y O 4 samples crystallise with the cubic spinel-like structure ( Fd3m S.G.). Optical spectra indicate that the vibrational mode frequencies and relative intensities of the bands are sensitive to the covalency of the (Co,Mn)-O bonds. Magnetic susceptibility and electron spin resonance measurements show the compositional dependence of the magnetic parameters, i.e. Curie temperature, Curie-Weiss constant and Neel temperature, when Mn is substituted by Co. The overall electrochemical capacity of LiMn 2-y Co y O 4 oxides have been reduced due to the 3d 6 metal substitution, however, a more stable charge-discharge cycling performances have been observed when electrodes are charged up to 4.3 V as compared to the performance of the native oxide.

A New Approach Toward Local Structure of Spinel Compounds Using Magnetic Properties

Abstract: In the system Li-Mn-O, spinels Li1+xMn2-xO4 exist as a continuous series of solid solution in the range 0.00≤x≤0.33 [1]. It is generally accepted that Li-rich spinels behave with improved electrochemical performance but physical properties of such materials are rather sparse. In this work, we report on the local structure of Li1+xMn2-xO4 compounds using magnetic measurements. Magnetic measurements were made on SQUID magnetometer using a liquid helium cooled amplifier to measure the magnetic moment in the range from 10 to 300 emu. Two kinds of measurements were carried out in the temperature range 4–300K: M(H) in the field range 0-30 kOe and temperature dependence of magnetic susceptibility recorded during heating the samples in the zero–field cooled (ZFC) and field– cooled (FC) mode at magnetic field 10 kOe. The χ(T) curves and isothermal M(H) plots were correlated to achieve a complete analysis of magnetic properties of the Li1+xMn2-xO4 materials. The temperature dependence of the magnetization of LiMn2O4 samples measured under an applied field of 10 kOe is reported in Fig. 1. The main difference is observed in the temperature range 40–50 K for the samples sintered at 800 °C. The magnetization curves for these samples show that the anomaly corresponds to the onset of an extrinsic magnetic component which saturates very fast with the magnetic field, superposed to the paramagnetic intrinsic component. This is the fingerprint of the ferrimagnetic Mn3O4 impurity phase identified by its spin ordering temperature at 42 K [2]. The comparison at T=4 K between the low field magnetization in our sample and the magnetization of Mn3O4 [3] allows us to deduce the amount of this impurity phase in the sample, namely 0.5%. It could not be detected by the X-ray diffractometry. The sample LiMn2O4 sintered at 750 °C shows anomaly at low temperature where the ZFC magnetic susceptibility goes through a maximum. It could be the onset of some antiferromagnetic domains of finite size. This behavior is consistent with the magnetic diffused peak in the range 10–65 K observed in the neutron spectra of some Mn3O4 samples [4]. The Curie-Weiss law is approximately satisfied for all samples in the temperature range 200-300 K, from which we can find the Curie (C) and Weiss (Θ) constant and calculate effective magnetic moment (μeff). The Θ constant decreases almost linearly with x from -267 K for the LiMn2O4 sample heated at 800 °C to -13 K for x=0.25 and then vanishes at x=0.3. The effective magnetic moment decreases with x from 5.93 μB for stoichiometric sample heated at 800 °C with mixed valence state (Mn) to 4.38 μB for Li1.33Mn1.67O4 heated at 750 °C in which the Mn ions are only in the Mn valence state (Fig. 2). Calculated value of effective magnetic moment for Mn (3.39 μB) differs from theoretical one (3.87 μB), but is in good agreement with experimental data.

Electrochemical Features of Li-Ni-Mn-Co Oxides

Abstract: We report the electrochemical behavior of various layered oxides in Li cells. A series of LiNiyMnyCozO2 materials (with z=1-2y) was synthesized by “chimie douce” and investigated as positive electrodes in rechargeable lithium batteries. Electrochemical performances of LiNiyMnyCozO2 oxides are tested cell using non-aqueous 1M LiPF6 dissolved in EC-DEC. Charge discharge profiles are investigated as a function of the rate capability, the voltage window and the synthesis parameters of the cathode. A relation is found between the gravimetric capacity and the cation disorder of materials as indicated by magnetometry analysis.

Electronic Properties of MoO3 Thin Films Grown by PLD and ARE Techniques and Their Performance Upon Lithium Intercalation

Abstract: Molybdenum trioxide (MoO3) is one of the interesting materials for optoelectronic applications. From optical application point of view, MoO3 is considered as chromogenic material and is potentially useful in the fabrication and development of electrochromic devices, electronic information displays, and memory devices. In addition, MoO3 films exhibit gas sensing properties and find application in sensor device technology where the controlled growth, crystal structure, surface morphology, and electronic band structure of MoO3 thin films are believed to play a significant role.

Local Structure and Magnetic Properties of LixFePO4 Glasses

Abstract: A series of LixFePO4 samples (0=x=1) have been synthesized in the vitreous state by fast quenching technique. Characterization methods include XRD, TG, FTIR, magnetic susceptibility and electrical measurements. FTIR studies reveal that in the entire composition range the local structure is identical to the olivine lattice including PO4 3- oxo-anions linked to FeO6 octahedra. Magnetic susceptibility data show that the antiferromagnetic ordering does not exist so far and the paramagnetic behaviour follows the Curie-Weiss law above 30 K. Both the Curie parameter and the Weiss temperature suggest a strong weakness of the superexchange interactions that are consistent with the analysis of infrared spectra. Electrical conductivity shows an activated conduction mechanism with Ea =0.67 eV.

Synthesis, structure, and electrochemical properties of Li 4Ti5O12

Abstract: Lithium titanium oxide (Li4Ti5O12) spinels are promising negative electrode materials for application in energy technology. In this work, we have synthesized Li4Ti5O12 and investigated its structure, electronic properties, and electrochemical features using several analytical spectroscopy and microscopy techniques. The equally spaced lattice fringes obtained using by the high-resolution transmission electron microscopy (HRTEM) along with electron diffraction reveal that the grown Li4Ti5O12 is well crystallized in the spinel structure without any indication of crystallographic defects such as dislocations or misfits. The electronic structure determination using high-resolution X-ray photoelectron spectroscopy (XPS) coupled with compositional studies using energy dispersive X-ray spectrometry (EDS) indicate excellent chemical quality of the Li4Ti5O12. Under the optimal synthetic condition, the sample delivers a discharge capacity of 161 mAh/g at C/12. The good cyclability of Li4Ti5O12 is attributed to the small expansion (δV≈1%) of the elementary unit-cell.

A New Lithium Iron Phosphate LiFe2P3O10 Synthesized at 600 °C from Precursor Obtained by Wet Chemistry

Abstract: We present the synthesis and characterization of a novel lithium iron polyphosphate LiFe2P3O10 prepared by wet-chemical technique from nitrate precursors. The crystal system is shown to be monoclinic (P2(1)/m space group) and the refined cell parameters are a=4.596 angstrom, b=8.566 angstrom, c=9.051 angstrom and beta=9-97.46 degrees. LiFe2P3O10 has a weak anti ferromagnetic ordering below the Neel temperature T-N=19 K. Electrochemical measurements carried out at 25 degrees C in lithium cell with LiPF6-EC-DEC electrolyte show a capacity 70 mAh/g in the voltage range 2.7-3.9 V.

High Performance of Lithium Iron Phosphates for HEV with Quality Control Made by Magnetometry

Abstract: Optimized LiFePO4 positive electrode for Li-ion batteries was obtained after severe control of the fundamental properties of material. The nanoscopic structure and magnetic properties of a series of carbon-coated LiFePO4 particles prepared under various conditions were analyzed with XRD, FTIR, Raman and SQUID magnetometry. We evaluate intrinsic and extrinsic properties. The existence of low content of nano-sized ferromagnetic particles was evidenced by magnetic measurements in samples grown from iron(II) oxalate; such ferromagnetic clusters do not exist in the optimised samples grown from FePO4(H2O)2. Other impurity phases such as Fe2P, Li3Fe2(PO4)3, FeP2O7 were also detected for particular conditions of preparation. The impact of the carbon coating on the electrochemical properties is reported. Li-ion cells show excellent cyclability after 200 cycles at 60 °C without iron dissolution.