A. Mauger

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

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

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...

An electron paramagnetic resonance study of n-type Zn1−xMnxO: A diluted magnetic semiconductor

We present the results of an electron paramagnetic resonance study of homogeneous single phase Zn1−xMnxO thin films with Mn concentrations varying between x=0.07 and x=0.34. Our results show antiferromagnetic (AF) coupling in the entire concentration range with an effective exchange integral of J/kB=−21.8 K for x0.16 much stronger than in the CdMn(S,Se,Te) series. We observe deviations from the Curie-Weiss behavior for concentrations above x=0.16 and show this to be a “universal” behavior of II-VI diluted magnetic semiconductors. Our results demonstrate that AF interactions are dominating in n-type Zn1−xMnxO (x>0.07) with a carrier concentration of 1018 cm−3 contrary to previous claims. These AF interactions are responsible for high spin freezing temperatures and absence of magnetic long range order.

Critical review on lithium-ion batteries: are they safe? Sustainable?

The goal of this critical review is to explain why the safety problem raised by the lithium batteries must be considered. The performance of the batteries with different chemistries is compared and analyzed, with emphasis on the safety aspects, in addition to the electrochemical properties of the cells. Problems encountered with cathode materials (layered compounds, spinel and olivine), anode materials (graphite and lithium titanate), electrolytes, lithium salts, and separators are pointed out. In this critical review, we also discuss the place of the lithium batteries in the context of sustainable energies (electric vehicles, smart grid).

A quasi-exact formula for Ising critical temperatures on hypercubic lattices

We report a quasi-exact power law behavior for Ising critical temperatures on hypercubes. It reads JkBTc = K0[(1 − 1/d)(q−1)]a where K0 = 0.8633747, d is the space dimension q the coordination number (q = 2d), J the coupling constant kB the Boltzman constant and Tc the critical temperature. Absolute errors from available exact estimates (d=2−7) are always less than 0.0005. Extension to other lattice is discussed.

Magnetic properties of CeyFe4−xNixSb12

The structural and magnetic properties of the metallic skutterudite CeFe4Sb12 have been investigated, with those of CeyFe4−xNixSbz with compositions approaching the relations y=(4−2x)/3y=(4−2x)/3, z=12z=12 for which the materials are semiconductors. The large and temperature-dependent contribution of Fe to the magnetic susceptibility of CeFe4Sb12 has been isolated from the Ce contribution, which makes possible a quantitative analysis of the Kondo lattice behavior. In CeFe4Sb12 the Wilson–Sommerfeld ratio is modified by a factor two with respect to former analyses where the magnetic contribution of Fe had been neglected. The main effect of the reduction of the free carrier (hole) concentration in CeyFe4Sb12 is to suppress the Kondo effect and the Fe contribution to the magnetic susceptibility. The samples with theoretical hole concentration per unit cell p∗=z−8−3y−2xp∗=z−8−3y−2x smaller than 0.5 behave like semiconductors in which the exchange between localized Ce3+ spins is negligible, and are most promising for a potential use as thermoelements.

Topology invariance in percolation thresholds

Abstract:An universal invariant for site and bond percolation thresholds (pcs and pcb respectively) is proposed. The invariant writes n{pcs}1/as {pcb}‒1/ab = δ/d where nas, ab and δ are positive constants, and d the space dimension. It is independent of the coordination number, thus exhibiting a topology invariance at any d. The formula is checked against a large class of percolation problems, including percolation in non-Bravais lattices and in aperiodic lattices as well as rigid percolation. The invariant is satisfied within a relative error of n±5% for all the twenty lattices of our sample at d=2, d=3, plus all hypercubes up to d=6.

Pure Phase Disordered LiMn1.45Cr0.1Ni0.45O4 by a Post-Annealing Treatment

A new post-annealing treatment at ca. 600 °C was used to modify the oxygen deficiency during the synthesis of the spinel LiMn1.45Cr0.1Ni0.45O4 cathode for Li-ion batteries. It is an effective way to eliminate the impurity phase without changing the crystal chemistry. Small amount of substituting Cr leads to better rate performance and cyclability at room temperature, compared to commercial LiMn1.5Ni0.5O4. LiMn1.45Cr0.1Ni0.45O4 delivered a reversible capacity of 104 mAh g-1 at 1C rate. After 125 cycles, about 99% of reversible capacity was retained for the LiMn1.45Cr0.1Ni0.45O4 in contrast with 6% capacity loss for the commercial LiMn1.5Ni0.5O4. Electrochemical impedance spectroscopy measurements revealed that the LiMn1.45Cr0.1Ni0.45O4 had a smaller surface resistance, which may be due to the segregation of Ni from the surface to the bulk.

Electrochemical features of Li-Ni-Mn-Co oxides

We report the electrochemical behavior of various layered oxides in Li cells. A series of LiNi y Mn y Co z O 2 materials (with z =1-2 y ) was synthesized by “chimie douce” and investigated as positive electrodes in rechargeable lithium batteries. Electrochemical performances of LiNi y Mn y Co z O 2 oxides are tested cell using non-aqueous 1 M LiPF 6 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.

Structural, Magnetic and Electrochemical Properties of the Spinel LiMn 2-y Co y O 4 Nanosized Powders

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.