Small magnetic polaron effect in lithium iron phosphates

The small polarons in LiFePO₄ are associated with the presence of Fe³⁺ ions introduced by the native defects in relative concentration in the samples known to be optimized with respect to their electrochemical properties. The nearest iron neighbours around the central polaron site are spin-polarized by the indirect exchange mediated by the electronic charge in excess. These small magnetic polarons are responsible for the interplay between electronic and magnetic properties that are quantitatively and self-consistently analysed. Comparison is made with other magnetic polaron effects in other members of the family of magnetic semiconductors to which this material belongs. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 September 2007.     

Some structural features of MoO3-1,10-phenanthroline intercalation compounds

Three MoO₃-1,10-phenanthroline intercalation compounds were synthesized at room temperature, by suspending α-MoO₃ in a water-ethanol solution of 1,10-phenanthroline. The obtained hybrid matrices were characterized by elemental analysis, infrared spectroscopy, thermogravimetry, X-ray diffractometry and scanning electron microscopy. Based on X-ray diffraction data, can be verified that ate two possible orientations of 1,10-phenanthroline into MoO₃ interlayer space: an orientation to allow the interaction of the nitrogen atoms with the acidic Lewis sites on the oxide surface and another orientation to allow the interaction of the π electrons with such sites The SEM micrographs show that as the 1,10-phenanthroline total amount is increased, there is a progressive exfoliation of the oxide grains, resulting in the total disruption of the initial platy morphology.     

Local structure and electrochemistry of LiNi y Mn y Co1 − 2y O2 electrode materials for Li-ion batteries

A series of LiNi _x Mn _y Co _z O₂ (x = y, z = 1 − 2y) oxides have been synthesized by “chimie douce” and investigated as positive electrodes in rechargeable lithium batteries. Layered LiNi _y Mn _y Co_1 − 2y O₂ materials with high homogeneity and crystallinity were synthesized using the wet-chemical method assisted by carboxylic acid as the polymeric agent. The long range and local structural properties are investigated with experiments including X-ray diffraction, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy. The evolution of the structure is discussed as a function of the cobalt content that confers layer-like behavior on the framework. Electrochemical performance of LiNi _y Mn _y Co_1 − 2y O₂ oxides is tested in cells using nonaqueous 1 M LiPF₆ dissolved in ethylene carbonate–diethyl carbonate. Charge–discharge profiles are investigated as a function of the rate capability and the voltage window. A relation is found between the gravimetric capacity and the cation disorder of the positive electrode as indicated by structural analysis. Fast lithium extraction attributed to the larger interslab space has been observed in the cobalt-rich oxides. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007.     

Experimental routes to in situ characterization of the electronic structure and chemical composition of cathode materials for lithium ion batteries during lithium intercalation and deintercalation using photoelectron spectroscopy and related techniques

Three different experimental routes to in situ characterization of electronic structure and chemical composition of thin film cathode surfaces used in lithium ion batteries are presented. The focus is laid on changes in electronic structure and chemical composition during lithium intercalation and deintercalation studied by photoelectron spectroscopy and related techniques. At first, results are shown obtained from spontaneous intercalation into amorphous or polycrystalline V₂O₅ thin films after lithium deposition. Although this technique is simple and clean, it is nonreversible and only applicable to the first lithium intercalation cycle into the cathode only to be applied to host materials stable in the delithiated stage. For other cathode materials, as LiCoO₂, a real electrochemical setup has to be used. In our second approach, the experiments are performed in a specially designed electrochemical cell directly connected to the vacuum system. First experimental results of RF magnetron sputtered V₂O₅ and LiCoO₂ thin film cathodes are presented. In the third approach, an all solid-state microbattery cell must be prepared inside the vacuum chamber, which allows electrochemical processing and characterization by photoelectron spectroscopy in real time. We will present our status and experimental difficulties in preparing such cells.     

Dip-coated silver-doped V2O5 xerogels as host materials for lithium intercalation

Vanadium pentoxide xerogels have shown high electrochemical performance in terms of energy content. The high specific energy and high intercalation capability make the materials promising for thin film lithium battery and electrochromic device applications. In order to enhance the rate capabilities of the host we increased the electronic conductivity by doping the V₂O₅ xerogels with silver. Samples were prepared by mixing various amounts of silver powder with V₂O₅ hydrogel. We were able to prepare silver-doped vanadium pentoxide dip-coated thin films with a molar ratio (Ag/V) ranging from 0.005 to 0.5 (Ag_yV₂O₃ with y = 0.01, 0.1 and 1). With the successful doping, the electronic conductivity of V₂O₅ was increased by 2 to 3 orders of magnitude. The insertion capacity of the material was maintained and up to 4 moles of lithium per mole of silver-doped V₂O₅ (XRG) were found to be reversibly intercalated.     

Nanoscopic scale studies of LiFePO4 as cathode material in lithium-ion batteries for HEV application

We present a review of the structural properties of LiFePO₄. Depending on the mode of preparation, different impurities can poison this material. These impurities are identified and a quantitative estimate of their concentrations is deduced from the combination of X-ray diffraction analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and magnetic measurements. An optimized preparation provides samples with carbon-coated particles free of any impurity phase, insuring structural stability and electrochemical performance that justify the use of this material as a cathode element a new generation of lithium secondary batteries.     

Insertion properties of LiFe<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>PO<Subscript>4</Subscript> electrode materials for Li-ion batteries

This paper addresses the synthesis structural and electrochemical properties of LiFe_0.5Mn_0.5PO₄ electrode materials for Li-ion batteries. The charge–discharge reaction of Li/LiPF₆-EC–DEC/LiFe_0.5Mn_0.5PO₄ cell carried out at the 1-C rate shows a capacity retention of 128 mAh/g. The local structure of the delithiated Li _x Fe_0.5Mn_0.5PO₄ phases have been studied by Fourier transform infrared spectroscopy and magnetometry. Spectral features indicate that the structure of the delithiated phase remains in the orthorhombic system. The compositional dependence of the magnetic moment is found to be in quantitative agreement with the theoretical value predicted for oxidation of M ²⁺ ions in the high spin state. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007     

Magnetic susceptibility of mixed valence compounds

Magnetic susceptibility of mixed-valence compounds has been calculated as a function of pressure using Falicov-Kimball model wherein thef-s hybridisation has been taken into account. This model can very well explain the continuous as well as discontinuous transitions from ground states of integral to intermediate valence. Our results for magnetic susceptibility are in qualitative agreement with recent experimental results on some mixed-valence compounds.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Silver intercalation in PbS1.18(TiS2)n, n=1, 2, misfit layer compounds

The reaction between metallic Ag and PbS_1.18(TiS₂)_n, n=1, 2, misfit layer compounds has been investigated by electrochemical technique, X-ray powder analysis and transmission electron microscopy. It was found that silver intercalation is possible only in the compound with n=2. The thermodynamic behavior and location of phase boundaries were studied in the temperature range 400–650 K. DC-conductivity and magnetic-susceptibility measurements were performed, and the data can be interpreted as an appearance of small polarons during silver insertion.     

Synthesis, structure characterization and magnetic properties of nanosize LiCo1 − y Ni y O2 prepared by sol–gel citric acid route

Single phase LiCo_1 − y Ni _y O₂ (y = 0.4 and 0.5) with fine particles and high homogeneity was synthesized by “chimie douce” assisted by citric acid as the polymeric agent and investigated as positive electrodes in rechargeable lithium batteries. The long-range and short-range structural properties are investigated with experiments including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and superconducting quantum interference device magnetometry. The physicochemical properties of the powders (crystallinity, lattice constants, grain size) have been investigated in this composition. The powders adopted the α-NaFeO₂ structure as it appeared from XRD and FTIR results. Magnetic measurements shows signal at low temperature attributed to the magnetic domains in the nanostructure sample from which we estimated that the cation mixing are 3.35 and 4.74% for y = 0.4 and 0.5 in LiCo_1 − y Ni _y O₂, respectively. LiCo_0.5Ni_0.5O₂ cathode yields capacity (135 mAh g⁻¹) compared to LiCo_0.6Ni_0.4O₂ cathode (147 mAh g⁻¹) when discharged to a cutoff voltage of 2.9 V vs. Li/Li⁺. Lower capacity loss and higher discharge efficiency percentage are observed for the cell of LiCo_0.6Ni_0.4O₂ cathode.     

Magnetic properties of mechanically alloyed Co–Ti powder

An amorphous phase containing traces of non-transformed Co and Ti powders was obtained by mechanical alloying nominal compositions of Co₆₇Ti₃₃ and Co₅₀Ti₅₀ in a high-energy ball-mill. These alloys were prepared from elemental powders of Co and Ti. The heat treatment of Co₆₇Ti₃₃ at 573, 873 and 1173 K crystallized nanoparticles of Co₂Ti and Co₃Ti compounds, while the same treatments conducted on Co₅₀Ti₅₀ resulted in the formation of Co₂Ti and CoTi nanoparticles. The saturation magnetizations reached a maximum value in the amorphous state and they decreased when the temperatures of the heat treatment rose. Demagnetizing interparticle interaction effects were estimated through hysteresis loops and initial magnetization curves using the Fourier technique.     

Magnetic and microstructural properties of the Ti-doped Barium hexaferrite

A study of the magnetic and microstructural properties of the M-type Ti⁴⁺-doped Barium hexaferrite according to the stoichiometric formulation BaFe_{(12−(4/3)x)}Ti_xO₁₉ with x=0, 0.6, 0.8 and 1.0, has been reported. The XRD and magnetic analysis show a variation of the host lattice parameters and a decrease in the values of remnant magnetization and magnetic anisotropy with Ti⁴⁺ content. The behavior of magnetic properties of materials is explained by the combined effect of the coherent rotation of the magnetic domains and the replacements of Fe³⁺ by Ti⁴⁺ ions in the octahedral and tetrahedral sites.     

Magnetic and optical properties of nitroxide radicals and their lanthanide complexes

This paper reports the structural, magnetic and optical properties of three series of lanthanide complexes [Ln(radical)₄](ClO₄)₃, [Ln(radical)₂(NO₃)₃] and [Ln(radical)(hfac)₃] (Ln=Gd(III), La(III) or Eu(III)) with nitronyl or imino nitroxide radicals.The magnetic properties of the gadolinium complexes were studied. Along the series, most gadolinium(III) complexes exhibit antiferromagnetic Gd^III-radical interaction. These results are discussed.The full absorption and luminescence spectra of some lanthanide complexes and their uncoordinated free radical ligands were measured. The rich vibronic structure in luminescence and absorption spectra indicates that several excited states define the absorption spectra between 400 and 800 nm. Qualitative trends can be established between magnetic ground state properties and the energies and vibronic structure of the title compounds.     

Magnetic properties of Mn doped BN compound

Electronic and magnetic properties of diluted B_1−xMn_xN alloys are calculated by means of the full potential linearized augmented plane wave (FP-LAPW) method and the generalized gradient approximation (GGA). A half-metallic state is predicted for a composition of 6.25%. The spin majority being metallic and minority being semiconducting. We found a total magnetic moment of 2 μ_B (Bohr-magnetons) per supercell, in agreement with the half-metallic behaviour. The main contribution of the cell magnetic moment is localized at the transition metal site Mn, with a local moment of 1.24 μ_B.     

Volume properties and compressibility of the graphite-potassium-mercury compounds

Abstract

The volume properties of graphite intercalation compounds (GIGS) C4KHg and of the initial intermetallic compounds KHg and KHg₂ have been investigated in the piston-cylinder apparatus, using the direct volumetric technique, under pressures up to 25 kbar. The compounds, average compressibility K₊, was determined to be 3.8×10^?3 kbar^?1 for C₄KHg, 3.0×10^?3 kbar-^?1 for C₈KHg, 4.8×10^?1 kbar^?1 for KHg, and 4.0 ×10^?1 kbar^?1 for KHg₂ at pressures of 0-20 kbar. The compressibility of the “two-dimensional” KHg layer in the GIC under various pressure conditions has been estimated. These estimates permit comparison of KHg properties in the “three dimensional” and “quasi-two dimensional” states. It was concluded that the influence of the graphite matrix on the intercalant is insignificant for this type ternary GIC.