A voltammetric study concerning the structural stability of Li-overstoichiometric Mg-doped LiCoO2 powders

The effect of Mg-doping and Li overstoichiometry on the structural stability of LiCoO₂ powders has been investigated with emphasis to voltammetric properties. Microparticle cyclic voltammetry (CV) conducted in caustic NaOH to best simulate a non-aqueous electrolyte shows a marked improvement of the structure stability of doped LiCoO₂. In contrast to the unsubstituted LiCoO₂ sample which shows voltammetric peaks associated to the well-known two-phase domain and monoclinic distortion reactions, in Li_1.08Mg_0.06CoO₂, LiMg_0.06CoO₂ and Li_1.08CoO₂ samples these peaks are strongly suppressed providing direct evidence for the existence of a stable solid solution with negligible phase transitions in the reversible intercalation region (3.8-4.2 V vs. Li) as well as in the overcharged region. The effect is higher with Mg-doping, irrespective of the Li overstoichiometry. However, the concomitant presence of Mg and Li excess in the structure is important for obtaining small particle sizes. Since Mg-doping induces a quasi metallic behavior in the samples, whereas the Li excess may provide an higher initial capacity, it is suggested that the Li_1.08Mg_0.06CoO₂ composition may be of interest as positive cathode for advanced Li-ion batteries.     

Long-term cyclability of nanostructured LiFePO4

Amorphous LiFePO₄ was obtained by lithiation of FePO₄ synthesized by spontaneous precipitation from equimolar aqueous solutions of Fe(NH₄)₂(SO₄)₂·6H₂O and NH₄H₂PO₄, using hydrogen peroxide as oxidizing agent. Nano-crystalline LiFePO₄ was obtained by heating amorphous nano-sized LiFePO₄ for different periods of time. The materials were characterized by TG, DTA, X-ray powder diffraction, scanning electron microscopy (SEM) and BET. All materials showed very good electrochemical performance in terms of energy and power density. Upon cycling, a capacity fading affected the materials, thus reducing the electrochemical performance. Nevertheless, the fading decreased upon cycling and after the 200th cycle the cell was able to cycle for more than 500 cycles without further fading.     

Adaptive pattern recognition with rotation, scale, and shift invariance

Mutually orthogonal pattern distortions are handled by an adaptive optical recognition system. A double-channel system is presented that implements pattern recognition with rotation, scale, and shift invariance. The recognition process is based on a two-stage operation: An object-independent determination of one distortion parameter (the scale, in the example presented here) is performed, and then the recognition is completed by a shift- and rotation-invariant optical correlator that is adapted to the measured parameter. Thus, complete invariance to three distortion parameters is achieved by the combination of two channels. The overall process is performed efficiently and can be executed in real time.     

Far-infrared absorption by phonons and librons in solid H2 and D2 in the ordered state

We present an experimental study of the infrared absorption spectra of solid, enriched ortho-hydrogen and para-deuterium in the ordered state for concentrations x of J = 1 molecules between 0.65 and 0.98 and temperatures from 0.9 to 11K. Comparison is made to theoretical expressions and numerical results for the absorption due to various microscopic processes, involving phonons and combinations oflibrons and phonons, which couple to the dipole moment induced by the electric quadrupole (EQ) fields of J = 1 molecules. The concentration dependence of the sharp, low-frequency optical phonon has also been studied. The integrated intensity of this feature is found to be quadratic in x and the frequency is well described by (x) = a + bx ²,which is the expected behavior for shifts arising from pairwise EQQ interactions. For constant x, the temperature dependence of the intensity of this phonon scales with the square of the long-range orientational order parameter. Our measurements of this quantity are compared to NMR determinations of the temperature dependence of the Pake splitting in the ordered state.Supported financially by the Stichting FOM.     

An improved experimental equation of state of solid hydrogen and deuterium

We have analyzed a large set of data in the literature as well as new data of our own to provide an improved equation of state of solid para-hydrogen and ortho-deuterium, with pressures ranging from 0 to 25 kbar (at the melting line). Results, including pressure, bulk modulus, and thermal expansion, are tabulated for a dense set of molar volumes as a function of temperature.Partial financial support provided by the Stichting FOM.     

Prospects for Bose-Einstein condensation in atomic hydrogen and other gases

Current experimental efforts to observe Bose-Einstein condensation in the laboratory are reviewed. New experiments in spin-polarized hydrogen at high density or very low temperatures offer promise of achieving this goal. The possibility of BEC in other elements, H₂,⁴He, and cesium are discussed.     

Morphological investigation of sub-micron FePO4 and LiFePO4 particles for rechargeable lithium batteries

Microstructural variations of amorphous FePO₄ and LiFePO₄ (the latter obtained by chemical lithiation of the former) as a result of the annealing temperature have been studied by Thermogravimetric Analysis (TGA)/Differential Thermal Analysis (DTA), chemical analysis, Brunauer-Emmet-Taylor (BET) and Scanning Electron Microscopy (SEM) techniques. Round-shaped amorphous FePO₄ particles 40-80 nm in size are obtained after heating (at 400 °C) amorphous FePO₄·2H₂O in air (previously prepared by a precipitation route). On further heating at 650 °C, in air, crystalline trigonal FePO₄ of crystallite size <200 nm is formed. Round-shaped amorphous LiFePO₄ particles 40-80 nm in size crystallize by heating at 550 °C in Ar+5%H₂ for 3 h. After thermal treatment, LiFePO₄ particles are interconnected by necks, which resembled a sintering process. The particle size of LiFePO₄ increases with an increase of temperature up to 750 °C, but an abnormal growth is evident at annealing temperatures above 650 °C. DTA analysis showed two exothermic peaks at 547 and 768 °C for FePO₄ due to phase transitions, whereas for LiFePO₄ two exothermic effects at 496 and 567 °C are shown.