Yu. Rosenberg

The role of anion additives in the electrodeposition of nickel–cobalt alloys from sulfamate electrolyte

Nickel–cobalt alloys have been deposited from sulfamate electrolyte with acetate and citrate-anion additives and evaluated for structure and properties, such as microhardness, tensile strength, internal stress and high-temperature oxidation. XRD data show that at low Co content, the alloys exhibit face-centered cubic (fcc) growth orientations. Above 60% Co, the deposit is completely hexagonal close packed (hcp) with pronounced (100) and (110) lines. It seems likely that the Ni–Co deposits from typical sulfamate electrolyte at pH 5, as well as at current density higher than 5 A/dm2, include metal hydroxides. This is followed by the formation of a more strained structure. The high-temperature oxidation rate of the Ni–Co coating from sulfamate electrolyte at pH 5 is twice that of the alloy deposited from the electrolyte with anion additives. We believe that, citrate complexes of Ni and Co, which are assumed to be involved in alloy deposition, eliminate the incorporation of hydroxides into the deposits and enable low-internal-stress coating. The anion-modified bath offers stability of structure and properties of the alloy over a wide range of acidity and current density.

STRAIN ENERGY DENSITY IN THE X-RAY POWDER DIFFRACTION FROM MIXED CRYSTALS AND ALLOYS

A correlation between precise x-ray powder diffraction patterns and atomic size mismatch in disordered mixed crystals (alloys and ionic crystals) is observed. The anisotropy of the elastic moduli has been taken into account for evaluation of the strain energy density of the mixed crystals revealed in x-ray powder diffraction measurements. The precursor of the phase transformation for a quenched disordered Au-Cu alloy is identified.

Tin Alloy-Graphite Composite Anode for Lithium-Ion Batteries

A composite anode material was prepared that contains nanosize (< 100 nm) particles of tin alloy Sn 65 Sb 18 Cu 17 and Sn 62 Sb 21 Cu 17 . The alloys were electroplated at high current densities (above i L ) from aqueous solutions, directly onto the copper current collector, and were coated by a polyvinylidene fluoride-graphite matrix at a ratio of alloy:graphite matrix 70:30 and 80:20 w/w, respectively. The processes involved in electrode production by this method are inexpensive, simple, and fast. Over 40 (100% depth of discharge) cycles were demonstrated, in half-cell, and over 30 were demonstrated with a LiCoO 2 battery containing 1 M LiPF 6 ethylene carbonate-diethyl carbonate electrolyte. The faradaic efficiency (Q De-ins /Q Ins ) is less than 100%. Lithium is fully deinserted from the host matrix only when the anode is cycled at low current densities. The kinetics of lithium insertion to and deinsertion from the composite anode material, slow gradually as the cycle number increases. X-ray diffraction patterns of the anode material show that the alloy becomes amorphous during cycling, while the graphite does not. X-ray photoelectron-spectroscopy measurements reveal that the solid electrolyte interphase consists of mainly LiF, small amounts of Li 2 O, and possibly, polymeric substances. The electrochemical behavior of the alloy changes with cycle number, while that of the graphite does not. The fall of the deinsertion capacity of the graphite from the first cycle to the 34th by more than 50% proves that the active material in the anode suffers from particle-to-particle break off.

Lithium-7 NMR studies of concentrated LiI/PEO-based solid electrolytes

Abstract Highly concentrated polymer electrolytes based on poly(ethylene oxide) (PEO) and LiI, with EO/Li ratio ≤3, were investigated by differential scanning calorimetry (DSC), powder X-ray diffraction (XRD) and 7Li solid state nuclear magnetic resonance (NMR) methods. The effect of 15-nm particle size Al2O3 additives and in several cases, other constituents ethylene carbonate and poly(methylmethacrylate) on structure and Li+ ion environment was explored. The addition of Al2O3 suppresses the formation of crystalline phases, including free LiI, which is present in EO/Li=1.5 samples without Al2O3. The conductivity jump observed in these concentrated electrolytes at around 80°C is correlated with an NMR-observed transition to a Li+ environment which is similar to that of free ions in a molten phase.

Conduction mechanisms in concentrated LiI-polyethylene oxide-Al{sub 2}O{sub 3}-based solid electrolytes

The ionic conductivity of concentrated LiI-polyethylene oxide P(EO){sub n} high surface area oxide composite polymer electrolytes has been investigated. Two different Arrhenius dependences for concentrated composite polymer electrolytes (CPEs) have been identified. The first one is characterized by an inflection point at about 80 C, and the second, by a conductivity jump. The authors have suggested that in CPEs, where 3T{sub k}orT{sub jump}) and on Ea have been studied (T{sub jump}=temperature of the conductivity jump). The addition of small quantities of ethylene carbonate, poly(methyl methacrylate), and polyacrylonitrile were found to be beneficial while poly(methyl acrylate), poly(butyl acrylate), and poly(vinylidene fluoride) additions made the polymer electrolyte stiffer and less conductive. MgO, Al{sub 2}O{sub 3}, and potassium aluminosilicate muscovite mica based CSEs have similar conductivity. Results clearly demonstrated the depression of CPE crystallinity by addition of fine Al{sub 2}O{sub 3} powder, ethylene carbonate, and poly(ethylene glycol) dimethyl ether, in agreement with the conductivity enhancement of the CPE.

A new approach to the understanding of ion transport in semicrystalline polymer electrolytes

Despite the conventional wisdom that ion transport in polymer electrolytes is mediated primarily by polymer segmental motion, we have performed measurements on semicrystalline complexes of poly(ethylene oxide) (PEO) with LiI which suggest that transport occurs preferentially along the PEO helical axis, at least in the crystalline phase. The principal basis for this claim is an observed enhancement by a factor of 5–20 in electrical conductivity in stretched polymer electrolyte films. The effect of uniaxial stress on the polymer electrolyte long and short range structures was investigated by X-ray diffraction and 7Li nuclear magnetic resonance (NMR) spectroscopy. Stretching results in partial alignment of the PEO helices and also induces small but observable changes in the Li+ solvation sheath. These results are correlated with the ionic conductivity enhancement in the stretched polymer.

Host-Guest Interactions in Single-Ion Lithium Polymer Electrolyte

The complex interplay between the ionic conductivity and structure of the LiCF 3 SO 3 :PEO polymer electrolyte, induced by the calix[6]pyrrole anion receptor, has been investigated by different experimental methods, including ac impedance, calorimetry, X-ray diffraction, and Fourier-transform infrared spectroscopy. It was found that calix[6]pyrrole, even at low concentrations, can form stable bidentate complexes with triflate anions, thus making cation transport dominating, which results in t + close to unity. It should be noted that the incorporation of an anion trap does not suppress the bulk ionic conductivity of polymer electrolytes at above 60°C. We attribute this effect to the dissociation of ion aggregates and structural changes imposed by the additive. Stable solid electrolyte interface resistance was achieved in the polymer electrolytes containing calix[6]pyrrole.

Stretching-induced changes in ion-polymer interactions in semicrystalline LiI-P(EO) n polymer electrolytes

Ionic conductivity, which is the most important polymer electrolyte (PE) property, is considered to be higher in the totally amorphous matrix, and ion transport is assumed to be mediated primarily by the motion of polymer segments. Despite this conventional wisdom, we suggest that fast ion transport occurs preferentially along the PEO helical axis, at least in the crystalline phase. In this work, we have studied the effect of hot and room-temperature stretching on the structural properties, ion–polymer interactions and ionic conductivity in dilute and concentrated LiI:P(EO)n (3VnV100) PEs. SEM and XRD data show evidence of the formation of a more oriented polymer-electrolyte structure. Significant changes in the FTIR spectra of the diluted LiI:P(EO)n electrolytes are found for the skeletal vibration mode of the C–O–C groups. The effect of stretching on the FTIR spectra of concentrated PEs was found to be less pronounced than that of the dilute PEs. The stretching process was found to influence the conductivity in the direction of the force more strongly than does an increase in temperature. D 2002 Elsevier Science B.V. All rights reserved.

Behavior of solid lubricant nanoparticles under compression

Inorganic fullerene-like materials have been identified as being of potentially utmost importance for many industrial applications. MoS2 and WS2 hollow nanoparticles have been identified as strong candidates for tribological applications such as solid lubricants. The main goal of this work was to evaluate the mechanical properties of solid lubricant particles in ensemble under hydrostatic pressure. The behavior of nanopowders under compression has been described on the basis of constitutive models of continuum mechanics. The model will be applied to an isotropic compaction of copper (well-studied medium), fullerene-like (IF-WS2) nanoparticles and a natural powder of 2H-WS2 platelets. The morphology of individual nanoparticles and nanoparticle ensembles will be examined and discussed. Another aspect of this work was to study the applicability and limitations of the proposed constitutive model for the understanding of the tribological behavior of solid lubricant nanoparticles. Compression with the maximal pressure (500 MPa) showed that the shape of the IF nanoparticles is preserved. The dominant mechanism of damage was found to be the delamination or peeling-off of the external sheets of hollow nanoparticles. Strong destruction of 2H-WS2 platelets was observed under compression.

Microstructure and phase characterization of diamond-like amorphous hydrogenated carbon films using STM/STS

Scanning tunneling microscopy and spectroscopy (STM/STS) are used to obtain nanoscale information on morphological and electronic properties of the surface of diamond-like amorphous hydrogenated carbon (a-C:H) films. The films are prepared by r.f. plasma decomposition of methane CH4. A two phase model of a-C:H involving sp2 clusters, embedded in a sp3-bonded matrix, is suggested. A new approach to a detection of graphite-like clusters at the surface of a-C:H films is proposed. An overlayer of indium tin oxide (ITO) which helps to detect graphite-like clusters is used. The ITO deposition is performed in the conditions which routinely cause ITO to grow as a good conductor with high electron density. The shape of current-voltage (I-V) characteristics obtained on the ITO/a-C:H, however, indicates nanoclusters of insulator within the matrix of the conductor. To explain the observed phenomenon the following results are considered. First, I-V characterization hints that thin films of ITO grown on weakly textured graphite normally have reduced electron density. Second, X-ray photoelectron spectroscopy measurements show that weakly textured graphite adsorbs oxygen much stronger than a-C:H. It is suggested, therefore, that it is oxygen, adsorbed by graphite-like clusters at the surface of a-C:H, which causes local drop of electron density in the ITO. As a consequence, I-V characterization of ITO/a-C:H can be used for obtaining a high resolution map of the location of graphite-like clusters over the a-C:H surface.