New type of the nanostructured composite Si/C electrodes

The silicon-carbon composite films, with the silicon and carbon relative content from 39.5: 60.5 to 87: 13 and thickness 100–480 nm, are prepared by magnetron sputtering with layer-by-layer deposition of the components. The film structure is studied by using X-ray diffraction analysis and atomic-force microscopy. All studied films were found to be roentgen-amorphous. Despite the films were deposited in a layer-by-layer mode, they have granular structure, with the granules sized 10–80 nm. The lithium incorporation from LiClO₄ solution in propylene carbonate-dimethoxyethane mixture is studied. All studied films reversibly incorporate lithium; thus, they can serve as the basis for negative electrodes of lithium-ion batteries. The initial capacity of the composite-film electrodes is 1.5 to 2.8 A h/g. As the films are submerged to cycling, the capacity decreases, mainly due to the films’ insufficiently strong adhesion to the substrate, which results in the film defoliation. The most cycling-resistant are the thinnest films containing no less than 30% carbon. The capacity of the best samples is as high as 1 A h/g after 200 cycles.     

Combined Method for Reducing Irreversible Capacity of the Negative Graphite Electrode in Lithium-Ion Battery

A method for the complete removal of the irreversible capacity of the negative electrode of lithium-ion batteries is suggested. The method consists of bringing the graphite electrode in contact with metallic lithium in the electrolyte. The distinguishing feature of the modified method is that all operations (assembling and filling cells, storing electrodes) are performed in a carbon dioxide atmosphere. Performing all these operations in a carbon dioxide atmosphere improves the reproducibility of the method. The balance between the weight of lithium and graphite, which is optimum for the complete removal of the irreversible capacity, is calculated. It is shown that applying the combined method does not lead to a decrease in the reversible capacity of the negative electrode.     

Temperature dependency of the intrinsic carrier density of hydrogenated amorphous silicon in MOS structures

The admittance versus frequency of a hydrogenated amorphous silicon metal oxide semiconductor capacitor is measured at a fixed bias in inversion and for temperatures in the range of 20–50 °C. The data are fitted to theoretical capacitance and conductance curves where the time constant of inversion is the result of the fit. In turn, the time constant can be converted to the (minority) carrier lifetime so that a lifetime value for each measurement temperature is available. The conversion from the time constant to the minority carrier lifetime requires the knowledge of the temperature-dependent intrinsic carrier density or rather its activation energy. The criterion for the correct choice is a temperature-independent carrier lifetime. Three published room temperature values of the intrinsic carrier density have been tested. The carrier lifetime activation energy is E _a = 0.70 ± 0.03 eV. Received: 17 June 1998 / Accepted: 23 October 1998     

Effect of Temperature on Reversible and Irreversible Processes during Lithium Intercalation in Graphite

Effect of temperature on reversible and irreversible processes during lithium intercalation in graphite from 1 M LiClO₄ solution in PC–DME is studied by galvanostatic cycling, cyclic voltammetry, and impedance spectroscopy. Reducing temperature diminishes both reversible and irreversible capacities. Conditions for the passive-film formation on graphite are discussed. If several first cycles are run at a negative temperature, the overall charge spent irreversibly decreases if the temperature is then elevated. The lower the initial-cycling temperature, the smaller the overall irreversible capacity.     

A highly cross-linked polymeric binder for high-performance silicon negative electrodes in lithium ion batteries

A support bandage for electrodes: A cross-linked polymeric binder inhibits mechanical fracture of silicon negative electrodes during cycling. Nanosized silicon powder with a 3D interconnected network of poly(acrylic acid) and sodium carboxymethylcellulose as binder exhibits high reversible capacity of over 2000?mAh?g(-1) after 100 cycles at 30?°C while maintaining a high capacity and high current density.     

Irreversible processes during the lithium intercalation into graphite: the passive film formation

Comparative studies of three type of carbonaceous materials—the modified oxidized graphite, thermoexpanded graphite, and carbon paper—prior to and after galvanostatic cycling in 1 M LiClO₄ solution in propylene carbonate-dimethoxyethane mixture are carried out using standard porosimetry. It was shown that the mean (effective) thickness of the passive film [solid electrolyte interface (SEI)] at the electrodes of the modified oxidized graphite and thermoexpanded graphite equals a few nanometers. The comparison of porosimetric and electrochemical data shows that the passive film comprises both lithium carbonate and alkylcarbonates. Additionally, this comparison allows corroborating the concept on the formation of polymer (or oligomer) component of the passive film at least at the thermoexpanded graphite electrodes.     

Deposition of silicon nitride thin films by reaction of SiH4 in a nitrogen post-discharge

Silicon nitride thin films are deposited on silicon wafers at room temperature when silane gas is injected in a nitrogen flowing post-discharge. Reactive processes involving siane molecules and long-lifetime nitrogen species are studied, pointing out the nonreactivity of the N₂(A³ _u ⁺ ) metastable state, the low contribution of the vibrationally excited nitrogen ground-state molecules, and the high reactivity of N(⁴S) atoms. Spectroscopic observations performed in the reaction region are correlated with thin-film characteristics.     

p－Si上电沉积Ni－W合金薄膜

利用恒电流沉积法，在ｐ－Ｓｉ上制备出不同Ｗ含量和不同结构的Ｎｉ－Ｗ薄膜，研究了镀液温度，ｐＨ值，电流密度对镀层组成的影响，结果表明，提高温度有利于获得高Ｗ含量的合金．随着镀层中Ｗ含量的增加，Ｎｉ面心立方晶格ｆ．ｃ．ｃ发生正畸变，晶粒平均尺寸变小，当Ｗ含量达到５６％以上时，晶粒小于２ｎｍ，薄膜呈非晶态结构．     

p—Si上电沉积Ni—W合金薄膜

利用恒电流沉积法，在ｐ－Ｓｉ上制备出不同Ｗ含量和不同结构的Ｎｉ－Ｗ薄膜，研究了镀液温度，ｐＨ值，电流密度对镀层组成的影响，结果表明，提高温度有利于获得高Ｗ含量的合金。     

Resolving the different silicon clusters in Li12Si7 by 29Si and (6,7)Li solid-state NMR spectroscopy

Structural signatures: The analysis of Si-Si and Si-Li connectivities by solid-state NMR spectroscopy allows the different types of silicon clusters to be discriminated in the model lithium silicide compound Li(12)Si(7) (see picture, Si clusters red and blue, Li ions gray). The results provide new NMR spectroscopic strategies with which to differentiate and study the structures formed in silicon-based electrode materials.     

Elimination of irreversible capacity of amorphous silicon: direct contact of the silicon and lithium metal

A method of elimination of the amorphous silicon irreversible capacity is suggested, which is based on the direct contact of the silicon and lithium metal under electrolyte. It is shown that this contact yields a solid-electrolyte film over the electrode surface even prior to its initial cathodic polarization, which results in the elimination of the irreversible capacity of amorphous silicon.     

Lithium insertion into silicon films produced by magnetron sputtering

Using the method of magnetron-plasma sputtering of polycrystalline silicon target, amorphous silicon films 32–214 nm thick were produced on various (copper and titanium, polished and rough) substrates. A study of their charge-discharge characteristics under the galvanostatic conditions showed that all thin-filmed electrodes are capable of reversible lithium insertion. The amount of lithium inserted in the first cycles is close to the theoretical one. An analysis of composition and morphology of surface layer and also the behavior of reversible and irreversible capacities during cycling showed that the degradation of capacity is caused by the exfoliation of films from the substrate (the effect is more pronounced for the specimens with polished substrates) and somewhat breaking (cracking) of films. The thicker are the films, the severer is the disruption of silicon films in the cycling. The adhesion of films to the substrate surface is favored by the film roughness. At sufficiently high adhesion of films, their electrochemical properties only slightly depend on the nature (copper or titanium) of substrate.     

Lithium intercalation in thin amorphous-silicon films

Electrochemical intercalation of lithium in thin films of amorphous hydrogenated silicon (a-Si:H), deposited at temperatures of 100 and 250°C on stainless-steel substrates, is studied. It is shown that the discharge capacity of films of identical thicknesses manufactured at a temperature of 250°C is greater than that for films produced at of 100°C. Dependence of the discharge capacity of the films manufactured at 250°C on their thickness is examined. It is established that an increase in the film thickness leads to acceleration of the decrease in the discharge capacity in the course of cycling. At a current density of 0.175 mA cm^?2, the discharge capacity of films 0.25 and 1.35 μm thick equals nearly 2 Ah g^?1 in a third cycle, whereas in a hundredth cycle it amounts to 1.10 and 0.37 Ah g^?1, respectively. The diffusion coefficient for lithium in the films is equal to ～10^-13 cm²s^?1.     

Endogenous and exogenous hydrogen influence on amorphous silicon thin films analysis by pulsed radiofrequency glow discharge optical emission spectrometry

During the last decade the photovoltaic industry has been growing rapidly. One major strategy to reduce the production costs is the use of thin film solar cells based on hydrogenated amorphous silicon (a-Si:H). The potential of pulsed radiofrequency glow discharge coupled to optical emission spectrometry (rf-PGD-OES) for the analysis of such type of materials has been investigated in this work. It is known that when hydrogen is present in the argon discharge, even in small quantities, significant changes can occur in the emission intensities and sputtering rates measured. Therefore, a critical comparison has been carried out by rf-PGD-OES, in terms of emission intensities, penetration rates and depth resolution for two modes of hydrogen introduction in the discharge, manually external hydrogen in gaseous form (0.2% H₂–Ar) or internal hydrogen, sputtered as a sample constituent. First, a comparative optimisation study (at 600 Pa and 50 W) was performed on conducting materials and on a silicon wafer varying the pulse parameters: pulse frequency (500 Hz–20 kHz) and duty cycle (12.5–50%). Finally, 600 Pa, 50 W, 10 kHz and 25% duty cycle were selected as the optimum conditions to analyse three types of hydrogenated samples: an intrinsic, a B-doped and a P-doped layer based on a-Si:H. Enhanced emission intensities have been measured for most elements in the presence of hydrogen (especially for silicon) despite the observed reduced sputtering rate. The influence of externally added hydrogen and that of hydrogen sputtered as sample constituent from the analysed samples has been evaluated.     

Irreversible deformation of macromolecular networks

Based upon a thermodynamical approach, the generalized Onsager type of relaxation of van der Waals networks is presented. By linearly and identically coupling the set hidden variables to the network, the memory function of the system can be related to the equilibrium strain-energy function. The relaxation behavior of real networks on stretching can quantitatively be described by means of a distribution of relaxation times known from small strain experiments. Some new and interesting conclusions are discussed as to how the macroscopically non-linear visco-elastic response might be interpreted.     

Parameters for Irreversible Inactivation of Monoamine Oxidase

The irreversible inhibitors of monoamine oxidases (MAO) slow neurotransmitter metabolism in depression and neurodegenerative diseases. After oxidation by MAO, hydrazines, cyclopropylamines and propargylamines form a covalent adduct with the flavin cofactor. To assist the design of new compounds to combat neurodegeneration, we have updated the kinetic parameters defining the interaction of these established drugs with human MAO-A and MAO-B and analyzed the required features. The K_i values for binding to MAO-A and molecular models show that selectivity is determined by the initial reversible binding. Common to all the irreversible inhibitor classes, the non-covalent 3D-chemical interactions depend on a H-bond donor and hydrophobic-aromatic features within 5.7 angstroms apart and an ionizable amine. Increasing hydrophobic interactions with the aromatic cage through aryl halogenation is important for stabilizing ligands in the binding site for transformation. Good and poor inactivators were investigated using visible spectroscopy and molecular dynamics. The initial binding, close and correctly oriented to the FAD, is important for the oxidation, specifically at the carbon adjacent to the propargyl group. The molecular dynamics study also provides evidence that retention of the allenyl imine product oriented towards FADH⁻ influences the formation of the covalent adduct essential for effective inactivation of MAO.     

Lithium intercalation into amorphous-silicon thin films: An electrochemical-impedance study

Lithium intercalation into 0.25-μm-thick films of amorphous silicon is studied using the electrochemical-impedance technique. An equivalent circuit, proposed for such electrodes, comprises the electrolyte resistance and three units connected in series, each unit being a parallel combination of a resistance and a constant-phase element. The units relate to the charge transfer processes at the silicon/electrolyte interface, charge transfer though the passive film on the silicon, and the lithium diffusion into the silicon bulk. During potential cycling, changes occur largely in the unit related to the passive film. The lithium diffusion coefficient in the amorphous silicon is estimated as ～ 10^?13 cm² s^?1.