Computer modeling of negative electrode operation in lithium-ion battery: Model of equal-sized grains,galvanostatic discharge mode,calculation of characteristic parameters

A computer simulation of the negative electrode (anode) operation in a lithium-ion battery is performed. A complete research program is carried out in accordance with the recommendations of the theory of porous electrodes: the “model of equal-sized grains of two types” was studied, percolation properties of the anode active layer were researched, values of effective coefficients were calculated for charge transfer and mass transport, a complete system of equations describing operation of the anode is presented. Two specific cases of galvanostatic mode of anode discharge are considered in detail: an “ideal” anode and anode with nanosize particles. Working anode parameters are calculated: optimum bulk concentration of graphite in the active layer, active layer thickness, time of complete anode discharge, its specific electric capacitance and final potential on the active/layer interelectrode space interface. Advisability of working with anodes with nanosize grains and electrolyte with enhanced specific conductivity is shown.     

Computer simulation of negative electrode operation in lithium—ion battery: Galvanostatic discharg active intercalating agent grains,role of diffusion limitations

Computer simulation of the structure and methods of operation (galvanostatic discharge) of the negative electrode of a lithium-ion battery is performed. Two possible models of the active anode layer were compared. 1. The model of porous active layer (mixture of active substance grains with grains of electrolyte). Here, the electrochemical process occurs within a porous active layer. 2. The film model (constant-thickness layer) of pure active substance (intercalating agent) grains without admixture of grains of electrolyte. In this case, the electrochemical reaction occurs only on the planar active electrode layer/interelectrode space interface. In both cases, the optimum working parameters of anode active layers were calculated: porous active layer thickness (in the film model, this was the calculation parameter), duration of full anode discharge, specific electric capacitance and finite difference between the intercalating agent/electrolyte potentials at the active anode layer/interelectrode space interface. It is found that each of these two models has its advantages and faults. Specific electric capacitance C cannot exceed the values of the order of magnitude of 10 C/cm² when a porous active layer is used. Whereas in the film model, much higher values of C may be obtained: tens and even hundreds of C/cm². On the other hand, in the case of anode discharge, the reasonable discharge current density value, its maximum value, at which practically full recovery of lithium atoms from active intercalating agent grains is still possible, proves to be by orders of magnitude higher in the case of an anode with a porous active layer, as compared with a film-type anode. Thus, in the case of development of electrode active layers of lithium-ion batteries, there is a possibility of choosing from two variants. There is the variant of an active film-type layer providing high capacitance values, but low discharge current density. Or there is another variant: a porous active layer with limited capacitance but then much higher values of discharge current density.     

Computer simulation of negative electrode operation in lithium—ion battery: Galvanostatic discharge,porous electrode model and film model

Computer simulation of the structure and methods of operation (galvanostatic discharge) of the negative electrode of a lithium-ion battery is performed. Two possible models of the active anode layer were compared. 1. The model of porous active layer (mixture of active substance grains with grains of electrolyte). Here, the electrochemical process occurs within a porous active layer. 2. The film model (constant-thickness layer) of pure active substance (intercalating agent) grains without admixture of grains of electrolyte. In this case, the electrochemical reaction occurs only on the planar active electrode layer/interelectrode space interface. In both cases, the optimum working parameters of anode active layers were calculated: porous active layer thickness (in the film model, this was the calculation parameter), duration of full anode discharge, specific electric capacitance and finite difference between the intercalating agent/electrolyte potentials at the active anode layer/interelectrode space interface. It is found that each of these two models has its advantages and faults. Specific electric capacitance C cannot exceed the values of the order of magnitude of 10 C/cm² when a porous active layer is used. Whereas in the film model, much higher values of C may be obtained: tens and even hundreds of C/cm². On the other hand, in the case of anode discharge, the reasonable discharge current density value, its maximum value, at which practically full recovery of lithium atoms from active intercalating agent grains is still possible, proves to be by orders of magnitude higher in the case of an anode with a porous active layer, as compared with a film-type anode. Thus, in the case of development of electrode active layers of lithium-ion batteries, there is a possibility of choosing from two variants. There is the variant of an active film-type layer providing high capacitance values, but low discharge current density. Or there is another variant: a porous active layer with limited capacitance but then much higher values of discharge current density.     

Active layer of an electrode with polymer electrolyte: Estimation of platinum utilization degree

A specific feature of the electrode active layer with polymer electrolyte consists in the fact that the current generation process can occur only on the condition of the direct contact of the catalyst support (carbon black) particles with Nafion. However, in reality, the support particle agglomerates (grains) contact the Nafion particle agglomerates (grains). Therefore, one must expect a low catalyst (platinum) utilization degree. A hypothesis is offered that a fractal film of Nafion is formed on the surface of the support grain pores in the case of manufacturing the “catalytic ink” used to form the active layer. It can significantly increase the platinum utilization degree. A detailed computer simulation of the process of Nafion penetration into the support grain pores is performed. Factors are established allowing reaching a high platinum utilization degree. The data of computer simulation agree with the experimental estimates of platinum utilization degree.     

A facile electrochemical synthesis of covellite nanomaterials at room temperature

A novel method for the electrochemical synthesis of covellite (CuS) nanoparticles (NPs) in aqueous phase was developed. In this experiment, thioglycerol (TG) is used as the catalyst for the hydrolysis of sodium thiosulfate, the sulfur source for the synthesis of CuS. Cu foil, which acts as the sacrificing anode, is oxidized to Cu²⁺ by applying a potential of 0.5 V while OH^- was produced on the cathode surface at the same time. The production of OH^- facilitates the reaction between Cu²⁺ and thiosulfate under the catalysis of TG. The evolution of hydrogen bubbles effectively prevents the deposition of copper sulfide on the cathode. Copper sulfide sols of “golden-brown”, and “dark-green” forms can be obtained by varying the concentration of TG. The “golden-brown” copper sulfide sols are also observed to convert to the green form with time, and the rate of this conversion process is faster at higher temperatures. X-ray diffraction (XRD) and chemical analysis indicate that the “dark-green” form of product is pure hexagonal phase CuS. The obtained CuS NPs were covered by a layer of TG as suggested by Fourier transform infrared (FTIR) data. The size and morphology of the particles are studied by transmission electron microscope (TEM).     

Small-angle X-ray scattering study of a poly(oxyphenylethylene)–poly(oxyethylene) diblock copolymer gel under shear flow

 The structure and flow behaviour of a micellar “cubic” phase is studied, using small-angle X-ray scattering (SAXS) and constant stress rheometry on a poly(oxyphenylethylene)–poly(oxyethylene) diblock copolymer in water. The predominant structure is a face-centred cubic (fcc) array of spherical micelles, which under shear undergoes layer sliding to give a scattering pattern from stacked hexagonal close-packed layers. A detailed analysis of the SAXS data indicates the presence of a fraction of grains with a structure distorted from a fcc phase. The additional reflections that characterize this structure can be indexed to a rhombohedral unit cell, space group R3ˉm, with the same volume as the fcc unit cell. The rhombohedral unit cell corresponds to a cubic cell that has been “stretched” along a [111] direction, and it is suggested that such a structure results from the gradient in shear velocity in the Couette cell employed. Shearing at high shear rates leads to a “smearing out” of the reflections, but upon cessation of shear under these conditions a highly oriented SAXS pattern is obtained, which confirms the persistence of rhombohedral ordering. The shear-induced changes in orientation are correlated to a plateau observed in the stress plotted against shear rate, such a plateau being a sign of inhomogeneous flow. Received: 8 September 2000 Accepted: 29 November 2000     

A bidimensional simulation of particle-cluster aggregation with variable active sites particles

 In this work a simple program has been developed which simulates the process of particle– cluster aggregation limited by diffusion. All the simulation have been carried out using 2d square lattices with square “particles” having a variable number of active inter-action sites (from 3 to 8) for each particle in order to analyze the effect of such limitation on the fractal dimension of the aggregates. The fractal dimension of such aggregates was calculated by the so-called “box counting” method. It has been shown that there is no change in the value of the fractal dimension (1.70) as the active site number is increased. Instead it appears that there is an average number of active sites of about 2.3 for all the structures no matter how many active interaction sites the particles have. This appears as an interesting result in connection with the aggregation of particles such as renneted casein micelles, which could present differences in the surface density of active sites. Received: 11 February 1997 Accepted: 8 January 1998     

On the application of the modeling of the relaxation spectrum to the prediction of linear viscoelastic properties of surfactant systems

 In this work, the linear viscoelastic properties of cetyl-trimethylammonium tosilate–water system are predicted by the modeling of the relaxation spectrum. The modeled spectrum of relaxation times is of the “wedge-box” type where the “wedge” portion is located at the short-time scale of relaxation times and the “box” part covers the long-time scale. The linear viscoelastic properties are calculated through the exact relationships with the suggested spectrum. Agreement between the calculated expressions and experimental data of the moduli and the stress relaxation function is found. Comparison is also made with predictions of the empirical expressions of the Cole–Cole and the Williams–Watts models. Received: 17 December 1996 Accepted: 1 July 1997     

Photoreduction of Ag<Superscript>+</Superscript> cations in a perfluorosulfonic membrane

The two-stage synthesis of nanosize silver in the pore space of a perfluorosulfonic membrane by ion-exchange fixation of Ag⁺ cations with the subsequent photoreduction in formaldehyde vapor was fulfilled. The study of the obtained composites shows that they contain silver “near-wall” form and nanoparticles weakly connected with the membrane surface.     

Computer simulation of positive electrode operation in lithium-ion battery: Optimization of active mass composition

The work of the positive electrode (cathode) of a lithium-ion battery is simulated. The model of equally sized grains of three types: the intercalating agent grains with a volume fraction g, the electrolyte grains with a volume fraction g _i, and the carbon black grains with a volume fraction g _e is studied. The optimal composition of cathode active mass providing maximum specific capacity of cathode is determined. It is shown that a fraction of carbon black grains should be as small as possible: g _e = 0.35. The variation in the fraction of intercalating agent grains within the allowable limits (0 ?? g ?? 0.3) changes the main parameters of cathode active mass: a fraction of electrochemically active intercalating agent grains g* (g* < g); a specific surface area S, on which the electrochemical process proceeds; and the conductivity k* by lithium ions in the ionic percolation cluster, which forms in the cathode active mass. The parameters g* and S decrease and parameter k* steeply increases with decreasing g. Therefore, in the range of possible values of g, specific capacity of cathode reaches the maximum value at g = g _opt. The value of g _opt is determined under the galvanostatic mode of cathode discharge. The cathode working parameters: the active layer thickness, discharge time, specific capacity, and potential at the cathode active layer/interelectrode space interface at the instant of discharge completion are calculated in relation to a fraction of intercalating agent grains g.     

Characterization of a covalent triazine-humic acid conjugate by gas chromatography

The aim of the described method was the characterization of a “atrazine-mercaptopropionic acid” humic acid conjugate, which was required for the calibration of immunoassays to determine bound residues. In order to estimate the amount of bound triazine, an oxidative nucleophilic substitution reaction of the covalently linked “atrazine-mercaptopropionic acid” to a new triazine derivative “atrazine-methoxyethanol” was performed. This cleavage product was quantified by gas chromatography. The conditions for this cleavage procedure were optimized and applied to the “atrazine-mercaptopropionic acid” humic acid conjugate and to a humic acid blank. The amount of bound “atrazine-mercaptopropionic acid” was calculated to 16.6 ± 2.5 μmol triazine per gram humic acid, which is equivalent to 0.39 ± 0.07% atrazine. Received: 7 August 1997 / Accepted: 12 September 1997     

Multivariate statistical analysis of coastal sediment data

Multivariate statistical analysis of sediment data (information matrix 123 × 16) from the Gulf of Mexico, USA shows that the data structure is defined by four latent factors conditionally called “inorganic natural”, “inorganic anthropogenic”, “bioorganic” and “organic anthropogenic” explaining 39.24%, 23.17%, 10.77% and 10.67% of the total variance of the data system, respectively. The receptor model obtained by the application of the PCR approach makes it possible to apportion the contribution of each chemical component for the latent factor formation. A separation of the contribution of each chemical parameter is achieved within the frames of “natural” and “anthropogenic” origin of the respective heavy metal or organic matter to the sediment formation process. This is a new approach as compared to the traditional “one dimensional” search with a limited number of preliminary selected tracer components. The model suggested divides natural from anthropogenic influences and allows in this way each participant in the sediment formation process to be used as marker of either natural or anthropogenic effects. Received: 20 March 1999 / Revised: 1 June 1999 / Accepted: 3 June 1999     

The effects of pretreatment of polycrystalline gold with OH<Superscript>•</Superscript> radicals on the electrochemical nucleation and growth of platinum

The nucleation and growth of platinum on polycrystalline gold was studied by chronoamperometry, cyclic voltammetry, and atomic force microscopy before and after treatment of the gold surface with hydroxyl (OH^•) radicals. Two different procedures of mechanical polishing of the gold surface (“coarse polish” and “fine polish”) were applied before the treatment with OH^• radicals. The nucleation and growth of Pt was much better reproducible on electrodes which underwent a “coarse polish”. The treatment of the Au surface with OH^• radicals decreased the number of active sites; however, the nucleation growth mode remained the same (3-D instantaneous). The spontaneous Pt deposition (no externally applied potential) on Au was unaffected by the treatment with OH^• radicals. In situ atomic force microscopy experiments showed that the Pt starts to grow only on some of the Au grains, most probably on those which have active sites on their surface. This leads to a roughening of the electrode surface upon Pt deposition. Treatment with OH^• radicals did only quantitatively diminish the amount of deposited Pt, but qualitatively the imaging of the Pt growth remained the same. Obviously, the OH^• radicals lead to a knockout (decreasing number) of active sites for Pt nucleation, while the nature of the remaining active sites stays unaffected.     

The use of small computers in large-scale activation analysis

This paper describes simple algorithms for evaluating gamma-ray spectra, readily applicable to small computers. A comparative assessment of the accuracy of the existing programs for experimental data analysis is presented. The use of “Nairi-K” computers suitable for “on-line” operation with multichannel analyzers enables as many as 300 element determinations to be carried out in a 7-hour working day.     

Multivariate statistical analysis of coastal sediment data

Multivariate statistical analysis of sediment data (information matrix 123 × 16) from the Gulf of Mexico, USA shows that the data structure is defined by four latent factors conditionally called “inorganic natural”, “inorganic anthropogenic”, “bioorganic” and “organic anthropogenic” explaining 39.24%, 23.17%, 10.77% and 10.67% of the total variance of the data system, respectively. The receptor model obtained by the application of the PCR approach makes it possible to apportion the contribution of each chemical component for the latent factor formation. A separation of the contribution of each chemical parameter is achieved within the frames of “natural” and “anthropogenic” origin of the respective heavy metal or organic matter to the sediment formation process. This is a new approach as compared to the traditional “one dimensional” search with a limited number of preliminary selected tracer components. The model suggested divides natural from anthropogenic influences and allows in this way each participant in the sediment formation process to be used as marker of either natural or anthropogenic effects. Received: 20 March 1999 / Revised: 1 June 1999 / Accepted: 3 June 1999     

Multireference perturbation CI I. Extrapolation procedures with CAS or selected zero-order spaces

We discuss the “three class” approximation to full multireference perturbation CI, which greatly reduces the computational effort by restricting the summation of diagrams to determinants belonging to a subspace of the zero-order space. In the framework of the CIPSI algorithm, we propose a new extrapolation procedure allowing recovery of the full “two class” results. The new procedure is applied to complete active spaces (CAS) and to individually selected zero-order spaces. Comparison with a full two class calculation on a CAS shows a reduction of computer time of one or two orders of magnitude in the tests presented here, with an accuracy in the order of 0.1 kcal/mol. Our procedure can thus compete with the CASPT2 algorithm, specifically conceived to deal with CAS. In the case of selected zero-order spaces, the speed-up is less dramatic but the method still retains its advantages. Received: 12 June 1997 / Accepted: 31 July 1997     

Danube River Water Data Modelling by Multivariate Data Analysis

 A data set (48×19) consisting of Danube river water analytical data collected at Galati site, Romania, during a four-year period has been treated by principal components analysis (PCA). The PCA indicated that seven latent factors (“hardness”, “biochemical”, “waste inlets”, “turbidity”, “acidity”, “soil extracts” and “organic wastes”) are responsible for the data structure and explain over 80 % of the total variance of the system. Its complexity is further proved by the application of multiple linear regression analysis on the absolute principal components scores (APCS) where the contribution of each natural or anthropogenic sources in the factor formation is shown. The apportioning makes clear that each variable participates to a different extent to each source and, in this way, no pure natural or pure anthropogenic influence could be determined. No specific seasonality for the variables in consideration is found. Received January 24, 2001. Revision July 6, 2001.