Yair Cohen

Carbon electrodes for double-layer capacitors. I. Relations between ion and pore dimensions

We characterized activated carbon electrodes for electrical double-layer capacitor (EDLC) systems. High-surface-area carbons were prepared by carbonization of cotton cloth at elevated temperatures (up to 1050°C), followed by activation at 900°C by oxidation with CO 2 during different time periods. Specific surface areas and characteristic pore sizes obtained from gas adsorption isotherms were correlated with those obtained from ion electroadsorption at the electrical double layer. Electrolytes studied included aqueous LiCI, NaCI, and KCl solutions and nonaqueous propylene carbonate solutions with LiBF 4 and (C 2 H 5 ) 4 NBF 4 salts. We found clear evidence that the porous carbons thus formed exhibit ion sieving properties, and that increasing activation time systematically increases the average pore sizes of these carbons. The electric double layer (EDL) capacity of these samples (calculated from voltammetric measurements) depends strongly on the adsorption interaction of the ions in the pores, and hence the relationship between the average pore size and the effective ion size determines the specific EDL capacitance of these samples. The following order of dimension of adsorbed species was found, based on the ion sieving of the various synthesized carbons of different average pore size N 2 ; Na + (aq); Cl - (3.6 A) < BF 4 - < TEA + (PC) < Li + (PC).

The Application of Atomic Force Microscopy for the Study of Li Deposition Processes

Li deposition on copper substrates was investigated using atomic force microscopy. The electrolyte solutions included LiClO{sub 4} and LiPF{sub 6} solutions in propylene carbonate. This work demonstrated the applicability of atomic force microscopy to the study of Li electrochemistry. It proved that the sensitive electrodes and solutions can be properly isolated from atmospheric contaminants and that surface scanning of the tip leaves the electrode`s morphology unchanged. Li deposition in LiPF{sub 6} solutions was found to be more uniform than Li deposition in LiClO{sub 4} solutions. This difference is attributed to the different surface chemistry developed in the two electrolyte solutions as known from previous studies. It was also possible to follow surface film formation on copper upon polarization to potentials higher than those of Li deposition, due to reduction of solution species at low potentials and precipitation of insoluble Li halides, LiOH-Li{sub 2}O and ROCO{sub 2}Li.

Morphological Studies of Li Deposition Processes in LiAsF6 / PC Solutions by In Situ Atomic Force Microscopy

The morphology of Li deposition-dissolution processes in propylene carbonate/LiAsF{sub 6} solutions was studied using in situ atomic force microscopy (AFM). Dry (20 to 30 ppm of water), water contaminated (200 ppm H{sub 2}O), and CO{sub 2} saturated solutions were tested. The substrates included copper foils, which are currently used as current collectors for Li electrodes and metallic lithium. The morphology of Li deposition in dry LiAsF{sub 6} is compared with that observed in LiClO{sub 4} and LiPF{sub 6} solutions. Basically, the morphology of Li deposition in LiAsF{sub 6} solutions is much smoother than in LiClO{sub 4} or LiPF{sub 6} solutions. The presence of water in solutions roughens the morphology of both the surface films formed on copper at low potentials and the Li layers deposited. The correlation between the previously known properties of the Li-solution interphase in these solutions and the present morphological observations is discussed. The results thus obtained further demonstrate the applicability of AFM for in situ morphological studies of highly reactive electrode surfaces.

The study of lithium insertion–deinsertion processes into composite graphite electrodes by in situ atomic force microscopy (AFM)

Li insertion–deinsertion into composite graphite electrodes, comprising synthetic graphite flakes (6 μm average size), polyvinylidene difluoride binder (PVdF), and copper current collectors, in commonly used alkyl carbonate solutions were studied by in situ atomic force microscopy (AFM). In this study, we were able to probe by in situ AFM the behavior of practical, composite graphite electrodes in ethylene carbonate–dimethyl carbonate (EC–DMC) solutions containing salts such as LiAsF6 and LiPF6 during entire lithiation–delithiation cycles. These in situ micro/nanomorphological studies could probe surface film formation on the graphite particles, as well as periodic volume changes in the graphite flakes during Li insertion–deinsertion cycles. These cyclic volume changes can explain the capacity fading of graphite electrodes upon prolonged cycling, in Li-ion batteries. While the overall morphology of these electrodes remains steady upon cycling in the appropriate solutions (in which the Li–C electrodes are efficiently passivated), there is a continuous problem in the extent of accommodation of the small volume changes in the graphite particles upon lithiation–delithiation, by the surface films. It is suggested that graphite electrodes fail during prolonged cycling due to small scale, continuous reactions of the active mass with solution species, which gradually increase their impedance and decrease the content of the lithium stored in the electrodes.

The use of a special work station for in situ measurements of highly reactive electrochemical systems by atomic force and scanning tunneling microscopes

In this article, we describe a special homemade workstation in which highly reactive electrochemical systems, such as lithium electrodes in polar aprotic systems, can be measured in situ by both atomic force microscopy and scanning tunneling microscopy (STM). The workstation includes an evacuable glovebox that maintains a pure atmosphere in which the microscopes are located, thus enabling measurements in a highly pure argon atmosphere. The system is based on a compact and functional evacuable glovebox which is placed in a special construction which provides full protection against vibrations. This is obtained by suspending the box by flexible cords during the experiments, while all the piping connections are removed. The concept of an evacuable glovebox, which can be back-filled by a pure atmosphere, enables measurements to be performed under a pure inert atmosphere, eliminating the need for noisy gas purification systems. Pure solutions and highly reactive electrode materials are introduced into this glo...

THE PREPARATION OF METAL-POLYMER COMPOSITE MATERIALS USING ULTRASOUND RADIATION

The propagation of ultrasound waves through fluid causes the formation of cavitation bubbles. 1 The collapse of these bubbles, described as an implos in the hot-spot theory, is the origin of extreme loca conditions: high temperatures (5000–25000 K) and hi pressures (1000 atm). 1 The cooling rates obtained during the collapse are greater than 10 7 Kys.2,3 These high cooling rates have been utilized by Suslick and his c workers in sonicating Fe(CO) 5 as a neat liquid or in solution,2,3 where amorphous iron nanoparticles were t sole reaction products. Suslick et al. have also prepared amorphous Co 4 and an amorphous Fe–Co alloy. 5 Following Suslick’s method, we have reported on bein able to control the particle size of the amorphous iro nanoparticles by varying the concentration of Fe(CO) 5 in its solution. 6 We have also prepared amorphous Ni 7 and amorphous Fe 2O3, all having nanometer size particles The application of high intensity ultrasound ra diation in polymer chemistry has been an activ research area. 9,10,11 Most of the published work in this area has been concerned with the degradation of polymer, where its molecular weight is reduced b sonication in dilute solutions. 10 The first polymerization reaction synthesis using ultrasound radiation was t of acrylonitrile in aqueous solution. 12 Two groups, Kruus and his co-workers and Price and his c workers, have published most of the work in this fiel Kruus has studied the polymerization of nitrobenzene 13

Simultaneous in-situ conductivity and cyclic voltammetry characterization of Li-ion intercalation into thin V2O5 films

Abstract Cyclic voltammetry (CV) and in-situ electronic conductivity curves have been simultaneously measured during Li-ion insertion/de-insertion into/from thin, partially crystallized, Li x V 2 O 5 film in 1 M LiBF 4 +propylene carbonate solution. A similarity has been observed between the CV curves and the plots of the derivative of conductivity, d σ /d E , versus potential. A semi-metal-to-insulator transition, and vice versa, has been observed (in intercalation and de-intercalation, respectively), ascribed to the e- to δ-phase transition of Li x V 2 O 5 . This transition is accompanied by an increase in the VV distance between adjacent VO 5 layers, and is probably responsible for the decrease in the film conductivity, realized via electron hopping between the high- and low-valence vanadium sites.

Atomic force microscopy study of the morphology of polythiophene films grafted onto the surface of a Pt microelectrode array

This work involves the application of atomic force microscopy (AFM) to the study of the morphology of polythiophene (PT) films grafted onto an array of six-band microelectrodes used for the measurements of in situ electronic conductivity of polymeric films. The following two types of the polymeric film can be distinguished: one film is related to the layer just above the microelectrodes, whereas the other type of film bridges the gap between the two microelectrodes. We have shown that the gaps located in the interior part of the array are completely bridged by the PT film; however, the thickness of the film above the microelectrodes is about one order of magnitude larger than that related to the center of the gap. The Z-profile of the film across the gap has a rounded shape. The lateral (friction) force image taken from both types of the PT films shows that they are similar. The AFM images prove that the PT film possesses mainly a granular-type morphology.