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The reductive decomposition of carbonate electrolyte solutions containing lithium ions was studied by differential electrochemical mass spectrometry (DEMS) and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS). The influence of the composition of the negative electrode on its electrochemical behavior was investigated by varying the graphite/binder ratio and using nongraphitic materials. The effect of different electrolyte salts was studied in a 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) mixture. In addition, EC/DMC-based electrolyte solutions containing different amounts of water were used to investigate the influence of H20 on the performance of graphite electrodes. Ethylene and hydrogen were the volatile decomposition products monitored by DBMS. Ethylene evolution is restricted to a potential region between about 0.8 and 0.3 V vs Li/Lit and occurs only during the first cycle.Hydrogen begins to evolve at about 1.3 V vs Li/Lit in the first cycle and the amount of gas decreases with increasing cycle number. The experimental results are discussed in terms of proposed reaction mechanisms.

Safety aspects of lithium-ion batteries are widely discussed nowadays. However, the prediction and understanding of the behavior of lithium-ion batteries upon overcharge are still a major challenge because all commonly used lithium metal oxides become highly oxidizing when delithiated, and decomposition of the electrolyte salt and/or solvent can be initiated.It is generally believed that one of the most important prerequisites for good cycling stability of Li-ion batteries is the formation of a complete and stable passivation film on the negative electrode during the initial charge/discharge cycles. The quality of this protective film strongly depends on the nature of the electrolyte solvents and also on the cycling conditions, and has been extensively investigated during the past few years. 1 In contrast, the existence of similar film formation processes at the positive electrode is still in question, although the oxidative electrochemical stability of a broad variety of organic electrolyte solvents has also been investigated in depth. 2 However, many of these studies were performed at model metal electrodes, and care has to be taken when applying such experimental results to real battery electrodes because the surfaces may exhibit different electrocatalytic activities.Organic carbonate solvents are thought to release CO 2 upon oxidation. 3 Such a behavior was observed for propylene carbonate (PC), which decomposes at platinum with the evolution of CO 2 at potentials as low as 2.1 V vs. Li/Li ϩ . The decomposition potential and the amount of CO 2 evolved depend strongly on the cycle number and water content of the electrolyte solvent. 4,5 An approach coming closer to real electrodes was recently chosen by Kanamura et al. 6 They deposited thin layers of LiCoO 2 or LiMn 2 O 4 onto current collectors and used subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) to detect potential-dependent changes at the electrode/electrolyte interface. However, due to experimental problems they could not determine whether or not CO 2 is evolved at these lithium metal oxide electrodes. Aurbach et al. detected Li 2 CO 3 on the surfaces of LiNiO 2 , LiCoO 2 , and LiMn 2 O 4 , and attributed this finding to a reaction of the pristine oxides with CO 2 of the air. 7 In addition, they suggested a mechanism based on the dissolution and migration of electrolyte decomposition products formed at the negative electrode, in order to account for the detection of lithium alkylcarbonates on the positive electrode after prolonged cycling.We recently studied the reductive decomposition of PC and ethylene carbonate (EC)/dimethylcarbonate (DMC) based electrolytic solutions by differential electrochemical mass spectrometry (DEMS) 8,9 in order to improve our understanding of the degradation paths of these solvents at real Li-ion battery electrodes. We demonstrated that this in situ technique gives valuable potential-dependent information about volatile decomposition products evolved during these complex electrode rea...

The vibrational structure of the highly symmetrical octahydridosilasesquioxane has been investigated in detail, and a harmonic force field in terms of internal force constants has been determined, based on extensive IR and FT-Raman data and on a normal coordinate analysis of H~SisOl2 and DsSigOl2. Group frequencies have been assigned according to a potential energy analysis, and relations to group frequencies of comparable silicon compounds have been discussed. A step by step procedure starting from Oh-H~Sig012 to the frameworks Oh-(-O)8SigO12, D4h-(-0)8Si~012, D4h-(=Si0)8Si8012, and finally to Dw(*0)8T8012, T = Si or A1 and Si/Al = 1, has turned out to be an excellent, enlightening approach for qualitatively and quantitatively describing the vibrational structure of the zeolite A framework. NIR FT-Raman spectra of Na+-exchanged zeolite Y and of Na+-and Ag+-exchanged zeolite A have been measured and compared with each other. An improved force field is reported, and a correlation of the experimental and the calculated IR and Raman spectra of zeolite A has allowed one to assign group frequencies to all fundamental modes. All fundamentals between 11 10 and 950 cm-l belong to antisymmetric T-0-T stretching vibrations while the symmetric T-O-T stretching modes are at 860-830 cm-l, 740-680 cm-l, and 610-570 cm-l. The fundamentals between 490 and 100 cm-l can be described as 0-T-0 bendingvibrations-with the exception of a double four ring and two sodalite cage breathing modes and the 8-ring pore opening-whereas the T-O-T bending vibrations and the torsional modes are below 100 cm-1.

Glassy carbon (GC) is a well known material frequently used in analytical electrochemistry. Numerous investigations of the catalytic surface properties of activated GC have been reported. 1 The GC electrodes can be activated along different routes such as wet chemical, dry chemical, or electrochemical oxidation. Laser activation has also been suggested by Pontikos and McCreery. 2 The numerous possibilities for carbon surface activation have been described in articles and book by McCreery 3 and by Kinoshita. 4 In electrochemical activation, a number of choices exist as to how to modify the surface, such as galvanostatic, potentiostatic, or cyclic polarization in various electrolytes. Film growth by cycling and the corresponding optical properties of the active layer were investigated earlier in our laboratory using spectroscopic ellipsometry. 5 Electrochemical double-layer capacitors (EDLC), also called supercaps or ultracaps, utilize high-surface area electrodes in order to achieve a high-double-layer capacitance. Three main categories of electrode materials typically are used in these EDLCs, viz., carbons, polymers, and metal oxides. 6 For noble metal oxides such as RuO 2 specific capacitance of more than 700 F/g was reported, 7 but these materials are generally considered as being too costly. Redox active polymer films are also considered to be potential electrode materials for EDLCs, 8 but most of them are rather slow, and their long-term stability and cycle life are still uncertain. High-surface area carbons are relatively inexpensive EDLC electrode materials with a relatively high specific capacitance of up to 100 F/g. 9 Therefore, in most of the capacitors available today, carbon materials are used for the electrodes. Problems still arise from the contact resistance between carbon powder particles and from that between the active layer and the current collector sheet. Metal particles or fibers have been added to the carbon powder in order to overcome the grain-to-grain resistance. 10 The use of GC for electrochemical EDLCs was suggested about 20 years ago in a patent by Miklos et al. 11 Activation of the GC surface was attained by gas-phase oxidation at elevated temperatures. Electrochemical activation was not considered in that patent.Several advantages are expected from modified glassy carbon when this is used as an electrode material in EDLCs. It is a reasonably good electronic conductor (200 S/cm) 1,12 and can therefore also be used as the current collector. In addition, GC is impermeable to gases and ions, so that a bipolar plate/electrode assembly (BPEA) can be created by simply modifying a glassy carbon sheet on both sides. 13 However, when working with GC one has to be aware of the fact that depending on the precursor material and temperature used during the pyrolysis process, rather different kinds of GC exist. 1 Properties such as the conductivity, density, reactivity, number, and diameter of internal closed pores, etc. are determined by the pyrolysis temperature. GC is still considered an expen...

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The making and breaking of σ bonds is an integral part of almost all photochemical reactions. Yet, the electronic states of σ electrons are not nearly as well understood as the states of π-electron systems. Efforts in our laboratory to enhance the current state of their understanding are described, using the specific example of oligosilanes. We address the intrinsically cyclic nature of σ delocalization and its dependence on chain length and conformation, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated heptasilane. We also describe a new low-energy luminescence from certain conformers of permethylated oligosilanes.