Solvents alkyl carbonates

Kinetic stability of lithium and the lithiated carbons results from film formation which yields protective layers on lithium or on the surfaces of carbonaceous materials, able to conduct lithium ions and to prevent the electrolyte from continuously being reduced film formation at the Li/PC interphase by the reductive decomposition of PC or EC/DMC yielding alkyl-carbonates passivates lithium, in contrast to the situation with DEC where lithium is dissolved to form lithium ethylcarbonate [149]. EMC is superior to DMC as a single solvent, due to better surface film properties at the carbon electrode [151]. However, the quality of films can be increased further by using the mixed solvent EMC/EC, in contrast to the recently proposed solvent methyl propyl carbonate (MPC) which may be used as a single sol-... 

In contrast to DEC/LiC104 solutions, where typical reaction products of carbonate solvents including alkyl carbonates, alkoxides, and Li2C03 are formed at the lithium surface, DEC/LiPF6 solutions yield LiF and Li20 only [168]. 

Ito and co-workers observed the formation of zinc bound alkyl carbonates on reaction of carbon dioxide with tetraaza macrocycle zinc complexes in alcohol solvents.456 This reversible reaction was studied by NMR and IR, and proceeds by initial attack of a metal-bound alkoxide species. The metal-bound alkyl carbonate species can be converted into dialkyl carbonate. Spectroscopic studies suggested that some complexes showed monodentate alkyl carbonates, and varying the macrocycle gave a bidentate or bridging carbonate. Darensbourg isolated arylcarbonate compounds from zinc alkoxides as a by-product from work on polycarbonate formation catalysis.343... 

The activated nickel powder is easily prepared by stirring a 1 2.3 mixture of NiL and lithium metal under argon with a catalytic amount of naphthalene (1(7 mole % based on nickel halide) at room temperature for 12 h in DME. The resulting black slurry slowly settles after stirring is stopped and the solvent can be removed via cannula if desired. Washing with fresh DME will remove the naphthalene as well as most of the lithium salts. For most of the nickel chemistry described below, these substances did not affect the reactions and hence they were not removed. The activated nickel slurries were found to undergo oxidative addition with a wide variety of aryl, vinyl, and many alkyl carbon halogen bonds. 

There is no question that the development and commercialization of lithium ion batteries in recent years is one of the most important successes of modem electrochemistiy. Recent commercial systems for power sources show high energy density, improved rate capabilities and extended cycle life. The major components in most of the commercial Li-ion batteries are graphite electrodes, LiCo02 cathodes and electrolyte solutions based on mixtures of alkyl carbonate solvents, and LiPF6 as the salt.1 The electrodes for these batteries always have a composite structure that includes a metallic current collector (usually copper or aluminum foil/grid for the anode and cathode, respectively), the active mass comprises micrometric size particles and a polymeric binder. 

Experimental phase diagrams of CO2 with ethyl acetate [37] and alkyl carbonates [38] are also available. Molecular simulations that describe the binary phases diagrams of many organic solvents with CO2 were also recently published [39], Binary phase diagrams of methanol with fluoroform are also available [40], At any given pressure and temperature, fluoroform is more miscible with methanol than CO2. Ternary phase diagrams for a number of systems are also available, such as methanol/H20/C02 [41] acetonitrile/H20/C02, and methanol/H20/fluoroform [40]. 

In nonaqueous solvents based on mixed alkyl carbonates, LiPFe remains one of the most conducting salts. For example, in EC/DMC (1 1) the conductivity is 10.7 mS cm only fractionally lower than that of... 

Aurbach and co-workers performed a series of ex situ as well as in situ spectroscopic analyses on the surface of the working electrode upon which the cyclic voltammetry of electrolytes was carried out. On the basis of the functionalities detected in FT-IR, X-ray microanalysis, and nuclear magnetic resonance (NMR) studies, they were able to investigate the mechanisms involved in the reduction process of carbonate solvents and proposed that, upon reduction, these solvents mainly form lithium alkyl carbonates (RCOsLi), which are sensitive to various contaminants in the electrolyte system. For example, the presence of CO2 or trace moisture would cause the formation of Li2COs. This peculiar reduction product has been observed on all occasions when cyclic carbonates are present, and it seems to be independent of the nature of the working electrodes. A single electron mechanism has been shown for PC reduction in Scheme 1, while those of EC and linear carbonates are shown in Scheme 7. ... 

Zinc homoenolate reacts with allylic halides and diene monoepoxides under copper catalysis [29]. Treatment of the zinc nomoenolate with a catalytic amount of Cu(II) in a polar solvent (e.g. hexamethylphosphoramide, HMPA, N,N-dimethylacetamide, DMA) generates a copper species which undergoes clean Sn2 allylation reactions Eq. (40). Polar solvents not only accelerate the reaction but greatly improve the SN2 selectivity. A variety of allylating reagents can be employed in this reaction (Table 9). The SN2 /SN2 ratio is particularly high (close to 100%) when the alkylated carbon bears no substituents. The reaction of... 

Electrolyte solutions of various aprotic organic solvents are used in primary lithium batteries. Among the organic solvents are alkyl carbonates [PC (er = 64.4-), ethylene carbonate (EC, 89.640°c)> dimethyl carbonate (DMC, 3.1), diethyl carbonate (DEC, 2.8)], ethers [DME (7.2), tetrahydrofuran (THF, 7.4), 2-Me-THF (6.2),... 

Solvents were initially selected primarily on the basis of the conductivity of their salt solutions, the classical example being propylene carbonate (PC). However, solutions based on PC on its own were soon found to cause poor cyclability of the lithium electrode, due to uncontrolled passivation phenomena. Solvent mixtures or blends were therefore developed and selection currently focuses on a combination of high dielectric solvents (e.g. ethylene carbonate, EC) with an alkyl carbonate (e.g. dimethy(carbonate, DMC), to stabilize the protective passivation film on the lithium electrode, and/or with a low viscosity solvent [e.g. 1,2-dimethoxyethane (DME) or methyl formate (MF)], to ensure adequate conductivity. 

In dissociating solvents, perfluoroalkyl iodides react with a zinc-copper couple to give perfluoroalkylzmc compounds, R Znl, which react with alkyl carbonates or pyrocarbonates to give perfluorocarboxylic acids or esters, respectively [55] (equations 47 and 48)... 

Acylation differs from alkylation in that the reaction usually is carried out in a solvent, commonly carbon disulfide, CS2, or nitrobenzene. Furthermore, acylation requires more catalyst than alkylation, because much of the catalyst is tied up and inactivated by complex formation with the product ketone ... 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

For instance, the reduction potential of many solvents depends on the salt used and, in particular, on the cation. The reduction potentials of alkyl carbonates and esters in the presence of tetraalkyl ammonium salts (TAA) are usually much lower than in the presence of alkaline ions (Li+, Na+, etc.). Similar effects were observed with the reduction potential of some common contaminants (e.g., H20, 02, C02). Moreover, the reduction products of many alkyl carbonates and esters are soluble in the presence of tetraalkyl ammonium salts, while in the presence of lithium ions, film formation occurs, leading to passivation of the electrode [3],... 

Tetraalkyl ammonium (TAA) salts are characterized by very low reduction potentials, along with good solubility in many organic solvents. Thus, nonaqueous solutions composed of such salts (e.g., tetrabutyl ammonium perchlorate and organic solvents such as ethers, esters, and alkyl carbonates) can be electrolyzed using noble metal electrodes. In contrast to lithium salt solutions, in TAA-based solutions there is no precipitation of insoluble products on the electrode, which leads to its passivation. Therefore, it is possible to isolate and identify the electrolysis products and thus outline precise reduction mechanisms for the various systems. 

This section deals only with solvents whose reduction products are insoluble in the presence of lithium ions. The list includes open chain ethers such as diethyl ether, dimethoxy ethane, and other polyethers of the glyme family cyclic ethers such as THF, 2Me-THF, and 1,4-dioxane cyclic ketals such as 1,3-dioxolane and 1,3-dioxane, esters such as y-butyrolactone and methyl formate and alkyl carbonates such as PC, EC, DMC, and ethylmethyl carbonate. This list excludes the esters, ethyl and methyl acetates, and diethyl carbonate, whose reduction products are soluble in them (in spite of the presence of Li ions). Solutions of solvents such as acetonitrile and dimethyl formamide are also not included in this section for the same reasons. Figure 6 presents typical steady state voltammo-grams obtained with gold, platinum, and silver electrodes in Li salt solutions in which solvent reduction products are formed and precipitate at potentials above that of lithium metal deposition. These voltammograms are typical of the above-mentioned solvent groups and are characterized by the following features ... 

In the absence of trace H20 and 02 in esters, ethers and alkyl carbonate solutions, the initial voltammograms obtained with noble metals in these solutions are featureless and show the irreversible reduction wave with an onset at 1.5 V (Li/Li+), which corresponds to salt anion and solvent reduction. In addition, in the absence of H20 and 02, the peaks related to Li UPD shown in Figure 6 and the peaks related to the Au/Au(OH)3 [or Au/Au(OH)ads] couple described in the previous section do not appear. This is demonstrated in Figure 10. 

Figures 11-15 present typical FTIR spectra obtained ex situ and in situ from nonactive electrodes polarized to different low potentials in alkyl carbonate solutions. These figures demonstrate the effect of potential, solvent, salt, and storage conditions of the electrodes at open circuit potential after film formation on their surface chemistry.

At the onset potential for reduction of about 1.5 V (Li/Li+), alkyl carbonate-based solvents, such as PC, EC, and DMC, are reduced in the presence of Li ions on nonactive electrodes to R0C02Li compounds. The most probable compounds formed in the case of PC and EC are propylene and ethylene lithium dicarbonates, respectively. The mecha-... 

Surface film formation on noble metal electrodes at reduction potentials was studied extensively with solutions of DME, THF, 2Me-THF, and DN. Basically, these solvents are much less reactive at low potentials than are alkyl carbonates and esters. However, in contrast to ethereal solutions of TBA+ whose electrochemical window is limited cathodically by the TBA+ reduction at around OV (Li/Li+), in Li+ solutions, ether reduction processes that form Li alkoxides occur at potentials below 0.5 V (Li/Li+) [4], It should be emphasized that the onset potential for surface film formation on noble metals in ethereal solutions is as high as in... 

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