Li alkyl carbonates

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-... 

T < 600 K, it seems quite plausible that ethylene oxide is derived from the thermal decomposition of a lithium alkoxide (most probable propoxide) produced by the (lower temperature) release of C02 from the corresponding Li alkyl carbonate. In fact, the fragments observed for the thermal decomposition of BuOLi were consistent with at least two different epoxides. 

In the LiPFe salt and organic carbonates containing electrolytes, the SEI film is a passivating surface film on the anode (e.g., a graphite particle) consisting of Li-alkyl-carbonates, polymeric carbonates, and lithium fluoride and is formed on the surface of particles via either the direct decomposition of the solvents to form a precipitate layer, or the co-intercalation of solvated lithium ions and their decomposition beneath the surface in the initial stage of lithium intercalation (exfoliation), during the first few cycles. 

Li halides, hydroxide, oxides, carbonate, Li alkyl carbonates, carboxylates, Li nitride, Li sulfide, etc.) conduct hthium ions. Hence, Li-ion can migrate through the surface films under an electrical field (see the SEI model [4,5]). As a result, lithium can be dissolved and deposited through the surface films, which cover the lithium electrodes, while their basic structure can be retained. 

While the initial surface species formed on lithium in alkyl carbonates consist of ROC02Li compounds, these species react with water to form Li,CO, C02, and ROH. This reaction gradually changes the composition of the surface films formed on... 

For example, the reaction enthalpy for the reduction of PC proceeding at lithium amalgam to form propylene gas and lithium carbonate is estimated to be -I41kcal (molPC)-1 [149]. PC is reduced at noble-metal electrodes at potentials below 1.5 V vs. Li, and yields lithium alkyl carbonates when lithium salts are the supporting electrolytes. Reduction occurs at 0.7-0.8 V vs. Li with Bu4NC104as supporting electrolyte [150],... 

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-... 

Li PC / LiAsF6 FTIR, XPS, IR alkyl carbonates, Li2C03 (from water), LiF [149]... 

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. 

Figure 1 provides several electrochemical windows of important, relevant processes, including the reduction of alkyl carbonates, ethers, Li insertion into graphite, and Li metal deposition. Recent studies revealed two major failure mechanisms of graphite electrodes in repeated Li insertion/ deinsertion processes 21... 

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],... 

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. 

Flence, aged surface films formed on nonactive electrodes at low potentials in alkyl carbonate solutions of these two salts contain LiF and other salt reduction products of the Li PF, Li BFy,... 

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... 

One possible explanation for the broad mle = 44 feature may be found in the thermal decomposition of a lithium alkyl carbonate produced by the reaction between PC and Li, for which C02 would be released at a much lower temperature than the corresponding inorganic carbonate. The formation of such a species has been suggested by Aurbach and co-workers on the basis of in situ and ex situ external reflection FTIR measurements performed in PC-based electrolytes [18]. Support for this assignment was obtained from experiments in which a genuine alkyl carbonate was prepared in UHV by exposing to C02 a layer of lithium alkoxide formed by the adsorption of an alcohol onto the Li surface, as described in Section I.E. 

The primary aim of these experiments was to examine the TPD spectra of a genuine lithium alkyl carbonate formed on the surface of Au(poly) to determine whether the thermal decomposition of this UHV-synthesized material exhibits an m/e = 44 TPD spectra which resembles that observed in the TPD of d6-PC adsorbed on Li/Au(poly). The synthetic pathway employed is based on the sequential condensation of an ultrapurified alcohol (n-butanol, BuOH) onto a Li/ Au(poly) surface followed by exposure to C02 to form the desired alkyl carbonate. 

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