Lithium alkyls, carbonation

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],... 

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. ... 

Aside from cyclic carbonates, the decomposition products from linear carbonates were also identified in the forms of either lithium alkyl carbonates or alkoxides, as shown by Scheme 7 and also in Table 6a. 1 5,271,279 Relatively, the reduction of linear carbonates was thought to be less consequential as compared to their cyclic counterparts, apparently due to their smaller presence in the solvation sheath of lithium cations. i H 1 1... 

In a further effort to identify the active intermediate that initiates the reaction, they tested the effect of a few possible ingredients on the production of EMC based on the knowledge about the chemical composition of the SEI on carbonaceous anodes. These model compounds included Li2C03, LiOCHs, and LiOH, while lithium alkyl carbonate was not tested due to its instability and therefore rare avail-ability. The results unequivocally showed that LiOCHs effectively catalyzes the ester exchange. 

When conducting a differential scanning calorimetry (DSC) study on the stability of carbonaceous anodes in electrolytes, Tarascon and co-workers found that, before the major reaction between lithiated carbon and fluorinated polymers in the cell, there was a transition of smaller thermal effect at 120 °C, marked peak (a) in Figure 28. They ascribed this process to the decomposition of SEI into Li2C03, based on the previous understanding about the SEI chemical composition and the thermal stability of lithium alkyl carbonates.Interestingly, those authors noticed that the above transition would disappear if the carbonaceous anode was rinsed in DMC before DSC was performed, while the other major processes remained (Figure 28). Thus,... 

Scheme 23. Thermal Decomposition of Lithium Alkyl Carbonate in the SEI...

However, Li2COs was not observed by Andersson and Edstrom in their XPS analysis of the graphitic anode that had been precycled and then stored at 60 °C for 7 days. They found that, during the storage, the original SEI consisting of lithium alkyl carbonate was indeed unstable and disappeared with time, as... 

On the basis of the above observation, Dahn and co-workers proposed a thermal reaction scheme for the coupling of carbonaceous anodes and electrolytes. The initial peak, which was almost identical for all of the anode samples and independent of lithiation degrees, should arise from the decomposition of the SEI because the amount of SEI chemicals was only proportional to the surface area. This could have been due to the transformation of the metastable lithium alkyl carbonate into the stable Li2C03. After the depletion of the SEI, a second process between 150 and 190 °C was caused by the reduction of electrolyte components by the lithiated carbon to form a new SEI, and the autocatalyzed reaction proceeded until all of the intercalated lithium was consumed or the thickness of this new SEI was sufficient to suppress further reductions. The corresponding decrease in SHR created the dip in the least lithiated sample in Eigure 35. Above 200 °C (beyond the ARC test range as shown in Eigure 35), electrolyte decomposition occurred, which was also an exothermic process. 

Irreversible Capacity. Because an SEI and surface film form on both the anode and cathode, a certain amount of electrolyte is permanently consumed. As has been shown in section 6, this irreversible process of SEI or surface layer formation is accompanied by the quantitative loss of lithium ions, which are immobilized in the form of insoluble salts such as Li20 or lithium alkyl carbonate. Since most lithium ion cells are built as cathode-limited in order to avoid the occurrence of lithium metal deposition on a carbonaceous anode at the end of charging, this consumption of the limited lithium ion source during the initial cycles results in permanent capacity loss of the cell. Eventually the cell energy density as well as the corresponding cost is compromised because of the irreversible capacities during the initial cycles. 

Figure 54. Peculiar surface chemistry of BOB anion on graphitic anode material XPS C Is spectra for a graphitic anode surface cycled in LiBOB- and LiPF6-based electrolytes. The peaks were resolved into three major contributions representing (1) hydrocarbon at 284.5 eV, (2) oligo-ether linkages at 286.5 eV, and (3) lithium alkyl carbonates at 289.37 eV, respectively. (Reproduced with permission from ref 489 (Figure 3). Copyright 2003 The Electrochemical Society.)...

Therefore, the authors concluded that, although direct identification was not available through spectroscopic means, the FAP anion must have participated in the formation of surface layers, which served as protection against sustained decompositions on one hand but were also responsible for the high impedance across the interfaces on the other. These robust surface films might exist on both anode and cathode surfaces and consist mainly of lithium alkyl carbonates because of the low level of HF in the solution. 

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. 

The direct evidence of this reaction mechanism is the observation of carbonyl stretching signature at -1,650 cm" in FTIR spectrum. The decomposition products from lithium salt were also found through the XPS surface analysis, such as alkoxides or oxides, a competition reaction between solvents and salts. However, the formation of alkyl carbonate seems to be predominant when EC is the component of electrolyte because of the more reactive nature of EC toward cathodic reductions [28]. The formation of lithium alkyl carbonate was also confirmed in an independent work, where the reduction products of EC in a supporting electrolyte were hydrolyzed by DjO and then subject to NMR analysis, which identified ethylene glycol as the major products formed, as indicated by the singlet at H spectrum [29]. Therefore, Aurbach and co-workers concluded the reduction products of EC and PC, lithium ethylene dicarbonate (LEDC) and lithium propylene dicarbonate (LPDC), respectively ... 

In an effort to gain fundamental understanding on those key ingredients in the SEI, Xu et al. from the US Army Research Laboratory (ARL) synthesized a series of model lithium alkyl carbonate compounds to simulate the proposed chemical species on the anode surface, including lithium methyl carbonate (LMC), lithium ethyl carbonate (LEC), LEDC, and LPDC, as summarized in Scheme 5.6 [38]. 

Fig. 5.25 Thermal instability of the lithium alkyl carbonate (a) weight retention as function of temperature as recorded with TGA (b) onset temperatures of the multistage thermal decomposition as demonstrated by DTG plots (reproduced with permission by the American Chemical Society from [38])...

Borodin, O. Smith, G.D. Fan, P., Molecular dynamics simulations of lithium alkyl carbonates, J. Phys. Chem. B 2006,110, 22773-22779. 

The electrochemical stability of liquid electrolytes containing carbonates has been studied in relation to a lithium electrode. It has been shown that the reduction reactions on lithium involve an O2 radical to form a lithium alkyl carbonate (R0-(C=0)-0Li) and lithium carbonate (Li2COs). This shows that carbonate-type solvents greatly influence the reaction products, which mns counter to the redox mechanisms discussed above. 

In each of the above spectrum, Li COj can easily be recognized by observing its characteristic bands at 1510cm", 1435 cm and 868cm . However, it is difficult to identify ROCO Li exactly because ROCO Li actually represents a series of lithium alkyl carbonates. Their peak positions depend on the structure of the R group and are determined by the composition of the electrolyte, the surface properties of the electrode and the reduction processes on it. 

CO2 reduction products. Based on the results of electron energy loss (EEL) and FTIR spectroscopy, Naji et proposed a mechanism of EC reduction on the surface of graphite electrode in the presence of LiClO.,. They believed that LijCOj is formed above 0.8 V by a direct two-electron reduction of EC. Then free radical termination reactions lead to the formation of lithium alkyl carbonate below 0.8 V. 

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