EC/DMC-mixtures

Note that lithium salts with nonfluorinated alkylsulfonate (e.g., LiSOsCHs) or benzenesulfonate (e.g., LiS03(C6H5)) anions have a very low solubility in aprotic solvents and a correspondingly low conductivity (Table 1.3) [219]. Lithium salts with oligoethersulfate anions (Fig. 1.19c) are soluble in EC DMC mixtures, but these... 

Not long after Dahn s work, Shenoy et al. employed molecular dynamics simulations to study the formation and growth of solid electrolyte interphase for the case of EC, DMC, and mixtures of these two solvent on lithium metalhc electrode [61]. In their work, they investigated the constitutes and structures of SEI on lithium metal electrode with the dependence of electrolyte composition and temperature change. The results show that the SEI films grow faster in the case of EC compared to DMC, with EC+DMC mixtures falling in between, as shown in Fig. 5.22. 

The increase of the liquid range of binary mixtures based on a polar (e.g., EC) and a nonpolar component (e.g., DMC) by salt addition refiects the association of the electrolyte. Large freezing point depressions are obtained in EC-rich mixtures, whereas DMC-rich mixtures yield only small depressions. As a consequence the minimum of the eutectic phase diagrams shifts to higher EC contents. For example, EC/DMC-mixtures show an eutectic point at a molar ratio of EC x c = 0.348 at —7.76°C. The temperature minimum of the ternary mixture EC/DMC/LiPFg is obtained at —16.04°C and %ec = 0.476 [55]. 

Typically, the liquidus lines of a binary system curve down and intersect with the solidus line at the eutectic point, where a liquid coexists with the solid phases of both components. In this sense, the mixture of two solvents should have an expanded liquid range with a lower melting temperature than that of either solvent individually. As Figure 4 shows, the most popular solvent combination used for lithium ion technology, LiPFe/EC/DMC, has liquidus lines below the mp of either EC or DMC, and the eutectic point lies at —7.6 °C with molar fractions of - 0.30 EC and "-"0.70 DMC. This composition corresponds to volume fractions of 0.24 EC and 0.76 DMC or weight fractions of 0.28 EC and 0.71 DMC. Due to the high mp of both EC (36 X) and DMC (4.6 X), this low-temperature limit is rather high and needs improvement if applications in cold environments are to be considered. 

In summary, these trends in the change of conductivity with m, xec, and T can be consistently interpreted in terms of the change of e and r] with these same variables. Since these factors and their effect on ion conductivity are not unique to the system illustrated, LiPFe/EC/DMC, these trends should provide general guidance as to how ion conductivities of other electrolyte systems with similar compositions would change with these same variables, and they should constitute a useful database for the understanding of more complex systems, such as ternary or quaternary mixtures. 

In the early era of lithium ion cell research, Aurbach et al. noticed that the presence of CO2 in the electrolyte had pronounced effects on the lithia-tion behavior of graphitic anodes. A number of electrolytes, which were thought to be incompatible with graphite because they are based on solvents such as methyl formate or THE, delivered much improved performance under 3—6 atm of C02. ° They proposed that CO2 participated in the formation of the SEI by a two-electron process, yielding Li2C03, which assisted in the buildup of the protective surface film. However, in PC-based electrolytes. CO2 presence proved to be ineffective, while, in electrolytes based on carbonate mixtures such as EC/DMC, the... 

Vielstich et al. [112] studied the oxidation of PC by DEMS and FTIR spectroscopy, and Kanamura et al. studied its oxidation using in situ FTIR spectroscopy [108,109], We also studied the oxidation of other alkyl carbonate solutions (EC-DMC and EC-DEC mixtures) using in situ FTIR spectroscopy [114],... 

The density of Li consumed in the SEI tends to be of asymptotic limit, which depends on the electrolyte composition but weakly on the temperature. In addition, the Li density in SEI formed in EC-based electrolyte is slightly higher than that in DMC and EC-i-DMC mixtures, suggesting that EC is more reactive compared to... 

FIGURE 20.4 (a) DSC curves for mixtures of 4 mg U0.92C6 and given amounts of EC + DMC solvent (b) DSC curves for mixtures of 4 mg Lio.48C6 and given amounts of EC - - DMC solvent [8], (For color version of this figure, the reader is referred to the online version of this book.)... 

In its most classic structure, a lithium-ion (Li-ion) battery contains a negative electrode made of carbon graphite, a positive electrode made of a layered oxide LiM02 (M transition metal, e.g. LiCo02) and a polypropylene separator soaked in an electrolyte made of a lithium salt (e.g. LiPFe) dissolved in a mixture of alkyl carbonate organic solvents (e.g. ethylene carbonate-dimethyl carbonate (EC-DMC)). The reversible electrochemical process is as follows ... 

---