Lithium binary solvent mixture

Since a high dielectric constant and low viscosity usually cannot be integrated into a single solvent, a solvent mixture, usually binary with one of the components selected for e and the other for tj, was used to formulate electrolytes for lithium batteries with the hope that a balance between these two properties could be arrived at via such mixing. " The concept was rapidly accepted by researchers of the 1980s, usually using cyclic carbonates for their high e and linear or cyclic ethers for their low... 

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 1910 Serkov (15) determined the conductance of several salts (including lithium bromide) at 25°C in water, methanol, ethanol, acetone, and binary mixtures of these solvents, reporting a value of 144 12 1 cm2 eq-1 for Ao for lithium bromide in acetone. He found that, unlike the other mixtures, acetone solutions exhibit no parallelism between conductance and fluidity, and concluded that when the surveyed ionophores are dissolved in acetone, the complexity of the solvates formed increases as Aq for the ionophores decreases. 

TTie important role of the solvent/electrolyte system is considered finally. In principle, none of the aprotic solvents which are commonly used is thermodynamically stable against lithium metal. The same is true for LiCg, although some improved metastability is found. Binary mixtures of PC (propylene, carbonate)/EC (ethylene carbonate) are widely used [196]. PC alone is not suitable, but it is convenient as a liquefier for the solid EC. Another binary system is EC/DEC (diethyl carbonate). 1 m LiPFe in 30-50 vol% EC has a specific conductivity of k = 7 mS/cm at 20 °C. In DEC alone, k is only 2 mS/cm. In this case, DEC is the stable thirmer. Ternary systems comprising EC/PC. DEC (1 1 2-3 by volume) are also employed. In both cases, the co-insertion of solvent molecules is totally inhibited (see below). A test of... 

GPEs, having the composition PMMA-PC/EC/yBL-LiCFsSOs with PMMA as the polymer, PC, EC, yBL) and their binary mixtures as solvents, and lithium triflate (LiCFsSOs) as the salt, have been prepared and studied. The conductivity values at all concentrations increase in the order PC < EC... 

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