Ethyl methyl carbonate (EMC) has been found to be a suitable cosolvent, along with three other carbonate solvents, for incorporation into electrolytes to enhance the low-temperature performance of rechargeable lithium-ion electrochemical cells. EMC is an asymmetric aliphatic carbonate, and, as noted in the first of the two immediately preceding articles, asymmetric carbonates confer certain benefits. In the research described in that article, the asymmetric carbonates were formed in situ, in reactions catalyzed by lithium alkoxide additives. In contrast, the present finding that EMC is a suitable cosolvent was made by following a different approach; namely, formulating the electrolyte solvents to include an asymmetric aliphatic carbonate — EMC — in the first place.
The table shows the compositions of electrolytes used in experiments on the effects of using EMC as a cosolvent. These compositions were chosen on the basis of the expectation of the beneficial effects of adding a low-viscosity, low-melting-temperature solvent (in this case, EMC) to carbonate solvent mixtures that had previously been observed to have desirable stabilizing and passivating qualities. Another purpose for some of the choices was to minimize the proportion of EC and maximize the proportion of low-viscosity, low-melting cosolvents, provided that doing so would not impair the dissolution of the LiPF6. As in the research described in the two immediately preceding articles, the basis for comparison in these experiments was established by a previously discovered optimal electrolyte formulation; namely, a 1.0 M solution of LiPF6 in a solvent comprising equal volume parts of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).
The experiments included charge/discharge tests of lithium/graphite half cells containing the various electrolytes, and ac-impedance and dc-micropolarization tests to characterize the films [solid/electrolyte interfaces (SEIs)] that formed on the graphite electrodes. The results of the experiments were interpreted in terms of stability of SEIs, kinetics of intercalation of lithium into graphite electrodes, and electrical conductivities of electrolytes. In the formulations studied, the addition of EMC exerted no observable adverse effects on the SEIs or on the kinetics; instead, the addition of EMC was found to reduce low-temperature film resistances and to enhance the kinetics and the discharge characteristics. The best low-temperature electrical performance was observed in the case of the electrolyte with the highest EMC content; this is consistent with the lower (relative to the other carbonate solvents) viscosity and freezing temperature of EMC.
This work was done by Marshall Smart, Ratnakumar Bugga, and Subbarao Surampudi of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.