Aliphatic esters have been found to be useful as electrolyte additives for improving the low-temperature performances of rechargeable lithium-ion electrochemical cells. The discovery of the beneficial effects of these additives was made during continuing research directed toward extending the lower limit of operating temperatures of these cells. Other aspects of this research have been described in the immediately preceding article and in prior NASA Tech Briefs articles referenced therein.
In experiments, the effects of aliphatic esters as additives were investigated with respect to a baseline optimal electrolyte formulation described in the noted previous articles; namely, a 1.0 M solution of LiPF6 in a solvent that consists of equal volume parts of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). In order of increasing molecular weight, the aliphatic esters investigated were methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), and ethyl butyrate (EB). These esters have freezing temperatures ranging from –73 to –98 °C — lower than the freezing temperatures of the carbonate solvents. They are fully miscible into the baseline electrolyte solution. In each case, the volume proportion of aliphatic ester incorporated into the electrolyte was equal to the volume proportion of one of the carbonate solvents.
The experiments included measurements of the temperature-dependent electrical conductivities of the ester-containing electrolytes, charge/discharge tests of lithium/graphite half cells containing these electrolytes, and ac-impedance and dc-micropolarization tests to determine the effects of electrolyte compositions on the electrochemical characteristics of films that formed on the graphite electrodes. The low-temperature electrical conductivities of the electrolytes were found to be increased by the addition of the esters, the greatest increase occurring in the cases of the esters of lowest molecular weight (see figure). However, the films formed in the presence of the higher-molecular-weight esters were found to be more stable and to exhibit better kinetics for lithium intercalation/de-intercalation, especially at lower temperatures. Taking both of these trends into account, it appears that the higher-molecular-weight esters are more promising as electrolyte additives.
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.
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