To maintain high conductivity in low temperatures, electrolyte co-solvents have been designed to have a high dielectric constant, low viscosity, adequate coordination behavior, and appropriate liquid ranges and salt solubilities. Electrolytes that contain ester-based co-solvents in large proportion (>50 percent) and ethylene carbonate (EC) in small proportion (<20 percent) improve low-temperature performance in MCMB carbon-LiNiCoO2 lithium-ion cells. These co-solvents have been demonstrated to enhance performance, especially at temperatures down to –70 °C. Low-viscosity, ester-based co-solvents were incorporated into multi-component electrolytes of the following composition: 1.0 M LiPF6 in ethylene carbonate (EC) + ethyl methyl carbonate (EMC) + X (1:1:8 volume percent) [where X = methyl butyrate (MB), ethyl butyrate EB, methyl propionate (MP), or ethyl valerate (EV)]. These electrolyte formulations result in improved low-temperature performance of lithium-ion cells, with dramatic results at temperatures below –40 °C. [See "Ester-Based Electrolytes for Low-Temperature Li-Ion Cells," (NPO-41097) NASA Tech Briefs, Vol 29, No. 12 (December, 2005), p. 59.]
Improved low-temperature performance can also be realized with ester-based electrolytes containing high salt concentrations and by using mixed salt formulations — specifically with (a) 1.0 M LiPF6 + 0.40 LiBF4 and (b) 1.40 M LiPF6 dissolved in EC+EMC+MP (1:1:8 volume percent) and EC+EMC+MB (1:1:8 volume percent) solvent mixtures. The rate capability has been observed to increase dramatically at low temperatures (i.e., –60 °C) using this approach. It is anticipated that increased salt concentrations and the use of mixed salt systems will also improve the low-temperature performance characteristics of other solvent blends of carbonates and esters. ["Mixed-Salt/Ester Electrolytes for Low-Temperature Li + Cells" (NPO-42862), NASA Tech Briefs, Vol. 30, No. 11 (November 2006), p 66.]
A number of these electrolytes have been demonstrated in both experimental and aerospace-quality, high-capacity prototype cells. In more recent work, these ester-containing electrolytes have been further optimized to provide both good low-temperature performance (down to –60 °C) while still offering reasonable high-temperature resilience. This has primarily been achieved by fixing the EC-content at 20 percent and the ester co-solvent at 20 percent, in contrast to the previously developed ultra-low temperature systems, which have the EC-content and ester-content at 10 percent and 80 percent, respectively. Using this approach, a prototype cell containing a 1.0 M LiPF6 EC+EMC+MP (20:60:20 volume percent) electrolyte was capable of delivering over six times the amount of capacity delivered by the baseline ternary, all-carbonate blend, and was able to support reasonably aggressive rates at low temperature (–50 and –60 °C). Cells containing other esters also performed well at low temperature, with the lower-molecular-weight, lower-viscosity co-solvents generally yielding better performance at low temperatures.
Although slightly less favorable in terms of electrolyte conductivity, the higher-molecular-weight esters [i.e., propyl butyrate (PB), and butyl butyrate (BB)] are expected to result in cells with more favorable high-temperature resilience (>40 °C), compared to the lower-molecular-weight esters.
This technology has relevance for manned and unmanned space missions, aircraft batteries, and Land Warrior applications, as well as for terrestrial applications that require good performance and a high level of safety over a range of temperatures, including portable electronics like camcorders, cellular phones, laptop computers, and radio communication sets. Electric vehicle applications can also use this innovation where high-power batteries must operate at low temperatures, such as in monitoring stations in Antarctica.
This work was done by Marshall Smart and Ratnakumar Bugga of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-44974, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Optimized Carbonate and Ester-Based Li-Ion Electrolytes
(reference NPO-44974) is currently available for download from the TSP library.
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