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
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail:
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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Overview
The document titled "Optimized Carbonate and Ester-Based Li-Ion Electrolytes" from NASA's Jet Propulsion Laboratory presents research on enhancing the performance of lithium-ion cells, particularly under low-temperature conditions. The study focuses on the development of electrolytes that combine carbonate and ester components to improve discharge capacity and overall efficiency.
Key findings include the evaluation of various electrolyte formulations containing 1.0M LiPF6 in a solvent mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with different ester co-solvents (designated as X, which includes MP, EP, MB, EB, PB, and BB). The research highlights the discharge capacity of these experimental lithium-ion cells at extremely low temperatures, specifically at -40°C, demonstrating the potential for improved performance in challenging environments.
Figures and tables within the document provide detailed data on charge-discharge characteristics and performance metrics of the lithium-ion cells. For instance, Table 1 compares the formation characteristics of cells using EC-based electrolytes with various ester co-solvents against traditional carbonate-based formulations. Additionally, Table 2 summarizes the discharge performance at various low temperatures, indicating that cells charged at room temperature can still perform effectively when discharged in colder conditions.
The document also includes a summary of conditioning cycling at 20°C for prototype 7 Ah lithium-ion cells, showcasing the performance of the optimized electrolytes compared to baseline carbonate-based electrolytes. This comparative analysis is crucial for understanding the advantages of the new formulations in practical applications.
Overall, the research underscores the importance of optimizing electrolyte compositions to enhance the viability of lithium-ion cells for aerospace and other applications where low-temperature performance is critical. The findings contribute to the broader field of energy storage technology, with implications for both scientific and commercial advancements.
The document serves as a technical support package under NASA's Commercial Technology Program, aiming to disseminate aerospace-related developments that have potential wider applications. For further inquiries or assistance, the document provides contact information for the Innovative Technology Assets Management team at JPL.
