Electrolytes comprising LiPF6 dissolved at a concentration of 1.0 M in five different solvent mixtures of alkyl carbonates have been found to afford improved performance in rechargeable lithium-ion electrochemical cells at temperatures as low as -70° C. These and other electrolytes have been investigated in continuing research directed toward extending the lower limit of practical operating temperatures of Li-ion cells. This research at earlier stages, and the underlying physical and chemical principles, were reported in numerous previous NASA Tech Briefs articles, the most recent being "Low-ECContent Electrolytes for Low-Temperature Li-Ion Cells" (NPO-30226), NASA Tech Briefs, Vol. 27, No. 1 (January 2003), page 46. The ingredients of the present solvent mixtures are ethylene carbonate (EC), ethyl methyl carbonate (EMC), methyl butyrate (MB), methyl propionate (MP), ethyl propionate (EP), ethyl butyrate (EB), and ethyl valerate (EV). In terms of volume proportions of these ingredients, the present solvent mixtures are
  • 1EC + 1EMC + 8MB,
  • 1EC + 1EMC + 8EB,
  • 1EC + 1EMC + 8MP,
  • 1EC + 1EMC + 8EV, and
  • 1EC + 9EMC.

These electrolytes were placed in Li-ion cells containing carbon anodes and LiNi0.8Co0.2O2 cathodes, and the low-temperature electrical performances of the cells were measured. The cells containing the MB and MP mixtures performed best.

This work was done by Marshall Smart and Ratnakumar Bugga of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-41097

Topics:
Chemicals Electrolytes Electromagnetic compatibility Logistics Physical Sciences Research and development
This Brief includes a Technical Support Package (TSP).
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Ester-Based Electrolytes for Low-Temperature Li-Ion Cells

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This article first appeared in the December, 2005 issue of NASA Tech Briefs Magazine (Vol. 29 No. 12).

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Overview

The document titled "Ester-Based Electrolytes for Low-Temperature Li-Ion Cells" from NASA's Jet Propulsion Laboratory discusses advancements in lithium-ion battery technology, particularly focusing on the performance of these batteries at low temperatures. Traditional lithium-ion systems operate effectively within a temperature range of -40°C to +40°C, but their performance significantly deteriorates below -40°C due to factors such as poor electrolyte conductivity, limited lithium ion mobility, and inadequate ionic diffusion within the electrodes.

To address these challenges, the document emphasizes the importance of optimizing electrolyte formulations. It identifies key parameters that influence low-temperature performance, including electrolyte conductivity, cell design, electrode thickness, and separator properties. The research highlights that the electrolyte's properties are crucial, with high conductivity being essential for effective battery operation in cold conditions.

The document presents a promising electrolyte formulation consisting of a ternary mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). This formulation aims to enhance conductivity at low temperatures. Additionally, the research explores the use of co-solvents with low viscosities and freezing points to further improve electrolyte performance. These co-solvents can help create a more conductive solution by optimizing properties such as dielectric constant, viscosity, and overall stability.

Experimental evaluations at -40°C demonstrated that most of the electrolytes tested, except for those containing ethyl valerate, exhibited excellent low-temperature performance, delivering 60-70% of their room temperature capacity at moderate rates. Notably, electrolytes based on methyl butyrate and methyl propionate showed the best performance, indicating good compatibility with electrodes and minimal polarization effects from the solid electrolyte interphase (SEI) film.

The document serves as a technical support package under NASA's Commercial Technology Program, aiming to disseminate aerospace-related developments with broader applications. It underscores the potential of ester-based electrolytes in enhancing the viability of lithium-ion cells for use in extreme environments, which is critical for various applications, including space exploration and other industries requiring reliable energy storage solutions in cold conditions. Overall, the research represents a significant step toward improving the performance and reliability of lithium-ion batteries in low-temperature scenarios.