A report describes a study of electrolyte compositions selected as candidates for improving the low-temperature performances of primary electrochemical cells that contain lithium anodes and fluorinated carbonaceous (CFx) cathodes. This study complements the developments reported in “Additive for Low-Temperature Operation of Li-(CF)n Cells” (NPO-43579) and Li/CFx Cells Optimized for Low-Temperature Operation (NPO-43585), which appear elsewhere in this issue of NASA Tech Briefs.
Similar to lithium-based electrolytes described in several previous NASA Tech Briefs articles, each of these electrolytes consisted of a lithium salt dissolved in a nonaqueous solvent mixture. Each such mixture consisted of two or more of the following ingredients: propylene carbonate (PC); 1,2-dimethoxyethane (DME); trifluoropropylene carbonate; bis(2,2,2-trifluoroethyl) ether; diethyl carbonate; dimethyl carbonate; and ethyl methyl carbonate. The report describes the physical and chemical principles underlying the selection of the compositions (which were not optimized) and presents results of preliminary tests made to determine effects of the compositions upon the low-temperature capabilities of Li-CFx cells, relative to a baseline composition of LiBF4 at a concentration of 1.0 M in a solvent comprising equal volume parts of PC and DME.
This work was done by Marshall C. Smart, Jay F. Whitacre, and Ratnakumar V. Bugga of Caltech; and G. K. Surya Prakash, Pooja Bhalla, and Kiah Smith of the Loker Hydrocarbon Institute at the University of Southern California for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-43587, volume and number of this NASA Tech Briefs issue, and the page number.
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Electrolytes for Low-Temperature Operation of Li-CFx Cells
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Overview
The document titled "Electrolytes for Low-Temperature Operation of Li-CFx Cells" (NPO-43587) from NASA's Jet Propulsion Laboratory discusses advancements in electrolyte formulations aimed at improving the performance of lithium-carbon fluoride (Li-CFx) primary batteries in extremely low temperatures, specifically down to -60°C.
Traditionally, Li-CFx batteries utilize electrolyte solutions composed of approximately 1.0 M concentrations of lithium salts such as LiBF4, LiClO4, LiPF6, or LiAsF6, dissolved in solvent mixtures like propylene carbonate (PC) and 1,2-dimethoxyethane (DME) or γ-butyrolactone. While these electrolytes perform adequately at moderate temperatures (between -20°C and 60°C), their conductivity diminishes significantly at lower temperatures, leading to poor battery performance due to freezing or reduced ionic mobility.
To address these challenges, the research team has explored several strategies to enhance the low-temperature performance of Li-CFx cells. Key approaches include optimizing salt concentration, adjusting solvent blends, incorporating low-viscosity solvents, and adding stable fluorinated co-solvents to improve the wetting characteristics of the CFx cathode. Additionally, film-forming additives have been investigated to enhance interfacial properties at the electrode interface.
The document outlines various candidate electrolyte formulations that have shown promise in experimental Li-CFx cells. These include variations of the baseline electrolyte (1.0 M LiBF4 in a 50:50 v/v mixture of PC and DME) with different solvent ratios and salt concentrations, such as 1.0 M LiBF4 in a 20:80 v/v mixture of PC and DME, and 0.50 M LiBF4 in the same solvent blend.
The findings indicate that optimizing the electrolyte composition can significantly improve the specific conductivity of the solutions at low temperatures, thereby enhancing the overall performance of the batteries. The document emphasizes the importance of these advancements for applications in aerospace and other fields where reliable battery performance in extreme conditions is critical.
Overall, this technical report serves as a valuable resource for understanding the innovative approaches being developed to enhance the functionality of Li-CFx batteries in low-temperature environments, showcasing NASA's commitment to advancing battery technology for future applications.
