Electrolytes comprising, variously, LiPF6 or LiPF6 plus LiBF4 dissolved at various concentrations in mixtures of alkyl carbonates and alkyl esters have been found to afford improved low-temperature performance in rechargeable lithium-ion electrochemical cells. These and other electrolytes have been investigated in a continuing effort to extend the lower limit of operating temperatures of such 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 “Ester-Based Electrolytes for Low- Temperature Li-Ion Cells” (NPO-41097), NASA Tech Briefs, Vol. 29, No. 12 (December 2005), page 59. The ingredients of the solvent mixtures include ethylene carbonate (EC), ethyl methyl carbonate (EMC), methyl butyrate (MB), and methyl propionate (MP). The electrolytes were placed in Li-ion cells containing carbon anodes and LiNi0.8Co0.2O2 cathodes, and the electrical performances of the cells were measured over a range of temperatures down to –60 °C. The electrolytes that yielded the best low-temperature performances were found to consist, variously, of 1.0 M LiPF6 + 0.4 M LiBF4 or 1.4 LiPF6 in 1EC + 1EMC + 8MP or 1EC + 1EMC + 8MB, where the concentrations of the salts are given in molar units and the proportions of the solvents are by relative volume.
This work was done by Marshall Smart and Ratnakumar Bugga of Caltech for NASA’s Jet Propulsion Laboratory.
This Brief includes a Technical Support Package (TSP).
Mixed-Salt/Ester Electrolytes for Low-Temperature Li+ Cells
(reference NPO-42862) is currently available for download from the TSP library.
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Overview
The document titled "Mixed-Salt/Ester Electrolytes for Low-Temperature Li+ Cells" from NASA's Jet Propulsion Laboratory discusses advancements in electrolyte formulations designed to improve the performance of lithium-ion batteries, particularly at low temperatures, down to -70°C. The research focuses on the development of advanced electrolytes that incorporate mixed salt formulations and ester-based co-solvents to enhance conductivity and overall battery performance in extreme conditions.
Key findings indicate that high concentrations of lithium salts, such as LiPF6 and LiBF4, when combined with specific solvent mixtures (e.g., ethylene carbonate (EC) and ethyl methyl carbonate (EMC)), significantly improve discharge performance and rate capability at low temperatures. The document highlights two specific formulations: 1.00 M LiPF6 + 0.40 M LiBF4 and 1.40 M LiPF6 dissolved in EC+EMC+MP and EC+EMC+MB solvent mixtures. These formulations have shown remarkable improvements in rate capability, particularly at -60°C.
The research emphasizes the importance of selecting solvents with critical properties such as high dielectric constant, low viscosity, and adequate coordination behavior to achieve high conductivity at low temperatures. The incorporation of low-viscosity ester-based co-solvents in multi-component electrolyte formulations has been a significant focus, leading to enhanced performance compared to traditional low-temperature electrolytes.
The document also outlines ongoing efforts to further understand the electrochemical characteristics of these systems through various techniques, including Electrochemical Impedance Spectroscopy and Tafel polarization measurements. The goal is to optimize the electrolyte compositions to maximize low-temperature performance across different solvent blends.
In summary, this technical report presents a comprehensive overview of innovative electrolyte technologies that leverage mixed salts and ester-based co-solvents to address the challenges of lithium-ion battery performance in cold environments. The findings have implications for a range of applications, particularly in aerospace, where reliable battery performance in extreme temperatures is critical. The document serves as a resource for further exploration and collaboration in the field of advanced battery technologies.
