A number of future NASA missions involving the exploration of the Moon and Mars will be “human-rated” and thus require high-specific-energy rechargeable batteries that possess enhanced safety characteristics. Given that Li-ion technology is the most viable rechargeable energy storage device for near-term applications, effort has been devoted to improving the safety characteristics of this system. There is also a strong desire to develop Li-ion batteries with improved safety characteristics for terrestrial applications, most notably for hybrid electric vehicle (HEV) and plugin hybrid electric vehicle (PHEV) automotive applications. Therefore, extensive effort has been devoted recently to developing non-flammable electrolytes to reduce the flammability of the cells/battery.
A number of electrolyte formulations have been developed, including systems that (1) incorporate greater concentrations of the flame-retardant additive (FRA); (2) use di-2,2,2-trifluoroethyl carbonate (DTFEC) as a co-solvent; (3) use 2,2,2-trifluoroethyl methyl carbonate (TFEMC); (4) use mono-fluoroethylene carbonate (FEC) as a co-solvent and/or a replacement for ethylene carbonate in the electrolyte mixture; and (5) utilize vinylene carbonate as a “SEI promoting” electrolyte additive, to build on the favorable results previously obtained.
To extend the family of electrolytes developed under previous work, a number of additional electrolyte formulations containing FRAs, most notably triphenyl phosphate (TPP), were investigated and demonstrated in experimental MCMB (mesocarbon microbeads) carbon-LiNi0.8Co0.2O2 cells. The use of higher concentrations of the FRA is known to reduce the flammability of the electrolyte solution, thus, a concentration range was investigated (i.e., 5 to 20 percent by volume). The desired concentration of the FRA is the highest amount tolerable without adversely affecting the performance in terms of reversibility, ability to operate over a wide temperature range, and the discharge rate capability.
The use of fluorinated carbonates, much in the same manner as the incorporation of fluorinated ester-based solvents, was employed to reduce the inherent flammability of mixtures. Thus, electrolyte formulations that embody both approaches are anticipated to have much lower flammability, resulting in enhanced safety.
This work was done by Marshall C. Smart and Ratnakumar V. Bugga of Caltech and G. K. Surya Prakash and Frederick C. Krause of the University of Southern California 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-47465, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Optimized Li-Ion Electrolytes Containing Triphenyl Phosphate as a Flame-Retardant Additive
(reference NPO-47465) is currently available for download from the TSP library.
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Overview
The document titled "Optimized Li-Ion Electrolytes Containing Triphenyl Phosphate as a Flame-Retardant Additive" is a technical support package prepared by NASA's Jet Propulsion Laboratory (JPL). It focuses on advancements in lithium-ion battery technology, particularly the development of safer and more efficient electrolytes by incorporating triphenyl phosphate (TPP) as a flame-retardant additive.
The research highlights the performance of various lithium-ion cells, specifically those using different electrolyte compositions. Key findings include the evaluation of cells containing 1.0 M LiPF6 in various solvent mixtures, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), and TPP. The document presents data on discharge capacities at low temperatures, demonstrating that cells with TPP and trifluoroethyl methyl carbonate (TFEMC) or fluoroethylene carbonate (FEC) exhibit improved performance compared to baseline systems. Notably, a specific electrolyte composition (1.0 M LiPF6 in FEC+EMC+TFEMC+TPP) showed the best performance at low temperatures.
The document also discusses the impact of different additives on battery performance. For instance, while the addition of DTFEC (difluoroethylene carbonate) was anticipated to enhance safety, it resulted in decreased performance at higher discharge rates and lower temperatures. This highlights the trade-offs between safety and performance in electrolyte formulations.
Additionally, the document includes data on the cycling performance of 7 Ah Li-ion cells with a FRA-containing electrolyte, comparing it to a baseline electrolyte. The results indicate that the optimized formulations can maintain better discharge performance under various conditions, which is crucial for applications in aerospace and other demanding environments.
Overall, the research underscores the importance of optimizing electrolyte compositions to enhance the safety and efficiency of lithium-ion batteries. The findings are significant for both aerospace applications and broader technological advancements, as they contribute to the development of batteries that are not only high-performing but also safer for use in critical applications.
The document serves as a valuable resource for researchers and industry professionals interested in the latest developments in battery technology, particularly in the context of enhancing safety through innovative chemical additives.
