Given that lithium-ion (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. Therefore, extensive effort has been devoted to developing non-flammable electrolytes to reduce the flammability of the cells/battery. A number of promising electrolytes have been developed incorporating flame-retardant additives, and have been shown to have good performance in a number of systems. However, these electrolyte formulations did not perform well when utilizing carbonaceous anodes with the high-voltage materials. Thus, further development was required to improve the compatibility.
A number of Li-ion battery electrolyte formulations containing a flame-retardant additive [i.e., triphenyl phosphate (TPP)] were developed and demonstrated in high-voltage systems. These electrolytes include: (1) formulations that incorporate varying concentrations of the flame-retardant additive (from 5 to 15%), (2) the use of mono-fluoroethylene carbonate (FEC) as a co-solvent, and (3) the use of LiBOB as an electrolyte additive intended to improve the compatibility with high-voltage systems. One of the promising electrolytes identified of the group investigated is l.0M LiPF6 in EC+EMC+TPP (20:70:10 vol %) + 0.15M LiBOB, which was demonstrated to have comparable performance to that of the baseline ternary electrolyte in MPG- 111/Toda (LiNiMnCoO2) coin cells, in terms of reversible capacity and discharge rate capability at room temperature. Thus, improved safety has been provided without loss of performance in the high-voltage, high-energy system.
The use of higher concentrations of the flame-retardant additive is known to reduce the flammability of the electrolyte solution, with 15% concentration resulting in solutions of substantially reduced flammability. Thus, the desired concentration of the flame-retardant additive is the greatest 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 FEC was used to reduce the inherent flammability of mixtures and improve the compatibility at the interfacial regions, due to desirable surface reactions.
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 NPO-47980
JPL
Mail Stop 321-123
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This Brief includes a Technical Support Package (TSP).
Li-Ion Electrolytes With Improved Safety and Tolerance to High-Voltage Systems
(reference NPO-47980) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing advancements in lithium-ion (Li-ion) electrolytes aimed at improving safety and performance for high-voltage applications, particularly for future NASA missions to the Moon and Mars. As these missions will be "human-rated," there is a critical need for rechargeable batteries that not only provide high specific energy but also possess enhanced safety characteristics.
The document outlines the challenges associated with traditional Li-ion battery technology, particularly its flammability and performance limitations when paired with high-voltage materials. To address these issues, extensive research has been conducted to develop low-flammability electrolytes that incorporate flame retardant additives. These new formulations aim to reduce the risk of fire while maintaining or improving battery performance.
Among the promising electrolytes identified, a formulation consisting of 1.0M lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and triethyl phosphate (TPP) has shown comparable performance to baseline electrolytes in terms of reversible capacity and discharge rate capability at room temperature. This formulation also demonstrates improved safety without sacrificing performance, making it suitable for high-energy systems.
The document emphasizes the importance of developing electrolytes that not only exhibit low flammability but also provide good cycle life, rate capability at low temperatures (down to 0°C), and tolerance to high voltages (up to 5.0V). The baseline electrolyte for the Energy Storage Project was originally developed for the Mars Surveyor Project and consists of a mixture of organic carbonates with LiPF6.
In summary, the document highlights NASA's commitment to advancing Li-ion battery technology for both space exploration and terrestrial applications, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs). The ongoing research aims to create safer, more efficient energy storage solutions that can meet the rigorous demands of future missions and commercial applications. The findings and developments presented in this document are part of a broader effort to enhance the safety and performance of rechargeable battery systems in various high-stakes environments.
