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SUMMARYLithium-ion batteries have been used on a number of NASA missions and have been base-lined for use on a number of up-coming aerospace applications. The Li-ion cells and batteries that have been developed together with Yardney Technical Products are especially attractive due to their high specific energy and energy density, good performance over a wide operating temperature range, as well as their good calendar and cycle life performance. However, given that the Li-ion technology is relatively new to the aerospace community and that mature, large capacity prototype cells have continued to evolve over the last 10 years, real-time performance test data is especially valuable in demonstrating the capabilities over a number of environmental and application specific conditions. For this reason, we have focused on performing a number of generic and mission specific performance life tests to establish the viability of the technology to meet current and future applications. In this work, we will describe the results of a number of generic cycle life tests, including 100% depth-of-discharge (DOD) cycling at various temperatures, partial DOD cycling simulating planetary orbiters and low earth orbit (LEO) satellite conditions, as well as partial DOD mission specific testing. We will also describe the results obtained from a number of calendar life tests where the cells were stored under different conditions, such as at different temperatures and different storage modes (i.e. open circuit voltage (OCV) conditions and storage under trickle charge conditions). Methods to quantify the capacity degradation and impedance growth of the batteries when subjected to different electrical and environmental conditions will be discussed, as well as, possible degradation mechanisms that can lead to reduced lifetime.

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With the intent of improving the performance of Li-ion cells over a wide operating temperature range, we have investigated the use of co-solvents to improve the properties of electrolyte formulations. In the current study, we have focused upon evaluating promising electrolytes which have been incorporated into large capacity (7 Ah) prototype Li-ion cells, fabricated by Yardney Technical Products, Inc.The electrolytes selected for performance evaluation include the use of a number of esters as co-solvents, including methyl propionate (MP), ethyl propionate (EP), ethyl butyrate (EB), propyl butyrate (PB), and 2,2,2-trifluoroethyl butyrate (TFEB). The performance of the prototype cells containing the ester-based electrolytes was compared with an extensive data base generated on cells containing previously developed all carbonate-based electrolytes. A number of performance tests were performed, including determining (i) the discharge rate capacity over a wide range of temperatures, (ii) the charge characteristics, (iii) the cycle life characteristics under various conditions, and (iv) the impedance characteristics.