The Impact of Radiation Exposure upon Lithium-Ion Batteries for Future Planned NASA Missions to Europa
Authors: Smart Marshall C. , Krause Frederick C. , Ratnakumar Bugga V. , Ulloa-Severino Antonio , Mnatsakanian Armen , Bienvenu Lindsey , Dembec Jacob , Mault Travis 
Published: February 2020

NASA is planning to launch a mission entitled the Europa Clipper which is intended to provide a detailed reconnaissance of the Jovian icy moon Europa. With one major objective being to determine the habitability of life on the moon, the mission will involve multiple flybys and will utilize a number of instruments, including an ice-penetrating radar that is intended to measure the thickness of the icy shell and possibly detect a subsurface ocean. As one would expect due to distances involved, the destination of Europa presents a number of technical challenges for the spacecraft, including being exposed to high levels of radiation and extreme temperature environments. Furthermore, NASA has stringent planetary protection protocols that must be followed that involves sterilizing the spacecraft to to avoid any potential contamination.
As described previously1,2, the current power subsystem architecture for the Europa Clipper mission involves the use of solar arrays, power management electronics, and a rechargeable lithium-ion battery.3 With respect to the Li-ion battery, we are planning to utilize a “small cell” approach, which will consist of multiple high specific energy small 18650-size Li-ion cells that are assembled into large capacity multi-string batteries. Due to the excellent cell-to-cell reproducibility and proper cell matching, individual cell monitoring and balancing circuitry is not required with this approach. NASA has previously utilized this approach on a number of past missions, including Kepler, Aquarius, and SMAP. To meet the challenging mission requirements, we identified leading candidate cell chemistries that are projected to meet the mission requirements. However, in contrast to these previous missions which have utilized heritage Sony HCM 18650 cells, we are planning on using much higher specific energy cells to reduce the overall spacecraft mass. Given that we would like to use a new cell chemistry, it necessitates performing a cell qualification activity. Furthermore, the candidate cell chemistry must be demonstrated to have good cycle and storage life characteristics, especially since the cruise period to Europa is extremely long and could potentially be over 6.5 years in duration, depending upon the launch vehicle selected.
To reduce the bio-burden of spacecraft components, dry heat microbial reduction (DHMR) is typically employed, which involves subjecting the samples to exposure at high temperatures (in excess of 150oC). However, since Li-ion cell technology will rapidly degrade at high temperatures, this methodology is not acceptable and alternative methods of achieving sterility must be adopted. The preferred method with respect to the Li-ion batteries is to subject them to high levels of gamma-irradiation, which has previously been demonstrated to have a minimal to low impact upon the performance characteristics.4,5 To assess the impact that would be sustained by exposure to γ-rays prior to launch to comply with planetary protection protocols, as well as to address the radiation exposure during the course of the mission, we have exposed a number of Li-ion cells and a large capacity battery module to gamma-irradiation (60Co source) at very high levels (up to 20 Mrad) to evaluate the potential impact.
In order to demonstrate the impact of irradiation, a number of performance characterization tests were implemented on samples subjected to varying levels of γ-rays (either 12 Mrad or 20 Mrad), including: (i) 100% DOD cycling under various conditions, (ii) charge and discharge rate characterization over a range of temperatures, (iii) module level testing and, (vi) verification of proper functionality of internal safety protection devices. As will be discussed, good performance has been obtained with candidate high specific energy Li-ion cells, and minimal capacity loss was observed after being subjected to high levels of radiation.
ACKNOWLEDGEMENT
The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and supported by the Europa Clipper Mission.

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