NASA is planning to launch a mission to perform a detailed reconnaissance of the Jovian icy moon Europa. This mission, entitled the Europa Clipper, would involve a Jupiter-orbiting spacecraft that performs a science investigation of the surface during multiple flybys of the moon by utilizing a number of instruments, including an ice-penetrating radar that is intended to measure the thickness of the icy shell and the possible presence of a subsurface ocean. Ultimately, one objective of future missions to icy moons is to determine their habitability for life. The destination of Europa presents a number of technical challenges, since the spacecraft would be exposed to harsh radiation and extreme temperature environments. Furthermore, to comply with planetary protection protocols, the spacecraft would need to be sterile to avoid any potential contamination.

The baseline power subsystem architecture for the Europa Clipper mission involves the use of solar arrays, power management electronics, and a rechargeable lithium-ion battery. ¹ The preliminary design involves the use of high specific energy small 18650-size Li-ion cells that are assembled into large capacity multi-string batteries. This design approach capitalizes upon the presence of internal cell safety devices, excellent cell-to-cell reproducibility, and the fact that individual cell monitoring and balancing circuitry is not required. To meet the challenging mission requirements, we are currently evaluating a number of potential cell chemistries that could deliver the desired performance. Promising candidates should provide (i) good tolerance to high levels of radiation, (ii) good performance characteristics over a wide temperature ranges, and (ii) excellent storage life characteristics, 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.

In order to demonstrate the viability of candidate 18650-size Li-ion cells for use, a number of performance tests have been performed, including the following: (i) 100% DOD cycling under various conditions, (ii) charge and discharge rate characterization over a range of temperatures, (iii) short term storage tests at high temperatures, (iv) long-term storage tests at +25^oC and 0^oC, (v) module level 100% DOD cycle life testing at +20^oC, (vi) module level long term storage testing at 0^oC, and (vii) characterization after exposure to gamma-irradiation (⁶⁰Co source). A number of these tests are focused upon establishing the long-term storage characteristics, with emphasis upon storing the cells on the bus at a partial state of charge at a fixed potential to mimic the spacecraft environment.² Effort was also devoted to determining if the cell-to-cell voltage dispersion within 8-cell strings diverges when subjected to cycling and/or long term storage.³ As previously mentioned, the cell chemistries must be able to tolerate high doses of radiation, and there is some concern that performance loss may be observed due to the presence of organic electrolyte solutions and polymer binders and separators.^4,5 To address this, cells were incrementally exposed to high levels of gamma-radiation using a ⁶⁰Co source, and periodically characterized in terms of the capacity and impedance. In summary, promising candidate cell chemistries have been identified, and the test program implemented is essential to enable proper end of life performance projections and appropriate battery sizing 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.

 

REFERENCES

^1. A. Ulloa-Severino, G. A. Carr, D. J. Clark, S. M. Orellana, R. Arellano, M.C. Smart, B.V. Ratnakumar, A. Boca, and S. F. Dawson, “Power Subsystem Approach for the Europa Mission”, 11^th European Space Power Conference (ESPC), October 5, 2016.

^2. B. V. Ratnakumar, M. C. Smart, and L. D. Whitcanack, “Storage Characteristics of Lithium-Ion Cells”, ECS Transactions, 25 (36), 297-306 (2010).

^3. F. C. Krause, A. Lawrence, M. C. Smart, S. F. Dawson, A. Ulloa-Severino, and B. V. Ratnakumar, “Evaluation of Commercial High Energy Lithium-Ion Cells for Aerospace Applications”, 227th Meeting of the Electrochemical Society, Abstract #47580, May 28, 2015.

^4. B. V. Ratnakumar, M. C. Smart, L. D. Whitcanack, E. D. Davies, and K. B. Chin, “Behavior of Li-Ion Cells in High-Intensity Radiation Environments”, J. Electrochem. Soc., 151 (4), A652- A659 (2004).

^5. B. V. Ratnakumar, M. C. Smart, L. D. Whitcanack, E. D. Davies, K. B. Chin, F. Deligiannis, and S. Surampudi, “Behaviour of Li-Ion Cells in High-Intensity Radiation Environments: II. Sony/AEA/ComDEV Cells”, J. Electrochem. Soc., 152 (2), A357- A363 (2005).