On November 26, 2018, the NASA InSight spacecraft successfully landed on the surface of Mars, with the intent of investigating the interior structure of the planet. The Mars InSight mission, which is an acronym for “Interior Exploration using Seismic Investigations, Geodesy and Heat Transport”, consists of a single lander performing geophysical measurements and studying its deep interior.^1,2 The spacecraft, which was built by Lockheed Martin Space Systems Company, is based upon a design that was successfully used for NASA’s 2007 Phoenix Mars lander. However, unlike the previous lander and rover missions to Mars, the InSight mission required a higher specific energy battery that can operate over a wider temperature range, with both charging and discharging from -30^oC to +35^oC. To meet these challenging mission requirements, the project adopted the use of the next generation Li-ion chemistry, which consists of graphite-LiNiCoAlO₂ (NCA), coupled with a JPL developed low temperature electrolyte comprised of 1.0M LiPF₆ in ethylene carbonate (EC) + ethyl methyl carbonate (EMC) + methyl propionate (MP) (20:60:20 vol%)^3-7. This advanced Li-ion chemistry, which was manufactured by Eagle Picher-Yardney Division, has proven to be mission enabling and has displayed good performance over a wide temperature range, including excellent storage and cycle life characteristics. To meet the primary mission requirements, the battery must support a calendar life of four years and operation on the surface of Mars for 709 sols (a duration of over one Martian year).

As noted above, one of the more challenging mission requirements is the ability to both charge and discharge the battery at very low temperatures (-30^oC) using moderately aggressive rates (i.e., C/5 rates based on the nameplate capacity of 25Ah). In addition to having to support the power and energy requirements under these conditions, there was some concern that lithium plating on the anode could occur during the low temperature charging which may lead to performance degradation.⁸ Furthermore, there was some concern that the low temperature capability of the battery could be compromised by being subjected to long, high temperature operation, making end of mission requirements difficult to meet.

To address these concerns, a comprehensive performance test program was under-taken, which included the following: (i) determining the impact of high temperature exposure, (ii) simulating the launch pad storage conditions, (iii) continuously cycling between the temperature extremes (+30^oC, +20^oC, -25^oC, and -30^oC) with periodic diagnostic capacity and impedance characterization at +30^oC, -25^oC and -30^oC, (iv) low temperature charge and discharge characterization (-25^oC to -40^oC), and (v) accelerated mission relevant testing, consisting of 60% DOD cycling over a wide temperature range.

In summary, the InSight Li-Ion chemistry with an ester-containing low temperature electrolyte delivered improved performance compared to the compared to the heritage NCO-based chemistry which has been used on previous rover and lander missions to Mars, including: (i) >15% higher capacity and energy at ambient temperature, (ii) superior low temperature performance (-25^oC and below), and (iii) improved resilience to high temperature exposure. Furthermore, the battery chemistry was demonstrated to be very resilient to continuous operation at -30^oC, with no indirect evidence of lithium plating occurring or noticeable performance degradation observed. Thus far, the flight batteries have completed over 100 Sols of operation of the surface of Mars and have displayed excellent performance and health characteristics.

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 Mars InSight Mission.

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