Another big NASA project is heading to Mars – this time to study the interior of the planet. The InSight lander plans to touchdown at a soft surface near the equator, activate bunch of probes, and start sending data to Earth. It should operate for at least one Martian year (the equivalent of ~2 Earth years). In the meantime, it’s only lifeline in this terribly hostile environment will be a pair of multijunction solar panels and a lithium-ion battery. Quite a unique battery.
Li-ion power in space
The NASA-Jet Propulsion Laboratory (JPL) already has a few similar research missions under its belt, with much of its technology having functioned in mind-blowing conditions and some with surprising reliability. For example, Mars Exploration Rover (MER) Opportunity has been performing measurements since 2004. That is 14 years in a place where the temperatures can swing more than 100ºC from day to night and the radiation is orders of magnitude higher than here on Earth. Mars Science Laboratory (MSL) Curiosity rover has been driving around since 2012.
The essential power comes from what one could call a truly ‘sustainable energy’ couple – solar cells and batteries. Solar panels are the main source, while batteries store excessive energy and help with periods of peak power demand. The past 20 years have seen a shift from nickel-hydrogen technology (low specific energy, but very high cycle life) to nickel-based lithium ion (higher specific energy, yet still ‘satisfactory’ cycle life). The Li-ion chemistry has gone from nickel-cobalt oxide (NCO) to nickel-cobalt-aluminum oxide (NCA, where the addition of Al improves stability). The latter one, NCA, is having its ‘debut’ on the InSight mission.
Note: On a few occasions (e.g. 2012 Curiosity and 1976 Viking 1&2), the main power source was a plutonium-based thermoelectric generator, known as RTG, but this technology might get dedicated to deep-space missions due to 238Pu scarcity issues and lack of sunlight beyond Jupiter. Fuel cells have never been used for planetary science missions.
Mars peculiarities and requirements
Not all missions have the same technical requirements (if interested, see NASA-JPL’s recent technical report on current and future energy storage technologies for planetary missions). The main problem for the Mars lander and rover batteries is the extreme changes in surface temperature.
Mars is a cold planet with an average temperature of about –55 ºC (–67 ºF). Moreover, the temperatures near the equator range from about 35 ºC (95 ºF) on a summer day down to about –110 ºC (–166 ºF) on a winter night. The swings are enormous – regularly almost 100 ºC (180 ºF) within one day. External battery heaters and radiators help to bring the lower limit up to –30 to –20 ºC, but it shouldn’t come as a surprise that the whole electrochemistry is optimized to handle the rest. Radiation does not appear to be a critical issue(1).
Batteries for Mars missions
Li-ion chemistry used for Mars surface missions actually didn’t change for many years. The first real test of the technology was 2004 MER rovers Spirit and Opportunity, where nickel-cobalt oxide (NCO, or LiNi0.8Co0.2O2) was used as cathode, meso-carbon microbeads were used as anode, and the low-temperature electrolyte was based on a ternary mixture of carbonate solvents(2).
Apparently, NASA was so happy with the battery performance that it decided to use the same chemistry in many of its subsequent missions, including 2007 Phoenix and 2011 MSL Curiosity. The Opportunity rover has been operational for more than 5000 sols (sol = solar/Martian day ≈ 1 Earth day) and the Curiosity for more than 2000 sols so far. That’s what one would call a good cycle and calendar life.
Batteries for these robots are all custom made. NASA-JPL researchers have developed them in collaboration with the US manufacturer Yardney Technical Products (now part of EaglePicher Technologies). Typically, the individual cells have a prismatic format (~10 Ah) with a moderate specific energy (~140 Wh/kg at 20 ºC), and – here it is – a wide operating temperature range from –20 to +30 ºC(2). What is remarkable is that at –20 ºC they are capable of delivering ~75% of their ambient-temperature capacity.
Batteries on InSight lander
The battery technology has improved since 2002 and also 2012, so the InSight mission will have new-generation chemistry on board. The cathode is nickel-cobalt-aluminum oxide (NCA, or LiNi0.8Co0.15Al0.05O2), the anode is a modified graphite, and the electrolyte is based on a mixture of carbonate solvents (ethylene carbonate, EC, and ethyl methyl carbonate, EMC) with ester co-solvent (methyl propionate, MP; in vol % ratio 20 EC : 60 EMC : 20 MP). This NCA/MP-containing electrolyte combination has shown significantly better capacity and stability over the previous chemistry(2). The operating temperature range has increased to –30 to 35ºC.
The battery configuration remains the same as in the previous Mars surface missions – two 8-cell batteries connected in parallel (i.e. 8s2p). An on-board battery management system (BMS) is essential for controlling and balancing the cells. And as the NASA-JPL’s performance requirements state:
- ‘The battery shall support 709 sols of surface operations over a temperature range of –30 ºC to +35 ºC
- Each 8-cell battery shall provide at least 25 Ah at -25 ºC beginning of life over the voltage range of 24.0 V to 32.8 V using a C/5 rate (5 A)
- Each 8-cell battery shall be able to support a 5 A charge rate over the entire allowable flight temperature range of –30 ºC to +35 ºC’
It takes some time and rigorous examination to decide on a battery that would be sent on a ~500 million kilometer journey costing 800+ million USD. The NASA-JPL has been testing the new chemistry for multiple years now. See this article and presentation, if you want more details about test conditions and results. The test conditions basically simulated those on Mars: charging and discharging the cells at varying temperatures between –30 and +35 ºC and at different rates (mainly C/5). The cells were also tested for full flight simulations – including launch pad storage, cruising period to Mars (storage at 70% SoC for 7 months), and entry–descent–landing pulse load profile simulation. And then, of course, years-long cycle life testing.
This battery chemistry has been chosen also for the Mars 2020 rover mission, so good luck!
Please follow the page or DOI (Digital Object Identifier) link to see the individual articles:
(1) NASA/Jet Propulsion Laboratory-Caltech report, Energy Storage Technologies for Future Planetary Science Missions (Publication ID: JPL D-101146)
(2) Smart et al., The use of lithium-ion batteries for JPL’s Mars missions (DOI: 10.1016/j.electacta.2018.02.020)
(3) Smart et al., Life verification of large capacity Yardney Li‐ion cells and batteries in support of NASA missions (DOI: 10.1002/er.1653)
(4) Ratnakumar et al., Lithium batteries for aerospace applications: 2003 Mars Exploration Rover (DOI: 10.1016/S0378-7753(03)00220-9)
(5) Ratnakumar et al., Lithium batteries on 2003 Mars Exploration Rover (DOI: 10.1109/BCAA.2002.986367)