Energy Storage Technologies for Planetary Science and Astrobiology Missions

Coauthors: Erik Brandon (Jet Propulsion Laboratory, California Inst. Tech) Eric Darcy (NASA Johnson Space Center ) Richard Ewell (Jet Propulsion Laboratory, California Inst. Tech) Peter Faguay (Department of Energy) Brent Futz (California Institute of Technology) Rob Gitzendanner (EaglePicher Technologies) Gerald Halpert (JPL Retiree) Chris Iannello (NASA NESC ) Ian Jakupca (NASA-Glenn Research Center ) Leo Leonine (NASA Goddard Space Flight Center) Simon Liu (Aerospace Corporation) Patricia Loyselle (NASA-Glenn Research Center) Chengsong Ma (SAFT America) Ram Manthiram (University of Texas, Austin) Edward Plichta (General Technical Services LLC) Jeff Sakamoto (University of Michigan, Ann Arbor) Marshall Smart (Jet Propulsion Laboratory, California Inst. Tech) Rao Surampudi (Jet Propulsion Laboratory, California Inst. Tech) Joe Troutman (Enersys / Quallion) William West (Jet Propulsion Laboratory, California Inst. Tech)

Kuiper Belt exploration mission and ii) In-situ Geochronology for the next decade. For these missions the energy storage systems are required to have i) long calendar, operational and cycle lives for outer planetary missions, ii) resilience to either high temperatures on Venus and Mercury, or low temperatures on Mars and Ocean Worlds, iii) ability to tolerate high intensity radiation for Jovian missions and iv) high energy densities for future landers, rovers and probes.
These missions could be categorized as: i) orbiters or fly-bys, ii) aerial vehicles including probes, iii) surface missions (landers and rovers), and sample return missions. Orbital and fly-by missions require rechargeable ES systems (batteries and capacitors) with long cycle life and calendar life, e.g., capacitors in Voyager, Galileo and Cassini or Ni-H 2 or Li-ion batteries for Mars orbiters, Juno and Europa Clipper. Planetary landers and rovers require primary and rechargeable batteries with high specific energy, and an ability to operate over wide temperatures, e.g., Li-ion batteries in Mars Exploration, Curiosity and Perseverance rovers, or primary batteries in Mars Sojourner and future Europa lander. Sample return missions need primary batteries with long shelf life, e.g., Li-SO 2 in Stardust and Genesis. Table-1 lists the performance targets for these missions. Planetary science missions have key performance needs that are similar to the commercial and defense applications, with emphasis on reducing mass and volume of the energy storage systems to increase payload, and improving cycle and calendar life to enable long-life missions. NASA may take advantage of the technology developments being supported by the Department of Energy, Department of Defense and the industry for commercial applications, but will need to adapt and validate them for the missions. Additionally, there are unique technologies for the extreme environments, as in some future missions, which will need to be exclusively developed by NASA. Below is a brief description of the capabilities State of Art (SOA) ES systems and the projected performance from future technologies of relevance to NASA's decadal missions.

State of Art (SOA) Rechargeable Batteries
Rechargeable batteries are being used in solar-powered missions to provide electrical power during eclipse periods and for load-leveling. Likewise, they are also being used in conjunction with radioisotope power systems, e.g., Mars Curiosity and Mars 2020 rover. Li-ion batteries are the baseline batteries in almost all the current and near-term NASA missions, aided by their higher specific energy and energy density compared to the previous aqueous chemistries, e.g., nickel-hydrogen. Cells for the Li-ion batteries could be commercial offthe shelf (COTS), pioneered by Sony HC 18650 cells, and now available from LG Chem, Samsung or Panasonic, or the large-format custom cells, i.e., flatplate or wound prismatic cells manufactured by Yardney Technical Products (EaglePicher Technologies) or GS Yuasa, or cylindrical cells developed by SAFT. By virtue of their high-volume and autonomous production, the COTS 18650 cells from reputed manufacturers have impressive cell-to-cell consistency and uniformity, which eliminate the need for cell balancing and simplify battery charging. Cells are arranged in standard series-parallel or parallel-series configuration to design a battery of desired capacity and voltage with adequate redundancy. Custom Li-ion cells, on the other hand, have the advantages of tailoring the size and chemistry of the cell for the intended application, as was successfully done in the Mars surface missions with low temperature electrolytes. Table-2 shows the performance characteristics of current Li-ion technologies. Current battery designs are prone to (thermal) propagation of the failure of a single cell to the entire module. Propagation-resistant designs are under development but will add to additional mass and volume of the battery. Even without the propagationresistance feature, the battery-level specific energy is only 60-70% of the cell specific energy. With the high specific energy of ~250 Wh/kg from today's COTS 18650 cells, it is possible to achieve 200 Wh/kg at the battery level, with advanced packaging, e.g., with additively manufactured (3d printed) battery structures. Finally, only metal-encased cells are used thus far, and pouch cells, which are the preferred cell formats for the new technologies, are yet to be qualified for space use.

Advanced Rechargeable Battery Technologies
New battery chemistries have been evolving which can potentially improve both the specific energy and energy density beyond the current Li-ion technologies, as listed below: • Si-C-Ni-rich nickel manganese cobalt (NMC) oxide 275 Wh/kg and 600 Wh/l • Li -Ni-rich NMC cathode with liquid electrolyte 300 Wh/kg and 600 Wh/l • Li-Ni-rich NMC cathode with solid electrolyte 300 Wh/kg and 650 Wh/l • Lithium-Sulfur cells in liquid/solid electrolyte 350 Wh/kg and 650 Wh/l Table-3 illustrates the projected performance of these technologies, as well as their technology readiness levels. Among these high energy technologies, solid electrolyte based batteries with Li anode offer the highest specific energies. Ooverall, despite the enhanced performance possible with these technologies relative to Li-ion batteries, their availability at a TRL 5 by 2025 for the decadal missions is uncertain. Among the various systems, all solid state batteries with Li anode and solid electrolytes are the most promising to meet the energy, cycle life and safety targets of future NASA missions and likely to achieve technical maturity by 2025. Li-ion batteries may benefit from further incremental growth at the cell level combined with efficient cell packaging designs and may provide 'safe' batteries with 200Wh/kg and good life characteristics. These batteries may lack adequate low temperature performance, and need to be modified with appropriate lowtemperature electrolytes. Additionally, there are a few high temperature (250-350 o C) systems relevant to Venus missions: i) Sodium-Sulfur (Na-S) and ii) Sodium-Metal Chloride (Na-MCl 2 ), both fully developed for commercial applications with 100 Wh/kg and 1000 cycles, but not used yet in space. Li(Al)-FeS 2 molten salt batteries (465 o C) are being developed at JPL for Venus and Mercury surface missions.

Li Primary Batteries:
Primary batteries are typically used for providing power in missions that require a single use of electrical power for a period of minutes to several hours, or even a few days. Primary batteries used in space missions are: silver-zinc (Ag-Zn), lithium-sulfur dioxide (Li-SO 2 ), and lithium-thionyl chloride (Li-SOCl 2 ). They have been used in planetary probes (Galileo, and Huygens), sample return capsules (Stardust and Genesis) and landers (Mars Exploration Rover) and Deep Impact. Both Li-SO 2 and Li-SOCl 2 batteries have moderate specific energy (150-250 Wh/kg), excellent shelf life of ~10y and operate over a temperature range of −40°C to 60°C.
Advanced Li-primary systems under development include Li-CF x , and Li-O 2 , which offer higher specific energy: 500 Wh/kg (battery) for Li-CF x and >600 Wh/kg for Li-O 2 . While Li-O 2 is still at a low TRL, the Li-CF x technology is now being developed for the Europa Lander mission concept. Table 4 lists the performance characteristics of the SOA and advanced primary batteries.

Fuel Cells
Viable fuel cell technologies under development for space applications include: polymer electrolyte membrane (PEM) fuel cells, solid oxide fuel cells (SOFC), and regenerative fuel cells (RFC) ES systems. Among these systems, H 2 -O 2 PEM primary FC and RFC ES systems are more widely applicable, because of their performance and advanced stage of development, while SOFCs have unique role for Venus aerial and select surface missions due to high temperature operation and fuel flexibility. H 2 -O 2 PEM primary FC are attractive for applications with dynamic power profiles and system sizes ranging from 100's Watts to kW-MW sizes (e.g., Ocean World landers) with potentially higher specific energy than primary batteries. RFCs present an enabling massefficient solution for high energy applications such as surface energy storage for future longduration human lunar (Lunar Surface Initiative) and Martian missions. and can offer >2X specific energy vs. rechargeable batteries at the system level. It should be noted that since the energy storage and energy conversion portions of an RFC are separate and can be individually optimized for the application, the specific energy of similar systems can vary significantly depending on the anticipated power profiles and energy storage quantity. These values do not scale linearly based on solely energy storage quantities and can range from below 250 Wh/kg to over 550 Wh/kg.

Capacitors
Capacitors are typically used in most spacecraft to meet peak power demands. Tantalum capacitors (solid and electrolytic designs) were used in the Galileo and Cassini missions. The advantage of capacitors is their ability to supply high pulses over short durations repeatedly for hundreds of thousands of cycles. Supercapacitors, especially the most recent versions, have improved specific energy at the sacrifice of some power density. This is achieved through the substitution of one of the high surface area electrodes, with a lithium intercalating electrode. These capacitors, termed asymmetric or Li-ion capacitors, can achieve 15 Wh/kg and power density of 5-10kW/kg. The cycle life is many orders of magnitude greater than a Li-ion battery. A hybrid system of suitable capacitor and high-energy battery will have both high energy and power.

Energy Storage Requirements of NASA's Next Decadal Missions
The next decadal mission concepts to Outer planets, Inner planets missions, Mars, Moon and to Small Bodies are discussed below in terms of their environments and energy targets.

Energy Storage Needs for Inner Planet Missions
Venus orbital missions and high-altitude aerial missions are relatively benign, but the lower atmosphere and surface missions are the most challenging, contending with highly corrosive gases, high pressure (90 bars at the surface), high temperatures (465 o C at the surface). However, at an altitude of ~55 km, where the winds are strong enough to enable aerial missions, the conditions are benign and earth-like with 0°C and 1 bar. If aerial missions are contemplated in lower atmosphere, then the batteries need to operate at higher temperatures (350°C) at an altitude of 15 km. Table-5 lists the desired ES technologies and their advancements for inner planet aerial and surface missions.

Energy Storage Needs for Outer Planetary Mission Concepts
There are two categories of outer planetary missions being considered for the next decade: a) Missions to Ocean Worlds and b) missions to the Ice Giants. Potential Ocean World mission destinations include: Enceladus, Europa, Titan, Ganymede, Callisto, and Ceres, while the Ice Giant destinations are Neptune and Uranus. Outer planet missions pose several technical challenges for energy storage systems, which include: a) long calendar life and cycle life capability, b) radiation tolerance (Jupiter system missions) and c) heat/radiation sterilization endurance. Table-6 lists the desired energy storage technologies and their advancements for outer planet missions.

Energy Storage Needs and Technologies for Mars Missions
Possible Mars robotic missions the next decade include: a) Mars orbiters, b) potential Mars sample return missions (includes Mars ascent vehicles, landers, and sample-fetching rovers), c) Mars helicopters and other forms of proposed aerial vehicles, and d) human Mars precursor missions (large landers, rovers, In-Situ Resource Utilization [ISRU] demonstration missions, etc.). Mars surface missions pose different challenges for energy storage systems: lowtemperature operational capability (<−40°C), long-life, high specific energy and high energy density. In addition, Mars aerial missions require very high power capability (>3000 W/kg). Table-7 lists the requirements of energy storage technologies and their desired advances for Mars missions.

Energy Storage Needs for Lunar Surface Missions
Advanced ES systems will be needed for both rovers and central power stations. Power levels for stations can be expected to be >100 kW, which favor RFCs over batteries. Lunar surface missions present two challenges:  Recoverable batteries with 300 Wh/kg and wide operating temperature (-100 to +160 C) with advanced thermal technologies, e.g., heat-pipe embedded additive manufacturing.
 RFCs with >300 Wh/kg at the system level, long life of >5 years, minimal or no required maintenance and the capability to operate over a wide temperatures (-173 C to +125 C). These capabilities will enable long-term operations of central power stations. Additionally, the two concept studies, "Intrepid Planetary Mission (Lunar Rover)" and "Lunar Geophysical Network" will be powered by high specific energy batteries with COTS 18650 cells.

Summary and Recommendations
Further advances in ES technologies are required to fulfil the needs of future NASA planetary science and astrobiology missions in the next decade. Though some of these requirements are synergistic with the industry, many mission architectures and designs are unique, and warrant further improvements in lifetime, thermal capability, environmental compatibility, mass and volume of the ES devices. NASA PSD will need to undertake its own technology programs to alleviate these constraints, while leveraging the DoE / DoD efforts. Below are the recommended ES technologies, which will need to be developed to enable new mission concepts or enhance the current missions. •