Designing Batteries for Space Exploration

The challenge of designing batteries for the Europa Lander mission

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“Space, the final frontier… to boldly go where no man has gone before.”

— Star Trek

Space exploration involves venturing into some of the universe's most inhospitable regions where human presence is impractical, necessitating the use of robotic spacecraft. These spacecraft require dependable power sources for their missions, which are typically supplied by energy generators such as photovoltaic solar arrays or radioisotope thermoelectric generators. However, there are times when this energy production is insufficient, particularly when there's a demand for peak power that exceeds what the generators can provide. This is why energy generators are often coupled with energy storage systems like batteries.

Batteries can be categorized as either primary or secondary. Primary batteries are designed for single use, providing power without the need for recharging, which makes them ideal for space missions where no additional energy generation or storage is required. Examples of planetary probes that used Li-SO2 primary batteries include Galileo and Cassini.

Figure 1. Schematic of a typical secondary lithium-ion battery cell. Source: JPhys Energy

Conversely, secondary batteries are rechargeable (e.g., lithium-ion; see Figure 1), allowing them to be cycled through discharge and recharge multiple times, thus providing sustainability for extended space missions. The European Space Agency's experimental Proba-1 Earth-observing mission in 2001 was the first to use rechargeable lithium-ion batteries in space.

What kind of constraints are batteries subjected to in these extreme environments? How different are these batteries from those we use every day in our smartphones or electric vehicles? How does one design such batteries, and who manufactures them? These are some of the questions we will address in this article. We will begin by describing the rigorous conditions batteries face in space, followed by a brief history of batteries used in space missions, their performance requirements and design recommendations. We will conclude with a discussion on the spacecraft for Europa missions and a list of companies that produce batteries specifically for space applications.

Let's dive in! 🔋

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Extreme Conditions

Energy storage requirements for space exploration vary depending on the destination and the nature of the mission. Nevertheless, these systems consistently need to withstand a variety of severe environmental conditions such as extreme temperatures, radiation, microgravity, and vibration, all while providing sufficient power and energy output.

To grasp the severity of these conditions, consider Venus. Landing a rover on its surface would require energy storage capable of operating at approximately 500°C under about 90 atm of pressure. This is roughly 20 times the ideal temperature and 90 times the standard pressure for commercial lithium-ion battery energy storage.

Extreme pressures present challenges for the structural design of battery packs. A certain amount of pressure can enhance contact between the various layers of a battery cell, but excessive pressure can disrupt ion conduction, leading to a reduction in battery cycle life.

Standard lithium-ion batteries operate optimally between 10°C and 40°C; temperatures outside this range result in degraded performance. Too low temperatures increase the viscosity of the electrolyte and slow down reaction kinetics, thereby increasing cell resistance and the risk of electrical shorts. Conversely, high temperatures cause electrolyte degradation, melting of separators, and eventually cathode decomposition, which releases oxygen and triggers thermal runaway.¹

Beyond temperature and pressure, radiation is another critical factor. Earth's magnetosphere protects the planet from much of the harmful radiation, but once beyond this shield, spacecraft are exposed to intense irradiation without adequate shielding. Radiation exposure can alter material properties, leading to unpredictable and often degraded behavior (see Figure 2).

Ionizing radiation, like gamma rays, harms liquid organic electrolytes by generating free radicals, which then react with other components within the battery cell. This radiation also cross-links polymer binders, impacting the structural integrity of battery electrodes. Additionally, it can modify the pore structure of polymer separators, changing their wetting properties. Neutron or ion irradiation induces defects in the crystal structure of materials. The ramifications include a decrease in battery capacity, reduced cycle life, changes in the solid-electrolyte interface, and an increase in cell resistance.

Figure 2. Gamma ray radiation effect on batteries: a) increase in cathode grain size with irradiation and b) delayed capacity loss. Source: Carbon Neutralization

Mechanical phenomena like vibration, acceleration, and impacts are additional considerations, particularly during events such as liftoff or landing. Vibration can compromise interconnections, including electrode-to-tab welds or terminal-to-busbar connections within a battery module or pack.