Lithium-Sulphur batteries as a key enabling technology for future space applications
The electrical power subsystems designed for space applications face uniquely harsh environments over their lifetime. Yet, today they are required to cope with new trends driven by the fast-growing space business: increasingly energy hungry payloads, longer mission durations, increasingly stringent safety requirements, lower satellite masses and smaller volumes. The particular characteristic that all battery engineers and researchers strive to optimise is the specific energy, i.e. the energy per weight and/or volume ratio. It is a major criterion in the battery choice as every gram sent in orbit has to be paid for in launch costs.
The current state of the art of energy storage for space applications is represented by the Lithium-Ion technology, which came as a true evolution in comparison to the previous Nickel-Hydrogen technology. It has allowed to double the specific energy in electrical systems over the last 15 years. However, Lithium-Ion technology seems to have reached the end of its product lifecycle, as ongoing Li-Ion developments have reached a stage where the chemistry performances have peaked to an extent that further developments remain unlikely.
Fortunately, the next generation battery technology is around the corner; this time its chemistry is based on Lithium-Sulphur (Li-S). Li-S batteries, developed by OXIS Energy Ltd, have shown impressive performances, particularly in terms of specific energy. Li-S could therefore represent the new breakthrough technology for space batteries. The specific energy is expected to double when compared to the currently available Lithium-Ion batteries.
The figure below illustrates the energy density map of recent battery technologies and outlines the advantages of Lithium-Sulphur technology in terms of battery mass and volume.
The schematic setup of a Li-S cell is similar to that of conventional batteries. The cells are composed of an anode, a cathode and two half-cells of electrolyte separated by a porous diffusion separator. During discharge, lithium dissolves from the anode surface, just as sulphur polymers dissolve at the cathode to form Lithium-sulphides. While each sulphur atom can host two lithium ions, lithium-ion batteries can accommodate just 0.5 to 0.7 lithium ions per atom. Therefore, Li-S allows for a much higher lithium storage density than conventional lithium-ion batteries.
The key advantages of the Lithium-Sulphur (Li-S) technology can be summarised as follows:
- Lightweight System: Power systems based on Li-S technology will be lighter than systems with the same energy content relying on Lithium-Ion cells.
- High Safety: The Li-S chemistry is safer then Li-ion and the cells have been proven to survive a barrage of electrical and physical abuse, without showing any adverse reaction.
- Extended Temperature Tolerance: The OXIS chemistry and technology is ideally suited for a deployment at both low (‐20°C) and high temperatures (+80°C).
- Great Cycle life: Li-S can be cycled over 1500 times with an 80% Depth of Discharge before its capacity reduces to 80%.
- Full Depth of Discharge Capability: Li-S cells have the potential for 100% Depth-of-Discharge and remain safe during over-discharge, in stark contrast to Li-ion batteries.
- Maintenance Free: Li-S cells have an indefinite shelf-life with no charging required to prevent damage even when stored for an extended period.
- Environmentally Friendly: The Li-S chemistry is considered to have a lower environmental impact in comparison to other battery technologies.