Batteries for Oxygen Concentrators

Advanced high energy battery systems are sought for use in Exploration Medical Capabilities mission applications such as power for mobile oxygen concentrators. There are only a few battery chemistries with a reasonable chance of achieving the target specific energies (Fig. 1). Metal-air battery systems are the most likely candidates. The most common type of commercial metal-air battery utilizes zinc-air chemistry and has a practical specific energy of ~370 Wh/kg. While this battery chemistry has a theoretical specific energy of 1350 Wh/kg, it is not possible for this chemistry to meet the specific energy goals for these applications (>2000 Wh/kg). In addition to zinc-air batteries, aluminum-air batteries are also available in the commercial market, although only in a limited fashion. Aluminum-air batteries have a much greater theoretical specific energy (8140 Wh/kg) and although they currently have a practical specific energy of ~350 Wh/kg the potential for significant near‐term improvement exists. The highest theoretical specific energy for a metal-air battery chemistry is lithium-air at 11,500 Wh/kg giving it and aluminum-air batteries the best potential to realize the high specific energy values needed for Exploration Medical Capabilities mission applications.

Figure 1. Top – Energy densities of various battery technologies (adapted from NREL).
Bottom – Theoretical specific energies of various metals which can be utilized in metal-air battery technology.

A lithium-air battery has three main components: an anode, electrolyte, and cathode (Fig. 2). The anode is the source of lithium-ions and is typically lithium metal. The electrolytes can be aqueous, aprotic (organic), mixed aqueous/aprotic, or solid state. Each of these types of electrolyte systems is being researched today and each has its own set of advantages and disadvantages. The final component of a lithium-air battery is the cathode, which as is stated in the name of this technology, is air – or more accurately stated, the oxygen in the air. Being that the cathode materials is supplied by the oxygen in the air the mass of the cathode is very small, thus imparting a significant savings in the mass of the overall system and the theoretical specific energy. However, the oxygen still needs a platform for the electrochemical reactions of the battery to take place. These reactions are supported by the use of porous carbon materials that are in some cases coated with a catalytic metal oxide, such as MnO2 or CoO2.

Figure 2. a – General schematic of a lithium-air battery ¹

Figure 2. b - Four types of possible architectures for lithium-air battery technology ²

References
1. Takeshi Ogasawara et al. (2006) Rechargeable LiO₂ Electrode for Lithium Batteries. J. Am. Chem. Soc., 128 (4), pp 1390–1393 doi: 10.1021/ja056811q
2. G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke (2010) Lithium-Air Battery: Promise and Challenges. J. Phys. Chem. Lett., doi: 10.1021/jz1005384

Electrochemistry