Overview of LiO₂ Battery Systems, with a Focus on Oxygen Handling Requirements and Technologies

Abstract

The reactions of Li and O₂ to form Li₂O₂, and of Li, O₂, and H₂O to form LiOH·H₂O, have exceptional energy content but are adversely affected by components of air such as CO₂ (for both cases) and H₂O (for the Li₂O₂ case). Hence, a method is required to supply O₂ while excluding contaminants. In this chapter we focus on O₂ supply for both a closed system (in which tanks store pure O₂ at pressures up to 350 bar) and an open system (in which CO₂ and possibly H₂O are removed through a series of unit operations). In particular, we consider the implications of the O₂ supply method on the specific energy and energy density at the system level, as well as other system attributes such as cost. For the closed (tank) system we find that with the use of a carbon fiber tank, for the reaction forming Li₂O₂, the specific energy is twice that of a comparison cell (one pairing Li metal with an advanced intercalation metal oxide), but the energy density is about 30 % lower. For the reaction forming LiOH·H₂O, the specific energy is about 40 % above that of a Li/metal oxide cell, but the energy density is 50 % lower. A unique challenge for the closed system is the need for high-pressure compression. An open system may be enabled through the combined use of several gas separation steps (including a membrane and solid adsorption) as well as a compressor to drive the air. The required purity of an O₂ supply stream remains unclear, but for a reduction of CO₂ and H₂O to levels of 1 ppm, the mass and volume of the O₂ supply equipment for the open system is comparable to that of the closed system. A unique challenge for the open system is safely charging in closed environments where the O₂ emitted does not quickly dissipate. For both types of systems, handling any volatile cell components (e.g., solvents) may be a challenge (for the closed system they may enter the high-pressure O₂ tanks, while in the open system they may be lost to the atmosphere), and potential technologies to address volatiles are not included in this analysis. We encourage Li/O₂ researchers to investigate sets of nonvolatile materials that may improve the robustness of the cell chemistry to the presence of air contaminants.