Toward Practical Lithium-Air Batteries by Avoiding Negative Effects of CO2

The gradual popularization of new energy technologies has led to rapid development in the field of electric transportation. At present, the demand for high-power density batteries is increasing and next-generation higher-energy battery chemistries aimed at replacing current lithium-ion batteries are emerging. The lithium-air batteries (LABs) are thought to be the ultimate energy conversion and storage system, because of their highest theoretical specific energy compared with other known battery systems. Current LABs are operated with pure O-2 provided by weighty O-2 cylinders instead of the breathing air, and this configuration would greatly undermine LAB's energy density and practicality. However, when the breathing air is used as O-2 feed for LABs, CO2, as an inevitable impurity therein, usually leads to severe parasitic reactions and can easily deteriorate the performance of LABs. Specifically, Li2O2 will react with CO2 to form Li2CO3 on the cathode surface. Compared with the desired discharge product Li2O2, the Li2CO3 is an insulating solid, which will accumulate and finally passivate the electrode surface leading to the "sudden death" phenomenon of LABs. Moreover, Li2CO3 is hard to decompose and a high overpotential is required to charge LABs containing Li2CO3 compounds, which not only degrades energy efficiency but also decomposes other battery components (e.g., cathode materials and electrolytes). In recent years, researchers have proposed many strategies to alleviate the negative effects brought about by Li2CO3, such as catalyst engineering, electrolyte design, and so on, in which O-2 selective permeable membranes are worth noting. This review summarizes the recent progresses on the understanding of the CO2-related chemistry and electrochemistry in LABs and describes the various strategies to mitigate and even avoid the negative effects of CO2. The perspective of CO2 separation technology using selective permeable membranes/filters in the context of LABs is also discussed.