Promoting Solution Phase Discharge in Li-O2 Batteries
Authors: Johnson Lee , Chen Yuhui , Gao Xiangwen , Bruce Peter G. 
Published: February 2020

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the capacity of Li-ion batteries. We must explore alternatives to Li-ion if we are to have any hope of meeting the long-term needs for energy storage. One such alternative is the Li-air (O2) battery, Fig. 1; its theoretical specific energy exceeds that of Li-ion, but many hurdles face its realization.[1-5]

One spin-off of the recent interest in rechargeable Li-O2 batteries, based on aprotic electrolytes is that it has highlighted the importance of understanding the fundamental processes at the positive electrode within the battery.[6-
12
] Based on these studies, it is generally accepted that a solution growth mechanism will be required to achieve high rates and capacities. One way to achieve discharge in solution is use of high donor or acceptor number (DN/AN) electrolytes, which might be less stable towards LiO2 and Li2O2 than their low DN/AN counterparts. To solve this dilemma, a reduction mediator was introduced into the low DN/AN electrolyte to encourage the discharge in solution. By forming an intermediate complex, it suppresses the direction reduction to Li2O2 on surface and thus leads to higher capacity on discharge with higher rates. Furthermore, if Li2O2 is formed in solution a charge mediator will be required, as the traditional electrode configuration is unable to oxidize the product. The implication of this is complete solution phase cycling where solid Li2O2is simply a storage material for lithium ions and electrons.
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