Lithium-oxygen battery has the highest theoretic energy density among all rechargeable batteries. But its discharge product Li₂O₂ is difficult to be oxidized back in the charge process, which causes high charge overpotential, serious electrolyte decomposition and poor cyclability. An effective way to solve this problem is adding redox mediator (RM) in the electrolyte to catalyze the charge reaction via a solution phase process. However, the reaction kinetics between the oxidized RM and Li₂O₂ is seldom investigated. Here, taking iodine species as the model RM, we find that the efficiency of the solution phase catalysis is limited by the reaction rate between the oxidized RM and Li₂O₂, which is largely dependent on the electrolyte solvent. Protic solvents, such as water and alcohols lead to high reaction rate because of the hydrogen bond induced solvation of Li₂O₂. However, too high proton activity may also bring unwanted peroxide disproportionation. Among a series of alcohols, n-butanol is found to show proper proton activity and significantly improves the solution phase catalytic efficiency.

Some issues on objectively evaluating the cathode performance of lithium-oxygen battery will also be discussed in this talk. Because of the complexity of the multi-phase electrochemical reaction, conventional half-cell testing methods and parameters to evaluate the performance of lithium-ion cathodes are not suitable for lithium-oxygen cathodes. It is common to use unlimited lithium and unlimited oxygen in half-cell tests. But such tests are just testing the “reusability of the catalyst” instead of the “reversibility of the electrochemical reaction”. The result cannot objectively reflect the parasitic reactions. To objectively evaluate the electrochemical reversibility, we propose a Li₂O₂-graphite cell with limited amount of lithium-ion. Such configuration can eliminate the “fake reversible capacity” caused by parasitic cathodic reactions, and thus is more reliable. We found that the real reversible ratio of the cathode reaction is much improved with the presence of redox mediator and butanol, but still far from satisfactory.

ref:

1. Shen, Y.; Zhang, W.; Chou, S.-L.; Dou, S.-X. Science 2016, 352, 667.

2. Sun, D.; Shen, Y.; Zhang, W.; Yu, L.; Yi, Z.; Yin, W.; Wang, D.; Huang, Y.; Wang, J.; Wang, D.; Goodenough, J. B. J. Am. Chem. Soc. 2014, 136, 8941.

3. Zhang, W.; Shen, Y.; Sun, D.; Huang, Z.; Zhou, J.; Yan, H.; Huang, Y. Nano Energy 2016, 30, 43.

4. Zhang, W; Shen, Y. ; Sun, D.;Huang, Z.; Huang, Y. Adv. Energy Mater. 2017, DOI:aenm201602938.