My research experience reaffirms me the significance of energy storage devices to society, strengthening my resolve to develop rechargeable batteries. I am interested in exploring them comprehensively in my doctoral study. New generations of computing and numerous technological systems, aided by the worldwide presence of the internet infrastructure are placing rechargeable batteries as necessities in the society. The increasing demand for energy storage batteries requires the development of rechargeable batteries.
Rechargeable non-aqueous Li-O2 batteries are regarded as promising alternatives to traditional Li-ion batteries due to their ultrahigh theoretical energy density of 3,500 Wh kg-1, which is about 10-fold higher than that of Li-ion batteries. The key issues for Li-O2 batteries are the high charge voltage and the corrosion of Li anode. The high charge voltage is relevant to the O2 evolution reaction (OER) process, where bulky and insulating Li2O2 particles are decomposed. A general method to tackle it is employing the soluble redox mediator (RM) to facilitate the decomposition of Li2O2 particles. In my previous work, I have developed a new bi-functional redox mediator to accelerate the kinetics of ORR and OER, greatly improving the rate performance of Li-O2 batteries. However, the Li-O2 batteries with this RM did not exhibit excellent cycling performance. This is because micron-sized Li2O2 particles formed during ORR process are found to be difficult to decompose during OER process. Thus, although the anion of the RM is of benefit for the oxidation of bulky Li2O2, the charging over-potentials are not effectively decreased. As the cycle proceeds, the over-potentials needed for OER process increases, leading to more side reactions and early fade of Li-O2 batteries. It is necessary for me to replace the anion of this RM with a more effective one to lower the activation energy process, promote the electrochemical decomposition of Li2O2, and reduce the OER over-potentials. Besides, the RM+ could chemically oxidize the Li anode, which would result in the corrosion of the Li anode. Therefore, it is also a critical factor on long-term cycles to protect Li anode. Building a stable protection layer is a delicate strategy to protect Li anode. The combination of RM and protection layer could work synergistically to reduce the over-potentials and side reactions, achieving great cycling performance. Prof. Grey’s specialization in solid-state nuclear magnetic resonance (ssNMR) could be of great help for me to investigate the electrochemistry inside the cells as ssNMR is sensitive to the electrochemical products formed during cycling, such as LiOH and Li2CO3. The formation of insoluble LiOH and Li2CO3 on the Li anode result from side reactions in the Li-O2 batteries. With the clear signatures for these products, I could gain valuable information that assist me to better design RM and protection layer. In conclusion, I intend to open up a path to promote the cycling performance of Li-O2 batteries using ssNMR.