Unravelling the degradation pathways and charging mechanism in the Li-O2 battery: the role of singlet oxygen

Thesis

Lithium-oxygen batteries (LOBs) have attracted immense research interest as they have the highest theoretical energy density among the advanced battery systems. Achieving such high energy densities can substantially reduce the need for fossil fuels, enable greater use of renewable energy, and revolutionise the transportation sector. However, due to the complexity of oxygen reduction and evolution reaction chemistries and reactivity of reduced oxygen species, LOBs still need significant improvement in their fundamental understanding to overcome the electrolyte and cathode stability issues.

In the Thesis, firstly, the redox mediated charging process was studied to understand the mechanism to enable fast charging with low overpotentials and side reactions. It is demonstrated that the redox mediated oxidation follows Marcus theory of electron transfer and the rate limiting step is the first 1-electron oxidation, Li₂O₂ → LiO₂. The kinetically dominant step is the LiO₂ disproportionation. It is also shown that the singlet oxygen (¹O) yield does not correlate with the amount of degradation. This casts doubt on whether ¹O₂ is the main cause of degradation in LOBs.

Secondly, due to the discrepancy in ¹O and electrolyte degradation amounts, the stability of the common LOB salts and solvents towards ¹O₂ were tested. It is shown that tetraglyme-LiTFSI, one of the most widely used electrolytes, is stable towards ¹O₂ under chemical conditions. In the LOB literature, ¹O₂ is mainly detected and quantified using DMA, a ¹O₂ trap. Here, it is revealed that DMA can react with superoxide depending on the salt-solvent combination and DMAO₂, the reaction product of ¹O₂ with DMA, can degrade into by-products depending on the solvent environment.

Finally, the stability of the electrolyte and the cathode towards ¹O₂ under electrochemical conditions and the real cause of faradaic efficiency loss are investigated. It is demonstrated that ¹O₂ is not the main source of degradation and fresh Li₂O₂ surfaces is the culprit. This finding should redirect the focus of LOB research from avoiding ¹O₂ to developing electrolytes inert towards peroxide and peroxide-derived side products for an electro/chemically stable cell.

Authors: Zor, C

• DOI: 10.5287/ora-ny501pkgk
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Rechargeable batteries; Singlet Oxygen; Li-air batteries; Li-O₂ batteries
• Subjects: Batteries; Energy storage