Investigation of oxidation in nonaqueous lithium-air batteries

Abstract

The demand for clean energy in portable applications is driving the development of high specific energy batteries, which will enable automobiles powered by electricity derived from renewable energy sources such as solar and wind. Lithium-air batteries are a promising avenue for advancing the energy storage capabilities beyond that of current lithium-ion technology. These batteries face a number of challenges which prevent their practical implementation in devices. This thesis explores possible mitigations for two of these challenges: (1) the high charging overpotential and (2) the volatility and poor oxygen conduction of liquid electrolytes in Li-air batteries. In the first part, Vulcan carbon-based electrodes were developed where chemically-synthesized lithium peroxide was included during the electrode preparation process. Variants of these electrodes which further included noble metal catalyst nanoparticles (Au, Pt, and Ru) were also prepared, and Pt and Ru were both demonstrated to begin oxidizing Li₂O₂ 500 mV lower than required for carbon-only or Au-containing electrodes. Using a differential electrochemical mass spectrometer (DEMS) designed and built over the course of this thesis, we showed that Ru-containing electrodes produce oxygen throughout the oxidation of Li₂O₂, while Pt generated both carbon dioxide and oxygen, indicative of electrolyte decomposition. These results served as a foundation for future efforts to develop solid catalysts for the oxidation of Li₂O₂ in Li-air batteries. In the second part, Li-O₂ devices using a solid electrolyte based on poly(ethylene oxide) (PEO) were developed. The discharge performance at room temperature and 60 °C was characterized, with dramatically higher discharge capacity and rate capability achievable at the elevated temperature. DEMS was used to show that the gases evolved during charging in argon were sensitive to the temperature of charging, with additional carbon dioxide observed at and above 50 °C. Finally, the autoxidation of PEO at 60 °C in Li-O₂ environments was studied, where NMR and DEMS measurements showed that the rate of PEO autoxidation increases with increasing applied potential, and that this reaction has a significant impact after only one charging cycle, identifying another condition that must be met for stable and practical Li-air batteries.