Characterization of Reaction Products Formed in Metal-Oxygen Batteries with Multinuclear Solid-State NMR

Abstract

This thesis has investigated electrolyte stability for the Li-O2 and Na-O2 batteries and the viability of applying multi-nuclear solid-state NMR to characterize their respective electrochemistries. Metal-oxygen batteries have extremely high energy densities, making them very attractive candidates for electric vehicle applications. However, the exact nature of the metal-oxygen battery chemistry is still not well understood. In this work, 7Li nutation spectroscopy was demonstrated to be a diagnostic tool for distinguishing Li2O2 (desired product) from Li2CO3 (undesired species) in electrochemically-cycled electrodes. The TEGDME electrolyte was shown to display superior stability to the TMP electrolyte. With 17O NMR, Li2O2 and evidence of electrolyte breakdown was observed in the TEGDME cell, whereas only electrolyte breakdown products were discovered within the TMP cell. A detailed breakdown mechanism for TMP was proposed, which accounts for the reaction products observed in the 1H, 7Li, 17O and 31P NMR spectra of the cycled electrodes. The expected Na-O2 reaction products were establish to have unique 23Na NMR signatures, allowing the desired Na2O2 and NaO2 to be distinguished from the undesirable Na2CO3 using; 1D, 2D 23Na-3QMAS, VT NMR and T1 filtering experiments. 23Na NMR of cycled Na-O2 electrodes revealed a newly observed species, NaF in addition to the expected products; Na2O2 and Na2CO3. 23Na NMR and PXRD of a mock electrode confirmed that NaO2 degrades the carbon electrode (carbon C65 + PVDF), producing NaF and Na2CO3, in the absence of the electrolyte. This thesis highlights the value of solid-state NMR, which revealed the instability of NaO2, and points to future research in topics of reagent stability and lifetimes for metal-air cells.