Alkali Metal-O₂ Batteries. Performance and Lifetime Limiting Effects

The rechargeable Na-O2 and Li-O2 batteries are attractive battery technologies as they potentially are very cheap and as they theoretically possess about 3 and 10 times higher energy density than the current Li-ion technologies. This PhD thesis is dedicated to studying the effects that limit cell performance of these two technologies. 

The Li-O2 battery was first introduced in 1996 and focus in the field is still on understanding the fundamental mechanisms controlling discharge and charge. This PhD thesis was mainly dedicated to the Li-O2 battery and initially charge conduction through the discharge product, Li2O2, was investigated. This was done by using of a conventional three electrode cell in which the heterogeneous electron transfer rate of three different redox couples were studied on Li2O2 coated glassy carbon electrodes to provide a measure of the conductivity of the Li2O2 layers. Charge transport through Li2O2 gives further evidence that hole transport dominates charge-transfer through Li2O2. Electrochemical impedance spectroscopy was also used to conduct detailed investigations of surface capacitance, ion transport, and chargetransfer reactions in the cathode of the Li-O2 cell. The capacitance of the cathode was shown to be sensitive to the thickness of the deposited Li2O2 layer. These investigations also explored the influence of the composition of the electrolyte and conditions, which favors a solution mediated Li2O2 deposition mechanism. On charge, an electrochemical “safe” operating voltage was identified until 3.30 V were an interface layer was formed, which activates side reactions and further increases the cell potential. A number of ionic liquids were also investigated for their oxygen diffusivity and solubility and while these were in the order of currently employed aprotic electrolytes as the ionic liquids significantly decompose under electrochemical operation. Last, the influence of CO2 was investigated and it was suggested that CO2 blocks the step valleys of the deposited Li2O2 forcing Li2O2 growth away from the electrode surface hereby increasing cell capacity, as the discharge becomes less limited by the cathode surface area.

The Na-O2 battery is an even newer technology as it was first introduced to the scientific community in 2010. The two batteries are experimentally quite similar as the only difference is the choice of anode. However, when one studies the two systems, the mechanisms controlling each type of battery are quite different. The discharge and charge processes of the non-aqueous Na-O2 battery were studied in this thesis. On discharge, the deposition mechanism of NaO2 was shown to be highly dependent on the current density and cell limitations could be correlated to the depositions mechanisms. On charge, three regions of NaO2 oxidation were identified, each corresponding to a different type of NaO2 oxidation.