Fundamental mechanisms in Li-air battery electrochemistry

The lithium-air (or Li-O2) batteries have received wide attention as an enabling technology for a mass market entry of electric vehicles due to a potential capacity much higher than current Li-ion technology. The technology is a relatively new battery concept proposed in 1996, and the current research still focuses on developing an understanding of the reactions inside the battery.

This thesis is dedicated to increase this understanding and use the knowledge to improve the performance of the battery, and the work span from detailed investigation of the atom positions to the proposal of a system used to manage a full size electric vehicle battery. An automated differential electrochemical mass spectrometer (DEMS) was built to investigate the relationship between current and the consumption and release of gases, which is important to identify and quantify degradation reactions.

The setup was used to characterize our carbon-based reference system as well as new ionic liquid-based electrolytes. Electrochemical impedance spectroscopy (EIS) has been used extensively to describe reaction mechanisms inside the battery; the origin of the measured overpotentials; and the onset potential for electrochemical degradation.

It was confirmed that the rapid potential loss near the end of discharge could be explained by an increase in the charge transport resistance; that the initial Li2O2 oxidation at 3.05 V was blocked by the formation of an SEI layer; and that the voltage increase during charge was primarily due to the formation of a mixed potential between competing oxidation reactions needed to maintain a constant current.

The knowledge about impedance spectroscopy was used to propose and investigate a novel battery management tool to estimate the state of charge and the state of health of a Li-O2 battery system better than any other method available. Finally, calculations were made to support that an open system configuration is a realistic option in terms of air purification, if H2O and CO2 levels at 1 ppm are allowed.