Combining Electrochemistry and Photoelectron Spectroscopy for the Study of Li-ion Batteries

Sammanfattning: In this thesis photoelectron spectroscopy (PES) is combined with electrochemistry to investigate the electrochemical processes that occur at the electrode/electrolyte interfaces in lithium-ion batteries (LIBs). LIB systems are studied by the use of both ex situ PES, where electrodes are electrochemically pre-cycled and subsequently measured by PES, and operando PES, where electrodes are cycled during PES measurements. Ex situ PES is used to determine the main degradation mechanisms of a novel high capacity material, Li2VO2F. The capacity fade seen for Li2VO2F. is found to be related to an irreversible oxidation of the active material at high voltages, and a continuous surface layer formation at low voltages. To decrease the capacity fading three strategies for optimizing the interface are investigated. The results show that a surface coating of AlF3 most efficiently can mitigate electrolyte reduction, while boron containing electrolyte additives and transition metal substitution more successfully limit the oxidation of the active material. A large part of the work performed in this thesis has been devoted towards developing a methodology suitable for conducting operando ambient pressure photoelectron spectroscopy (APPES) measurements on LIB systems. A general connection between the theory of PES and electrochemistry is made, where in particular a model suitable for interpreting operando APPES results on solid/liquid interfaces is suggested. The model is further developed for the specific case of LIB interfaces. The results from the operando studies show that the kinetic energy shifts of the liquid electrolyte measured by APPES can be correlated to the electrochemical reactions occurring at the interface. If no charge transfer occurs, the kinetic energy shift is proportional to the applied voltage. During charge transfer the behavior is more complex, and the kinetic energy shifts are related to the change in chemical potential of the working electrode. In summary, this thesis exemplifies how both ex situ and operando PES are highly useful techniques for the study of LIB battery interfaces. The possibilities of both techniques are highlighted, and important considerations for an accurate interpretation of the PES results are also discussed.