To accommodate the need for greener energy solutions renewable energy sources aswell as reliable energy storage is a prerequisite. For the latter, high energy densitybatteries with long-term cycling stability are necessary. The cycling properties of abattery is to a large extent dependent on the functionality of the battery interfaces. Assuch, there is a need to understand the reactions occurring between the electrode andelectrolyte, and to limit those that are detrimental to the battery performance. Thetopic of this thesis is these interfaces in Li-ion batteries seen through photoelectronspectroscopy (PES).PES is due to its surface and chemical sensitivity one of the most suitable techniquesto study battery interfaces. In this thesis, PES is used to follow the oxidationstate and chemical environment of different atoms to understand the reactions occurringin the battery. This work uses a combination of soft and hard X-ray photoelectronspectroscopy as well X-ray absorption spectroscopy (XAS) to investigate the degradationmechanisms in high energy density cathode materials. The materials investigatedare in the class of Li-rich disordered rock-salts (DRS) and provide very highinitial capacities, but unfortunately lacks in cycling stability. In this thesis it is shownthat the reason for this is an unstable surface, possibly related to the occurrence ofanionic redox in the material, leading to breakdown of both electrolyte and electrodematerial. In addition, it is shown that the interface stability can be improved by choosingtransition metals that promotes the DRS structure and thus increases the chemicalstability of the material and long term cycling of the battery.Even though ex situ measurements provide many insights into the properties ofbattery interphases, there is still a need for operando measurement to completely answerthe puzzling question of their full functionality. In this thesis first steps towardsoperando measurements are taken by identifying the measurements conditions necessaryto probe a battery electrolyte with ambient pressure photoelectron spectroscopy(APPES) and a thorough characterization of a typical battery electrolyte is performed.The results show that the liquid can be stabilized by using the solvent as ambient gas,and also that care should be taken to avoid radiation damage when synchrotron lightis used. For the electrolyte characterization it is shown that a salt enrichment of particularlyLi+ and ionic fluoride is found at the droplet surface. These results are crucialto be able to single out contributions from the interphase in future operando measurements.When the method of operando APPES has matured and can be performed routinely,this could possibly be the key needed to understand how the interfaces in batteriescan be controlled to unlock the potential of stable high capacity materials infuture batteries.

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.