Investigating Lithium-Ion Battery Electrode/Electrolyte Interfaces using Ambient-Pressure X-ray Photoelectron Spectroscopy

Licentiate thesis, 2026

Rechargeable batteries store and release energy through interfacial electrochemical reactions, making interfaces central to battery performance, lifetime, and safety. In lithium-ion batteries, the solid electrolyte interphase (SEI) formed between the electrode and electrolyte is essential for stable operation. The SEI is chemically and structurally heterogeneous, participates in lithium-ion transport, may evolve during battery operation and appear to host a permanent electric potential gradient. However, many of its kinetic characteristics remain poorly understood, largely due to the experimental challenges associated with probing buried interfaces between the battery materials under operation. This thesis addresses these challenges by combining well defined model systems with operando ambient-pressure X-ray photoelectron spectroscopy (APXPS). By probing the bulk electrolyte meniscus rather than the buried interface directly, SEI-related processes can be investigated under working conditions. An electrochemical test protocol exploiting the self-discharge and relaxation of the electric double layer at the interface is introduced, expanding on existing electrochemical models used with APXPS, to also capture information about the SEI. This approach enables identifying the SEI onset and provides insight into surface reaction kinetics. Additionally, in situ and operando experimental approaches are compared, clarifying what interfacial information can be extracted from each and outlining their respective advantages and limitations. Together, these results contribute to a deeper understanding of SEI formation and interfacial dynamics in lithium-ion batteries.