Cryogenic X-ray photoelectron spectroscopy for battery interfaces

Understanding the chemical environment of pristine interfaces is a long-sought goal in electrochemistry, materials science, and surface science. A substantial understanding of one such interface, the solid electrolyte interphase (SEI) in lithium anodes, originates from X-ray photoelectron spectroscopy (XPS). However, room temperature (RT) combined with ultra-high vacuum (UHV) can induce major SEI evolution from reactions and volatilization during XPS. Subsequently, a technique is necessary for SEI stabilization. Here, we develop cryogenic (cryo)-XPS with immediate plunge freezing and demonstrate SEI preservation. We discover completely different SEI speciation and a thicker pristine SEI with cryo-XPS, free from RT-associated thickness reduction and alterations to major species including LiF and Li2O in UHV. This new access to pristine SEI composition enables performance correlations across diverse electrolyte chemistries. Primarily, we highlight the necessity of studying sensitive interfaces under cryo conditions.