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)^1,2. However, room temperature (RT) combined with ultrahigh vacuum (UHV) can induce major SEI evolution from reactions and volatilization during XPS^1,2. Thus, a technique is necessary for SEI stabilization. Here we develop cryogenic (cryo)-XPS with immediate plunge freezing and demonstrate SEI preservation. We discover substantially different SEI speciation and a thicker pristine SEI with cryo-XPS, free from RT-associated thickness reduction and alterations to important 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 cryogenic conditions.