Cryogenic X-ray photoelectron spectroscopy for battery interfaces

Abstract

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 XPS1,2. 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. Cryogenic X-ray photoelectron spectroscopy, combined with immediate plunge freezing, can be used to probe the pristine solid electrolyte interphase in lithium metal batteries.