Advances in energy storage technology beyond traditional lithium ion battery (LIB) materials are necessary to meet future energy demands. These include the growing number of electric vehicles and grid storage applications. Next generation technologies such as Li-S and Li-O₂ batteries employ metallic lithium as an anode material due to its high specific capacity and low electrochemical potential. However, lithium presents a number of challenges including uncontrolled electrolyte consumption, formation of dendrites and “dead” lithium, as well as formation of a non-passivating and inhomogeneous solid electrolyte interphase (SEI). The composition, morphology, and electrical properties of the SEI are critical to overall battery performance and lifetime. Nevertheless, there exists a lack of fundamental understanding regarding its initial formation and how these early processes relate to cell performance after many cycles.

In this work, we use soft X-ray absorption spectroscopy (XAS) and electrochemical measurements to study the initial stages of SEI formation in Li-Cu cells as a function of cycling parameters in different electrolytes. Using these methods, the potential and rate dependence of electrolyte decomposition and the corresponding SEI formation is illuminated and the effects on initial Li metal plating are investigated. Further, the continued evolution of the SEI during the first several plating and stripping half cycles is studied. Controlled formation of the SEI during the first several half cycles may lead to large effects on the long-term composition and structure of the SEI and determine overall battery performance and cycle life.