The Electrochemical Mechanisms of Solid–Electrolyte Interphase Formation in Lithium-Based Batteries

In lithium-based batteries, the solid–electrolyte interphase (SEI) is a layer of material that forms between the negative electrode and the liquid electrolyte; it is produced spontaneously by the breakdown of electrolyte compounds at the highly reducing potentials inherent to these systems. The SEI is perhaps the most important factor controlling the efficiency, safety, and lifetime of lithium batteries, and many empirical approaches have been developed to control the SEI's properties. In this work, we adapt methods from electrocatalysis to allow for the potential-dependent, atomistic simulation of SEI formation on lithium surfaces via electronic structure calculations. We use a computational lithium electrode (CLE) technique, in which the potential scale is thermodynamically linked to the lithium reference electrode, to study the decomposition of ethylene carbonate, one of the most prevalent battery electrolytes, into SEI components on lithium-based surfaces. On Li metal surfaces, we find that the most favorable process is forming the inorganic carbonate phases (accompanied by the liberation of ethylene gas), while forming the organic SEI component lithium ethylene dicarbonate (LiEDC) is unfavorable. In contrast, we find LiEDC to be favorable on the inorganic lithium surfaces (Li2CO3 and Li2O). This gives a mechanistic interpretation of a common physical picture of SEI formation: inorganic species (e.g., Li2CO3) are formed more heavily near the electrode, while organic species (e.g., LiEDC) are formed more heavily near the electrolyte. Both electrochemical and non-electrochemical pathways are explored and found to have similar energetics at this level of theory. This study rationalizes experimental findings and sets the stage for mechanism-based control of SEI formation.