Molecular Probes Reveal Chemical Selectivity of the Solid–Electrolyte Interphase

The solid–electrolyte interphase (SEI) is well-known to provide critical protection between the strongly reducing negative electrode and the organic electrolytes of nonaqueous batteries. Batteries with a poorly passivating SEI will suffer from rapid capacity fade and short lifetimes. Despite its importance and extensive study of its structure and composition, the mechanism of SEI passivation remains poorly understood. In this work, we demonstrate using electrochemical collector-generator measurements that the SEI is chemically selective in its passivation and propose a model based on catalytic active sites to explain its performance. Electrochemically interrogating the SEI with functionalized ferrocene mediators shows that the through-film mediator reduction is much more sensitive to mediator functional group than size, indicating preferential partitioning into the organic SEI layer. Additional experiments with controlled electrode crosstalk show that incorporation of dissolved transition metals increases both the density and the activity of active sites within the SEI. We conclude that the inner, inorganic layer is responsible for preventing charge transfer through the SEI while the outer, organic layer is minimally important. Our model reconciles contradictory observations from the literature and identifies the most important components of a functional battery interface.