How Transition Metals Enable Electron Transfer through the SEI: Part I. Experiments and Butler-Volmer Modeling

Transition metal dissolution from high-voltage Li-ion battery cathodes disrupts the formation and performance of the solid-electrolyte interphase (SEI). SEI contamination by transition metals results in continual Li loss and severe capacity fade. Fundamental understanding of how metals undermine SEI passivation is necessary to mitigate this degradation. This two-part study interrogates the mechanisms by which transition metals facilitate through-film charge-transfer and SEI failure. Part I presents experimental results in which we intentionally contaminate SEIs with Mn, Ni, and Co. Rotating disk electrode voltammetry of a redox mediator quantifies how each metal impacts the charge-transfer characteristics of the SEI. A physics-based model finds that all three metals disrupt the electronic properties of the SEI more than the morphology. Surprisingly, the Butler-Volmer kinetics of charge-transfer through a Mn-contaminated SEI are an order of magnitude faster than for a Co-contaminated SEI, even with similar embedded metal concentrations. Such trends between metals are inconsistent with bandgap predictions from density functional theory, implying an alternative redox-cycling mechanism, which is mathematically developed and compared to experiment in Part II.