(625b) Determining Passivation Mechanisms in the Solid-Electrolyte-Interphase with Functionalized Molecular Probes and Electrochemical Collector-Generator Measurements

The Solid-Electrolyte-Interphase (SEI) is a critical component in Li-ion batteries because it passivates the negative electrode and prevents continual electrolyte decomposition. Since its discovery in 1979, the chemical composition and bilayer structure of the film have been studied extensively¹, yet the mechanisms by which the SEI provides this crucial passivation remain elusive. The interphase is thought to be composed of two layers: a compact, inorganic inner layer and a loose, organic outer layer. Disagreement over the role of each layer hampers concerted efforts to diagnose failure mechanisms in high-voltage Li-ion batteries, thereby impeding commercialization of important battery technology.

High-voltage Li-ion batteries with transition metal oxide cathodes (e.g. LNMO, LMO, LMR-NMC) are known to experience accelerated capacity fade resulting in unacceptably short battery lifetimes. This is attributed to dissolution of Mn from the cathode matrix and deposition of the metal on or within the SEI². The mechanism by which Mn causes premature battery failure has been an area of concentrated investigation for the last decade and remains almost completely unknown. Some groups propose the metal corrupts the inner SEI layer and forms a conductive pathway for electron transfer to the electrolyte, while others argue that the metal decomposes the interphase and cracks allow facile electrolyte transport to the highly reductive negative electrode.

In this work we leverage generator-collector electrochemical experiments to study the SEI and discover that the film is chemically selective. Our approach involves model experiments on a glassy carbon-glassy carbon rotating ring-disk electrode (RRDE) whereby we deploy functionalized ferrocene-based redox mediators to interrogate the electrochemical properties of the SEI. We find that transport through the SEI is affected only slightly by the molecule’s bulk diffusivity in the electrolyte. The chemistry of its functional groups has much stronger effects, demonstrating that the SEI preferentially solvates more polar species. Additional experiments simulate the effects of Mn dissolution during electrode cross-talk on the passivation of the SEI. We propose a new model for SEI passivation based on distributed active sites and conclude that the inner layer of the SEI plays the critical role in inhibiting accelerated capacity fade. . This work reconciles decades of experimental observations to provide new insight into the passivation mechanism of the SEI.

List of References

1. Peled, E. (1979). Journal of the Electrochemical Society, 126(12), 2047–2051.
2. Abraham, D. P. et al. (2008). Electrochemical and Solid-State Letters, 11(12), A226.