Interrelation between Redox Molecule Transport and Li+ Ion Transport across a Model Solid Electrolyte Interphase Grown on a Glassy Carbon Electrode

The solid electrolyte interphase (SEI) on the graphite anode of lithium ion batteries plays a crucial role for the battery performance. It is believed that the SEI blocks electrons and solvent molecules, while Li+ can easily migrate across the SEI. However, quantitative measurements of transport coefficients for these species in the SEI are problematic due to the complex structure of graphite composite anodes. Here we have grown model SEIs on glassy carbon electrodes and have characterized them by a combination of scanning electron microscopy, AFM-based scratching experiments, impedance spectroscopy and redox probe experiments. SEM and AFM experiments reveal a dual-layer structure of the SEI. The redox probe experiments with ferrocenemolecules provide strong indication that the diffusion of the redox molecules across pores in the inner SEI layer is faster than electron transport across the SEI. Remarkably, the effective diffusion coefficient of ferrocene in the SEI is virtually identical to the effective diffusion coefficient of Li+ obtained from the SEI semicircle in the impedance spectra. Moreover, both diffusion coefficients show the same temporal evolution after SEI formation. This suggests that in our model SEIs, Li+ is primarily transported in the liquid electrolyte phase inside the pores of the inner layer. (c) 2017 The Electrochemical Society. All rights reserved.