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

Abstract

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 ferrocene molecules 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.

Influence of the Formation Current Density on the Transport Properties of Galvanostatically Formed Model‐Type Solid Electrolyte Interphases

“…Comparing the values of and in Table reveals that and exhibit similar values at the lowest formation current density of −4 μA cm −2 . This was also observed in previous works for SEIs formed by means of slow‐scan cyclic voltammetry . However, with increasing formation current density, decreases much more strongly than .…”

“…Above the half‐wave potential of ferrocene ( E 1/2 =3.24 V), the current density becomes almost constant, indicating a diffusion‐limited current. Such diffusion‐limited currents were also observed previously for SEIs formed by slow‐scan cyclic voltammetry . A control experiment with a ferrocene‐free electrolyte shows only very small (most likely capacitive) currents in the same potential range.…”

“…In previous studies, we have shown that assuming relative permittivity values for the SEI in the range of ϵ r =10–20 yields good agreement between the thickness estimated from impedance spectroscopy, d EIS, and the thickness of the passivating inner layer of the SEI from AFM‐based scratching experiments and FIB/SEM imaging . Assuming ϵ r =10, we obtain d EIS values of 22 nm, 19 nm and 14 nm for current densities of −4 μA cm −2 , 18 μA cm −2 , and −71 μA cm −2 , respectively.…”

“…Using the Nernst‐Einstein equation, the effective diffusion coefficient of the Li + ions can be estimated by means of Equation (7), assuming identical diffusivities of cations and anions: …”

“…Due to these contradictory experimental results on graphite SEIs and due to the high interest in faster SEI formation in commercial battery production, we carried out a systematic study on galvanostatic formation of model‐SEIs on planar glassy carbon electrodes. In previous studies, it was shown that such model‐type planar electrodes are well suited for studying the transport of ions and molecules across the SEI . This is in contrast to graphite composite electrodes, which exhibit a very complex morphology strongly complicating the understanding of transport processes.…”