SEI Characterization Using Ferrocene/Ferrocenium As Redox Shuttle

The characterisation of the solid electrolyte interphase (SEI) formed at the negative electrode in lithium-ion batteries is proposed by Tang et al. 1 Its principle is to introduce a redox shuttle in the electrolyte after the passive film (SEI) is formed at the electrode surface and to study how the electrochemistry of the shuttle is modified by the presence of the film. It is then possible to derive some film properties such those related to the transport of the redox molecule in the film and those related to the kinetics of the shuttle oxidation/reduction. So far, the majority of the work on shuttle experiments was done on a glassy carbon electrode using ferrocene/ferrocenium as redox shuttle. In this communication, we will describe experiments dedicated to form SEIs at glassy carbon electrodes and provide details on shuttle characterization experiments. We will show that the electrochemistry of the ferrocene/ferrocenium couple is greatly dependent on the formation charge involved in the SEI formation step. The SEI both slows down the kinetics of ferrocenium reduction (low overpotential) and also has an effect on the value of the limiting current density (high overpotential). For thin SEIs, there is a marked influence of the current density with the electrode rotation rate. Koutecky-Levich analysis can be performed on these data in order to derive the so-called kinetic current density. For electrodes on which “thick” SEIs are grown, the current density for ferrocenium reduction is much smaller and much less sensitive to the electrode rotation rate, which indicates that there is little ferrocenium diffusion limitations in the electrolyte at this low current density value. Performing Koutecky Levich analysis on these data is not possible, but the current density value measured at the RDE somehow can be considered as a direct reading of the kinetic current density. Acknowlegdment: This work was supported by the European Union’s 7th Framework programme for research, technological development and demonstration under the SIRBATT project, which has received agreement No. 608502. References: Tang, M.; Newman, J. J. Electrochem. Soc. 2011 , 158(5), A530.