Electrochemical Impedance Spectroscopy (EIS)

A significant part of our research focuses on Electrochemical Impedance Spectroscopy (EIS). For lithium-ion battery electrodes, the use of EIS based on a transmission line model (TLM) analysis to extract the effective transport and kinetic parameters is an active research area.

Semicircle does not  (always) represent Rct

Using direct analogy from flat electrode, often battery impedance spectra gets evaluated assuming flat electrode (which of course they are not) and the semi circle diameter (suppressed semi-circle in most cases) is thought be only Rct (charge transfer resistance), despite several work in this regards. One of the reason for such oversight is that fitting a model (TLM) having Rion (ionic resistance), Rct, Qdl (double layer capacitance) is not straightforward since most EIS fitting software does not provide this TLM implementation.

In the following publication, we present a simple method to deconvolute charge transfer resistance from the battery EIS. This publication essentially boils down a sophisticated EIS fitting to a reading a value from a graph and makes it super easy for anybody to measure the value of charge transfer resistance. Figure below shows the step-by-step method to use the plot to essentially do the TLM fitting of your impedance data. More info on this work done in collaboration with TUM can be found in the link below. 

R. Morasch, J. Keilhofer, H. Gasteiger, B. Suthar, Methods—Understanding Porous Electrode Impedance and the Implications for the Impedance Analysis of Li-Ion Battery Electrodes. J. Electrochem. Soc.,  (2021). (Link)

Deconvolution of Rct from EIS
Figure: describing the deconvolution of Rct from the available EIS of a battery electrode