(Keynote) Electrochemical Impedance Spectroscopy Characterization of Advanced Materials for Energy
Authors: Collet Thomas , Van Laethem Dries , Zhu Xinhua , Van den Steen Nils , Montoya Rodrigo , Fernandez Macia Lucia , Deconinck Johan , Hubin Annick 
Published: February 2020

Several electrochemical energy technologies (batteries, fuel cells, supercapacitors, …) have been developed. Many have been commercialized, but a large number is still under development. Systems are optimized in terms of life time, performance and cost, and new systems are being designed.
The electrode and electrolyte materials must have the required characteristics with respect to ionic and/or electronic transport properties, electrochemical properties, chemical stability, compatibility with other materials, thermal properties, … To be able to tailor the energy materials, characterization of their electrochemical behavior is of prime importance. The electrochemical behavior is influenced by a high number of processes, all occurring at a different rates. Knowing the quantitative mechanism of an electrochemical system is an essential step in the prediction of its behavior. Electrochemical Impedance Spectroscopy (EIS) is the designated experimental technique to that purpose. Yet, identifying and quantifying the characteristic parameters of the reactions governing the system from the experimental EIS results is complex.
In our group, we developed a new approach to EIS [1-4]. It consists of an integrated measuring and modelling methodology based on an odd random phase multisine excitation signal (ORP-EIS) and on mathematical modeling via parameter estimation. The mathematical model representing the system is formulated on the relations between the input and output to the system, including the model parameters. These parameters stand for inherent properties of the system; they describe the stability and control the behavior of the system.
This methodology is applied in this paper for the investigation of two types of energy materials.
The first application concerns Lithium Ion Batteries (LiB’s), used for Hybrid Electrical Vehicles (HEV’s). This study contributes to the quest for electrode materials with high cycling stability. Li4Ti5O12 (LTO) has been proposed as a promising candidate to replace the market-dominating carbon-based anodes (graphite and amorphous carbon). Aged batteries are post-mortem analyzed by EIS to have insight into the origin of the aging phenomena. Reliable impedance modelling is performed to quantitatively address the origin of the rising impedance.
The second application is in the field of Solid Oxide Fuel Cells (SOFC’s). Solid electrolytes exhibit electrical conductivity due to ionic conduction through the crystalline structure. For their use in SOFC’s, a sufficient electrical conductivity is achieved at high temperatures. To reduce the application temperature, research is dedicated to increase the conductivity of the ceramic material by heterogeneous doping or reducing the grain size or the film thickness. How the microstructure of the material affects the electrical conduction is still a matter of debate in literature. A good understanding of the relationship between microstructure and ion conductivity would open the possibility to engineer advanced materials for SOFCs. Oxides with fluorite structure are measured by EIS at low to high temperatures, varying the oxygen flow. Thin film and bulk electrolytes of doped ceria with different grain size and dopants are investigated.

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[4] L. Fernández Macía, M. Petrova, A. Hubin, Journal of Electroanalytical Chemistry, 737 (2015) 46