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EIS pseudocapacitance – Electrochemistry, Battery & Corrosion – Application Note 20

Latest updated: May 6, 2020

Abstract

In equivalent circuit modeling, it is sometimes useful to substitute an RQ circuit with an RC circuit. In this application note, the impedance spectra of a Li-ion battery is modeled using an overall equivalent circuit that consists of an R/Q circuit from which the pseudocapacitance can be calculated automatically by simply selecting the “PseudoC” option after fitting the data to the model. This note also demonstrates how to build and add a new circuit to the ZFit tool.

 

Introduction

With some systems, the plot of the Nyquist diagram shows a semi-circle whose center is below the real axis. This characteristic happens when the surface of the electrode is not totally smooth. This “depressed” semicircle can be simulated with an R/Q circuit instead of an R/C circuit for the perfect semicircle. It can then be useful to calculate an equivalent capacitance called pseudo capacitance when using a Q element instead of a C element (Figure 1). This calculation is only available for a circuit (R/Q). It corresponds to the determination of a capacitance value C at an angular frequency ωc = 2πfc corresponding to the maximum imaginary part on the Nyquist circle obtained by fitting with the circuit (R/Q). Then, this value is the solution to the following equation

 

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