Short communication FEM modelling of a coaxial three-electrode test cell for electrochemical impedance spectroscopy in lithium ion batteries Stefan Klink a,1 , Daniel Höche b,1 , Fabio La Mantia c, * , Wolfgang Schuhmann a, c a Analytische Chemie e Elektroanalytik & Sensorik, Ruhr-Universität Bochum, Universitätstr. 150, D-44780 Bochum, Germany b Department of Corrosion and Magnesium Surface Technology, Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Max-Planck-Straße 1, D-21502 Geesthacht, Germany c Center for Electrochemical Sciences e CES, Ruhr-Universität Bochum, Universitätstr. 150, D-44780 Bochum, Germany highlights graphical abstract  Asymmetries in test cells lead to dis- tortions of electrochemical imped- ance spectra.  FEM provides insight into the quality of impedance spectra for battery research.  Coaxial three-electrode test cells prove reliable with reasonable asymmetries.  Asymmetries, thin electrolytes and edge effects should be avoided. article info Article history: Received 7 February 2013 Received in revised form 22 March 2013 Accepted 27 March 2013 Available online 12 April 2013 Keywords: Electrochemical impedance spectroscopy Finite element method Reference electrode Lithium ion battery Geometry Distortions abstract Electrochemical impedance spectroscopy for lithium ion batteries has recently gained increasing attention due to its ability of non-invasive evaluation of important electrochemical parameters. Commonly used three-electrode test cells, however, proved unreliable due to asymmetric current line distributions, causing severe distortions of impedance spectra. Finite element method (FEM) simulations can visualize these current lines at different frequencies and simulate impedance spectra at given geometries. By applying FEM simulations to a recently developed coaxial impedance test cell, limiting conditions for reliable impedance measurements could be identified. Using a reference electrode in coaxial position yields sufficiently reliable results as long as the electrode misalignment is small compared to the electrolyte thickness and edge effects are prevented. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The recent impetus in lithium ion battery research requires a manifold of methods for the analysis of materials, electrodes and electrochemical reactions. Since batteries are rather inaccessible electrochemical systems, non-invasive techniques for in-situ analysis are highly required. Electrochemical impedance spec- troscopy (EIS) utilizes the frequency dependent current response of an electrochemical system following a sinusoidal voltage perturbation (or vice versa) to separate physico-chemical pro- cesses via their specific relaxation time constants in time domains from 10 5 e10 3 s. Electrochemical impedance spectra of two-electrode systems can be easily recorded. These spectra are, however, a sum of the * Corresponding author. Tel.: þ49 234 3229432. E-mail addresses: stefan.klink@rub.de (S. Klink), daniel.hoeche@hzg.de (D. Höche), fabio.lamantia@rub.de (F. La Mantia), wolfgang.schuhmann@rub.de (W. Schuhmann). 1 These authors contributed equally. Contents lists available at SciVerse ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.03.186 Journal of Power Sources 240 (2013) 273e280