Optimization of reference electrode position in a three-electrode cell for impedance measurements in lithium-ion rechargeable battery by finite element method

113

106

Electrochemical Impedance Spectroscopy (EIS) is a well-established technique for investigating the loss processes that take place in lithium-ion batteries with different characteristic time constants. Three-electrode setups are needed to separate the contributions of working electrode (WE) and counter electrode (CE), but often suffer from measurement artifacts. This paper is the second part of a two-part paper dealing with the roots of these distortions: (I) electrochemical and (II) geometric asymmetry. The first part presents a theoretical examination by FEM simulation and the second part details the corresponding real-world measurements. The simulation results were confirmed: electrochemical and geometric asymmetry lead to artifacts for point-like reference electrodes but not for mesh reference electrodes. The geometric characteristics of the mesh reference electrode are crucial (thin wire and an open weave are essential). Furthermore, a possible realization of a mesh reference electrode setup is presented, using an aluminum mesh coated with Li 4 Ti 5 O 12 powder as reference electrode. This combination was then validated by additional measurements in symmetric cells. Additionally, the benefits of using the proposed setup for recording quasi-equilibrium potential curves and for performing rate capability experiments are demonstrated. This Part II of the paper presents an experimental work, which validates the simulations of Part I. Part I presents the theory and results of FEM simulations on three-electrode cells, where the working electrode (WE), counter electrode (CE) and reference electrode (RE) differ by geometric characteristics and placement.Lithium-ion batteries receive ever-growing attention due to their potentially high energy and power densities, which would be essential for automotive applications. Electrodes with improved energy density are needed to increase vehicle ranges, but in practice only those with long lifetimes, good rate capability and a high safety level will be selected. Candidate materials are usually assessed by a combination of different electrochemical measurements, including capacity tests and impedance measurements.Electrochemical Impedance Spectroscopy (EIS) is well established for identifying dominant loss processes in electrodes, and across different time-scales.1 Such studies are usually performed in half-cell setups, using lithium metal as the counter electrode.2 However, this type of counter electrode often dominates the sum of impedance contributions and adds a stochastic component to the measurement data.3,4 To investigate the working electrode (WE) without influences from the counter electrode (CE), a three-electrode setup is necessary. This means that a third "reference" electrode (RE) is inserted into the cell, providing a constant reference potential by ensuring a constant lithium-ion activity. The setups for such measurements can be challenging and often lead to measurement errors. Several groups have already detected these errors and some possible solutions have bee...

Section: Setup B -Mesh Reference Electrode (Li 4 Ti 5 O 12 (Lto)-coatmentioning

confidence: 99%

mentioning

confidence: 99%

Three-Electrode Setups for Lithium-Ion Batteries

Costard

Ender

Weiss

et al. 2016

113

106

“…However, a reliable 3-electrode setup can be experimentally challenging, considering cell design and referenceelectrode positioning. [31][32][33][34] Hence, the purpose of this section is to assess the error made by using 2-electrode data as an input for a 3-electrode based model analysis by fitting 2-electrode surrogate data simulated from the P2D model with the basecase 3-electrode P2D model. The difference between the two models is that the WE potential is either referred to the CE or the RE.…”

Section: Resultsmentioning

confidence: 99%

Guidelines for the Analysis of Data from the Potentiostatic Intermittent Titration Technique on Battery Electrodes

Malifarge

Delobel

Delacourt

2017

The Li-ion battery (LIB) modeling community needs to rely on a vast bank of measured Li-ion cell parameters. Most of them are determined by routine experiments but some can solely be estimated by fitting a model to a well-designed experiment. For instance, the determination of the lithium chemical diffusion coefficient has been the focus of numerous studies. Specific techniques have been proposed for its estimation, e.g., the potentiostatic intermittent titration technique (PITT), the galvanostatic intermittent titration technique (GITT), the cyclic voltammetry (CV), and the electrochemical impedance spectroscopy (EIS). Our study focuses on the PITT so as to provide guidelines for experimental measurements and corresponding data analysis. The validity of underlying assumptions of published analytic solutions used to fit PITT data is assessed using a pseudo-2D (P2D) model. Among the tested assumptions are the drop of the porous-electrode effect, of the particle size distribution and of the finite kinetics of Li insertion/de-insertion. Tests are also conducted to assess the error made on the chemical diffusion coefficient and reaction-rate constant determination by fitting 2-electrode simulated data with a 3-electrode P2D model. Lithium-ion battery (LIB) cell energy density has been greatly enhanced over the years owing to research in new materials and electrode engineering.1 Further improvement of LIB energy density is possible by packing more active material within the electrodes. However, doing this usually trades off with a power decrease as lithium-ion transport in the liquid phase becomes a major issue in thick and/or dense porous electrodes.2 Nonetheless, a compromise is still achievable through engineering of the electrode fabrication process, i.e., by tuning the electrode microstructure to optimize the ion and electron transport paths. This would decrease electrode nonuniformities that appear during cell operation, e.g., in terms of state of charge, liquidphase salt concentration, and solid/liquid-phase potentials.3 On this matter, LIB physical models prove an efficient tool to shed light on phenomena such as the development of concentration/potential gradients across the cell. Moreover, LIB models are tunable to the studied physics and cost effective compared to an experimental trial-and-error process, which makes them well-suited to determine parameters that optimize the porous-electrode design depending on the application. It also helps to assess the possible occurrence of unwanted phenomena such as Li metal plating. Among these models, the pseudo-2D (P2D) one introduced by Newman group in the early nineties is a widely used continuum model. 4,5 It relies on porous-electrode and concentrated-solution theories. In this model, the porous electrode is described as the superposition of the liquid and solid phases that are defined by their respective volume fractions and the interfacial surface area. Electrode properties are averaged over volume elements that are small compared to the overall dimension of t...

“…Hoshi et al 11 assessed the position of a point-like reference electrode using FEM simulations. They observed distortions and inductive loops in the spectra, caused by the electrochemical asymmetry between the working and counter electrode.…”

Section: A72mentioning

confidence: 99%

Three-Electrode Setups for Lithium-Ion Batteries

Ender

Illig

Ivers‐Tiffée

2016

Material and degradation effects in lithium-ion batteries are studied in three-electrode cells using electrochemical impedance spectroscopy. But half-cell impedance spectra are often superimposed by distortions caused by the individual cell arrangement. Finite Element Method simulations of the three-electrode cell were applied to identify and quantify these contributions. This study identified two basic mechanisms: (I) a radially inhomogeneous current distribution originating from geometric asymmetry of the electrodes; and (II) a frequency-dependent inhomogeneous current distribution in the electrolyte caused by an electrochemical asymmetry. Mechanism II is caused by different electrode materials, and enhanced when the electrolyte diameter exceeds those of both working and counter electrode. With the help of the FEM model, we evaluated three-electrode cells featuring different reference electrode geometries: (a) point-like, (b) wire and (c) mesh reference electrode. The results of these FEM simulations are shown as half-cell and full-cell impedance data, disclosing the magnitude of distortions and artifacts for each type of reference electrode geometry. The mesh reference electrode, proposed in literature but not widely adopted, showed the largest potential for error-free impedance spectra. The FEM simulations were supported by experiments, comparing a point-like with a mesh reference electrode in a three-electrode cell (see part II).