The importance of cell geometry for electrochemical impedance spectroscopy in three-electrode lithium ion battery test cells

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...

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confidence: 99%

Three-Electrode Setups for Lithium-Ion Batteries

Costard

Ender

Weiss

et al. 2016

113

106

“…A number of cell designs for impedance measurements with a reference electrode have been suggested, with the reference electrode either placed between anode and cathode, [5][6][7][8][9] or placed in-plane with anode or cathode through a central hole (also referred to as co-axial arrangement). [10][11][12] The more commonly used design, however, is a Swagelok T-cell design with the reference electrode (typically consisting of a lithium metal disc) being placed perpendicularly to the anode and cathode, outside the active area. 13 Yet, experiments and numerical simulations by Ender et al 14 showed that the impedance measurements with the latter reference electrode placement can display significant distortions caused by small in-plane offsets between anode and cathode (referred to as geometrical asymmetry) and/or by large differences in the impedance response of anode and cathode (referred to as electrical asymmetry), consistent with earlier work by Dees et al 15 This is also the case for coaxially located reference electrodes, for which the measured anode or cathode impedance is shown to be highly sensitive toward misplacements of the electrodes.…”

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confidence: 99%

“…13 Yet, experiments and numerical simulations by Ender et al 14 showed that the impedance measurements with the latter reference electrode placement can display significant distortions caused by small in-plane offsets between anode and cathode (referred to as geometrical asymmetry) and/or by large differences in the impedance response of anode and cathode (referred to as electrical asymmetry), consistent with earlier work by Dees et al 15 This is also the case for coaxially located reference electrodes, for which the measured anode or cathode impedance is shown to be highly sensitive toward misplacements of the electrodes. 10,12,16 * Electrochemical Society Student Member. * * Electrochemical Society Fellow.…”

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confidence: 99%

A Gold Micro-Reference Electrode for Impedance and Potential Measurements in Lithium Ion Batteries

Solchenbach

Pritzl

Kong

et al. 2016

140

227

Impedance measurements of lithium-ion batteries are a powerful tool to investigate the electrolyte/electrode interface. To separate the contributions of anode and cathode to the full-cell impedance, a reference electrode is required. However, if the reference electrode is placed inappropriately, the impedance response can easily be biased and lead to erroneous conclusions. In this study, we present a novel micro-reference electrode for Swagelok-type T-cells which is suitable for long-term impedance and reference potential measurements. The reference electrode consists of a thin insulated gold wire, which is placed centrally between cathode and anode and is in-situ electrochemically alloyed with lithium. The resulting lithium-gold alloy reference electrode shows remarkable stability (>500 h) even during cycling or at elevated temperatures (40 • C). The accuracy of impedance measurements with this novel reference electrode is carefully validated. Further, we investigate the effect of different vinylene carbonate (VC) contents in the electrolyte on the charge transfer resistance of LFP/graphite full cells and demonstrate that the ratio of VC to active material, rather than the VC concentration, determines the impedance of the anode SEI.

“…A main criterion for a micro-reference electrode suitable for highquality EIS measurements is a centered position of the reference electrode between working and counter electrode. [11][12][13] Several approaches are presented in the literature, as for example, a copper wire, where lithium is in-situ plated from anode or cathode, 14 a reference electrode consisting of a lithium-tin alloy, 15 or consisting of a lithium-bismuth alloy. 16 Our group has recently developed a micro-reference electrode = These authors contributed equally to this work.…”

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confidence: 99%

An Analysis Protocol for Three-Electrode Li-Ion Battery Impedance Spectra: Part I. Analysis of a High-Voltage Positive Electrode

Landesfeind

Pritzl

Gasteiger

2017

142

199

A key for the interpretation of porous lithium ion battery electrode impedance spectra is a meaningful and physically motivated equivalent-circuit model. In this work we present a novel approach, utilizing a general transmission line equivalent-circuit model to exemplarily analyze the impedance of a porous high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode. It is based on a LNMO/graphite full-cell setup equipped with a gold wire micro-reference electrode (GWRE) to obtain impedance spectra in both, non-blocking conditions at a potential of 4.4 V cell voltage and in blocking configuration achieved at 4.9 V cell voltage. A simultaneous fitting of both spectra enables the deconvolution of physical effects to quantify over the course of 85 cycles at 40 • C: a) the true charge transfer resistance (R CT ), b) the pore resistance (R Pore ), and c) the contact resistance (R Cont. ). We demonstrate that the charge transfer resistance would be overestimated significantly, if the spectra are fitted with a conventionally used simplified R/Q equivalent-circuit compared to our full transmission line analysis. Advanced analysis techniques for lithium ion batteries are a key requirement to deconvolute the complex interplay between the aging mechanisms occurring at the anode and the cathode. In principle, this can be accomplished by electrochemical impedance spectroscopy (EIS), if the individual contributions of anode and cathode to the overall cell impedance can be determined, and if this EIS response can be fitted unambiguously to physically motivated equivalent-circuit models. In general, the measured cell and/or electrode impedances are usually fitted with a serial connection of an ohmic resistor (R), with a parallel circuit of a resistor and a capacitor (C), commonly referred to as R/C element and often also modified to an R/Q element (Q representing a constant-phase element), as well as with a Warburg element (W). [1][2][3][4][5] Recently, more elaborate equivalentcircuits using a transmission line model are getting more and more attention. [6][7][8] In order to independently obtain the impedance of anode and cathode, there are two possible options: i) the assembly of symmetric cells as shown by Chen et al. 9 or Petibon et al., 10 where coin cells out of two anodes (impedance of the negative electrode) or two cathodes (impedance of the positive electrode) are assembled in a glove box or dry-room from two (aged) full-cells at a specified state-of-charge (SOC); ii) the use of three-electrode setups consisting of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE), which allows to individually determine the impedance of the anode and the cathode of a lithium ion battery full-cell. The latter is a more convenient approach, as individual impedance spectra can be recorded continuously during battery cycling, so that anode and cathode impedance can be monitored during cycle-life studies on a fullcell instead of obtaining only one set of anode and cathode impedance spectra after disassembly of a full-cell...