Measurements and Simulations of Electrochemical Impedance Spectroscopy of a Three-Electrode Coin Cell Design for Li-Ion Cell Testing

141

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.

mentioning

confidence: 99%

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

Solchenbach

Pritzl

Kong

et al. 2016

141

227

“…REs are commonly used in various areas of electrochemistry, but the close proximity of lithium-ion cell electrodes and the low conductivity of electrolytes (compared to aqueous systems) make it difficult to introduce a reliable third electrode. Several three-electrode configurations [1][2][3][4][5][6][7] have been employed for lithium-ion studies, including the following: i) T-cell set-up where the RE is positioned at the edge of the electrode sandwich ii) a Swagelok-cell design, where a coaxial micro-RE is position at each side of the cell sandwich with holes in both the working electrode (WE) and CE; (iii) a coin-cell * Electrochemical Society Member.…”

mentioning

confidence: 99%

Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li_1.03(Ni_0.5Co_0.2Mn_0.3)_0.97O₂/Silicon-Graphite Full Cells

Klett

Gilbert

Trask

et al. 2016

120

110

The capacity and power performance of lithium-ion battery cells evolve over time. The mechanisms leading to these changes can often be identified through knowledge of electrode potentials, which contain information about electrochemical processes at the electrodeelectrolyte interfaces. In this study we monitor electrode potentials within full cells containing a Li 1.03 (Ni 0.5 Co 0.2 Mn 0.3 ) 0.97 O 2 -based (NCM523) positive electrode, a silicon-graphite negative electrode, and an LiPF 6 -bearing electrolyte, with and without fluoroethylene carbonate (FEC) or vinylene carbonate (VC) additives. The electrode potentials are monitored with a Li-metal reference electrode (RE) positioned besides the electrode stack; changes in these potentials are used to examine electrode state-of-charge (SOC) shifts, material utilization, and loss of electrochemically active material. Electrode impedances are obtained with a Li x Sn RE located within the stack; the data display the effect of cell voltage and electrode SOC changes on the measured values after formation cycling and after aging. Our measurements confirm the beneficial effect of FEC and VC electrolyte additives in reducing full cell capacity loss and impedance rise after cycling in a 3.0-4.2 V range. Comparisons with data from a full cell containing a graphite-based negative highlight the consequences of including silicon in the electrode. Our observations on electrode potentials, capacity, and impedance changes on cycling are crucial to designing long-lasting, silicon-bearing, lithium-ion cells. As applications of lithium-ion systems expand beyond consumer electronics to the transportation and electricity storage markets, battery-cell longevity has become a primary barrier to widespread commercialization. While cells in smart phones are expected to function for about two years, energy storage units in electric vehicles are expected to maintain performance over a 10-15 year period. This performance is primarily defined by cell capacity and impedance characteristics, which change as electrode potentials and electrodeelectrolyte interfaces evolve over time. The knowledge of electrode potentials is very important for battery aging investigations because it can point to sources of performance degradation and, hence, lead to solutions that improve cell life. Such improvements are urgently needed for high-energy density cells, with silicon-containing-negative and layered-oxide-positive electrodes, being developed at Argonne National Laboratory as part of the U.S. DOE's Applied Battery Research (ABR) for Transportation program.The potentials of individual electrodes, such as that of a layeredoxide electrode, can be monitored in two-electrode cells by using a Li-metal counter electrode (CE) at low currents, or by using a CE with high surface areas, which reduce current densities and maintains a relatively stable potential. However, electrode potentials cannot be measured independently in commercial lithium-ion cells, which do not contain Li-metal. Yet aging investigations of ...

“…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...