Gamry will be holding a Workshop at the ECS Meeting in Cancun, Mexico. If you do EIS on very low impedance or very high impedance samples then you should attend Gamry’s workshop at the ECS meeting tomorrow. This workshop will cover precautions needed for analyzing very low impedance samples such as batteries and fuel cells as Read more about Electrochemical Impedance Spectroscopy of Difficult Samples[…]
Purpose of This Note
This application note discusses the differences between 2‑point and 4‑point measurements of batteries. These two typical setups are compared with Gamry’s battery holders for CR2032 coin cells and cylindrical 18650 batteries. Both holders allow direct‑contact Kelvin sensing.
EIS measurements are performed with two types of lithium‑ion batteries and different experimental setups. In addition, shorted lead measurements show the low‑impedance limits of Gamry’s 18650 and CR2032 battery holders.
Introduction
It is crucial to know the exact specifications when testing batteries or any other energy storage device. Many parameters affect the capability of a battery, e.g. electrolyte, electrode materials, and temperature.
Batteries have to pass different tests to check their capacity, voltage window, current rating, internal impedance, leakage current, cycle life, operational temperature range, as well as several impact tests.
In order to get correct, reliable, and reproducible results, researchers have to rely on their experimental setup. Wrong setups can heavily affect and falsify measurement results leading to inaccurate conclusions.
The following sections show by means of EIS experiments the influence of the setup on the actual result. Common battery setups are compared with Gamry’s direct‑contact 4‑terminal battery holders. Shorted lead measurements with dummy cells show the lower limits of Gamry’s battery holders.
Dual Cell CR2032 and 18650 Battery Holder
Figure 1 shows typical options to connect cylindrical batteries and coin cells. Some batteries can be purchased with soldered tabs on each electrode. They allow connecting alligator clips for measurements. If no tabs are available, simple battery holders with two contacts are often used.
Figure 1 – Selection of battery connectors for cylindrical batteries and coin cells.
Porous electrodes offer a high surface area to volume or weight ratio which is highly beneficial to a number of energy generation or storage devices. (e.g. Dye-Sensitized Solar Cells, Super-Capacitors, Fuel Cells, etc). Transmission lines are heavily used in modeling in Electrochemical Impedance Spectroscopy (EIS) experiments where porous systems are employed.
Though transmission lines are commonly used, an introductory paper was hard to find. This paper is intended to serve that purpose. It assumes basic knowledge of EIS and modeling using equivalent circuits as covered in our EIS application note.
We will distinguish between the base electrode and the porous electrode and the various interfaces as shown in Figure 1.
Figure 1. Porous Electrodes and the Nomenclature That Will Be Used in This Paper.
Dates: November 10-14, 2014
Where: Houston, Tx
Course OverviewImpedance spectroscopy is an extremely powerful non-destructive investigative technique that can obtain essential information about interfacial and bulk material parameters through the use of low energy, time varying electrical excitation. When applied to an electrochemical system, electrochemical impedance spectroscopy (EIS) can provide information on reaction parameters, corrosion rates, oxide characteristics and integrity, surface porosity, coating integrity, mass transport, and many other electrode/interface characteristics. EIS is possibly one of the most powerful methods available in electrochemistry today and is utilized in research and development in essentially every technical sector, e.g., transportation and infrastructure, batteries and fuel cells, medicine, among many others. However, effective utilization of this method has been hindered by the lack of a comprehensive and cohesive explanation of the theory, measurement, and analysis techniques. This course on EIS has been developed to fill this void.
This short course, now in its 26th year, was first taught in 1988 in Charlottesville, Virginia. It has recently had the tremendous fortune of moving to the vibrant and technologically rich city of Houston, Texas. Please read more about our course on this website, or contact us either by e-mail or telephone.
Take two different metals, place an ion transporting medium between them, wire them up to be connected outside the electrolyte solution (perhaps to a voltmeter or potentiostat) and you’ve made a battery. With the ever-increasing demand for portable power, batteries are becoming more and more important. As demands on battery performance increase, so do demands Read more about Potentiostats for Battery Research Testing[…]
The Constant Phase Element (CPE) is a non-intuitive circuit element that was discovered (or invented) while looking at the response of real-world systems. In some systems the Nyquist plot (also called the Cole-Cole plot or Complex Impedance Plane plot) was expected to be a semicircle with the center on the x-axis. However, the observed plot was indeed the arc of a circle, but with the center some distance below the x-axis.
These depressed semicircles have been explained by a number of phenomena, depending on the nature of the system being investigated. However, the common thread among these explanations is that some property of the system is not homogeneous or that there is some distribution (dispersion) of the value of some physical property of the system.
Mathematically, a CPE’s impedance is given by
1 / Z = Y = Q° ( j omega )n
where Q° has the numerical value of the admittance (1/ |Z|) at omega =1 rad/s. The units of Q° are S•sn (ref 1).
A consequence of this simple equation is that the phase angle of the CPE impedance is independent of the frequency and has a value of -(90*n) degrees. This gives the CPE its name.
When n=1, this is the same equation as that for the impedance of a capacitor, where Q° =C.
1 / Z = Y = j omega Q° = j omega C
When n is close to 1.0, the CPE resembles a capacitor, but the phase angle is not 90°. It is constant and somewhat less than 90° at all frequencies. In some cases, the ‘true’ capacitance can be calculated from Q° and n
The Nyquist (Complex Impedance Plane) Plot of a CPE is a simple one. For a solitary CPE (symbolized here by Q), it is just a straight line which makes an angle of (n*90°) with the x-axis as shown in pink in the Figure. The plot for a resistor (symbolized by R) in parallel with a CPE is shown in green. In this case the center of the semicircle is depressed by an angle of (1-n)*90°
One guideline that I have heard recommended (although I cannot give a reference for it) is that data over a decade range of frequency is required to support each circuit component.
All curve-fitting software should report some measure of the “goodness of fit.” Often this is the chi-squared parameter ( X2 ) or a value related to it. Boukamp makes the recommendation that the value of X2 should decrease by tenfold if a new circuit element is introduced into the circuit model. The tenfold decrease provides the justification for including the new circuit element. If the inclusion of an additional circuit element does not substantially improve the goodness-of-fit (as evidenced by the decrease in the X2 value), then based on Occam’s Razor, you should keep the simpler model, or continue your search for an improved one.
The old joke about the ability to “fit an elephant” if you use enough parameters is all too true with impedance data. Each component added to the model should have a physical explanation. Adding components only because they make the fit look better (smaller X2) without a physical interpretation is the equivalent to “fitting an elephant.”
This application note presents an introduction to Electrochemical Impedance Spectroscopy (EIS) theory and has been kept as free from mathematics and electrical theory as possible. If you still find the material presented here difficult to understand, don’t stop reading. You will get useful information from this application note, even if you don’t follow all of the discussions.
Four major topics are covered in this Application Note.
No prior knowledge of electrical circuit theory or electrochemistry is assumed. Each topic starts out at a quite elementary level, then proceeds to cover more advanced material.
(1)
Almost everyone knows about the concept of electrical resistance. It is the ability of a circuit element to resist the flow of electrical current. Ohm’s law (Equation 1) defines resistance in terms of the ratio between voltage, E, and current, I.
While this is a well known relationship, its use is limited to only one circuit element — the ideal resistor. An ideal resistor has several simplifying properties:
“Although the equivalent circuit approach is looked down upon by some, analyzing EIS data by fitting it to equivalent circuit models can be a valid and rewarding approach, particularly in the early stages of an investigation.” When you first begin an electrochemical investigation, very often you may know little or nothing about the process or Read more about Fitting EIS Data to Equivalent Circuits[…]
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