- In general, most of the literature refers to the first semi-circle to SEI, while the second semi-circle to charge transfer.
- However, it is not explicitly mentioned whether it is SEI on the Li-metal or graphite (in case of full-cell) or this SEI is actually the CEI on the cathode. Also, the same goes for the charge transfer, either this CT (semi-circle) represents the anode or cathode.
- Therefore, my question is how to distinguish whether these phenomena are related to cathode, anode or both.
The only way to be sure is to use a 3-electrode cell. Assign the interface of interest to the working electrode, the other as the counter electrode, then connect a relevant material (usually lithium metal for Li-ion) as a reference electrode; this will make the impedance of the counter-electrode transparent in the measurement. Otherwise, with a 2-electrode system, you can't be sure which parts of the EIS spectrum are from anode and which are from cathode. You can buy a 3-electrode cell (sometime called Swagelok) from a few different suppliers, though they're not cheap. Some have also tried custom-inserting into a coin cell, though this would be difficult to crimp/seal.
While its not possible to log the correct contribution of the two electrodes individually in a 2 electrode system, one can play with the applied voltage (from OCV to lower near zero values) in the EIS and separate the interfacial resistance contributions of the two electrodes from each other. This can be easily achieved by mimicking the system with an equivalent circuit model and assigning the known values of conductivity/resistance to the materials used.
'While its not possible to log the correct contribution of the two electrodes individually in a 2 electrode system, one can play with'
1) the electrode Area, A, focusing, your SEI-study, on the reduced, A, electrode
in correlation with
2) the DC-Voltage stress[1], Vdc (Vdc > Vminimum,dc >> 0V), an important parameter in EIS for battery cells, usually selected as (Vdc =) OCV[2], the minimum (DC-Voltage) stress value.
1. This path/way is already proposed, above, by Waqas Tanveer.
2. or, sometimes, under a low stress study, near (Vdc ~) OCV.
Hi Umair,
This is a really good question. You can do this using symmetric cells. One for negative|negative and the other for positive|positive symmetric cell. Then do EIS measurements on symmetric cells. 1/2 of the sum of positive and negative symmetric cell impedance will match with the impedance of the full cell. Therefore, you will be able to measure the contribution of SEI and charge transfer impedance from each negative and positive electrode. You need to be very careful about the SoC and disssembly of full cells and reassembly of the symmetric cells. For more information please read the following articles from Jeff Dahn group.
- Study of Electrolyte Additives Using Electrochemical Impedance Spectroscopy on Symmetric Cells
- Introducing Symmetric Li-Ion Cells as a Tool to Study Cell Degradation Mechanisms
- Comparative study of electrolyte additives using electrochemical impedance spectroscopy on symmetric cells
- Designing Positive/Positive and Negative/Negative Symmetric Cells with Electrodes Operating in the Same Potential Ranges as Electrodes in a Full Li-Ion Cell
In solid-state electrochemistry, we have the same problem when researching solid-state fuel cells with a film electrolyte membrane, i.e. when the reference electrode cannot be used. To determine the contributions of the anode and cathode separately, we use approaches to change the external conditions at the anode and cathode independently of each other, followed by a detailed analysis of the impedance spectra by the DRT method. This approach is described in my work Electrochim. Acta 372 (2021) 137858. Maybe in the case of lithium-ion cells, there is a similar approach.
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