Dear all,
I performed an EIS analysis on the LiNi0.5Mn1.5O4/Li half-cell at 50% SOC (figure attached). Generally, most literature reports two semi-circles corresponding to two relaxation processes (RSEI and RCT). In my case, I can see three semi-circles, which is not surprising considering the 2-electrode setup. However, I am having problems evaluating the data.
From my understanding, there can be two scenarios,
Scenario 1 (1):
R2= Contact resistance.
R3= Charge transfer at LNMO cathode.
R4 =Charge transfer at Li anode.
Scenario 2 (2):
R2= Charge transfer at Li anode.
R3= Charge transfer at LNMO cathode
R4 =Originiate from some other process, however not sure.
Some important observations:
- Rs and R2 remain unchanged throughout the SOC (0–100%).
- R3 was initially high, rapidly decreased till 20% SOC, and later slowly decreased till 100% SOC.
- R4 was high at 0% SOC, then rapidly decreased to a low value at 10%SOC and remained unchanged through different SOC (10-90%).
- C2, C3 and C4 remain unchanged at different SOC.
Based on the EIS spectra and data shared, I would be happy to hear the point of view of experienced researchers.
Some details regarding the experiment,
Cathode/WE: LiNi0.5Mn1.5O4
Anode/CE/RE: Li-metal
Cell setup: Coin-cell (2032)
SOC at EIS: 50%
Frequency range: 500 kHz–5 mHz
(1) https://onlinelibrary.wiley.com/doi/epdf/10.1002/aenm.202003400
(2) Article Lithium Iron Aluminum Nickelate, LiNi x Fe y Al z O 2 —New S...
Based on the details provided, here is my interpretation and suggestions on analyzing the EIS spectrum of your LiNi0.5Mn1.5O4/Li half cell:
- Your assignment of the high frequency semicircle (R2) to charge transfer resistance (RCT) at the Li metal anode makes sense. Unchanging R2 and C2 with SOC agrees with stable Li/electrolyte interface.
- The second semicircle (R3) decreasing with SOC likely does originate from the DEL-LNMO/electrolyte interphase and RCT associated with Li+ insertion/deinsertion. This is the key cathode response.
- The third semicircle (R4) that diminishes after 10% SOC is likely from the cathode electrolyte interphase (CEI) layer formation initially but becomes very conductive later.
- I would fit and model the spectra with the typical Randles circuit model including a Constant Phase Element rather than pure capacitor to account for non-ideal behavior.
- Evaluate LNMO RCT vs SOC by fitting R3. Check if CEI stabilization is complete by 10% SOC based on R4 evolution.
- The high frequency x-axis intercept (Rs+RCT Li) could allow you to deconvolute anode and cathode charge transfer resistances as well.
Modeling with suitable equivalent circuits and parameter tracking should provide further insights into the interfacial processes at the electrodes.
Ahmed Emad Fathy Abbas thank you very much for sharing your feedback. I would like to add few comments on your comments as below,
- I would fit and model the spectra with the typical Randles circuit model including a Constant Phase Element rather than pure capacitor to account for non-ideal behavior.
- The spectra are already fitted with randles circuit using constant phase element, no the capacitor. The circuit is show in the figure as well.
- Evaluate LNMO RCT vs SOC by fitting R3. Check if CEI stabilization is complete by 10% SOC based on R4 evolution.
- R3 has been evaluated. As mentioned, initially is drastically decreases with SOC, and later slowly decrease with high SOC as well.
Further, you can find I added another scenario possible, if you have anything to share on that as well, that would be great.
Regarding the additional scenario you raise about the third semicircle potentially originating from the passive layer formation on the lithium metal surface - that is an excellent point to consider as well. Some thoughts on distinguishing that:
- The evolution you describe where R3 changes significantly with SOC while R4 stabilizes after 10% SOC would still point more to R4 being related to the cathode CEI layer. Passivation on lithium metal should not vary with SOC.
- However, you could purposely form the passive SEI layer on lithium by holding at low potential before assembling the cell. If you still see the R4 semicircle disappear by 10% SOC, it would indicate that process is independent of the Li passivation.
- Comparing EIS spectra evolution just for the Li | electrolyte interface in a symmetric cell could also help establish its passive film response.
- The CEI is known to evolve more over the first few cycles reaching a stable composition later. So the rapid decrease you see in R4 fits well with that explanation.
Ahmed Emad Fathy Abbas thank you very much for sharing your thoughts on the data, it is really helpful.
Dear Umair Nisar ,
we might be more helpful, if you would like to share[1], some multiple electrical impedance/s (Z, in colored curves[2]) SOC-cases, say, at least two more, near the extreme cases: '0% SOC', and '100% SOC'[2].
1. Apart that shared case of '50% SOC' (EIS_LNMO-Li.jpg) .
2. All Z-curves in a single plot.
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