the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Mar 2024
Second harmonic electron paramagnetic resonance spectroscopy and imaging reveal metallic lithium depositions in Li-ion batteries
Abstract. We have investigated the metallic lithium particles nucleation following lithiation and delithiation steps of the graphite electrode using X-band electron paramagnetic resonance (EPR). Metallic lithium aggregates like dendrites and/or filaments which are formed during electrochemical cycling on the graphite anode are complex structures which may lead to internal short-circuit and safety issues. Understanding and following, in real conditions, this nucleation process is necessary to improve the development of Li-ion batteries. The complexity to detect metallic lithium structures inside Li-ion batteries depends on the number of EPR lines and their linewidth. The presence of lithiated graphite phases affects the detection of micrometric Li-metal elements. Herein, we report a new approach using cw-EPR spectroscopy and imaging combining the first and the second harmonic detection schemes to provide evidence of the metallic lithium aggregates nucleation in these negative electrodes. Although the first harmonic gives all the EPR signals present in the sample, it is found that the second harmonic EPR signal is mainly sensitive to metallic lithium depositions.
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RC1: 'Comment on mr-2024-5', Hu Bingwen, 28 Mar 2024
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This paper is interesting.
However, the main problem is that the modulation is set to 0.2 mT,
which is much larger than the signal of lithium detrite which is less than 0.05mT.
Therefore I suggest to repeat the experiments with the modulation of 0.05 mT, especially Figure 4.
Citation: https://doi.org/10.5194/mr-2024-5-RC1 -
AC1: 'Reply on RC1', Charles Dutoit, 03 Apr 2024
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We thank the reviewer for his detailed reading of our manuscript and for this comment, which is absolutely right.
Indeed, in the case of dendritic lithium structures, the EPR spectrum is expected very sharp (around 0.05mT and less) as reported in a variety of EPR investigations. But also, the EPR spectrum should exhibit a pure Lorentzian line shape unlike the micrometric lithium aggregates exhibit a Dysonian shape. This effect on the line shape is due to the skin depth (δmw = 1.1 µm for Li0 at X-band) which limits the microwave penetration inside the conductor (i.e. through the metal thickness d). If d>δmw the EPR line exhibits an asymmetric peak (Dysonian with A/B > 1) whereas if d<δmw, the EPR line is a symmetric and intense (Lorentzian with A/B=1).
Initially, we performed EPR measurements using an amplitude modulation of 0.2mT to follow the EPR signature of the lithiated graphite which gives a linewidth around 0.4mT. At this step of investigations, we did not expect to observe a metallic lithium EPR signature. But after analysis using two Dysonian functions, we performed the second harmonic investigation to confirm the presence and the origin of the other hidden contribution which is assimilated to micrometric lithium structures.
In our case, we did not detect dendritic structures and the reason is twofold. On one hand, no sudden drop to 0V characterizing a short-circuit was detected during electrochemical cycling. On the other hand, no symmetric EPR spectrum was observed as expected for sub-micrometric structures like dendrites.
We detected micrometric lithium particles with a size much higher than 1.5 µm which is much larger than the skin depth. As a consequence, the linewidth of such particles is much larger than 0.05mT. Even if the EPR signal of the metal lithium aggregates, recorded using the first harmonic detection scheme is possibly over-modulated, the second harmonic mode does not show an excessive over-modulation for the EPR peak. In contrary, recorded a slightly over-modulated EPR line using the second harmonic mode increase the signal-to-noise ratio S/N.
As suggested by the reviewer, in the case of dendritic structures, an amplitude modulation less than 0.05mT will be necessary to get semi-quantitative information about such particles.
Citation: https://doi.org/10.5194/mr-2024-5-AC1 -
RC2: 'Reply on AC1', Hu Bingwen, 10 Apr 2024
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Thank you for the response.
Since graphite is conducitive, the Dysonian lineshape might comes from lithiated graphite rather than the dentrites.
Citation: https://doi.org/10.5194/mr-2024-5-RC2 -
AC2: 'Reply on RC2', Charles Dutoit, 12 Apr 2024
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Thank you for this comment. Exactly, the EPR signature of the lithiated graphite (LixC6) exhibits a relatively large Dysonian line shape compared to the metallic lithium signal. In our study and on the aged sample, we detect such LixC6 spectrum characterized by an asymmetric ratio A/B around 1.6 and a linewidth of around 1mT. However, an additional signal is also detected and exhibits an asymmetric ratio A/B around 1.8 with a linewidth of around 0.2mT. This signal is possibly over-modulated (as explained in the previous comment) and its “real” linewidth is necessary smaller than 0.2mT as expected for Li-metal with micrometric size. It is worth noting here that we do not observe dendritic lithium structures which gives a symmetric line shape (A/B=1) but only micrometric lithium aggregates (much larger structures than dendrites) which give a Dysonian line shape (A/B>1). This last point is important because while micrometric lithium structures display a Dysonian line shape like LixC6, the difference between both materials comes from their respective linewidth. This is the reason why, using the second harmonic detection mode which is sensitive to the slope of the spectrum and hence sensitive to the sharpest line, we detect only the micrometric lithium contribution.
Citation: https://doi.org/10.5194/mr-2024-5-AC2 -
RC3: 'Reply on AC2', Hu Bingwen, 16 Apr 2024
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The explanation sounds OK.
However, in my personal experiences, it is very difficult to observe metallic lithium depositions ( which is not dentrites) in first cycle with C/2. That is why I doubt the results.
It would be better if you show the photo of the electrode to see whether the metallic lithium depositions exist.
Citation: https://doi.org/10.5194/mr-2024-5-RC3 -
AC3: 'Reply on RC3', Charles Dutoit, 16 Apr 2024
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This comment is absolutely right. As we showed in our supplementary (see figure S1) during the first cycle no spectroscopic signature of metallic lithium aggregates was observed. Indeed, we recorded the EPR spectrum of a graphite anode after the first half charge of the first cycle. This result is shown in Figure S1 and provide evidence of no additional signal from Li-metal but only the EPR spectrum of LixC6 at this state of charge (SOC). However, in our EPR investigation, the aged sample, which presents an EPR spectrum from LixC6 and an additional EPR spectrum from Li-metal, has been analyzed by EPR spectroscopy and imaging after undergoing more than 2000 (dis)charge cycles and not only 1. Even if the number of cycles is not clearly reported in our manuscript, this information is given by the sentence: Aged cell was cycled until 30% of capacity loss (experimental details). This last sample having undergone a large number of electrochemical cycles may present some degradation signature such as traces of metallic lithium aggregates.
We cannot show the photo of the electrode because this investigation was carried out several months ago and we no longer have the sample.
Citation: https://doi.org/10.5194/mr-2024-5-AC3 -
RC4: 'Reply on AC3', Hu Bingwen, 16 Apr 2024
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Thank you for the information. Now it is more or less OK.
At last, for the end of this conversion, it would be useful if some photos were taken at that time. Besides, thin dentrites might be as important to probe as the thick metalic deposition.
Citation: https://doi.org/10.5194/mr-2024-5-RC4 -
RC5: 'Reply on AC3', Hu Bingwen, 16 Apr 2024
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By the way, in the manuscript, there is no clear text stating that EPR is only detecting thick lithium deposits, not thin lithium deposits or lithium dendrites. This makes it easy for the reader to think that this article is detecting lithium dendrites, which are often more common. It was therefore suggested that the two different deposits should be clarified in the main text.
Citation: https://doi.org/10.5194/mr-2024-5-RC5 -
RC6: 'Reply on AC3', Hu Bingwen, 16 Apr 2024
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By the way, in the manuscript, there is no clear text stating that EPR is only detecting thick lithium deposits, not thin lithium deposits or lithium dendrites. This makes it easy for the reader to think that this article is detecting lithium dendrites, which are often more common. It was therefore suggested that the two different deposits should be clarified in the main text.
Citation: https://doi.org/10.5194/mr-2024-5-RC6 -
RC7: 'Reply on AC3', Hu Bingwen, 16 Apr 2024
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By the way, in the manuscript, there is no clear text stating that EPR is only detecting thick lithium deposits, not thin lithium deposits or lithium dendrites. This makes it easy for the reader to think that this article is detecting lithium dendrites, which are often more common. It was therefore suggested that the two different deposits should be clarified in the main text.
Citation: https://doi.org/10.5194/mr-2024-5-RC7 -
AC4: 'Reply on RC7', Charles Dutoit, 19 Apr 2024
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Thank you for that remark which shows that we have not been clear on this point. In our study, we chose to use the term “micrometric size” to characterize the lithium aggregates rather than “sub-micrometric size” which, usually, characterize dendritic structures.
As suggested by the reviewer, we clarified this point on page 4, line 96 and on page 6, line 111 concerning the non-dendritic feature of these metallic lithium structures.
Citation: https://doi.org/10.5194/mr-2024-5-AC4 -
RC9: 'Reply on AC4', Hu Bingwen, 22 Apr 2024
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I would like to recommend this paper for publication after minor revisions of those mentioned points.
Citation: https://doi.org/10.5194/mr-2024-5-RC9
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RC9: 'Reply on AC4', Hu Bingwen, 22 Apr 2024
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AC4: 'Reply on RC7', Charles Dutoit, 19 Apr 2024
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RC4: 'Reply on AC3', Hu Bingwen, 16 Apr 2024
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AC3: 'Reply on RC3', Charles Dutoit, 16 Apr 2024
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RC3: 'Reply on AC2', Hu Bingwen, 16 Apr 2024
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AC2: 'Reply on RC2', Charles Dutoit, 12 Apr 2024
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RC2: 'Reply on AC1', Hu Bingwen, 10 Apr 2024
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AC1: 'Reply on RC1', Charles Dutoit, 03 Apr 2024
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RC8: 'Comment on mr-2024-5', Anonymous Referee #2, 22 Apr 2024
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The authors report a new cw-EPR approach to selectively detect Li metal signals by making use of the second harmonic detection. This method was demonstrated on a lithiated graphite electrode for Li-ion batteries. This new approach offers a certain degree of selectivity of the EPR detection towards Li metal, may find broad applications in understanding Li-ion batteries. This reviewer would like to recommend it for publication after minor revisions, as suggested below:
- Since the major novelty of this work is the application of second harmonic detection to differentiate the Li metal signal from the LixC6, the theory behind second harmonic detection and its selectivity towards narrow components should be (at least briefly) described. Furthermore, where is the boundary between being selective and non-selective in terms of EPR signal linewidth?
- Is it possible to distinguish dendritic and mossy Li structures by the second harmonic detection?
- How the simulation is set up in EasySpin should be described in detail.
Citation: https://doi.org/10.5194/mr-2024-5-RC8 -
RC10: 'Comment on mr-2024-5', Anonymous Referee #3, 25 Apr 2024
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The second harmonic detection scheme is presented as a means to distinguish between EPR signals of different conductive species in a graphite anode of a Li-ion battery. Some of the features of the investigated samples appear to be unexpected by the authors. As a consequence, EPR spectra were recorded with a modulation amplitude that was too large for some of the signal components, leading to overmodulation. To facilitate the separation of different signal contributions, second harmonic detection was employed. Thereby, a different contrast was obtained, favoring signals from metallic lithium. It would be interesting to also see the effect of varying the modulation amplitude. However, as discussed in previous comments, some of the experiments cannot be easily reproduced. Nonetheless, the use of second harmonic detection provides an additional, independent method for signal separation. This technique represents a welcome addition to the tool set available to EPR spectroscopists for the investigation of battery components and batteries. It should be reported to the magnetic resonance community, even though some questions regarding the investigated samples may remain unanswered at this point. The presented data are suitable and sufficient to substantiate the claims of the manuscript.
As a basis for decision making for other researchers, a discussion of the signal amplitudes caused by the different sources (or, more precisely, their attenuation) would be helpful. (This comment is directly related to comment 1 of reviewer #2 in RC8.) Furthermore, the A/B ratio is used as a qualitative measure to identify the signal origin. Would a variation of A/B be expected with second harmonic detection, considering that the signal originates from multiple sources, or from species with a continuous distribution of relevant length scales? After these minor revisions, this reviewer recommends publication of the manuscript.
Citation: https://doi.org/10.5194/mr-2024-5-RC10
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