Exploring Thermal Runaway: Role of Battery Chemistry and Testing Methodology

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I believe that the paper is suitable for publication; however, we recommend a few minor revisions to further refine its quality and clarity.

What is the main question addressed by the research?

The primary focus of the research is understanding and predicting thermal runaway  in battery cells, specifically examining the impact of different battery chemistries and initiation protocols. The goal is to improve the safety of battery electric vehicles  by creating a solid experimental basis for modeling and simulating TR events. The paper analyzed  the following  different chemistries:NMC, LFP, and SIB and triggers:constant heating, heat-wait-seek, nail penetration.

Originality and Relevance

Although this is not my specific field of research, after conducting a literature review, I found several studies that focus on testing various battery chemistries and trigger methods but I'm not able to find a unified work that deals with all these aspects. However, I am unable to provide a comprehensive assessment of the novelty of the results obtained and reported, despite the validity and robustness of the experiments conducted. In my opinion, the main innovation lies in the systematic aggregation and comparison of results across different chemistries and trigger methods for thermal runaway.  This synthesis provides significant added value, as it offers a comprehensive overview that, according to the authors, is fundamental for studying propagation phenomena in multi-cell battery packs.

Consistency of Conclusions with Evidence

The conclusions are consistent with the presented results.  However a separated section with a conclusion should be added to summarize  the concept.

Appropriateness of References

In my opinion, the references are appropriate and well-cited, covering foundational studies on thermal runaway mechanisms, battery chemistry, and safety evaluations.

Comments on Tables and Figures

As stated in the previous review, the figures must be well-formatted but, most importantly, they should be more detailed in the paper.  

Comments on the Quality of English Language

The manuscript presents an interesting and promising study; however, it would greatly benefit from a more comprehensive and detailed description. In particular, a more thorough discussion concerning the issues of thermal runaway in batteries  is recommended. Furthermore, the figures in the results section should be more clearly explained and appropriately centered to enhance their readability and comprehensibility. Additionally, the use of more technical language throughout the manuscript would enhance its clarity and alignment with the standards of scientific writing.

Author Response

Originality and Relevance

Although this is not my specific field of research, after conducting a literature review, I found several studies that focus on testing various battery chemistries and trigger methods but I'm not able to find a unified work that deals with all these aspects. However, I am unable to provide a comprehensive assessment of the novelty of the results obtained and reported, despite the validity and robustness of the experiments conducted. In my opinion, the main innovation lies in the systematic aggregation and comparison of results across different chemistries and trigger methods for thermal runaway.  This synthesis provides significant added value, as it offers a comprehensive overview that, according to the authors, is fundamental for studying propagation phenomena in multi-cell battery packs.

We are grateful to the reviewer to acknowledge the originality of our work and to have an added value when compared to the state-of-the-art.

Consistency of Conclusions with Evidence

The conclusions are consistent with the presented results.  However a separated section with a conclusion should be added to summarize  the concept.

We thank the reviewer for considering our result to be consistent.

We are surprised by such a request because we believe the concept of this work was already presented in the submitted manuscript:

• The concept of this work is summarized at the end of the introduction, as it is normally done in scientific publications, at least based on our experience, page 5 lines 199-193 in the revised manuscript.

“The goal of this work, in a first step, is to create an experimental base to evaluate the influence of both battery chemistry and trigger method on the characteristics of the thermal runaway. The trigger methods chosen aim to investigate scenarios representing the three main triggers of a thermal runaway event: (a) the heat-wait-seek method, mimicking internal self-heating, mainly due to internal failure; (b) the continuous heating method, representing the effect of an intense external heat source (i.e., close to a hot spot in the vicinity of the cell) and (c) nail penetration mimicking a mechanical damage, for example in case of a vehicle crash”.

 

• And page 5 line 211 to 218 in the revied manuscript.

“In a second step, the experimental results are used to develop a methodology to simulate, and thus predict, thermal runaway events even for advanced battery technologies, enabling the transfer of the presented methodology to investigations of thermal propagation phenomena in multi-cell configurations. Due to the high complexity of the thermal runaway and propagation phenomena, we employ a combination of physical testing procedures with virtual development methods to drastically reduce the number of tests needed for the development of a “no thermal propagation” battery system and thus accelerate the development of safe and cheap electrical vehicles.”

 

• The conclusion part includes:
  • summarize of the work done p22 lines 738-739 in the revised manuscript: “Different thermal runaway tests have been conducted to simulate the various possible causes for thermal runaway and propagation occurrence in electric vehicle batteries”
  • The main important information obtained from the discussion based on our experimental results p22 line 742-744 of the revised manuscript the main information: “The presented results clearly confirm the high influence of both the testing conditions and battery chemistry on the thermal runaway phenomenon, i.e., the duration, the venting, the peak temperature and gas release”
  • The use of the experimental data for the simulation part page 22 line 753-755 of the revised manuscript: “The presented measurement results are a solid experimental basis that has been further employed for modeling in 3D and predicting the thermal runaway with chemistry-specific reaction mechanisms”
• However, it may appear to the reader that the concept of our work was not sufficiently highlighted. Consequently, a small paragraph has been added at the beginning of the conclusion. We hope that the addition of this paragraph contributes to a clear understanding of the chosen concept.

Appropriateness of References

In my opinion, the references are appropriate and well-cited, covering foundational studies on thermal runaway mechanisms, battery chemistry, and safety evaluations.

Comments on Tables and Figures

As stated in the previous review, the figures must be well-formatted but, most importantly, they should be more detailed in the paper.  

We do not understand what the reviewer means by “as stated in previous review”. It is the first time this manuscript and the related figures are submitted to any scientific journal.

Following the reviewer’s request, we have addressed the resolution of the figures, as requested by another reviewer, and plan to submit high resolution figures separately as well. Furthermore, the discussion related to the figures has been revised to be more detailed, as further requested in the following comment.

 

Comments on the Quality of English Language

The manuscript presents an interesting and promising study; however, it would greatly benefit from a more comprehensive and detailed description. In particular, a more thorough discussion concerning the issues of thermal runaway in batteries  is recommended. Furthermore, the figures in the results section should be more clearly explained and appropriately centered to enhance their readability and comprehensibility. Additionally, the use of more technical language throughout the manuscript would enhance its clarity and alignment with the standards of scientific writing.

We are delighted to know the reviewer judges our study to be of interest.

We apologize that our manuscript has issues regarding explanation and technical language.

• “a more thorough discussion concerning the issues of thermal runaway in batteries is recommended”

Following the reviewer’s suggestion, a detailed paragraph at the end of section 3 has been added in the corrected manuscript on the issues of the thermal runaway.

• “the figures in the results section should be more clearly explained and appropriately centered”

The figures have been centered as much as we could within the WEVJ template. We further expect that the final formatting of the manuscript will be done in the proofing phase of the manuscript submission after the manuscript was accepted.

The manuscript has been adapted to increase the clarity of the discussion and comply with scientific writing.

We hope these changes are able to satisfy the reviewer’s request.

Submission Date

18 December 2024

Date of this review

27 Dec 2024 19:43:07

Reviewer 2 Report

Comments and Suggestions for Authors

The content of the manuscript is within the scope of the journal and can be of broad interest to readers. However, in terms of specific content, there is still room for improvement. Therefore, I decided to give the decision of minor revision. It is recommended that the author properly absorbs the reviewers' comments and make corresponding improvements and enhancements.

 

1. For the keywords, 'battery chemistry', 'Arrhenius approach', and 'thermal propagation' should be added to attract a broader readership.

 

2. Page 1, 'In the past decade, BEVs have emerged as challengers to the vehicles relying on conventional internal combustion engines (ICEs).'

      It should be noted if the electricity for batteries comes from a fossil fuel-based thermal power plant, electric vehicles do not help decarbonization and reduce emissions. For renewable energy sources, the authors should introduce that solar energy and wind energy are unstable and intermittent during generation, and thus these valuable electric energies are difficult to apply continuously and stably. To tackle this issue, the employment of batteries as energy storage systems combined with renewable energy may greatly improve the utilization rate and stability of renewable energy (10.3390/app14083316). In this way, electric vehicles become more and more popular in the context of global decarbonization.

 

3. Besides understanding the mechanism of thermal runaway, some experimental measured approaches should also be mentioned briefly. Especially for non-destructive technologies, state of charge and temperature joint estimation based on ultrasonic reflection waves (10.3390/batteries9060335) are very promising techniques since it covers both the thermal runaway/safety issues and the SOC situations.

 

4. If the topic is mainly for BEVs, are SIB and NMC widely used in BEVs? I consider the majority of EV batteries should be lithium-ion batteries. Hence, I am quite wondering whether there is so much need to evaluate two batteries with minor usage.

 

5. Since the working conditions of electric vehicles are complex and changeable, can the simulation in this manuscript accurately replace all experimental scenarios? Or are the experimental results in some special scenarios still open to discussion or improvement? This issue should be further clarified.

Author Response

Review 2

 

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

Quality of English Language

(x) The quality of English does not limit my understanding of the research.
( ) The English could be improved to more clearly express the research.

 

		Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?		( )	(x)	( )	( )
Is the research design appropriate?		( )	(x)	( )	( )
Are the methods adequately described?		( )	(x)	( )	( )
Are the results clearly presented?		(x)	( )	( )	( )
Are the conclusions supported by the results?		(x)	( )	( )	( )

Comments and Suggestions for Authors

The content of the manuscript is within the scope of the journal and can be of broad interest to readers. However, in terms of specific content, there is still room for improvement. Therefore, I decided to give the decision of minor revision. It is recommended that the author properly absorbs the reviewers' comments and make corresponding improvements and enhancements.

We are delighted by the fact that the reviewer sees our manuscript to be of broad interest to readers.

1. For the keywords, 'battery chemistry', 'Arrhenius approach', and 'thermal propagation' should be added to attract a broader readership.

We thank the reviewer for its interesting suggestion. Battery chemistry and Arrhenius approach have been added to the keywords list.

However, we prefer not to add thermal propagation. Even if this phenomenon is mentioned in the manuscript as an outlook for simulation approaches, no propagation phenomenon (i.e. from cell to cell), neither experimental, nor simulated, has been reported. We are afraid that adding “thermal propagation” in the keywords will lead to misunderstanding expectations from the readers and make our manuscript less consistent.

 

2. Page 1, 'In the past decade, BEVs have emerged as challengers to the vehicles relying on conventional internal combustion engines (ICEs).'

It should be noted if the electricity for batteries comes from a fossil fuel-based thermal power plant, electric vehicles do not help decarbonization and reduce emissions. For renewable energy sources, the authors should introduce that solar energy and wind energy are unstable and intermittent during generation, and thus these valuable electric energies are difficult to apply continuously and stably. To tackle this issue, the employment of batteries as energy storage systems combined with renewable energy may greatly improve the utilization rate and stability of renewable energy (10.3390/app14083316). In this way, electric vehicles become more and more popular in the context of global decarbonization.

The reviewer is right that the carbon impact of electrical vehicles depends on the energy used to produce and charge them. However, transport decarbonization is not within the scope of this work and we have not mentioned it in our manuscript. Our work focusses on the battery safety for EVs and more particularly on the thermal runaway phenomenon as mentioned in the title of our manuscript.

Independently of their real ecological impact, EVs accounted for 10 % of the worldwide automotive sales volume (reference 1 in the submitted manuscript) in 2023, with the trend further continuing in 2024. Consequently, in our point view, safety issues related to cell thermal runaway are of broad interest to the battery community.

3. Besides understanding the mechanism of thermal runaway, some experimental measured approaches should also be mentioned briefly. Especially for non-destructive technologies, state of charge and temperature joint estimation based on ultrasonic reflection waves (10.3390/batteries9060335) are very promising techniques since it covers both the thermal runaway/safety issues and the SOC situations.

We are thankful to the reviewer for this insight. We do agree that new diagnostic approaches for temperature and/or SOC estimation are generally able to provide valuable data, leading to a better understanding of the cells’ state. However, the focus of our manuscript is not on the diagnostics of thermal runaway but rather a mechanistic approach to the thermal runaway phenomena and their simulations within single cells. From an experimental point of view, we have chosen a simplistic approach using thermocouples to provide a measure of component temperature. Since the cells were placed in a fully charged state, representing a worst-case scenario prior to the induction of thermal runaway, our concept furthermore does not rely on estimation methods for the SOC. We thus have not further explored the addition of citations covering diagnostic approaches for state estimations at cell level.

4. If the topic is mainly for BEVs, are SIB and NMC widely used in BEVs? I consider the majority of EV batteries should be lithium-ion batteries. Hence, I am quite wondering whether there is so much need to evaluate two batteries with minor usage.

It is true that the topic of this work is related to BEVs. Its focus is on the thermal runaway at cell level. As mentioned in the introduction page 2 lines 77-78 of the revised manuscript, the NMC-based battery is a lithium-ion battery. It is a very popular chemistry due to its high energy density as mentioned in the same paragraph on page 2. We humbly suggest to the reviewer to look at the reference 23, available online. LFP is getting more popular (especially in China) due to its low cost and superior safety performance. However, NMC-based LIBs are still commonly used within BEVs (> 30% of the world market in 2023) and will remain superior to 10%, even in a low demand scenario (see reference 3 of the revised manuscript). We thus are convinced that highlighting any safety issues of the NMC-based LIBs is an obvious need for the automotive industry.

We agree with the reviewer that SIB is not presently a major player for the BEV market. As mentioned in the introduction page 2 line 85 of the revised manuscript “sodium-ion batteries (SIB) can be reasonably considered to be the among the most promising. This technology has a very high level of maturity, as it is already used in commercial products, for example electric tools and micro-cars”. In page 27 of the reference 3 of the revised manuscript, the increased innovation scenario considers the SIB will represent approx. 15% of future market share. Even in the current trend scenario, the expected share market of SIBs is 5%, which is not negligeable. Consequently, investigating the safety of emerging battery technologies is of prime importance for the car producers when strategic choices must be made for the development of the next generation of BEVs.

We understand from the reviewer’s comments that this point was not sufficiently stressed in the submitted manuscript. Thus, the related text at the end of the introduction (page 5 lines 206-210) has been adapted accordingly in the revised manuscript.

5. Since the working conditions of electric vehicles are complex and changeable, can the simulation in this manuscript accurately replace all experimental scenarios? Or are the experimental results in some special scenarios still open to discussion or improvement? This issue should be further clarified.

The reviewer is right, BEVs, and more particularly their batteries, are complex dynamic systems. It is the reason combining safety, performance and cost is a challenge. Furthermore, we agree with the reviewer that the transfer of experimental results to a modeling environment can be challenging and is never going to be able to fully represent all real-world influences and related errors. We are, however, confident that our approach covers the main effects occurring during cell thermal runaway. As pointed out in the manuscript, our simulation model covers the chemical reactions based on an Arrhenius approach and furthermore covers heat transfer to surroundings and environment using a 3D CFD approach. To guarantee the safety of passengers and compliance with the regulations, we think that experimental tests will always be needed to validate the final product. As already mentioned in the submitted manuscript page 5 line 215 of the revised manuscript “we employ a combination of physical testing procedures with virtual development methods to drastically reduce the number of tests needed for the development of a “no thermal propagation” battery system and thus accelerate the development of safe and cheap electrical vehicles”. Consequently, we consider the related request of the reviewer as already implemented.

We furthermore want to emphasize that in addition to be faster, cheaper, safer and more environmentally benign than physical tests, simulation results can deliver valuable information on the internal processes occurring in the cell during thermal runaway which are usually not accessible by experimental tests. This has already been stated in the conclusion: “this approach yields an accurate 3D temperature field providing insight into physical processes that are not accessible with typical experimental methods. “

Finally, we want to highlight that our approach was able to cover different initiation methods for the same chemistry. By calibrating the Arrhenius model with the data of one trigger method, we were able to cover other trigger methods as well, even though the timescales and initial temperature levels are considerably different. For this, of course, changes to the simulated 3D environment, such as the changing experimental setup, were considered. Thus, we are confident that our simulation approach appropriately covers the trigger methods highlighted within the manuscript.

To make this information clearer to the reader, we have implemented changes to the respective part of the conclusion. We hope to have sufficiently complied with the reviewer’s request.

 

Submission Date

18 December 2024

Date of this review

31 Dec 2024 23:06:45

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents an in-depth study on the thermal runaway behavior of different battery chemistries (LFP, NMC, and SIB) under various testing methods. While the study presents valuable experimental insights and modeling efforts, however some clarity is required , I suggest the author should revise the article and address below comments

 

1. explain why specific heating rates and protocols such as heat-wait-seek and constant heating were chosen and how these parameters influence battery failure scenarios.
2. What is the relation between weight loss of cells during thermal runaway and the thermal stability and structural integrity of different chemistries.
3. short-circuit mechanisms inside the cells is required to justify the surface temperature variations in the nail penetration tests
4. what is the role of electrolyte decomposition products in the thermal runaway process,
5. I suggest including the bandgap influence on the thermal behavior of the materials .
6. Provide some link between electronic properties of cathode materials which impact thermal stability.
7. The manuscript lacks a comprehensive comparison of failure onset with different SOC (State of Charge) levels, which can significantly impact safety assessments.
8. Provide some data such as SEM to analysis internal electrode structure damage in Nail Penetration
9. Figures 5 and 6 need clearer legends and higher resolution to improve readability,
10. The author should provide a justification for the observed differences in smoke and gas emissions, particularly between NMC and SIB, and how cell design affects these outcomes.
11. how the thermocouples were calibrated and whether heat losses to the environment were accounted for in the simulations.

Author Response

Reviewer 3

 

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

Quality of English Language

(x) The quality of English does not limit my understanding of the research.
( ) The English could be improved to more clearly express the research.

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	( )	(x)	( )	( )
Is the research design appropriate?	(x)	( )	( )	( )
Are the methods adequately described?	(x)	( )	( )	( )
Are the results clearly presented?	( )	(x)	( )	( )
Are the conclusions supported by the results?	( )	(x)	( )	( )

Comments and Suggestions for Authors

The manuscript presents an in-depth study on the thermal runaway behavior of different battery chemistries (LFP, NMC, and SIB) under various testing methods. While the study presents valuable experimental insights and modeling efforts, however some clarity is required , I suggest the author should revise the article and address below comments

We are very pleased to read that the reviewer considers our work as an in-depth study on thermal runaway.

1. explain why specific heating rates and protocols such as heat-wait-seek and constant heating were chosen and how these parameters influence battery failure scenarios.

There are two reasons as to why we have chosen different triggering methods, and specifically both the heat-wait-seek and constant heating protocols:

• Each trigger method was chosen to represent a reasonably inferable real-world scenario, as stated at the end of the introduction when detailing the aims and scope of the manuscript.
• Both HWS and CH protocols as well as the nail penetration method are well-established and commonly used in scientific and technical literature to trigger thermal runaway. We want to refer the reviewer to references 8, 10, 11 and 16 (in the revised manuscript) as well as regulatory documents including reference 2 and the equivalent Chinese regulation GB

We have thus considered these protocols to be state of the art in terms of trigger methods for cell thermal runaway investigations.

With regard to specific heating rates and power, we have chosen a 5 K heating step for the HWS protocol in line with general practice in the scientific literature (see the aforementioned references). In the CH protocol, the heating rate was limited by the thermal power output of the employed heating pad. However, the power uptake of 27 W is reasonably close to the 30 W stipulated within the aforementioned regulatory documents for the 18650 cells used in our experimental campaign.

This information was made available to the reader by implementing changes to the Materials and Methods part of the manuscript.

We hope this appropriately addresses the reviewer’s comment.

 

2. What is the relation between weight loss of cells during thermal runaway and the thermal stability and structural integrity of different chemistries.

We would like to thank the reviewer for pointing this out.

The observed weight loss is in line with the observed thermal stability of the cathode materials of the cells. This information is accessible to the reader in Table 2 of the submitted manuscript.

Usually, the weight loss is higher for thermally less stable materials (like NMC) due to the high temperatures and high pressures involved during the reactions. This leads to a higher conversion of active materials, increasing weight loss. Furthermore, due to the high pressure, more materials can be ejected during the violent thermal runaway reactions.

To clarify the point to the reader, we added a sentence covering the typical expectations on the weight loss during thermal runaway in relation to the thermal stability of the cell materials.

The related discussion to Table 2 was modified accordingly.

3. short-circuit mechanisms inside the cells is required to justify the surface temperature variations in the nail penetration tests

We thank the reviewer for the comment. We understand this comment to involve both the experimental and the simulation part.

Experimental:

Due to the increased complexity of using redundant thermocouples and limited signal inputs we only have employed a single thermocouple for each lateral cell position. We therefore treated the lateral cell surface as a whole, trying not to over-emphasize a specific location on the lateral surface within the discussion of the lateral cell temperatures as we consider this not to be best scientific practice.

Nevertheless, the involvement of short circuit mechanisms has already been reported in the literature (reference 17 in the revised manuscript). Various other reports (e.g. references 18 and 19 in the revised manuscript) are consistent in their description of the cell thermal runaway as being initiated by an internal short circuit that causes the vicinity of the nail to heat up and initiate the thermal runaway which is then propagating through the whole cell.

Based on the already cited literature, it appears intuitive that the heat generated is sufficient to lead to similar follow-up mechanisms as observed for the other TR scenarios (e.g. the heat is sufficient to initiate electrolyte/electrode reactions). These reactions are themselves exothermic as reported in the introduction. Thus, the cascade scenario of the TR is initiated.

The specifics of the TR scenario triggered by the nail-penetration including additional related references were added to the introduction part of the manuscript. With this, we hope to satisfy the reviewer’s demand.

 

Simulations:

Within nail penetration simulations, a local heat source was used to simulate the short circuit introduced by the nail.

For the heating protocols, no short circuits were implemented. Instead, the simulations were calibrated using experimental data, so the heat generated during internal short circuiting is already considered within the Arrhenius approach for the cell reactions.

To clarify the reviewer’s point for the readers, we have added a sentence detailing the simulative approach for short circuit scenarios within the cells in the Materials and Methods part.

 

4. what is the role of electrolyte decomposition products in the thermal runaway process,

The role of the electrolyte decomposition product is described in the introduction page 4. Where the electrolyte is forming a non-stable SEI on the anode side in the temperature range of 90-10°C, with important gas emission, leading to an increase of the cell pressure, and eventually venting.

5. I suggest including the bandgap influence on the thermal behavior of the materials.

We are troubled by such a request. This work focuses on the influence of battery chemistry and testing methodology on the thermal runaway and its corresponding simulation. Knowing the thermal stability of the different cathodic materials appears sufficient. Adding the band gap influence on the thermal behavior appears in our point of view very “material” focused and is not the topic of this work.

6. Provide some link between electronic properties of cathode materials which impact thermal stability.

Similarly, to remark 5 we are very troubled by the request of the reviewer. We do not see why linking the electronic property of cathodic materials with their thermal stability is the topic of this work. We think that knowing the thermal stability of the cathodic material and thus how it influences the thermal runaway scenario of the cell is what matters.

7. The manuscript lacks a comprehensive comparison of failure onset with different SOC (State of Charge) levels, which can significantly impact safety assessments.

We agree with the referee that the SoC influences the stability of the cells versus the TR. This is well known for battery cells (see e.g. reference 15 for NMC, and 21 for LFP in the revised manuscript). A sentence has been added to the introduction to inform the reader.

We agree with the referee that investigating the influence of the state of charge of each cell on the TR scenario is of interest but that is not the topic of this experiment, and it can be reasonably considered as a full-study by itself similarly to the work reported by Kurzawski et al. (reference 22 of the revised manuscript), all cells were tested in fully charged state, which is standard (see reference 8, 10, 16 and 25 from the revised manuscript) and, based on the literature, is considered as a worst case  corresponding to a real-world scenario at the end of a charge. Thus, the knowledge acquired is sufficient for the safety assessment. In addition, testing different SoC for each cell and each trigger method will overload the content of this manuscript while it is not the topic of this work and will not impact the safety assessment as we are already considering the worst possible case, SoC = 100% as already reported. This point has been added to the Materials and Methods part to inform the reader.

8. Provide some data such as SEM to analysis internal electrode structure damage in Nail Penetration

We understand the point of view of the reviewer of the potential interest in post-mortem analysis.

• However, we do not see any improvement by adding postmortem data in the manuscript. We hardly see how the SEM analysis can be linked with the safety issue of thermal runaway as the present scope of this work is not involved in any possible improvements to the cell’s internals and materials in order to mitigate or avoid thermal runaway completely.
• In addition, especially SEM images only show a limited part of the material investigated which may not be representative of the overall appearance of the cell’s leftovers. Thus, we do not see that this additional experimental request adds specific value to the manuscript. Finally, we think that most of the jelly-roll structure has been lost after TR as illustrated by Wang et al. (DOI: 10.1016/j.echem.2020.07.028) and Wei et al.(DOI: 10.3889/fengrg.2023.1230429).

9. Figures 5 and 6 need clearer legends and higher resolution to improve readability,

We would like to thank the reviewer for his/her help to improve our manuscript. The resolution of the figure in the submitted manuscript is very high (on our end, when zooming to 4500% on the word file our figures do not appear blurry). Thus, we do not see how we can improve the original quality of our figures.

It is possible that -depending on the settings- the figure resolution is decreased during conversion of the original manuscript word-file to a pdf-file. To improve the readability of our figures, all figures were in addition uploaded as a pre-converted PDF document. The conversion induced a little loss of resolution, but the figures appear all readable and understandable even at the maximum zoom allowed by the PDF viewer of our computer, we hope the reviewer will find the resolution sufficient on his/her computer screen and are confident that the resolution is best in the accepted version.

We hope that this addresses both the legends and resolution issues, otherwise we kindly ask the reviewer to further clarify the improvements needed for better readability of the stated figures. In our point of view, the figure legends adequately assign the depicted measurement traces to specific properties and/or settings (e.g. the position of a thermocouple) Each axis (temperature and time) is labelled with the unit used (°C or seconds). Each temperature variation of each sensor is properly labelled by a color (consistent coloring for equivalent sensor positions). For Figures 5 and 6, significant events observed on the video camera (e.g. the start of venting) are highlighted by a dashed line and a label indicating the time. The most significant ones are even illustrated by a picture with the corresponding experimental time.

10. The author should provide a justification for the observed differences in smoke and gas emissions, particularly between NMC and SIB, and how cell design affects these outcomes.

The exact composition of the electrolyte is not known. But based on literature the electrolyte is similar based on organic carbonates using LiPF₆ in LIB and NaPF₆ in SIB as supporting electrolyte (reference 32 of the revised manuscript, and DOI: 10.1039/d0cp03639k and 10.1016/j.powsour.2023.234008). Similarly, we do not know the exact cell design of each cell, but we consider them to be reasonably similar. Cells are of the same format and all cells are using a jellyroll made from a two electrode, coated metallic foil with a polymeric separator between them (reference 32 of the revised manuscript). In addition, we do not know the threshold pressure of the venting for each cell, which is likely to influence the overall occurrence of the smoke and gas emission of individual cells.

The gas emission occurring during a TR is dependent on multiple factors, it is challenging to clearly explain the exact causes of differences observed. The gas emission is mainly initiated by reaction between the electrolyte and the surface of the anode. The electrode composition is different for the NMC cell and the SIB, for the former the anode is a graphite and silicon mixture while for the SIB it is hard carbon, thus the anode chemistry is likely to have an influence on the electrolyte degradation and the smoke and gas emission. In addition, we do not know what the electrolyte content present in each cell, and the electrode surface/ electrolyte amount ratio. Finally, even if the electrolyte reacts with the anode first, the TR usually involves a plethora of reactions with other materials when temperature is sufficiently high. In case of the SIB, temperatures of 500-600°C are reached, but no flame has been observed while clearly combustion occurs for the NMC-based LIB in all cases.

To improve our manuscript accordingly, we added a sentence detailing the complexity of the smoke and gas emissions for the different chemistries and trigger methods investigated in our approach. We hope this satisfies the reviewer’s request.

11. how the thermocouples were calibrated and whether heat losses to the environment were accounted for in the simulations.

The thermocouples were not specifically calibrated; we relied on the specification of the provider. Of course, our measurement approach only allows for the measurement of surface and gas temperatures, as the introduction of a thermocouple into the cell would obviously compromise the experimental outcome. The temperatures recorded are dependent on the heat transfer from the cell core to the cell surface as well as the heat transfer between the cell surface and the thermocouple.

Still, the temperatures recorded are above several hundred-degree Celsius, even superior to 1,000°C for the NMC cell. We believe the accuracy of the temperature data to be sufficient in this case. Furthermore, the temperatures we recorded in our experimental campaign are in line with similar experimental investigations (see references 8, 11, 16 and 24 of the revised manuscript). Consequently, we believe our data to be sufficiently reliable to support our discussion and conclusion.

Regarding the second part of the reviewer’s request, we want to emphasize that heat losses to the environment are always difficult to address in simulations. We have addressed heat losses to the environment by incorporating convective heat transfer to the ambient gas phase as well as the heat transfer to the materials surrounding the cells (e.g. clamps and holders) within our CFD calculations. In order to clarify the methodology regarding the reviewer’s comment, we have added a sentence detailing the approach in the Materials and Methods part of the manuscript.

 

Submission Date

18 December 2024

Date of this review

20 Jan 2025 21:31:49

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The author response is satisfactory for all my comments and hence i agree to accept it 

Author Response

Open Review ( ) I would not like to sign my review report
(x) I would like to sign my review report Quality of English Language (x) The English is fine and does not require any improvement.
( ) The English could be improved to more clearly express the research.            

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	(x)	( )	( )	( )
Is the research design appropriate?	(x)	( )	( )	( )
Are the methods adequately described?	(x)	( )	( )	( )
Are the results clearly presented?	(x)	( )	( )	( )
Are the conclusions supported by the results?	(x)	( )	( )	( )

    Comments and Suggestions for Authors

The author response is satisfactory for all my comments and hence i agree to accept it 

 

We are thrilled that the reviewer is pleased with our response and has agreed to accept the revised manuscript.