Investigating the Influence of Three Different Atmospheric Conditions during the Synthesis Process of NMC811 Cathode Material

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

“Investigating the influence of three different atmospheric conditions during the synthesis process of NMC811 cathode material” by Tiozzo et al.

 

The above manuscript concerns the effect of different synthesis conditions on the final electrochemical performance of NMC811 cathode materials, a member of highly important class of active materials for Li-ion battery cathodes. Overall, I am generally satisfied with the quality of current manuscript and recommend its publication after addressing the below comments:

1. The authors describe their PXRD data at length for drawing a number of key conclusions such as presence of lithium nitride and additional TM oxide phases, as well as Li-Ni antisite mixing (line 226). It is highly desirable to perform Rietveld refinements on the resulting data to gain quantitative insights on these aspects, including the fractions of each phase present in the final product and the extent of ion mixing. In particular, I am not sure how the exact stoichiometry of “Mn0.78Co2.22O4” was derived purely from the PXRD data unless some estimation using e.g., Vegard’s law was performed.

2. Why did the authors use different Li:TM ratios for the SCS and CPT cases (Table 1)?

3. Line 87: N2 is clearly not inert (formation of Li3N) so this phrase should be modified accordingly.

Author Response

The above manuscript concerns the effect of different synthesis conditions on the final electrochemical performance of NMC811 cathode materials, a member of highly important class of active materials for Li-ion battery cathodes. Overall, I am generally satisfied with the quality of current manuscript and recommend its publication after addressing the below comments:

1. The authors describe their PXRD data at length for drawing a number of key conclusions such as presence of lithium nitride and additional TM oxide phases, as well as Li-Ni antisite mixing (line 226). It is highly desirable to perform Rietveld refinements on the resulting data to gain quantitative insights on these aspects, including the fractions of each phase present in the final product and the extent of ion mixing. In particular, I am not sure how the exact stoichiometry of “Mn0.78Co2.22O4” was derived purely from the PXRD data unless some estimation using e.g., Vegard’s law was performed.

Thanks a lot for your observation. We decided to perform Rietveld refinements and we got more insight in the structure of our materials.

Regarding the presence of Mn0.78Co2.22O4, we initially used the stoichiometry coming from the PDF4+ database. We have not applied the Vegard’s law to calculate the ratio between Mn and Co. On the other hand, since the weight fraction of that phase is very low in the SCS-N2 sample and the quality of our data is not sufficient to support any hypothesis on the composition, we decided to use the name mixed manganese cobalt oxide with general composition Mn_xCO_3-xO₄.

Regarding the Li-Ni mixing, our Rietveld refinement demonstrates that all the samples treated in oxygen and air atmospheres show a mixing of the order of 7-9%, fully in line with other works present in the literature.

We modified the paper and the supplementary material to include this new information.

2. Why did the authors use different Li:TM ratios for the SCS and CPT cases (Table 1)?

This is because in the SCS synthesis some lithium is lost due to the high temperature of the combustion reaction. The text was modified accordingly.

3. Line 87: N2 is clearly not inert (formation of Li3N) so this phrase should be modified accordingly.

Thanks for pointing this out. We modified the text accordingly. The term “inert” has been substituted with “non-oxidizing”.

Reviewer 2 Report

Comments and Suggestions for Authors

In this study, the authors investigate synthetic factors impacting the structure and performance of NMC811 cathode materials. Two commercial synthesis methods, SCS and CPT, were employed, alongside evaluating three atmospheric conditions (air, O2, and N2) during the calcination process. Through extensive characterization, the study juxtaposes the morphology and electrochemical properties of the cathodes produced under these varying conditions. It concludes that the CPT method, when combined with an oxygen-rich calcination environment, yields superior material properties and performance. The authors also offer valuable insights for future synthetic studies based on their findings. Thus I endorse the publication of this manuscript in crystals, but two critical points below should be addressed before that:

1. On line 253, the authors claim negligible differences in morphology among materials treated in different atmospheric conditions. Contrarily, Figure 7 reveals notable variations in particle size and other features. This discrepancy needs to be elaborated.

2. Figures 9 and 10 indicate that both the reference and commercial materials significantly outperform those synthesized in this study. This necessitates a detailed explanation.

Author Response

In this study, the authors investigate synthetic factors impacting the structure and performance of NMC811 cathode materials. Two commercial synthesis methods, SCS and CPT, were employed, alongside evaluating three atmospheric conditions (air, O2, and N2) during the calcination process. Through extensive characterization, the study juxtaposes the morphology and electrochemical properties of the cathodes produced under these varying conditions. It concludes that the CPT method, when combined with an oxygen-rich calcination environment, yields superior material properties and performance. The authors also offer valuable insights for future synthetic studies based on their findings. Thus I endorse the publication of this manuscript in crystals, but two critical points below should be addressed before that:

1. On line 253, the authors claim negligible differences in morphology among materials treated in different atmospheric conditions. Contrarily, Figure 7 reveals notable variations in particle size and other features. This discrepancy needs to be elaborated.

Thanks for pointing this out. We modified the text accordingly.

2. Figures 9 and 10 indicate that both the reference and commercial materials significantly outperform those synthesized in this study. This necessitates a detailed explanation.

The commercial cathode materials were used as a reference and were optimized by the seller to reach high performances. On the other hand, the goal of this work is s to study the effect of the calcination atmosphere on the structure, morphology, and performances. So, we don’t expect to get results superior or equal to the benchmark materials, since we know that we must optimize many other synthesis parameters. The text was modified accordingly.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript furnishes invaluable insights into the synthesis of NMC811 materials, playing a pivotal role in elucidating the ramifications of varying atmospheric conditions on the synthesis trajectory. Nevertheless, further endeavors are requisite to augment the depth and breadth of the research, as well as its pertinence to practical implementations.

1. A more elaborate delineation or annotation of Figure 1's assembly process is recommended to facilitate comprehension of the battery's pragmatic assembly among the readership.

2. The authors might consider the controlled monitoring of additional variables that could potentially influence the outcomes during the synthesis, such as temperature and humidity levels.

3. There should be a discussion of the potential mechanisms, explicating why disparate atmospheric conditions precipitate fluctuations in NMC811 performance.

4. The environmental implications of by-products or waste generated during the synthesis process warrant appropriate discourse.

5. The discussion section of the article is deficient in considerations of economic viability and sustainability, which are paramount for the material's commercialization. To enhance the manuscript's utility, the authors are advised to discuss the scalability of laboratory synthesis methods to industrial production.

Comments on the Quality of English Language

 Moderate editing of English language required.

Author Response

This manuscript furnishes invaluable insights into the synthesis of NMC811 materials, playing a pivotal role in elucidating the ramifications of varying atmospheric conditions on the synthesis trajectory. Nevertheless, further endeavors are requisite to augment the depth and breadth of the research, as well as its pertinence to practical implementations.

1. A more elaborate delineation or annotation of Figure 1's assembly process is recommended to facilitate comprehension of the battery's pragmatic assembly among the readership.

Thanks for this comment. The caption of the figure was modified adding some details about the coin cell setup.

2. The authors might consider the controlled monitoring of additional variables that could potentially influence the outcomes during the synthesis, such as temperature and humidity levels.

Thanks for this suggestion. The temperature of the coprecipitation bath is controlled and set to 55 °C and in our future studies we will explore the effect of changing the synthesis temperature on the materials properties. On the other hand, since the procedure is a wet chemistry process, the control of humidity is not possible.

Regarding the SCS procedure, the reaction temperature is determined by the exothermic combustion reaction, so it cannot be controlled. Moreover, we don’t think humidity would play a major role during the combustion.

Regarding the calcination, we will study the effect of changing the temperature in a future paper. Moreover, we will use also an oxidizing atmosphere containing oxygen water.

The perspectives at the end of the paper were modified.

3. There should be a discussion of the potential mechanisms, explicating why disparate atmospheric conditions precipitate fluctuations in NMC811 performance.

Thanks for the comment. Our paper is related to exactly this aspect, but the correlation between the electrochemical performances and the crystal structure/microstructure of the cathode materials is not easy, since many parameters determine the electrochemical behaviour. The fact that nitrogen-treated samples show no electrochemical capacity is related to the fact that the calcination under nitrogen produces a mixture of oxides that cannot intercalate lithium (see the Rietveld treatment included now in the paper). On the other the difference between the air and oxygen treated samples is tiny and can be explained in terms of morphology and reduced cation mixing in the oxygen treated samples.

4. The environmental implications of by-products or waste generated during the synthesis process warrant appropriate discourse.

This is a general concern related to cobalt (and nickel) containing cathode materials for Li-ion batteries. Regarding the co-precipitation, the reaction is performed in a closed environment and the wastes can be controlled in an accurate way. The SCS procedure on the other hand is more prone to release side products in the environment due to the violent exothermic combustion. The calcination step, if oxygen or air are used has a high yield of final product with a negligible content of by-products. As a general remark, any Li-ion battery using cobalt, should be recycled at the end of life to fully recover the raw materials and avoid any release of cobalt in the environment. But this is independent from the preparation procedure selected for the cathode preparation.

5. The discussion section of the article is deficient in considerations of economic viability and sustainability, which are paramount for the material's commercialization. To enhance the manuscript's utility, the authors are advised to discuss the scalability of laboratory synthesis methods to industrial production.

The two synthesis procedures (SCS and CPT) are very different in terms of economic investment and sustainability. The SCS is probably the most simple and cheaper approach, but it does not guarantee a fine control of the particle size and morphology. Moreover, during the combustion process, some lithium is lost so the synthesis requires more lithium precursor and in addition the exothermic nature of the reaction does not allow to increase to much the reaction bath. On the other hand, the co-precipitation process is more controllable but requires a higher investment for the infrastructure. As a matter of fact, co-precipitation is already one of the used techniques for the preparation of this class of materials. Some more details are found in references 9 and 10 of our bibliography as described in the introduction. A short comment was added at the end of the paper.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I am satisfied with the response from the authors and recommend publication in its current form.

Reviewer 3 Report

Comments and Suggestions for Authors

I have no further comments and agree to accept the manuscript.

Comments on the Quality of English Language

Minor editing of English language required.