Effect of Water-Soluble CMC/SBR Binder Ratios on Si-rGO Composites Using µm- and nm-Sized Silicon as Anode Materials for Lithium-Ion Batteries

Round 1

Reviewer 1 Report (New Reviewer)

The article “Effect of Water-Soluble CMC/SBR Binder Ratios on Si-rGO Composites using µm- and nm-sized Silicon as Anode Material for Lithium-Ion Batteries” summarizes the role of binder composition on the cycling stability of the micro- and nano sized Si-rGO composites. The overall design including the methodology and characterization is reasonable considering the issues associated with Si material for instance, rapid capacity fading, pulverization, and mechanical degradation. However, I find it difficult that the authors have made a substantial contribution to improve the performance of the composite material.   

1.       Did you check the electrode swelling after 51 cycles for the um sized Silicon particles? Figure 8, the capacity drops drastically, do you have any strong evidence to verify this capacity loss.

SEM micrograph after 50 cycles 0.1C and 65 cycles at 1C should be included to check the morphology, loss of connections, and how the SEI layer looks like. The degradation was observed for all binder compositions. What is the use of this binder material and using three different compositions?  

2.       The author pointed out several reasons for the capacity fading.

“This strong degradation at the beginning of the cycling is assumed to be caused by the µm-silicon contained in the composite, due to the well-known degradation mechanisms including pulverization because of its particle size of 4.5 µm”.

I do not agree with their statement. The initial capacity loss during the initial cycles is mainly attributed to the formation of the SEI layer. Which upon further cycling reduces due to the formation of thick, unstable SEI layer due to volume expansion /contraction of Si (that consumes more Li ions), low porosity, electrode swelling, and finally the pulverization of the Si particles. These issues were addressed in a recent publication using advanced electron Microscopy tools. Check this recent article:  https://doi.org/10.1002/smll.201906812

3.       The authors also highlight the well-known reasons for Si capacity fading. However, they did not give proper citations to the explanation. I would recommend the authors to check the recent articles published on Si based composites to explain their results.

“Due to the strong volume expansion, the particles might have been fractured, resulting in a loss of contact between individual particle fragments and the rest of the active material, causing an irreversible capacity loss. In addition, the increased silicon surface area due to particle fragmentation may have led to an increased SEI formation, thus enhancing the irreversible capacity loss. Furthermore, the strong volume expansion could have led to the known degradation by delamination of the active material from the current collector or the conductive network, respectively.”

4.       For nm sized Si, again the reduced capacity was observed, and the authors linked this capacity with the increasing SEI layer formation. Although, it seems plausible that due to high surface/volume ratio of thesis particles the surface becomes more reactive which can lead to the formation of a thick SEI layer. Again, they did not provide any evidence of this. Relevant articles should be cited to support this claim.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report (Previous Reviewer 1)

In this resubmitted manuscript,  the author had answered most of my questions. However,  the key problems have not been solved. I will  reconsider the decision after the author solved the following two questions

(1) The author should provide the original charge-discharge curves under various current densities in which  X-axis is the Capacity and Y-axis is Voltage.  Fig 8a, 9a, 10a, S2, S3, S4 only show the cycling performance，and it is indeed not enough to prove the electrochemical performances.  The charge-discharge curves is the most basic date in battery related paper, if these data were absent, readers would assume the cycling capacities were falsifed.

(2) The original EIS curves should be given. Since the values of the resistance have been provided in Table S1, it would be easy for authors to show us the original curves.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report (Previous Reviewer 3)

Authors addressed all issues. I agree to be pulibshed in Battereis MDPI journal.

Author Response

Comment of the Reviewer:

Authors addressed all issues. I agree to be pulibshed in Battereis MDPI journal.

 

Answer to the Reviewer:

Thanks again for the time and effort to review our manuscript.

Round 2

Reviewer 1 Report (New Reviewer)

The authors have a made significant improvement by providing additional data and adding new references to confirm their findings. However, they did not provide the electrode swelling data which is important to understand the volume variation of the overall electrode.

 

This manuscript can be accepted for publication in Batteries.

Reviewer 2 Report (Previous Reviewer 1)

All my questions have been issued, and I think this paper can be accepted for publication now. 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

I suggest it could be rejected.

In this paper, the author demonstrated the optimal binder ratio of CMC to SBR depends on the particle size of the used silicon in Si-rGO composites. However, this work has little theoretical value or practical significance. Besides, some raw electrochemical test results (such as CV curves, charge curves, EIS curves) are missing in this manuscript. This manuscript should be transferred to other journals which focuses on engineering applications.

 

1. What is the objective of this manuscript？rGO/Si electrode has not been commercialized now due to the high cost of rGO, and the investigation of the manufacturing parameter of CMC/SBR binder ratios is little important.

2. For rGO, the C/O ratio is not constant even the preparation parameter is the same. The C/O ratio of rGO also affects the required amount of binders. So the rGO used in this manuscript is not actually the same.

3. The manuscript only shows the raw data of discharge curves, the CV curves、charge curves and EIS curves should be added.

4. The SEM images of electrodes after cycling should be given to show the differences.

 

5. As for the nanosized Si/rGO, the performance of samples with CMC/SBR ratio of 1:3 and 1:1 are nearly the same, while the sample with ratio of 3:1 shows the best performance. How about the performance sample with the ratio of 4:1 or 5:1，they may be even better.

Author Response

Attached

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript reported the influence of two aqueous binders (CMC/SBR  ratios) on the electrochemical performance of Si/rGO anodes for Li-ion storage.  Moreover, two different Si particles with nano and micrometres were explored in this work.  Especially, the electrical conductivities of Si/rGO anodes with different CMC/SBR binder ratios were studied. However, there are still many issues that need to be addressed before its possible publication in Batteries.

(1) The wt% of active Si in the composite is not high, only 15-17%. Even so, these two Si-rGO anodes still had bad constant cycling performances (Figures 8-10). Are water-soluble CMC/SBR suitable binders for Si anodes? Could the authors check their cycle data using traditional binders (PVDF)?

 (2) Columbic efficiency is a very important parameter. It is better to provide the columbic efficiency during the cycling of µm-Si-rGO and nm-Si-rGO electrodes, especially the initial columbic efficiency.

(3)What is the areal mass loading of active Si for µm-Si-rGO and nm-Si-rGO electrodes? What’s the meaning of “mAh g-1 AM” ? Does the rGO belong to the active material for Li-ion storage? The contribution of pure rGO should be added.

(4)  Is necessary to consider the areal capacities of µm-Si-rGO and nm-Si-rGO electrodes? Some latest publication on binders for Si anodes is suggested to be referred, such as “Design of high-energy-dissipation, deformable binder for high-areal-capacity silicon anode in lithium-ion batteries”.

(5) Apart from CMC and SBR, there are still some new aqueous binders, such as phosphorylate binder (“Reversible cross-linked phosphorylate binder for recyclable lithium-sulfur batteries”), which should be introduced in the Introduction part.

 

(6) The SEM images are difficult to demonstrate that the silicon particles are homogeneously incorporated into the rGO matrix. 

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The authors addressed the effects of designing of binder in the silicon-graphene composite electrode. With controlling the ratio of CMC and SBR, the micron and nano-size silicon-graphene composite shows higher performance than other cases. The experiments were performed systematically and the manuscript was written well. It is worth to be published after addressing some comments. 

 

1) In the measurement of the electrical conductivity of the composite electrode, the difference between micron and nano-sized silicon results from the dispersion of graphene and binder. Do authors address the size effects on dispersion?

 

2) In SEM images (Figures 5 and 6), it is hard to detect of binder in the porous electrode. Can authors explain more clearly the effects of binders?

 

3) The nano-sized Si shows slower capacity degradation than the micron-sized Si, but the rate capability of nano-sized Si at 1C rate shows almost zero capacity which indicates the lower performance. Why does the nano-sized Si show the lower performance than the micron-sized Si under high C-rate? Is there any XPS data to validate SEI formation?

 

4) The initial CE of nm-Si-rGO_3.75CMC_1.25SBR shows the highest values among the tested cases. Si has a issues on the initial CE, so the values of 52.88% is too lower as anode for LIBs. Furthermore, other related works reported almost 80~90% of ICE. (ACS Nano, 13, 8, 9607-9619)

 

5) Is there any reference cell test of CMC and SBR only case? For the nano-sized Si, the CMC binder plays an important role in cycle performance. The cycle performance data of CMC mono-binder is also needed.

 

6) To valid authors' arguments of tailoring dual binder, is there any theoretical works?

 

Author Response

Attached

Author Response File: Author Response.docx