Selection of Optimum Binder for Silicon Powder Anode in Lithium-Ion Batteries Based on the Impact of Its Molecular Structure on Charge–Discharge Behavior
Round 1
Reviewer 1 Report
This paper reports details of electrochemical and mechanical behaviors of Si anodes with various binders for lithium ion batteries. This paper builds on the extensive knowledge of Si based anodes. It is well known that the sole Si anode is not suitable as an anode for lithium ion batteries. Moreover, micrometer-sized silicon particles have been shown to fracture on lithiation. In general, using a binder with strong adhesive to Si would be able to cause a deformation of Cu current collector, leading to a safety problem. However, the paper suffers from a lack of information regarding the above mentioned issue.
Author Response
Reply to reviewers’ comment.
We kindly thank you for very valuable advices to our manuscript. This letter is a reply to reviewers’ comment. We attached the revised file. Please receive it.
We revised contents (sentences, figures, tables) in our manuscript greatly to keep from confusion of the manuscript. Some figures are changed about captioning position and caption number for re-building the flow of our contents. The status of figures and tables before and after revision are as follows,
Figure 1 before revision is moved to “Figure 1 in 1. Introduction”.
Figure 2 is added the thickness of binder with Si and KB.
The abbreviation on horizontal axis of Figure 3 is revised.
Figure 4 before revision is same as Figure 4 after revision.
The abbreviation on horizontal axis of Figure 5 is revised.
The abbreviation on horizontal axis of Figure 6 is revised.
Figure 7 before revision is changed to “Figure 8”.
Figure 8 before revision is moved to “Figure 9 in 3. Discussion and conclusion”, and added a new data.
Figure 9 before revision is moved to “Figure 10 in 3. Discussion and conclusion”.
Figure 10 before revision is moved to “Figure 11 in 3. Discussion and conclusion”.
Figure 11 before revision is deleted.
Figure 12 before revision is deleted.
New Figure 7 after revision is inserted as new caption in 2. Experiments of the manuscript. It explains electrochemical function of binder for charge-discharge cyclic characteristics.
Table 1 before revision is deleted, and the information of each binder is inserted to the main text.
Table 2 before revision is changed to “Table 1”, and the contents are revised.
The Abstract and main contents for each figure and table are revised and moved in the manuscript, and they are highlighted in yellow. And the number and caption of sub-titles are arranged and highlighted too. (Please see and confirm the manuscript.)
New captions of sub-titles in 2. Experiments are
2.1 Shear peeling
2.2 Compressive strength
2.3 Evaluation of electrochemical properties of resins as active materials.
Please do not confuse the figure number and captioning location before and after revision.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript attempts to study the impact of a series of binders for anode of LIBs, in views of the mechanical and electrochemical behaviors. It is interesting and significant to find optimal binders and understand related mechanisms. However, this manuscript needs a great revision to reach creditable conclusions. Some suggestions as follows are made for reference.
The numbers of sub-titles should be consecutive, for examples, 2. Experiments, 3. Discussion and conclusions, and 4. Summary. Some description in section 2.1 is better to place it into 1. Introduction. For shear peeling test, detailed information such as thickness of testing sample, the instrument and operations, as well the discussion on experimental results are necessary. What’s the relation between the molecular structure of organic binder and their mechanical properties? It needs further explanation and discussion. Authors have chosen seven kinds of binders to combine with Si powders and tested the electrochemical cycling performances, however from the results (Figure 7 and 8), all have not shown excellent roles. Furthermore, the cycling performances are though determined by many factors, to set up a relation between peeling strength and cycled capacity is so farfetched. Actually, the combination state between binder and active component would be decided by many reasons, even though the molecular structure of binder is crucial one. What’s the purpose using Si-CuO composite or Si nanowhiskers as new active materials? It seems no any help to understand the binders. The description in Table 2 is lack of essential evidences to support it.
Author Response
Reply to reviewers’ comment.
We kindly thank you for very valuable advices to our manuscript. This letter is a reply to reviewers’ comment. We attached the revised file. Please receive it.
We revised contents (sentences, figures, tables) in our manuscript greatly to keep from confusion of the manuscript. Some figures are changed about captioning position and caption number for re-building the flow of our contents. The status of figures and tables before and after revision are as follows,
Figure 1 before revision is moved to “Figure 1 in 1. Introduction”.
Figure 2 is added the thickness of binder with Si and KB.
The abbreviation on horizontal axis of Figure 3 is revised.
Figure 4 before revision is same as Figure 4 after revision.
The abbreviation on horizontal axis of Figure 5 is revised.
The abbreviation on horizontal axis of Figure 6 is revised.
Figure 7 before revision is changed to “Figure 8”.
Figure 8 before revision is moved to “Figure 9 in 3. Discussion and conclusion”, and added a new data.
Figure 9 before revision is moved to “Figure 10 in 3. Discussion and conclusion”.
Figure 10 before revision is moved to “Figure 11 in 3. Discussion and conclusion”.
Figure 11 before revision is deleted.
Figure 12 before revision is deleted.
New Figure 7 after revision is inserted as new caption in 2. Experiments of the manuscript. It explains electrochemical function of binder for charge-discharge cyclic characteristics.
Table 1 before revision is deleted, and the information of each binder is inserted to the main text.
Table 2 before revision is changed to “Table 1”, and the contents are revised.
The Abstract and main contents for each figure and table are revised and moved in the manuscript, and they are highlighted in yellow. And the number and caption of sub-titles are arranged and highlighted too. (Please see and confirm the manuscript.)
New captions of sub-titles in 2. Experiments are
2.1 Shear peeling
2.2 Compressive strength
2.3 Evaluation of electrochemical properties of resins as active materials.
Please do not confuse the figure number and captioning location before and after revision.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The manuscript is a little improved with the addition of more resut and revision. However the authors shoud address the following issues before the manuscript can be accepted.
The sections on evaluation of electrochemical properties are not understandable to this reviewer. Cell test cnditions (full cell test? or half test?, electrolyte) and electrode conditopns|(preparation method, electrode composition, loading level, electrode density) should be described in detail. If Si particle of 5 um is used , it is fractured during cycling in general. If any this happens, its effects should be described.
Author Response
The manuscript is a little improved with the addition of more resut and revision. However the authors shoud address the following issues before the manuscript can be accepted.
The sections on evaluation of electrochemical properties are not understandable to this reviewer. Cell test cnditions (full cell test? or half test?, electrolyte) and electrode conditopns|(preparation method, electrode composition, loading level, electrode density) should be described in detail. If Si particle of 5 um is used, it is fractured during cycling in general. If any this happens, its effects should be described.
We deeply appreciate to give us important indications. The reply to reviewer’s comments was added on the yellow highlighted lines in the chapter of “2.3 Evaluation of electrochemical properties of resins as active materials“ with new figure (Fig. 9) as follows;
“We examined the dependence of charge-discharge properties on the binder resins. We evaluated the charge-discharge properties and cycle properties of retention shown in Fig. 7. For the anode, the mixing weight ratio of binder: conductive additive (copper powder; Kojundo Chemical Laboratory Co., Ltd., Japan) was 1: 8. Electrochemical testing of the composite anode electrodes was conducted using two-electrode 2032 type test coin cells (Housen, Japan) caulking metal cups with a gasket to maintain the electrode assemblies, a polypropylene/polyethylene stacked film separator and a 1 M lithiumhexafluorophosphate (LiPF6) electrolyte within solvents of DFEC/FEC/EC/DMC=1/2/2/5 v/v%. The electrochemical performance of the two-electrode coin cells with LiCoO2 cathode were evaluated by a potentiostat machine (Hokuto-denko Co. Ltd, Japan). The initial electrochemical performance of the composite anode in Fig. 7(a) was evaluated in a coin cell using a constant current charge–discharge test in the voltage range of 0.3–2.0V with a current density of 0.075 mA/cm2 at room temperature. The state of the charge was set to 100 %. The charge-discharge property of Fig. 7(a) indicates measured result using PI in an anode. We could find there was a tendency of the properties changing due to binder resin in Fig. 7(b).
(※ Figure 7 is same as the last version.)
Figure 7. Electrochemical property of binder with Cu powder as conductive additive for cyclic characteristics.
Charge-discharge characteristics of Poly-imide (PI). Specific capacity indicates on a capacity per weight of only PI included in an anode electrode of a 2032 type coin cell. Retention of charge-discharge properties using binders as active materials in a 2032 type coin cell.
Each binder has a function of active material for lithium ion battery, and the irreversible capacity was large. Especially, binders, which had no benzene ring in molecular skeleton of binder, were low retention shown in Fig. 7(b). It presumed that benzene ring molecular structure of a binder had a role of occlusion and release of lithium ion in graphite-like structure as anode active material.
Figure 8 was shown to evaluate the dependence of cycle properties on the composition of binder resins and the amount of conductive additives. A 2032 type coin cell using LiCoO2 as a cathode electrode and a 1 M lithiumhexafluorophosphate (LiPF6) electrolyte was used to evaluate the cyclic properties of charge-discharge. For the anode, the mixing ratio of Si power: binder: conductive additive (KB) is 10: 1.5: 1. The electrochemical performance of the composite anode in Fig. 8 was evaluated in a coin cell using a constant current charge–discharge test in the voltage range of 1.6–4.1V with a current density of 1.5 mA/cm2 at room temperature. The state of the charge was set to 60 %. We could find a tendency of the properties changing due to binder resin in an anode electrode.
The cross-sectional scanning electron microscopy (SEM) views with overviews of anode electrode as insets in each SEM image of Fig. 9 showed the status of anode electrodes after 100th cyclic charge-discharge measurement. Fig. 9(a) using PVDF as a binder had unclear boundary between Si powders and PVDF by the crystal collapse owing to expansion and contraction of Si in the composite. It seems that the shape of Si powders in Poly-imide (PI) of Fig. 9(b) was maintained, however some small crack on the surface of each Si powder were occurred. It is believed that the binder layer covering each Si particle plays a buffering role by preventing cracking following drastic changes in volume with numerous repetitions of the charge–discharge cycle. We could find the polymer having large molecular structures and strong adhesion was effective to prevent the crystal collapse of Si by occlusion and exudation of Li-ion.
(※ Figure 8 and caption are same as the last version.)
(Please see Figure 9 on the attached file.)
Figure 9. Overviews (inset in each SEM views) and cross-sectional SEM views of anode electrodes with the composites of Si powder, KB and binder ((a) PVDF, (b) PI) after 100th cycle. Dark area in each SEM image indicated Si active materials.”
The caption number of Figures 10 and 11 in the last manuscript are changed to Figure “11” and “12”.
Reviewer 2 Report
This manuscript has been well revised to be acceptable.
Author Response
We kindly thank you for very valuable advices to our manuscript.
