Facile synthesis of reduced graphene oxide by modified Hummer's method as anode material for Li-, Na- and K-ion secondary batteries

Reduced graphene oxide (rGO) sheets were synthesized by a modified Hummer's method without additional reducing procedures, such as chemical and thermal treatment, by appropriate drying of graphite oxide under ambient atmosphere. The use of a moderate drying temperature (250°C) led to mesoporous characteristics with enhanced electrochemical activity, as confirmed by electron microscopy and N2 adsorption studies. The dimensions of the sheets ranged from nanometres to micrometres and these sheets were entangled with each other. These morphological features of rGO tend to facilitate the movement of guest ions larger than Li+. Impressive electrochemical properties were achieved with the rGO electrodes using various charge-transfer ions, such as Li+, Na+ and K+, along with high porosity. Notably, the feasibility of this system as the carbonaceous anode material for sodium battery systems is demonstrated. Furthermore, the results also suggest that the high-rate capability of the present rGO electrode can pave the way for improving the full cell characteristics, especially for preventing the potential drop in sodium-ion batteries and potassium-ion batteries, which are expected to replace the lithium-ion battery system

1. In the manuscript, there are few literatures in latest three year (from 2017). I used "rGO" and "battery" to search in Web of Science, I found lots of relate literatures. Such as: APPLIED SURFACE SCIENCE, 2019, 465: 470-477 It would be better if the author can add more latest relate literatures. 2. It would be better, if the authors can put four figures in Fig. 5 together to see the difference of different samples. 3. It would be better, if the authors can compare their BET result and anode capacity with other literatures. 4. For the electrochemical performance, the authors should provide the CV curve and the Nyquist plots. 5. It seems that, the performance in this manuscript is not good enough. a) The author should compare the performance of the anode using the Hummer's method. b) Why only temperature was chosen to do sensitivity? c) Please give some comment on how to further improve the performance?

15-Feb-2019
Dear Professor Kim: Title: Facile synthesis of reduced Graphene oxide by modified Hummer's Method as anode material for Li, Na, and K-ion secondary batteries Manuscript ID: RSOS-181978 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The authors disclosed the preparation of rGO by modified Hummer's method and discussed the influences of the drying temperature to the properties of rGO product. The morphology, BET and C/O ratio of the material have been investigated. Though these points have been discussed and written comprehensively in the manuscript, there are still some issues requiring to be considered. 1) rGO in this paper was synthesized by a modified Hummer's method. Which is the difference between the process used in this paper and previous reports. Is there any difference of the rGO properties compared to that of published before? 2) Table 1 listed the element content of as-synthesized products. Why the oxygen content increased from 22.81% of rGO-250 to 24.46% of rGO-300? Pleased discussed in detail.
3) As shown in the SEM images from Figure 1, the rGO aggregated very much. This would effect the performance of the electrochemical properties. Is there any method to separate the layers of the rGO and obtain the fine dispersion? 4) After 50 cycles, the reversible specific capacity of the half-cell remained at 53%. This is quite poor! Why did the cycle stability deteriorate so quickly? 5) The author should state the novelty of this work more clearly, such as process of preparation, properties of product, or electrochemical performance.

Reviewer: 2
Comments to the Author(s) This manuscript shows a facile synthesis of reduced graphene oxide by modified Hummer's method as anode materials. It seems interesting and there is lot of work. But this manuscript still needs to be revised before it can be accepted.

Reviewer: 1
Comments to the Author(s) The authors disclosed the preparation of rGO by modified Hummer's method and discussed the influences of the drying temperature to the properties of rGO product. The morphology, BET and C/O ratio of the material have been investigated. Though these points have been discussed and written comprehensively in the manuscript, there are still some issues requiring to be considered.
Comment 1: rGO in this paper was synthesized by a modified Hummer's method. Which is the difference between the process used in this paper and previous reports. Is there any difference of the rGO properties compared to that of published before?
The authors wish to thank the reviewer for the valuable time, favorable and useful comments/suggestions toward the publication of this manuscript. The Hummer's method is usually followed to prepare reduced graphite oxide. During the final washing/filtering with water, the solvent molecules tend to occupy the interlayer galleries of the hydrophilic graphite oxide. Upon subsequent annealing of the as-prepared product, the trapped molecules facilitate the formation of the porous morphology in the final graphite oxide powder. Since the porous features of the graphite oxide powder influences their physico-chemical properties, researchers have used various solvents like HBr, NH3 with different vapor pressures to control the porous morphology and thereby enhance their electrochemical properties. A very recent report indicated that the use of HCl with high vapor pressure (than water) as the final filtering solvent aid in enhancing their porous morphology and simultaneously facilitating partial reduction during the subsequent drying process at only 120 C. This study demonstrated the enhanced electrochemical properties with respect to rechargeable lithium batteries.
Inspired by the earlier work on HCl, the present work performed a systematic study on using concentrated HCl as the filtering solvent to prepare the reduced graphene oxide (rGO) at various drying temperatures of 120, 200, 250, and 300 C. The variation of the drying temperatures on the porous morphology and electrochemical properties in the rGO material was studied in detail for rechargeable lithium, sodium and potassium battery applications. As expected, our work confirmed that the drying temperatures also significantly influenced the surface and electrochemical properties. All these statements have been included in the revised manuscript. The authors are sincerely thankful to the reviewer for raising up this valuable point as the motivation and the uniqueness of the present work could be stated more clearly.
Comment 2: Table 1 listed the element content of as-synthesized products. Why the oxygen content increased from 22.81% of rGO-250 to 24.46% of rGO-300? Pleased discussed in detail.
The authors thank the reviewer for the comment. The authors wish to state that, in addition to drying temperature, annealing environment also can influence the thermal reduction of GO. Usually, controlled reaction environments like inert air atmosphere or vacuum conditions are followed for the thermal reduction of GO. In the present case, as the surface functional groups containing oxygen groups are removed at high temperatures in open-air conditions. In general, the absorbed water are evaporated and hydroxyl/carboxyl group are decomposed under 250 C. Given that the openair conditions are oxidizing atmospheres, there can be increase in oxygen content via some chemical reactions like the chemisorption of oxygen by the active surface carbon. Also, it is possible that the impurities present in open air could interfere with the sample and cause undesired reactions. Hence, more studies are required and currently underway to identify the exact reasons for the slight increase in the oxygen content at 300 C. Nevertheless, the authors are sincerely thankful to the reviewer for his careful analysis of our data and providing crucial comments for the authors to dwell upon and increase the quality of the manuscript.

Statements included in the revised manuscript:
"Interestingly, the oxygen content slightly higher for the rGO prepared at 300 C than that prepared at 250 C. This can be related to the open-air environments used for the drying process in the present study as the annealing environment can influence the thermal reduction of GO. Usually, controlled reaction environments like inert air atmosphere or vacuum conditions are followed for the thermal reduction of GO. In the present case, the surface functional groups containing oxygen (absorbed water and hydroxyl/carboxyl group) are decomposed at temperatures under 250 C. 22 Given that the open-air conditions are oxidizing atmospheres, there can be a slight increase in oxygen content via some chemical reactions like the chemisorption of oxygen by the active surface carbon. Also, it is possible that the impurities present in open air could interfere with the sample and cause undesired reactions. Hence, more studies are required to identify the exact reasons for the slight increase in the oxygen content at 300 C." Comment 3: As shown in the SEM images from Figure 1, the rGO aggregated very much. This would effect the performance of the electrochemical properties. Is there any method to separate the layers of the rGO and obtain the fine dispersion?
The authors wish to thank the reviewer for the comment. In this work, we were tried to use 'ultra-sonication' before drying at moderate temperatures. However, it appears that this method was less effective to separate the layers of the rGO and obtain the fine dispersion. This can be one of the reasons for the slight cycling instability observed in the prepared samples. Therefore, an addition step towards exfoliating the sheets need to be considered as an immediate direction of research with these materials. Strategies of performing solvents/surfactants-assisted ultra-sonication or mechanical or thermal methods to improve the exfoliation in the rGO materials. These statements have been included in the revised manuscript. However, the authors are sincerely thankful to the reviewer again for the very useful comment since it has helped to not only improve the quality of the manuscript but also help in finding inroads to improve the properties of the prepared materials.

Statements included in the revised manuscript:
"Overall, the SEM images reveal the slight aggregation of the layers in the prepared samples, especially for the lower temperature samples." "Although the aspect of gradual specific capacity decrease, in general, under repeated cycling is most likely related to the aggregated layers in the rGO samples, there is room for further improvement in the electrochemical properties using simple strategies like solvent/surfactant-assisted ultrasonication and/or mechanical/thermal methods." Comment 4: After 50 cycles, the reversible specific capacity of the half-cell remained at 53%. This is quite poor! Why did the cycle stability deteriorate so quickly?
The authors wish to thank the reviewer for the comment. The authors agree to the reviewer that the electrochemical property is still subject to improvement. The initial decrease in the specific capacity during cycling is mostly related to the stabilization of the SEI film formed. The SEI layer was formed as a result of the reaction of transport ions (Li + , Na + , K + ) with residual oxygen containing functional groups on the electrode surface. The probable case for the decrease in capacity on short-term cycling is probably related to the irreversible ion-insertion. This can be related to the possibility that the reduced graphene oxide layers require to be more separated to facilitate stable insertion. This is an area that needs to be understood and more studies are underway to improve their electrochemical properties as this work is only a part of the major work aimed to find the practical potentiality of these materials in the field of rechargeable batteries. The present work is only an attempt to show the effect of the drying temperatures upon the electrochemical properties of the prepared rGO samples as anodes in rechargeable battery applications. Finally, the discussed points have been included in the revised manuscript. Nevertheless, the authors wish to sincerely thank the reviewer for raising up this issue and encouraging us to dwell deep on the subject of further improving the electrochemical property of the prepared samples.

Statements included in the revised manuscript:
"The general decrease in the specific capacities during the initial few cycles of the present samples can arise from the stabilization of the solid electrolyte interphase (SEI) layer on the electrode surface. 8 However, the slight instability in the specific capacities during repeated cycling can be related to the slightly aggregated layers in the prepared electrode samples and further investigations towards layer exfoliation in the rGO samples via chemical or mechanical or thermal methods are required." Comment 5: The author should state the novelty of this work more clearly, such as process of preparation, properties of product, or electrochemical performance.
The authors thank the reviewer for the valuable comment. In this work, we have synthesized reduced graphene oxide (rGO) by a modified Hummer's method using HCl with higher vapor pressure (than water) as the filtration solvent followed by subsequent drying in air at various temperatures. The influence of the drying temperatures upon the porous morphology and the electrochemical properties of the prepared rGO samples as anode materials have been investigated in detail for lithium, sodium and potassium battery applications. This study clearly showed that the drying temperature of 250 C promoted the formation of optimum pore-sizes in the mesopore range and the corresponding rGO material has potential as anode for emerging rechargeable battery applications, especially, in NIBs and KIBs. Moreover, the presentation of a simple drying process with no further chemical/thermal reduction processes to enhance the porous morphology and improve the electrochemical properties in graphene oxide electrodes for useful energy storage applications is promising. These points have been included in the revised manuscript The authors are thankful again for the comment as it enabled the description of the work in detail with the purpose and the achievement and thereby improve the readability of the manuscript.

Statements included in the revised manuscript:
"Moreover, recently, the solvent molecules from the water washing/filtration step in the Hummer's method was identified to remain in the interlayers of the hydrophilic as-prepared product. Upon subsequent annealing, the trapped molecules facilitate the formation of the porous morphology in the final product. Since porosity features are known to influence electrochemical properties, various solvents (other than water) like HBr, NH3 and recently, HCl with different vapour pressures were utilized to tune the porous formation of GO and thereby enhance their electrochemical performances. Specifically, the use of HCl with high vapor pressure (than water) enabled to obtain porous morphology and simultaneously promote partial reduction in the GO material during drying at 120 C for anode application in rechargeable lithium batteries.
Inspired by these works, the present work performed a systematic study on using concentrated HCl as the filtering solvent in a modified Hummer's method 13 to prepare reduced graphene oxide (rGO) at various drying temperatures of 120, 200, 250, and 300 C. No further chemical/thermal reduction procedure is followed. Electron microscopy and surface analyses confirmed the accordion-morphology of rGO and their mesoporous characteristics. The variation of the drying temperatures on the porous morphology and electrochemical properties in the rGO material was studied in detail. In other words, the feasibility of using the prepared rGO host for the insertion/de-insertion of various carrier ions (such as Li + , Na + , and K + ) for energy storage applications is demonstrated. As expected, our work confirmed that the drying temperatures also significantly influenced the surface and electrochemical properties. The present study thus showcases the possibility of using rGO as an electrode material for alternative energy storage systems."

Reviewer: 2
Comments to the Author(s) This manuscript shows a facile synthesis of reduced graphene oxide by modified Hummer's method as anode materials. It seems interesting and there is lot of work. But this manuscript still needs to be revised before it can be accepted. Firstly, the authors wish to thank the reviewer for the favorable comments and suggestions toward the publications of the manuscript. In accordance to the reviewer's suggestion, we have included a few references in the Introduction section. The suggested/above said reference has also been included in the revised manuscript. The authors are thankful to the reviewer for suggestion of adequate literature and providing a reference to be cited too.
Comment 2: It would be better, if the authors can put four figures in Fig. 5 together to see the difference of different samples.
The authors are thankful to the reviewer for the comment. In accordance, the figure was merged in the revised manuscript.
Comment 3: It would be better, if the authors can compare their BET result and anode capacity with other literatures.
The authors thank the reviewer for the comment. In agreement to the reviewer's suggestion, the BET results of a few reduced graphene oxide prepared for various applications have been mentioned in the revised manuscript. In addition, one specific result on the rGO used as anode for sodium battery has been included in the revised manuscript. These points have been included in the manuscript. Overall, the authors thank the reviewer for the very useful suggestion and giving us a clear discourse on preparing for the high quality readership of the esteemed journal.

Statements included in the revised manuscript:
"The maximum surface area values obtained here are competitive to those reported for the case of reduced graphene oxide prepared for various applications. 32-35" "For example, Wang et al., 35 developed porous reduced graphene oxide with high surface area (~330 m 2 g -1 ) and the electrochemical measurement revealed that reversible sodium storage capacities of 174 mAh g -1 at 0.2 C. Although the surface area is higher than that measured in the present case, the reversible specific capacity attained here is quite competitive with the value reported." Comment 4. For the electrochemical performance, the authors should provide the CV curve and the Nyquist plots.