Influence of MnO₂-Birnessite Microstructure on the Electrochemical Performance of Aqueous Zinc Ion Batteries

Round 1

Reviewer 1 Report

The submitted paper “Influence of MnO2-Birnessite Microstructure on the Electro-2 chemical Performance of Aqueous Zinc Ion Batteries” by María Luisa López et al. represents good research, about the improvement of cathode materials for Zn-ion battery. Introduction, Experimental and Results, and discussion sections are well elaborated. The paper represents significant contributions in the field.

The reviewer opinion is that the paper could be accepted as is.

Author Response

All is ok

Reviewer 2 Report

In this manuscript, the authors reported Influence of MnO₂-Birnessite Microstructure on the Electro-chemical Performance of Aqueous Zinc Ion Batteries. The manuscript explored the effects of structural features and particle morphology of different birnessite-type manganese oxides on electrochemical performance. The paper structure is complete and has certain data support. Some issues can be addressed before possible publication as follows:

1. Some English expressions and types can be improved such as:

Page 1 Line 35, environments should be environment.

Page 1 Line 39, Zn²⁺ cations should be Zinc cations.

Page 1 Line 42, Page 8 Line 224 and Page 11 Line 299, Zn²⁺ ions should be Zn²⁺ or Zinc ions.

Page 9 Line 240, H⁺ ions should be H⁺ or hydrogen ions.

Page 3 Line 98, at air should be in the air.

Page 4 Line 143-144, ‘the’ in this sentence is redundant.

Page 10 Line 262, contrary should be on the contrary.

2. Some papers maybe are useful for background and explaination, such as Nano-Micro Lett, (2022) 14: 6, Electrochim. Acta 2020, 353, 136570, J Colloid Interf Sci, 599 (2021) 467.
3. In Experimental, authors should provide supplier information of all the used chemicals and materials.
4. The use of Beyreuther et al. 's Eq1 suggests citing the corresponding references.
5. The SEM image in Figure 4a is vague, so the microscopic morphology cannot be clearly observed.
6. Fonts with spacings of ~7 Å are shown incompletely in Figure 4e and 4f.
7. The abscissa of cycle number in Figure 5b should be uniform and without A/g unit.
8. The electrochemical test image in the article is not clear enough.

Author Response

Author Response File: Author Response.pdf

Reviewer 3 Report

Title:  Influence of MnO2-Birnessite Microstructure on the Electro-chemical Performance of Aqueous Zinc Ion Batteries

Manuscript ID: applsci-1544421

The current manuscript discusses the preparation of birnessite-type K_xMnO₂ materials to be used as a cathode for Zn-ion batteries. They have synthesized three cathode samples with two different methods [a) thermal treatment and b) hydrothermal method]. The characterization of the prepared samples are well presented. The authors also studied the electrochemical behavior of the half-cells and the reasons for the variations in the electrochemical behavior is well explained. The subject matter is appropriate for the journal readership. It can be published after resolving the following issues.

The authors should pay attention to the following comments.

1. PDF Page 2, line 67, I believe the obtained powders are dried ‘on’ a stove instead of ‘in’ a stove, please correct the grammar.
2. Is there any way to control the concentration of K in between the octahedral layers? As the authors mentioned that is the key in the thermal stability of the structure. If so, then please mention it in the manuscript.
3. In Fig. 3, for both (a) and (b), the XPS spectral lines are crossing with the other figure data. It doesn’t seem professional. As the BE range is same for the three compounds, either you can remove the X-axis for KMn4 and KMn8 or adjust the spectral lines in such a way it won’t interact with the other figure data.
4. In PDF Page 5, line 165 is in between the figure and the figure caption. Move the line appropriately.
5. Figure 4 needs clear adjustments. For example, the SEM image of KMn4 is not clear. Fig. 4 (e) overlaps with the scale of figure c. Also, in Fig. 4 (d), the spacing of 7 Å is not clearly visible. The same goes for Fig. 4 (f). Please adjust them accordingly. The order of the figures is KMn8, KMn4 and HKMn but not for Figure 4. Please follow the same order as the rest of the figures.
6. In Fig. 5 (a) and (b), follow the same notation for the units, either mA.h/g or mA.h.g^-1. Also, in Fig. 5 (a), put Y-axis as E_cell (V vs Zn²⁺/Zn).
7. In Fig. 5 (b), for KMn4 there should not be any units for cycle number.
8. In Fig. 6, put Y-axis as I (mA). X-axis as E_cell (V vs Zn²⁺/Zn).
9. In PDF page 8 line 221, it can be “In agreement with the above statement...”.
10. In Fig. 7, put Y-axis as E_cell (V vs Zn²⁺/Zn).
11. In Fig. 8, correct the subscript for the formula ZnMn₂O₄.
12. I wonder why the authors did not include the ex-situ XRD for KMn8. Is there any reason for it?

Author Response

The authors would like to thank you and the reviewers for your invaluable comments and suggestions. All of them have been taken into account in the new version of our manuscript and we think that they have improved it. All the changes have been highlighted in the text. We hope our manuscript is now suitable for publication.

1. PDF Page 2, line 67, I believe the obtained powders are dried ‘on’ a stove instead of ‘in’ a stove, please correct the grammar.

The grammatical mistake was corrected

2. Is there any way to control the concentration of K in between the octahedral layers? As the authors mentioned that is the key in the thermal stability of the structure. If so, then please mention it in the manuscript.

In general, the amount of K cations and H₂O located between octahedral layers seem to be related with the stabilization of the a-, and d- MnO₂ phases. At the same time, both K and water concentration as well as the structure adopted by MnO₂ seem to be related to the synthesis methods. Intensive research was made in this sense by research community. In this case, our results based on EDX analyses and thermogravimetry data seem to support the following conclusions:

The EDX shown a greater heterogeneity in the concentration of K in the HKMn sample, therefore some of the particles crystalize with small amount of K

The thermogravimetric data indicated that only the HKMn sample is fully changed to structure type a-MnO2.

Therefore, in principle, hydrothermal method led to phases where the K amount is smaller than the obtained by thermal treatment. This result is reproducible. In this sense, we have included this sentence in the manuscript.

3. In Fig. 3, for both (a) and (b), the XPS spectral lines are crossing with the other figure data. It doesn’t seem professional. As the BE range is same for the three compounds, either you can remove the X-axis for KMn4 and KMn8 or adjust the spectral lines in such a way it won’t interact with the other figure data.

The figure 3 has been changed in agreement with the given suggestions

4. In PDF Page 5, line 165 is in between the figure and the figure caption. Move the line appropriately.

This line has been correctly located.

5. Figure 4 needs clear adjustments. For example, the SEM image of KMn4 is not clear. Fig. 4 (e) overlaps with the scale of figure c. Also, in Fig. 4 (d), the spacing of 7 Å is not clearly visible. The same goes for Fig. 4 (f). Please adjust them accordingly. The order of the figures is KMn8, KMn4 and HKMn but not for Figure 6.  Please follow the same order as the rest of the figures.

A new figure 4a has been included with high quality. Moreover, the data on the figures 4e and 4f are now clearly seen.

7. In Fig. 5 (a) and (b), follow the same notation for the units, either mA.h/g or mA.h.g^-1. Also, in Fig. 5 (a), put Y-axis as E_cell (V vs Zn²⁺/Zn).

8. In Fig. 5 (b), for KMn4 there should not be any units for cycle number.

9. In Fig. 6, put Y-axis as I (mA). X-axis as E_cell(V vs Zn²⁺/Zn).

10. In PDF page 8 line 221, it can be “In agreement with the above statement...”.

11. In Fig. 7, put Y-axis as E_cell(V vs Zn²⁺/Zn).

12. In Fig. 8, correct the subscript for the formula ZnMn₂O₄.

All the figures have been checked and corrected

13. I wonder why the authors did not include the ex-situ XRD for KMn8. Is there any reason for it?

In principle, ex-situ XRD for KMn8 was not included due to the fact that XRD profiles of discharged and charged electrode shown the same features that the pattern of KMn4 electrode. Moreover, as it is commented in the text, these charge and discharge profiles were also observed by other authors (reference 25 in the manuscript). 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I suggest the publication of this paper after the modifications.