Towards Thin Calcium Metal Anodes—An Essential Component for High-Energy-Density Calcium Batteries

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, thin Ca battery anode was prepared by electrodeposition using 1.0 M Ca(BH₄)₂ in THF as electrolyte. The parameters of substrate pretreatment, current density, fluid dynamics and area charge density are compared in detail. It is concluded that mechanical pretreatment and high current density can obtain more ideal Ca negative electrode. But there are still some areas for improvement.

Here are some suggestions:

1. The "and" after the reference [10] in line 49 on page 2 should be "And", which is a capitalization issue.

2. The article as a whole uses more separators, and the reading experience is not smooth, so adjust it appropriately.

3. The format of references [13][14] in the text is different from other serial numbers, so it should be adjusted to [13,14].

4. "Results and Discussion" should go to the beginning of page 5.

5. Figures 3, 4, and 8 should not take up a whole page and need to be resized to make the layout more reasonable.

6. The annotation of SEM pictures in Figure 5(a)(b) should be deleted.

7. " 2.00 mAh cm^-2" in line 245, page 8 should be "1.00 mAh cm^-2".

8. The "Table 1" appear twice in the text.

Author Response

We thank the reviewer for the detailed formatting and layout suggestions. We have revised the manuscript accordingly, as follows:

1. The "and" after reference [10] in line 49 on page 2 has been changed.
2. We have reduced the use of separators throughout the manuscript to create a smoother reading experience.
3. The format of references [13][14] has been adjusted to [13,14] for consistency with other reference citations.
4. The "Results and Discussion" section has been moved to the beginning of page 5.
5. Figures 3, 4, and 8 have been resized to avoid occupying entire pages, resulting in a more balanced layout.
6. The annotations on the SEM images in Figure 5(a) and (b) have been removed.
7. The value "2.00 mAh cm⁻²" in line 245 on page 8 has been corrected to "1.00 mAh cm⁻²."
8. Duplicate instances of "Table 1" have been eliminated.

We believe these revisions improve the manuscript’s clarity and overall presentation, and we thank the reviewer for these constructive suggestions.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors systematically investigated the electrodeposition behavior of Ca using a 1.0 M of Ca(BH₄)₂/THF electrolyte, and evaluated various parameters such as substrate pretreatment, current density, hydrodynamics, and charge density to optimize the conditions for producing compact and uniform Ca deposits. While the proposed concept is novel, the manuscript should be revised before further consideration for publication.

1. It is suggested that the authors cite some recent references, such as Energy Mater., 2024, 4: 400023, and Adv. Energy Mater. 2022, 12, 2103112, which are closely related to modification on metal anode materials.
2. The size of the figures in the main text is not harmonized. It is recommended that the size of the figures be harmonized.
3. Please provide a more detailed description of the result of the contact angle measurements and explain the relationship between the contact angle and surface energy.
4. The authors mentioned“Given the need for high substrate coverage, increasing the current density proves advantageous for manufacturing thin Ca anodes.” However, in the subsequent tests, the current density used was 0.5 mA cm^-2 instead of the higher 2.0 mA cm^-2, please explain the reasons.
5. Could the authors provide a more detailed explanation of why 1.0 M of Ca(BH₄)₂/THF was chosen as the electrolyte and what advantages it offers?

Author Response

1. We thank the reviewer for the detailed analysis and constructive suggestions. The two references suggested—Two-dimensional (2D) Materials for 3D Printed Micro-Supercapacitors and Micro-Batteries (Energy Mater., 2024, 4: 400023) and Li4Ti5O12 Coating on Copper Foil as Ion Redistributor Layer for Stable Lithium Metal Anode (Adv. Energy Mater., 2022, 12, 2103112)—indeed address innovative approaches in electrode and anode design. The first article discusses a manufacturing process for 3D printed 2D materials applied in micro-supercapacitors and micro-batteries. Although the underlying motivation—to overcome current limitations in energy storage—is similar to that of our work, the specific process and targeted application differ significantly from our approach. Similarly, the second reference focuses on the modification of the artificial SEI on lithium metal anodes. While this is an impressive and active research area, lithium metal is far more extensively studied compared to other battery metals. In contrast, our research is centered on the development of thin calcium metal anodes by electrodeposition—a relatively underexplored field. Our study systematically investigates how the interplay of electrodeposition parameters can be fine-tuned to achieve a compact Ca layer, thereby addressing critical challenges in battery performance and energy density. In summary, while the suggested references provide valuable insights into alternative strategies for anode modification, they pertain to different systems and methodologies that do not directly impact the core scope of our work on electrodeposited Ca anodes. We appreciate the reviewer’s suggestions and trust that the explanation clarifies our focus and the distinct contributions of our study.
2. The sizes of the figures have been harmonized throughout the manuscript to ensure consistency and improved clarity.
3. We have added a more detailed description of the contact angle measurement results in the "Results and Discussion" section starting at line 204 on page 5. These results indicate that mechanically pretreated substrates exhibit lower contact angles, which is indicative of enhanced wettability and higher effective surface energy. This increased surface energy promotes stronger interactions with the electrolyte and, in turn, leads to a higher nucleation density and more uniform Ca deposition.
4. While increasing the current density can improve substrate coverage during Ca electrodeposition, our tests revealed that a current density of 0.5 mA/cm² is sufficient to synthesize a 10 µm thick Ca anode with good homogenity. Although higher current densities (e.g., 2.0 mA/cm²) may enhance surface coverage, they also lead to undesirable side effects, such as increased electrolyte degradation due to accelerated decomposition mechanisms. Specifically, at higher current densities, the decomposition of the electrolyte and the formation of undesired secondary products can reduce the effective lifetime of the electrolyte, ultimately impacting the long-term stability of the anode and overall battery performance. Therefore, 0.5 mA/cm² was chosen as the optimal parameter, balancing electrolyte stability with deposition efficiency. The manuscript has been updated in the "Results and Discussion" section (starting at line 275 on page 8) to reflect these findings.
5. The 1.0 M Ca(BH₄)₂/THF electrolyte was selected after testing it in our laboratory and comparing it with other Ca-containing electrolytes (results not included in this manuscript). This electrolyte exhibited a high coulombic efficiency and generally lower polarization hysteresis during plating and stripping at room temperature. In contrast, other electrolyte systems showed significantly lower performance, primarily due to the formation of passivation layers during electrodeposition. This rationale has been integrated into the "Introduction" section of the manuscript (line 109, page 3) to provide clearer context for our choice of electrolyte.

We appreciate the reviewer’s constructive feedback and believe that these revisions have substantially improved the clarity and overall quality of our manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

Comments on the manuscript nanomaterials-3474898 entitled: “Towards Thin Calcium Metal Anodes-An Essential Component for High-Energy-Density Calcium Batteries”

This study provides a thorough examination of the electrodeposition behavior of calcium (Ca) using a 1.0 M Ca(BH₄)₂/THF electrolyte, offering valuable insights into the potential of electrodeposition for fabricating a thin, compact Ca layer suitable for balanced battery anodes. The investigation into key parameters such as substrate pretreatment, current density, hydro-dynamics, and charge density is commendable, as it demonstrates a systematic approach to optimizing the conditions for uniform Ca deposits.

The finding that mechanical pretreatment significantly enhances nucleation density and coverage compared to chemical pretreatment is particularly noteworthy. This is a crucial observation that underscores the importance of surface energy in promoting a uniform Ca layer. Furthermore, the influence of current density on substrate coverage and the transition in deposit morphology with hydrodynamics (from hemispherical structures to coalesced island growth at higher stirring speeds) adds valuable detail to understanding the deposition process.

Overall, this study contributes to the growing body of knowledge on electrodeposition for battery anodes and paves the way for future developments in deposition efficiency, energy loss reduction, and practical applications in energy storage technologies.

This paper has high qualities and suggested to be published.

Author Response

Thank you for your feedback. We have carefully reviewed the manuscript and made several improvements in response to your comments. In particular, we have refined the discussion on substrate pretreatment to emphasize how mechanical treatment increases surface energy and nucleation density, as supported by our contact angle and roughness profile measurements (starting at line 199 on page 5).

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript "Towards Thin Calcium Metal Anodes – An Essential Component for High-Energy-Density Calcium Batteries" explores the development of Ca battery anodes prepared by electrodeposition, by discussing the effects of varying different electrodeposition parameters. The work is interesting, but some points need revision.

 

1. On page 7, lines 187 to 198, the authors discuss the differences between the pre-treatment steps. The higher nucleation density observed in the mechanically treated substrate is explained to be due to an increase in surface energy. This is corroborated by the contact angle measurements that suggest mechanically pretreated surfaces have lower contact angles, indicating improved wettability and stronger interactions with deposited Ca. However, surface roughness is another important parameter that can influence nucleation. The manuscript could be improved if the authors could provide for example atomic force microscopy studies to study and quantify surface roughness.

 

2. Also related to my previous comment, the justification between both pre-treatment procedures is currently justified on the basis of wettability (based on contact angle measurements). Nonetheless, it should be noted that the chemical pre-treatment can also modify the surface composition. In this regard, how can the authors ensure that the starting point for nucleation is the same in terms of surface chemistry? X-ray photoelectron spectroscopy studies would be useful to provide more information regarding the surface chemical composition between substrates. Please note that if the surface chemistry differs significantly between treatments, it may contribute to variations in nucleation behavior, making it difficult to attribute differences solely to wettability.

 

3. In Figure 8d, the EDX analysis shows the presence of oxygen (5 at%) in the electrodeposited Ca layer. Since EDX gives us the elemental composition but doesn’t clearly identify the chemical state resolution, how can the authors distinguish whether this oxygen comes from residual electrolyte decomposition, from the natural oxidation of Ca during the electrodeposition, or from exposure after transfer? Would using complementary techniques like X-ray photoelectron spectroscopy (XPS) or electron energy loss spectroscopy (EELS) help in confirming the oxidation state of Ca (possibly distinguishing between Ca metal, CaO, and Ca(OH)₂, etc.)? It seems like that could really assist in figuring out if an oxide or hydroxide layer has formed.

Comments on the Quality of English Language

Please carefully revise the manuscript for missing punctuation and spelling errors.

Author Response

1. 1. We thank the reviewer for highlighting the role of surface roughness in nucleation. We agree that additional quantitative data—for example, from atomic force microscopy (AFM) studies—would further enhance our understanding. In response, we have integrated a discussion of substrate roughness into the "Results and Discussion" section, (starting at line 199, on page 5) including roughness profile measurements. Our results demonstrate that mechanical pretreatment substantially reduces the substrate’s roughness. Despite this smoother surface, the lower contact angles measured on mechanically pretreated substrates indicate an increase in effective surface defects, which promotes a higher nucleation density and facilitates the formation of a more uniform and compact Ca anode. We acknowledge that further quantitative analysis using AFM would be valuable, and we consider this an important avenue for future investigation.
   2. Regarding surface chemistry, we currently lack the capability to verify our results with XPS measurements. However, as we utilize high-grade Cu substrates, we speculate that the primary impact on nucleation behavior arises from the mechanically induced surface modifications—specifically, an increased density of grain boundaries—which in turn enhances the effective surface energy.
   3. The chemical state of the sample is expected to remain largely unchanged during transport, as a vacuum gas-tight transfer box was used. However, the EDX elemental distribution map for O (Figure 9h) shows that most of the oxygen is near the surface and not in the bulk layer. This might be an indication of a possible air exposure during the transport. Nonetheless, we recognize that there is ongoing discussion in the literature regarding the nature of interphases, particularly at the interface between the electrolyte and the Ca electrode. We intend to incorporate complementary techniques, such as XPS or EELS, in future studies to more accurately determine the oxidation state of Ca and to distinguish between Ca metal, CaO, and Ca(OH)₂. Such analyses will further elucidate whether the observed oxygen originates from residual electrolyte decomposition, inherent oxidation during electrodeposition, or post-transfer exposure.

We appreciate the reviewer’s constructive comments and believe that these clarifications and planned future experiments will further strengthen the manuscript.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Now this submission can be accepted by the journal.

Reviewer 2 Report

Comments and Suggestions for Authors

/

Reviewer 4 Report

Comments and Suggestions for Authors

I appreciate the efforts made by the authors for providing a revised version. The manuscript can now be accepted.