Effect of the Annealing Temperature of Lithiophilic Ag–Cu Co-Deposition on the Cycling Performance of Li-Metal Anodes

Round 1

Reviewer 1 Report

This study entitled ”Effect of the Annealing Temperature of Lithiophilic Ag–Cu Co-2 Deposition on the Cycling Performance of Li-Metal Anode” could be interesting. However, this form is not suitable for publication in Inorganics Journal. There are some unclear aspects regarding the method of preparing, analyzing, and also for characterization. The results and conclusions are not enough convincing concerning the studied subject.

Here are some of them:

-        The prepared method is too briefly described. In Fig. 1 seems to be magnetron sputtering. In this case, some experimental data have to be presented. Also the system of annealing.

-        The elemental distribution on the Ag–Cu CC surface from EDS analysis is not presented, even if it is declared (row 89)

-        In rows 91-93 in the phrase: ”The resistivity of 91 the film co-deposited on the glass substrate was measured using a four-point-probe surface resistivity meter (CMT-SR1000N, AIT, Gyeonggi-do, Korea) to confirm the annealing  effect. ”, obviously is not related to this subject (glass substrate...)

-        In row 131 after Figure 2 there is a reference to Figure 5, without relevence with the text.

-        Table 1 is not presented in good condition, the unit measures are missing.

-        The sentence from Rows 142-143 refers to other caracterization.

-        Rows 154-155: ”A comparison between the Ag–Cu CC obtained at 500  and 400 °C reveals the appearance of similar large Ag particles and approximately surrounding rough surface.”  This phrase is not a scientific one.

-        Overall, the whole content has to be improved. It is difficult in this form to understand the real innovative study, which in principle, exists.

The English should be improved.

Author Response

Dear Reviewer:

We are pleased to re-submit a revised version of our manuscript titled “Effect of the Annealing Temperature of Lithiophilic Ag–Cu Co-Deposition on the Cycling Performance of Li-Metal Anode” for Materials publication. We appreciate the thorough review and constructive criticisms of the reviewers. We have addressed each of their concerns below and have revised sections of the paper for clarity. We hope the revision has improved the paper to a level of the reviewers’ satisfaction.

This study entitled ”Effect of the Annealing Temperature of Lithiophilic Ag–Cu Co-2 Deposition on the Cycling Performance of Li-Metal Anode” could be interesting. However, this form is not suitable for publication in Inorganics Journal. There are some unclear aspects regarding the method of preparing, analyzing, and also for characterization. The results and conclusions are not enough convincing concerning the studied subject.

Here are some of them:

1. The prepared method is too briefly described. In Fig. 1 seems to be magnetron sputtering. In this case, some experimental data have to be presented. Also the system of annealing.

We have considered the reviewer's comments and added experimental procedures accordingly.

The revised content can be found on page 2, line 74 to line 89.

 “In this study, co-sputtering was performed using a magnetron DC-sputter system (DC-Sputter, BLS, Gyeonggi-do, Korea). To achieve Ag area ratios of 62.5%, 75%, and 100%, Cu foil pieces were precisely cut to match the desired area ratios and placed on the Ag circular target (99.99%, VTM, Incheon, Korea), followed by co-deposition. To remove surface oxide layers and impurities prior to the co-deposition of lithiophilic Ag and Cu, a diluted HCl solution (10 mL HCl and 100 mL DI water) was used for acid treatment, and the treatment was carried out for 10 minutes. Subsequently, the cleaning process was performed in 100 mL acetone for 10 min. After the pickling process, the Cu foil (thickness of 18 µm) was used as the substrate for the deposition [25]. The pressure in the sputter chamber was increased to 1 × 10−5 Torr, and Ar gas was injected at 10 sccm and 0.15 Torr for 30 s with a power of 40 W. After the completion of co-deposition of Ag-Cu, the as-deposited CC (Copper-Clad) samples were annealed under a 4% H₂/96% Ar atmosphere in a furnace (PyroTech, Gyeonggi-do, Korea) at temperatures of 400°C, 500°C, and 600°C, respectively. The heating rate was set at 5°C/min, and a mixed gas flow of 20 SCCM was maintained until the end of the process. Subsequently, the samples were cooled to room temperature in the furnace after reaching the desired process temperature.”

2. The elemental distribution on the Ag–Cu CC surface from EDS analysis is not presented, even if it is declared (row 89)

We conducted EDS mapping to examine the distribution on the surfaces of Ag and Cu CC, and the results are presented in Figure 4. Due to the slight atomic number difference between Ag and Cu, distinguishing them was challenging. Therefore, we enhanced visibility through image analysis, as shown in Figure 4. The raw data is depicted in the left image below, and Figure 4, the analyzed data, is shown on the right. (Please see attached)

3.  In rows 91-93 in the phrase: ”The resistivity of 91 the film co-deposited on the glass substrate was measured using a four-point-probe surface resistivity meter (CMT-SR1000N, AIT, Gyeonggi-do, Korea) to confirm the annealing  effect. ”, obviously is not related to this subject (glass substrate...)

We deposited the Ag-Cu alloy without substrate effect on a glass substrate to measure its intrinsic sheet resistance. Since the resistance is almost negligible when Ag-Cu is deposited on a Cu substrate, it was impossible to distinguish the sheet resistance difference based on the heat treatment conditions. Therefore, to measure the sheet resistance difference of the Ag-Cu alloy, we used a non-conductive glass substrate.

4. In row 131 after Figure 2 there is a reference to Figure 5, without relevence with the text.

We have deleted the statement indicating that the results in Figure 2 match those in Figure 5, as per the reviewer's suggestion, which we found appropriate based on the context.

5. Table 1 is not presented in good condition, the unit measures are missing.

To enhance reader understanding, we have changed the representation of sheet resistance from Ω/□ to ohm/sq.

6. The sentence from Rows 142-143 refers to other caracterization.

We observed crystallization starting from 500°C in the XRD results and attempted to correlate these findings with decreased sheet resistance. As per the reviewer's suggestion, we have decided to delete this sentence to enhance reader comprehension.

7.  Rows 154-155: ”A comparison between the Ag–Cu CC obtained at 500  and 400 °C reveals the appearance of similar large Ag particles and approximately surrounding rough surface.”  This phrase is not a scientific one.

”A comparison between the Ag–Cu CC obtained at 500  and 400 °C reveals the appearance of similar large Ag particles and approximately surrounding rough surface.” The mentioned sentence was deleted and revised in lines 183 to 193.

   “In Figures 3c and 3d, the influence of Ostwald ripening can be observed, leading to the formation of Ag particles in island-like structures that continue to grow. Furthermore, in Figures 3d and 3e, it can be observed that the Ag particles remain fixed in island-like shapes without continuous growth. Moreover, the surface of the Ag–Cu CC obtained at 600°C exhibits grains of the same size because the Cu diffusion rate increases from that at 500°C, thereby inhibiting the growth of Ag particles. In contrast, at 600°C, the diffusion coefficient of Cu increased, and the size of the Ag and Cu became similar [27]. Based on these observations, it is expected that by adjusting the heat-treatment conditions, large Ag particles will be embedded and exist in the form of islands. Figure 4 shows the EDS mapping images of the CC subjected to different heat-treatment conditions after the Ag–Cu deposition.”

8. Overall, the whole content has to be improved. It is difficult in this form to understand the real innovative study, which in principle, exists.

Considering the insights you provided, we have revised the content to enhance the reader's understanding of the research findings as much as possible.

Reviewer 2 Report

In this work, the authors present a lithiophilic collector with a homogeneous distribution and immobilization of Ag and Cu atoms by the co-deposition of Ag and Cu on a Cu collector followed by heat treatment. This fabricated collector not only provides the nucleation sites and distributes the current density, enabling a dendrite-free Li plating in Li-metal anode, but also has the beneficial effect on the cycling performance because the unstable Ag atoms are immobilized as the cycle progresses. Therefore, I recommend that this paper can be published in inorganics. There are some minor issues as listed below:

(1) The authors tested the characteristics of the fabricated current collectors at increasing heat-treatment temperature to 400, 500 and 600 °C, and suggested that the best results were obtained at 600 °C. How about increase the annealing temperature above 600 °C? More characterization analysis should be supplied.

(2) In Figure 7, the fit result of impedance spectra using the equivalent circuit can be provided.

(3) In Figure 8 the coulombic efficiency plot range can be narrowed down to 70 - 120% to illustrate the change more clearly.

(4) Some recent progresses of Li metal anodes also benefit from the lithiophilic thin film decoration and alloyable coating, e.g. Energy Storage Mater. 37, 466-475, 2021; Angew. Chem. Int. Ed., 60, 14040-14050, 2021.

(5) In section 3.2, “The Ag-Cu CCs were nucleated by the well-known Volmer-Weber growth mode, in which metal thin films initially grow into stable islands after nucleation on the substrate”, I would suggest to provide a more detailed description about “grow into stable islands”. Relevant analyses would be interesting and helpful for understanding the advantages of the embedded island-shaped Ag atoms.

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Author Response

We are pleased to re-submit a revised version of our manuscript titled “Effect of the Annealing Temperature of Lithiophilic Ag–Cu Co-Deposition on the Cycling Performance of Li-Metal Anode” for Materials publication. We appreciate the thorough review and constructive criticisms of the reviewers. We have addressed each of their concerns below and have revised sections of the paper for clarity. We hope the revision has improved the paper to a level of the reviewers’ satisfaction.

In this work, the authors present a lithiophilic collector with a homogeneous distribution and immobilization of Ag and Cu atoms by the co-deposition of Ag and Cu on a Cu collector followed by heat treatment. This fabricated collector not only provides the nucleation sites and distributes the current density, enabling a dendrite-free Li plating in Li-metal anode, but also has the beneficial effect on the cycling performance because the unstable Ag atoms are immobilized as the cycle progresses. Therefore, I recommend that this paper can be published in inorganics. There are some minor issues as listed below:

(1) The authors tested the characteristics of the fabricated current collectors at increasing heat-treatment temperature to 400, 500 and 600 °C, and suggested that the best results were obtained at 600 °C. How about increase the annealing temperature above 600 °C? More characterization analysis should be supplied.

This paper discusses the control of atomic distribution by exploiting the difference in diffusion rates between Ag and Cu, achieved through co-sputtering followed by thermal treatment. Below 600°C, Ag exhibits faster diffusion than Cu, leading to the formation of Ag islands. However, above 600°C, Cu's diffusion coefficient becomes similar to Ag's, significantly hindering the formation of Ag islands. Consequently, due to the Cu substrate, uniform Ag islands are not generated on the surface. Upon examining the Coulomb efficiency of CC treated at 650°C and 700°C, it was observed that the Coulomb efficiency sharply decreases after more than 40 cycles.

(2) In Figure 7, the fit result of impedance spectra using the equivalent circuit can be provided.

We have revised Figure 7 on page 9 by incorporating the fitting results.

(3) In Figure 8 the coulombic efficiency plot range can be narrowed down to 70 - 120% to illustrate the change more clearly.

The modifications to Figure 8 on page 10 have been made in response to the reviewer's feedback.

(4) Some recent progresses of Li metal anodes also benefit from the lithiophilic thin film decoration and alloyable coating, e.g. Energy Storage Mater. 37, 466-475, 2021; Angew. Chem. Int. Ed., 60, 14040-14050, 2021.

Thank you for your valuable feedback. We will incorporate the suggestions from this paper into our subsequent research.

(5) In section 3.2, “The Ag-Cu CCs were nucleated by the well-known Volmer-Weber growth mode, in which metal thin films initially grow into stable islands after nucleation on the substrate”, I would suggest to provide a more detailed description about “grow into stable islands”. Relevant analyses would be interesting and helpful for understanding the advantages of the embedded island-shaped Ag atoms.

We have added the following content to page 5, lines 175-177. The content is as follows.

“Specifically, co-sputtering was conducted until the point where separate, round nuclei in island-like shapes formed simultaneously on the extensive substrate surface [26].”

Reviewer 3 Report

In this study, an island shape of Ag was deposited on a Cu substrate by sputtering of Ag and Cu and subsequent annealing.  The authors demonstrated the electrochemical performance of obtained film as the current collector for Lithium metal batteries in the half cell setup. Minor changes need to be addressed in the manuscript:

1.       1. Line 56, typo “Li+/L”

2.      2.  There are other metals are studied as metal alloy with Lithium for the current collector in lithium metal batteries, such as gold and nickel.  Please add references on these metals and explain why Ag is selected in this study instead of other metals.

3.     3.  Line 141, is it a typo of “500 °C”? If not, why there is still sheet resistance of 600 °C annealed film in the table?

4.      4. What the mass loading of deposited Cu and Ag on the tested film? The amount of deposited metals should have effects on the performance of the current collectors.

Minor editing of English language is recommended.

Author Response

We are pleased to re-submit a revised version of our manuscript titled “Effect of the Annealing Temperature of Lithiophilic Ag–Cu Co-Deposition on the Cycling Performance of Li-Metal Anode” for Materials publication. We appreciate the thorough review and constructive criticisms of the reviewers. We have addressed each of their concerns below and have revised sections of the paper for clarity. We hope the revision has improved the paper to a level of the reviewers’ satisfaction.

In this study, an island shape of Ag was deposited on a Cu substrate by sputtering of Ag and Cu and subsequent annealing.  The authors demonstrated the electrochemical performance of obtained film as the current collector for Lithium metal batteries in the half cell setup. Minor changes need to be addressed in the manuscript:

1. Line 56, typo “Li+/L”

I have corrected the typo in the 62nd line to 'Li+/Li'.

2. There are other metals are studied as metal alloy with Lithium for the current collector in lithium metal batteries, such as gold and nickel.  Please add references on these metals and explain why Ag is selected in this study instead of other metals.

I have made revisions to lines 56-62 on page 2, and I have referenced sources 22 and 23. The revised content is as follows.

“Similar studies are being conducted using Ni, Zn, and Au. However, Ni and other elements have lower Li nucleation overpotential compared to Cu, but they still exhibit Li nucleation overpotential. Additionally, Au and Pt are disadvantaged from a cost perspective. Based on various combinations of Li–Ag alloys, including AgLi12, Among noble metals, Ag, which is reasonably priced, is considered a promising metal candidate for an anode-free battery because it has the highest electrical conductivity and lower potential (0–0.250 V vs. Li+/Li) than other metals [22,23].”

3. Line 141, is it a typo of “500 °C”? If not, why there is still sheet resistance of 600 °C annealed film in the table?

I have deleted the sentence in the 141st line, and sorted the equipment. The samples treated at 500°C and 600°C did not show any resistance in the re-measurement results. I have made the necessary modifications to Table 1 on page 5.

4. What the mass loading of deposited Cu and Ag on the tested film? The amount of deposited metals should have effects on the performance of the current collectors.

The coating thickness of the thin film is 100 nm, and a ø13 electrode puncher was used. Considering the theoretical density of Ag and Cu, the weights were calculated and summarized in the table. A difference of 0.0001g was confirmed in the measured results using a scale. I have added this content to lines 112-113 on page 3.

“(Ag-Cu film thickness 100nm, weight 0.0001g)” with Ag:Cu weight ratio 9:1

Round 2

Reviewer 1 Report

The paper is drastically improved, so I recommend for publication in this form