Examination of Over-Discharge Effects on a Cylindrical Lithium-Ion Battery via the Immersion of a Copper Strip in a Salt Solution

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study presents a novel approach to discharging cylindrical lithium-ion batteries using a saltwater-copper strip system, addressing a critical safety issue in battery recycling. The experimental design is systematic, and the results offer valuable insights into optimizing discharge efficiency and minimizing corrosion risks. The work aligns well with the scope of Corrosion and Materials Degradation. However, several aspects require clarification or enhancement before publication.

1. Mechanistic Clarification of Copper Corrosion 

The manuscript attributes copper corrosion to electrochemical reactions but lacks detailed analysis of the corrosion mechanism (e.g., galvanic vs. pitting corrosion). Provide SEM/EDS characterization of the copper strip surfaces after discharge to visualize morphological changes and elemental distribution. This would strengthen the link between copper dissolution and precipitate formation.

2. Control Experiment Validation

While varying gaps (2–6 mm) are tested, no control experiment is conducted without the copper strip. Including a baseline comparison (e.g., discharging in saltwater alone) would isolate the effect of copper and validate its necessity for accelerating voltage drop. Additionally, testing different salt concentrations (e.g., 10 wt.% vs. 30 wt.%) could identify optimal conditions.

3. Practical Scalability and Safety

The discussion focuses on laboratory results but does not address industrial challenges (e.g., handling large volumes of corrosive solutions, gas emissions during electrolysis). A brief section on safety protocols (e.g., hydrogen gas mitigation) and cost-benefit analysis compared to cryogenic methods would enhance the study’s real-world relevance.

Comments on the Quality of English Language

The manuscript is generally well-written, but minor improvements in grammar, terminology consistency, and sentence structure would enhance readability and professionalism. The authors are encouraged to proofread carefully or seek assistance from a native English speaker for final polishing.

Author Response

Reviewer_1

Dear Reviewer, 

I, along with my coauthors, would like to thank you for the very thoughtful considerations regarding the manuscript entitled “Examination of over–discharge effects on a cylindrical Lithium-Ion Battery via copper strip immersion in a salt solution”, submitted to MCD of MDPI. The manuscript was revised accordingly to the considerations received from your review. We have included the author’s comment in the Response Letter and responded to them individually, indicating how we addressed each concern or problem and describing the changes we have made in the manuscript.

Comments and Suggestions for Authors

This study presents a novel approach to discharging cylindrical lithium-ion batteries using a saltwater-copper strip system, addressing a critical safety issue in battery recycling. The experimental design is systematic, and the results offer valuable insights into optimizing discharge efficiency and minimizing corrosion risks. The work aligns well with the scope of Corrosion and Materials Degradation. However, several aspects require clarification or enhancement before publication.

1. Mechanistic Clarification of Copper Corrosion 

The manuscript attributes copper corrosion to electrochemical reactions but lacks detailed analysis of the corrosion mechanism (e.g., galvanic vs. pitting corrosion). Provide SEM/EDS characterization of the copper strip surfaces after discharge to visualize morphological changes and elemental distribution. This would strengthen the link between copper dissolution and precipitate formation.

А. The reviewers' recommendations will be incorporated into our forthcoming research, which will be conducted as a separate study on producing copper precipitate during discharge. The following article will provide a comprehensive analysis of the copper corrosion mechanism, as you requested. Moreover, we intend to conduct an analysis of the copper conductor's surface characteristics. This work, however, focused on determining the influence of the gap spacing between the copper conductor and the battery on the battery's voltage drop. Please acknowledge our limited expertise in the field of corrosion.

2. Control Experiment Validation

While varying gaps (2–6 mm) are tested, no control experiment is conducted without the copper strip. Including a baseline comparison (e.g., discharging in saltwater alone) would isolate the effect of copper and validate its necessity for accelerating voltage drop. Additionally, testing different salt concentrations (e.g., 10 wt.% vs. 30 wt.%) could identify optimal conditions.

А. Several researches indicate that immersion in saltwater causes corrosion of steel battery caps and aluminum valves, leading to the contamination of the solution with battery electrode materials. This study, therefore, focused on developing a new methodology for preventing corrosion during battery discharge. A more detailed explanation can be found in the introduction (lines 76–83). Our experimental results were compared to study of Sever et al.'s using a 20 wt.% salt solution concentration, selected based on several studies.’s studies.

3. Practical Scalability and Safety

The discussion focuses on laboratory results but does not address industrial challenges (e.g., handling large volumes of corrosive solutions, gas emissions during electrolysis). A brief section on safety protocols (e.g., hydrogen gas mitigation) and cost-benefit analysis compared to cryogenic methods would enhance the study’s real-world relevance.

А. Our study focused solely on the new methodology. This suggestion will be part of our next research study.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper is a study on the impregnation method of electrolyte in the disposal process of LIB (positive electrode type NCA). The authors facilitated the discharge of the LIB by inserting a copper plate into the solution. This would reduce disposal processing time. In addition, by having the copper plate contact only the cathode side of the LIB, the corrosion of the LIB was prevented by the corrosion of the copper plate. This study contributes to LIB disposal technology. As stated by the authors, it is very important to determine if recycling is feasible. It should be clarified whether LIBs treated by the authors' method are recyclable. To do so, the treated LIBs should be disassembled to verify the condition and composition of the electrodes. In addition, the relationship between gap and treatment time is shown, but the relationship with precipitates is not signified. Therefore, the importance of gap distance in this study is not known. There are several other points that need clarification and are noted below.

Question1

The No contact method is concerned about the generation of chlorine gas. Since chlorine gas is toxic, a gas analysis should be performed and the authors should demonstrate the safety of the proposed method.

Question2

I do not understand the relationship between gap distance and precipitates. Can you state the relationship between gap and precipitates in the experiment in Figure 1b?

Question3

Is the active material of the anode 100% graphite?

Question4

Please add copper, size, and gap to Figure 1. for NoContact, does the gap refer to the total value of the positive and negative poles?

Question5

Section 3.1 is similar to the discussion. It should be moved to the appropriate section.

Question6

How many times have you done the test? The authors should include standard deviations in the results figures to ensure reproducibility.

Question7

Provide photographs of unused and corroded batteries and clearly show that the battery caps and valves have corroded.

Question8

Do you mean that Cu-01 is Figure 1a and Cu-02 is the precipitate in Figure 1b? Please clarify. Also, please add for Cu-01 and Cu-02 to the experimental method.

Question9

Page 12 Line 350「Because of the high concentration of copper and chloride ions,・・」

Which results show that the concentrations of copper and chloride ions are high?

Question10

Photographs of the inside of the battery and its composition should be provided to show the recycling potential of the treated battery electrodes.

Author Response

Reviewer_2

Dear Reviewer, 

I, along with my coauthors, would like to thank you for the very thoughtful considerations regarding the manuscript entitled “Examination of over–discharge effects on a cylindrical Lithium-Ion Battery via copper strip immersion in a salt solution”, submitted to MCD of MDPI. The manuscript was revised accordingly to the considerations received from your review. We have included the author’s comment in the Response Letter and responded to them individually, indicating how we addressed each concern or problem and describing the changes we have made in the manuscript.

Comments and Suggestions for Authors

This paper is a study on the impregnation method of electrolyte in the disposal process of LIB (positive electrode type NCA). The authors facilitated the discharge of the LIB by inserting a copper plate into the solution. This would reduce disposal processing time. In addition, by having the copper plate contact only the cathode side of the LIB, the corrosion of the LIB was prevented by the corrosion of the copper plate. This study contributes to LIB disposal technology. As stated by the authors, it is very important to determine if recycling is feasible. It should be clarified whether LIBs treated by the authors' method are recyclable. To do so, the treated LIBs should be disassembled to verify the condition and composition of the electrodes. In addition, the relationship between gap and treatment time is shown, but the relationship with precipitates is not signified. Therefore, the importance of gap distance in this study is not known. There are several other points that need clarification and are noted below.

 Question1-The No contact method is concerned about the generation of chlorine gas. Since chlorine gas is toxic, a gas analysis should be performed and the authors should demonstrate the safety of the proposed method.

Using Figure 7 and Reaction 7, we showed that Cl from dissociated NaCl reacts with metallic copper to form copper chloride. XRF analysis detected chlorine in the salt solution precipitate, and XRD analysis confirmed the presence of FeCl₃ and Cu₂Cl(OH)₃. We also noted the impracticality of discharging without contact with the copper conductor.

Question2-I do not understand the relationship between gap distance and precipitates. Can you state the relationship between gap and precipitates in the experiment in Figure 1b?

Figure 1 provides a schematic diagram of the experimental setup. As shown in Figure 7b, a green product of the copper chloride product formed on the solution surface because of the copper conductor dissolving. The main goal of our research was to investigate the direct relationship between gap spacing and battery voltage decrease using a copper conductor. Our findings confirmed conductor dissolution and precipitation in the solution during discharge, necessitating additional research. We'll be focusing our next research efforts in this direction.

 Question3-Is the active material of the anode 100% graphite?

We can't say for sure about this. Without research on the battery's anode and cathode materials, we cannot answer. Readings and studies suggest a typical composition of graphite and silicon additives.

 Question4-Please add copper, size, and gap to Figure 1. for NoContact, does the gap refer to the total value of the positive and negative poles?

We modified Figure 1.

 Question5-Section 3.1 is similar to the discussion. It should be moved to the appropriate section.

We changed to as Discussion section.

Question6-Provide photographs of unused and corroded batteries and clearly show that the battery caps and valves have corroded.

We battery samples provided in Fig. 2 and 4

 Question7-Do you mean that Cu-01 is Figure 1a and Cu-02 is the precipitate in Figure 1b? Please clarify. Also, please add for Cu-01 and Cu-02 to the experimental method.

We revised.

 Question8-Page 12 Line 350「Because of the high concentration of copper and chloride ions,Which results show that the concentrations of copper and chloride ions are high?

We removed the text.

 Question9-Photographs of the inside of the battery and its composition should be provided to show the recycling potential of the treated battery electrodes.

We added the Figure 7.

Reviewer 3 Report

Comments and Suggestions for Authors This manuscript describes studies on the discharge of cylindrical lithium-ion batteries (NCA type) using saltwater and copper conductors, in both contact and non-contact methods. The research is significant as a complete discharge of a battery is needed before recycling to prevent explosions due to residual charge. The manuscript can be published after correcting numerous language errors and some mistakes.   General comments:  1. A comparison to the battery discharge without a copper conductor should be addressed. 2. Corrosion of the battery's steel cap and aluminum valve leads to dissolution of anode and cathode components into the solution. Thus, a number of both soluble and insoluble products were detected in the research. The authors should address their impact to the environment. For instance, MSDS for copper oxychloride states that the chemical can cause several adverse health effects.   Specific comments: Line 63: Clarify the meaning "expensive nitrogen gas" in the quick-freezing method. Or should it be "expensive liquid nitrogen"? Line.100-101 Clarify " The sea salt is composed of approximately 93-94.5% of its total composition." Line. 110 Table 1: There is no need to use a table that has one row only. Lines.132-133: The sentence "At the negative electrode, the electrochemical process involving salt water leads to the production of hydrogen and chlorine ions that transform into gases." should be either clarified or modified. How could negatively charged "chlorine ions" (chlorides?) be attracted to the negatively-charged electrode, and further neutralized to chlorine gas? Line 231: Hydrogen gas cannot evaporate. Perhaps authors meant that it was carried away into the solution. Lines 344 and 353 (Table 2): There is a difference in the reported figure for iron(III) oxide in the chemical composition. Lines 360 and 353 (Table 2): The Table 2 reads "CuO" (cupric oxide) whereas the the text talks about Cu2O (cuprous oxide). Comments on the Quality of English Language

Numerous English language errors throughout the manuscript makes reading and understanding the research difficult. It should be proof-read by an English native. Some examples are listed below:

Line 17: Should "where" be changed to "with"?

Line 56: Perhaps "In" should be changed to "Before".

Line 86: Should "informed" be replaced with "justified"?

Author Response

Reviewer_3

Dear Reviewer, 

I, along with my coauthors, would like to thank you for the very thoughtful considerations regarding the manuscript entitled “Examination of over–discharge effects on a cylindrical Lithium-Ion Battery via copper strip immersion in a salt solution”, submitted to MCD of MDPI. The manuscript was revised accordingly to the considerations received from your review. We have included the author’s comment in the Response Letter and responded to them individually, indicating how we addressed each concern or problem and describing the changes we have made in the manuscript.

Comments and Suggestions for Authors

This manuscript describes studies on the discharge of cylindrical lithium-ion batteries (NCA type) using saltwater and copper conductors, in both contact and non-contact methods. The research is significant as a complete discharge of a battery is needed before recycling to prevent explosions due to residual charge. The manuscript can be published after correcting numerous language errors and some mistakes.  

General comments: 

1. A comparison to the battery discharge without a copper conductor should be addressed.

Several researches indicate that immersion in saltwater causes corrosion of steel battery caps and aluminum valves, leading to the contamination of the solution with battery electrode materials. This study, therefore, focused on developing a new methodology for preventing corrosion during battery discharge. A more detailed explanation can be found in the introduction (lines 76–83). Our experimental results were compared to study of Sever et al.'s using a 20 wt.% salt solution concentration, selected based on several studies.’s studies.

2. Corrosion of the battery's steel cap and aluminum valve leads to dissolution of anode and cathode components into the solution. Thus, a number of both soluble and insoluble products were detected in the research. The authors should address their impact to the environment. For instance, MSDS for copper oxychloride states that the chemical can cause several adverse health effects.

The battery cap, anode, and cathode materials all corroded and dissolved, contaminating the solution. We clearly stated in the article, and further explained in the Conclusion section, that this method is unsuitable for practical applications. 

Specific comments:

• Line 63: Clarify the meaning "expensive nitrogen gas" in the quick-freezing method. Or should it be "expensive liquid nitrogen"?

We revised.

• 100-101 Clarify " The sea salt is composed of approximately 93-94.5% of its total composition."

We revised.

• 110 Table 1: There is no need to use a table that has one row only.

We revised.

• 132-133: The sentence "At the negative electrode, the electrochemical process involving salt water leads to the production of hydrogen and chlorine ions that transform into gases." should be either clarified or modified. How could negatively charged "chlorine ions" (chlorides?) be attracted to the negatively-charged electrode, and further neutralized to chlorine gas?

We revised.

• Line 231: Hydrogen gas cannot evaporate. Perhaps authors meant that it was carried away into the solution.

We revised.

• Lines 344 and 353 (Table 2): There is a difference in the reported figure for iron(III) oxide in the chemical composition.

We revised

• Lines 360 and 353 (Table 2): The Table 2 reads "CuO" (cupric oxide) whereas the the text talks about Cu2O (cuprous oxide).

This depends on how the XRF and XRD analyses differ. Because XRF uses the familar copper oxide form for its calculations, it's represented as CuO.

Comments on the Quality of English Language

• Numerous English language errors throughout the manuscript makes reading and understanding the research difficult. It should be proof-read by an English native. Some examples are listed below:
• Line 17: Should "where" be changed to "with"?

We revised

• Line 56: Perhaps "In" should be changed to "Before".

We revised.

• Line 86: Should "informed" be replaced with "justified"?

We revised.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for taking the time to answer my questions. I also confirmed that the paper has improved. However, the answer to question 1 is not a response. The authors should also describe for the benefit of the reader the environmental impact of this method and the precautions (dangers) that should be taken in its implementation.

Author Response

 Question1-The No contact method is concerned about the generation of chlorine gas. Since chlorine gas is toxic, a gas analysis should be performed and the authors should demonstrate the safety of the proposed method.

A. Using Figure 7 and Reaction 7, we showed that Cl from dissociated NaCl reacts with metallic copper to form copper chloride. XRF analysis detected chlorine in the salt solution precipitate, and XRD analysis confirmed the presence of FeCl₃ and Cu₂Cl(OH)₃. We also noted the impracticality of discharging without contact with the copper conductor.