Surface Reduction of Li₂CO₃ on LLZTO Solid-State Electrolyte via Scalable Open-Air Plasma Treatment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper reports an ambient atmosphere plasma process to remove Li2CO3 formed on the surface of LLZO solid electrolytes upon exposure to CO₂ and moisture. Upon review, it was found that additional details of the plasma process are required. The characterization of the process efficacy was performed through XPS and EIS, and the results are qualitatively understandable. There are major issues when it comes to interpreting the XPS results quantitatively. However, EIS and XPS indicated that this process may not be fully optimized as there is still evidence of Li2CO3 peaks and the impedance decrease is of the order of 43%. The authors do a good job in comparing the potential drawbacks of their method to other methods based on thermal annealing. This reviewer recommends publication, after major revision:

 

Comments:

Comment 1: Figure 1, what does the purple skewed line at the top indicate? In general, the schematic is good to have but please mark each part so that readers who are not familiar with plasma instrumentation can comprehend your setup. Likewise, please expand acronyms – what do RONS refer to in the figure? If possible, please provide an image of your setup since this is the key point of this paper. Likewise, please provide a typical process workflow, for example:

Exposed LLZTO pellet placed in shroud, compressed air injected → Plasma ignited in ambient atmosphere → Pellet exposed for xx minutes → Plasma turned off → Shroud flooded with inert N₂

Comment 2, Line 137: Typo – plasma

Comment 3, All XPS figures in main script and SI: Please show the raw experimental data in addition to the curve fit. Please describe the fitting parameters – what was the peak shape, FHWM, background subtraction etc. How are the authors sure about the C-O-C assignment real in Figure 2a and 2c? The FHWM appears to be abnormally high. Same comment regarding FHWM applies to C-O-C assignments in Figure 3. Please reanalyze your XPS data and address this issue – your fit shows in multiple cases that C-O-C peak contributions are ranging between 285-290 eV – that is impossible and is clearly wrong.

Comment 4, Figure 2: The problem with this XPS measurement is that it does not convey any quantitative information about the rate at which Li2CO3 forms.

First, XPS is surface sensitive – and in Figure S3a, you are not seeing any LLZTO contributions to the XPS spectra. Therefore, all the Li2CO3 signal in Figure 2a, is coming from Li2CO3 alone and there maybe additional Li2CO3 underneath, so Figure S2 values may not be accurate. Likewise, Figure S2 is hard to interpret – the title states ‘atomic concentration analysis of C=O associated with Li2CO3, C-C, C-O’. Why are the sums not adding upto 100% in that case?

Second, in Figure S2, case b and case c, there is no denying the fact that Li2CO3 levels are increasing – but what is the absolute magnitude of the increase? XPS percentages are not quantitative and there is no internal fixed standard (of known concentration) to normalize the Li2CO3 values. Therefore, the only interpretation of this XPS data is that Li2CO3 is increasing upon exposure to ambient air and that is well known.

Comment 5, Figure S8: What is normalized intensity?

Comment 6: From reading the entire paper, the conclusion that was drawn is that the open-air plasma treatment process is clearly not as effective in reducing impedance in cells as compared to the Ar-atmosphere heating methods. If a method does not ultimately solve the issue then there is little chance that it will be adopted for practical use. Nevertheless, this reviewer commends the authors for presenting a good and fair comparison highlighting the drawbacks of this reported process compared to traditional approaches and these findings should be made available to the academic community.

What this reviewer wants to see is a comprehensive description of the failure issues in a separate section and possible solutions. This will ensure that the readers understand what the limitations are and hopefully they can decide if this process stands a chance for future scale up. An example in this regard, it was mentioned that the origin of the impedance could be from the side of the LLZO not exposed to the plasma. The authors claim that this can be overcome if the LLZO is bonded to a current collector – this reviewer assumes that Li metal is being spoken of here. The scenario is a fair assumption – but is this a practical scenario? Is it being done? If not, then there is no point in artificially creating a scenario and stating that the issues can be overcome. Please highlight this point and all other drawbacks and possible solutions in this paper.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have demonstrated a sufficiently novel approach on removing Li2CO3 from garnet SE via open-air plasma treatment. The work is pretty well written with results analyzed logically. Therefore, I would think this work a publication in MDPI Batteries after carefully addressing all of my comments.

1. The authors have, however, missed significant work on removing Li2CO3 on garnet SE. One most recent and good example is: Advanced Energy Materials, 13 (42), 2301656, 2023, which should be highlighted in the introduction section. The work showed heat treatment process for removing Li2CO3 by heating up to 800C under inert atmosphere and it is a significant and scalable methodology in garnet SE field. 

2. The authors mentioned the designed technique is "scalable". However, there is lack of evidence showing the scalability and relevant analysis. Thus, I would suggest the authors include the detailed analysis on these aspects.

3. It is not clear and understandable to me for the open circuit shown in Figure 4a-b. It looks like they were electrically shorted circuit. A double-layer related circuit parallel to a cable?

4. In Figure 4, I am assuming the authors used blocking electrode for the measurement. However, it looks like there is no diffusion tails. Why? Additionally, could the authors please calculate the ionic conductivity based on the EIS in Figure 4? This would give a more reasonable and direction evidence on Li2CO3 removal.

5. Can the authors show surface morphology (e.g., via SEM) before and after the treatment? This also gives a direct evidence for comparison.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thanks for the point by point response, no further comments.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have well addressed all of my comments. Thus, approval for a publication in MDPI Batteries.