Improved Electrochemical Behavior and Thermal Stability of Li and Mn-Rich Cathode Materials Modified by Lithium Sulfate Surface Treatment

Round 1

Reviewer 1 Report

1. Details in experimental setup should be provided, e.g., speed for ball milling treatment, the half-cells description, etc...
2. The figure 2d - the insert should have a better resolution.
3. For XRD the Rietveld refinement and HRTEM measurements are recommended.
4. The sulfur content and distribution should be employed. 
5. The DSC measurements should be compatible with the performed synthesis protocol.

Author Response

 

1. Details in experimental setup should be provided, e.g., speed for ball milling treatment, the half-cells description, etc...

Answer:

Thank you for this note.

We have added the following information into the experimental setup, as requested:

Ball milling was carried out for 1 hour, speed was 400 rpm (page 6).

We described the half-cell configuration in the manuscript, as follows (page 6):

“Electrochemical testing of untreated and Li₂SO₄-treated HE-NCM cathode materials was carried out by galvanostatic cycling in half-cells of coin-type 2325 (parts from NRC-CNRC, Ottawa, Ontario, Canada). They comprised HE-NCM cathodes, Li metal foil (~200-mm thick from Honjo Metal Co., Osaka, Japan) as counter electrodes, Celgard 2500 polypropylene separators, and ethylene carbonate (EC) – ethyl methyl carbonate (EMC) (3:7 v/v %) 1M LiPF₆ Li-battery grade electrolyte solutions LP57 from BASF, Ludwigshafen, Germany. Electrode loading was ~3 mg of active HE-NCM material per cm², corresponding to 0.75 mAh/cm² areal loading.”

 

2. Figure 2d - the insert should have a better resolution.

Thank you for this valuable note. Probably, the reviewer meant the insert in Figure 1, not Figure 2d.

We have improved the resolution of the insert in Figure 1.

3. For XRD the Rietveld refinement and HRTEM measurements are recommended.

We have performed the Rietveld analysis for the untreated and treated HE-NCM cathode materials and included the corresponding figures, calculations, and discussion in the revised version (Figure 2 page 11). Table 1 and its caption were also modified slightly due to the inclusion of the data of the Li₂SO₄ phase from the Rietveld analysis (along with LiMnO₃ and Li(TM)O₂ phases).

4. The sulfur content and distribution should be employed. 

Point EDS using both SEM and TEM techniques was used for measuring the sulfur content in HE-NCM material. The S-content was around 1 at. % as measured from several points on a sample. Because of this low content, the EDS mapping would be too noisy while trying to collect the data; that is why the maps are not shown in the paper.

The distribution of the conformal Li₂SO₄ coating on the HE-NCM particles can be seen from the HR-TEM images (Figure 4).

5. The DSC measurements should be compatible with the performed synthesis protocol.

The DSC studies in our paper aimed to measure the heat evolved during the thermochemical reactions between the HE-NCM materials (untreated and Li₂SO₄-treated) and the solution species of EC-EMC/LiPF₆ (Figure 12). These measurements do not relate at all to the synthesis protocol of HE-NCM. This included co-precipitation synthesis of this material and annealing at 750⁰C provided by the manufacturer BASF, Germany.

6. Moderate English changes are required.

We have improved the English through the entire paper, as requested by this reviewer and by reviewer 3. For instance, we have transformed the Passive Voice expressions for Active Voice where needed according to the grammar requirements (pp. 5, 6, 7, 24, 26) and changed or omitted a few sentences or their parts, for clarity and style (for example, in p. 3, 8, 12).

Reviewer 2 Report

The paper is highly comprehensive with a few possible improvements as follows:

(1) The authors should provide more information or references about their "on-line electrochemical mass spectrometry" instrument beyond just stating that "For the in-operando 207 measurements of the evolved gases as a function of applied potential Mid 208 mode was selected for H2, CO2, and O2 gases."

(2) The authors should provide evidence for "elastic" and "plastic" in the statement that "It is suggested that at 575 °C lithium sulfate underwent first-order phase transition from ordered, low symmetry (monoclinic), elastic, poorly conducting β-phase to disordered high symmetry, plastic, high Li+ conducting α-phase that promotes the kinetics at the electrode/solution interface,..."

(3) The authors should explain why the mass loading is so low (0.53 mAh/cm^2). Does the lithium sulfate surface treatment cause degradation of the graphite electrode if sulfur atoms migrate to the negative electrode?

(4) Is there any evidence that the surface treatment mitigated cracking of the HE-NCM particles after many lithiation and de-lithiation cycles? 

Author Response

The paper is highly comprehensive with a few possible improvements as follows:

1. The authors should provide more information or references about their "on-line electrochemical mass spectrometry" instrument beyond just stating that "For the in-operando 207 measurements of the evolved gases as a function of applied potential Mid 208 mode was selected for H₂, CO₂, and O₂"

We have added the following information to the revised version: a link to the OEMS instrument used in our studies [https://www.hidenanalytical.com/products/gas-analysis/hpr-40-dems/] and the reference [Chem. Mater. 2019, 31, 3840−3847] of our previous paper, in which OEMS was also explored.

2. The authors should provide evidence for "elastic" and "plastic" in the statement that "It is suggested that at 575 °C lithium sulfate underwent first-order phase transition from ordered, low symmetry (monoclinic), elastic, poorly conducting β-phase to disordered high symmetry, plastic, high Li⁺ conducting α-phase that promotes the kinetics at the electrode/solution interface,..."

     Thank you for this valuable suggestion.

We have not performed any physicomechanical studies of the phase transitions for Li₂SO₄ at various temperatures (500 – 600⁰C) but rather referred to the literature data. For clarity, we shortened the description of such transitions in the revised version. We believe that readers of this paper can find the detailed information in the corresponding references.

3. The authors should explain why the mass loading is so low (0.53 mAh/cm^2). Does the lithium sulfate surface treatment cause degradation of the graphite electrode if sulfur atoms migrate to the negative electrode?

Thank you for this interesting question.

First, we have recalculated the areal loading to be 0.75 mAh/cm² considering a capacity of 250 mAh/g at a 1C rate. This is indicated in the revised version (page 7).

Second, we used relatively thin HE-NCM electrodes with the geometrical loading of ~3 mg/cm² to obtain well-resolved impedance spectra (Nyquist plots) measured during cycling, while taking into account the loading does not affect much the profiles of the voltage vs. capacity and capacity vs. cycle number. 

In this research, we have used only half coin-cells configuration with Li anodes and have not tested HE-NCM cathodes with graphite anodes. Since Li₂SO₄ coating layer is shown to be stable on the surface of HE-NCM material upon cycling in Li-cells (100 cycles at 30⁰C), we assume that it would be stable and preserve its structure in full cells HE-NCM vs. graphite as well. However, some degradation of Li₂SO₄ layer at elevated temperatures (>50⁰C) upon prolonged cycling at high rates cannot be ruled out.

(4) Is there any evidence that the surface treatment mitigated cracking of the HE-NCM particles after many lithiation and de-lithiation cycles? 

Thank you for this interesting question.

We have not studied the cycled electrodes from the point of view of cracks formation and plan to perform such studies in the future. However, according to our previous research dedicated to the SO₂ thermal gas treatment of HE-NCM electrodes (resulted in the formation of the Li₂SO₄ coating on the active material) as well as of other surface coatings, the surface layer on the active materials can protect them from reactions with the electrolyte solution and maintain the integrity of the particles after prolonged cycling.

Reviewer 3 Report

This manuscript introduces the improved electrochemical behavior and thermal stability of Li and Mn-rich cathode materials modified by lithium sulfate surface treatment, which are promising for advanced lithium-ion batteries. The introduction provide sufficient background and include all relevant references. The research design is appropriate, and the results are clearly presented. However, the following contents should be slightly modified:

1. The conclusion should be summarized into clear points, such as (1)…(2)…(3)…
2. Figure 2. (a) TEM image of untreated HE-NCM displaying the typical ball-shaped “primary” agglomerates of several micrometers in diameter, which showed the minimum particle size of about 2um. This description conflicts with the description on line 217-218 (From the morphological      viewpoint,      HE-NCM … particles are ball-shaped of around ~5-15 µm in diameter as measured by scanning electron microscopy [27]), which should be explain or revise.
3. Figure 5.(c) should be correction curve description text, is Li2SO4-treated HE-NCM, not is Li2SO4-treated.

Author Response

1. The conclusion should be summarized into clear points, such as (1)…(2)…(3)…We have modified the Conclusion section accordingly.

2. Figure 2 (a) TEM image of untreated HE-NCM displaying the typical ball-shaped “primary” agglomerates of several micrometers in diameter, which showed the minimum particle size of about 2um. This description conflicts with the description on lines 217-218 (From the morphological viewpoint, HE-NCM … particles are ball-shaped of around ~5-15 µm in diameter as measured by scanning electron microscopy [27]), which should be explained or revised.

We have corrected the description of the morphology images in the revised version (p. 12 and p.13, Figure 3 caption). Typically, the primary particles of the HE-NCM materials prepared by BASF are ball-shaped of several micrometers in diameter. Of course, there are smaller particles as well due to some particle size distribution according to the BASF data.

3. Figure 5(c) should be correction curve description text, is Li₂SO₄-treated HE-NCM, not is Li₂SO₄-treated.

Page 16: Figure 5c (now it's Figure 6c) was corrected to “Li₂SO₄-treated HE-NCM”

Round 2

Reviewer 1 Report

The manuscript can be published in the present form.

Reviewer 2 Report

I am satisfied with the revisions and would recommend the publication of the paper.