Al2O3-Embedded LiNi0.9Mn0.05Al0.05O2 Cathode Engineering for Enhanced Cyclic Stability in Lithium-Ion Batteries
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors1. Lack of Al₂O₃ Phase Confirmation in LNMA Samples via XRD, and Need for Supplementary Surface Characterization
Although the authors emphasize the presence of a γ-Al₂O₃ coating layer on the LNMA cathode materials, the XRD patterns of the coated LNMA samples (Figure 2 and Figure 4a) do not show any distinguishable diffraction peaks attributable to Al₂O₃. While the authors mention that the absence of Al₂O₃ peaks is due to the nanoparticulate size and broad peak width being obscured by the background signal, this explanation remains insufficient. Given that the Al₂O₃ phase plays a central role in the claimed performance enhancement, its presence on the LNMA particles should be rigorously verified beyond indirect inference.
In particular, the XRD of isolated Al₂O₃ nanoparticles (Fig. 4a) cannot substitute for direct phase identification on the LNMA surface. Therefore, I strongly recommend incorporating XPS analysis of the LNMA samples to confirm the chemical state and surface presence of Al-containing species. This would help confirm whether the coating layer is indeed Al₂O₃, and whether any in situ reactions (formation of Li–Al–O species) occur during the annealing step.
Furthermore, the current characterization of the coating layer relies primarily on TEM-based thickness observation, which, while useful for morphology, cannot provide sufficient chemical specificity. A combined XPS depth profiling and high-resolution elemental mapping would significantly strengthen the evidence for the uniformity and nature of the Al₂O₃ coating.
2. Insufficient Electrochemical Differentiation between Samples – Need for Statistical Reproducibility
As shown in Figure 7, the electrochemical performance differences among the various LNMA samples (especially LNMA-1, -2, and -3) are relatively minor, with capacity values and retention rates overlapping within narrow margins. This makes it difficult to clearly distinguish the true performance advantage of the optimized coating condition (ex: LNMA-2) over other samples.In such cases, presenting electrochemical data from multiple independent coin cells for each composition is essential to ensure statistical reliability and to avoid overinterpreting cell-to-cell variability. I recommend that the authors include cycling data (capacity retention and Coulombic efficiency) from at least three separate cells per sample. Box plots or error bars showing standard deviations would help clarify the reproducibility and significance of the performance trends.This would greatly enhance the credibility of the claimed improvements and help establish the practical robustness of the coating strategy.
3. Lack of Mechanistic Justification for LNMA-2 as the Optimized Sample – Need for Deeper Electrochemical Analysis
While the manuscript highlights LNMA-2 as the optimal sample based on cycling stability and rate capability, the rationale for its superior performance remains somewhat underdeveloped. The authors attempt to explain this by referencing EIS results and Li⁺ diffusion coefficients, yet these indicators alone do not fully elucidate why LNMA-2 achieves the best balance between coating thickness and electrochemical function. To convincingly establish LNMA-2 as the optimized condition, additional electrochemical investigations (such as direct measurements of ionic conductivity, Li⁺ transference number, or electrochemical Li⁺ mobility) are recommended. These metrics would provide quantitative insight into how the Al₂O₃ shell influences ion transport pathways at the electrode/electrolyte interface and could clarify whether the observed performance is due to enhanced interfacial kinetics, reduced side reactions, or improved Li⁺ accessibility. Such supplemental data would greatly strengthen the mechanistic argument and distinguish LNMA-2 beyond empirical observation.
Author Response
Response 1:
We are very grateful to the reviewers for their constructive suggestions. In response, we have supplemented the high-resolution mapping and the surface XPS/depth-profiling XPS analyses of LNMA-2. For specific figures S1 and S2, please refer to the attached file.
Figure S1b presents the TEM-EDS elemental mapping of LNMA-0. We observe that aluminum (Al) exhibits uniform distribution across the analyzed region, leading us to infer that Al is homogeneously dispersed throughout the entire particle. Figure S1d displays the TEM-EDS mapping of LNMA-2. Notably, Al demonstrates significantly enhanced distribution at the particle edges, evidenced by higher signal intensity (brighter contrast) in these areas. Meanwhile, aluminum maintains uniform distribution in other regions. This observation allows us to conclude that Al₂O₃ is uniformly coated on the particle surfaces.
To confirm the formation of the Al2O3 coating, characteristic Al 2p, Ni 2p, and Mn 2p XPS peaks of pristine LNMA-2 and depth-profiled LNMA-2 materials were measured and compared. As shown in Figures S2a and S2c, characteristic peaks of O and C are clearly observable in the spectra of all samples, while the primary constituent elements Ni, Mn, and Al also exhibit distinct peaks, confirming the presence of these elements in all samples. Figure S2b demonstrates that the LNMA-2 sample displays an Al 2p XPS peak, verifying the successful formation of an Al2O3 coating on the surface of LNMA-2 particles. Compared to the reported peak position of pure Al2O3 oxide (74.6 eV) in the NIST XPS Database, the collected Al 2p peak shows a slight shift, suggesting possible chemical bonding between the Al2O3 coating layer and the LNMA-2 particles. However, analysis of Figure S2d after depth profiling of LNMA-2 reveals fitted peaks corresponding to the characteristic binding energy position of Al2O3. Therefore, based on the XPS analysis, we conclude that the structure of the surface coating layer on LNMA-2 is indeed that of Al2O3.
Response 2:
We sincerely thank the reviewer for this insightful and constructive comment regarding the statistical reliability of the electrochemical differentiation between LNMA samples. The reviewer is absolutely right to emphasize the importance of multiple-cell testing to distinguish true performance advantages, especially when differences appear subtle, and to mitigate cell-to-cell variability. We fully agree that presenting data from multiple independent cells is the gold standard for establishing statistical significance and reproducibility in battery performance evaluation. We regret that the original manuscript presented data primarily from single cells for each composition in the long-term cycling tests (Figure 7). We acknowledge that this limitation makes the differentiation between samples, particularly LNMA-1, -2, and -3, less statistically robust than desired. The comprehensive electrochemical characterization across numerous samples and conditions generated a very large dataset. Furthermore, due to practical constraints during the initial testing phase (material availability, time, and resource allocation), not every sample underwent cycling with the full complement of three independent cells specifically under the long-term cycling protocol presented in Figure 7. In direct response to the reviewer’s valid concern, we have prioritized performing additional cycling tests on the key sample representing the optimized coating condition (LNMA-2). We have now successfully completed cycling tests on three independent coin cells fabricated from the LNMA-2 material under the identical conditions as Figure 7 (2.7-4.3 V, 0.5 C rate).
Figure S3a presents the capacity retention from three independent cycles of LNMA-2 tested at 0.5C between 2.7 V and 4.3 V, yielding results of 90.6%, 90.2%, and 90.0%, respectively. Figure S3b displays the calculated mean value and standard deviation of this triplicate data. The average capacity retention is 90.2%, with a standard deviation of less than 1%. This demonstrates excellent data reproducibility and enhances the statistical reliability of the results.
Response 3:
We sincerely thank the reviewer for their insightful suggestions aimed at deepening our mechanistic understanding. While supplementary tests like ionic conductivity or Li⁺ transference number could offer additional insights, we respectfully submit that our current electrochemical characterization provides robust quantitative evidence for LNMA-2's superiority.
Our conclusions are primarily based on Electrochemical Impedance Spectroscopy (EIS) and Li⁺ diffusion coefficients calculated from EIS data. These are established, standard methods in battery cathode research for quantifying interfacial kinetics and Li⁺ transport:
- EIS (Table 4):LNMA-2 exhibits the lowest charge transfer resistance (), indicating superior interfacial kinetics for enhanced rate capability.
- Li⁺ Diffusion (Table 4):LNMA-2 shows the highest value, providing direct quantitative evidence for its optimal Li⁺ transport efficiency.
- Performance Correlation:The combined evidence of minimized , maximized , and the best cycling performance (Fig.7) strongly supports that LNMA-2 achieves the ideal balance between protective interface optimization (reducing side reactions) and unimpeded Li⁺ transport.
Given that EIS-derived parameters are core, widely accepted metrics providing a clear kinetic evidence chain for LNMA-2's performance advantage within our study's screening objective, we believe supplementary tests are less critical at this stage and would introduce redundancy.
Nevertheless, we highly value the reviewer's call for deeper mechanistic insight. In the revised manuscript, we will:
Strengthen the Discussion: Fit the post-cycling data using a new equivalent circuit model and analyze the variation patterns.
Acknowledge Future Work: Explicitly mention Li⁺ transference number measurements as a valuable direction for future studies probing interfacial ion transport pathways in more detail.
We appreciate the reviewer's constructive feedback, which significantly enhances the rigor of our work.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript presents surface engineering of Al₂O₃-coated LiNi₀.₉Mn₀.₀₅Al₀.₀₅O₂ cathodes with enhanced electrochemical performance. The topic is of interest to the field of lithium-ion batteries, particularly in the context of high-Ni cobalt-free cathode design. However, several concerns must be addressed before the manuscript can be considered for publication.
- The reported initial Coulombic efficiency in the voltage profiles appears unusually high compared to typical values for Ni-rich cathode systems. Please clarify whether a formation cycle was performed prior to the initial cycle. If so, provide details and explicitly state them in the manuscript.
- In connection to the above point, the cycling stability figures should include Coulombic efficiency on the right y-axis for clarity and to assess degradation behavior over long-term cycling.
- The EIS analysis is oversimplified. Please incorporate an appropriate equivalent circuit model and conduct a more detailed fitting to extract interface-specific parameters (R_ct, R_SEI, C_dl). Comment on how these evolve with cycling.
- While the manuscript attributes performance enhancement to the protective function of the Al₂O₃ coating layer, there is insufficient mechanistic evidence to support this claim. Post-cycling characterization, such as TEM, XRD, or surface analyses (e.g., XPS, TOF-SIMS), should be included to validate suppression of CEI degradation or transition metal dissolution. These analyses are essential to substantiate the interpretation.
Author Response
Response 1:
We appreciate the reviewer’s insightful observation. To clarify, all cells underwent a activation process before the initial cycle measurements. The specific protocol was as follows:
- Voltage range: 2.7–4.3 V
- Current rate: 0.1C
- Number of cycles: 3 full charge/discharge cycles
This activation step stabilizes the electrode/electrolyte interface, thereby yielding a higher initial Coulombic efficiency. We acknowledge that omitting these details in the original manuscript may have caused ambiguity. Therefore, we have added relevant clarifications.
Thank you for highlighting this important methodological point.
Response 2:
Thank you for your valuable suggestion. We have incorporated the Coulombic efficiency data into the corresponding figure 6 and Figure 7 in the main text.
Response 3:
We sincerely thank the reviewer for their insightful suggestion regarding the electrochemical impedance spectroscopy (EIS) analysis. We agree that a more refined equivalent circuit model is essential for accurately deconvoluting interface-specific parameters. In response, we have implemented the following revisions:
Pre-cycling EIS Model Justification:
The equivalent circuit model used for the pre-cycling analysis (Fig. X, original manuscript) employed a simplified but well-established configuration widely adopted in literature for pristine electrodes [32]. This approach is validated for initial-state characterization, where interfacial complexities are negligible.
Post-cycling EIS Re-analysis:
For cycled electrodes, we acknowledge the limitations of the initial model. Therefore, in the revised manuscript (Fig. 9c), we have re-selected an appropriate equivalent circuit model to fit the relevant parameters. The fitted parameters have been reorganized into Tables 4 and 5, and the evolving patterns of post-cycling parameters have been redescribed, as follows:
In addition, after 50 cycles, the of the LNMA-0 and LNMA-2 change small. The of the LNMA-0 positive electrode becomes 273.1 Ω in the fully discharged state, while the of the LNMA-2 positive electrode is 204.3 Ω, which indicates that the values of both electrodes increase after cycling, and the of the LNMA-2 is comparatively smaller. however, the of LNMA-0 and LNMA-2 are 11.64 Ω and 8.37 Ω in table 5, respectively.
The reviewer’s valuable suggestion significantly strengthened our interfacial analysis. The updated methodology provides deeper mechanistic insights into degradation pathways, and we believe the revised manuscript now comprehensively addresses these points.
Response 4:
We sincerely thank the reviewer for their insightful suggestion regarding the CEI degradation or transition metal dissolution. Therefore, we have primarily supplemented post-cycling scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) data to demonstrate this protective effect.
Figure SI. presents the scanning electron microscopy (SEM) images of LNMA-0 and LNMA-2 cathode materials after cycling. Compared to the significant microcracks observed on the particle surface of LNMA-0 (Fig. SI(a)), the Al2O3-coated LNMA-2 particles (Fig. SI(c)) exhibit a marked reduction in microcrack formation, primarily attributed to the protective effect of the Al₂O₃ coating. Furthermore, the magnified particle interface of LNMA-0 (Fig. SI(b)) reveals extensive pore formation, whereas the enlarged surface of LNMA-2 (Fig. SI(d)) maintains dense stacking characteristics. This contrast clearly indicates that the massive dissolution of metal cations from LNMA-0 compromises its structural framework, leading to substantial interfacial changes and subsequent microcrack generation.
Figure SII displays high-resolution transmission electron microscopy (HRTEM) images of LNMA-0 and LNMA-2 after cycling. Clearly, a cathode electrolyte interphase (CEI) film has formed on the surfaces of both materials, primarily due to oxidative decomposition of electrolyte solvents (e.g., EC/DMC) and lithium salts (e.g., LiPF₆) at the cathode surface. Concurrently, electrolyte-induced dissolution of metal cations accelerated CEI growth. Comparative analysis reveals that the Al₂O₃ coating provides a stabilizing effect on the CEI layer. By suppressing metal cation dissolution, it effectively prevents excessive CEI accumulation.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsAll reply is suitable for my comments.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors have properly addressed the concerns from my end. I find the revised manuscript to be acceptable in its current form.
