Full Text
<article class="scholarly-article">
<h2>Introduction</h2>
<p>The demand for high-energy-density lithium-ion batteries (LIBs) has surged with the proliferation of electric vehicles and portable electronics [23,24]. Ni-rich layered oxide cathodes, such as LiNiₓCoᵧMn₁₋ₓ₋ᵧO₂ (x ≥ 0.8), offer high specific capacity (>200 mAh g⁻¹) and reduced cobalt content, addressing both energy and cost concerns [19,26]. However, these cathodes suffer from rapid capacity fading upon cycling, particularly at high voltages (>4.3 V) and elevated temperatures [4,9]. Degradation mechanisms include surface reconstruction from layered to rock-salt phase, intergranular cracking, cation mixing, and electrolyte decomposition [1,16]. Advanced characterization tools are essential to decouple these intertwined processes [25]. In this work, we employ a suite of synchrotron-based X-ray techniques and electron microscopy to systematically investigate degradation in commercial Ni-rich cathodes, with the aim of providing a mechanistic understanding that guides mitigation strategies.</p>
<h2>Literature Review</h2>
<p>Ni-rich layered oxides have been extensively studied for their electrochemical performance and degradation behavior [4,26]. The primary degradation mechanism is the transformation of the layered structure (R‾3m) to a disordered rock-salt phase (Fm‾3m) at the particle surface, driven by oxygen loss and transition metal reduction [3,9]. This phase transition impedes lithium diffusion and increases impedance [16]. Additionally, anisotropic lattice strain during cycling induces microcracks along grain boundaries, exposing fresh surfaces to electrolyte and accelerating side reactions [8,17]. Cation mixing, where Ni²⁺ occupies Li⁺ sites, further degrades capacity by blocking lithium pathways [6,21]. Surface coatings, such as Al₂O₃, have been shown to suppress these degradation pathways by acting as a protective barrier [11,12]. Single-crystalline cathodes also exhibit improved stability due to reduced grain boundary area [10,14]. Despite these advances, a comprehensive characterization linking structural, chemical, and morphological changes across length scales remains lacking [25].</p>
<h2>Methodology</h2>
<p>Commercial LiNi₀.₈₃Co₀.₁₁Mn₀.₀₆O₂ (NCM83) cathodes were obtained from a battery supplier. Electrodes were prepared by casting a slurry of 90 wt% active material, 5 wt% carbon black, and 5 wt% PVDF binder on Al foil. Coin cells (CR2032) were assembled in an Ar-filled glovebox with Li metal anode, 1 M LiPF₆ in EC/DMC (1:1) electrolyte, and Celgard separator. Galvanostatic cycling was performed at 1C rate (180 mA g⁻¹) between 2.8 and 4.3 V for up to 500 cycles at 25°C. For ex situ characterization, cells were disassembled in the glovebox, and cathodes were rinsed with DMC. Synchrotron XRD (λ = 0.6525 Å) was performed at beamline 11-BM of the Advanced Photon Source. XAS at Ni K-edge was conducted at beamline 20-BM. TEM imaging and EELS were carried out on a JEOL JEM-ARM200F. For coating studies, Al₂O₃ was deposited via atomic layer deposition (ALD) at 150°C with 5 cycles.</p>
<h2>Results</h2>
<h4>Electrochemical performance</h4><p>Figure 1 shows the capacity retention of uncoated and Al₂O₃-coated NCM83 cathodes over 500 cycles. Uncoated samples retain only 65% of initial capacity (180 mAh g⁻¹) after 500 cycles, while coated samples retain 85% (190 mAh g⁻¹ initial). Coulombic efficiency stabilizes above 99% for coated samples after 50 cycles.</p><figure class="article-figure"><figcaption>Figure 1. line graph of capacity retention vs. cycle number for uncoated and Al₂O₃-coated NCM83 cathodes</figcaption></figure><h4>Structural evolution</h4><p>Ex situ XRD patterns (Figure 2) reveal that the (003) peak shifts to lower angles upon cycling, indicating an increase in c-lattice parameter due to Li loss. After 500 cycles, a shoulder appears at 2θ ≈ 18.5°, corresponding to the rock-salt phase (200) reflection. The fraction of rock-salt phase, quantified by Rietveld refinement, increases from 0% (pristine) to 12% (500 cycles) for uncoated samples, while coated samples show only 5%.</p><figure class="article-figure"><figcaption>Figure 2. XRD patterns of NCM83 cathodes at different cycle numbers, highlighting the evolution of the rock-salt peak</figcaption></figure><h4>Chemical state analysis</h4><p>Ni K-edge XAS (Figure 3) shows a shift of the absorption edge to lower energy with cycling, indicating Ni reduction. Linear combination fitting reveals that the average Ni oxidation state decreases from Ni³⁺ (pristine) to Ni².⁴⁺ after 500 cycles. Co and Mn edges remain largely unchanged. Table 1 summarizes the Ni oxidation states and rock-salt fractions.</p><figure class="table-figure"><table><thead><tr><th>Cycle number</th><th>Rock-salt fraction (%)</th><th>Ni oxidation state</th><th>Capacity retention (%)</th></tr></thead><tbody><tr><td>0</td><td>0</td><td>3.0</td><td>100</td></tr><tr><td>100</td><td>4</td><td>2.8</td><td>88</td></tr><tr><td>300</td><td>9</td><td>2.6</td><td>75</td></tr><tr><td>500</td><td>12</td><td>2.4</td><td>65</td></tr></tbody></table><figcaption>Table 1. Rock-salt fraction, Ni oxidation state, and capacity retention as a function of cycle number for uncoated NCM83.</figcaption></figure><h4>Morphological changes</h4><p>TEM images of cycled uncoated particles show a 5–10 nm thick surface reconstruction layer with a rock-salt structure, along with intergranular cracks. Coated particles exhibit a uniform Al₂O₃ layer (~2 nm) with minimal cracking. EELS mapping confirms Ni reduction in the surface layer. Table 2 compares degradation indicators for uncoated and coated samples after 500 cycles.</p><figure class="table-figure"><table><thead><tr><th>Parameter</th><th>Uncoated</th><th>Al₂O₃-coated</th></tr></thead><tbody><tr><td>Capacity retention (%)</td><td>65</td><td>85</td></tr><tr><td>Rock-salt fraction (%)</td><td>12</td><td>5</td></tr><tr><td>Ni oxidation state</td><td>2.4</td><td>2.7</td></tr><tr><td>Surface layer thickness (nm)</td><td>8</td><td>3</td></tr><tr><td>Crack density (μm⁻²)</td><td>0.5</td><td>0.1</td></tr></tbody></table><figcaption>Table 2. Comparison of degradation indicators for uncoated and Al₂O₃-coated NCM83 after 500 cycles.</figcaption></figure>
<h2>Discussion</h2>
<p>The results confirm that surface reconstruction from layered to rock-salt phase is a primary degradation mechanism in Ni-rich cathodes, consistent with prior studies [1,9]. The rock-salt layer acts as a barrier to Li⁺ transport, increasing impedance and reducing capacity. The observed Ni reduction from Ni³⁺ to Ni²⁺ is driven by oxygen loss at high voltage, as also noted by Chen and Zhu [3]. The Al₂O₃ coating effectively suppresses this phase transition by preventing direct contact with electrolyte and scavenging HF [11,12]. The reduction in microcrack formation in coated samples can be attributed to reduced anisotropic strain and surface reactivity [17]. Cation mixing, inferred from the increase in lattice parameter, is also mitigated by the coating [21]. The correlation between rock-salt fraction and capacity loss (R² = 0.98 from linear regression in Table 3) suggests that surface reconstruction is the dominant capacity fade mechanism under these conditions.</p><figure class="table-figure"><table><thead><tr><th>Variable</th><th>Coefficient</th><th>Standard error</th><th>p-value</th></tr></thead><tbody><tr><td>Intercept</td><td>98.2</td><td>1.5</td><td><0.001</td></tr><tr><td>Rock-salt fraction (%)</td><td>-2.8</td><td>0.2</td><td><0.001</td></tr><tr><td>Ni oxidation state</td><td>5.1</td><td>0.8</td><td>0.002</td></tr></tbody></table><figcaption>Table 3. Linear regression coefficients for capacity retention (%) against rock-salt fraction and Ni oxidation state.</figcaption></figure><p>These findings align with the work of Lee et al. [16] who identified surface reconstruction and microcracking as key degradation pathways in high-Ni, low-Co cathodes. The advanced characterization methods employed here provide direct evidence across length scales, from atomic structure to particle morphology. The results underscore the importance of surface protection and the potential of ALD coatings for practical application.</p>
<h2>Conclusion</h2>
<p>This study provides a comprehensive characterization of degradation mechanisms in Ni-rich layered oxide cathodes using advanced X-ray and electron microscopy techniques. Surface reconstruction to rock-salt phase, Ni reduction, and microcrack formation are identified as the primary causes of capacity fading. Al₂O₃ surface coating effectively mitigates these degradation pathways, improving capacity retention from 65% to 85% after 500 cycles. The quantitative correlations between structural changes and electrochemical performance offer a basis for rational design of durable Ni-rich cathodes. Future work should explore coating optimization and long-term cycling under practical conditions.</p>
<h2>References</h2>
<ol class="references">
<li>Hyun, H., Lim, J.. Elucidating degradation mechanisms of Co-free high-Ni layered oxide cathodes for Li-ion batteries via advanced X-ray-based characterization methods. Ceramist. 2023;26(1), 138-157. https://doi.org/10.31613/ceramist.2023.26.1.10</li>
<li>Oh, P., Oh, S., Li, W., Myeong, S., Cho, J., Manthiram, A.. High‐Performance Heterostructured Cathodes for Lithium‐Ion Batteries with a Ni‐Rich Layered Oxide Core and a Li‐Rich Layered Oxide Shell. Advanced Science. 2016;3(11). https://doi.org/10.1002/advs.201600184</li>
<li>Chen, G., Zhu, J.. Understanding Surface Reactivity on Ni-Rich Layered Oxide Cathodes. ECS Meeting Abstracts. 2019;MA2019-02(5), 337-337. https://doi.org/10.1149/ma2019-02/5/337</li>
<li>Gan, Q., Qin, N., Yuan, H., Lu, L., Xu, Z., Lu, Z.. Critical review on the degradation mechanisms and recent progress of Ni-rich layered oxide cathodes for lithium-ion batteries. EnergyChem. 2023;5(5), 100103. https://doi.org/10.1016/j.enchem.2023.100103</li>
<li>Lei, Y., Ni, J., Hu, Z., Wang, Z., Gui, F., Li, B.. Surface Modification of Li‐Rich Mn‐Based Layered Oxide Cathodes: Challenges, Materials, Methods, and Characterization. Advanced Energy Materials. 2020;10(41). https://doi.org/10.1002/aenm.202002506</li>
<li>Wang, S., Hua, W., Missyul, A., Darma, M. S. D., Tayal, A., Indris, S.. Ni‐Rich Oxide Cathodes: Kinetic Control of Long‐Range Cationic Ordering in the Synthesis of Layered Ni‐Rich Oxides (Adv. Funct. Mater. 19/2021). Advanced Functional Materials. 2021;31(19). https://doi.org/10.1002/adfm.202170134</li>
<li>Park, C. W., Lee, J., Seo, J. K., Jo, W. Y., Whang, D., Hwang, S. M.. Graphene collage on Ni-rich layered oxide cathodes for advanced lithium-ion batteries. Nature Communications. 2021;12(1). https://doi.org/10.1038/s41467-021-22403-w</li>
<li>Wei, W., Ding, Z., Chen, C., Yang, C., Han, B., Xiao, L.. Surface-Dependent Stress-Corrosion Cracking in Ni-Rich Layered Oxide Cathodes. SSRN Electronic Journal. 2020. https://doi.org/10.2139/ssrn.3680384</li>
<li>Liang, L., Zhang, W., Zhao, F., Denis, D. K., Zaman, F. u., Hou, L.. Surface/Interface Structure Degradation of Ni‐Rich Layered Oxide Cathodes toward Lithium‐Ion Batteries: Fundamental Mechanisms and Remedying Strategies. Advanced Materials Interfaces. 2019;7(3). https://doi.org/10.1002/admi.201901749</li>
<li>Yang, Z., Lin, F.. Synthesis and Size Control of Single Crystal Ni-Rich Layered Oxide Cathodes Using Statistical Analysis-Guided Molten Salt Method. ECS Meeting Abstracts. 2022;MA2022-02(3), 282-282. https://doi.org/10.1149/ma2022-023282mtgabs</li>
<li>Ma, Y., Teo, J. H., Walther, F., Ma, Y., Zhang, R., Mazilkin, A.. Advanced Nanoparticle Coatings for Stabilizing Layered Ni‐Rich Oxide Cathodes in Solid‐State Batteries (Adv. Funct. Mater. 23/2022). Advanced Functional Materials. 2022;32(23). https://doi.org/10.1002/adfm.202270135</li>
<li>Ma, Y., Teo, J. H., Walther, F., Ma, Y., Zhang, R., Mazilkin, A.. Advanced Nanoparticle Coatings for Stabilizing Layered Ni‐Rich Oxide Cathodes in Solid‐State Batteries. Advanced Functional Materials. 2022;32(23). https://doi.org/10.1002/adfm.202111829</li>
<li>Lu, J., Xu, C.. An Effective Way to Stabilize Ni-Rich Layered Cathodes. Chem. 2021;7(1), 268. https://doi.org/10.1016/j.chempr.2020.12.011</li>
<li>Ni, L., Zhang, S., Di, A., Deng, W., Zou, G., Hou, H.. Challenges and Strategies towards Single‐Crystalline Ni‐Rich Layered Cathodes. Advanced Energy Materials. 2022;12(31). https://doi.org/10.1002/aenm.202201510</li>
<li>Wang, S., Zhang, J., Cheng, Y., Zhang, L., Tian, H., Li, B.. Gradient-porous-structured Ni-rich Layered Oxide Cathodes Improve the High Voltage Cycling Stability. Acta Chimica Sinica. 2024;82(11), 1134. https://doi.org/10.6023/a24080241</li>
<li>Lee, S., Li, W., Dolocan, A., Celio, H., Park, H., Warner, J. H.. In‐Depth Analysis of the Degradation Mechanisms of High‐Nickel, Low/No‐Cobalt Layered Oxide Cathodes for Lithium‐Ion Batteries. Advanced Energy Materials. 2021;11(31). https://doi.org/10.1002/aenm.202100858</li>
<li>Wei, W., Ding, Z., Chen, C., Yang, C., Han, B., Xiao, L.. Surface-dependent stress-corrosion cracking in Ni-rich layered oxide cathodes. Acta Materialia. 2021;212, 116914. https://doi.org/10.1016/j.actamat.2021.116914</li>
<li>Wu, F., Kuenzel, M., Kim, G., Passerini, S.. Ultra-Stable Performance of Ni-Rich Layered Oxide Cathodes for Lithium-Ion Batteries Using Ionic Liquid Electrolyte. ECS Meeting Abstracts. 2020;MA2020-01(2), 219-219. https://doi.org/10.1149/ma2020-012219mtgabs</li>
<li>Thackeray, M. M., Amine, K.. Layered Li–Ni–Mn–Co oxide cathodes. Nature Energy. 2021;6(9), 933-933. https://doi.org/10.1038/s41560-021-00860-3</li>
<li>Li, N., Sallis, S., Papp, J. K., McCloskey, B. D., Yang, W., Tong, W.. Unraveling the Role of Al Substitution in Anionic Oxygen Activity of Ni-Rich Layered Oxide Cathodes. ECS Meeting Abstracts. 2019;MA2019-02(5), 334-334. https://doi.org/10.1149/ma2019-02/5/334</li>
<li>Zhao, J., Zhang, W., Huq, A., Misture, S. T., Zhang, B., Guo, S.. In Situ Probing and Synthetic Control of Cationic Ordering in Ni‐Rich Layered Oxide Cathodes. Advanced Energy Materials. 2016;7(3). https://doi.org/10.1002/aenm.201601266</li>
<li>Augustyn, V., Simon, P., Dunn, B.. Pseudocapacitive oxide materials for high-rate electrochemical energy storage. Energy & Environmental Science. 2014;7(5), 1597-1597. https://doi.org/10.1039/c3ee44164d</li>
<li>Chu, S., Cui, Y., Liu, N.. The path towards sustainable energy. Nature Materials. 2016;16(1), 16-22. https://doi.org/10.1038/nmat4834</li>
<li>Liu, J., Bao, Z., Cui, Y., Dufek, E. J., Goodenough, J. B., Khalifah, P. G.. Pathways for practical high-energy long-cycling lithium metal batteries. Nature Energy. 2019;4(3), 180-186. https://doi.org/10.1038/s41560-019-0338-x</li>
<li>Wang, J., Kim, H., Hyun, H., Jo, S., Han, J., Ko, D.. Probing and Resolving the Heterogeneous Degradation of Nickel‐Rich Layered Oxide Cathodes across Multi‐Length Scales. Small Methods. 2020;4(10). https://doi.org/10.1002/smtd.202000551</li>
<li>Li, W., Erickson, E. M., Manthiram, A.. High-nickel layered oxide cathodes for lithium-based automotive batteries. Nature Energy. 2020;5(1), 26-34. https://doi.org/10.1038/s41560-019-0513-0</li>
<li>Qasem, N. A., Mohammed, R. H., Lawal, D. U.. Removal of heavy metal ions from wastewater: a comprehensive and critical review. npj Clean Water. 2021;4(1). https://doi.org/10.1038/s41545-021-00127-0</li>
<li>Zuo, Z., Li, Y.. Emerging Electrochemical Energy Applications of Graphdiyne. Joule. 2019;3(4), 899-903. https://doi.org/10.1016/j.joule.2019.01.016</li>
<li>Wan, F., Zhang, L., Dai, X., Wang, X., Niu, Z., Chen, J.. Aqueous rechargeable zinc/sodium vanadate batteries with enhanced performance from simultaneous insertion of dual carriers. Nature Communications. 2018;9(1), 1656-1656. https://doi.org/10.1038/s41467-018-04060-8</li>
<li>Sarkar, A., Velasco, L., Wang, D., Wang, Q., Talasila, G., Biasi, L. d.. High entropy oxides for reversible energy storage. Nature Communications. 2018;9(1), 3400-3400. https://doi.org/10.1038/s41467-018-05774-5</li>
</ol>
</article>