Documentary information
First author: Wang Fan, Liang Wenbiao
Corresponding Authors: ZHAO Yin*, YUAN Shuai*
Affiliation: Shanghai University
Research the background
Commercial NCM cathode materials are generally micron-scale polycrystalline secondary particles composed of nano-scale primary particles, and their structural stability is crucial to cell performance. Previous studies have shown that continuous surface reconstruction on the primary particle boundary of the polycrystalline NCM cathode will result in kinetic obstacles, resulting in capacity decay. In addition, the surface oxygen loss and continuous phase transformation of the primary particles can further trigger intergranular cracks, which affect the chemical-mechanical stability of the cathode material. Obviously, the phenomenon of primary interparticle degradation should be regarded as a key factor affecting the electrochemical performance and structural stability of the NCM cathode, but it is still challenging to optimize the primary grain boundary structure of the NCM cathode with a simple and effective strategy.
Brief introduction of the text
鉴于此，上海大学袁帅研究员和赵尹副研究员在国际化工类知名期刊Chemical Engineering Journal上发表题为“Surface atomic arrangement of primary particles through pre-oxidation to enhance the performance of LiNi0.8Co0.1Mn0.1O2 cathode materials”的研究论文。 该工作通过原位生成超薄岩盐相表面重构层作为一次颗粒的保护层，来改性LiNi0.8Co0.1Mn0.1O2多晶正极材料。 多种光谱分析和原子级成像表征技术表明，这种超薄的一次颗粒保护层有助于抑制NCM811正极材料的不可逆相变，并增强正极-电解质界面结构稳定性。 与原始NCM811和二次颗粒表面改性NCM811相比，基于一次颗粒改性的NCM811正极具有良好的结构和循环稳定性。
Key points of this article
Point 1: A high-nickel ternary cathode material with a thin and uniform halite facies surface reconstituted layer on the surface of the primary particles
Combined with microchannel technology, pre-oxidation treatment and spray granulation process, MP-NCM cathode materials with one-time particle surface reconstruction were successfully prepared. Figure 1a shows the primary particle surface reconstruction process of the LiNi0.8Co0.1Mn0.1O2 cathode material (the untreated sample is counted as NCM, and the secondary particle modified sample is counted as MS-NCM). HRTEM confirmed the formation of an ordered β-NiOOH phase on the surface of the modified primary particle precursor with fewer defects and impurities compared to the unmodified sample (Fig. 1e,f).
Fig. 1. (a) Schematic illustration of the synthesis process of MP-NCM. SEM image of (b) MP-NCMOH and (c) MP-NCM, and (d) magnified SEM image of the MP-NCM secondary particles and the corresponding elemental mapping. HRTEM images of (e) MP-NCMOH and (f) NCMOH secondary particles.
The XPS results showed that there was a NiOOH phase on the surface of the precursor after pre-oxidation treatment, which confirmed that the average oxidation state of transition metal (TM) was increased after pre-oxidation treatment. The subsequent solid-phase lithiation in an oxygen atmosphere further facilitated the oxidation of Ni2+ to Ni3+ (Figure 2a). The XRD refinement results showed that the lithiated MP-NCM sample exhibited a lower Li/Ni mixing, indicating that the pre-oxidation of the primary particles had a more pronounced effect on inhibiting cation mixing (Fig. 2b,c). The results of HAADF-STEM and EELS showed that MP-NCM had a higher Ni3+ content and formed an ultra-thin epitaxial NiO lithic phase on the surface of the primary particles, which could be used as a conformal protective layer for the primary particles to stabilize the layered structure of the cathode during cycling (Fig. 2d-i).
Fig. 2. (a) Ni 2p spectra, (b) XRD patterns, (c) I(003)/I(104) and cation mixing values for all cathodes. FIB cross-section images and EELS elemental valence mapping, low- and high-resolution HAADF-STEM images of (d-f) NCM and (g-i) MP-NCM.
Point 2: Excellent electrochemical performance
Figure 3a-c shows the initial charge-discharge curves and cycling performance of unmodified NCM, primary grain boundary modification (MP-NCM), and secondary particle surface modification (MS-NCM) samples over the 2.75-4.3 V voltage range. The results show that MP-NCM has a higher discharge capacity (204.7 mAh g−1) and a coulombic efficiency of 93.4%, as well as better cycling performance (200-turn capacity preservation rate of 94%). At the same time, the MP-NCM also exhibits good cycling stability at high cut-off voltages (4.5 V) and high temperatures (50 °C) (Fig. 3d,e). The rate performance test results show that the capacity retention rate of MP-NCM is higher under different current densities. The higher Li+ diffusion coefficient of MP-NCM was confirmed by GITT (Fig. 3f), which was due to its fewer defects and lower degree of cation mixing.
Fig. 3. (a) Initial capacity-voltage curves for all cathodes. Long-term cycling stability of all cathodes at (b) 4.3 V (25 ℃), (d) 4.5 V (25 ℃), and (e) 4.3 V (50 ℃). (c) Capacity-voltage curves of MP-NCM at various cycles. (f) Rate capacity. (g) Li+ diffusion coefficients for all cathodes calculated by the GITT method.
Point 3: Study on the primary grain boundary stabilization mechanism of polycrystalline NCM cathode
The phase transitions of NCM and MP-NCM during charge-discharge were analyzed by in-situ XRD, and the lattice structure of both of them showed a similar evolution trend (Fig. 4a,b). However, compared with NCM, MP-NCM as a whole showed a small change in lattice parameters. The results of in-situ XRD showed that the H2-H3 phase of MP-NCM became significantly inhibited, which significantly alleviated the accumulation of anisotropic strain between primary particles, thereby reducing crack formation. The effectiveness of the primary grain boundary modification on structural stability was further confirmed by dQ/dV, and the MP-NCM exhibited small H2→H3 reduction peak intensities in cycling, and the difference in peak intensities was not significant (Fig. 4c-e).
Fig. 4. Charge-discharge curves and the corresponding 2D contour plots for (a) NCM and (b) MP-NCM. The dQ/dV curves of (c) NCM and (d) MS-NCM and (e) MP-NCM.
The mechanical integrity of the cathode during long-term cycling was characterized by SEM (Fig. 5a-c). The results showed that the unmodified NCM cathode showed serious mechanical damage and microcracks, while no obvious cracks were observed in the MP-NCM, showing good mechanical stability. The HRTEM results showed that compared with the NCM AND MS-NCM materials, the phase transition from layered to halite phase of MP-NCM was the lightest (Fig. 5d-f). XPS was used to analyze the chemical composition of the cathode surface after cycling, and it was found that the LiF content on the MP-NCM surface was low (Fig. 5g). The HRTEM and XPS results confirmed that surface modification of nanoscale primary particles could effectively inhibit interfacial side reactions.
Compared with the intrinsic surface reconstruction layer of the unmodified NCM sample, it was found that the ultra-thin epitaxial lithic phase formed on the surface of the primary particles of the MP-NCM sample had a denser structure and fewer defects. Secondly, the surface layer of MP-NCM contains more Ni3+, resulting in a higher energy barrier for Ni migration and Li/Ni mixing. The dense and stable ultra-thin epitaxial halite phase formed on the surface of the MP-NCM primary particles (the constructed conformal surface reconfiguration layer) is thought to help prevent the continuous growth of the surface reconstituted layer, including further oxidation of the electrolyte and etching of the cathode material by HF, thereby improving the structural and cyclic stability of the NCM cathode (Fig. 5h).
Fig. 5. Cross-section SEM and HRTEM of (a, d) NCM, (b, e) MS-NCM and (c, f) MP-NCM after 200 cycles. (g) Cycled F 1 s spectrum. (h) Schematic diagram of the structural evolution of NCM and MP-NCM particles after cycling.
conclusion
In this work, a strong and dense ultra-thin epitaxial rock salt phase was formed on the surface of the primary particles by a combination of microchannel technology, pre-oxidation treatment and spray granulation process, which enhanced the stability of the interface structure and reduced the generation of microcracks, and the prepared high-nickel ternary polycrystalline cathode materials exhibited high discharge capacity (204.7 mAh g−1 at 0.1C), initial coulombic efficiency (93.4%) and capacity retention (94.0% after 200 cycles). This work reveals the potential of grain boundary modification strategies to improve the electrochemical performance of high-nickel ternary cathodes.
Links to articles
Surface atomic arrangement of primary particles through pre-oxidation to enhance the performance of LiNi0.8Co0.1Mn0.1O2 cathode materials
https://doi.org/10.1016/j.cej.2024.153503
Brief introduction of the communication operator
Researcher Yuan Shuai is a researcher and doctoral supervisor of the Nanoscience and Technology Research Center of the School of Science/Engineering Research Center for Materials Composite and Advanced Dispersion Technology of the Ministry of Education, and serves as the deputy dean of the School of Science, the deputy director of the Engineering Research Center of the Ministry of Education for Material Composite and Advanced Dispersion Technology, and the deputy dean of the Institute of Emerging Industries of Shanghai University (Jiaxing, Zhejiang). The main research directions are: (1) high-performance electrode materials, membrane materials and solid electrolyte materials; (2) charge transport/transfer behavior of micro/nanostructured materials; (3) High-safety, high-energy-density electrochemical energy storage devices. He has undertaken more than 10 national and ministerial projects, and participated in the national science and technology support projects of the Ministry of Science and Technology as a key personnel.
已在Angewandte Chemie International Edition, Advanced Energy Materials, Advanced Functional Materials, ACS Nano, Energy Storage Materials等期刊发表SCI论文150余篇，被引6000余次，H-index 48。 为两部英文专著撰写章节。 申请发明专利110余项，获得WO国际专利4项，及50余项中国授权专利。 获省部级奖励1项。 担任中国颗粒学会青年理事、上海颗粒学会秘书长、Research on Chemical Intermediates (Springer), Batteries (MDPI) 期刊编委等。
Associate Researcher Zhao Yin: Associate Researcher and Doctoral Supervisor of the Center for Nanoscience and Technology, School of Science, Shanghai University. His research focuses on the design, controllable preparation and application of functional nanomaterials in the field of electrochemical energy storage. He has undertaken the National Natural Science Foundation of China, the Natural Science Foundation of Shanghai Municipal Science and Technology Commission, the Science and Technology Research Project of Shanghai Municipal Science and Technology Commission, the International Cooperation Project of Shanghai Municipal Science and Technology Commission, the Innovation Project of Shanghai Municipal Education Commission, and the project commissioned by Essilor Group. At present, he has published more than 60 SCI papers in internationally renowned journals such as Angewandte Chemie International Edition, Chemical Engineering Journal, Applied Catalysis B: Environmental, ACS Applied Materials & Interfaces, J. Phys. Chem. B, etc. It has been cited more than 2300 times, and the H-index is 28.
Brief introduction of the first author
Wang Fan's profile: 2020 master's degree student of the Nanoscience and Technology Research Center of Shanghai University, his main research direction is the synthesis and modification of high-nickel ternary cathode materials for lithium-ion batteries.
Liang Wenbiao is currently a Ph.D. candidate in materials science and engineering at the School of Materials Science and Engineering, Shanghai University, and his main research direction is the synthesis and modification of ultra-high nickel single crystal ternary cathode materials for lithium-ion batteries. As the first author, he has published many research papers in academic journals such as Angewandte Chemie International Edition, Chemical Engineering Journal., Journal of Materiomics, etc.