【Background】
Lithium-rich manganese substrate oxide (LMR) is expected to be a cathode material for lithium-ion batteries (LIB) due to its high reversible capacity. However, issues such as oxygen release and structural degradation associated with oxygen redox can lead to voltage degradation and capacity decay during cycling. In LMR, it corresponds to U≫△. In this case, oxygen can participate in charge compensation, which releases a higher capacity. However, during the delithiumation process, oxygen oxidation leads to an increase in electron-hole density in the O2p orbital. The O(2-n)-(0<n<2) ions of localized oxygen holes are thermodynamically unstable, prompting a violent rearrangement of the crystal structure to stabilize these unstable localized O-holes. Thus, O(2-n)- ions move closer to each other along the O-O axis, resulting in O-O dimerization. This dimerization results in the release of oxygen, which triggers irreversible migration of transition metal ions, resulting in a gradual densification of the crystal structure from the surface region to the bulk region. When U/2≈△, these two bands overlap, and the electrons can achieve |The O2p state and (TM-O)* occupy the band together, so that the depth of oxygen oxidation can be adjusted, and the reversible anionic redox can be realized, so that the crystal structure has a certain flexibility. Therefore, there is an urgent need to adjust you and △ to regulate (TM-O)* to occupy the state with |The overlap between O2p states enhances the reversible oxygen redox while improving the mechanical properties of the crystal structure.
【Job Introduction】
Recently, Professor Xia Dingguo of Peking University collaborated with Chu Wangsheng, an associate researcher at the University of Science and Technology of China, to doped magnesium, zinc, copper and niobium into monodisperse Li1.2Ni0.13Co0.13Mn0.54O2 particles to construct a surface high-entropy structure. Density functional theory calculations show that the incorporation of multiple ions within the crystal lattice produces different electrostatic fields, which widen the |O2p Belt. This extension enhances the |The overlap between the O2p band and (TM-O)* occupies the band promotes reversible oxygen redox and increases the flexibility of the crystal structure. As a result, the anisotropy of modulus and lattice strain is effectively suppressed. The synthesized cathode material exhibits excellent electrochemical properties, with the MZCN-LMR cathode showing an impressive 80.5% capacity retention after 400 cycles over a voltage range of 2.1-4.6 V, with a voltage decay of only 0.7 mV per cycle. This surface high-entropy strategy provides a versatile method to enhance the redox reversibility of anions, contributing to the application of LMR. The article was published in the internationally renowned journal Energy Storage Materials. Yang Yali and Cai Junfei are the first authors of this article.
【Content Description】
Because the structural degradation mainly starts from the surface of the material, the surface energy level structure can be effectively modulated by manipulating the surface structure and improving the stability of the crystal structure. In a previous article, we synthesized a monodisperse monocrystalline lithium-rich manganese-based cathode material (MP-LMR), which makes it easier to uniformly manipulate the surface structure due to the absence of grain boundaries. Therefore, we chose MP-LMR as the starting material and introduced new elements for component optimization. Inspired by the high-entropy stabilization effects found in the fields of metal alloys, electrocatalysis and energy storage electrodes, we introduced four elements Mg, Zn, Cu and Nb into the MP-LMR in this study and named it MZCN-LMR. Mg acts as a pillar effect, effectively inhibiting the migration of transition metal ions. Zn and Nb can form strong covalent bonds with oxygen to inhibit oxygen loss. Cu can effectively improve electronic conductivity. It is expected to achieve a comprehensive improvement from the perspective of electronic structure and crystal structure.
The synchrotron radiation XRD spectra with better monochromaticity, higher luminous flux and higher phase resolution were analyzed, and no impurities related to Mg, Zn, Cu and Nb elements were found. Atomic-resolution HAADF-STEM images show the surface atomic arrangement and elemental composition of the MZCN-LMR. No cladding or amorphous layer was found, which further ruled out the possibility of doped elements existing on the crystal surface in the form of heterogeneous layers. In addition, there is also a surface layer of about 2 nm. In the surface layer, there are brighter atomic columns present in both the Li and TM layers. This may be due to the large difference in size of the four elements, doped in the transition metal layer and lithium layer, resulting in structural distortion. GPA analysis shows that there is indeed a large structural distortion in the surface layer. The results of EDS spectroscopy showed that Nb had obvious surface enrichment characteristics, Mg and Zn showed slight surface enrichment, and Cu showed uniform distribution.
Figure 1. Structural characterization. (a) Rietveld refinement of synchrotron radiation XRD plots for MZCN-LMR and (b) MP-LMR. (c) XPS depth profile of Nb 3d. (d) Atomic-resolution HAADF-STEM images of the MZCN-LMR. (e) Distortion assessment of (d) by geometric phase analysis (GPA). (f) EDS profiles of doped elements and their overlap plots with manganese. "Mixed" means that the doping elements are all overlapping. (g-i) XAS spectra of Zn K-edge, Cu K-edge, and Nb K-edge of MZCN-LMR.
2. Electrochemical performance
The MZCN-LMR has superior rate performance, with discharge specific capacities of 263, 251 and 210 mAh g-1 at 1C, 2C and 5C, respectively. It is worth noting that after the 5C test, the MZCN-LMR sample returned to the specific capacity of 293 mAh g-1 at 0.1C magnification, exceeding the 281 mAh g-1 of MP-LMR, indicating superior structural reversibility. After 400 cycles, the MZCN-LMR exhibits an ultra-high capacity retention rate of 80.5% and a very low voltage degradation of 0.7 mV/turn, and the MZCN-LMR cathode exhibits significantly enhanced electrochemical stability compared to the MP-LMR's 62.5% capacity retention rate and 1.06 mV/turn voltage decline (Fig. 4.14).
Figure 2. Electrochemical properties. (a) Comparison of charge/discharge curves of MP-LMR and MZCN-LMR cathodes at 0.1 C (20 mA g-1) and voltage ranges of 2.1-4.5 V and 2.1-4.8 V. (c) discharge capacity and (d) average discharge voltage of the two cathodes in the voltage range of 2.1-4.6 V at 1 C vs. the number of cycles.
3. DFT calculation and anionic redox reversibility
In the LMO, |The O2p concentration is located near the Fermi level, which is consistent with the results in the literature. In MZCN-LMO, O2p shows a more diffuse and uniform distribution below the Fermi level. This stems from the variety of different cations in the high-entropy structure, resulting in different electrostatic fields, which make |The O2p energy level is broadened and relatively far away from the Fermi level. The bonding and antibonding states of Mn and O were analyzed by COOP calculation (4.19c, d). In LMO, only 33.7% of (Mn-O)* occupy states and |O2p bands overlap. This condition results in highly reactive On-species that may escape from the crystal lattice by reductive elimination, as shown in Figure 4.20. In contrast, in the MZCN-LMO structure, 87.1% of the (Mn-O)* occupy the state with |The overlapping of O2p energy levels indicates that the overlap between the two bands is enhanced, and the reversibility of oxygen redox is improved.
Figure 3. Density of states distributions of (a) MZCN-LMO and (b) LMO. Shaded pink areas indicate | below the Fermi levelO2p energy range. (c) Anisotropic distribution of the shear modulus of the two samples at different lithium contents.
Figure 4. Oxygen redox reversibility Oxygen redox reversibility. (a) XPS profile evolution of the MZCN-LMR cathode in the first cycle. (b) Evolution of the O K edge RIXS of MZCN-LMR and (c) MP-LMR in different states: the first 4.8 V charge, the first 2.1 V discharge, and the 4.8 V charge at the 50th cycle. (d) mRIXS results for cycle 50 of MZCN-LMR and (e) MP-LMR at 4.8 V state of charge. Comparison of O K edge sXAS spectra collected in MZCN-LMR and MP-LMR electrodes in (f) TEY mode and (g) PFY mode.
4. Strain evolution
Throughout the charging process, the unit cell parameter c undergoes a continuous increase, which is due to the increased repulsion between the oxygen layers during the delithiumation process. It is worth noting that compared with the first discharge state, the c-value of MZCN-LMR changes by only 0.8%, while the c-value of MP-LMR increases by 0.9%, which is 12.5% higher than that of MZCN-LMR. In addition, the a-value of MZCN-LMR also changed less than that of MP-LMR. The c/a ratio, a routine measure for assessing lattice strain and reflecting the degree of variation in lattice anisotropy, shows significant differences between the two samples. Compared with the first discharge state, the c/a ratio of MP-LMR increased by 1.96%, while the growth rate of MZCN-LMR was 1.69%, which was 13.8% lower than the change trend of MP-LMR. It is worth noting that in MZCN-LMR, the c/a value successfully returns to the initial state, indicating that the anisotropic lattice strain is recoverable.
Figure 5. 25 MZCN-LMR cathode and MP-LMR cathode second turn in-situ XRD test results (test current density: 50 mA g-1).
【Conclusion】
A lithium-rich manganese-based cathode material with a surface high-entropy structure was successfully synthesized, Li1.2Ni0.13Co0.13Mn0.54O2. Firstly, the existence of surface high-entropy structures was proved by AC-STEM, Feff calculation based on absorption spectrum, DFT calculation and XPS. Then, DFT density of states analysis and crystallographic orbital overlap analysis were performed, and the high-entropy structure was conducive to broadening the O2p nonbonding orbital, strengthening the overlap with (M-O)* occupied orbitals, thereby regulating the depth of oxygen redox. Combined with DFT modulus analysis and in-situ XRD results, it is proved that the high-entropy structure can help to alleviate the stress and strain during charging and discharging, and improve the flexibility of the crystal structure. The post-cycling XAS, SEM and GPA analyses further confirmed that the surface high-entropy structure led to the improvement of oxygen stability and crystal structure flexibility, which was conducive to stabilizing the electronic and mechanical structures, and significantly improved the overall electrochemical performance. In addition, this study can also be extended to the modification of high-voltage lithium cobalt oxide, which shows mobility. The proposed high-entropy stabilization strategy provides valuable insights for the application of high-performance lithium-ion battery cathode materials, and guides the development of strategies aimed at achieving highly stable electronic and crystal structures in layered oxide cathode materials.
Yali Yang, Junfei Cai, Yuxuan Zuo, Kun Zhang, Chuan Gao, Limin Zhou, Zhenhua Chen, Wangsheng Chu, Dingguo Xia, Enhancing the stability of Li-Rich Mn-based oxide cathodes through surface high-entropy strategy, Energy Storage Materials, 2024.
https://doi.org/10.1016/j.ensm.2024.103587
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