【Background】
With the continuous development of lithium-ion battery technology, high-Ni cathode materials have become one of the mainstream cathode materials. However, the residual lithium on the surface of the high-Ni cathode leads to high alkalinity, which seriously affects the performance, which leads to the sintering of the cathode in a specific atmosphere and environment, which increases the cost. Different from the traditional oxygen atmosphere synthesis conditions, the sintering of high Ni materials in the air atmosphere will have many problems, too high Ni content will lead to the instability of the material in the air atmosphere, but the influence mechanism of Mn and Co elements in this is not clear, as an important component of the material, clear the role of Mn and Co is very important for the efficient design and synthesis of materials.
Based on this, the team of Professor Arumugam Manthiram from the University of Texas at Austin conducted a research conference at Chem. In a research paper titled "Roles of Mn and Co in the Air Synthesizability of Layered Oxide Cathodes for Lithium-Based Batteries", the authors pointed out that the key parameter affecting the air stability of the cathode is the average oxidation state of Ni, which has a lot to do with the content of Mn and Co. The substitution of Mn for Ni reduces the oxidation state of Ni because Mn is present in the form of Mn4+ and reduces the formation of residual lithium on the surface, which greatly improves overall air stability and thus increases the synthesis capacity in the air, but at the cost of reduced capacity. In contrast, the substitution of Co for Ni maintains the charge balance of Ni3+, as Co exists in the form of Co3+, providing a higher initial capacity, but the air stability and circulation capacity are weakened to a certain extent due to the driving force of residual lithium formation and the increase in surface reactivity.
【Graphic Guide】
The morphology of the NM70, NMC70 and NC70 cathodes after calcination with air and O2 is shown in Figure 1. SEM images of air-calcined cathode secondary particles and focused ion beam (FIB) cross-section images are first shown in Figure 1a-c. All three cathodes have a spherical secondary particle form with a diameter of ~ 10-12 μm, which is composed of this particle. From air-NM70 to air-NC70, the particle size of primary particles showed an increasing trend. The scanning electron microscopy (SEM) cross-section of the positive electrode showed that the secondary particles were aggregated and grown throughout the system, the largest of which was air-NC70. Figure 1d-f shows the SEM and cross-sectional view of the positive electrode calcined in O2. The morphology of the secondary particles of these three cathodes is similar to the diameter of air analogues, but with this particle growth is finer, longer, and denser.
Figure 1. SEM and cross-sectional SEM plots of NM70, NC70 and NCM70.
The refinement results of the powder XRD peak and the cathode powder are shown in Figure 2, indicating that the cathode is a hexagonal α-NaFeO2 structure with R3-m space group. The results of the air roasting cathode are shown in Figure 2a-c, where the Li+/Ni2+ mixture from air-NM70 to air-NC70 decreases and the I(003)/I(104) peak area ratio increases. These general trends are consistent with the current understanding of the cathode structure of Ni layered oxides. Air-NM70 has the largest proportion of Li+/Ni2+ mixing, which is due to the highest content of Mn4+ in the structure (30 mol %), which will drive the formation of an equal amount of Ni2+ to maintain charge neutrality, which in turn will exacerbate the cation mixing. The empty atmosphere-NMC70 contains a moderate proportion of Li+/Ni2+ and equal amounts of Mn4+ and Co3+ in the structure. In contrast, the O2 roasted cathode in Figure 2d-f followed the same trend as the air roasted cathode, but with a decrease in Li+/Ni2+ mixing and an increase in the I(003)/I(104) peak area ratio in all samples. This indicates that O2 calcination can make the layered crystal structure have better ordering. However, the XRD results showed that the cathode crystal structure remained good even after air calcination.
Figure 2. XRD refinement of NM70, NC70, and NCM70.
In order to further explore the influence of the structural composition of the material on the properties, the authors performed electrochemical tests on the materials. Fig. 3a-c shows the charge-discharge curves of the NM70, NMC70 and NC70 cathodes at C/10 magnification. The NM70 cathode of the O2 and air calcined samples showed significant differences, with discharge capacitances of 184 and 172 mAh g-1, respectively. The dQ dV-1 curve of the positive electrode at C/10 magnification is shown in Figure 3d-f. The peak distributions of the O2 and air calcination cathodes were similar across all tested components, which means that the calcination environment had little effect on the overall phase transition. Fig. 3 The cycling performance of O2 and air calcination cathode in g-i shows that the capacity retention rates of NM70 are 95% and 98%, that of NMC70 are 97% and 89%, and that of NC70 are 82% and 73%, respectively. NM70 cathode is the only group of materials with better calcination performance in air atmosphere than O2 atmosphere. From NM70 to NC70, the difference in the recyclability of O2 and air calcined cathodes deteriorated significantly. The dQ dV-1 curve again shows that samples with different calcination atmospheres under the same cathode composition have similar profiles, but this does not fully explain the different rate performance and cycling test results. This also implies that the phase transitions that occur in the dQ dV-1 curves are not a major factor influencing these performance differences. Therefore, there must be other deep reasons for the difference in the performance of NM70, NMC70 and NC70 calcined in O2 and air.
Figure 3. Constant current charge and discharge, capacity differentiation and cycle performance curves of three materials at room temperature.
In order to further investigate the effects of different roasting and Mn/Co environments on the cathode performance, the authors determined the oxidation state and oxygen content of Ni by inductively coupled plasma spectroscopy (ICP-OES) and iodine titration redox method. Figure 4a shows the average Ni oxidation state of all tested cathodes, from NM70 to NC70, with a tendency to increase as the Co content increases. The Ni oxidation states of the O2 calcined NM70, NMC70 and NC70 cathodes were 2.64, 2.85 and 3.0, respectively. In fact, the average Ni oxidation state of O2-NC70 was quantified as 3+ during all measured titrations. This couples well with our XRD results, which show that the least Li+/Ni2+ mixing is in O2-NC70 due to the large amount of Ni3+ retained in the transition metal layer. Oxygen content can also be quantified by incorporating specific Li and Ni ratios from ICP-OES data into redox titration results, as shown in Figure 4b. The positive oxygen content obtained by calcination of each O2 is basically the same, which is 1.99. The oxygen content of the air calcination cathode is slightly reduced, but it is also very similar, with a deviation between ~1.92-1.93. The results showed that the lack of oxygen content in the high-Ni layered oxides led to more Li+/Ni2+ mixing, which aggravated the deterioration of Li+ diffusion kinetics and the degradation of cycling performance of the cathode.
Figure 4. The average valence state of sintered Ni and the stoichiometric ratio of O in the oxygen atmosphere of different materials.
The authors measured the residual lithium content on the surface of the cathode by acid-base titration, as shown in Figure 5a. The content of LiOH and Li2CO3 is determined by methanol and water titration to ensure accurate measurement of both substances. This is consistent with the authors' previous study showing that water-based titration can induce lithium leaching from the positive electrode in the form of LiOH. The results show a sharp increase in the total residual lithium in the NC70 material, dominated by Li2CO3, because it contains the highest Ni3+ content among the three cathodes. It also has a significantly longer exposure time in air compared to O2-NC70, which further catalyzes the complete reaction of Li2CO3. Figure 5b plots the difference in LiOH and Li2CO3 content between the O2 calcination and air calcination cathodes from the data from Figure 5a. From NM70 to NMC70, the difference between the LiOH content of the O2 calcination cathode and the air calcination cathode decreased steadily, and the LiOH content difference of NC70 decreased faster, to -93 ppm. This negative value indicates that the LiOH residue on air-NC70 is less than that of O2-NC70, as shown in Figure 5a. In contrast, the difference between the Li2CO3 of the O2 roasted cathode and the air roasted cathode increased dramatically in NC70 to 588 ppm, nearly 6 times that of NMC70. This further indicates that the conversion of LiOH to Li2CO3 in air-NC70 is more complete compared to O2-NC70.
Figure 5. The residual Li content and the proportion of LiOH and Li2CO3 in different materials.
Ar+ sputtering XPS was used to detect the depth of the inner layer under the surface of the cathode. The O1s spectra in Figure 6a-c show the evolution of the main peaks in the NC70, NMC70, and NM70 cathodes during 0-10 min sputtering. The O1s peaks were OH-, metallic carbonate, and transition metal-lattice oxygen (TM-O) absorbed at 534.0, 531.7, and 529.1 eV, respectively. This coincides with previous studies of the inner layer, which consists mainly of metallic carbonates and Ni oxide oxides, as well as small amounts of hydroxides. These non-conductive species hinder the intercalation kinetics of Li+. The TM-O peak area normalized to the total area of all O1s peaks was used to analyze the depth distribution of the inner layer, as shown in Figure 6d-f. After 5 min of sputtering, the concentration of TM-O increased sharply from 17.4% to 40.2%. Even after 10 minutes of sputtering, the concentration remains largely constant, meaning that the bulk material is almost completely detected. At the same time, the air-NC70 increases much more slowly throughout the sputtering process, from 16.2% of the surface to 32.6% after 10 minutes of sputtering. The profile does not appear to have reached a steady-state value like O2-NC70. This indicates that the bulk lattice is not fully detected in the air-NC70, i.e., a thicker inner layer is formed. This is in good agreement with the residual lithium titration results and helps explain the significant loss of performance compared to O2-NC70.
Figure 6. XPS test results of sintering of different materials under air and oxygen atmosphere.
[Summary and outlook]
The composition design of the NMC cathode needs to be determined according to the Mn and Co content and the advantages brought by it, so as to achieve the best match between benefits and costs. The incorporation of a large amount of Mn can be used as a potential solution to promote cathode air synthesis, which reduces the Co content, improves the stability, and reduces the cost. A higher amount of Co incorporation increases the initial energy density, but exacerbates air instability and increases production costs. This work can promote a basic understanding of the intrinsic composition and chemical properties of high-Ni cathodes, so as to better assist in the design of high-nickel cathode materials with excellent performance.
Michael Yi, Zehao Cui, Hugo Celio, . Roles of Mn and Co in the Air Synthesizability of Layered Oxide Cathodes for Lithium-Based Batteries. Chem. Mater. 2023, DOI: 10.1021/acs.chemmater.3c02177
https://doi.org/10.1021/acs.chemmater.3c02177
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