Research on Preparation and Electrochemical Performance of the High Compacted Density Ni-Co-Mn Ternary Cathode Materials ()
1. Introduction
Li-ion batteries (LIBs) have been widely used in portable electronic devices and electric vehicles due to their high specific energy, cyclic stability, and low environmental pollution [1] [2] . Especially, Nickel-cobalt-manganese ternary cathode material has become the largest incremental LIBs cathode material in the global market due to its advantages of high specific capacity. In recent years, the demand for energy density has gradually increased, so the focus of cathode material is on the nickel-rich ternary material, high voltage ternary material and high compacted density ternary material [3] . In applications, such as in the new energy vehicle market, high-nickel multi-element cathode materials and high voltage ternary materials are gradually improving their price/performance ratio by increasing their nickel content while reducing their cobalt content. However, they still face challenges, such as poor structural stability, surface microcracks and particle fragmentation, rapid decay in cycle life, poor thermal stability, and poor safety performance [4] - [12] . In addition, since the structure of spherical particles, namely the secondary particles, is inevitably fractured during the electrode rolling and the repetitive lithiation/delithiation process as a result of transient diffusion. Compared with the single crystal large particles of lithium cobalt oxide material, the ternary cathode material belongs to the small agglomerates (≤0.5 µm), the compacted density (≤3.5 g/cm3) is lower, and the energy density cannot be effectively improved. Thus, it is of significance to understand the inner structures of agglomerates.
In this study, the inner structure of agglomerates is investigated, which refers to the arrangement of plate-like primary particles in the secondary particles. The Nickel-cobalt-manganese ternary cathode material with a larger size of the inner structure of agglomerates (~2 µm) is prepared, which can not only enhance the density of the secondary spherical particles, but also improve the compaction density (~3.70 g/cm3). Therefore, the larger size of the inner structure of agglomerate has a more complete crystal structure, showing a better electrochemical performance.
2. Experimental
Synthesis of Materials. The spherical precursor Ni0.5Co0.2Mn0.3(OH)2 was synthesized by a modified co-precipitation method. Briefly, proper amounts of NiSO4, CoSO4 and MnSO4 (cationic ratio of Ni:Co:Mn = 5:2:3) were added to a strongly stirred tank reactor under nitrogen atmosphere, forming 2.0 mol∙L−1 solution. 4.0 mol∙L−1 NaOH solution and 2.0 mol∙L−1 ammonia were added at the same time. The obtained precursor was then dried at 100˚C for 12 h in the air. After drying, the precursor and Li2CO3, MgO were ball milled for 6 h. The Li/TM molar ratio was fixed at 1.05. Meanwhile, Mg element at 0%, 0.10%, 0.15%, 0.20%, 0.25% stoichiometric ratio was added. The cathode material was heated to 500˚C at a rate of 3˚C/min, then to 900˚C at a rate of 2˚C/min and finally calcined for 12 h. For comparison, the different samples were heated to 910˚C, 920˚C at a heating rate of 2˚C/min, respectively. Three cathode materials were denoted as NCM-1, NCM-2 and NCM-3.
The post-treated LiNi0.5−xCo0.2Mn0.3MgxO2 was pulped, filtered, washed, dried, broken and sieved under the solid-liquid ratio (1:5); After sifting, LiNi0.5−xCo0.2 Mn0.3MgxO2 was heated to 910˚C at a rate of 2˚C/min and finally calcined for 8 h. By comparison, the different samples were heated to 930˚C, 950˚C at a heating rate of 2˚C/min and finally calcined for 8h, respectively. Three cathode materials were denoted as NCM-4, NCM-5 and NCM-6. The powder with high pressure solid density was obtained by air flow crushing.
Materials Characterization. The particle size distribution was measured on a laser diffraction instrument (Malvern Mastersizer 3000, Malvern, UK) with dynamic light scattering methods. The sample structures were identified by using powder X-ray diffraction (XRD, Cu Ka radiation, Bruker D8 Advance). The morphology and element distribution were analyzed using a scanning electron microscope (SEM, JSM-6700F) equipped with an energy dispersive spectrometer (EDS, Horiba, EX-250). The metal ion content was determined by the inductively coupled plasma atomic emission spectrometry (ICP-AES).
Electrochemical Tests. The working electrode was prepared by mixing active materials (LiNi0.5−xCo0.2Mn0.3MgxO2, 85 wt%), carbon black conductive additive (Super P, 8 wt%), and polyvinylidene fluoride binder (PVDF, 7 wt%) dissolved in N-methylpyrrolidone (NMP). The slurry was then casted on aluminum foil and followed by drying at 120˚C for 24 h in vacuum oven. Electrolyte was a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) containing lithium hexafluorophosphate (LiPF6) and Celgard 2400 film was used as separator. The cells were assembled in an argon-filled glove box with H2O and O2 concentrations below 0.1 ppm. All the electrochemical performances were performed on a LAND CT2001A (Wuhan, China) battery program-control test system between 3.0 and 4.3 V at different charge/discharge rate 0.2C/0.2C and 1C/1C (1C = 150 mAh∙g−1) at 23 ± 2˚C.
3. Results and Discussion
3.1. Characterization of Physical Properties
Figure 1 shows the XRD patterns of three cathode samples, which corresponds to those with a-NaFeO2 structure with the R3m space group.
The structural stability is closely related to “cation mixing”, which means Ni2+ ions tend to migrate to Li+ sites resulting in various problems including capacity
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Figure 1. XRD patterns of LiNi0.5−xCo0.2Mn0.3MgxO2 cathode materials prepared at different calcination temperatures.
loss, structure deterioration and surface side reactions. Therefore, decreasing the degree of the disordering can originally reduce the performance degeneration and maintain the structural stability. Generally speaking, the intensity ratio of (003) to (104) peak (I(003)/I(104)) indicates the degree of cation mixing in the layered structure [13] . Previous result has shown that I(003)/I(104) decreases as the degree of the disordering increases. Before cycling, the peak intensity ratios of I(003)/I(104) are greater than 1.2 and the (006)/(102), (108)/(110) peaks of the three materials show obvious splits, indicating that the three materials have good layered structure. By comparison, The I(003)/I(104) ratio of NCM-2 is much higher than the others in Table 1. It is evident that the crystal structure changes less and the structure is more ordered, which is conducive to Li+ insertion/extraction. These results indicate that NCM-2 material has a good cycling performance (Figure 2).
SEM images of five LiNi0.4985Co0.2Mn0.3Mg0.015O2 cathode materials after
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Table 1. Lattice parameters of LiNi0.5−xCo0.2Mn0.3MgxO2 cathode materials prepared at different calcination temperatures.
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Figure 2. SEM images of LiNi0.5−xCo0.2Mn0.3MgxO2 cathode materials prepared at 910˚C.
slurrying treatment prepared at 910˚C, 930˚C and 950˚C calcination temperature heating rate of 2˚C/min and finally calcined for 8h are presented in Figure 3. With the increase of temperature, as a particle size increases gradually, LiNi0.4985Co0.2Mn0.3Mg0.015O2 cathode material prepared at 930˚C has the best particle size, density of compaction and electrochemical performance.
3.2. Electrochemical Performance
Subsequent electrochemical measurements were carried out to evaluate the electrochemical performances. Charge-discharge performance was characterized at 0.2C in the voltage window of 3.0 - 4.3 V by button batteries. The cycle performances were characterized at 1C for 100 cycles in the voltage window of 3.0 - 4.3 V by cylindrical batteries.
The electrochemical performances of the three groups of ternary cathode materials are presented in Table 2. The first charge-discharge specific capacities of NCM-5 are 199.0 mAh∙g−1 and 175.9 mAh∙g−1. In contrast, the first discharge specific capacities of NCM-4 and NCM-6 cathode materials are 174.5 mAh∙g−1 and 169.5 mAh∙g−1, respectively. It can be seen that the NCM-5 cathode material shows higher capacity characteristics.
The results show that LiNi0.4985Co0.2Mn0.3Mg0.0015O2 cathode material prepared at calcination temperature 930˚C has a good layered structure, and the compacted density of the electrode plate is above 3.68 g/cm3. The discharge capacity retention rate is more than 97.5% after 100 cycles at a charge-discharge rate of
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Figure 3. SEM images of LiNi0.4985Co0.2Mn0.3Mg0.015O2 cathode materials taking the Mg element prepared at different calcination temperatures.
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Table 2. Electrochemical performance of LiNi0.4985Co0.2Mn0.3Mg0.0015O2 cathode materials prepared at different calcination temperature.
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Figure 4. Cycling performance curves of LiNi0.4985Co0.2Mn0.3Mg0.0015O2 cathode material.
1C, which shows a good cyclic performance. The results are in good agreement with XRD results (Figure 4).
4. Conclusion
In this paper, the high compacted density LiNi0.4985Co0.2Mn0.3Mg0.0015O2 cathode material for lithium-ion batteries has been prepared. XRD results reflect that the cathode material shows good crystallinity and layered structure. SEM reveals that the cathode material delivers good sphericity and concentrated particle size distribution. The compacted density of the electrode plate is above 3.68 g/cm3. Electrochemical tests show that it exhibits a better cycle performance of more than 97.5% capacity retention after 100 cycles at 1C.
Acknowledgements
The authors acknowledge funding support from the Key Research and Development Program of Lanzhou Resources and Environment Voc-Tech University (No. X2022ZD-07), and Natural Science and technology development guidance plan Fund Project of Lanzhou (No. 2022-ZD-149).