4LR44 battery.Research progress on high-nickel lithium-ion battery cathode materials

Under the dual pressure of environmental pollution and energy crisis, finding clean green energy is the direction of joint efforts in the world today. Lithium-ion batteries occupy a very important place in clean energy, especially cars in which lithium-ion batteries are used as power sources. They have developed rapidly in recent years, and have proposed solutions to global energy and environmental issues.

Under the dual pressure of environmental pollution and energy crisis, finding clean green energy is the direction of joint efforts in the world today. Lithium-ion batteries occupy a very important place in clean energy, especially cars where lithium-ion batteries are used as power sources. They have developed rapidly in recent years, proposing a new development path in response to global energy and environmental issues [1-2]. The performance of lithium-ion batteries mainly depends on the active materials participating in the electrode reaction, and the development speed of negative electrodes is faster than that of positive electrodes [3]. Therefore, studying lithium-ion battery cathode materials has important economic and practical significance for improving the performance of lithium-ion batteries and broadening their application fields.


Lithium-ion batteries have a variety of cathode materials. LiCoO2 with a layered structure is currently an important commercial lithium-ion battery cathode material. It has excellent overall performance, but its high cost and the toxicity of Co restrict its larger-scale application. LiNiO2 has a similar crystal structure, is lower cost and more environmentally friendly, but has poor structural stability. Compared with traditional layered LiCoO2, high-nickel cathode material (Ni80%) has the advantages of high specific capacity, low cost, and long life. It is currently a research hotspot at home and abroad. It has gradually entered the commercial application stage and is considered to be extremely promising. Lithium-ion power lithium battery cathode materials with promising application prospects.


High-nickel cathode materials have the characteristics of high specific capacity and low cost, but they also have shortcomings such as low capacity retention and poor thermal stability, as shown in Figure 1 [4,5], making their commercialization difficult. The performance and structure of high-nickel cathode materials are closely related to the preparation process. Different preparation methods and modification methods directly affect the performance of the product. Lithium-ion high-nickel cathode materials, especially high-nickel ternary cathode materials LiNi0.8Co0.15A10.0502 (NCA) and LiNi0.8Co0.1Mn0.1O2 (NCM811), are currently very popular lithium-ion battery cathodes for research and application. Material. Therefore, this article reviews two popular cathode materials, NCA and NCM811, including their important preparation methods and modification research progress, and compares their important properties.


1. Preparation method of high-nickel cathode materials
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Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%
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Since high-nickel cathode materials are very sensitive to the preparation environment, battery production environment, and storage environment (temperature, humidity, oxygen value), finding a suitable preparation system has certain implications for the industrialization of the entire high-nickel cathode material. Reference value. The preparation method has a great influence on the microstructure and electrochemical properties of high-nickel layered materials. Common preparation methods include: high temperature solid phase method, co-precipitation method, sol-gel method, spray drying method and combustion method, etc.


1.1 Preparation of NCA


Cao et al. [6] prepared LiNi0.8Co0.2-xAlx02 (0x0.2) cathode material using conventional co-precipitation method. First prepare a 2mol/L mixed solution of nickel, cobalt and aluminum nitrates, drop in 4mol/L ammonia to adjust the pH to 8.5, then drop in sodium hydroxide solution until the pH reaches 11, and then add PVP dispersant. The precipitate is washed, filtered and dried to obtain the Ni-Co-A1 hydroxide precursor. Mix LiOH and precursor according to the material ratio Li/Me=1.05, roast at 600℃ for 6h, then place it in oxygen flow and bake at 750℃ for 8-24h to obtain LiNi0.8Co0.2-xAlx02. Preparation by roasting at 750℃ for 16h The NCA sample showed the highest first discharge specific capacity of 160.8mAh/g and a first coulombic efficiency of 89%, and the discharge specific capacity was still 150mAh/g after 40 cycles.


Han et al. [7] used the sol-gel method to prepare a sol at 140°C. Then it is calcined at 800°C to obtain LiNi0.8Co0.2-xAlx02 powder material. The results show that no matter how much Al (x0.05) it contains, the powder is a single-phase layered compound. In addition, it was found that with the addition of Al, the initial discharge capacity of the material decreased, but the charge and discharge performance improved. Ju et al. [8] used nitrates of Ni, Co, and Al as raw materials, citric acid and ethylene glycol as chelating agents, and used spray pyrolysis to prepare the Ni-Co-A1-O precursor, and then added LiOH at 800°C. After roasting for 0.5 to 12 hours, the obtained NCA material has a spherical morphology with an average size of 1.1μm. The discharge specific capacity is as high as 200mAh/g, and it has good cycle performance, high temperature performance and rate performance.


The NCA cathode material prepared by Hu[9] through co-precipitation method can be charged and discharged at a current density of 0.2C in the charge and discharge cut-off voltage range of 2.8V-4.3V. The material has a discharge specific capacity of 196mAh/g and remains unchanged after 50 cycles. Has a capacity retention rate of 96.1%. Chung et al. [10] used chemical adsorption method to coat the surface of NCA with a layer of amorphous carbon with a thickness of 2 to 3 nm. The carbon coating layer effectively inhibited the reaction between the matrix material and HF in the electrolyte, improving the performance of the matrix material. Thermal stability and improved electrochemical performance under high current. In fact, the use of electrochemical inert substances to modify LiNiO2-based cathode materials for lithium-ion batteries improves cycle performance and safety performance, but reduces the specific discharge capacity or energy density. Kim et al. [11] used a precipitation method to prepare A1F3 surface modified NCA cathode material. The capacity retention rate of the base material after 50 cycles was only 86.5%, while the modified material reached 96%, and the rate performance of the modified material was and thermal stability have been improved.
Low temperature and high energy density 18650 3350mAh-40℃ 0.5C discharge capacity ≥60%

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1.2 Preparation of NCM811


Xiao et al. [12] used transition metal acetates with different lithium sources to prepare NCM811 cathode materials under different conditions. The results show that the charge and discharge performance of the obtained NCM811 samples are significantly different. The specific capacity of the samples using LiOHH2O or LiNO3 as the lithium source is significantly lower than that of the samples with Li2CO3 lithium source. The sample obtained by pretreating Li2CO3 and transition metal acetate at 550℃ and then sintering at 800℃ has the best electrochemical performance. The maximum capacity of the first 20 cycles of charge and discharge at 0.2C rate is 200.8mAh/g, and the average capacity is 188.1mAh/g. .


Lu et al. [13] used sol-gel method and coprecipitation to prepare NCM811 cathode material respectively, and studied the effects of the two methods on material properties. The results showed that compared with the co-precipitation method, the NCM811 cathode material particles prepared by the sol-gel method have a hexahedral structure, the particle size is concentrated around 500nm, the layered structure is obvious, the degree of cation mixing and particle aggregation is low, and the tap density High, the first discharge specific capacity is 200.2mAh/g, and the capacity retention rate is 82.2% after 50 cycles at 0.5C.


Xiong et al. [14] prepared LiF in-situ coated NCM811 material. Figure 2 shows the EDX analysis results after NCM811 material coating. As can be seen in Figure 2, a uniform LiF layer is distributed on the surface of NCM811. The LiF coating layer effectively hinders the side reaction between HF and the electrode. After 200 cycles, the capacity retention rate of the material is 10.4% higher than that of the uncoated material. The rate performance and 60°C high-temperature cycle performance are also higher than that of the uncoated material.


The performance of high-nickel cathode materials depends largely on the size and morphology of the particles [15]. Therefore, most preparation methods focus on uniformly dispersing different raw materials to obtain spherical particles of small size and large specific surface area. The combination of co-precipitation method and high-temperature solid phase method is currently the mainstream method. The raw materials are mixed evenly in the early stage, the prepared material has uniform particle size, regular surface morphology, and the process is easy to control. It is an important method in current industrial production. The spray drying method has a simpler process and faster preparation than the co-precipitation method. The morphology of the obtained material is no less than that of the co-precipitation method, and has the potential for further research.


2. Research on modification of high-nickel cathode materials


2.1 Modification research of NCA


An important issue with NCA materials during use is capacity fading. On the one hand, the radii of Ni2+ and Li+ are very close during charging, and part of Ni2+ will occupy the vacancies of Li+, causing ion mixing and causing irreversible capacity loss of the material; on the other hand, when Ni in the material is in a high oxidation state (Ni3+ or Ni4+) has strong instability, which will cause the material structure to change at high temperatures, and is prone to side reactions with the electrolyte, causing capacity fading [16].


At present, the important improvement methods are to synthesize LiNi1xyzCoxAlyMzO2 quaternary materials by doping Mg, Mn and other elements [17,18] and surface coating of ternary materials to improve the performance of the materials. Doping can stabilize the lattice structure of the material, reduce the degree of cation mixing, reduce irreversible capacity loss during charge and discharge, and improve performance from within the material. Surface coating can reduce the direct contact area between the electrode material and the electrolyte, reduce the corrosion of the material by HF in the electrolyte, and solve the problem from outside the material [19,20]. Compared with doping, people more often use surface coating to modify materials [21].


Chung et al. [22] mixed sodium dodecyl sulfate and NCA and calcined at 600°C in air for 5 hours to obtain carbon-coated NCA/C materials. When charging and discharging at 0.1, 0.5, 1 and 3C rates in the voltage range of 2.8~4.3V, the discharge specific capacities of NCA/C are 183, 165, 140 and 83mAh/g respectively, compared to 181 of the uncoated material. , 160, 128 and 46mAh/g, which are greatly improved under high rate conditions. At the same time, the cycle performance of the material has also been improved. The capacity retention rate of NCA/C after 40 cycles at 0.1C rate is 93%, while the capacity retention rate of the uncoated material is 86%.


Huang et al. [23] found that FePO4 coating improved the cycle performance of NCA materials, but the first charge and discharge capacity of the material decreased. When electrochemically inert substances are used for coating, the discharge specific capacity and energy density of the material will be lost. On this basis, researchers proposed electrochemically active material coating. Liu et al. [24] coated the surface of NCA materials with 3.0% by weight LiCoO2 through the molten salt method. After 50 cycles under test conditions of 0.5C, 2.75~4.3V, the first discharge specific capacity of the NCA/LiCoO2 material is 163.6mAh/g, and the capacity retention rate is 95.8%, while the first discharge specific capacity of the uncoated material is 154.3 mAh/g, capacity retention rate is 87.9%. After coating, the cycle and rate performance of the material have been improved to a certain extent. The electrochemical impedance test results show that the reduction of NiO phase generated on the surface of the coating layer is an important reason for the improvement of material performance.


Yoon et al. [25] used high-energy mechanical ball milling to ball-mill NCA and graphene at 200 r/min for 30 minutes under argon protection to obtain NCA-graphene composite materials. After 80 cycles at a current of 55.6mA/g, the first discharge specific capacity of the NCA-graphene composite is 180mAh/g, and the capacity retention rate is 97%, while the first discharge specific capacity of the uncoated material is 172mAh/g. The capacity retention rate is 91%. The coated graphene enhances the material's electrical conductivity, thereby reducing the polarization of the battery. Compared with other carbon coating experiments, this method uses graphene for coating, which does not require high-temperature calcination to directly obtain the carbon source, which is more energy-saving and environmentally friendly. However, the additional cost and improvement of graphene coating caused by adding graphene must also be considered. Uniformity of coating. Chung et al. [26] used an in-situ polymerization method to coat the NCA material with a layer of PAN, which not only stabilized the material structure and delayed the increase in impedance during the material cycle, but also improved the rate performance of the material.


Lim et al. [27] prepared Li2O-2B2O3 (LBO)-coated NCA/LBO materials through a solution method. The formation process of the LBO coating layer and the transport mechanism of Li+ in the coating layer are shown in Figure 4. It can be seen that the coating layer prevents HF from corroding the electrode material and provides a good diffusion channel for Li+. After coating, the structural collapse of the electrode material and the dissolution of transition metals are inhibited, thereby improving the material's cycle performance at high temperatures of 55°C. Tested at 55°C with a current of 180mA/g, the capacity retention rate of the NCA/LBO material with a coating weight percentage of 2% after 100 cycles was 94.2%, which was much higher than the 75.3% of the uncoated material. Because of its high ionic conductivity, the rate performance of the coated material has also been improved accordingly. In addition, since the coating layer inhibits side reactions between the electrode material and the electrolyte, the coated material also exhibits good thermal stability. It can be seen that using lithium-oxide as a composite oxide to coat the NCA material can greatly improve the performance of the electrode material. Based on this, trying other oxide compositions may become a future research direction.


The nanoscale primary particles of layered high-nickel NCM materials can expand the reaction interface and shorten the diffusion path of Li+, improving the capacity and rate performance of the material, but there is also the risk of side reactions. The NCM layered material reacts with the electrolyte to form an SEI film, which increases the boundary impedance and leads to rapid capacity fading [2830]. In addition, when NCM layered materials are deeply charged at high voltage, Li/O vacancies will cause the oxidized Ni3+/4+ ions to become unstable, and the cations will migrate and form a NiO phase and a spinel phase on the electrode surface. surface reconstruction layer [31,32]. The appearance of the surface reconstruction layer will increase the diffusion kinetic resistance of Li+, leading to capacity fading. High-nickel NCM layered materials also have shortcomings such as poor high-temperature performance and low tap density, which restricts the commercial application of this material. Doping and surface coating modification are considered important methods to effectively reduce side reactions and improve the electrochemical performance and thermal stability of materials.


Wang et al. [33] found that partial replacement of O2- by F- is beneficial to stabilizing the surface structure of NCM811 material and improving the high-temperature cycle performance of the material. Yuan et al. [34] used a coprecipitation method to prepare NCM811 materials, and investigated the effects of doping of three elements, Li, Mg, and Al, on the material properties. The doping of Mg and Al makes the NCM811 material lattice regular.