The latest research progress of nickel-cobalt-manganese ternary materials for lithium batteries

　　This article will systematically introduce the latest research progress and results of several major nickel-cobalt-manganese ternary materials in recent years, as well as some research progress in doping and coating in order to improve the performance of these materials.

　　Nickel-cobalt-manganese ternary material is a new type of lithium-ion battery cathode material developed in recent years. It has important advantages such as high capacity, good cycle stability, and moderate cost. This type of material can effectively overcome the high cost of lithium cobalt oxide materials at the same time. , The stability of lithium manganate materials is not high, and the capacity of lithium iron phosphate is low. Successful applications have been achieved in batteries, and the application scale has been rapidly developed.

　　In 2014, the output value of China's lithium-ion battery cathode materials reached 9.575 billion yuan, of which ternary materials accounted for 2.74 billion yuan, with a share of 28.6%; in the field of power batteries, ternary materials are rising strongly. The BAIC EV200 and Chery eQ launched in 2014 , JAC iEV4, Zotye Cloud 100 and others all use ternary power batteries.

　　At the 2015 Shanghai International Auto Show, in new energy vehicles, the share of ternary lithium batteries surpassed lithium iron phosphate batteries and became a highlight, including Geely, Chery, Changan, Zotye, China and other mainstream domestic auto companies have launched them. A new energy model with a ternary power battery. Many experts predict that ternary materials are expected to replace expensive lithium cobalt oxide materials in the near future due to their excellent performance and reasonable manufacturing costs.

　　It has been found that the ratio of nickel, cobalt and manganese in the nickel-cobalt-manganese ternary cathode material can be adjusted within a certain range, and its performance varies with the ratio of nickel-cobalt-manganese. Therefore, in order to further reduce high-cost transition metals such as cobalt and nickel And the purpose of further improving the performance of cathode materials; countries around the world have done a lot of work in the research and development of ternary materials with different nickel-cobalt-manganese compositions, and a number of different nickel-cobalt-manganese compositions have been proposed. Ternary material system. Including 333,523,811 systems and so on. Some systems have successfully realized industrialized production and application.

　　1 Structure characteristics of nickel-cobalt-manganese ternary cathode materials Nickel-cobalt-manganese ternary materials can usually be expressed as: LiNixCoyMnzO2, where x+y+z=1; depending on the molar ratio of the three elements (x:y:z ratio), They are called different systems, such as the ternary material with a molar ratio of nickel, cobalt and manganese (x:y:z) of 1:1:1 in the composition, referred to as type 333 for short. The system with a molar ratio of 5:2:3 is called the 523 system and so on.

　　Ternary materials such as type 333, type 523 and type 811 belong to the hexagonal α-NaFeO2 layered rock salt structure, as shown in Figure 1.

　　In the nickel-cobalt-manganese ternary material, the main valence states of the three elements are +2, +3, and +4, and Ni is the main active element. The reaction and charge transfer during charging are shown in Figure 2.

　　Generally speaking, the higher the content of active metal components, the greater the capacity of the material, but when the content of nickel is too high, it will cause Ni2+ to occupy the Li+ position, which will aggravate the mixing of cations and reduce the capacity. Co can inhibit the mixing of cations and stabilize the layered structure of the material; Mn4+ does not participate in the electrochemical reaction, which can provide safety and stability while reducing costs. 2 The latest research progress in the preparation technology of nickel-cobalt-manganese ternary cathode materials. Solid-phase method and co-precipitation method are the main methods of traditional preparation of ternary materials. In order to further improve the electrochemical performance of ternary materials, the solid phase method and co-precipitation method are improved At the same time, new methods such as sol-gel, spray drying, spray pyrolysis, rheological phase, combustion, thermal polymerization, template, electrospinning, molten salt, ion exchange, microwave assist, infrared assist, ultrasonic assist, etc. have been proposed.　　2.1 solid phase method　　 ternary materials founder OHZUKU first used the solid phase method to synthesize 333 materials. Because the traditional solid phase method only uses mechanical mixing, it is difficult to prepare ternary materials with uniform particle size and stable electrochemical performance. For this reason, HE, etc., LIU, etc. use low-melting nickel, cobalt and manganese acetate, roasted at a temperature higher than the melting point, the metal acetate becomes a fluid state, the raw materials can be mixed well, and a certain amount of oxalic acid is mixed in the raw materials to relieve agglomeration. Out of 333, scanning electron microscopy (SEM) shows that its particle size is evenly distributed around 0.2-0.5μm, and the first-cycle discharge specific capacity of 0.1C (3-4.3V) can reach 161mAh/g. TAN and others use nanorods as manganese sources to prepare 333 particles with a uniformly distributed particle size of 150-200nm.　　The primary particle size of the material prepared by the solid phase method is 100-500nm, but due to high-temperature calcination, primary nanoparticles are easily agglomerated into secondary particles of different sizes. Therefore, the method itself needs to be further improved. 2.2 Co-precipitation method Co-precipitation method is a method based on the solid phase method. It can solve the problems of uneven mixing and excessive particle size distribution in the traditional solid phase method by controlling the concentration of raw materials, the dropping rate, the stirring speed, and the pH value. And the reaction temperature can be used to prepare ternary materials with core-shell structure, spherical shape, nano-flowers and other morphologies and relatively uniform particle size distribution. The raw material concentration, dropping rate, stirring speed, pH value and reaction temperature are the key factors for preparing ternary materials with high tap density and uniform particle size distribution. LIANG et al. controlled pH=11.2, complexing agent ammonia concentration 0.6mol/L, and stirred Speed 800r/min, T=50℃, prepared 622 material with a tap density of 2.59g/cm3 and a uniform particle size distribution (Figure 3), 0.1C (2.8～4.3V) cycle for 100 cycles, the capacity retention rate is as high as 94.7 %.

　　In view of the high specific capacity of the 811 ternary material (up to 200mAh/g, 2.8～4.3V), the 424 ternary material can provide excellent structure and thermal stability characteristics. Some researchers tried to synthesize a ternary material with a core-shell structure (core is 811, shell l is 424). HOU et al. used distributed precipitation and first pumped 8:1:1 (nickel) into a continuous stirred reactor (CSTR). Cobalt-manganese ratio) raw material, after the 811 core is formed, pump the raw material solution with the ratio of nickel-cobalt-manganese of 1:1:1 to form the first shell layer, and then pump the original solution with the composition of 4:2:2 Finally, a 523 material with a core composition of 811 and a double-layer shell with a shell composition of 333 and 424 was finally prepared. Under 4C magnification, the capacity retention rate of this material after 300 cycles is 90.9%, while that of 523 prepared by traditional precipitation method is only 72.4%.

　　HUA et al. used the co-precipitation method to prepare the linear gradient type 811. From the core of the particle to the surface, the nickel content decreases successively, and the manganese content increases successively. From Table 1, it is obvious that the linear gradient distribution of the 811 ternary material has a large rate of discharge capacity. And the circulation is obviously better than the 811 type with evenly distributed elements.

　　Nano ternary materials, with large surface area, short Li+ migration path, high ion and electronic conductance, excellent mechanical strength, etc. can greatly improve the battery performance at large rates.

　　HUA et al. used the rapid co-precipitation method to prepare the nano-flower-like type 333. The 3D nano-flower-like type 333 not only shortens the Li+ migration path, but its special surface morphology provides enough channels for Li+ and electrons, which also It explains well why the material has excellent rate performance (2.7～4.3V, fast charge at 20C, discharge specific capacity up to 126mAh/g).

　　Due to the excellent complexing performance of ammonia and metal ions, the co-precipitation method generally uses ammonia as a complexing agent, but ammonia is corrosive and irritating, harmful to humans and aquatic animals, even at very low concentrations (>300mg/ L), therefore KONG et al. tried to use low-toxic complexing agent oxalic acid and green complexing agent sodium lactate to replace ammonia. Among them, sodium lactate was used as a complexing agent to prepare 523 materials, and its 0.1C and 0.2C performance were superior to ammonia as a complexing agent. Type 523 prepared by the agent.

　　2.3 Sol-gel method

　　The biggest advantage of the sol-gel method (sol-gel) is that the reactants can be uniformly mixed at the molecular level in a very short time, and the prepared material has uniform chemical composition distribution, accurate stoichiometric ratio, small particle size and distribution Narrow and other advantages.

　　MEI and others use an improved sol-gel method: add citric acid and ethylene glycol to a certain concentration of lithium nickel cobalt manganese nitrate solution to form a sol, and then add an appropriate amount of polyethylene glycol (PEG-600), PEG is not only used as a dispersion It is also used as a carbon source to synthesize a 333 ternary material with a particle size distribution of about 100nm and a carbon-coated core-shell structure. The capacity retention rate of 100 cycles at 1C is 97.8% (2.8～4.6V, the first cycle of discharge) Capacity 175mAh/g). Yang et al. investigated the influence of different preparation methods (sol-gel, solid-phase method and precipitation method) on the performance of the 424 type. The charge and discharge test results show that the 424 material prepared by the sol-gel method has a higher discharge capacity.

　　2.4 template method

　　The 　　 template method has a wide range of applications in the preparation of materials with special morphology and precise particle size by virtue of its spatial confinement effect and structure-oriented effect.

　　WANG et al. used carbon fibers (VGCFs) as a template (Figure 4), used the surface of VGCFs—COOH to adsorb metallic nickel, cobalt, and manganese ions, and calcined at high temperature to prepare nanoporous 333 ternary materials.

　　On the one hand, the nanoporous 333 particles can greatly shorten the lithium ion diffusion path, on the other hand, the electrolyte can infiltrate into the nanopores to increase another channel for Li+ diffusion. At the same time, the nanopores can also buffer the volume change of the long-cycle material, thereby improving Material stability. The above advantages make the 333 type to obtain excellent rate and cycle performance on the water-based lithium-ion battery: 45C charge and discharge, the first cycle discharge specific capacity reaches 108mAh/g, 180C charge, 3C discharge, cycle 50 cycles, the capacity retention rate reaches 95% .

　　XIONG et al. used porous MnO2 as a template and LiOH as a precipitant to precipitate nickel and cobalt on the pores and surfaces of MnO2. The 333 type was obtained by high-temperature roasting. Compared with the traditional precipitation method, the 333 ternary material prepared by the template method has More excellent rate performance and stability.

　　2.5 spray drying　　The spray drying method is regarded as a method with very broad application prospects for the production of ternary materials due to its high degree of automation, short preparation cycle, fine particles and narrow particle size distribution, and no industrial wastewater generation.

　　OLJACA et al. prepared a ternary material with a composition of 333 by spray drying. At a high temperature of 60 to 150 ℃, the nickel cobalt manganese lithium nitrate is rapidly atomized, the water evaporates in a short time, and the raw materials are quickly mixed. The final powder is obtained The final 333 ternary material is obtained by firing at 900°C for 4 hours.

　　OLJACA and others believe that by controlling the temperature and residence time during the pyrolysis of raw materials, high-temperature roasting can be greatly shortened or even completely avoided, so as to achieve continuous, large-scale, and one-step preparation to obtain the final material; in addition, the particle size can be adjusted by controlling the solution concentration, Factors such as nozzle droplet size. OLJACA and other materials prepared by this method have a discharge specific capacity of 167mAh/g at 0.2C, and a discharge specific capacity of 137mAh/g at a high rate of 10C.

　　2.6 Infrared, microwave and other new roasting methods

　　Compared with traditional resistance heating, new electromagnetic heating such as infrared and microwave can greatly shorten the high-temperature baking time and at the same time can prepare carbon-coated composite cathode materials in one step.

　　HSIEH et al. prepared the ternary material using a new infrared heating roasting technology. First, the nickel, cobalt, manganese, lithium acetate and water were mixed uniformly, and then a certain concentration of glucose solution was added. After firing at 900°C (under N2 atmosphere) for 3 hours, a carbon-coated 333 composite cathode material was prepared in one step. SEM showed that the particle size of the material was about 500nm with slight agglomeration. X-ray diffraction (XRD) spectra showed that the material has good properties. The layered structure; in the voltage range of 2.8~4.5V, 50 cycles of 1C discharge, the capacity retention rate is as high as 94%, the first cycle discharge specific capacity is 170mAh/g (0.1C), 5C is 75mAh/g, and the high rate performance needs to be improve.

　　HSIEH and others also tried intermediate frequency induction sintering technology, using 200℃/min heating rate, in a relatively short time (900℃, 3h) to prepare 333 material with a uniform particle size distribution between 300 and 600 nm. This material has excellent cycle performance, but The high-rate charge and discharge performance needs to be improved.

　　It can be seen from the above that although the solid-phase method has a simple process, it is difficult to control the morphology and particle size of the material; the co-precipitation method can prepare electrochemical products such as narrow particle size distribution and high tap density by controlling the temperature, stirring speed, and pH value. Ternary materials with excellent performance, but the co-precipitation method requires filtration, washing and other processes, resulting in a large amount of industrial wastewater; the sol-gel method, spray pyrolysis method and template method obtain the material element stoichiometric ratio accurately and controllable, and the particles are small and dispersed It has good performance and excellent battery performance, but these methods have high production costs and complex processes.

　　Sol-gel has serious environmental pollution, spray pyrolysis waste gas needs to be recycled, and the preparation of new excellent and cheap template agents needs to be developed; new infrared and intermediate frequency heating technology can shorten the high-temperature roasting time, but the heating and cooling rate is difficult to control, and the material rate is Performance needs to be improved. Such as spray pyrolysis, template method, sol-gel, etc., if the synthesis process can be further optimized and cheap raw materials are used, it is expected to realize large-scale industrial application.