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

Nickel-cobalt-manganese ternary material is a new type of lithium-ion battery cathode material developed in recent years. It has the advantages of high capacity, good cycle stability and moderate cost, because such materials can effectively overcome the high cost of lithium cobaltate materials at the same time. The problem of low stability of lithium manganate material and low capacity of lithium iron phosphate has been successfully applied in the battery, and the scale of application 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 were 2.74 billion yuan, accounting for 28.6%. In the field of power batteries, ternary materials are rising strongly. Beiqi EV200 and Chery eQ were listed in 2014. The Jianghuai iEV4 and Zhongtaiyun 100 all use ternary power batteries.

At the 2015 Shanghai International Auto Show, in the new energy vehicles, the occupancy rate of ternary lithium batteries exceeded the lithium iron phosphate battery, which became a highlight, including Geely, Chery, Changan, Zotye, Zhonghua and many other domestic mainstream car companies, a new energy vehicle with a ternary power battery. Many experts predict that ternary materials will 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 to cobalt 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 the high-cost transition metal such as cobalt-nickel The content of the cathode material and the purpose of further improving the performance of the cathode material; countries all over the world have done a lot of work in the research and development of ternary materials with different compositions of nickel, cobalt and manganese, and have proposed a number of different compositions of nickel, cobalt and manganese ternary material system, including 333, 523, 811 system and so on. Some systems have successfully achieved industrial production and application.

1Ni-cobalt-manganese ternary cathode material structure characteristics

The nickel-cobalt-manganese ternary material can be generally expressed as: LiNixCoyMnzO2, where x+y+z=1; depending on the molar ratio of the three elements (x:y:z ratio), they are respectively referred to as different systems, such as A ternary material having a molar ratio of nickel to cobalt manganese (x:y:z) of 1:1:1 in the composition, referred to as 333 for short. A system having a molar ratio of 5:2:3 is referred to as a 523 system or the like.

The ternary materials such as 333, 523 and 811 belong to the hexagonal α-NaFeO2 layered rock salt structure, as shown in Fig. 1.

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

In general, the higher the content of the active metal component, the larger the material capacity, but when the content of nickel is too high, Ni2+ will occupy the Li+ position, which will aggravate the cation mixing, resulting in a decrease in capacity. Co just inhibits the cation mixing and stabilizes the layered structure of the material; Mn4+ does not participate in the electrochemical reaction, providing safety and stability while reducing costs.

The Latest Research Progress of Preparation Technology of Nickel-Cobalt-Mn Oxide Cathode Materials

The solid phase method and the coprecipitation method are the main methods for the traditional preparation of ternary materials. In order to further improve the electrochemical performance of ternary materials, new methods such as sol-gel, spray drying, and the like, while improving the solid phase method and the co-precipitation method, Spray pyrolysis, rheological phase, combustion, thermal polymerization, stencil, electrospinning, molten salt, ion exchange, microwave assisted, infrared assisted, ultrasonic assisted, etc. are proposed.

2.1 solid phase method

The ternary material founder OHZUKU originally used solid phase method to synthesize 333 materials. The traditional solid phase method is difficult to prepare ternary materials with uniform particle size and stable electrochemical properties because of simple mechanical mixing. To this end, HE, etc., LIU, etc. use low-melting nickel-cobalt-manganese, calcined at a temperature higher than the melting point, the metal acetate is in a fluid state, the raw materials can be well mixed, and a certain amount of oxalic acid is mixed in the raw material to alleviate agglomeration. The 333, scanning electron micrograph (SEM) showed that the particle size was evenly distributed around 0.2-0.5μm, and the discharge capacity of the first cycle of 0.1C (3~4.3V) reached 161mAh/g. TAN and other 333 particles prepared by using nanorods as a manganese source have a uniform particle size distribution of 150 to 200 nm.

The primary particle size of the material prepared by the solid phase method is 100-500 nm. However, since the primary nanoparticles are easily agglomerated into secondary particles of different sizes due to high-temperature calcination, the method itself needs to be further improved.

2.2 coprecipitation method

The coprecipitation method is a method based on the solid phase method, which can solve the problems of uneven mixing and wide particle size distribution in the conventional solid phase method, and can control the raw material concentration, the dropping rate, the stirring speed, the pH value and the reaction temperature. The ternary materials with various morphologies such as core-shell structure, spherical shape and nano-flowers and uniform particle size distribution are prepared.

The raw material concentration, the dropping rate, the stirring speed, the pH value and the reaction temperature are the key factors for preparing a uniform ternary material with high vibrating density and particle size distribution. LIANG and the like are controlled by pH=11.2, the complexing agent ammonia concentration is 0.6 mol/L, and stirring. The speed of 800r/min, T=50°C, prepared 622 material with a tap density of 2.59g/cm3 and uniform particle size distribution (Fig. 3), 0.1C (2.8~4.3V) cycle 100 cycles, capacity retention rate up to 94.7 %.

In view of the high specific capacity of the 811 ternary material (up to 200 mAh/g, 2.8 to 4.3 V), the 424 ternary material provides excellent structural and thermal stability characteristics. Some researchers have tried to synthesize a ternary material with a core-shell structure (nuclear 811, shell l is 424). HOU et al. use distributed precipitation and pump 8:1:1 (continuously) into a continuous stirred reactor (CSTR). The raw material of cobalt-manganese ratio) after the formation of the 811 nucleus, pumped into a raw material solution with a ratio of nickel to cobalt manganese of 1:1:1, forming a first shell layer, and then pumping a raw solution having a composition of 4:2:2. Finally, a 523 material having a core composition of 811 and a double-layered shell having a shell composition of 333 and 424 was obtained, which was excellent in cycle performance. At 4C rate, this material has a capacity retention rate of 90.5% for 300 cycles, while the 523 prepared by conventional precipitation method is only 72.4%.

HUA et al. prepared a linear gradient of type 811 by co-precipitation method. From the core to the surface, the nickel content decreased in turn, and the manganese content increased in turn. From Table 1, it can be seen that the discharge capacity of the 811 ternary material at a large magnification is linearly distributed. And the cyclicity is significantly better than the 811 type with evenly distributed elements.

The nano ternary material has a large surface area, a short Li+ migration path, high ion and electronic conductance, and excellent mechanical strength, which can greatly improve the performance of the battery at a large rate.

HUA et al. prepared a nanoflower-like 333 type by rapid co-precipitation method, and the 3D nanoflower-like 333 types not only shortened the Li+ migration path, but also provided a special channel for Li+ and electrons. It is a good explanation why the material has excellent rate performance (2.7 ~ 4.3V, 20C fast charge, discharge specific capacity of 126mAh / g).

Due to the excellent complexing properties of ammonia and metal ions, ammonia is commonly used as a complexing agent in coprecipitation, but ammonia is corrosive and irritating, harmful to both humans and aquatic animals, even at very low concentrations (>300 mg/ L), therefore KONG and other attempts to use the low toxicity complexing agent oxalic acid and green complexing agent sodium lactate instead of ammonia, of which 523 type material prepared by sodium lactate as a complexing agent, its 0.1C, 0.2C performance is superior to ammonia as a complex Form 523 prepared by the preparation.

2.3 Sol-gel method

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

MEI and the like adopt a modified sol-gel method: adding citric acid and ethylene glycol to a certain concentration of lithium nickel cobalt manganese nitrate solution to form a sol, and then adding an appropriate amount of polyethylene glycol (PEG-600), PEG is not only dispersed And as a carbon source, a 333 ternary material with a particle size distribution of about 100 nm and a carbon-coated core-shell structure was synthesized in one step. The capacity retention rate of the 1 C cycle of 100 cycles was 97.8% (2.8 to 4.6 V, the first cycle discharge) Capacity 175mAh/g). YANG et al. investigated the effects of different preparation methods (sol-gel, solid phase method and precipitation method) on the properties of Type 424. The results of charge and discharge tests showed that the 424 material prepared by sol-gel method had 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 due to its spatial confinement and structure guiding.

WANG et al used carbon fiber (VGCFs) as a template (Fig. 4), and used VGCFs surface-COOH to adsorb metal nickel-cobalt-manganese ions and high-temperature roasting to obtain nanoporous 333 ternary materials.

On the one hand, the nanoporous 333 type particle can greatly shorten the lithium ion diffusion path. On the other hand, the electrolyte can be infiltrated into the nanopore to increase the Li+ diffusion to increase another channel, and the nanopore can also buffer the volume change of the long circulating material, thereby improving Material stability. These advantages make the Model 333 achieve excellent rate and cycle performance on water-based lithium-ion batteries: 45C charge and discharge, the first cycle discharge capacity of 108mAh / g, 180C charge, 3C discharge, cycle 50 cycles, capacity retention rate of 95%.

XIONG and the like use porous MnO2 as a template, LiOH as a precipitant, nickel-cobalt precipitated on the pores and surface of MnO2, and 333 type is obtained by high-temperature baking. 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

Spray drying method is regarded as a method for producing ternary materials due to its high degree of automation, short preparation cycle, fine particle size and narrow particle size distribution, and no industrial wastewater.

OLJACA and other methods were prepared by spray drying method. The composition was 333 ternary materials at 60-150 °C, nickel-cobalt-manganese-lithium nitrate was rapidly atomized. The water evaporated in a short time, and the raw materials were quickly mixed. The final powder was obtained. The final 333 ternary material was obtained by calcination at 900 ° C for 4 h.

OLJACA and others believe that by controlling the temperature and residence time in the pyrolysis process of raw materials, the high temperature roasting can be greatly shortened or even completely avoided, thereby achieving continuous, large-scale, one-step preparation of the final material; in addition, the particle size can be controlled by controlling the solution concentration, Factors such as nozzle droplet size. OLJACA and other materials prepared by this method have a specific discharge capacity of 167 mAh/g and a discharge specific capacity of 137 mAh/g at a large 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 can simultaneously prepare carbon-coated composite positive electrode materials.

HSIEH and other new infrared heating roasting technology used to prepare the ternary material. Firstly, the nickel-cobalt-manganese-lithium acetate salt was mixed with water, and then a certain concentration of glucose solution was added. The powder obtained by vacuum drying was calcined in an infrared box at 350 ° C for 1 h then the carbon-coated 333 composite cathode material was prepared by calcination at 900 ° C (N 2 atmosphere) for 3 h. The SEM showed that the material had a particle size of about 500 nm and slightly agglomerated. X-ray diffraction (XRD) showed that the material was good. The layered structure; in the voltage range of 2.8 ~ 4.5V, 1C discharge 50 times, the capacity retention rate is as high as 94%, the first ring discharge specific capacity is 170mAh / g (0.1C), 5C is 75mAh / g, large rate performance needs to be improve.

HSIEH also tried the medium frequency induction sintering technology, and adopted a heating rate of 200 ° C / min, in a shorter time (900 ° C, 3 h) prepared 333 material with a uniform particle size distribution of 300 ~ 600nm, the material has excellent cycle performance, but The large rate charge and discharge performance needs to be improved.

It can be seen from the above that although the solid phase method is simple in process, the material morphology and particle size are difficult to control; the coprecipitation method can prepare an electrochemical solution with narrow particle size distribution and high tap density by controlling temperature, stirring speed, pH value, etc. The ternary material with excellent performance, but the coprecipitation method requires filtration, washing and other processes to produce a large amount of industrial wastewater; the stoichiometric ratio of the material elements obtained by the sol-gel method, the spray pyrolysis method and the template method is precisely controllable, the particles are small and dispersed. Good properties, excellent material battery performance, but these methods are expensive to prepare and complex.

Sol-gel has large environmental pollution, and the spray pyrolysis waste gas needs to be recycled. The preparation of new excellent and inexpensive template reagents needs to be developed; the new infrared and medium frequency heating technology can shorten the high temperature baking time, but the heating and cooling rates are difficult to control, and the material magnification is difficult. Performance needs to be improved. For example, spray pyrolysis, templating, sol-gel, etc., can further optimize the synthesis process, using inexpensive raw materials, and is expected to achieve industrialized large-scale applications.

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