Usage of nickel-cobalt-manganese ternary materials in 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 problems of high cost of lithium cobalt oxide materials, low stability of lithium manganate materials and low capacity of lithium iron phosphate at the same time, they have been successfully applied in batteries, and the application scale has been rapidly developed.

According to reports, 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, and Beiqi EV200 was launched in 2014. Chery EQ, Jianghuai iEV4, Zhongtaiyun 100, etc. all use ternary power batteries.

At the 2015 Shanghai international auto show, the share of ternary lithium battery surpassed that of lithium iron phosphate battery as a major highlight in new energy vehicles. Most domestic mainstream automobile companies, including Geely, Chery, Changan, Zotye, Zhonghua, have launched new energy models with ternary power battery. Many experts predict that ternary materials with their excellent performance and reasonable manufacturing costs are expected to replace the expensive lithium cobalt oxide materials in the near future.

It was found that the proportion of nickel-cobalt-manganese in the nickel-cobalt-manganese ternary anode materials could be adjusted within a certain range, and its performance changed with the different proportion of nickel-cobalt-manganese. A lot of work has been done in the research and development of ternary materials with different proportions of nickel-cobalt-manganese in the world. Including 333,523,811 systems and so on, some systems have been successfully industrialized and applied.

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

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 811nucleus, is 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 materials 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 333type 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, and harmful to both humans and aquatic animals, even at very low concentrations ("300mg/ 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 greatest advantage of sol-gel method is that it can achieve uniform mixing of reactants at the molecular level in a very short time, and the prepared materials have the advantages of uniform distribution of chemical components, accurate stoichiometric ratio, small particle size and narrow distribution, etc..

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 were 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. 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.

3 Nickel-cobalt-manganese ternary cathode material problem and modification

Compared with lithium iron phosphate and lithium cobalt oxide, nickel-cobalt-manganese ternary materials have the advantages of moderate cost and high specific capacity, but there are also some problems that need to be solved urgently. The main problems include: low electronic conductivity, poor stability of large rate, high voltage. Poor cycle stability, cation mixing (especially nickel-rich ternary), poor high and low temperature performance, etc. In response to these problems, it is currently mainly improved by element doping and surface coating.

3.1 ion doping modification

Adding trace amounts of other elements such as Na, V, TI, Mg, Al, Fe, Cr, Mo, Zr, Zn, Ce, B, F, and Cl to the LiNixCoyMnzO2 lattice can increase the electrons of nickel, cobalt and manganese And ionic conductivity, structural stability, reduce the degree of cation mixing, thereby improving the electrochemical properties of the material. Ion doping can be divided into cationic doping and anionic doping.

3.1.1 cationic doping

The cation doping can be further divided into equivalent cation doping and unequal cation doping.

Equivalent cation doping generally stabilizes the structure of the material, expands the ion channel, and increases the ionic conductivity of the material. GONG et al. mixed Ni1/3Co1/3Mn1/3(OH)2 prepared by coprecipitation with LiOH and NaOH and calcined at a high temperature to obtain Li0.95Na0.05Ni1/3Co1/3Mn1/3O2. The radius of Na+(0.102nm) is larger than Li+(0.076). Nm), after equivalent doping of Na+, not only the unit cell parameters c and a increase, but also c/a and I003/I104 increase, which indicates that Na doping increases the layer spacing and broadens Li+. The diffusion channel is beneficial to the rapid deintercalation of Li+. On the other hand, the Na doping reduces the degree of cation mixing, and the layered structure is more orderly and complete;

Li+ rapid deintercalation is helpful to improve the material's rate performance. The charge and discharge test shows that the performance of Na-doped materials is better than that of undoped at different magnifications (0.1~5C): 0.1C (27mA/g) doped with Na+ , 2.0~4.5V) The first ring discharge specific capacity is 250mAh/g, the undoped is only 155mAh/g, and the doping ratio of Na is 99% after 110 cycles, while the undoped top 10 The circle has been attenuated by 2.5%; electrochemical impedance shows that doping Na reduces the electron transfer impedance.

HUA and other similar methods were used to dope Na+, and Li0.97Na0.03Ni0.5Co0.2Mn0.3O2 was prepared. The conclusions obtained are consistent with GONG.

The unequal cation doping generally changes the material band structure and improves the material's electronic conductance. For nickel-rich ternary elements such as 523, 622, 811, etc., ion doping can reduce the degree of cation mixing and thus improve the electrochemical performance of the material.

In view of the fact that vanadium oxide is a good conductor of ions and electrons, ZHU et al. prepared different contents of vanadium-doped Li[Ni0.5Co0.2Mn0.3]1 by solid phase method. xVxO2 (X=0, 0.01, 0.03, 0.05), XPS shows that V is mainly V5+, and electrochemical impedance indicates that V5+ unequal doping reduces electron transfer impedance;

The XRD spectrum shows that V doping reduces the cation mixing, and the increase of the unit cell parameter c makes the Li+ deintercalation easier at different magnifications. Therefore, the material has better performance at 0.1 to 5C than the undoped; but due to V5+ electricity chemically inactive, the first discharge specific capacity of the material after doping is reduced.

HENG and other Al doping improves the high temperature cycle and storage performance of Type 523 materials. When the charge cut-off voltage is higher than 4.3V, the ternary material cycle performance deteriorates.

NAYAK et al. increased the charge cut-off voltage to 4.6V, and the 333 type capacity was rapidly attenuated. It was found by high-power lens and Raman spectroscopy that the high-pressure cycle destroyed the layered structure of 333material, and the layered structure changed to the spinel-like structure. EIS test when the charge cut-off voltage is higher than 4.4V, the electron transfer impedance increases, resulting in rapid decay of material capacity under high pressure.

In order to improve the structural stability of the material, MARKUS et al. prepared LiNi0.33Mn0.33Co0.33 by combustion method yTIyO2, found that TI4+ substituted Co3+ can inhibit the formation of secondary rock salt phase, and TI4+ radius is larger than Co3+, Ti-O bond energy M-O (M=Ni, CoMn), which can inhibit the volume change of materials during lithium deintercalation.

The Cr-doped 333 material prepared by co-precipitation method such as LIU, electrochemical impedance (EIS) test shows that Cr doping reduces the electron transfer resistance, and the capacity retention rate of the coil is up to 97% at 4.6V cut-off voltage, only 86.6 percent of the samples were unadulterated.

3.1.2 Anion doping

The anion doping is mainly F, Cl instead of O2. The chemical bond energy of F -- M (M=Ni, Co, Mn) is higher than that of M -- O, which is conducive to enhancing the stability of the material. Moreover, F doping can alleviate the corrosion of HF on the anode material in the electrolyte.

Zhang et al. prepared Cl-doped LiNi1/3Co1/3Mn1/3O2 by sol-gel method? On the one hand, xClx, Cl doping reduces the average valence of the transition metal, while the radius of the low-valent metal ion is larger, causing the unit cell parameter to increase, and on the other hand, Cl Is the radius greater than O2? , the cell parameter c is increased, the Li+ migration channel is broadened, the Li+ deintercalation is also faster, and the material rate performance is improved;

Cl doping also improved the high temperature performance of the material (x = 0.1, 55 ° C, 100 lap capacity retention rate of 91.8%, and unadulterated 82.4%); when the charge cut-off voltage rises to 4.6V, the capacity is rapidly decayed, However, the attenuation of unadulterated is more severe. YUE et al. used the low temperature solid phase method to mix and prepare the 811 or 622 ternary materials mixed with NH4F, and calcined in air at 450 °C for 5 h to obtain 811 and 622 ternary materials doped with different F content.

Although the F-doped 811 and 622 materials have a slight decrease in the first-discharge specific capacity (0.1C) at room temperature, the F-doped 811 is at a high temperature of 55 ° C, and the 50-cycle discharge specific capacity is reduced from 207 MAH/g to 204 MAH/ g, while unadulterated is reduced from 205 MAH/g to 187 MAH/g. It is clear that F-doped significantly improves the high-temperature cycle stability of the material, and XRD shows that the F-doped 811-type cycle still maintains a good layer after 100 cycles. Structure, while the unadulterated structure is changed, wherein the value of I003/I104 of doping F is greater than that of unadulterated, indicating that F doping reduces the degree of cation mixing;

Transmission electron microscopy (TEM) showed that the surface of F-doped 811 particles remained smooth after 100 cycles, while the surface morphology of unadulterated particles changed significantly. YUE et al considered that the improvement of material cycle stability is due to F-doping. The electrode is protected from HF corrosion. The cycle stability and rate performance of the F-doped 622 ternary material were improved.

3.1.3 Multi-ion co-doping

Multi-ion co-doping, the synergistic effect can significantly improve the electrochemical performance of the material.

SHIN et al. prepared a Mg-doped Type 424 precursor by carbonate co-precipitation method, and then mixed with LiNO3 and LiF and ground at a high temperature to obtain Mg and F co-doped LiNi0.4Co0.2Mn0.36Mg0.04O2. yFy (y = 0, 0.08). 1C cycle 100 cycles (3 ~ 4.5V, 1C = 170mA / g), Mg, F co-doping reduces the material's first ring discharge specific capacity, but the unadulterated retention rate is only 87%, single-doped Mg retention The rate is up to 91%, while the retention ratio of Mg and F co-doping is as high as 97%. Even if the charge cut-off voltage rises to 4.6V, Mg and F co-doped 424 ternary materials have no attenuation in 50 cycles, but unadulterated. Cycle stability deteriorates rapidly.

The EIS test shows that the co-doping of Mg and F reduces the electron transfer resistance. The differential thermal analysis shows that the Mg and F co-doped exothermic peaks shift positively and the heat of reaction decreases. The significant improvement of thermal stability is considered to be the particle surface M-F (M Protection of =Ni, CoMn). SHIN et al. believe that the significant improvement in cycle stability results from the F-doping protecting the surface of the positive electrode material from HF corrosion.

MOFID and other preparations Fe and Al co-doped LiNi0.6Mn0.2Co0.15Al0.025Fe0.025O2 by combustion method. Co-doping of Fe and Al reduces the degree of cation mixing and enhances the stability of 622 structures. Thus, the electrochemical properties of the materials are improved.

3.2 Surface coating modification

Excellent thermal stability and cycle stability are the premise of LiNixCoyMnzO2 application. Increasing the charge cut-off voltage can increase the ternary material gram capacity, but it will aggravate the side reaction between the electrolyte and the positive electrode material, and deteriorate the material cycle stability; the thermal stability and cycle stability of LiNixCoyMnzO2 are also severely tested at the working temperature or at a large rate. At low temperatures, the conductivity of the nickel-cobalt-manganese ternary material is drastically reduced and the capacity is also significantly reduced;

It is found that the surface of LiNixCoyMnzO2 particles is coated, and the coating layer as a protective layer can alleviate the corrosion of the positive electrode material by the electrolyte and inhibit the collapse of the structure, which can significantly improve the cycle stability and thermal stability of the ternary material; the conductive coating layer It can also improve the electronic conductance and ionic conductance of ternary materials, thereby improving their electrochemical performance.

3.2.1 Metal oxide coating

Al2O3 has poor conductivity but is chemically stable, which can slow down the side reaction between the electrolyte and the material, thereby improving the structural stability and electrochemical performance of the material.

YANO et al. prepared Al3O3 coated 333 ternary material by sol-gel method. Al2O3 coating significantly improved the cycle stability of the material at high charge cut-off voltage (cycle at 4.5V, 4.6V, 4.7V charge cut-off voltage). For 100 cycles, the capacity retention of the coating was 98%, 90%, and 71%, respectively, and the uncoated were 25%, 16%, and 32%, respectively. YanO et al. considered that the uncoated capacity was rapidly attenuated because of the electrode pole. The polarization of the electrode is likely to be a change in the surface structure of the electrode, and the improvement of the stability of the coated type 333 is precisely because the Al2O3 coating suppresses the polarization of the electrode and enhances the structural stability of the type 333.

YANO et al. confirmed by STEM (scanning transmission electron microscope) and EELS (electron energy loss spectrum) that rock salt phase formation occurred in the surface region of uncoated particles.

LIU et al [35] and CHEN et al. used Y2O3 and TiO2 as cladding layers respectively to improve the cycle stability of 523 and 622 ternary materials at high charge cut-off voltage: Y2O3 coated 523 with a thickness of 5-15 nm 2.8 ~ 4.6V, 1600mA / g cycle 100 ring capacity retention rate of 76.3% (114.5mAh / g on the 100th ring), while undoped only 8.3%; thickness of 25 ~ 35nm TiO2 coated 622 The capacity retention rate was 88.7% for the 50 cycles of 1C cycle at 3.0 to 4.5 V, and 78.1% for the uncoated.

Conventional humidification method is used to coat ternary materials, and the coating thickness and uniformity are difficult to control. KONG et al. deposited ZnO with a thickness of only 4.3 nm on the surface of 523 ternary materials by atomic layer deposition (ALD). ALD technology coating is more uniform, ultra-thin ZnO layer can effectively reduce the dissolution of metal ions in the electrolyte, relieve the electrode from the corrosion of the electrolyte, while its ultra-thin shape is conducive to Li+ rapid migration, ALD coating significantly improved The electrochemical performance of 523 ternary materials (at 2.5 ~ 4.5V, 55 ° C, 1C, 5C cycle 30 cycles, 60 cycles after discharge specific capacity ≥ 225.5mAh / g, and uncoated cycle to 60 laps Below 140mAh/g).

3.2.2 Metal fluoride coating

SHI et al. used a humidification method to disperse the 333 ternary materials in the LiNO3 solution, then add the NH4F solution dropwise, and evaporate at 70 ° C, and then calcine it at 500 ° C for 2 h to obtain a LiF-coated 333 ternary material. Because of the strong binding energy of F-M, the particle surface structure can be stabilized, the electrode can be protected from HF corrosion, and the surface layer conductivity of the particle is also enhanced.

Whether at high temperature (60 ° C) or low temperature (0, 20 ° C), LiF coating is better than uncoated

YANG et al. also prepared AlF3 coated 523 ternary material by humidification method, and the cycle performance at high rate was greatly improved. The retention rate of 100 cycles of 4C cycle was 98% (4C first cycle specific capacity 150mAh/g).

3.2.3 Lithium salt coating

Some lithium salts such as Li3VO4 and Li2ZrO3 are excellent conductors of Li+, and coating these lithium salts is advantageous for improving the positive electrode material ratio and low temperature performance.

WANG et al. coated a layer of 10 nanometers of Li2ZrO3 on the surface of 333. The PITT test showed that the diffusion coefficient of Li+ was increased by two times. The diffusion rate of lithium ions directly affected the electrochemical properties of the material. At a high rate of 50C, the coated 333 type discharge specific capacity is as high as 104.8mAh/g, and the 50C cycle 100-turn retention rate is 89.3%; At 20 ° C, the coverage of the coated 1 C cycle was 73.8%, while the uncoated was only 9.9%.

HUANG et al. coated 3% Li3VO4 on the surface of Type 523 material, and the capacity retention of 100 cycles of 10C cycle was 41.3% (149mAh/g in the first cycle), while only 1.4% was undoped. The test results show that the Li+ diffusion coefficient decreases with the cycle, but the decrease is less than that of Li3VO4.

3.2.4 Carbon or polymer coating

Low electron conductivity is an inherent disadvantage of nickel-cobalt-manganese ternary materials. Conductive carbon or polymer coating can improve its electronic conductance and improve its electrochemical performance. Polyethylenedioxythiophene (PEDOT) is a good electron conductor and electrochemically stable. Polyethylene glycol (PEG) is also a good conductor of Li+. The general coating does not have these two properties.

JU et al first dissolved PEDOT and PEG with N-methylpyrrolidone (NMP), then dissolved the 622 material powders in the polymer solution, stirred at 60 ° C for 4 h, and dried by filtration to obtain PEDOT-PEG double polymer coated 622 ternary materials. The electrochemical inertness of the coating, excellent ionic and electronic conductivity significantly improved the cycle stability of the 622 ternary material (100 cycle reduction of 0.5C cycle from 10.7% to 6.1%) and structural stability (TEM display cycle 100 The surface coating is still in the back of the circle, and the surface morphology is basically unchanged.)

XIONG et al. prepared a polypyrrole-coated 811 material by chemical polymerization. The electrochemically inert coating improved the stability of the material at high temperature and high charge cut-off voltage, while the good conductivity of polypyrrole improved the magnification of the 811 type performance.

MEI et al. used PEG (600) as a dispersant and carbon source, and coated a layer of carbon on the surface of Type 333 to improve the cycle stability of 333material at high charge cut-off voltage (2.8-4.6V, 1C cycle 100-turn capacity). Attenuation is less than 3%).

3.3 Other modifications

The excellent electrical conductivity and special morphology of carbon nanotubes and graphene can significantly improve the electronic conductance of LiNixCoyMnzO2.

ZHOU et al. prepared 333/Ag composites by thermal decomposition method. The multi-armed carbon nanotubes (CNTs) were dispersed in NMP. After ball milling for 2 hours, 333/Ag composites were added. After drying, 333/Ag/CNT composites were obtained. The excellent electrical conductivity of Ag and CNT and the three-dimensional conductive structure formed by CNTs significantly improved the electrochemical properties of the material: the composite capacity retention rate was 94.4% for 1 cycle of 1C cycle and 63% for pure 333.

JAN et al. mixed the graphene and 811 materials in a ratio of 1:20 for 0.5 h, dispersed in ethanol, and then ultrasonicated, and then stirred at 50 ° C for 8 h. After drying, the graphene/811 composite material was obtained, and the graphene-modified 811 type was obtained. Its capacity, cycle stability, and rate performance are all significantly improved.

WANG et al. added graphene when preparing the ternary precursor by precipitation method. The addition of the lamellar structure of the graphene structure reduces the agglomeration of the primary particles, relieves the external pressure and reduces the crushing of the secondary particles, and the three-dimensional graphene. The conductive network improves the material's high rate and cycle performance.

Different from coating and doping, HAN and so on only by simple mechanical ball milling (nano Sb2O3 and 333 or 424 material mixed at 3:100), without high temperature roasting, Sb2O3 modified 333 and 424 type, Sb2O3 addition inhibited Electrode polarization reduces electron transfer resistance and stabilizes the SEI film (electrode interface film), thereby improving the electrochemical performance of the 333 and 424 materials.

Improved synthesis process and to explore new preparation methods which can improve the performance of LiNixCoyMnzO2, such as doping, cladding and preparation can further improve the ternary composite material at high temperature, high charge cut-off voltage and low temperature conditions, the thermal stability of the structure stability, so as to improve the capacity and cycle stability, ratio of material performance.

4. Conclusion

LiNixCoyMnzO2 has gradually emerged as a positive material in the positive electrode materials due to its low preparation cost, high energy density and excellent cycle life. In the future, ternary materials in the field of electric vehicle power batteries will be one of the favorable competitors. In the future, the focus of research on ternary materials is to optimize the synthesis process and further reduce the preparation cost; to explore new preparation methods to prepare high-magnification density ternary materials with high-magnification properties such as nano-ternary and special morphology;

To the nickel-rich ternary with higher specific capacity, such as 424, 523, 622, 811, etc.; to improve the structural stability of the ternary material by doping and coating, so as to improve the specific capacity of LiNixCoyMnzO2 by increasing the charge cut-off voltage. The purpose of capacity, of course, the development of high-pressure electrolytes that match it is also one of the research priorities.

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