Brief analysis of the top 10 research advances in 18650 rechargeable battery lithium 3.7v 3500mah ternary materials

Nickel, cobalt and manganese have the characteristics of high specific capacity, long cycle life, low toxicity and low cost. In addition, the three elements have good synergistic effects, so they are widely used. For lithium-ion battery positive electrode materials, nickel is an important component in redox energy storage. How to effectively increase the specific capacity of the material by increasing the nickel content in the material is one of the current research hotspots.

1 High nickel ternary materials

Generally speaking, high nickel ternary positive electrode materials refer to materials with a molar fraction of nickel greater than 0.6. Such ternary materials have the characteristics of high specific capacity and low cost, but they also have defects such as low capacity retention and poor thermal stability.

The material performance can be effectively improved by improving the preparation process. The micro-nano size and morphology of the particles largely determine the performance of high nickel ternary positive electrode materials. Therefore, the current important preparation method is to evenly disperse different raw materials and obtain nano-spherical particles with large specific surface area through different growth mechanisms.

Among the many preparation methods, the combination of coprecipitation and high-temperature solid phase method is the current mainstream method. First, the coprecipitation method is used to obtain a precursor with uniform raw material mixing and uniform material particle size, and then high-temperature calcination is used to obtain a ternary material with regular surface morphology and easy process control. This is an important method for industrial production at present.

The spray drying method is simpler than the coprecipitation method, with a faster preparation speed. The morphology of the obtained material is not inferior to the coprecipitation method, and there is potential for further research. The shortcomings of high-nickel ternary positive electrode materials such as cation mixing and phase change during charging and discharging can be effectively improved by doping modification and coating modification. While inhibiting the occurrence of side reactions and stabilizing the structure, improving conductivity, cycle performance, rate performance, storage performance, and high temperature and high pressure performance will continue to be a hot topic of research.

2 Lithium-rich ternary materials

This material has the characteristics of high voltage, and the first charge and discharge mechanism is different from the subsequent charge: the first charge will cause structural changes, which are reflected in the charging curve with two different platforms separated by 4.4V. During the second charge, its charging curve is different from the first curve. During the first charge, Li2O is irreversibly removed from the layered structure of Li2MnO3, and the platform around 4.5V disappears.

Lithium-rich ternary positive electrode materials with different structures can be prepared by solid phase method, sol-gel method, hydrothermal method, spray pyrolysis method and co-precipitation method. Among them, the co-precipitation method is used more, and each method has its own advantages and disadvantages.

Lithium-rich ternary materials show good application prospects and are one of the key materials required for the next generation of high-capacity lithium-ion batteries, but about large-scale application.

The future research directions of this material are mainly in the following aspects:

(1) Insufficient understanding of the mechanism of lithium insertion and extraction cannot explain the phenomenon of low Coulomb efficiency of materials and large differences in material performance;

(2) The research on doping elements is not sufficient and is relatively single;

(3) Due to the erosion of the positive electrode material by the electrolyte under high voltage, poor cycle stability is caused;

(4) There are few commercial applications and the investigation on safety performance is not comprehensive. 3 Single crystal ternary positive electrode materials

Under high voltage, as the number of cycles increases, the secondary particles or agglomerated single crystals may experience primary particle interface powderization or agglomerated single crystal separation in the later stage of 18650 rechargeable battery lithium 3.7v 3500mah ternary materials, resulting in increased internal resistance, rapid battery capacity decay, and poor cycle performance.

Single crystal high voltage ternary materials can improve the efficiency of lithium ion transfer and reduce the side reactions between the material and the electrolyte, thereby improving the material's cycle performance under high voltage. First, the ternary material precursor is prepared by coprecipitation method, and then single crystal LiNi0.5Co0.2Mn0.3O2 is obtained under high temperature solid phase.

This material has a good layered structure. At 3-4.4V, the discharge capacity of button battery 0.1 can reach 186.7mAh/g. After 1300 cycles of the full battery, the discharge capacity is still 98% of the initial discharge capacity. It is a ternary positive electrode composite material with excellent electrochemical performance.

The positive electrode material production line is the first large-scale production of micron-sized single crystal particles modified spinel lithium manganese oxide and nickel cobalt lithium manganese oxide ternary positive electrode materials in the world, reaching an annual production capacity of 500 tons.

4 Graphene doping

Graphene has a two-dimensional structure with a single-layer atomic thickness, a stable structure, and a conductivity of up to 1×106S/m. Graphene has the following advantages when used in lithium-ion batteries: ① Good electrical and thermal conductivity, which helps to improve the rate performance and safety of the battery; ② Compared with graphite, graphene has more lithium storage space, which can improve the energy density of the battery; ③ The particle size is micro-nanoscale, and the diffusion path of lithium ions is short, which is conducive to improving the power performance of the battery.

5 High-voltage electrolyte

Ternary materials have received more and more attention and research due to their high voltage window. However, due to the low electrochemical stability window of the current commercial carbonate-based electrolyte, high-voltage positive electrode materials have not yet been industrialized.

When the battery voltage reaches about 4.5 (vs. Li/Li+), the electrolyte begins to undergo violent oxidative decomposition, resulting in the inability of the battery's lithium insertion and extraction reaction to proceed normally. Improving the stability of the electrode/electrolyte interface by developing and applying new high-voltage electrolyte systems or high-voltage film-forming additives is an effective way to develop high-voltage electrolytes. In energy storage systems, ionic liquids, dinitrile organics and sulfone organic solvents are currently important as electrolytes for high-voltage ternary materials. Ionic liquids with low melting point, non-flammability, low vapor pressure and high ionic conductivity have shown excellent electrochemical stability and have been widely studied.

Replacing the currently commonly used carbonate solvents with new solvents with high-voltage stability in whole or in part can indeed effectively improve the oxidation stability of the electrolyte. In addition, most of the new organic solvents have the advantages of low flammability, which is expected to fundamentally improve the safety performance of lithium-ion batteries. However, most of the new solvents have poor reduction stability and high viscosity, which leads to reduced cycle stability of battery negative electrode materials and reduced rate performance of batteries.

In high-voltage electrolytes, film-forming additives are also essential components, and common ones include tetraphenylphosphine amide, LiBOB, lithium difluorobisoxalate borate, tetramethoxytitanium, succinic anhydride, trimethoxyphosphine, etc.

Adding a small amount (<5%) of film-forming additives to carbonate-based electrolytes allows them to undergo oxidation/reduction decomposition reactions before solvent molecules, and form an effective protective film on the electrode surface, which can inhibit the subsequent decomposition of carbonate-based solvents. The film formed by the additive with excellent performance can even inhibit the dissolution of metal ions in the positive electrode material and the deposition on the negative electrode, thereby significantly improving the electrode/electrolyte interface stability and the battery cycle performance.

6 Surfactant-assisted synthesis

The performance of ternary positive electrode materials depends on the preparation method. It is prepared by coprecipitation method, through the synergistic use of surfactants, ultrasonic vibration and mechanical stirring, and finally the prepared sheet precursor and lithium carbonate are annealed at high temperature to grow into a ternary layered structure. It is a new type of ternary positive electrode material synthesis process currently used.

It was found that the use of OA and PVP as surfactants can prepare a positive electrode material precursor with excellent morphology in the shape of regular hexagonal nanosheets, and the particle size distribution of the obtained nanosheets is relatively uniform, with a size of about 400nm. The surfactant has a good shape control function for the precursor. The assembled battery has a first discharge capacity of 157.093mAh˙g-1 at a discharge rate of 1C. The capacity retention rate is greater than 92% after 50 cycles at discharge rates of 1C, 2C, 5C and 10C, reflecting good electrochemical performance.

7 Microwave synthesis method

Among the important methods for preparing ternary positive electrode materials, the solid phase method, coprecipitation method and sol-gel method all require high-temperature sintering for several hours, which consumes a lot of energy and has a complex preparation process. Microwave heating is a volume heating caused by dielectric loss in the material in the electromagnetic field. The heating speed is fast and uniform. The synthesized materials often have better structure and performance. It is a very promising way to synthesize positive electrode materials.

The structure, micromorphology and electrochemical properties of the synthesized materials were characterized by XRD, SEM and charge and discharge. The experimental results show that the cathode material synthesized in a microwave with an output power of 1300W has a first discharge capacity of up to 185.2mAh/g and a coulombic efficiency of 84% under 0.2C charge and discharge conditions. After 30 cycles, it maintains 92.3% of the capacity (2.8-4.3V), showing good electrochemical performance and application potential. 8 Infrared synthesis method

When infrared rays irradiate the heated object, when the wavelength of the emitted infrared rays is consistent with the absorption wavelength of the heated object, the heated object absorbs the infrared rays, and the molecules and atoms inside the object resonate, resulting in strong vibration and rotation, and the vibration and rotation increase the temperature of the object to achieve the purpose of heating.

Using this heating principle, it can be used to prepare ternary cathode materials. HSIEH uses a new infrared heating calcination technology to prepare ternary materials. First, nickel, cobalt, manganese, lithium acetate is mixed with water, and then a certain concentration of glucose solution is added. The powder obtained by vacuum drying is calcined at 350℃ in an infrared box for 1h, and then calcined at 900℃ in a nitrogen atmosphere for 3h. The carbon-coated 333-type ternary cathode material is obtained in one step. In the voltage range of 2.8-4.5V, 1C discharge is performed for 50 cycles, and the capacity retention rate is as high as 94%. The first cycle discharge capacity is 170mAh/g, and 5C is 75mAh/g. The high rate performance needs to be improved.

When using the traditional high-temperature calcination method to prepare ternary cathode materials, the synthesis temperature is high, the calcination time is long, and the energy loss is large.

The study found that in a low-temperature plasma environment, the chemical activity of each reactant is high and the chemical reaction rate is fast, which can achieve the rapid preparation of ternary cathode materials. Nickel, cobalt and manganese oxides are mixed evenly with lithium carbonate, and then placed in a plasma generator. Under the condition of oxygen, the reaction is carried out at 600℃ for 20~60 minutes to obtain the ternary positive electrode material Li(Ni1/3Co1/3Mn1/3)O2.

The prepared positive electrode material has a high initial discharge capacity of 218.9mAh˙g-1, and the cycle stability, rate performance and high temperature performance are also due to the material prepared by the traditional method.

10 Preparation of ternary positive electrode materials from waste batteries

The cost of positive electrode materials for lithium-ion batteries accounts for 30%-40%. Therefore, the cost of lithium-ion batteries can be greatly reduced by recycling the positive electrode materials of waste batteries and using the preparation process to restore the energy storage performance of the positive electrode materials. Moreover, a complete lithium-ion battery industry chain should include the recycling of lithium-ion batteries.

GEM invested 100 million yuan to build the largest production line for processing waste batteries and scrapped battery materials in my country, recycling more than 4,000 tons of cobalt resources annually, accounting for more than 30% of my country's strategic cobalt resource supply, forming a special recycling route for GEM battery materials from waste batteries to new batteries.

The entire production line is made of nickel, cobalt and manganese recycled from waste batteries into a solution, adding a synthetic agent, and after a series of processes, it becomes a nickel-cobalt-manganese ternary power lithium-ion battery positive electrode material. Since it was put into production in October 2014, it has achieved an output value of nearly 200 million yuan, and it is expected to achieve an output value of 500 to 600 million yuan in the future.

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