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How to control the alkalinity of the surface of high nickel ternary materials?

Release date:2018-08-22  source:粉体网  Browse times:828
        With the rapid development of the power battery market, the cathode material of the battery has gradually become one of the main directions of industrialization research, in which the high-nickel component ternary nickel-cobalt-manganese 811 material has a high discharge specific capacity (200 mAhg-1) and low cost. The characteristics have become the hot spot of the next industrialization.

However, in the process of industrialization of high-nickel ternary materials, there is one “blocking tiger” one after another. One of the most important ones is that the high-nickel ternary material has too high pH (alkaline) during homogenization and coating. It is easy to absorb water and cause the jelly to be jelly-like, which deteriorates the processing property and affects the performance of the electrode material.

Why is the pH of the ternary material too high?

At present, the most common and mature synthesis method of ternary NCM nickel-cobalt-manganese material is to synthesize ternary material precursor by co-precipitation method and re-mix lithium high-temperature solid phase sintering. This method can achieve controlled particle size distribution and high tap density. The spherical ternary cathode material.

Preparation process of ternary materials
Experiments have confirmed that the active oxygen anion on the surface of the positive electrode material reacts with CO2 and moisture in the air to form carbonate, while lithium ions migrate from the bulk to the surface and form Li2CO3 on the surface of the material. This process is accompanied by deoxidation of the surface of the material to form a structure. A twisted surface oxide layer. Any positive electrode material, as long as it is exposed to the air, will produce carbonate, just a matter of the amount. The surface alkalization is different in the degree of difficulty in forming the surface of different kinds of positive electrode materials. The excess of lithium salt in the synthesis of ternary materials makes the excess lithium salt calcined at high temperature mainly as oxide of Li, which reacts with H2O and CO2 in the air to form LiOH and Li2CO3, which remain on the surface of the material, making the material The pH is higher.

It is well known that the higher the nickel content of the ternary material (including NCA), the lower the sintering temperature. When the molar ratio of the lithium salt to the transition metal ion is constant, the decrease in the sintering temperature leads to a decrease in the amount of volatilization of the lithium salt, which in turn leads to an increase in the lithium salt content remaining on the surface of the material, and the alkalinity of the material becomes large.

In addition, in the high Ni system, due to the limitation of the valence balance, a part of Ni in the material exists in the form of 3+, and excess Li easily forms LiOH and Li2CO3 on the surface of the material, and the higher the Ni content, the larger the alkali content on the surface. The more easily the water is absorbed during the homogenization and coating process, the jelly is formed.

At the same time, it should be noted that these residual lithium salts are not only electrochemically active, but also cause flatulence of the battery during charge and discharge of the battery due to decomposition of lithium carbonate or the like under high pressure.

How to reduce the pH of ternary materials?

We all know that high Ni ternary materials are the future application direction of high energy density power batteries, but one of the most important reasons why it has not been industrialized is that the materials are alkaline, and the requirements for production environment and process control ability are high. It is very easy to cause jelly, which is difficult in practical applications. Therefore, reducing the surface residual alkali content is very important for the application of ternary materials in batteries.

At present, the means to reduce the excessive alkalinity of the surface of high-nickel ternary materials mainly starts from four aspects:

Generally control the pH and production environment of the precursor from the source, control the temperature, atmosphere and environmental humidity of the entire production line, and strictly control the contact of materials with air.

In the mixed lithium sintering stage, the proportion of lithium salt is lowered, and the sintering system is adjusted to allow lithium to rapidly diffuse into the interior of the crystal.

Washing the material and then sintering it to reduce the surface residual alkali content, but correspondingly lose some electrical properties, which is currently a method commonly used in commerce.

Surface coating modification is also an effective method to reduce the residual alkali content on the surface of ternary materials. High-nickel NMC generally requires surface coating modification.

 
 
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