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Naming rules

Traditionally, when we talk about ternary materials, we generally refer to nickel cobalt manganese oxide lithium NCM positive electrode materials (in fact, there are also negative electrode ternary materials). Ni, Co, Mn, and other three metal elements can be used in different ratios to obtain different types of ternary materials.

The general formula is LiNi1-x-yCoxMnyO2, and common ratios include 11142452362811. Please note that the order of the above ratios is N: C: M, which is different in China and abroad.

Another point to mention is that although NCA materials are often mentioned together with NCM, they can be accurately classified as binary high Ni materials and cannot be classified as ternary materials.

Comparison of synthesis methods

Chemical co precipitation method: Generally, chemical raw materials are mixed in a solution state, and appropriate precipitants are added to the solution to co precipitate the various components that have been uniformly mixed in the solution according to the stoichiometric ratio, or an intermediate product is first reacted and precipitated in the solution, and then calcined and decomposed to prepare fine powder.

The chemical co precipitation method is divided into direct chemical co precipitation method and indirect chemical co precipitation method.

The direct chemical co precipitation method involves co precipitation of salts of Li, Ni, Co, and Mn, followed by filtration, washing, and drying before high-temperature calcination. The indirect chemical co precipitation method is to first synthesize a ternary mixture of Ni, Co, and Mn co precipitation, then filter, wash, and dry it, and mix it with lithium salt for sintering; Alternatively, after the formation of Ni, Co, Mn ternary mixed co precipitates, the solution containing lithium salts and mixed co precipitates can be evaporated or freeze-dried without filtration, and then the dried material can be subjected to high-temperature calcination.

Compared with traditional solid-phase synthesis techniques, the use of co precipitation method can achieve molecular or atomic stoichiometric mixing of materials, making it easy to obtain precursors with small particle size and uniform mixing. The calcination temperature is lower, the composition of the synthesized product is uniform, the reproducibility is good, the conditions are easy to control, the operation is simple, and this method is adopted for commercial production.

Solid phase synthesis method: Generally, nickel cobalt manganese and lithium hydroxides, carbonates or oxides are used as raw materials, mixed according to the corresponding amounts of substances, and calcined at 700-1000 ℃ to obtain the product. This method mainly uses mechanical means to mix and refine raw materials, which can easily lead to uneven micro distribution of raw materials, making the diffusion process difficult to proceed smoothly. At the same time, impurities are easily introduced during the mechanical refinement process, and the calcination temperature is high, the calcination time is long, the reaction steps are multiple, the energy consumption is high, the lithium loss is severe, it is difficult to control the stoichiometric ratio, and it is easy to form impurities. The products have significant differences in composition, structure, particle size distribution, etc., so the electrochemical performance is unstable.

Sol gel method: first, mix the raw material solution evenly to make a uniform sol, and make it gel. During the gel process or after the gel, it is formed and dried, and then calcined or sintered to obtain the required powder materials. Sol gel technology requires simple equipment and easy process control. Compared with the traditional solid phase reaction method, it has lower synthesis and sintering temperatures, and can produce materials with high chemical uniformity and chemical purity. However, the synthesis cycle is relatively long, the synthesis process is relatively complex, and the cost is high. It is difficult to industrialize.

The functions, advantages and disadvantages of the three elements

NCM622 Material Structure Diagram

Introducing 3+Co: reduces cation mixing occupancy, stabilizes the layered structure of the material, lowers impedance, improves conductivity, and enhances cycling and rate performance.

Introducing 2+Ni: can increase the capacity of the material (increase the volumetric energy density of the material), but due to the similar radius of Li and Ni, excessive Ni can also cause lithium nickel mixing due to dislocation phenomena with Li. The higher the concentration of nickel ions in the lithium layer, the more difficult it is for lithium to be deintercalated in the layered structure, and the electrochemical properties deteriorate.

Figure (b) shows a schematic diagram of the mixed arrangement of Ni and Li

Introducing 4+Mn can not only reduce material costs, but also improve material safety and stability. However, excessive Mn content can easily lead to spinel phase and damage the layered structure, resulting in reduced capacity and cyclic decay.

Where does the high pH of ternary materials come from and what are their impacts?

We all know that high Ni ternary materials are the application direction of future high-energy density power batteries, but why have they not been used well? One of the most important reasons for this is the high alkalinity of the material, which makes it easy for the slurry to form jelly after absorbing water. We simply cannot meet its requirements for production environment and process control capabilities. Reducing the surface residual alkali content is of great significance for the application of ternary materials in batteries.

This is because there is an excess of lithium salt in the synthesis of ternary materials. The excess lithium salt, after high-temperature calcination, mainly produces Li oxide, which reacts with H2O and CO2 in the air to generate LiOH and Li2CO3 again, remaining on the surface of the material, resulting in a higher pH value of the material. In addition, due to the limitation of valence balance in high Ni systems, a portion of Ni in the material exists in the form of 3+, while excess Li is prone to form LiOH and Li2CO3 on the surface of the material. The higher the Ni content, the greater the surface alkalinity, and the easier it is to absorb water during homogenization and coating, resulting in a jelly like slurry.

Meanwhile, it should be noted that these residual lithium salts not only have high electrochemical activity, but also cause gas production during battery charging and discharging due to the decomposition of lithium carbonate and other substances under high pressure.

How to reduce the pH of ternary materials?

Generally, the pH and production environment of the precursor are controlled from the source, the proportion of lithium salt is reduced, and the sintering system is adjusted to allow lithium to quickly diffuse into the crystal. Washing the material with water and then secondary sintering can reduce the residual alkali content on the surface, but this will result in a loss of some electrical properties. Surface coating is also an effective method to reduce the residual alkali content on the surface of ternary materials.

Three element material modification method?

The surface of ternary materials is modified with metal oxides (Al2O3, TiO2, ZnO, ZrO2, etc.) to mechanically separate the material from the electrolyte, reduce side reactions between the material and the electrolyte, and suppress the dissolution of metal ions. The coating of ZrO2, TiO2, and Al2O3 oxides can prevent impedance increase during charge and discharge processes and improve the cycling performance of the material. Among them, the coating of ZrO2 causes the smallest increase in surface impedance of the material, while the coating of Al2O3 does not reduce the initial discharge capacity.

How to improve the safety of ternary materials?

How to improve the safety of ternary materials? Simply put, there are several important points: firstly, from the perspective of ternary materials themselves, ceramic alumina coating should be carried out to control the Ni content within a reasonable range. Secondly, efforts should be made to study the compatibility with other materials in the battery system, such as the matching of electrolyte additives and the selection of ceramic isolation films.

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