1. What is a ternary material?

Ternary (LiCoxMnyNi1-x-yO2) has a-NaFeO2 type layered structure (R-3m space group) and has a theoretical capacity of approximately 275 mAh/g. In ternary materials, Mn always maintains +4 valence and has no electrochemical activity. It only serves as the material skeleton to stabilize the crystal structure. Ni and Co are electrochemically active, with +2 valence and +3 valence respectively. As the composition ratio of Ni, Co, and Mn changes, the material's capacity, safety performance and many other properties can be controlled to a certain extent. People in the industry are accustomed to naming materials according to the ratio, such as 111/442/532 (representing Ni, Mn , the proportion of the three elements Co), etc. Affected by the mutual occupancy of nickel and lithium, the structural stability of the material is better when the ratios of Ni, Mn, and Co are 1:1:1 and 4:4:2. However, in order to obtain more reversible capacity, the development of ternary materials tends to increase the nickel content, such as 532/622/721/811, etc.

LiCoxMnyNi1-x-yO2 crystal structure

Preparation methods of binary and ternary materials

Currently, there are many synthesis methods to prepare LiCoMnNiO2 ternary cathode materials , mainly including: *** precipitation method, sol-gel method, spray drying method and solid phase reaction, etc. The precipitation method is the most common method for preparing spherical ternary materials, including: hydroxide precipitation method and carbonate precipitation method.

***Precipitation method process flow chart:

3. Modification

Although ternary materials have good electrochemical properties, in practical applications , there are still many problems that need to be solved, such as: mixing of lithium ions, improving first efficiency, improving lithium ion diffusion coefficient and electronic conductivity. The main modification methods of LiNi1/3Co1/3Mn1/3O2 are: ion doping and surface coating. Surface coating, appropriate doping ratio and uniform doping can make the structure of the material more stable and improve the cycle performance and thermal stability of the material.

(1) Ion doping

The output power of lithium-ion batteries is directly related to the electronic conductivity in the material and the ionic conductivity of lithium ions, so different means are used to improve the electronic conductivity and ionic conductivity are key to improving materials.

(2) Surface coating

Modify the surface of the ternary material with metal oxides (Al2O3, ZnO, ZrO2, etc.) to mechanically separate the material from the electrolyte and reduce the risk of material and electrolyte contact. Liquid side reactions, inhibiting the dissolution of metal ions, and optimizing the cycle performance of materials. At the same time, surface coating can also reduce the collapse of the material structure during repeated charge and discharge processes, which is beneficial to the cycle performance of the material.

4. Are ternary materials really unsafe?

What may bother everyone about ternary materials may still be safety issues, especially high-nickel ternary materials. In fact, it can be seen from the national new energy policy that we can feel the limited application of ternary batteries in buses and buses. This is because it is difficult for ternary batteries to pass the acupuncture item in the national standard GBT 31485-2015 "Safety Requirements and Test Methods for Power Batteries for Electric Vehicles". In 2017, the country canceled this test, and ternary batteries are used in passenger cars. It has developed rapidly and has occupied half of the new energy market in the past two years.

Schematic diagram of acupuncture in GBT 31485-2015

Are ternary materials really unsafe? In fact, this question cannot be answered simply by whether it is safe or unsafe. Energy is like a bomb, it depends on how you control it. If it is controlled well, it can kill the enemy and serve the country; if it is not controlled well, it may crash the car and kill people. It can be seen from the historical development process of oil, natural gas, nuclear energy, etc. that these energy sources have experienced uncontrollable situations in the world. In fact, the safety of any kind of energy depends on our control ability. The same goes for ternary batteries. Now that ternary batteries are unsafe, it just shows that we have not yet mastered how to control them. Whether the battery is safe or not, in addition to having a certain relationship with the material itself, is also directly related to the environment we use, the battery management system, and the vehicle control system.

Excerpted from a foreign ternary pure electric car burning and fire accident that appeared in the news:

Of course, ternary materials themselves do still have many unresolved problems, which is why ternary materials have always been Considered unsafe by everyone. For the ternary battery itself, the poor thermal stability of the raw material itself and the battery manufacturing process should be the two main factors that make the ternary battery unsafe.

The ternary material is a layered compound. The thermal stability after delithiation is not ideal and can easily cause oxygen loss and phase change. Moreover, the material will decompose at around 200°C and thermal runaway will occur.

How to improve the safety of ternary materials, here are a few important points: First, from the perspective of the ternary material itself, it is coated with ceramic alumina and the Ni content is controlled within a reasonable range, and secondly, it is combined with other materials in the battery system. Efforts should also be made to study the coordination of materials, such as the matching of electrolyte additives and the selection of ceramic isolation membranes.

The main problems currently encountered in the application of NMC to power batteries include:

1) Due to the cation mixing effect and the changes in the surface microstructure of the material during the first charging process, NMC’s ??first The charging and discharging efficiency is not high, and the first efficiency is generally less than 90%;

2) The safety of ternary material battery cells is more serious due to serious gas production, and high-temperature storage and cyclability need to be improved;

3) The lithium ion diffusion coefficient and electronic conductivity are low, making the rate performance of the material not very ideal;

4) The ternary material is a secondary spherical particle formed by agglomeration of primary particles. The secondary particles will break under high compaction, thus limiting the compaction of the ternary material electrode, which also limits the further improvement of the energy density of the battery core.

I talked about ternary modification earlier, so I won’t go into details here. Just say a few words! In response to the above problems, modification measures currently widely used in the industry include: 1. Heteroatom doping. In order to improve the relevant properties required by the material (such as thermal stability, cycle performance or rate capability, etc.), doping modification research is usually conducted on cathode materials. 2. Surface coating. Ternary surface coatings can be divided into two types: oxide and non-oxide. The most common oxides include MgO, Al2O3, ZrO2 and TiO2. Common non-oxides mainly include AlPO4, AlF3, LiAlO2, LiTiO2, etc. 3. Optimization of production process. Improving the production process is mainly to improve the quality of ternary products, such as reducing the surface residual alkali content, improving the integrity of the crystal structure, reducing the content of fine powder in the material, etc. These factors have a greater impact on the electrochemical performance of the material.

In conclusion, the safety performance of ternary batteries is a systematic project. In addition to the material itself that needs to be optimized, the battery manufacturing process also needs to be considered. Another important thing behind is the battery management system. Once this is done, battery safety will be more guaranteed.

National Energy Information Platform contact number: 010-65367702, email: hz@people-energy.com.cn, address: People’s Daily, No. 2 Jintai West Road, Chaoyang District, Beijing