What about ternary materials for lithium batteries?

The cost of anode materials in lithium ion batteries is about 40%, and that of anode materials is only about 5%. Therefore, it can be seen that anode materials play an important role in lithium ion batteries. Anode materials are mainly divided into three categories, namely, layered structure, spinel structure and olivine structure. The representative materials of spinel structure and olivine structure are LiMn2O4 and LiFePO4, while the representative materials of layered structure are LiMO2(M=Ni, Co, Mn) and ternary materials.

Introduction to ternary anode materials

In 1999, ZhaolinLiu et al. from the national university of Singapore first proposed the ternary layered Li(Ni,Co,Mn)O2 materials with different components. Through the synergistic effect of ni-co-mn, they combined the advantages of good circulation performance of LiCoO2, high specific capacity of LiNiO2 and low cost and safety performance of LiMnO2.

In the case of electric vehicles, higher battery power is required to go further, and ternary materials have higher energy and promise better endurance than the LFP, which is widely used in power batteries. At present, the price of electric cars in the industry is very high, and the high cost of power battery is one of the important reasons, which accounts for almost half of the price of the whole car.

Ternary anode material has a longer life, so that the power battery can be used for longer, thus improving the cost performance of electric vehicles. However, in January last year, the state suspended the use of ternary lithium ion batteries in passenger cars, mainly due to the unstable safety performance of ternary materials. After all, in today's knowledge explosion, nothing can stop people from exploring new technologies, except security.

With the change of the proportion of the three elements of ni-co-mn, the ternary materials can be roughly divided into two types: Ni:Mn isotype and nickel-rich type. In the former, Co is +3, Ni is +2, and Mn is +4. The invariant valence of Mn plays a role in stabilizing the structure. When charging, Ni loses 2 electrons, maintaining the high-capacity characteristics of the material.

In order to improve the battery capacity and increase the content of Ni, it is called nickel-rich type. In such materials, Co is +3 valence, Ni is +2/+3 valence, and Mn is +4 valence. When the charging voltage is lower than 4.4v (relative to Li+/Li),Ni+2/+3 is oxidized to form Ni+4. Continue to charge. At a high voltage, Co3+ reacts to produce Co4+. Under 4.4v, the higher the Ni content, the greater the reversible capacity of the material.

The NCA formed by replacing Mn4+ with Al3+ also belongs to the high nickel ternary material. Al3+, like Mn4+, has the same stable valence state, and the Co content affects the ionic conductivity of the material. Figure 2 compares the properties of ternary materials of different components.

Figure 2 relationship between discharge specific capacity, thermal stability and capacitance retention rate of ternary materials with different components

As can be seen from the figure, with the increase of Ni content, the specific capacity of the discharge point increased from 160mA·h·g-1 to more than 200mA·h·g-1, and the thermal stability and capacity retention rate decreased.

3. Problems existing in ternary materials

· influence of increased Ni content

By increasing the content of Ni in ternary materials, the capacity of the battery can be improved. However, circularity and thermal stability become worse. When the Ni content increases, the phase transition will occur during the REDOX process, resulting in the attenuation of capacity. The increase of Ni content also reduces the temperature of thermal decomposition and increases the heat release, resulting in poor thermal stability of the material. For high nickel Li[NixCoyMnz]O2 materials, x>0.6 materials are easy to react with CO2 and H2O in the air to generate Li2CO3, and LiOH, the former is the main cause of gas inflation, and the latter will react with LiPF6 in the electrolyte. When x is higher, the effect is more severe.

· matching with electrolyte

The reaction and charge transfer at the interface of electrolyte and anode materials will affect the performance of lithium ion batteries, and the corrosion of active materials and the decomposition of electrolyte will seriously affect the charge transfer at the interface of electrode/electrolyte.

· uneven surface reaction

SooyeonHwang[3] et al. from Korea institute of science and technology found that the structure of NCA would change during charging, and the Li on the particle surface would be more likely to come out, resulting in uneven crystal and particle structure on the surface. Such change would cause rapid capacity attenuation and impedance rise of the material.

4. Research and development direction of ternary materials

Professor ai xinping from wuhan university made a prediction for the next generation of power batteries. He mentioned that it is not difficult for the battery system with NCM and NCA as the positive electrode and graphite carbon as the negative electrode to reach the short-term goal of 150 ~ 170Wh/kg, but safety is the main obstacle to its loading application.

In the future development, the following four directions are worth exploring: higher capacity ternary materials; Higher power ternary materials; Improvement of synthesis method; Study on electrolyte additives matched with ternary materials.

The page contains the contents of the machine translation.