Is the power battery positive 811 about to replace 622?

In the current policy and market demand have forced the entire industrial chain will go higher nickel route, the lithium-ion battery ternary cathode material LiNi0.8Co0.1Mn0.1O2 (hereinafter referred to as NCM811) has high reversible specific capacity and low cost advantages, broad application prospects, is regarded as the first choice for the next generation of high specific energy cathode materials. However, in the next 2019, can the NCM811 material really skip the NCM622 and quickly dominate the field of power battery positives? According to the author's years of experience in the development and application of high-nickel ternary materials, the following points are analyzed the reason:

1. Structural characteristics determine thermal stability and cycle disadvantage

NCM811 material is closer to LiNiO2 in nature and can be regarded as a doped modified product of LiNiO2. It has the same α-NaFeO2 layered rock salt structure similar to LiCoO2, belonging to hexagonal crystal system, spatial point group and layered embedded lithium composite oxide. In the crystal, Li is located at 3a O is at 6c, and transition metal Co is at position 3b. The average valence of Ni is +3, a certain proportion of Ni2+ exists, Co is +3, and Mn is +4. Due to the high Ni content in NCM811 material, the disadvantages are more significant. Firstly, the cation mixing is more serious, Ni2+ still occupies the Li position, and secondly, Ni+ is incompletely reduced during discharge, which easily reacts with the electrolyte, making the material reversible specific capacity and cycle performance are affected. In addition, the Ni2+ oxidation is incomplete, and it is easy to form a NiO layer on the surface, which hinders the migration of Li ions. Therefore, the above reasons may result in low initial charge and discharge efficiency, fast decay of the cycle process, and poor rate performance. , largely limited its large-scale application.

Electrochemical and thermal properties as a function of Ni content, found that electrochemical properties and safety mainly depend on microstructure (morphological and volume structure stability) and physicochemical properties (Li + diffusion coefficient, electron conductivity, volume expansion ratio and chemical stability) Sexuality) Although the increase in Ni content leads to an increase in capacity, the thermal stability continues to decrease and accelerates deterioration, seriously affecting the safety of the cell. Further charging the battery to 4.3V, the DSC experiment results show that with the increase of Ni, the decomposition temperature is gradually reduced, the heat release is increased, the thermal stability of the material is poor, and in addition, the Li which is released at the same potential is higher than the low The nickel ternary cathode material has a high content of Ni4+ and has strong reducibility, and is prone to oxidation reaction, thereby oxidizing the electrolyte, releasing gas, and destroying the crystal structure of the material, resulting in deterioration of stability.

Noh et al studied the electrochemical properties of nickel-cobalt-manganese different proportions of ternary cathode materials. Studies have shown that the redox peaks of low-nickel materials are very stable during the cycle, and the redox peaks increase with the increase of Ni content. The NCM811 series material was cycled 100 times, and the voltage of the oxidation peak was shifted from 3.62V to 3.76V. The differential voltage curve in the figure below shows that the structure changes from H2 to H3, resulting in volume shrinkage, which directly leads to its capacity decay. At present, the cell cycle of the NCM811 product sample in the test is still less than 1500 times. To some extent, the difficulty of the NCM811 material in the material industry is still not well solved.

2. Chemical properties determine processing difficulties

The difficulty of the process is mainly due to the serious cation mixing caused by the high Ni content of the material itself, causing the precipitation of lithium, forming soluble salts such as lithium carbonate and lithium hydroxide on the surface of the material, so that the PH value of material is alkaline, in the compounding. In the process, it is easy to absorb water and form jelly, which affects coating, has poor moisture control, and is difficult to process.

On the other hand, this chemical reaction of water absorption is irreversible. As the surface lithium is consumed, it will further cause more lithium to precipitate, thereby destroying the structure of the material, causing irreversible material phase transition and affecting the late cycle performance of the battery. The research community is currently trying to prevent further precipitation of precipitated lithium by surface coating, such as surface passivation of Li2ZrO3 on the surface of the material, or attempting to strengthen the structure of the material by cation doping, but these practices are still immature in the industry, even Some companies are improving in this regard, but they still cannot be eliminated.

3. The technology is not perfect, resulting in slow release of capacity

At present, the data research of domestic mainstream ternary manufacturers Rongbai, Dangsheng, Shanshan and Bamo shows that 2018~2019 is expected to have a production capacity of 40,000 tons, which is far lower than the current production capacity of NCM523 and NCM622. It is expected that it will not be until 2020. With a capacity of 64,000 tons, material suppliers are more cautious in this response and the production capacity is slower.

Conclusion

Although the nickel-rich NCM811 material has the advantages of high capacity and low price, due to its short structure such as structure and chemical properties, its cycle, thermal stability, safety and other issues need to be further optimized and stabilized, and the cell manufacturer introduces NCM811 at the same time. It also needs to be fully evaluated, and must not be blindly enterprising! The promotion and application of high-nickel NCM811 materials requires a systematic solution, not only the innovation of materials, but also the application side (field process, product structure, environmental control, BMS and so on) have a deep understanding of the material, in order to develop strengths and avoid weaknesses, give full play to the advantages of NCM811.

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