NCM111 Material Failure Analysis -Lithium - Ion Battery Equipment

NCM111 Material Failure Analysis -Lithium - Ion Battery Equipment

NCM111 material is the most mature and commonly used ternary material. NCM111 material has the advantages of low cost, simple synthesis process and good rate performance, so it is widely used in power tools and electric vehicles and other fields. Especially in recent years, the electric vehicle industry has developed rapidly, so the market demand for ternary materials continues to rise. According to incomplete statistics, the annual demand for ternary materials by Lishen alone is as high as 2,000 tons.

However, the biggest problem facing NCM111 materials at present is the problem of cycle life. In use, the battery decay rate is significantly faster than that of lithium cobalt oxide lithium ion batteries. A big factor is due to the NCM111 material itself. It is caused by the decay, especially the dissolution of transition metal elements, which causes the structural damage of the NCM111 material. At the same time, the dissolved Mn element will also cause damage to the SEI film of the negative electrode, which is an important reason for the rapid decline of NCM battery life, so it is very important. It is necessary to conduct in-depth research on the dissolution mechanism of transition metals.(Lithium - Ion Battery Equipment)

It should be noted that transition metal dissolution is not the only mechanism for the performance degradation of NCM materials. Under the condition of high cut-off voltage, other mechanisms include: 1) release of oxygen; 2) increase in battery impedance during cycling; 3) Irreversible phase transitions of electrode materials.

Marco Evertz et al. from the University of Münster in Germany conducted an in-depth study on the dissolution mechanism of transition metal elements in NCM111 materials at different cut-off voltages. The NCM material used in the experiment was supplied by Toda Industry Co., Ltd. The battery adopts a soft-pack square battery structure. At a current density of 150mA/g, the battery was charged to 4.3V and 4.6V respectively, and the discharge cut-off voltage was controlled at 2.5V .

The experiment found that under the cut-off voltage of 4.3V, the capacity retention rate can reach 91.4% after 100 cycles, but when the cut-off voltage is increased by 4.6V, the capacity retention rate is only 36.8%. However, it should be noted that, according to the research of Kasnatscheew et al., a large part of the capacity loss of NCM materials is the apparent capacity loss, not the irreversible loss. This part of the capacity loss can be recovered.

The study on the dissolution of transition metal elements found that at the cut-off voltage of 4.3 V, the transition metal content on the negative electrode surface only increased slightly with the increase of the number of cycles.

After 100 cycles of the battery, the concentrations of Mn, Co, and Ni on the surface of the negative electrode were 52, 37, and 41 ppm, respectively. The total loss of transition metal elements on the surface of the negative electrode, the separator, etc. was counted, and the cut-off voltage was 4.3V. The total loss of transition metal elements accounts for about 0.021wt% of the total weight of the positive active material.

However, when the cut-off voltage was increased to 4.6V, the loss of transition metal elements reached 0.45wt%, indicating that the cut-off voltage has a decisive influence on the stability of NCM111 materials.

According to the above findings, Marco Evertz et al. proposed a new dissolution mechanism of transition metal elements in NCM materials: due to the large volume expansion of the lattice structure of the NCM111 material during the intercalation and extraction of lithium, and a large stress, so Cracks will be formed on the active material particles. Under the use of pF6-, the metal elements will be further solvated, resulting in the dissolution of transition metal elements. This mechanism of use is more obvious at high cut-off voltages.

This study shows that there are three important factors that cause the dissolution of transition metal elements: 1) lattice defects in the synthesis process, such as oxygen defects, etc.; 2) material structure distortion caused by atomic scale, such as lithium ion insertion and desorption. The lattice expansion/contraction caused by the intercalation leads to the fracture of the material; 3) During the cycling process, the material transforms from a layered structure to a spinel structure.

These are all things that we should pay attention to in the process of research and development of NCM materials in the future, and corresponding measures should be taken to stabilize the material structure and improve the cycle stability of the material under high voltage.



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