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Lithium Manganese Oxide
Lithium manganate, as a lithium battery material with a long history of use, has high safety, especially strong resistance to overcharge, which is a major advantage. Due to the good structural stability of lithium manganate itself, the amount of positive electrode material does not need to exceed that of the negative electrode when designing the battery cell. In this way, the number of active lithium ions in the entire system is small, and after the negative electrode is fully charged, there will not be too many lithium ions in the positive electrode. Even if overcharge occurs, there will not be a situation in which a large amount of lithium ions are deposited on the negative electrode to form crystals.
In addition, the material price is low, and the production process requirements are relatively low, is relatively early to obtain widely used cathode materials.
But it also has obvious drawbacks. The high temperature performance of spinel lithium manganate is not good. The existence of oxygen defects makes the cell prone to capacity attenuation in the high voltage stage, and at the same time, recycling at high temperature will cause similar capacity attenuation. The reason lies in the disproportionation effect of trivalent manganese ions. The way to prevent high temperature attenuation mainly focuses on reducing the point of trivalent manganese.
Lithium manganate, limited by its high temperature performance, is generally not used in high power or high ambient temperature occasions, such as high-speed passenger cars, plug-in hybrids, etc., and lithium manganate is rarely used as a power source. But for electric buses, in-city logistics vehicles, etc., lithium manganate is fully capable.
Lithium Iron Phosphate
The advantages of lithium iron phosphate are mainly reflected in safety and cycle life. The main determinant is the olivine structure of lithium iron phosphate. Such a structure, on the one hand, leads to low ion diffusion capacity of lithium iron phosphate, on the other hand, it also has good high temperature stability, and good cycling performance.
The disadvantages of lithium iron phosphate are also obvious, such as low energy density, poor consistency and poor low temperature performance.
The low energy density is determined by the chemical properties of the material itself. A large molecule of lithium iron phosphate can accommodate only one lithium ion.
Consistency, especially poor batch stability, is not only related to the level of production management, but also related to its own chemical properties. Lithium iron phosphate is one of the most difficult cathode materials for lithium batteries. The difficulty of the consistency and uniformity of the chemical reaction also brings another problem. The iron elemental and iron ion impurities in the lithium iron phosphate materials are always present, which brings the potential failure of the battery.
Due to its high safety, lithium iron phosphate battery is still the main type of power lithium battery for electric vehicles in China, although the energy density partly affects its use range. In particular, the use of lithium iron phosphate batteries is mandatory for buses that involve the safety of a large number of people.
The ternary lithium cathode material combines the advantages of LiCoO2, LiNiO2 and LiMnO2 materials, forms a synergistic effect inside the same cell, and takes into account the three requirements of material structure stability, activity and low cost. One of the highest energy density in cathode materials. Its low temperature effect is also significantly better than that of lithium iron phosphate batteries.
Among the three elements, the higher the content of Ni, the higher the energy density of the cell, and the lower the safety of the cell. In practical applications, the proportional relationship of the three materials in the cell has been changing over time. People's pursuit of energy density is getting higher and higher, so the proportion of Ni is also higher and higher.