Cell energy density

Each cathode material has its own theoretical energy density and by choosing a cathode material, the upper limit of the energy density of the cell is chosen. The amount of cathode material designed and the vibrational density during processing and fabrication also have an impact on the energy density of the finished core.

Cell power density

The different types of cathode material determine the general range of battery charge and discharge power. Some details of the material, as a secondary factor, can also have an impact on the power characteristics. For example, the crystal structure stability of the cathode material, the particle size, the doping atoms, the carbon cladding process, the method of material preparation, etc. All of these factors ultimately affect the power density of a Li-ion battery by influencing the ability of the cathode material to hold lithium ions and the fluidity of the de-embedding channels.

Cell cycle life

There are many factors that affect the cycle life of a battery cell. The main ones related to the cathode material are the loss of the active material in the cycle and the decay of the cathode's ability to hold lithium ions caused by the breakdown of the material structure during the charging and discharging process. And impurity components in the cathode material, such as singlet and trivalent iron, can interact with the electrolyte and produce undesirable side effects or cause internal micro-short circuits.

Important characteristics of the three mainstream cathode materials

Lithium manganate, as a lithium battery material with a long history of use, has the advantage of being safe and particularly resistant to overcharging. Due to the good structural stability of lithium manganate, the amount of positive electrode material does not have to exceed the negative electrode by much when designing the battery cell. In this way, the number of active lithium ions in the whole system is small, and after the negative electrode is full, there will not be too many lithium ions stored in the positive electrode. Even if there is an overcharge, there will not be a large number of lithium ions deposited in the negative electrode to form crystals. Lithium manganate is therefore one of the best overcharge-resistant materials in use.

In addition, it is an early and widely used cathode material due to its low price and relatively low production process requirements.

However, it also has obvious drawbacks. The high temperature performance of lithium spinel manganate is poor. The presence of oxygen defects makes the cores susceptible to capacity degradation at high voltages, while cycling at high temperatures can cause similar capacity degradation. The cause lies in the trivalent manganese ions that trigger the disproportionation effect. The way to prevent high temperature degradation is focused on reducing the trivalent manganese.

Lithium manganate, due to its high temperature performance, is not generally used in high power or high ambient temperature applications, such as high speed passenger cars and plug-in hybrids, where it is rarely used as a power source. But for electric buses, logistics vehicles, etc., lithium manganese acid can be completely competent.

The advantages of lithium iron phosphate are mainly in terms of safety and cycle life. The main determining factor comes from the olivine structure of lithium iron phosphate. This structure, on the one hand, leads to a lower ion diffusion capacity of lithium iron phosphate, on the other hand, it also has a better high-temperature stability, and good cycling performance.

The disadvantages of lithium iron phosphate are also obvious: low energy density, poor consistency and poor low temperature performance.

The low energy density is a result of the material's own chemistry, with one lithium iron phosphate macromolecule holding only one lithium ion.

Consistency, especially poor batch stability, is related to its own chemistry, in addition to production management levels. Lithium iron phosphate is one of the more difficult of the various lithium battery cathode materials to prepare. This high degree of chemical consistency and uniformity also presents another problem, with the consistent presence of iron monomers and iron ion impurities in lithium iron phosphate materials, posing a potential failure risk to the battery.

Lithium iron phosphate battery, due to its high safety, although the energy density part of the impact of its use, but is still the current main power lithium battery varieties of electric vehicles in China. Especially involving a large number of people's lives and safety of public transport, national policy mandates the use of lithium iron phosphate batteries.

Ternary lithium cathode material, a combination of LiCoO2, LiNiO2 and LiMnO2 in the advantages of the three materials, in the same core to form a synergistic effect, taking into account the stability of the material structure, activity and lower cost of the three requirements, is the three main cathode materials in the highest energy density of one. Its low temperature effect is also significantly better than that of LiFePO4.

The higher the content of Ni among the three elements, the higher the energy density of the cell, and the lower the safety of the cell. In practice, the proportional relationship between the three materials in the cell has been changing over time. The quest for higher energy density has led to a higher proportion of Ni.

The most cited disadvantage of ternary materials is their safety. In the event of thermal runaway, the products of the side reactions contain a large amount of gas, making the risk of accidents and their ability to spread much higher. Secondly, the cycle life of ternary materials is also a bottleneck, currently not up to the level of lithium iron phosphate; finally, the special microstructure of ternary materials makes it unsuitable for high pressure compaction operations, and therefore the common processing methods to increase energy density are not applicable to it.

Ternary materials are gradually expanding their market share, driven mainly by the pursuit of vehicle range. To match or even surpass the range of fuel cars, electric vehicles must pack as much power as possible into a limited space, which makes energy density particularly important. The subsidy policy introduced by the state last year is also for the purpose of stimulating the research and development of high energy density electric cores, setting a threshold for energy density, and those who cannot enter will not be subsidized. From vehicle manufacturers to pack manufacturers to core manufacturers, each link must comply with the general trend of improving product energy density, so ternary lithium batteries are increasingly used. The improvement of the safety performance of the battery itself and the improvement of the system monitoring and handling of accidents will also advance the expansion of the ternary lithium battery market.