The cathode material is one of the key materials that determine the performance of lithium-ion batteries. It is also the main source of lithium ions in commercial lithium-ion batteries. Its performance and price have a greater impact on lithium-ion batteries. The cathode materials that have been successfully developed and applied at present mainly include lithium cobalt oxide, lithium iron phosphate, lithium manganate, and the ternary material lithium nickel cobalt manganate (NCM) and lithium nickel cobalt aluminate (NCA).
- Lithium cobalt oxide (LCO): Suitable for small batteries, actual capacity is not high Lithium cobalt oxide is the first generation of commercial cathode material. It has been gradually modified and improved over decades of development and can be considered the most mature cathode material for lithium-ion batteries. Lithium cobalt oxide has the advantages of a high discharge platform, high specific capacity, good cycle performance, and a simple synthesis process. However, this material contains highly toxic cobalt and is expensive, making it difficult to ensure safety when making large power batteries.
Lithium cobalt oxide remains the best choice for small lithium batteries. Most still use lithium cobalt oxide instead of ternary materials with higher specific capacity. The reason is that the compacted density of lithium cobalt oxide materials is greater than that of ternary materials, that is, more lithium cobalt oxide can be accommodated per unit volume. Among small batteries that pay more attention to volume density, lithium cobalt oxide occupies a place.
Lithium cobalt oxide has a high theoretical capacity, but its actual capacity is only half of the theoretical capacity. The reason is that lithium ions are released from the lithium cobalt oxide material during the charging process, but the amount released is less than 50%, the morphology and crystal form of the material can remain stable. As the number of lithium ions extracted increases to 50%, the lithium cobalt oxide material will undergo a phase change. If charging continues at this time, the cobalt will dissolve in the electrolyte and generate oxygen, seriously affecting the battery cycle stability and safety performance. Therefore, the general lithium cobalt oxide charging cut-off voltage is 4.2V.
- Lithium Iron Phosphate (LFP): low energy density, outstanding safety Lithium iron phosphate is one of the cathode materials that has attracted much attention at present, with a theoretical specific capacity of 170mAh/g, the actual specific capacity can reach 150mAh/g. Above, its main features are that it does not contain harmful elements, is low in cost, is very safe, and has a cycle life of up to 10000. Nowadays, these characteristics make lithium iron phosphate materials quickly become a research hotspot, and lithium iron phosphate batteries are also widely used in the field of electric vehicles.
The disadvantage of lithium iron phosphate is also obvious, that is, low energy density. There are two reasons. First, the voltage of lithium iron phosphate material is only 3.3V, lower than other cathode materials, which makes lithium iron phosphate batteries store less energy; secondly, lithium iron phosphate has poor conductivity and needs to be nanometer-sized and coated to obtain good electrochemical performance, which makes the material fluffy, the compacted density is lower.
The combined effect of the two makes the energy density of lithium iron phosphate batteries lower than that of lithium cobalt oxide and ternary batteries. Therefore, lithium iron phosphate batteries are mainly used in electric buses and a small number of passenger cars.
Will lithium iron phosphate be phased out shortly? Recently, new energy vehicle safety accidents have occurred frequently. Lithium iron phosphate, which is considered to be soon replaced by ternary materials, has once again entered people’s field of vision. People hope to increase its capacity by modifying lithium iron phosphate.
At present, some scholars have adopted the method of incorporating lithium iron phosphate into Mn Elements enable it to have higher voltage and higher energy density. There are also related studies using composite technology to combine lithium iron phosphate and NCM, the mixing of three-element materials can effectively improve the safety performance of three-element batteries while maintaining a high energy density.
- Ternary materials (NCM,NCA): Performance can be adjusted, how to choose the path?
The ternary material is lithium nickel cobalt manganese oxide (lithium nickel cobalt manganese oxide) which has a very similar structure to lithium cobalt oxide. (LiNixCoyMn1-xyO2), this material can be balanced and regulated in terms of specific energy, cyclicity, safety and cost.
Different configurations of the three elements nickel, cobalt, and manganese will bring different properties to the material: Increased nickel content will increase the capacity of the material, but will worsen the cycle performance; the presence of cobalt can make the material structure more stable, but too high a content will reduce the capacity; the presence of manganese can reduce costs and improve safety performance, but the content If it is too high, the layered structure of the material will be destroyed.
Therefore, finding the proportional relationship between the three materials to optimize the overall performance is the focus of the research and development of ternary materials. Common ratios include NCM 111, 523, 622, and 811. NCA (LiNi0.8Co0.15Al0.05O2) is to replace the manganese element with an aluminum element, which improves the structural stability of the material to a certain extent, but its aluminum content is small and can be approximately regarded as a binary material.
How does increasing nickel content change material properties?
(1) The higher the nickel content, the higher the specific capacity of the material.
(2) The higher the nickel content, the more difficult it is to store and develop the material. High-nickel ternary materials are easy to absorb water and deteriorate, reducing capacity and cycle life. Moreover, part of the water will be stored in the crystal, causing the battery to produce gas in a high-temperature environment, causing the battery to bloat and bring safety risks.
(3) The higher the nickel content, the worse the thermal stability of the ternary material.
(4) Increased nickel content will cause electrolyte matching problems.The surface of high-nickel materials is caused by water absorption and deterioration. LiOH Substances such as these will react with the electrolyte, causing capacity fading and safety issues.
Therefore, the modification technology of high-nickel materials is an important development direction. Modification technology includes doping with other elements, surface coating, etc., such as nano-coating the particle surface with conductive polymers or inorganic materials, which can increase cycle life, and improve high temperature performance and safety.
In conclusion, cathode materials play a pivotal role in determining the performance and safety of lithium-ion batteries. From the well-established lithium cobalt oxide to the safety-focused lithium iron phosphate and versatile ternary materials, each comes with its own set of advantages and limitations. Ongoing research is focused on optimizing ternary materials, especially those with higher nickel content, to strike a balance between specific capacity, cycle life, and safety. As technology evolves, the quest for cathode materials that offer superior energy density and reliability remains a driving force in advancing battery technology.