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What is a Lithium Battery Cathode Material? The Easiest Explanation Ever!

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Industry information
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2022/08/30 09:56

The cathode material is one of the key materials that determines the performance of lithium-ion batteries, and it is also the main source of lithium ions in commercial lithium-ion batteries. Its performance and valence have a great impact on lithium-ion batteries. At present, the successfully developed and applied cathode materials mainly include lithium cobalt oxide, lithium iron phosphate, lithium manganate, ternary materials nickel cobalt lithium manganate (NCM) and nickel cobalt lithium aluminate (NCA). Lithium cobalt oxide (LCO): suitable for small batteries, the actual capacity is not high

 

Lithium cobalt oxide is the first-generation commercial cathode material, which has been gradually modified and improved in decades of development, and can be considered as the most mature cathode material for lithium-ion batteries. Lithium cobalt oxide has the advantages of high discharge platform, high specific capacity, good cycle performance, and simple synthesis process. However, the material contains more cobalt and the cost is higher.

 

Lithium cobalt oxide remains the best choice for small lithium batteries. At present, most of the 3C electronic batteries still use lithium cobalt oxide instead of ternary materials with higher specific capacity, because the compaction density of lithium cobalt oxide materials is higher than that of ternary materials, that is, the lithium cobalt oxide can be accommodated in a unit volume. more. Lithium cobalt oxide has a place in small batteries that place more emphasis on bulk density.

 

The theoretical capacity of lithium cobalt oxide is high, but the actual capacity is only half of the theoretical capacity. The reason is that lithium ions are extracted from the lithium cobalt oxide material during the charging process, but when the amount of extraction is less than 50%, the morphology and crystal form of the material can be kept stable. As the lithium ion extraction increases to 50%, the lithium cobalt oxide material will undergo a phase transition. If charging continues at this time, the cobalt will dissolve in the electrolyte and generate oxygen, which will seriously affect 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 wide attention at present. The theoretical specific capacity is 170mAh/g, and the actual specific capacity can reach more than 150mAh/g. Its main features are low cost, very good safety, and high cycle life. These features 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 about 3.3V, which is lower than other cathode materials, which makes the storage energy of lithium iron phosphate battery lower; second, lithium iron phosphate has poor conductivity and needs to be nanosized and coated to Good electrochemical properties were obtained, which resulted in a fluffy material with a lower compaction density. 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.

 

Is iron phosphate going to be phased out in the near future? Recently, there have been frequent safety accidents in new energy vehicles. Lithium iron phosphate, which is thought to be replaced by ternary materials soon, has entered people's field of vision again. People hope to improve its capacity by modifying lithium iron phosphate. At present, some scholars have mixed lithium iron phosphate and NCM ternary material by doping Mn element in lithium iron phosphate to make it have higher voltage and higher energy density. The higher energy density of the battery can effectively improve its safety performance.

 

Ternary materials (NCM, NCA): performance can be adjusted, how to choose the road?

Ternary material is a common name for lithium nickel cobalt manganese oxide (LiNixCoyMn1-x-y02) with a very similar structure to lithium cobalt oxide. This material can be balanced and regulated in terms of specific energy, cyclability, safety and cost. The different configurations of the three elements of nickel, cobalt, and manganese will bring different properties to the material: the increase of nickel content will increase the capacity of the material, but the cycle performance will be worse; the presence of cobalt can make the material structure more stable, but the content of too high will make the material more stable. The capacity is reduced; the presence of manganese can reduce costs and improve safety performance, but too high a content will destroy the layered structure of the material, so finding the proportional relationship of the three materials to optimize the overall performance is the focus of the research and development of ternary materials . Common ratios include NCM111, 523, 622, 811, etc. NCA (LiNio.8C0015Ah0502) replaces the manganese element with aluminum element, which improves the structural stability of the material to a certain extent, but its aluminum content is small, which can be approximately regarded as a binary material.

 

How does increasing nickel content change the material properties?

(1) The higher the nickel content, the higher the specific capacity of the material. The specific capacity of NCM811 material can reach 210mAh/g, which is nearly 25% higher than that of NCMIII material.

(2) The higher the nickel content, the greater the difficulty in material storage and development. High-nickel ternary materials are very easy to absorb water and deteriorate, reducing capacity and cycle life. Moreover, part of the water will also be stored in the crystal, which will cause the battery to generate gas in a high-temperature environment, causing the battery to swell and bring security risks.

(3) The higher the nickel content, the worse the thermal stability of the ternary material. For example, NCM111 material decomposes at around 300°C, while NCM811 decomposes at around 220°C.

(4) The increase of nickel content will bring about the problem of electrolyte matching. Substances such as LiOH generated on the surface of high-nickel materials due to water absorption and deterioration will react with the electrolyte, causing capacity decay and safety problems. Therefore, the modification technology of high nickel materials is an important development direction. Modification techniques include doping with other elements, surface coating, etc., such as nano-coating on the surface of particles with conductive polymers or inorganic materials, which can improve cycle life, high temperature performance and safety.

 

Is the future route NCM811 or NCA? Both are high nickel ternary materials with similar properties, but there are differences in the following points:

(1) The cobalt content in NCM811 is 0.1, and the cobalt content in NCA is 0.15, which makes the cost of NCA raw materials slightly higher due to the high price of cobalt;

(2) Replacing manganese with aluminum can enhance the stability of the material and improve the cycle performance of the material, but in the production process, since aluminum is an amphoteric metal and is not easy to precipitate, there is a higher barrier in the production process of NCA material than NCM811;

(3) In battery manufacturing, NCA has more stringent requirements on humidity and other conditions, and there are technical barriers to battery production.

At present, both ideas are feasible. In the future, which material will be the first to overcome the technical difficulties and achieve mass production, which material will be the first to occupy the market.

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