Different from lifepo4 battery, NMC material is lithium nickel cobalt manganese oxide Li(NiCoMn)O2, a precursor product of ternary composite cathode material. The appearance is black powder. It is made of nickel salt, cobalt salt and manganese salt. The ratio of nickel, cobalt and manganese can be adjusted according to actual needs.
1. Comparison of properties of different proportions of NMC material
NMC material is named after it contains three transition metal elements: Ni, Co, and Mn. By adjusting the ratio of the three elements, NMC material with different electrochemical properties such as NCM111, NCM523, NCM622, and NCM811 can be obtained.
Among them, the Co element has the functions of reducing cation mixing, increasing ionic conductivity, increasing rate and cycle performance. Ni element has a high specific capacity characteristic, which ensures the capacity output of the material. The Mn element does not participate in the electrochemical reaction in the working voltage range, and has a wide range of sources, which is cheap and safe.
Although the discharge specific capacity of the material gradually increases with the increase of Ni content, which can greatly increase the energy density of lithium batteries, but the thermal stability decreases significantly and the safety performance deteriorates. In addition, the degree of cation mixing is intensified, and the diffusion of Li+ is hindered, resulting in a decrease in the electrochemical and cycle performance of the material.
2. NMC material performance analysis
High nickel NMC material phase transition leads to poor cycle performance
The main reason for the poor cycle performance of NMC material with the increase of Ni content is that multiple phase transitions occurred during the charging and discharging process. For low Ni NMC material, its oxidation/reduction peak corresponds to 3.76/3.72V, and the reversibility is very good.
For the NMC material with high Ni, there are four pairs of redox peaks, and a phase transition from H2 to H3 occurs at a higher voltage, and microcracks appear in the final particles, resulting in attenuation of cycle performance.
Comparison of residual alkali content on the surface of NMC material
As the Ni content increases (especially high-nickel NMC material with Ni content > 60%), the sintering temperature decreases, resulting in a decrease in the volatilization of lithium salts, an increase in the content of lithium salts remaining on the surface of the material, and the formation of lithium oxides.
It is easy to absorb CO2 and H2O in the air, and form Li2CO3 and LiOH on the surface of the material. Li2CO3 will cause the battery to decompose and produce gas when it is stored at high temperature, full charge and high potential. LiOH will react with the lithium salt of the electrolyte to generate HF, deteriorating battery performance.
In addition, during the homogenization process, the bases on the surface of the material will also attack the C-H and C-F of PVDF to undergo an elimination reaction, forming a C=C double bond to produce chemical cross-linking. Due to the high regularity of PVDF chains, continuous HF removal reactions are prone to occur, which eventually leads to high-nickel NMC material gel, which increases the difficulty of slurry processing.
Poor thermal stability and safety performance
As the Ni content increases, the thermal decomposition temperature of NMC material decreases, the heat release increases, and the safety performance decreases significantly. This is because at the same potential, the higher the content of Ni element, the more Li+ is released, so the content of Ni4+ is more, and Ni4+ is easily reduced to Ni3+. In order to maintain the charge balance, some oxygen escapes, resulting in poor stability.
In view of the above problems, measures such as ion doping, surface coating and element concentration gradient design can be used to improve NMC material. Ion doping (such as Mg2+, Al3+, F-) can improve electronic conductance and ion conductance, improve material structure stability and thermal stability, etc., thereby improving the electrochemical performance and cycle life of NMC material.
3. Classification of NMC material
NMC material can be divided into NCM and NCA according to the type of elements. According to the proportion of elements, it can be divided into low nickel, medium nickel, medium high nickel and high nickel NMC material. NMC material can be divided into single crystal ternary and polycrystalline ternary according to particle morphology. NMC material can be divided into conventional ternary and high voltage ternary according to the voltage system.
① NCM vs NCA
The difference between NCM and NCA is that one uses Mn element and the other uses Al element. From the perspective of manufacturing process, NCA has more stringent production conditions, higher technical barriers and higher production costs. From the perspective of energy density, NCA has a higher specific capacity and can meet longer battery life.
From the perspective of safety performance, the thermal stability of ternary materials containing Mn is better, and the thermal stability of NCA is not as good as that of NCM. From a cost point of view, NCA uses less Co element, and the cost of raw materials is lower. From the perspective of cycle life, NCA has better long-cycle performance.
Although NCA has a certain energy density advantage, its core patented technology is basically controlled by Japanese and Korean companies, and the manufacturing conditions are harsh and the production cost is high. China top 10 ternary cathode material companies mainly uses the NCM ternary technology route.
In addition, NCA materials have a disadvantage that cannot be ignored, that is, more serious side reactions and gas generation will occur during charging and discharging. As a result, the application of NCA materials is limited to hard shell systems, such as 18650 battery, and it is difficult to apply them in pouch cell systems.
② Material with different nickel content
For Ni and Mn equivalent NMC material (such as 333 and 424), the valence of Ni element is +2, while for Ni and Mn non-equivalent NMC material, the valence of Ni element is +2/+3. The higher the Ni element content, the more electrons that can participate in the electrochemical reaction, and the higher the specific capacity of the material, but the discharge platform will decrease.
Although the increase in Ni content is beneficial to increase the energy density of lithium batteries, the manufacturing cost, cycle life and safety performance of the material are further deteriorated. For example, high-nickel NMC material requires more complicated doping and cladding technology. In order to alleviate the problem of cation mixing, high-nickel NMC material needs to be sintered in an oxygen furnace. The low-nickel NMC material only needs air furnace sintering.
In terms of sintering temperature, high-nickel NMC material requires low-temperature sintering, so LiOH, which is more expensive and has a lower melting point, needs to be used as a lithium source, while low-nickel NMC material only needs to use cheap Li2CO3.
③ Single crystalline NMC material vs polycrystalline NMC material
Single crystalline NMC material is a crystal grown from a crystal nucleus, generally composed of primary particles with a particle size of 2~5um. The polycrystalline NMC material is a secondary particle formed by the agglomeration of several submicron small single crystals, with a general particle size of 10~15um.
Due to the larger particle size of polycrystalline NMC material aggregates, the compaction density is higher, and the primary particle size is only a few hundred nanometers, the Li+ diffusion path is shorter, and the rate performance is better than that of single crystalline NMC material. However, there are many grain boundaries inside polycrystalline NMC material particles.
During the process of Li+ extraction and embedding, microcracks are likely to occur due to anisotropic lattice changes, which accelerates side reactions and increases impedance, resulting in worse cycle performance. The single crystalline NMC material particles are compact inside, have better structural stability and thermal stability, have better cycle performance and safety performance, and are more suitable for high-voltage systems.
④ Regular voltage NMC material vs high voltage NMC material
By increasing the charging upper limit voltage, the specific capacity and discharge platform of the material can be significantly improved, thereby increasing the energy density. However, the higher the charging voltage, the more Li+ will be extracted from the cathode, the stability of the crystal structure of the material will be significantly deteriorated, and the cation mixing and irreversible phase transition will be intensified, resulting in a significant decline in cycle performance.
This problem can be improved by ion doping (such as Al3+, Zr4+, Ti4+) to alleviate phase transition, surface coating to reduce side reactions and high-voltage electrolyte additives. Obviously, single crystalline NMC material with higher particle mechanical strength and smaller specific surface area is more suitable for high-voltage systems.