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The working mechanism and preparation of lithium battery cathode materials for electric vehicles

by:dcfpower     2021-03-10
【Abstract】Lithium cobalt oxide is the first lithium battery cathode material to be commercialized. However, due to its high price and environmental pollution, it cannot be used as a power battery for electric vehicles. Lithium nickelate has a layered crystal structure, which is suitable for the insertion and extraction of Li+. When Li+ leaves the crystal lattice, the crystal structure of lithium nickelate will change. Therefore, in order to ensure good reversibility, the amount of Li+ released should be controlled at 0u003cxu003c0.75, the upper limit of the charging voltage is 4.2V, and the actual cycle specific capacity is 200mAh/g.  【Keywords】 Lithium nickel oxide charge and discharge cycle   The exhaustion of fossil fuels has increased people's awareness of environmental protection, and new energy electric vehicles have gradually attracted consumers' attention. New energy electric vehicles mainly use lithium batteries as energy carriers. In lithium batteries, the positive electrode material has a decisive effect on the performance of the battery. Lithium cobalt oxide is the first lithium battery intercalation oxide cathode material to be commercialized. However, due to the lack of cobalt resources, the high price, and the impact on the environment, the development of lithium cobalt oxide as a power source for electric vehicles is severely restricted. Lithium nickelate also has a layered structure and is a very potential cathode material for lithium batteries. This article will introduce the crystal structure of lithium nickelate and its charging and discharging mechanism as a cathode material for lithium batteries.  1. Crystal structure and charge-discharge mechanism of lithium nickelate   Layered structure of lithium cobaltate has the advantage of long cycle life as a lithium battery pack cathode material. However, due to structural stability, its actual cycle capacity is only half of the theoretical capacity. In addition, problems such as high price and environmental pollution restrict its development as a cathode material for large-capacity batteries, especially in the development of electric vehicles. Lithium nickelate also has a hexagonal layered sodium ferrite crystal structure. Li+ occupies the 3a position in the lattice, and Ni3+ occupies the 3b position, and the adjacent NiO2 layer is separated by the Li+ layer. Therefore, Li+ has a good two-dimensional movement channel, which can maintain good cycle performance. 'The theoretical specific capacity of lithium nickelate is 275mAhg-1. During the charging process, Li+ gradually escapes from the octahedral position in the lithium nickelate lattice. When the amount of escape 0u003cxu003c0.25, the lattice constant c decreases. This shows that with the release of Li+, the layer spacing of NiO2 decreases. At this time, lithium nickelate still maintains a hexagonal crystal structure. When 0.25u003cxu003c0.75, the hexagonal structure (H) of lithium nickelate gradually transforms to the monoclinic structure (M), and the lattice constant c increases, that is, the distance between NiO2 layers increases and the lattice expands. When Li+ continues to escape from the crystal lattice (0.55u003cxu003c0.75), lithium nickelate transforms from a monoclinic structure to a hexagonal structure, and the lattice constant c continues to increase. The increase of the lattice constant c is the increase of the NiO2 layer spacing, which is beneficial to the diffusion of Li+. When the amount of Li+ released is in the range of 0.25 to 0.75, the interplanar spacing of the NiO2 layer remains in the range of 4.74 to 4.8?. However, continuing to increase the amount of Li+ released (xu003e0.75), the hexagonal crystal structure of lithium nickelate will undergo irreversible lattice distortion, accompanied by a sharp drop in the lattice constant c, which hinders the movement of Li+. At the same time, the decrease of NiO2 layer spacing reduces the cycle performance of lithium nickelate. When the charging voltage is higher than 4.2 V, the layer spacing of NiO2 decreases by 0.3? and the cycle performance decreases by 50%. Therefore, during the actual cycle of lithium nickelate electrodes, the amount of Li+ released should be controlled within the range of xu003c0.75, and the upper limit of the charging voltage should be controlled at 4.2 V to ensure good reversibility. The actual cycle specific capacity is 200mAh/g.  Second, the preparation method of lithium nickelate    At present, high temperature solid phase reaction method is a common method for synthesizing lithium nickelate. When the synthesis temperature is low, the degree of crystallization of lithium nickelate is low, and an ideal layered structure cannot be formed. However, at high temperatures, Ni3+ is easily reduced to Ni2+, and because it has a similar ionic radius to Li+, Ni2+ is extremely easy to occupy the 3a position of Li+ to form proton disordered lithium nickelate with charge vacancies. When the reaction temperature reaches 700°C or higher, the intensity of the (003) diffraction peak of lithium nickelate decreases significantly, while the intensity of the (104) diffraction peak increases. The intensity ratio of diffraction peaks is an important indicator to characterize the disorder of protons in lithium nickelate. The decrease in the intensity ratio of the diffraction peaks indicates that when the temperature is higher than 700°C, the lithium and nickel in the crystal lattice are transposed, that is, proton disorder occurs.   In addition, proton disorder can also cause changes in the crystal structure of lithium nickelate. When the degree of proton disorder reaches 25%, lithium nickelate transforms from a hexagonal crystal structure to a spinel structure similar to lithium manganate; when the degree of disorder reaches 50%, it transforms into a square structure (Fm3m). The two-dimensional motion channel of Li+ disappears completely, and the cycle performance decreases.  3. Summary   Lithium nickelate has the advantages of low price, environmental friendliness and high theoretical specific capacity. The layered structure of sodium ferrite makes it have excellent cycle performance similar to lithium cobalt oxide, so it is a very potential lithium battery pack cathode material. When Li+ leaves the crystal lattice, the crystal structure of lithium nickelate changes. Therefore, in order to ensure good reversibility, the amount of Li+ released should be controlled at 0u003cxu003c0.75, the upper limit of the charging voltage is 4.2V, and the actual cycle specific capacity is 200mAh/g.  
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