Study On Properties Modification Of Ternary Cathode Materials For Lithium Ion Batteries

With the progress of the times,people put forward higher requirements for the endurance and safety performance of power systems for electric vehicles and aerospace,which means higher requirements for the energy density and material stability of lithium-ion batteries.Ternary cathode materials have been widely used in the power battery industry because of their high energy density,high specific capacity and low cost.However,the cathode materials still have some disadvantages,such as the dissolution of transition metal ions,irreversible phase transition,side reaction between materials and electrolyte,which lead to the increase of electrode reaction impedance,cycle life and rate performance degradation.In this paper,LiNi_0.5Co_0.2Mn_0.3O₂ materials were prepared by sol-gel method,co-precipitation method and rheological phase method respectively.The electrochemical performances of the three materials were compared.In addition,some innovative ideas are put forward for the defects of the preparation method.Then,some innovative ideas are proposed on the basis of traditional synthesis methods.According to the research progress of ternary cathode materials,two modification methods of double ion doping and compound coating with different characteristics are studied,and the modification mechanism was explored.The main research results are as follows.LiNi_0.5Co_0.2Mn_0.3O₂ materials were prepared by sol-gel method,co precipitation method and rheological phase method respectively.The results show that the morphology of the polyhedra is spherical like by the sol-gel method and the rheological phase method.The morphology of the material prepared by coprecipitation method is spherical and has good dispersibility.The microspheres are composed of primary particles of about 100nm in size.The layered structure of the materials prepared by sol-gel method and co precipitation method is better and the crystallinity is higher.In the range of 0.1C and 3.0～4.4V,the material prepared by sol-gel process and co precipitation method has higher specific discharge capacity and better cycling performance.The first discharge specific capacity of the two materials is 168.4m Ah/g and 170.5m Ah/g respectively,and the capacity retention rate of the50 cycles is 85.2%and 87.8%respectively.The LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials were prepared by adding different amounts of citric acid,adding a certain amount of ethylene glycol in the sol-gel process,and using porous Mn O₂ as manganese source.The results show that when the proportion of citric acid is 0.5,the crystallinity and layered structure of cathode material prepared by rheological phase method are better,and the specific surface area of cathode material increases.In the range of0.1C and 3.0～4.4V,The first discharge capacity of the material is 168.5m Ah/g,and the coulomb efficiency is increased to 81.3%.The capacity retention rate of the material is 89.4%after 50 charge and discharge cycles.Mn CO₃ powder was prepared by precipitation method,and Mn O₂ porous material was obtained by calcination.The cathode material was prepared by using Mn O₂ as manganese source.The obtained material is spherical,which is composed of primary particles with large specific surface area.The initial discharge capacity of the material prepared by porous Mn O₂ is 179.4m Ah/g,and the cycling performance reaches90.8%.Al-F co-doping,Na-Ti co-doping and K-Cl co-doping were applied to the LiNi_0.5Co_0.2Mn_0.3O₂cathode materials.The results show that Al-F co doped materials are prepared by sol-gel method.The metal spacing of Li(Ni_0.5Co_0.2Mn_0.3)_1-xAl_xO_2-xF_x materials increases,the particle size increases after doping,and the morphology boundary is clearer.When the Al-F content is 0.03,in the range of 0.1C and 3.0～4.6V,the cycling performance and rate performance of the material are better.The first discharge capacity of the material is168.6m Ah/g at room temperature and 181.5m Ah/g at high temperature,which is slightly lower than that of the undoped material.After 50 cycles of charge and discharge,the specific discharge capacity of the material at room temperature is 157.8m Ah/g,and the capacity retention rate is 93.6%.At high temperature,the specific discharge capacity of the material is155.1m Ah/g,and the capacity retention rate is 85.5%.Al-F doping focuses on alleviating crystal microcracks and phase transitions.The layer spacing of the Li_1-xNa_x(Ni_0.5Co_0.2Mn_0.3)_1-xTi_xO₂materials prepared by co-precipitation method increases,the degree of cation mixing decreases,the primary particles increase,and the space between the primary particles increases.When the content of Na-Ti is 0.02,in the range of 0.1C and 3.0～4.6V,the initial discharge capacity at high temperature and room temperature is smaller than that of undoped material,which is 179.2m Ah/g and 189.0m Ah/g respectively,but the initial coulomb efficiency increases to 85.2%and 82.6%,respectively.After 50 cycles,the capacity retention at room temperature and high temperature increased to 94.7%and 86.6%respectively.Na-Ti doping increases the diffusion coefficient of crystal lithium ions while maintaining lattice stability.The layer spacing and lattice volume of the Li_1-xK_x(Ni_0.5Co_0.2Mn_0.3)O_2-xCl_xmaterials prepared by co-precipitation method increase,the degree of cation mixing decreases and the degree of sintering increases.When the doping amount of K-Cl is 0.02,in the range of 0.1C and 3.0～4.6V,the initial discharge capacity of the material is 188.6m Ah/g at high temperature and 206.6m Ah/g at room temperature,which is higher than that of the undoped material.The capacity retention rate of 50 cycles is 94.1%and 87.3%in two environments.K-Cl doping can increase the specific capacity and reduce the polarization of the electrode.The LiNi_0.5Co_0.2Mn_0.3O₂cathode materials were coated with relatively stable Mg F₂ and ionic conductive Na Al O₂,respectively.The results show that the Mg F₂ coating has no effect on the structure of the raw material,and the coating can make the particle dispersion of the material better and the corners more rounded.Mg F₂ coating can reduce the first discharge capacity of the material,and improve the cycle performance and rate performance.When the Mg F₂coating content is 1wt.%,the first discharge capacity is 166.8m Ah/g,and the capacity retention rate is 95.2%after 50 cycles.Na Al O₂ coating can increase the layer spacing of crystal structure to a certain extent,but has little effect on the degree of cation mixing.In addition,the coating can reduce the agglomeration of material particles.The Na Al O₂ coating can increase the initial discharge capacity,improve the cycle performance and rate performance of the material.When the Na Al O₂coating content is 3wt.%,the first discharge capacity is 180.6m Ah/g,the discharge capacity is 169.4m Ah/g and the capacity retention rate is 93.8%after 50 cycles.