Study On Nickel-rich Ternary Cathode Material And Its Lithium Storage Performance

The traditional cathode materials LiCoO2,LiNiO2 and LiMnO2 for lithium ion batteries have some disadvantages,such as poor thermal stability and harsh synthesis conditions.The layered structure of LiNixCoyMn1-x-yO2 positive material has fully played its own advantages through the coordination of three elements Ni Co Mn:Ni element improves capacity,Co reduces cation mixing and improves ratio performance,and Mn element stabilizes the structure of material.Therefore,LiNixCoyMn1-x-yO2 cathode material has become one of the cathode materials with great practical value and research significance.LiNi0.5Co0.2Mn0.3O2 cathode material has been industrialized due to its relatively high reversible specific capacity,good cycle life and rate performance,low cost and low toxicity.However,the structure of LiNi0.5Co0.2Mn0.3O2 is unstable during high voltage charging and discharging,and it is prone to irreversible phase transition,which aggravates the corrosion of electrolyte on nickel,cobalt and manganese,and seriously affects the safety performance and cycle performance of the battery.In this paper,we first synthesized LiNi0.5Co0.2Mn0.3O2 cathode materials by solid-state method as reference samples,and then improved the interface and structural stability,energy density and long cycle life of LiNi0.5Co0.2Mn0.3O2 cathode materials by various modification methods such as Cation Anion co doping,surface single-layer coating and surface double-layer coating.The main research contents are as follows:Using Li2CO3,Ni0.5Co0.2Mn0.3(OH)2 and KCl as raw materials,K and Cl ions were Co doped into LiNi0.5Co0.2Mn0.3O2 bulk phase by high temperature solid state method to modify LiNi0.5Co0.2Mn0.3O2.XRD patterns show that the samples with different doping amount have high crystallinity,single phase and good layered structure,and the cell volume increases slightly,which indicates that doping ions expand the lithium layer space and facilitate the migration of Li+.The structure refinement data show that after doping,the number of Li sites occupied by K+,O sites occupied by Cl-,and Li sites occupied by N2+ decreases,and the cation mixing decreases obviously,which is conducive to the stability of crystal structure.The charge discharge test in the voltage window of 2.7-4.6V shows that the initial discharge capacity of the sample increases with the increase of K and Cl Co doping.When the K and Cl content is 3%,the specific capacity is 210 mAh·g-1 at 0.1C.At 1C rate,the discharge capacity of LiNi0.5Co0.2Mn0.3O2 doped with 1 mol%KCl is about 28 mAh·g-1 higher than that of pure LiNi0.5Co0.2Mn0.3O2.LiNi0.5Co0.2Mn0.3O2 doped with 1 mol%KCl has the best performance.Aluminum isopropoxide and 1 mol%K and Cl Co doped LiNi0.5Co0.2Mn0.3O2 were used as raw materials.Al(OH)3 was prepared by hydrolysis of aluminum isopropoxide.Al(OH)3 was deposited on lmol%K and Cl Co doped LiNi0.5Co0.2Mn0.3O2 particles.After sintering,Al2O3 was formed on the surface of LiNi0.5Co0.2Mn0.3O2 cathode material.The crystal structure,morphology and surface chemical state of the modified materials were analyzed by XRD,SEM and TEM.The electrochemical properties of the modified materials were characterized by high voltage(4.6V).The results show that the coating does not change the morphology of LiNi0.5Co0.2Mn0.3O2 cathode material,and there is a layer of Al2O3 coating on the surface of LiNi0.5Co0.2Mn0.3O2 cathode material.The results show that when the content of K and Cl and the content of Al2O3 are 1%mol,2%mass fraction,the comprehensive performance of the material is the best,the first discharge specific capacity of 0.1C is 210 mAh·g-1,the coulomb efficiency is improved,and the irreversible capacity loss is reduced.The first discharge capacity at 0.1C is 210 mAh·g-1,the coulomb efficiency is improved and the irreversible capacity loss is reduced.Compared with pure LiNi0.5Co0.2Mn0.3O2,the specific capacity at 5C is increased by 26 mAh·g-1,and the capacity retention rate at 1C is increased by 16%after 100 cycles.The effects of PAS content on the morphology and properties of LiNi0.5Co0.2Mn0.3O2 were studied by using phenolic resin and 1mol%K and Cl Co doped LiNi0.5Co0.2Mn0.3O2 as raw materials.The results show that the electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material coated with 2wt%PAS is the best.After 100 cycles,the capacity of LiNi0.5Co0.2Mn0.3O2 cathode material is 130 mAh·g-1,and the capacity retention rate is 70%,which is 38 mAh·9-1 higher than that of pure LiNi0.5Co0.2Mn0.3O2 cathode material.The discharge specific capacity of 5C rate is 162 mAh·g-1,which is 23 mAh·g-1 higher than that of pure LiNi0.5Co0.2Mn0.3O2.On the basis of single layer coating,the NCM523 positive material was modified by Al2O3 and PAS double coating.The battery with excellent electrochemical performance was explored by using the advantages of the two materials.The crystal structure,morphology and surface chemical state of the modified materials were analyzed by XRD,SEM and TEM,and the charge discharge test was carried out at high voltage.The results show that after 100 cycles,the capacities of LiNi0.5Co0.2Mn0.3O2 materials coated with 2wt%Al2O3-1wt%PAS and 1%PAS-1%Al2O3 are 129 mAh·g-1 and 113 mAh·g-1 respectively,and the discharge specific capacities are 158 mAh·g-1 and 155 mAh·g-1 respectively at 5C rate.The coulomb efficiency is also improved.After coating with 2%Al2O3-2%PAS and 5%PAS-2%Al2O3 the capacity retention rate is 63%and 71%respectively,which is increased by 11%and 19%compared with the uncoated material.