Study On The Modification Of LiNi_0.5Co_0.2Mn_0.3O₂ Ternary Cathode Materials

The gradual depletion of the earth's fossil energy and the increasing environmental problems have continuously promoted the rise of green energy.Lithium ion batteries have attracted much attention as a kind of reusable new energy devices.Lithium ion battery has many advantages,such as green environmental protection,high energy density,long cycle life,low self-discharge rate and simple reaction mechanism.It has become a research hotspot in recent years.The lithium ion battery in 3C digital has developed mature,and the performance of lithium ion power battery in the new energy vehicle still needs to be improved.The development of lithium ion battery anode materials with high energy density,good cycle performance,will be the focus of current research in lithium battery.The nickel based three element LiNixCoyMn2O2(NCM)series has a high theoretical specific capacity(280 mAh g-1),and is the most potential type of the next generation of lithium ion cathode materials.The comprehensive performance of LiNi0.5Co0.2Mn0.3O2(NCM523)is outstanding in terms of energy density,cost and safety.It is the first choice for cathode materials of Li ion battery.However,under the condition of high temperature(55?),the stability of LiNi0.5Co0.2Mn0.3O2 cycle becomes worse and the safety performance decreases.The problem of fast fading of capacity at high cut-off voltage(?4.6 V)seriously limits its large-scale industrial production.In this paper,in order to solve these problems,to further improve the comprehensive electrochemical properties of LiNi0.5Co0.2Mn0.3O2 materials,we use LiMnPO4(LMP)as the coating layer of modified LiNi0.5Co0.2Mn0.3O2 materials;LiNi0.497Co0.i92Mn0.3Mg0.003Ti0.008O2 materials were prepared by Ti4+ and Mg2+ codoped system.In order to improve the secondary reaction and structural collapse of the cathode material of lithium ion battery in the process of charging and discharging,the NCM523 was coated modified.After a variety of researches were carried out by solid phase coating method and two liquid phase coating methods,the sol-gel method in liquid phase was finally selected,and LMP was successfully coated on the surface of NCM523 cathode material.When the coating amount is 3 wt.%and the coating thickness is about 25nm,the sample has the best electrochemical performance.Through the constant current charge discharge cycle test,it is found that the coating is very effective for improving the cyclic stability of the high temperature condition.After 200 cycles of 1C ratio at 55 ?,3 wt.%LMP coated samples still retain the capacity of 125.2mAh g-1(Capacity retention rate 70.3%),which is much higher than that of the pristine samples 92.2mAh g-1(Capacity retention rate 51.1%).This remarkable improvement can be attributed to the change in the structure of the sample after LMP coating,which makes the surface structure of the material more stable.In summary,the electrochemical performance of LiNi0.5Co0.2Mn0.3O2 can be improved by LMP coating.For lithium ion battery materials,the attenuation of the working capacity at high cut-off voltage will be accelerated.In order to improve the stability of high pressure cycling,LiNi0.497Co0.i92Mn0.3Mg0.003Ti0.008O2 was prepared by codoping Ti4+ and Mg2+ with n precursor:nTi= 1:0.008 and n precursor:nMg= 1:0.003 dopants.XRD refinement results show that Ti49+ and Mg2+ codoped samples of I(003)/I(104)ratio increased.It is indicated that doping improves the mixing of cations in the crystal structure.Through the constant current charge discharge cycle test,the metal cation doping is very effective in improving the cyclic stability of high cut-off voltage.After 500 cycles of 1C ratio in the range of 2.8-4.6V voltage,Ti4+ and Mg2+ codoped samples still retain the capacity of 118.6mAh g-1(Capacity retention rate 63.4%),which is much higher than that of the pristine samples 57.1 mAh g''(Capacity retention rate 31.4%).