Synthesis And Doping Modification Of LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Material For Li-ion Batteries

For promoting the electromonility revolution,it is critically important for electric vehicles to develop power sources which guarantees long driving ranges of electric vehicles.Besides H₂/O₂ fuel cell vehicles,lithium-ion batteries are the most suitable power source for electrochemical propulsion of electric vehicles,which mainly benefits from its ability to provide necessary energy density,cycle life,stability and reasonable safety performance.As the main factor limiting the energy content of LIBs,cathode materials have been widely concerned.Among them,the layered lithiated oxides LiNi_xCo_yMn_1-x-yO₂（NCM;0≤x,y,x+y≤1）of transition metals（TMs）with a dense lattice exhibit high specific capacity of unit volume storage,especially for the high nickel oxides(LiNi_0.6Co_0.2Mn_0.2O₂,LiNi_0.8Co_0.1Mn_0.1O₂,etc.).However,as the increase of nickel content,the stability deteriorates,which seriously affects the electrochemical performance.Therefore,effective measures must be taken to improve this disadvantage.In this paper,the homogeneous precursor Ni_0.6Co_0.2Mn_0.2（OH）₂ has been synthesized via co-precipitation with Na OH and NH₃·H₂O as precipitant agent and complexing agent respectively.The Ni-rich oxide LiNi_0.6Co_0.2Mn_0.2O₂ product has been prepared by sintering a stoichiometric mixture of lithium salt.The effects of different experimental conditions on the morphology,crystal structure and electrochemical properties of the materials are studied by different test and characterization methods.The results show that the precursor particles prepared under the condition of the p H value 11.2 and ammonia concentration of 0.8 mol L^-1 are regular and dense.After evenly mixing with the Li OH·H₂O（1:1.05）,the layered LiNi_0.6Co_0.2Mn_0.2O₂ material not only has a good layered structure,but also has excellent cycling performance,which sintering at 800℃for 12 hours.After 100 cycles,the capacity of LiNi_0.6Co_0.2Mn_0.2O₂ sample decreases from the initial 165.5 m Ah g^-1 to 149.2 m Ah g^-1with a retention rate of 90.2%.In order to improve the electrochemical performance of the ternary nickel-rich cathode material,the LiNi_0.6-xCo_0.2Mn_0.2M_xO₂（M=Cr,Cu,Fe,Sn and Zn,0≦x≦0.02）and LiNi_0.6Co_0.2-xMn_0.2M_xO₂（M=Cu&Mg,Cu&Fe,x=0,0.01）products have been prepared respectively by doping with different types of single or double elements.A preliminary research is conducted on the doping effects of the morphology comparison,crystal structure analysis and electrochemical properties test to compare the doping effects.The results show that,in terms of single element doping,a small amount of Cu has the most significant effect on improving material properties.After 200 cycles,the Cu-doped product has a higher discharge capacity of 142.5 m Ah g^–1,and the capacity retention rate is 86.2%,which is better than that of LiNi_0.6Co_0.2Mn_0.2O₂ sample（82.7%）.It can be seen that Cu,as a new type of doping agent for layered LiNi_0.6Co_0.2Mn_0.2O₂ cathode material,has great potential in improving the structure and performance of the material.In the aspect of double elements doping,the doping effect of the Cu&Mg is better than that of the Cu&Fe.Combined with the good doping effect of the Cu dopant,a novel Cu-NCM cathode material,LiNi_0.6Co_0.19Mn_0.2Cu_0.01O₂（Cu-NCM）,has been constructed and synthesized by in-situ doping method.A systematically research is conducted on the doping effect of the surface morphology,crystal structure and electrochemical properties.The mechanism of Cu doping has been aslo analyzed in detail.The results indicate that the incorporation of Cu effectively promotes lithium ion diffusion,reduces the degree of Li/Ni mixing and stabilizes the structure of the LiNi_0.6Co_0.2Mn_0.2O₂ during repeated cycles.As a consequence,the doped Cu-NCM sample delivers excellent electrochemical performances.Even at a large rate of 5C,the Cu-NCM exhibits a high discharge capacity of 140.8 m Ah g^–1 with an increased capacity retention of 77%as compared to that of the bare LiNi_0.6Co_0.2Mn_0.2O₂（62%）after350 cycles.The CV and EIS results also provide evidence and explanation for the improvement of electrochemical performance.The structural transformations of the Cu-NCM cathode material under different cycles are further evidenced by XRD and Raman techniques.