Synthesis And Modification Of Lithium Nickel Cobalt Manganese Oxide Cathode Materials For Lithium-ion Batteries

Li-Ni-Co-Mn oxide cathode materials have becoming one of the most important cathode materials for power lithium ion battery because of their high specific capacity,good cycle performance,moderate production cost and high safety.In this paper,different preparation methods and their optimum preparation conditions were explored based on LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials.In view of the drawbacks of poor cycle performance at high voltage,high cationic mixing phenomenon and low electronic conductivity,the LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials with better performance were prepared by ion doping and coating.The LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials were synthesized by co-precipitation method.The synthesis conditions were optimized as follows,Li₂CO₃ as the source of lithium,NaOH as the precipitant,pH value 11.0,stirring rate 600 r/min,reaction temperature 50?and sintering temperature 900?.On this basis,the conductivity of LiNi_1/3Co_1/3Mn_1/3O₂ were improved by adding a certain amount of graphene?GO?as conductive agent.The results showed that the discharge capacity of the material was obviously increased to 189.4 mAh/g at 0.2C when the amount of GO was 15.0%.In order to improve the structural stability of LiNi_1/3Co_1/3Mn_1/3O₂,the LiNi_1/3Co_?1/3-x?Al_xMn_1/3O₂?x=0.005,0.010,0.030?cathode materials were prepared by aluminium doping.The results showed that LiNi_1/3Co_?1/3-x?Al_x Mn_1/3O₂ materials were still?-NaFeO₂ layer structure,Co³⁺sites were occupied by Al³⁺,the c/a value increased while the lattice parameters c and a decreased,hence the layered structure of the material was more stable,the phenomenon of cation mixing was reduced and the Li⁺stripping efficiency was improved.The discharge capacity of LiNi_1/3Co_?1/3-0.01?Al_0.01Mn_1/3O₂ was 148.0 and125.8mAh/g in 2.5-4.6V at 1.0 and 5.0C respectively,the capacity retention was 66.2%and58.8%after 50 cycles,which was increased by 11.4%and 16.4%compared with those undoping samples.LiNi_1/3Co_1/3Mn_1/3O₂ materials were prepared by high temperature solid-state method and the preparation condition were optimized.The Li(Ni_1/3Co_1/3Mn_1/3)_1-xMg_xO₂?x=0.01,0.03,0.05?cathode materials were prepared by doping magnesium.The results showed that Ni²⁺sites were occupied by Mg²⁺which resulted in the increase of the oxidation state of Ni and electron generation,therefore the band structure was changed and the intrinsic electron conductivity was improved.In addition,Mg²⁺doping reduced the content of Ni²⁺,the probability of occupation of the 3a sites of Li⁺by Ni²⁺was reduced and the phenomenon of cationic mixing was reduced correspondingly.The materials showed the best performance when x(Mg²⁺)was 0.03.The discharge capacity was 106.8mAh/g and the capacity retention remained 86.0%after 50 cycles in 2.5-4.3V at 5.0C.LiNi_1/3Co_1/3Mn_1/3O₂ were modified by coating,and the coating material,coatingmethod and coating amount were explored.The results showed that the properties of the materials were the best when the Al2O3 coating amount was 1.0%by chemical precipitation.In this case,the specific surface area of the material was enlarged and the charge diffusion rate was increased,in the meantime the Al2O3 effectively relieved the dissolution of the active substance by the electrolyte,the concentration of the active substance was increased and the electrochemical performance of the material was improved.Under the test voltage of 2.5-4.3V,the specific capacity of the coated material was 149.6,140.6,130.2,111.9 and92.3mAh/g at 0.1,0.2,0.5,1.0and 5.0C respectively,which increased by 6.25%,6.8%,10.3%,11.8%and 19.1%compared with the bare samples.