Synthesis And Modification Of LiNi_xCo_yMn_zO₂ As A Cathode Material For Li-ion Batteries

In order to solve the energy crisis and environmental pollution problems,the research of lithium-ion batteries attracts more and more attention.LiNi_x Co_yMn_zO₂ and Li-rich materials with high specific capacity are currently promising cathode materials.But there are still some shortcomings in the material that need to be improved.Firstly,the synthetic methods of Ni-rich material,LiNi_0.8Co_0.1Mn_0.1O₂,are studied in this paper.We have prepared different morphology and particle sizes of Ni-rich materials by the solvothermal method,coprecipitation method and sol-gel method.Results show that LiNi_0.8Co_0.1Mn_0.1O₂ prepared by solvothermal method is spherical,with uniform particle size and good specific capacity.When the LiNi_0.8Co_0.1Mn_0.1O₂electrode is charged/discharged at 0.1 C and 0.5 C between 2.7-4.5 V,the first cycle discharge capacities respectively are 200.5 and 159.5 mA h g^-1,which are higher than samples prepared by other two methods.Secondly,the LiNi_x Co_yMn_zO₂ materials of different nickel,cobalt and manganese have been synthesized by the solvothermal method mentioned above,including five samplesLiNi_0.8Co_0.1Mn_0.1O₂,LiNi_0.7Co_0.15Mn_0.15O₂,LiNi_0.6Co_0.2Mn_0.2O₂,LiNi_0.5Co_0.2Mn_0.3O₂,LiNi_0.3Co_0.3Mn_0.3O₂.The LiNi_0.8Co_0.1Mn_0.1O₂ sample has the best discharge performance that the first cycle discharge capacity is 200.5 mA h g^-1charged/discharged at 0.1 C,but the sample has maximum capacity attenuation.The LiNi_0.3Co_0.3Mn_0.3O₂ sample has the best cycle performance with the capacity retention81.6%after 100 cycles at 0.5 C.As the content of Ni in the samples decreases,the discharge specific capacity gradually decreases,and the capacity retention rate gradually increases.Surface modification of the material LiNi_0.5Co_0.2Mn_0.3O₂ has been investigated by coating Al₂O₃,MgO and LiFePO₄/C.The appropriate amount of coating can effectively improve the rate performance and cyclic stability of the material.The electrochemical performances of the modified samples are better when 1%Al₂O₃,1%MgO and 10%LiFePO₄/C are covered on the surface of the LiNi_0.5Co_0.2Mn_0.3O₂ material.The first cycle discharge capacities respectively are 191.7,197.6 and 187.2 mA h g^-1 charged/discharged at 0.1 C,and the capacity retentions respectively are 82.3%,81.4%and82.1%after 100 cycles at 0.5 C.Those are hingher than the LiNi_0.5Co_0.2Mn_0.3O₂ sample,which the first cycle discharge capacity is 160.8 mA h g^-1 and the capacity retention is79.5%after cycling.In addition,the Li-rich cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has been prepared to study the effect of different calcination times on the morphology and electrochemical properties.When the calcination time is 6 h,the sample Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has uniform particle and moderate size with good rate and cycle cyclic performance.The sample has the maximum discharge capacity,168.9 mA h g^-1,after 100 cycles at 0.5 C.A ball milling method is adopted to cover LiFePO₄/C material on the surface of Li-rich materials.The sample with coaing 15%LiFePO₄/C exhibits better electrochemical performances with discharge capacities of 249.7 mA h g^-1 and capacity retention 86.6%after 100 cycles at 0.5 C.