Preparation And Electrochemical Properties Of The Lithium-rich Manganese-based Cathode Material

Because of high energy density and wide discharge platform,lithium ion batteries（LIBs）as an energy storage device are widely used in portable electronic products and electric vehicles.In LIBs,the cathode material is a key factor to limit their real-world applications.Currently,the commercial cathode material of Li Co O₂has been unable to meet the ever-increasing demand due to the restrictions from low discharge capacity.Lithium-rich manganese-based cathode materials[x Li₂Mn O₃·（1-x）Li MO₂,M=Ni,Co,Mn or combination]have a relatively high specific discharge capacity and then it is considered as an excellent alternative for Li Co O₂.However,there are some problems such as poor cycle performance,low initial Coulombic efficiency and fast voltage attenuation.In this work,to address these problems,the morphology,structure and electrochemical properties of lithium-rich manganese-based oxide of Li_1.2Mn_0.6Ni_0.2O₂were investigated from the aspects of synthesis method and material modification.The following research results are obtained:（1）The cathode material precursor was synthesized by ultrasound-assisted chemical reduction.After heat treatment at 850℃for 12 h,the layered cathode material of with high purity was successfully prepared.The experimental results show that the as-prepared Li_1.2Mn_0.6Ni_0.2O₂exhibits a fluffy structure composed of primary irregular blocks,which is conducive to a comprehensive contact between electrolyte and cathode,thereby giving rise to a superior electrochemical performance.At a current density of 0.2C（40 m A/g）,the first charge and discharge capacity are 337.5 and 236.9 m A·h/g,respectively.After 100 cycles,the specific discharge capacity is maintained at about 210m A·h/g with an average voltage attenuation of only 0.0043 V.At 0.5 and 2 C,the discharge capacity after 100 cycles is kept at about 190 and 150 m A·h/g.After rate performance test at different current densities,the discharge capacity can be back to 260m A·h/g when the current density is converted back into 0.1 C.（2）The as-prepared Li_1.2Mn_0.6Ni_0.2O₂was modified with sodium doping by melt impregnation.The influence of sodium doping amount on the morphology,structure and electrochemical properties of the material was systematically studied.There is no obvious change in the morphology and crystal structure of Na-doped samples and also no new phase is formed.Both cell parameters of a and c are slightly increased with increasing amount of doping sodium,which could be conducive to the migration of lithium ions.At a current density of 0.1 C,the initial charge and discharge capacity is340.2 and 249.0 m A·h/g,respectively.After 100 cycles at 0.5 C,the discharge capacity is maintained at about 220 m A·h/g.Moreover,the voltage decay during the repeated cycles is also suppressed.（3）Li_1.2Mn_0.6Ni_0.2O₂was doped with Ce by means of high-temperature combustion.After heating at 500℃for 30 min,the sample consisting of regularly hexagonal flake were prepared.The experimental results show that when the doping Ce amount is at x=0.01,the sample exhibits the superior electrochemical performance.At 0.1 C,the specific capacity for the first charge and discharge cycle is 348.8 m A·h/g and 275.3 m A·h/g with an initial coulomb efficiency of 78.9%.After 50 cycles at 0.2 C,the discharge capacity is kept at 231.4 m A·h/g.