Solvothermal Synthesis Of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ And Its Modification

Li-rich layered solid solution material xLi2MnO3·(1-x) LiMO2(M=, Mn, Ni, Co) is consider to be one of the most promising cathode materials in the field of high performance Li- ion batteries becouse of its discharge capacity can over 250 mAh/g between 2.0V and 4.6V. However, this kind of material also has fatal flaws: low tap density, poor rate performance and high initial irreversible capacity loss. This paper takes 0.5Li2MnO3?0.5Li[Mn1/3Co1/3Ni1/3]O2 as the object,and study on the modifications of nanoparticles, doping and surface modification :(1) Synthesis of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 by solvothermal method and the effects on structure and properties caused by different lithium salt content: The solvothermal method make material proportion controllable(evaporating off solvent), the process of sintering process is simple,and can get high purity and nanometer particles. Nonaparticle Li[Li0.2Mn0.54Ni0.13Co0.13]O2 has high purity through XRD and SEM characterization. Change the amount of lithium salt, namely Li:M=1.525, Li:M=1.545 and Li:M=1.575(M= transition metal), to study different lithium salt content effects on properties of materials, and Li:M=1.545 material performance better: its discharge capacity and first coulombic efficiency is 249.2 mAh/g and 78.5% respectively in the voltage range of 2.0-4.8V at 0.1 C. Under the continuously variable current density of 20 mA/g, 40 mA/g and 200 mA/g, discharge capacity is 249.2 mAh/g, 164.9 mAh/g and 88.6 mAh/g respectively. And the discharge capacity can be stabilized at 180 mAh/g when returning to 0.1 C.(2) Study on the modification of doping Sn and V into Li[Li0.2Mn0.54Ni0.13Co0.13]O2 : To improve electronic and ionic conductivity of material, reduce material charge transfer impedance, dopeselectively V and Sn into the Li[Li0.2Mn0.54Ni0.13Co0.13]O2.Through distribution analysis of elements,the doping elements V and Sn can be uniformly distributed into Li[Li0.2Mn0.54Ni0.13Co0.13]O2 material structure, electrochemical tests show that the metal Sn and V doping can improve the first discharge capacity, high rate performance and the initial efficiency.The discharge capacity and the first time efficiency of 1% Sn doped sample is 259.6 mAh/g and 79.8 % respectively at 0.1C, 0.5 % Sn doped sample discharge capacity can be up to 135.4 mAh/g at 1C; 3% V doped sample discharge capacity and coulombic efficiency is 270.2 mAh/g and 80% respectively at 0.1C, and its discharge capacity is 151.3 mAh/g at 1 C.(3) Study on the modification of coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 material: P2O5 is used as the raw material, because P2O5 easily absorbs moisture and partly invert to phosphoric acid, and Li[Li0.2Mn0.54Ni0.13Co0.13]O2 surface is alkaline, therefore P2O5 can be uniformly coated on the surface of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 with the help of acid-base reaction in the mixing process. P2O5 also can react with the surface make th surface a part of Li2 O pre out, combines advantages of surface treatment and surface coating. P2O5 coating amount is 3wt %,5wt % and 7wt %, respectively. When the coating amount is 7wt %, the initial discharge capacity is as high as 281.3mAh/g in 2.0-4.8V voltage range at 0.1C, the efficiency reached 80.4% for the first time, and after cycling 30 times the discharge capacity can remaine at 250mAh/g, the capacity retention rate is up to 88.9%, the original sample without coating under the same condition, only 76.9%. Obviously, the P2O5 coating can greatly improve the discharge capacity and cycle performance of material.