Modification Of LiNi_0.5Co_0.2Mn_0.3O₂-type Cathode Materials

Layered LiNi0.5Co0.2Mn0.3O2-type cathode material has attracted much more attention due to its higher theoretical capacity than those of LiNi1/3CO1/3Mn1/3O2 and its many other advantages.In this dissertation,a "two-step-drying and calcination method"was used to synthesis series of LiNi0.5Co0.2Mn0.3O2-type cathode materials.The optimization synthesis condition,such as aging temperature,different manganese sources,5-Nitro-2-furaldehyde diacetate quality,theoretical composition of the sample,investigated according to the relationship among synthesized condition,structure and electrochemical performance of as-synthesized sample.The many advanced techniques,such as XRD,charge-discharge cycling,cyclic voltammetry,AC impedance,Fourier transform infrared spectroscopy and scanning electron microscopy,employed to investigate which is the key factor to determine sample performance.The research results as follows:1.The effect of aging temperature of the precursor on the structure and electrochemical performance studied used Li2CO3,Ni(CH3COO)2·4H2O,Co(CH3COO)2·4H2O and Mn(CH3COO)2·4H2O as start agents.The experimental result show the as-synthesized sample exhibits the best electrochemical performance while the aging temperature is 50 C in the temperature zone of 40 to 60 C.In the voltage range of 2.5V to 4.3V,the sample with the particle size of 0.2?m exhibits the capacities of 172.3 mAh/g(0.1C rate),161.4 mAh/g(1C rate),136.9 mAh/g(5C rate),121.4mAh/g(10C rate)(1C=180mAh/g),respectively.2.The effect of manganese sources on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2-type cathode materials studied used Li2CO3,Ni(CH3COO)2·4H2O,Co(CH3COO)2·4H2O and MnCO3 as start agents.The experimental result shows the as-synthesized sample exhibits the capacities of 147.7mAh/g at the 1st cycle,142.9mAh/g at the 40th cycle and the capacity retention rate is 96.8%in 40 cycles in the voltage range of 2.5 to 4.3 V and at the 1C rate.While MnCO3 selected as a manganese source instead of Mn(CH3COO)2-4H2O during the synthesis,as-synthesized sample exhibits improved cycle life and decreased capacity.3.The effect of 5-Nitro-2-furaldehyde diacetate used in the synthesis of precursor on the electrochemical performance of LiNi0.5Co0.2Mn0.3O2-type cathode materials studied used Li2CO3,Ni(CH3COO)2·4H2O,Co(CH3COO)2·4H2O and MnCO3 as the start agents.The experimental result shows the optimization sample synthesized while 5-Nitro-2-furaldehyde diacetate added into the start agents according to the ratio of 1mol Li+to 0.1 Og 5-Nitro-2-furaldehyde diacetate during synthesis of the precursor.The optimization sample exhibits the capacities of 157.7mAh/g at the 1st cycle,154.4mAh/g at the 40th cycle and the capacity retention rate is 97.9%in 40 cycles in the voltage range of 2.5 to 4.3 V and at the 1C rate.The optimization sample exhibits the capacities of 141.8(2C rate),127.2 mAh/g(5C rate),113.4mAh/g(10C rate)(1C=180mAh/g),respectively.And its charge-transfer resistance is 19.3?;its Li+ diffusion coefficient is 5.52 10-16cm2/s.4.The theoretical composition of the cathode materials is optimization used Li2CO3,Ni(CH3C00)2·4H2O,Co(CH3COO)2·4H2O,MnCO3 and 5-Nitro-2-furaldehyde as start agents.The experimental result shows the optimization sample possesses the theoretical composition of LiNi0.51Co0.2Mn0.29O2.In the voltage zone of 2.5V to 4.3V,the optimization sample exhibits the capacities of 186.0(1C rate),146.5(5C rate),128.6mAh/g(10C rate)(1C=180mAh/g),respectively.And its charge-transfer resistance decreased with the proper increasing of nickel content in the as-synthesized sample.