Ultrasonic Spray And Freeze Drying Method To Synthesize Cathode Materials For Lithium Ion Secondary Batteries

On account of the toxicity and high cost of the current widely used LiCoO₂ cathode materials,the researchers are in urgent need to find an alternative cathode material for it.The layered ternary material LiNi_xCo_yMn_1-x-yO₂ combines the advantages of LiCoO₂,LiNiO₂ and LiMnO₂ with big reversible capacity,stable structure,high safety,and lower cost.LiMnPO4 has a charge-discharge platform as high as 4.1 V,which indicates a high energy density and a vast potential for development.These two kinds of cathode materials with excellent performances and no obvious shortages,are now the very hotpots for cathode developing today.In this thesis,LiNi_1/3Co_1/3Mn_1/3O₂ and LiMnPO4 materials were synthesized by ultrasonic spray and freeze drying method,and the mainly accomplished jobs are as follow:?1?The layered ternary material LiNi_1/3Co_1/3Mn_1/3O₂ was synthesized through ultrasonic spray,freeze drying,pre-sintering and sintering,using lithium acetate,nickel acetate,cobalt acetate and manganese acetate as the raw materials.Thermogravimetric analysis?TG?,X-ray diffraction?XRD?,scanning electron microscope?SEM?,Energy dispersive spectroscopy?EDS?and X-ray photoelectron spectroscopy?XPS?tests were conducted to analyze the preparation of the material and the prepared product.The appropriate temperature for pre-sintering was finally determined to be at 500?,and the temperature for sintering to be above 800?.The LiNi_1/3Co_1/3Mn_1/3O₂ material sintered at 850? had an accumulated porous structure with a particle size range of 300 nm-400 nm and an uniformed element distribution.The main valences of the nickel,cobalt and manganese elements in the material were respectively+2,+3,+4,conforming to the literatures.Finally,the software simulation and crystal structure modeling were utilized to explain the contact between the 003/104 peak intensity ratio in the XRD of LiNi_1/3Co_1/3Mn_1/3O₂ and the lithium-nickel cation mixing.?2?The heating rate during pre-sintering,sintering temperature,sintering time and the quantity of lithium salt in the solution were controlled to add parallel experiments,and X-ray diffraction?XRD?,scanning electron microscope?SEM?and charge-discharge tests were used to optimize these preparation conditions.The optimized processes were:adding stoichiometric ratio of lithium salt in the solution,pre-sintering at 500? for 5 h with a heating rate of 2?/min,sintering at 850? for 9 h after grinding.It showed that the finally achieved material had a discharge capacity of 181.87 mAh/g at 0.05 C,79.52 mAh/g at 2 C.The discharge capacity declined to 88.35 mAh/g from 140.61 mAh/g at 0.5 C after 55 cycles,displaying a capacity retention of 62.83%.?3?The oxide Mn₂O₃ was prepared by ultrasonic spray and freeze drying method,using Mn?CH3COO?2·4H2O as the raw material.Carbon-coated LiMnPO4 was further synthesized after ball-milling and sintering,with sucrose as the carbon source,lithium hydrogen phosphate as the lithium source.It proved that the olivine structure was successfully prepared after sintering at 650??850?,and the diffraction peak was very sharp,which indicated high crystallinity.The product had an accumulated porous structure with a particle size range of 90 nm?230 nm,and the carbon layer was uniformly formed on the surface of the particles.