Synthesis And Electrochemical Properties Of Lithium-rich Manganese-based Cathode Materials By Polyacrylamide Gel Method

Lithium-rich manganese base material has become one of the most promising cathode materials because of its high specific energy,high energy density,low cost and environmental protection.In this thesis,lithium-rich manganese-based Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanomaterials were prepared via polymer-network gel method.The polymer-network gel method has advantages in experiment and process.It can prepare nanoparticles with smaller,uniform particles and non-agglomerated.Subsequently,the materials were modified by doping and coating,thus proposing a new means of synthesis as well as ideas for property improvement.The main work includes:（1）Uniform,small particle size and no agglomeration nanoparticles were prepared by polymer-network gel method.The effects of different calcination temperatures on the structure,morphology and electrochemical properties of the materials were investigated.Determine the optimal calcination temperature of the pure samples.The experimental results shows that the sample have good structural morphology and the best electrochemical properties when the calcination temperature is 850°C.The initial discharge capacity is 272.5 mAhg^-1 at 0.1 C,the initial coulomb efficiency is 76%.After 100 cycles at 1 C,the capacity retention rate is 80%.（2）The surface of material particles was coated with fast ion conductor LiAlO₂coating.The experimental results shows that when the thickness of the LiAlO₂ is about 8.7 nm,the sample has good structural morphology,and the electrochemical performance is also greatly improved.the initial discharge capacity is 293.2 mAhg^-1.The reason for the increase capacity may be that the fast ion coating on the surface of the sample provides more lithium-ion channels.The surface coating slows the corrosion of the material by the electrolyte.At the same time,it also reduces the resistance of the material and improves the rate performance.The discharge capacity is 106.1 mAhg^-1 at the high current density of 5 C.（3）The lithium-rich material was modified by Li₂O-2B₂O₃coating.Li₂O-2B₂O₃protective layer was formed on the surface of the particles by a simple wet chemical method combined with secondary calcination.Coatings of different thicknesses have a great effect on material properties.The results shows that when the thickness of Li₂O-2B₂O₃ coating is 4.7 nm,the material can obtain the best electrochemical performance.The initial discharge capacity is 291.2 mAhg^-1 and the initial coulomb efficiency 86.7%.The capacity retention rate is 86.3%after 100 cycles at 1 C.At the high current density of 5 C,the discharge capacity is 116.8 mAhg^-1.Li₂O-2B₂O₃coating isolates the direct contact between the material and the electrolyte,reduces the corrosion effect on the material,stabilizes the structure and improves the performance of the material.（4）The lithium-rich manganese-based materials was modified by aluminum doping.The material was doped with metal element Al,and the optimal doping amount was discussed.The results shows that the appropriate amount of doping Al（x=0.04）has little effect on the structure and morphology of the material.The sample has the initial specific capacity of 297.4 mAhg^-1,and the initial coulomb efficiency is 83%.The capacity retention rate is 84%after 100 cycles at 1 C.In the rate performance test,the capacity is 114.1 mAhg^-1at 5 C.Al doping can improve the electrochemical performance of the materials because it can inhibit the migration of transition metal ions.（5）Boron doping modification for lithium-rich manganese-based Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ materials.Firstly,discussing the influence of the samples with different doping amounts on the structure and properties.The experimental results shows that when the doping amount is x=0.03,the structure of the sample is good and the electrochemical performance is improved obviously.The initial discharge capacity is 292.7 mAhg^-1 at 0.1 C,and the initial coulomb efficiency is88.7%.The specific capacity is 148.5 mAhg^-1at high current density of 5 C in the performance test.In the long cycle,the discharge capacity is 197.9 mAhg^-1 at 1 C,after 100 cycles,the discharge capacity is 177.6 mAhg^-1,and the capacity retention is89.7%.Boron doping stabilizes the structure of the main crystal lattice inside the material,improves the stability of oxygen,inhibits phase transition,and improves the material performance.