Preparation And Modification Of Manganese-based Lithium-rich Phase Materials For Lithium-ion Battery

At present,the main contradiction in the electric vehicle consumer market is the contradiction between the growing requirement for the endurance mileage and the low energy density of power batteries.The fundamental reason for the limited energy density of power batteries is the limited capacity of the positive and negative electrodes in commercial lithium-ion batteries.In this thesis,the manganese-based lithium-rich phase material aLi2MnO3·(1-?)LiMO2(M=Mn,Co,Ni)is focused as it is the promising lithium-ion battery cathode materials with the highest energy density,simple preparation method and low cost.Through surface coating,ion doping and material compounding strategies,the key scientific problems such as the low initial coulombic efficiency,voltage drop during cycling,poor cycle/rate performance and poor high/low temperature performances of lithium-rich phase materials are treated.It provides an experimental basis for the manganese-based lithium-rich phase cathode material to be widely used in the next generation of high energy density lithium-ion batteries.The 1st chapter of the thesis first introduces the background of the birth of lithium-ion batteries and the working principle of lithium-ion batteries.Then,a brief overview is given on the structure,advantages,disadvantages,and modification strategies of several important cathode materials and anode materials.Finally,the background and research content of this thesis are outlined.The 2nd chapter summarizes the drugs and experimental instruments used in the experiment,and introduces the methods of lithium-ion battery assembly and electrochemical performance testing.In chapter 3,the lithium-rich phase Li1.2Ni0.2Mn0.6O2 cathode material is prepared by the ?-MnO2 precursor method supplemented by solid phase sintering.The composition of residual lithium on the surface of the lithium-rich phase is explored,and the Ni/Mn acetates are used to coat the lithium-rich phase to eliminate the residual lithium on the surface and form a spinel coating layer.A great improvement in the comprehensive performance of the lithium-rich phase material is obtained.In chapter 4,the lithium-rich phase Li1.2Ni0.13Co0.13Mn0.54O2 nanoparticles are prepared by a sol-gel method.The in-situ decomposition of LiPF6 forms a uniform and thin LiF coating layer on the surface of the lithium-rich particles.The influences of LiF coating on the morphology and structure of the lithium-rich phase as well as the improvement mechanism of its electrochemical performance are investigated.Chapter 5 uses NaF to modify the lithium-rich phase Li1.2Ni0.13Co0.13Mn0.54O2 by surface coating.The study of the Li+/Na+exchange mechanism behind the double gradient modification is conducted,including the depth of gradient doping and the content of NaF.The optimal coating amount is obtained that results in a modified lithium-rich phase material with excellent electrochemical performance.Chapter 6 expands the research of the NaF-coated lithium-rich phase in Chapter 5 by using KF-coated lithium-rich phase Li1.2Ni0.13Co0.13Mn0.54O2 material.The Li+/K+ exchange mechanism is confirmed and,as a result of the double gradient coating modification,the electrochemical properties such as the cycle and rate performances of the lithium-rich phase are improved.Chapter 7 conducts a Cs doping study on the lithium-rich phase Li1.2Ni0.13Co0.13Mn0.54O2 material.We have explored the effect of Cs doping on the interlayer spacing of the lithium-rich phase lithium layer,optimized the doping amount,and realized greatly improved performance.In Chapter 8,the initial coulombic efficiency of Li1.2Ni0.13Coo.13Mn0.54O2 material with lithium-rich phase is improved.The lithium-deficient Cr2O5 cathode material and the lithium-rich phase are physically mixed as the electrode material.The composite electrode assembly is systematically explored.Based on the coulombic efficiency matching mechanism,the full cell is assembled with the matching electrode and the Li4Ti5O12 negative electrode.In this way,the originally low initial coulombic efficiency can be raised to be close to 100%.Finally,Chapter 9 summarizes the full text,summarizes the innovations and deficiencies,and gives work plans in the future.