The Modification Research Of 5V Cathode Material LiNi_0.5Mn_1.5O₄ For Lithium Ion Batteries

Recently,the growing market demand has expanded the application of lithium ion batteries(LIBs)from small sized moble electronics to large-scale electric vehicles(EVs)and energy storage system.The future application on the large-scale devices requires lithium ion batteries with even higher energy density,long cycling capability and much more safer.Hence,improving the energy density of cathode materials has become very important nowadays.In comparison with with the commercialized cathode materials(LiCoO2,LiFePO4,etc.),LiNi0.5Mn1.5O4 delivers a higher working voltage(4.7 V)and competitive energy density(658 Wh kg-1).Moreover,LiNi0.5Mn1.5O4 cathode material with source-rich raw material,environmentally benign and simple preparation is especially suitable for large-scale applications.So in this thesis,different modified methods will be adopted to further improve the electrochemical performance of LiNi0.5Mn1.5O4.Chapter 1 gives a general introduction of LIBs about the mechanisum,application,evolution,cathode materials and anode materials.Additionally,LiNi0.5Mn1.5O4 cathode material is mainly depicted with its recent electrochemical performance and modified methods reported in references.Chapter 2 lists the experimental reagents and equipments.Then the prepared process of the electrode,the assembled process of the coin-cells for lithium ion batteries and the electrochemical analyses are also detailed introduced.In Chapter 3,Cu-,Mg-and Zn-doped LiNi0.5Mn1.5O4 electrode materials are synthesized via a thermopolymerization method.Cu-and Mg-doped LiNi0.5Mn1.5O4 sample have a P4332 space group.While Zn-doped LiNi0.5Mn1.5O4 sample with Fd3m space group,Zn2+ ions occupy 8a sites of Fd3m space group by displacing some Li+ ions originally at 8a sites.The LiNi0.45Cu0.05Mn1.5O4 and LiNi0.45Mg0.05Mn1.5O4 samples exhibit excellent rate performance(with specific capacities of 98.3 mAh g-1 and 92.4 mAh g-1 at 10 C rate,respectively)and good capacity retention at high temperature(55 ?)(with the capacities of 117.6 and 119.5 mAh g-1 after 100 cycles at 1C).In Chapter 4,the electrochemical performance of LiNi0.45Cu0.05Mn1.5O4 cathode with Fd3m crystalline structure is superior to the one with P4332.In Chapter 5,the metal oxides coatings(CuO,MnO2,ZnO)are separately formed on the surface of LiNi0.5Mn1.5O4 sample.In comparision,MnO2-coated LiNi0.5Mn1.5O4 shows the best electrochemical performanc.Furthermore,the optimal Mn02 content(3%)in the Mn02-coated sample delivers a discharge capacity of 108 mAh g-1 at 10 C rate,and a high capacity retention 80%(vs.66%for the uncoated sample)at 55 ? after 100 cycles.The significantly improved electrochemical performance of 3%MnO2-coated sample may be ascribed to the increased content of Mn3+ on the surface of the coated sample.In Chapter 6,a Co3O4 coating is formed on the surface of LiNi0.5Mn1.5O4 particles via an ethanol mediated precipitation method.The existence of Co3O4 coating can be observed by scanning electron microscopy and transmission electron microscopy.This coating layer can greatly enhance the cycling stability and rate performance of LiNi0.5Mn1.5O4.The optimal sample 5%Co3O4-coated LiNi0.5Mn1.5O4 delivers a discharge capacity 126 mAh g-1 at 1 C rate with a capacity retention 96.8%after 300 cycles and a high discharge capacity 97.5 mAh g-1 at 10 C rate.In Chapter 7,LiNi0.5Mn1.5O4 microspheres with hollow,solid or core-shelled inner structures for the lithium-ion batteries are designed and fabricated by a multi-step synthesis procedure.The core-shell structured LiNi0.5Mn1.5O4 exhibits excellent cycling stability and outstanding rate performance,which delivers a discharge capacity of 111.5 mAh g-1 after 200 cycles at 1 C,and 98 mAh g-1 at 8 C,respectively.Further analyses with cyclic voltammetry and electrochemical impedance spectroscopy demonstrate that the core-shell structure can provide structural stability and shortened lithium diffusion path at both room and elevated temperatures.At last,in Chapter 8,the author not only summarizes the innovativeness and deficiencies of this thesis,but also puts forward the expectations and suggestions for the possible futher research.