Synthesis, Modification And Electrochemical Properties Of Electrode Materials For Lithium Ion Batteries

In this thesis, Li3V2(PO4)3, a-NiMoO4, restacked MoS2, and MoS2/CNT composites were synthesized using rheological phase method or hydrothermal/solvothermal routes. The synthesis, crystal structure, and morphology of these materials were deeply studied. In addition, as electrode materials for the lithium ion battery, the electrochemical properties of these compounds or composites were also investigated. Li3V2(PO4)3was prepared using a rheological phase method and modified via both carbon-coating and Yttrium-doped. The results show that Li3Y0.03V2(PO4)3has a higher capacity and better cycling stability than the pure Li3V2(PO4)3. a-NiMoO4particles were also prepared thorough a rheological phase method at different temperatures and modified via carbon-coating. MoS2/CNT composites were synthesized through a solvothermal method. The electrochemical properties of MoS2/CNT composites are far better than that of the sample without CNTs. Restacked MoS2was synthesized by exfoliation and hydrothermal process. As a cathode material for lithium ion batteries, the electrochemical properties of restacked MoS2are better than the raw MoS2. The main contents are as follows:(1) A rheological phase method was used to prepare Li3V2(PO4)3/C and Y-doped Li3YxV2(PO4)3/C (x=0.01,0.03,0.05) with several different amount. In this route, LiOH·H2O, NH4VO3, NH4H2PO4, and Y2O3were selected as raw materials and citric acid as chelating agent and carbon source. Crystalline structure and morphology of the products were characterized by XRD and TEM. The results show that the structure and morphology of the samples were changed from irregular particle into regular spherical or regular polyhedral shape. The samples after Y-doped have smaller cell parameters, compared with the samples before Y-doped. As an cathode material for lithium ion battery, the Li3Y0.03V2(PO4)3/C electrode exhibits a higher capacity and better cycling stability, compared to the Li3V2(PO4)3/C electrode without Y-doping. Therefore, the as-prepared Li3Y0.03V2(P04)3/C could be a potential cathode material for lithium ion battery.(2) By using citric acid as chelating agent, Ni(Ac)2-4H2O and (NH4)6Mo7O24-4H2O as raw materials, α-NiMoO4was synthesized through a rheological phase method at different temperatures (500℃,620℃,750℃). The a-NiMoO4prepared at620℃exhibits a higher discharge capacity than that prepared at500℃and750℃as anode materials for lithium ion batteries. Then, using konjac powder as the carbon source, a-NiMoO4synthesized at620℃was modified through sol-gel method and heat treatment. The TEM results indicate that the morphologies of the samples (a-NiMoO4/C) were changed from regular particles and rods into irregular particles. The a-NiMo4/C electrode exhibits the best electrochemical properties when the mass ratio of α-NiMoO4and konjac powder is1:0.5.(3) MoS2/Carbon nanotube (CNT) composites were prepared through a simple solvothermal method at220℃for24h, with Na2MoO4and KSCN as reactants and ethylene glycol (EG) as solvent in the presence of CNT. SEM results show that MoS2/CNT composite is composed of a2-dimensional nanoflake structure with the thickness of20-30nm and these nanoflakes self-assemble into microsphere with the size of500nm and were bound around by coating CNT. As an electrode material for lithium ion battery, the MoS2/CNT electrode exhibits larger capacity and better cycling stability. This method provided a direction to prepare new cathode materials of high specific capacity and stable cycle performance for lithium ion battery.(4) Restacked MoS2prepared through an exfoliation-restacking route was characterized as cathode material for lithium ion battery. XRD results indicate that the crystal phase of the sample is2H-MoS2TEM images show that the restacked sample consists of nanoflakes with the thickness of10nm. The estimated c-axis parameter of the raw material is0.620nm and0.635nm for the restacked from higher resolution TEM observation.The restacked MoS2exhibits235.7mAh/g for the first cycle and retains a reversible capacity of142.3mAh/g, in a voltage range of1.0-3.0V versus Li/Li+, after100cycles. The reasons for the improved electrochemical performance of the restacked MoS2electrode are also discussed. The present results suggest that the exfoliation-restacking route be a sample and effective way to improve the electrochemical performance of layered cathode material for lithium ion batteries.