Preparation And Electrochemical Performance Of Cobalt-based Oxide Anode Materials For Lithium-ion Batteries

With the rapid development of economy and science in 21th century,energy and environmental problems become increasingly prominent.Thus a new green energy source,lithium ion batteries are proposed to cope with these problems.At present,the commercial anode material of lithium ion battery is graphite.Theoretically,its specific capacity is only 372 mAh/g,and cannot meet the demand of high energy density battery.While the high specific capacity and abundant resources of cobalt-based oxides attract wide attention of researchers.In this thesis,Cobalt-based oxides are compounded with conductive polymers,carbon and oxides to prepare composites with special structure for lithium ion batteries,and the microstructures of the composites are measured by XRD,SEM,XPS,TEM and so on.It is also tested as anode material of the lithium ion batteries by CV,charge-discharge and EIS.The research is as follows:?1?The urchin like NiCo₂O₄ is synthesized by hydrothermal method and calcination,and then NiCo₂O₄/PPy composites are prepared by chemical oxidation polymerization with Py monomer as raw material.As a conductive agent,PPy can improve the conductivity of Ni Co₂O₄,and as a buffer substrate,it can effectively reduce the volume expansion of NiCo₂O₄.This thesis investigates the difference of electrochemical properties between NiO,Co₃O₄ and NiCo₂O₄ and the effect of calcination temperature and PPy content on the electrochemical properties of NiCo₂O₄/PPy composites.The results show that compared with NiCo₂O₄,the specific capacity and cyclic stability of Ni Co₂O₄/PPy composites signifiantly improves.The discharge specific capacity of 30%NiCo₂O₄/PPy composites is 1164.9 mAh/g at current density of 200 mA/g after 100 cycles.?2?NiCo₂O₄/C composites are prepared using polyacrylonitrile?PAN?as carbon source by electrospinning and calcination.The composite's hollow tubular structure provides more buffer space for the material.PAN as a carbon source,part of the carbon material is retained after high temperature calcinations which improves the conductivity of the composites.The thesis studies the electrochemical properties of composites with the variation of concentrations,calcination temperatures and carbon contents.The results show that the best electrochemical properties of the composite is NiCo₂O₄-6/C,with the 30mL solvent,400?and 6mmol salts in the raw material.At the current density of 200 mA/g,the first discharge specific capacity of NiCo₂O₄-6/C composite is1446.2 mAh/g,the coulombic efficiency is 76%.After 120 cycles,the discharge specific capacity is up to 1092.8 m Ah/g,and the capacity retention rate was99.5%.So the cycling stability of the composite proves excellent.?3?Precursor nanofibers are prepared by electrospinning with polyacrylonitrile as carbon source,synthesizing the NiO-CoO/C composites with heterostructure by two-step calcination.This theis studies the properties of composites with different calcination temperature and Ni:Co molar ratio.The composition of the composites are analyzed by X-ray diffraction and X-ray photoelectron spectroscopy.The morphology and structure of the materials are characterized by field emission scanning electron microscopy and transmission electron microscopy.The results show that the composite is hollow structure.The electrochemical properties of the NiO-CoO/C composite prepared when Ni:Co=1:2 and calcination temperature at 400?is the best,at current density of 200 mA/g,the first discharge specific capacity is 1621.2 mAh/g,the coulomb efficiency is 75%,and after 200 cycles,the reversible specific capacity is934 mAh/g,and the capacity retention rate is 82%.