Preparation And Investigation Of Multiwall Carbon Nanotube Based Electrodes For Efficient Long-life Batteries

In energy storage field,the supercapacitor is unique for higher specific capacitance than conventional capacitors and higher power than batteries.Besides,their cycle life can be up to tens of thousands of times with superiorities of low maintenance cost and green nature of materials.This it is very promising for applications in daily life,industry,and national defense.The energy density of the supercapacitor has been significantly improved with the constant efforts from researchers around the world,but is still less than those for batteries.The lithium ion battery is a kind of advanced rechargeable battery technology,with high energy density,low self-discharge,no memory effect,and small pollution to the environment,but its power density and lifetime are far less than the supercapacitor.Therefore,two types of energy storage devices have complementary advantages with similar structures.It is one of the important R&D field for new energy technologies to integrate these two cell structures to get a new type of energy storage device,i.e.,super capacitor-battery complex device,with advantages on many aspects.Transition metal oxide has always been an important research object during the development of electrode materials.Among them,manganese oxides are widely investigated with priorities of sufficient resources,low price,and environmental friendly nature.However,transition metal oxide does not conduct well.In order to make up this deficiency,the common method is to form composite materials with materials of good conductivity.Carbon nanotubes have excellent electrical conductivity and stable chemical properties.They are also popular electrode materials.In additional,carbon nanotubes are very flexible to form tubular network,beneficial to support the active material.In this thesis,the research activity is consist of two parts:in the first part,the supercapacitor electrode made by the MnO₂ and CNT_S composites are produced and investigated.Combining hydrothermal method with CVD technology,the MnO₂/CNTs composite electrodes are prepared.TheMnO₂/CNTs/Ni-foamelectrodeshowedgood electrochemical properties for the specific capacitance of up to 411.78 F g^-1,small resistance,and good cycle stability with retention of 90.05%after 5000 cycles.In the second part,the properties of spinel LiMn₂O₄ as the anode material for the lithium ion battery are studied.LiMn₂O₄ crystals prepared by hydrothermal method present high crystallinity.According to the electrochemical study,the initial discharge capacity of the LiMn₂O₄positive electrode was 0.055 mAh at 0.1C rate.There is an ideal voltage platform with two symmetrical platforms.The range of voltage platform is from 3.8 V to 4 V,accounting for about 80%of the total capacity,showing reasonable charging and discharging efficiency.In the short term test,this electrode demonstrates decent cycle stability.This thesis work provides valuable results to develop CNT based supercapacitor-lithium ion battery complex.