| Electrical energy storage devices and system is one of key technology in the modern society,with the increasing demand of power consumption.As a leading technological energy storage platform lithium-ion battery has attracted much attention due to the high energy density,long service life and high safety.It has been used in a wide range of energy-storage applications,from small batteries for portable devices to multi-megawatt containerized batteries for the provision of grid ancillary services.However,there are still many bottlenecks in the application of lithium-ion battery.The capacity and stability of the anode is one of the bottlenecks.The most commercially popular anode electrode is graphite or other carbon material.The low specific capacity(372 mAh/g)and poor charge and discharge performance of graphite limited the application of lithium-ion battery in electric vehicles,energy storage station and so on.So it is a hot research area to search new anode materials,which has high energy density,high rate performance and long cycle life.The theoretical study shows that germanium has large specific capacity(1384 mAh/g),good conductivity,and fast ion diffusivity at room temperature.It is promising anode material for the next generation of lithium ion battery.But the poor stability caused by the huge volume charge during the lithiation/delithiation process limited its application.The most effective approach is reducing germanium particle size to nanoscale and preparing thin film anode electrode.Moreover,the thinner electrode without any conductive additives and binders has great advantage in the application of all-solid-state thin film lithium-ion.However,the low surface area of the traditional current collector limited the loading of active material and increasing of the energy density.So fabrication of current collectors which has high surface area is one important process for germanium anode thin film lithium ion batteries.In this work,two kinds of current collectors with different nanostructure were prepared by thermal reduction method and chemical vapor deposition method.The structure of current collector and the effects of nanostructures on properties of germanium anode thin film lithium ion batteries have been studied.The main studies include the following parts:(1)Preparation of nano-porous foamed nickel current collector and the effects on the properties of lithium storage of germanium anodeThe foamed nickel current collector with nanoporous structure have been prepared by thermal reduction method.The structures have been studied by scanning electron microscope(SEM),X-ray diffraction(XRD)and Raman spectra.The anode displayed very high specific capacity and stability after depositing germanium film on these current collectors by electron beam evaporation technique.The initial specific capacity is 1280 mAh/g at 20 A/g current density and 740 mAh/g at 50 A/g current density.The ageing test at current density of 20 A/g and 50A/g showed that specific capacity did not drop and kept at about 1280 mAh/g and 740 mAh/g after 100 cycles.(2)Study of preparation and the properties of germanium/carbon nanotube coaxial nanorod anodeThe carbon nanotube has been prepared on the above nano-porous foamed nickel current collector without any catalyst and barrier layer with CVD.The germanium was deposited on the carbon nanotube/nano-porous foamed nickel substrate.The SEM and Raman spectra show that the anode has a germanium/carbon nanotube coaxial nanorod structure.The specific capacity and stability have been improved.The specific capacity is 800 mAh/g at 50 A/g and the specific capacity only dropped<10%after 300 cycles at a current density of 10 A/g. |
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