| In recent years,lithium-ion battery has made many breakthroughs and gradually been large-scale promoted of electric vehicles,hybrid electric vehicles,and electrochemical energy storage systems.Therefore,at present,the scientists pay much more attention on the separator and the electrode materials,which are parts of the lithium ion battery,carry out extensive and in-depth research.The first research point of this article is separator of lithium-ion battery.Now,commercial separators used for LIBs are typically micorporous polyolefin polymer films?polyethylene and polypropylene,PE and PP?due to their high mechanical strength,good electrochemical and chemical stability,cheap and thermal shut down property.Although the polyolefin separators are generally reliable for portable application,there are still some major issues for vehicular storage.One of these major issues is these polyolefin separators have hydrophobic surface property and low surface energy,which will result in their poor compatible with conventional liquid electrolytes.The poor wetting capability issue will cause pores or tunnels in the polyolefin separators to be not completely filled with liquid electrolytes,and then result in high ionic resistance between the separator and electrodes.It will not only directly affect the battery performance such as cycling stability and power performance,but also bring additional disadvantages in the manufacturing costs and speeds since the electrolytes soaking into separators is one of the slowest steps in battery manufacturing processes.To solve the poor wetting capability issue of these polyolefin separators,we coated the separators with tannic acid,which will increase the separators uptake ability.We test FTIR,XPS,Contact angle,SEM and DSC properties of the separators to compare the difference before and after coating,we assembled the lithium ion batteries with different separators to test the cycle stability and ratio properties.From the test result we got the information that after coating process,the hydrophilic ability improved,the electrolyte uptake improved,also the ion conductivities improved,finally the electrochemical performance of the batteries is improved.Silicon?Si?is considered to be one of the most promising anode materials for next-generation lithium-ion battery because of its natural abundance,low discharge potential,less prone to aggregation and most importantly its high theoretical specific capacity.However,pristine silicon material cannot be widely used in commercial lithium-ion batteries now.This is because a serious volume expansion effect will happen during the insertion of Li+,which can result in the collapse of the electrode structure and a sharp capacity fading.To solve these above problems,we use dopamine as the carbon precursor to design a“core-shell”composite structure of Si nanoparticles through the carbonization of polydopamine?PODA?under high temperature.Then NaOH treated material to give a"yolk shell"structure of composite nanomaterials.This special structure will inhibit the particle aggregation and suppress the volume expansion effect during the cell cycling in order to improve the cycling performance of the batteries.The mechanism of carbon coating modification to improve the cycling performance was investigated by Infrared spectroscopic analysis?IR?,X-Ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,Thermogravimetic analysis?TGA?,and Transmission electron microscopy?TEM?,AC impedance and so on.The results showed that the thickness of Si@void@C is about 5nm with the carbon content of 25%.We did the cycle test under the temperature of 25°C with the constant rate of 1Ag-1 and the result showed that the capacity of Si@void@C was 804 mAh/g after 50cycles,which was much higher than that of pristine Si nanoparticles.In addition,the rate performance?from 0.1 A/g to 5 A/g?of Si@void@C is much better than Si@C nanoparticles.The third research point of this paper is tannic acid as carbon source for coating Si nanomaterials.In the research of the anode and cathode materials of lithium-ion battery,we often used to coating carbon on the positive or negative electrode material so that we could obtain better electrochemical performance of electrode materials.In order to explore more convenient,effient and feasible method of carbon-coated,we use tannic acid as the carbon precursor to design a“core-shell”composite structure of Si nanoparticles through the carbonization of poly-tannic acid under high temperature.The results showed that the thickness of Si@C is about 5nm with the carbon content of 21%.We did the cycle test under the temperature of 25°C with the constant rate of 1Ag-1 and the result showed that the capacity of Si@C was 1343.7 mAh/g after 50 cycles,which was much higher than that of pristine Si nanoparticles. |
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