| Lithium ion batteries have been warmly praised in the field of new energy such as environment-friendly vehicles and portable electronic devices,which because of its many advantages.The theoretical specific capacity of silicon is4200 mAh·g-1,much higher than that common graphite anode material(372mAh·g-1)of lithium ion batteries.However,as an anode material of lithium ion batteries,silicon has a major defect.During the process of lithiation/delithiation,silicon particles have huge volume expansion?>300%?.This volume change caused the attenuation of specific capacity.Therefore,the key to the study of silicon based anode is to alleviate the volume expansion and improve the stability of the cycle.In order to buffer the volume change of Si,this thesis was based on nano silicon powder,and mixed Si with tubular polypyrrole or tubular amorphous carbon in mechanical ball-milling to prepared the silicon/tubular polypyrrole or silicon/tubular amorphous carbon composites.In addition,covered a layer of graphene on the surface of the composite to formed a double-layer structure composite anode.The structure,morphology and electrochemical properties of composites were characterized by infra-red spectrum,X-ray diffraction,scanning electron microscope and a series of electrochemical tests,respectively.The optimum composite ratio and covering thickness were obtained.The main contents and conclusions are as follows:?1?Tubular polypyrrole was synthesized under the action of methyl orange.Using mechanical ball-milling to mixed nano silicon with tubular polypyrrole in different mass ratio?4:2,3:3,2:4?to prepared three kinds of silicon/tubular polypyrrole composites.The electrochemical performance of pure silicon anode was taken as a reference to explored the optimum mass ratio of the composite.The result shows that,the addition of tubular polypyrrole effectively relieve the volume expansion and extend the cycle life of silicon.The composites which with the mass ratio of 3:3 shows a relatively optimal electrochemical performance.Under the current density of 500 mA·g-1,the initial reversible capacity is 1889.2 mAh·g-1,and reduce to 627.5 mAh·g-11 after 100 cycles,the capacity retention rate is 33.22%.During the step cycle,when current density restore to the normal value,the capacity can be recover to 831.4 mAh·g-1.?2?The nano silicon and tubular polypyrrole were mixed in different mass ratio?4:2,3:3,2:4?,and tubular polypyrrole as a precursor of carbonization.Silicon/tubular amorphous carbon composites were obtained by carbonization at800?.Study on the optimum mass ratio of composites by electrochemical performance tests.The result shows that the lithium intercalation capacity and capacity stability of the composites have significant improvement which compared with the pure silicon anode.The composites which with the mass ratio of 3:3 shows a relatively optimal electrochemical performance.Under the current density of 500 mA·g-1,the initial reversible capacity is 2393 mAh·g-1,and reduce to 549.5 mAh·g-11 after 100 cycles,the capacity retention rate is 22.96%.During the step cycle,when current density restore to the normal value,the capacity can be recover to 787 mAh·g-1.?3?Besides alleviate the volume expansion of silicon to increase the cycle stability,it also can be achieved by maintain the electrical contact between silicon and collector.Using the Si/tubular amorphous carbon composites as the inner layer of anode.And covered a graphene outer layer with different thickness?2?m,4?m,6?m?on the surface of inner layer to formed a double layer structure composites anode.The scanning electron microscope and electrochemical tests shows that the double layer structure significantly inhibited the shedding of the anode.The best thickness of graphene layer is 4?m.After100 cycles,the specific capacity is reduce from 1969.2 mAh·g-11 to 829.6mAh·g-1.And the capacity retention rate is 42.13%.During the step cycle,when current density restore to the normal value,the capacity can be recover to 931.8mAh·g-1. |
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