Modification And Electrochemical Properties Of One-dimensional Silicon Oxide(SiO_x) Nanowires

Lithium ion batteries are widely used in various electric devices and electronic products because of their high energy density,long cycle life and relatively environmental protection.As increased requirement of the capacity and long cycle life for all kinds of electronic devices,many researchers pay more attention to study anode/cathode materials for lithium ion batteries,and in the process of exploring,they found many materials in lithium ion battery can show the characteristics of high capacity and high rate performance.At present,the cathode material of lithium ion battery has been commercialized,and the research on anode materials is not enough.In the development and research of anode materials,the silicon oxide?SiO_x?materials with high theoretical capacity(1675-2650 mAh g^-1),rich resources and environmental friendliness are widely attract researchers'concern around the world,and are considered in the future to have large probability to replace existing graphite carbon anode materials.However,SiOx anode material has also been hindered by a series of problems:?1?the huge volume expansion occurs during the cycle,?2?the formation of the unstable solid electrolyte film?SEI film?and?3?the low conductivity of SiO_x.Therefore,to really apply SiOx to the anode of lithium ion battery,we should first think and solve the above problems.In this thesis,the porous carbon coated silicon oxide nanowires?pC-SiO_x NWs?were constructed,showing improved electrochemical properties compared with SiOx.Considering that graphene has higher conductivity and stability,we further prepared the composite structure of reduced graphene coated silicon oxide nanowires?rGO-SiO_x NWs?to improve its rate performance.The synthesized pC-SiOx NWs and rGO-SiOx NWs are compared with three kinds of samples,such as mesoporous silica?p-SiO₂ NPs?,porous carbon coated mesoporous silica?pC-p-SiO₂ NPs?and unmodified silica nanowires?nC-SiO_x NWs?in structure and performance,highlighting the advantages of the composite structure.1.In this thesis,we used the cheap mesoporous silica?BMS?as the raw material,and the composite structure was obtained by hydrothermal reaction and polymer cladding and the high-temperature carbonization.The core-shell structure of pC-SiOx NWs and rGO-SiOx NWs were obtained.The core-shell nanocomposite structure introduced the highly conductive porous carbon layer and graphene,so the electrode has excellent electrical conductivity,which significantly facilitate electron transmission.At the same time,the unique one-dimensional nanowires structure can effectively buffer drastic volume change to ensure the integrity of the electrode,and the outer layer of the stable carbon materials has beneficial effect on the stable growth of the SEI,therefore,the problems of SiOx NWs anodes are solved effectively and their electrochemical performance are greatly improved.2.Using modern testing methods to characterize the two kinds of composite,such as the characterization of phase,morphology and electrochemical performance of anode materials in lithium ion batteries,the main results are as follows:?1?XRD,TG,XPS,SEM,TEM and Raman were used to characterize the phase,morphology and structure of SiO_x NWs,pC-SiO_x NWs and rGO-SiO_x NWs.The results show that the composite structures of pC-SiOx NWs and rGO-SiOx NWs have the characteristics of pure phase,homogeneous and stable structure,and meet our expected design and preparation results.?2?The pC-SiOx NWs and rGO-SiOx NWs were used as lithium ion battery anode material,and lithium metal is employed as the counter electrode to assemble half cell for the constant current charge and discharge,such as CV and EIS tests.In the 0.01-3.0voltage range,the constant current charge and discharge tests results at the current density of 100 mA g^-1 show that the initial discharge capacity of the pC-SiO_x NWs and rGO-SiO_x NWs can reach 2,240 mA h g^-1 and 1912 mA h g^-1,respectively.1050 mA h g^-1 and 1005 mA h g^-1 were retained after 100 cycles respectively.At a high current density of 500 mA g^-1,the material still have the excellent capacity of 600 mA h g^-1 and800 mA h g^-1,respectively.Compared with the uncoated SiO_x NWs?nC-SiO_x NWs?,porous silica nanoparticles?p-SiO₂ NPs?and porous carbon coated porous silica nanoparticles?pC-p-SiO₂ NPs?,pC-SiO_x NWs and rGO-SiO_x NWs show better cycling stability and higher capacity.