Designed Synthesis And Lithium-Storage Properties Of Micro/Nano-Structured Silicon-Based Anode Materials

With the emerging of portable mobile electronic devices and electric vehicles,the market places higher requirements on the endurance mileage of lithium-ion batteries?LIBs?and the development of new high energy density electrode materials is imminent.Compared with traditional graphite anodes which show limited specific capacity(372 mA h g^-1),the ultra-high specific capacity(4200 mA h g^-1)of silicon?Si?has attracted the interest of researchers.As Si-based anodes also have some advantages of moderate working potentials,abundant reserves,and no pollution,they have been considered to be one of the most competitive candidate anodes for next-generation LIBs.Nevertheless,the large-scale commercialization of Si-based material is not going smoothly.There are two major challenges that need to be addressed:?1?Si undergoes a huge volume expansion?⁴20%?during cycling,and the consequent stress and strain will then lead to the crushing,pulverization,and deactivation of active silicon materials.Also,the SEI films on the surface of the active materials will be destroyed and reconstruct,consuming a large number of lithium sources,and eventually make the battery quickly fail;?2?Since Si is a kind of semiconductor material,pure Si is unable to meet the requirement of high-rate LIBs.In this thesis,we mainly focus on improving the electrochemical performance of Si-based anodes by designing unique micro/nano-structured electrodes,which can highly improve the conductivity and maintain the stability of the electrode during cycling.For this purpose,two types of Si-based anodes were rationally synthesized,and both of them show excellent lithium-storage properties.?1?Si/SiO_x@Void@C composite:SiO is prone to convert into Si/SiO₂ under high temperature,and the resulting SiO₂ phase can act as a sacrificial layer to create the internal void structure by HF acid etching.In the work,we introduced a CVD process to grow a carbon layer on the surface of Si/SiO₂ particles before etching,and finally obtained Si/SiO_x@Void@C composites with a unique yolk-shell structure.The results show that the abundant void spaces could effectively buffer the volume change of the inner active Si-based electrode materials.Simultaneously,the carbon shell greatly improves the electrical conductivity of the composites,and enhances the stability of the SEI film.As a result,the composites exhibit excellent cyclability and rate performance.When the cell was tested at a large current density of 6 A g^-1,the reversible specific capacity remains at 785 mA h g^-1,and there is almost no decay over 300 cycles.Considering that the entire experimental production process is easy to repeat and the cost of raw materials is low,this as-fabricated Si-based anodes may hold great promise in the future applications.?2?SiNPs@rGO1/rGO2 composite:The SiNPs were pre-coated with graphene sheets by a self-assembly process,which effectively overcame the problem of agglomeration and enabled them to be homogeneously embedded into the conductive skeletons of a three-dimensional graphene network to obtain a kind of SiNPs@rGO1/rGO2 composite.On the perspective of structural design,the synergistic effects between SiNPs and graphene sheets will effectively improve the performance of the Si anode.First,the rich voids inside the material would provide sufficient buffer spaces for the volume effect and enhance the structure stability of the electrode.Then,the graphene phase and the 3D network could greatly improve the electrical conductivity of the composite.Finally,the double graphene sheets cladding structure could avoid the direct formation of the SEI film on the surface of SiNPs,and promote the stable growth of the SEI layer.Therefore,the SiNPs@rGO1/rGO2composite shows better performance compared with pure SiNPs,SiNPs@rGO1 and SiNPs@rGO2,and it displays a high capacity of 880 mA h g^-1 after 200 cycles at a current density of 1000 mA g^-1.It is worth noting that this rational structural design can not only be applied in Si-based materials,but also in some other similar alloy-based anode materials.