| To meet the increasing demand for clean energy,the development of lithium-ion batteries with high energy density becomes quite essential.Silicon-based materials have been considered to be one of the most promising next-generation anode materials owing to abundant resources,high specific capacity and low working potential.However,the large volume expansion and low electrical conductivity of silicon,also lead to its poor electrochemical stability and kinetic performance.In this thesis,Si and Si O anode materials are investigated via the nanostructure design and surface coating strategies.The above problems are solved by surface coating of hard carbon,Ti O2 and Ti N,which is expected to enhance the electrical conductivity,suppress the volume expansion,stabilize the SEI layer,thus improve the electrochemical stability and kinetic performance.The main research conclusions are as follows:（1）Self-supporting Si@TiO2@NC thick film.The solution precipitation method was used to coat titanium dioxide on the surface of nano silicon,and then the obtained Si@Ti O2 was mixed with the multi-layer graphite plates and PAN solution.The thick film was produced by the tape-casting method,and Si@Ti O2@NC was obtained after heat treatment.The hierarchical structure consisted of a carbon skeleton derived from multi-layer graphite and hard carbon,and nano silicon coated with 10 nm TiO2.It can be directly used as the anode electrode without the conductive agent,binder and current collector.Si@Ti O2@NC delivers a capacity of 1019.9 m Ah g-1 with a capacity retention of 92.4%after 50 cycles at a current density of 0.8 A g-1.Compared with pure silicon,its cycling performance is significantly improved,because the carbon skeleton and Ti O2coating can significantly increase the electrical conductivity,effectively inhibit the volume expansion of the silicon material and stabilize the SEI film.（2）Double-coated SiO@NC@TiN composite material.A two-step liquid method was used to coat phenolic aldehyde and titanium dioxide on the surface of Si O.Si O@NC@Ti N composites were obtained after heat-treatment in an ammonia atmosphere.The surface of Si O particles was coated by a carbon layer of about 20 nm and a Ti N layer of about 10 nm.The results show that the Si O@NC@Ti N composite material delivers a specific capacity of 1114.9 m Ah g-1 with a capacity retention rate of81.1%after 200 cycles at a current density of 0.4 A g-1.And at the high current density of 6 A g-1,the capacity still maintains 1015.5 m Ah g-1.The excellent cycling stability and rate performance can be mainly attributed to the good electrical conductivity and the buffering effect of NC and Ti N.（3）Porous silicon composite material PSi@NC@TiN.The disproportionated SiO was coated with hard carbon and etched by hydrofluoric acid to form PSi@HC.After that,PSi@NC@Ti N was obtained by Ti O2 coating and heat treatment in an ammonia atmosphere.The thickness of carbon layer is about 25 nm and the thickness of Ti N layer is about 15 nm.The material shows a high capacity of 1352.2 m Ah g-1 and retains a capacity of 93.4%after 200 cycles at a current density of 0.8 A g-1.Even under the large current density of 8 A g-1,the capacity still maintains 1036.7 m Ah g-1.The design porous structure greatly improves the capacity and cycling stability of Si O.However,the increased specific surface area,also results in the reduction of the first coulombic efficiency（ICE）.After the chemical pre-lithiation,the ICE can be increased from 70.4%to 85.4%.The research results in this thesis show that the electrical conductivity of silicon anode can be significantly enhanced via the composition with conductive graphite and hard carbon,and the volume expansion of silicon anode can be greatly suppressed by hard carbon,robust Ti O2 and Ti N.Thus,the electrochemical stability and kinetic performance of silicon-based materials are successfully improved,which will accelerate their practical application in the next-generation Li-ion battery with high energy density. |
