Preparation And Electrochemical Performance Of Si/C Anode Materials For High Performance Lithium-Ion Batteries

In recent years,Si has been considered as the most promising and prospective anode material for the next generation lithium ion battery because of its ultrahigh theoretical capacity(3572 mAh/g)and appropriate operating voltage(<0.4 V vs Li/Li+).However,the huge volume expansion of Si causes the generation of pulverization,disintegration and separation from the electrode material during(de)lithiation processes,which result in poor cycling performance,seriously restricting the industrial application of Si as a high-performance anode material.Focused on the above problems,we make great efforts to design and further synthesize nano-Si materials with different morphologies and structures,and then combines them with highly conductive carbon and metal oxides with great stability,which solve these problems of silicon as an anode material and enable excellent electrochemical properties.The innovative results achieved in this paper are as follows:(1)Firstly,the cornlike SBA-15 was prepared by a hydrothermal reaction and used as a silicon source and template to fabricate three-dimensional ordered mesoporous silicon after magnesiothermic reduction procedure.Then,the cornlike nitrogen-doped carbon coated on ordered mesoporous silicon composite was synthesized with polypyrrole as source of C and N.The results show that the initial discharge specific capacity of the composite materials is up to 2548 mAh/g at the current density of 0.2 A/g and a voltage range of 0.01-1.5 V,with CE remained 71.4%,and furthermore the specific capacity is still 1336 mAh/g with average CE retained as high as 99.36%at 1 A/g after 200 cycles.The composite materials also exhibit excellent rate performance,at a high rate of 8 A/g,and the discharge specific capacity reaches as high as 781.2 mAh/g.In the charge/discharge process,the sufficient channel structure of ordered mesoporous silicon is used to effectively relieve the volume expansion,the surface coating of nitrogen-doped carbon can enhance the electron transfer rate,promote the rapid charge and discharge processes and inhibit the formation of volume expansion,pulverization and other issues,which further guarantee the structural stability of silicon during the charge/discharge processes.(2)Using a template-free method,hollow silica spheres with suitable wall thickness were prepared by tetraethylorthosilicate and ammonia water and used as a silicon source to obtain hollow silicon spheres by a magnesiothermic reduction method.Then the sol-gel method was utilized to coat TiO2 and C on the hollow Si spheres,and further fabricate a novel anode material of double-stabilized hollow Si@TiO2@C nanosphere.The results of electrochemical measurement show that the initial discharge capacity of the Si@TiO2@C material is 2557.1 mAh/g at 0.2 A/g,with CE remained 86.06%,and the reversible discharge capacity still maintain 1270.3 mAh/g with average CE retained as high as 99.53%at 1 A/g after 250 cycles.The material exhibits excellent rate performance,even at a current density of 8 A/g,and the discharge capacity is still 718.2 mAh/g.In the 250 cycles,Si@TiO2@C composite make use of its hollow structure with appropriate shell thickness to reserve room for volume expansion of Si whilst the rich channel structure shortens the diffusion and transfer distance of Li+ and electrons.Simultaneously,the external TiO2 and C layers can control the expansion to be inward not outward and improve the conductivity,respectively,with silicon under great stress/strain,which ensures the structural integrity of composite material during the cycle.(3)In order to select materials suitable for large-scale and batch production of micro-nano silicon,we designed and prepared a hierarchical Si/FG/C composite material with silicon content of 8.4%using high-purity photovoltaic mono-crystalline silicon scrap as a low-cost silicon source.The results show that the initial discharge capacity is up to 634 mAh/g at 0.2C(128 mA/g),with capacity retention rate maintained 81.5%and the average CE retained 98.9%after 200 cycles.The flake graphite with hierarchical structure not only can greatly improve the conductivity of the composite material,but also can be used as a mechanical skeleton to endow the nano-silicon in the interlayer with electrochemical activity.The carbon material coated on the surfaces of nano-silicon and flake graphite can greatly enhance the cycling stability of the material,and then limit the growth of the SEI film outside the composite material rather than on the surface of each nano-silicon particle,repeatedly.