Design And Synthesis Of Carbon-Coated Nano-Silicon-Based Anode Material For Lithium Ion Batteries

Lithium ion batteries?LIBs?are crucial for the use of portable consumer electronics,electric vehicles,communications storage and so on.With the increasing demand,people require the better endurance and higher energy density of LIBs.At present,LIBs can not reach the increasing demand of terminals for batteries with its low capacity.Silicon-based materials,one of the most promising anode materials,have a theoretical capacity of 4200 mAh/g,which is more than 10 times that of graphite.After materials had been industrialization,silicon-based batteries will greatly improve the capacity of the battery.However,silicon-based materials have many problems,including volume expansion,easy to generate unstable SEI film,poor cycle life,and low conductivity.In this regard,this paper mainly uses the ball milling and electrospinning/electrospraying techniques and design the micro-nano structure and carbon coating strategy,which is achieved to increase the conductivity of silicon anode material and inhibit the volume change during charge and discharge.The main research contents of the expanded carbon-coated nano-silicon negative electrode materials are as follows:?1?The synthesis of Si/G/C composites is based on a ball milling technique that embeds nano-silicon particles in a graphite layer.By electrostatic spraying technology,a layer of polymer is coated to prevent silicon from being directly exposed on the surface,and further thermal decomposition is performed to obtain the mesoporous Si/G/C material.The results of characterization and electrochemical performance tests were compared with Si/G and Si/C materials,show that the mesoporous Si/G/C composites have excellent properties.It had a reversible 652 mAh/g after 200 cycles at the current density of 200 mA/g,even over538 mAh/g at high current density of 2 A/g.The main reason for the excellent performance is that the laminar graphite composite of the support and the amorphous carbon layer can improve the structural integrity of the silicon-based material which sufficient buffer space is increased to reduce the volume expansion of the silicon base.?2?The synthsis of the flexible silicon/graphene multi-layer thin films is mainly based on nano-silicon particles and graphene oxide?GO?as raw materials,using GO and Zn in situ reaction-assisted deposition,to further synthesized by freeze-drying technology.Through structural characterization,it can be seen that the nano-silicon particles are evenly distributed between the graphene layers.This structure reduces the agglomeration of nano-silicon to a certain degree.At the same time,the graphene layer not only increases the conductivity of the silicon base,but also provides an effective buffer space.Electrochemical performance tests showed that the electrochemical performance of the flexible silicon/graphene multi-layer thin films mixed with 15%silicon was the best.After200 cycles at the current density of 200 mA/g,the reversible capacity was as high as 707mAh/g.?3?The synthesis of SiO₂@TiO_2/CNF composites was based on simple mechanical milling.Micron SiO was used as raw material and prepared by sol-gel coating TiO₂ method and electrospinning method.Comparison with SiO/CNF to investigate the effect of TiO₂ on the structure and electrochemical performance of composites The results show that TiO₂ is successfully coated on the SiO surface layer and forms a network structure under the support of three-dimensional conductive carbon nanofibers.This structure can reduce the huge volume stress c and increase the electron conduction and promote the Li⁺.diffusion.Electrochemical properties show that SiO@TiO₂/CNF composites have well structural stability,high reversible capacity,and excellent rate performance.The reversible capacity of 800 mAh/g can be achieved after 170 cycles at 200 mA/g.Furthermore,SiO@TiO₂/CNF composites display a high reversible capacity of 338 mAh/g at 3000 mA/g.