Preparation Of Silicon-based Materials For Lithium Ion Battery Anodes

As one of the most important energy storage devices,the development of high capacity rechargeable lithium ion batteries is crucial to the rapid development of electric vehicles and portable electronic devices.The biggest obstacle to develop high-energy density lithium ion batteries is the limited specific capacity of the most advanced anode and cathode materials.For anode materials,Si has been considered to be the most promising next-generation anode,because of its higher capacity(Li_4.4Si～4200 m Ah g^-1)than the traditional graphite anode(～372 m Ah g^-1).Besides,there are other merits of Si,such as high operational safety,high abundance in the earth crust,and good environmental compatibility.However,to commercialize silicon-based anodes,several challenges need to be solved,including huge volume changes during discharge/charge progresses,unstable solid electrolyte interphase,and poor electrical conductivity,which severely limit its practical application.Tremendous efforts have been devoted on different aspects to address the aforementioned issues with Si,such as carbon coating,acid etching,reducing the particle sizes,and so on.However,most of these methods require complex precursor preparations and post-processing.In addition,in order to relax the stress generated from the volume changes during cycles and possess more free space surrounded,most researchers adopt the method of etching by hydrofluoric acid,but this method is dangerous and has a bad influence on environment,which seriously limit its application in industry.Herein,we design a simple and scalable approach for the preparation of porous carbon coating layer on the Si nanoparticles composites via the hydrothermal reaction of glucose on the surface of Si,and subsequent carbonization treatment.During this process,the decomposition of oxalic acid produces gases that can make poles inside the materials.In this way,the silicon nanoparticles are successfully encapsulated into the carbon shell,and the pores inside the materials can further relieve volume expansion.When used as the anodes for lithium ion batteries,the composites exhibited excellent Li-storage performance.After 60 cycles,the Si-C-28%-650 ^oC electrode delivers an average capacity of 1050 m Ah g^-1 with a capacity retention of 72.15%under 0.1 A g^-1.In addition,in this process we can easily control the lithium storage capacity and particles sizes in gradient through changing the raw material ratio of glucose to Si nanoparticles.Hence,a simple method for preparing porous irregular sphere-like Si/C anodes is developed.But after a comprehensive investigation,we find that the method described above cannot completely solve the volume expansion of the Si nanoparticles.Because the Si O_x also has high specific capacity,and it was demonstrated that O element in Si O_x will react with Li and Si,forming Li₂O and Li₄Si O₄ in the process of first intercalation of Li⁺into Si,which can act as buffer matrix to relieve volume expansion.In this paper we further study the modification of Si O_x.First,effectively reduced sizes Si O_x particles are prepared by the method of high-energy ball-milling.Based on the obtained SiO_x anodes above,we further use the chemical vapor deposition（CVD）method to prepare Si O_x@C composites.Then the modified Si O_x@C composites are mixed with the self-made graphene in a certain proportion as the anodes.The result shows that the electrodes perform excellent cycle stability,the SCG-55 electrode delivers a capacity retention of 73.8%under 0.1 A g^-1 after 100 cycles.In general,the preparation methods of electrodes have the advantages of simple production,low cost and wide source of materials.The porous Si-C composites are synthesized by one-step synthesis,which is simple and does not require complicated post-processing.Besides,the graphene that used to improve the cycling performance of Si O_x is made by our laboratory and can be used in large-scale industrial production.And the applications of CVD method in industry are comparatively mature.These advantages are suitable for large-scale production,and the anodes prepared have excellent electrochemical properties.