Porous Si-based Composites:Synthesis And Application For Lithium-Ion Batteries

Lithium-ion battery(LIB)is one of the most popular secondary batteries since it was firstly introduced in 1991.Recently,the fast development of consumer electronics and electric vehicles drives LIBs to higher energy density and higher power density.Silicon(Si)has a ultrahigh theoretiscal capacity of 4200 mAh/g,which is identified as one of the most promising candidate of the next-generation LIBs anode materials.However,Si anodes generally suffer from tremendous volumetric change during Li+ insertion and extraction,which results in rapid capacity decay and poor storage performance.Therefore,it is still a great challenge for Si anodes to be commercialized in large scale.In this dissertation,we focus on the synthesis and application of porous Si-based composites due to the advantages:firstly,the porosity provides space for Si expansion and accommodates the mechanical stress caused by the volume change;Secondly,large specific surface area increases the contact with the electrolyte;Thirdly,there are more transmission routes for Li+ and electrons owing to the three-dimensional network structure.Specially,we propose a new structure of porous Si in carbon cages,a new and green carbon source to prepare the carbon layer,as well as two kinds of porous Si/metal composites based on source of Mg2Si.The main innovative results are displayed as follows.(1)The synthesis of porous Si in carbon cages(PoSi@CC)was demonstrated via the oxidation of Mg2Si,coating of carbon layer and subsequent acid washing.Different from the traditional porous Si@C core-shell(PoSi@CS)structure,new structure and morphology of PoSi@CC were obtained.Compared with PoSi@CS,PoSi@CC expressed better electrochemistry performance,which was due to the enough space in the PoSi@CC particles that can buffer the volume change during the charge/discharge process,while the outer carbon cages could alleviate the pulverization and stabilize solid electrolyte interphase(SEI)film.In addition,the"Breath effect" of the electrodes had also been suppressed.Therefore,less expansion was shown by the PoSi@CC electrodes and superior cycling stability was expressed.(2)The synthesis of porous Si/carbon core-shell(PoSi@C-CO2)structure was demonstrated via the reaction between Mg2Si and CO2 and subsequent acid washing.CO2 was a green carbon source for Si anodes,and the carbon layer from the in-situ deposition through magnesiothermic reduction of CO2 expressed higher graphitization.Besides,the deposition process of the carbon layer was integrated into the synthetic process of porous Si,which indicated the potential commercialization.As the anode materials for lithium-ion batteries,the PoSi@C-CO2 electrodes demonstrated enhanced cycling stability and rate capability.(3)The synthesis of Cu3Si nano-particle embedded 3D porous Si was demonstrated via the solid-state reaction between Mg2Si and CuO,and subsequent acid washing.Excellent battery performance was demonstrated owing to the structural advantages.At first,the porous structure was composed of interconnecting particles with small size,which could accommodate the volume and mechanical stress well.Then,the embedding of Cu3Si nano-particles improved the electrical conductivity,which could lead to the fast transport of Li+ and electrons.(4)The synthesis of porous SiGe@C was demonstrated via the oxidation of Mg2Si/Mg2Ge composites,acid pickling and subsequent carbon coating processes.Si and Ge was distributed uniformly in the porous micro-particles.Preeminent cycling stability and rate capability were expressed owing to the unique porous structure,synergistic effect of Si and Ge,inherent high lithium-ion diffusivity and good electrical conductivity of Ge.