Synthesis, Reduction And Electrochemical Studies Of Submicron Spherical SiO₂ Particles

Lithium-ion batteries（LIBs）have rapidly taken over the market for power supplies such as portable electronic devices because of its high energy density and long cycle life.However,LIBs will still not meet today’s demand,when applied to devices with vastly improved storage capacity,such as electric vehicle power supply（EVs）and energy（such as in photovoltaic plants or wind turbines）.The specific capacity of commercial LIBs-Graphite material is widely used in commercial LIBs anodes whose theoretical is only372m Ah·g^-1,but actual specific capacity has reached 365 m Ah·g^-1,which is difficult to be greatly improved.Therefore,it is urgent to improve the theoretical specific of anode electrodes.Silicon（Si）is the most attractive next generation high capacity anode material.Si has a high theoretical specific capacity of 3575m Ah·g^-1(form Li₁₅Si₄),which is nearly 10 times that of graphite,is environmentally friendly and rich in resources.However,in the electrochemical cycle of Si as an anode material.1)More than 300%volume expansion,resulting in conductive network has lost and another electrode damage phenomenon,the capacity will rapidly decay;2)The instability of SEI resulting in unstable cycling performance,the capacity will slowly decay.Therefore,the cycle life and stability of Si-based anode materials have become an urgent problem to be solved.The main existing solutions are to prepare a series of Si nanocrystalline structures such as:si nanowires,nanotubes structures,etc.In order to buffer the expansion of the body.But,experimental conditions of these nano structure Si to be strict,high cost,harmful to the environment,etc,are not suitable for commercialization.In order to solve the above problems,cheap silica fume and contrast samples（submicron/nano spherical SiO₂ prepared）were used as precursor templates in this paper,and then mixed with magnesium by balling milling to form a uniform Mg@SiO₂ reaction compositie structure.After magnesiothermic reduction,porous Si were successfully prepared.Through carbon modification methods,prepared Si@C composites.The prepared Si/C composites were uniformly with commercial graphite in different proportions to make anode electrodes,and the electrochemical performance was tested after assembling batteries.The electrochemical performance test results show that:coated Si@C show excellent electrochemical performance.The transformation of low value materials into high value is realized.At the same time,a process for the commercial production of Si under the conditions of low energy consumption and simple operation is put forward.The main research results of this paper are as follows:（1）Preparation of submicron and nano spherical SiO₂.Exploring TEOS synthetize SiO₂ with nucleation and growth mechanisms.Using ammonia water and L-lysine as catalysts for the hydrolysis and condensation of TEOS,submicron and nano spherical SiO₂ particles with uniform particle size were successfully prepared.（2）Preparation of porous Si by magnesiothermic reduction.Exploring the process mechanism of magnesiothermic reduction reaction found:in addition to the SiO₂/Mg mass ratio and reduction temperature,the homogeneity of the mixture also greatly affects the degree of reduction depending on the mechanism by which magnesium vapor is transported.The Mg and SiO₂ were uniformly mixed into Mg@SiO₂ reaction composites by mechanical ball milling method,which shortened the diffusion path and depth of magnesium vapor.Content of Si component after the reduction and HCl-etched sample can reach 85%-99.3%,it’s a very high degree of reduction.（3）Si@C composites were prepared by carbon coating modification.Using phenolic resin（PF）and dopamine（DA）as carbon precursors,the porous Si particles were modified by carbon coating.The precursors of both have good encapsulation properties for Si particles.Combined analysis of XRD and XPS show that Si C phase is formed at the interface of silicon and carbon,which proves that the interface is connected by strong Si C covalent bond.（4）Electrochemical properties of Si@C composities electrode.Due to the robust core shell structure of Si@C compact composites for the capacity of carrying and buffering the expansion of Si core,the 10wt%and 70wt%coated composite electrodes have excellent electrochemical performance and high dimensional stability.The Si@C composites obtained by using silica fume as precursor have excellent performance,and the raw materials are cheap,which realizes the transformation of low value materials into high value and promotes the possibility of the commercialization of silicon-based electrodes.