Magnesium Thermal Reduction To Prepare Silicon-based Electrode Materials With High Energy Storage Density

New energy industry,a new and booming industry in modern society.The new energy automobile is one of the most promising industries.As the power source of the new energy vehicle,the power battery has more and more requirements for its endurance,so the high energy density lithium-ion battery has been paid more and more attention.As far as commercial lithium-ion battery are concerned,the actual capacity of graphite of commonly used cathode materials is almost up to the theoretical capacity?372m Ah/g?,which is very limited in capacity improvement and can not meet the requirement of high density energy storage.Due to its high specific capacity of 3578m Ah/g,abundant natural resources and low potential,silicon has become one of the next generation of lithium-ion battery materials with high energy storage density.However,due to the intercalation and deintercalation of Li⁺,Si will undergo a huge volume expansion?about 300%?during the charging and discharging process,resulting in the fragmentation of Si particles,further causing the collapse of tissues and the loss of electrical contact between the quasi-active substance and the current collector,resulting in unstable cycle performance.The preparation of silicon nanostructures is also required to buffer.Researchers have proposed various nanostructures to alleviate the volume expansion of Si during cycling.However,the preparation process of nanostructures is expensive and difficult to realize large-scale mass production.However,Si has a large volume expansion??300%?and poor conductivity when applied to negative materials of batteries.A variety of nanostructures have been proposed to reduce the volume expansion of silicon in cycles,but the fabrication of nanostructures is expensive and difficult to scale up.It is a promising research direction that magnesium thermal reduction can be used to obtain low-cost and large-scale Si anode materials.However,the lower reduction degree?40%?60%?is the biggest defect,which not only reduces the utilization of Si,but also reduces the energy storage density.In this paper,how to improve the degree of magnesium thermal reduction is studied,mainly aiming at the influence of mixing uniformity on the degree of magnesium thermal reduction.Using fumed Silica?Si O₂ nanoparticles,FS?as Si source,nano-Si was prepared by magnesium thermal reduction.The thermodynamics and kinetics of magnesium thermal reduction were analyzed,and the effect of mixing uniformity on the degree of magnesium thermal reduction was explained Secondly,the effect of mixing uniformity on the reduction degree of magnesium is verified by several different mixing modes,and a coating model between Mg and Si O₂ is proposed to form a single microreactor between a single Mg particle and multiple Si O₂ particles,to improve the reaction degree of magnesium thermal reduction.The main findings are as follows:?1?The thermal reduction of magnesium is a two-step tandem reaction.In the first step,Mg₂Si is formed by the reaction of Mg with Si O₂,and the reaction is quick until Mg is consumed.In the second step,Mg₂Si is used as reducing agent to reduce the remaining Si O₂.The decomposition pressure of magnesium in the second step of the reaction is very low?100 times lower than the evaporation pressure of Mg at the same temperature?,and the second step of the thermal reduction of magnesium is a rate reaction.The migration rate and flux are low and the reaction is slow,whether it is the evaporation of Mg particles or droplets,the diffusion of solid Si O₂ materials between channels,or the diffusion from the surface of Si O₂ particles into the solid and into the reaction interface,thus,the whole reduction reaction of magnesium is hindered.Moreover,the mixing uniformity of the reactants is lower than mm size,and the dispersion of the macroscale distance is needed to decompose the Mg vapor,so the reaction time of the second step is long and the reaction speed is slow.?2?Through manual mixing,magnetic stirring,power stirring and ball milling,different materials with different mixing uniformity were obtained,and magnesium thermal reduction was carried out under certain conditions,and different reduction Si was obtained,and the conversion of Si O₂ to Si is increasing.?3?The surface of magnesium spheres was modified by hydrolysis and polycondensation of TEOS on the surface of magnesium powders to form Si O₂directly,and?Ps-Mg@Si O₂ particles with core-shell structure were obtained.After magnesium thermal reduction of?Ps-Mg@Si O₂,the morphology of the primary particles and the original clusters can be maintained,and finally a silicon sphere with double pore structure is formed.Moreover,the yield of silicon particles can reach92%,which further shows that the microreactor model shortens the distance of materials,shortens the diffusion distance of Mg vapor,shortens the reaction time and improves the reaction degree.?4?The reduced Si is coated with conductive C layer,and the composite is used as the lithium ion anode material.The electrochemical properties of the reduced Si are obviously different at different reduction degree.Among them,the spherical structure of lithium-ion composite materials has good conductivity,mainly due to its shell-like double pore structure,hollow structure for charging and discharging the volume expansion provides a buffer space,and the nano-channels on the shell provide channels for Li⁺during charging and discharging.The innovation and characteristics of this paper are as follows:This paper explains the process of magnesium thermal reduction from thermodynamics and kinetics,explains that the magnesium thermal reduction reaction is a series reaction,and explains the importance of material uniformity to magnesium thermal reduction.The influence of homogeneity on the degree of magnesium thermal reduction was verified by comparing the different mixing methods and the degree of magnesium thermal reduction.The?Ps-Mg@Si O₂ model was formed by coating magnesium particles on the surface,and the concept of microreactor was proposed.At the same time,the shell-like structure was retained after magnesium thermal reduction,and the micrometer silicon spheres with shell-like double pore structure were obtained.