Controllable Preparation And Electrochemical Performance Of Carbon-silicon Composite Electrode Materials

With the rapid development of the global economy,the energy issue is becoming one of issues that human society is facing.Lithium-ion batteries are widely used in a variety of electronic devices due to their small size,high energy power density and excellent cycle stability.Positive and negative electrode materials affect the improvement of lithium ion battery performance.The actual capacity of commercialized graphite materials is close to the theoretical capacity of 372 mA h/g,which is difficult to meet the high energy density requirements of power batteries today.The silicon-based anode material is expected to replace graphite as a new-generation lithium-ion battery anode material due to its higher theoretical specific capacity,lower lithium intercalation potential and low cost.However,the volume expansion of the silicon material itself will reduce the cycle stability of the battery.It is an effective method to improve the cycling stability of lithium-ion batteries by preparing silicon-carbon composite materials by means of nanometer silicon and optimization of silicon and carbon composite structures.In this study,a carbon-silicon composite electrode material was prepared by designing hollow structures of carbon-silicon materials,coating silicon with carbon and metal oxide materials,and thermal reduction of magnesium.X-ray diffraction（XRD）,scanning electron microscope（SEM）and transmission electron microscope（TEM）were used to characterize the morphology and composition of the samples.The materials were tested for electrochemical performance by making batteries.The specific research content and results are as follows:（1）A functional double-shell Si@void C@TiO₂ electrode material with a hollow structure is prepared by coating carbon and metal oxide materials on nano-silicon particles and then etching them with hydrofluoric acid.The performance of electrode materials with different structures and TiO₂ contents were compared.The first charge and discharge specific capacity of Si@void C@TiO₂ material can reach 1251 and 2053 mA h/g,and the coulomb efficiency of the first cycle is 61%.After a long cycling of 500 cycles at a current density of 0.1 A/g,it still has a high reversible capacity of 668 mA h/g.The average Coulomb efficiency is as high as 98%and the capacity retention rate is 53.4%.Compared with the Si@void C materials prepared under the same conditions,the capacity and stability of the double-shell Si@void C@TiO₂ Electrode materials are significantly improved.（2）Using sol-gel to hydrolyze butyl titanate to obtain metal oxide TiO₂ to cover the nano-silicon particles,and then embed the polyaniline porous carbon material prepared by aniline polymerization reaction to obtain Si@TiO₂@C electrode material.The electrochemical performance of electrode materials with different structures were tested.Results show that when the Si@TiO₂@C electrode material is stable in cycling,the charge-discharge specific capacity at the 500th cycle under the current density of 0.1 A/g is 1112.5 mA h/g and 1126 mA h/g,and the Coulomb efficiency is as high as 98.8%.In addition,when the current density is 0.5 A/g,the charge and discharge specific capacities are 508.9 mA h/g and 513.5 mA h/g in the 500th cycle,and the Coulomb efficiency is99.1%.In contrast,the capacity of the Si@C structure electrode will rapidly decay.（3）The carbon-coated silicon material（Si@C）was prepared by a hydrocarbon reaction of ethyl orthosilicate and glucose and then through a magnesium thermal reduction reaction.Acetylene was used to further improve the quality of carbon coating.The content of SiO₂ was adjusted by controlling the content of ethyl orthosilicate,and the electrochemical performance of the electrode materials were discussed.Results show that,the Si@C composite samples prepared with 1:1 carbon-coated and magnesium powder and NaCl added during the magnesium thermal reduction present the best performance.Its charge and discharge specific capacities in the first cycle are 672.8 mA h/g and 1052.7 mA h/g,respectively,and the Coulomb efficiency is 63.9%.After 400 cycles,the charge and discharge specific capacities are 608 mA h/g and 617.5 mA h/g,with the capacity retention rate about 70%.