Preparation And Electrochemical Properties Of Cellulose Based Carbon/Silicon Composite Aerogel

Lithium-ion battery is the most widely used energy storage device in modern society.It has the advantages of high energy density,safety,low pollution and no memory effect.As a negative electrode material,carbon materials have the characteristics of low lithium embedding potential,good conductivity,good cycling stability,good physical chemical properties,and low cost,which are widely used in commercial lithium-ion batteries.However,the theoretical specific capacity of carbon materials is relatively low,which limits its application field.Nitrogen atom doping can improve the electrical conductivity of carbon materials,improve the wettability of electrolyte on the surface of materials,and provide more lithium ion active sites by introducing defects,thus improving the electrochemical performance of materials.Silicon has a high theoretical specific capacity of 4200 m Ah g^-1,there will be serious volume expansion in the process of charge and discharge resulting in particle pulverization,poor cycle stability and other problems.Using carbon material structure stability and silicon composite can avoid silicon particle aggregation,effectively inhibit silicon volume expansion,prevent SEI film repeated formation,improve the negative electrode material cycle stability.Therefore,introducing nitrogen atoms into carbon materials or combining with silicon is an effective way to improve the electrochemical performance of carbon materials.Cellulose is the most abundant and sustainable natural polymer and can be obtained from plants,animals or bacteria.The aerogels prepared from cellulose have the characteristics of high porosity,high specific surface area,etc.,and have been applied in the fields of heat insulation,sound insulation,medicine and so on.In this study,cellulose based carbon/silicon composite aerogel with high silicon content was designed and synthesized using cellulose as carbon source and ethyl orthosilicate as silicon source.The effects of different cellulose precursor structures on the microstructure and electrochemical properties of cellulose based carbon aerogel were investigated.The effects of different carbonization temperatures on the microstructure,crystal structure and electrochemical properties of nitrogen-doped cellulose based carbon aerogel were analyzed.Cellulose based carbon/silicon composite aerogel with different silicon contents was prepared by changing the amount of ethyl orthosilicate.The lithium storage mechanism of composite aerogel was discussed by correlation between preparation parameters,structure and electrochemical properties.The specific research contents are as follows:（1）Cellulose based carbon aerogel with two different precursor structures was prepared by sol-gel method using cellulose powder as raw material and sodium hydroxide/urea and sodium hydroxide/polyethylene glycol as solvent,respectively.The effects of different preparation conditions on the micromorphology,pore structure and specific surface area of carbon aerogel were studied,and the correlation between the microstructure and electrochemical performance of carbon aerogel was investigated.The optimum preparation technology of cellulose based carbon aerogel was obtained.（2）Using cellulose as carbon source and urea as nitrogen source,using yeast"fermentation"and sodium hydroxide activation,using sol method to introduce nitrogen atoms in situ to prepare hierarchical porous nitrogen doped cellulose based carbon aerogel.Nitrogen doping can improve the electrical conductivity and wettability of the materials,and provide more Li⁺active sites by introducing defects,which can effectively improve the electrochemical performance of cellulose based carbon aerogel.The effects of different carbonization temperatures（500-700）on the micromorphology,pore structure and electrochemical properties of cellulose aerogel show that the carbon aerogel at 600 presents A three-dimensional network porous structure with a specific surface area of 319.8m² g^-1 and a specific capacity of 871.3 m Ah g^-1 at 0.1 A g^-1 current density.（3）Using cellulose as carbon source and ethyl orthosilicate（TEOS）as silicon source,cellulose/silicon dioxide composite aerogel was prepared by sol method and freeze-drying.Further carbonization and magnesium thermal reduction prepared high silicon content three-dimensional network structure cellulose carbon/silicon composite aerogel.The nano-silicon particles are uniformly anchored on the carbon aerogel skeleton junction,and the three-dimensional network structure provides a buffer space for the expansion of silicon particles.At the same time,carbon matrix can enhance the conductivity of composite aerogel,improve the first coulomb efficiency and cycle stability.By changing the amount of TEOS,the composite aerogel with different silicon content was prepared.The results show that FMCA-Si-3 has a stable three-dimensional network structure,the highest first coulomb efficiency and the most stable cycling performance.