| Silicon is a promising alternative anode material currently for large-scale commercialization,owning to its extremely high theoretical capacity(4200 m Ah/g),nature abundance and high insertion potential.However,the gigantic volume expansion during Li alloying and dealloying leads to structure failure of electrode material,the particles crumbling or pulverization,and more serious the active material Shedding from the collector,which is believed to be the main reason for rapid capacity fading and poor cycle stability of silicon.At the same time,silicon as a semiconductor has poor conductivity,which leads to poor rate performance of silicon anode.To overcome the shortcomings of silicon anode,the modification methods include nanocrystallization,compoundation,alloying and so on.Because of simple structure,low cost,silicon/carbon composite has been studied extensively to overcome the above problems.Using pitch and phenolic resin as carbon resource respectively,the Si/G/C composites are prepared by spray drying and pyrolysis method in this article.Effects of different process parameters on their physical and electrochemical properties are studied subsquently.At last,this article explores the processes of twice coating and twice spraying,character the physical and electrochemical properties of Si/G/C composites prepared by above processes.Using pitch as carbon resource,Si/G/C composite with core-shell structure is prepared by spray drying and pyrolysis method.Effects of pyrolytic temperature,pyrolytic time,silicon size on physical and electrochemical properties of Si/G/C composite are studied.After optimization,the prepared Si/G/C-1 composite exhibits an initial discharge specific capacity of 703.1 mAh/g with an coulombic efficiency of86.5%.and it exhibits good cycling performance that the capacity retention is 86.1%after 75 cycles at the current density of 100 mA/g.even at 500 mA/g,the discharge capacity can reach 630.5 mAh/g.In the Si/G/C-1 composite,the amorphous carbon layer can prevent the direct contact between silicon and electrolyte which reduces the irreversible capacity consumed by the formation of SEI and increases the initial coulombic efficiency.Moreover,the amorphous carbon layer and graphite not onlybuffer the volume expansion,maintain the structure stability but also enhance the conductivity.These collaborative function contributes to the excellent initial coulombic efficiency,cycle stability and rate performance.Using phenolic resin as carbon resource,Si/G/C composite with core-shell structure is prepared by spray drying and pyrolysis method.Effects of silicon content,graphite type,phenolic resin content and carbon source on physical and electrochemical properties of Si/G/C composite are studied.After optimization,the prepared Si/G/C-2 composite exhibits an initial discharge specific capacity of 621.8mAh/g with an coulombic efficiency of 82.5%.and it exhibits good cycling performance that the capacity retention is 85.4% after 85 cycles at the current density of 100 mA/g.even at 500 mA/g,the discharge capacity can reach 519.5mAh/g.Finally,this article explores the processes of twice coating and twice spray drying-pyrolysis.In the twice coating process,the Si/C-B2 is synthesised by coating a layer of amorphous carbon at the surface of Si/C-B1 again.The prepared Si/C-B2 exhibits an initial discharge specific capacity of 622.3 mAh/g with an coulombic efficiency of 82.6% and it exhibits good cycling performance that the capacity retention is 85.0% after 100 cycles at the current density of 100 mA/g.In the twice spray drying-pyrolysis process,the Si/C-P2 is synthesised by applying spray drying and pyrolysis two times,taking glucose and pitch as carbon source respectively.The prepared Si/C-P2 exhibits an initial discharge specific capacity of 648.8 m Ah/g with an coulombic efficiency of 85.0%.and it exhibits good cycling performance that the capacity retention is 85.6% after 80 cycles at the current density of 100 mA/g. |
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