Fabrication Of Si/C Composite Anode Materials For Lithium Ion Batteries By Electrospinning

Silicon has been considered a promising anode material for replacement of graphite because of its high theoretical specific capacity(4200 m Ah·g-1),a relatively low working potential(<0.5 V vs Li/Li+),environmentally friendly and abundant reserves.However,the volume change of silicon during the charge and discharge process is severe(about 300%),which leads to problems such as fragmentation of active material,destruction of electrode structure,and unstable solid electrolyte interface film,which seriously restricts the practical application of high-capacity silicon-based anode material for lithium ion batteries.The preparation of silicon-carbon composites by electrospinning can not only effectively alleviate the structural changes of silicon in the charge and discharge process,but also the carbon fibers can increase the conductivity of the electrodes.From the perspective of electrode material structure design,the one-dimensional silicon-carbon composite fibers,carbon-coated silicon-carbon composite fibers,and carbon-coated silicon-carbon composite fibers with a porous structure were constructed by electrospinning to prepare high-capacity,long-life anode materials for lithium-ion batteries,the main research content includes the following aspects:1.In order to study the effect of silicon particle size on the morphology,structure and electrochemical performance of composites,different sizes of nano-silicon(10 nm,20 nm,30 nm,80 nm)and polyvinylpyrrolidone(PVP)were electrospun and the subsequent high temperature calcination.The results show that when the small-size silicon particles(10 nm,20 nm)are elect rospun,the silicon particles are completely distributed in the carbon fibers.As the particle size increases,silicon gradually exposes on the surface of the fiber.The larger the particle size,the more serious the expose is.2.To solve the poor cycle performance of Si-C composite nanofibers with silicon exposed on the fiber surface,a carbon-coated Si-C composite nanofibers with “sandwich” structure is constructed by hydrothermal reaction and subsequent high-temperature carbonization.The effects of calcining temperatures and the thickness of the carbon coating layer on the electrochemical performance were studied.The presence of the carbon coating layer effectively coats the silicon particles exposed on the surface of the fiber,avoids direct contact between the silicon and the electrolyte,reduces the occurrence of side reactions,and is conducive to the formation of a stable SEI film,The composite material has better rate performance and cycle performance: the specific capacity can be maintained at 683.4 m Ah·g-1 under a current density of 2 A·g-1,it still has a specific capacity of 601.5 m Ah·g-1 after 100 cycles under the current density of 1 A·g-1,a capacity retention rate of 79.3%,and a capacity retention rate of 55.2% after 200 cycles.3.On the basis of the above structure,ZIF-8 was introduced as a template for constructing porous carbon,and a carbon-coated silicon-carbon composite fiber material with a porous structure was constructed.The cycle stability of the composites was further improved.The reversible discharge specific capacity reached 963.5 m Ah·g-1 under the current density of 1 A·g-1 and it deliver a reversible specific capacity of 664.4 m Ah·g-1 after 200 cycles,the capacity retention rate is 72.5%.The excellent electrochemical performance attributed to the introduction of the porous structure,which can increases the specific surface area and pore volume of the composite,and effectively relieves the volume expansion and contraction of silicon in the charge and discharge.