Synthesis and Electrochemical Study of Si/C Composite Anode Materials

Author:Li Bo

Supervisor:qian xue feng

Database:Doctor

Degree Year:2018

Download:32

Pages:165

Size:14888K

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Si has been regarded as the most promising anode material for lithium-ion capacitors due to its highest theoretical specific capacity,low discharging voltage,and natural abundance.However,the huge volume changes during cycling and its low electronic conductivity hinder the application of Si as an anode by limiting the cycling life and rate performance.Nanostructuring and carbon coating are demonstrated as efficient way to improve the electrochemical performance.In this paper,Si/C nanocomposites have been designed and fabricated and their electrochemical performances have been investigated.The main findings and results are listed as follow:(1)Both Si/C anode material and AC cathode material with hierarchically biomimetic structure are fabricated from natural rice husks,respectively.The interconnected porous structure of the RH-derived Si/C anode material can buffer the huge volume change of Si and faciliate the transportation of ions.As a result,the RH-derived Si/C anode material have a capacity of more than 500 mAh g-1 after 250 cycles at 6.4 A g-1,and a capacity of500 mAh g-1 is still kept at a high rate of 12.8 A g-1.One the other hand,the RH-derived AC cathode material(RAC)shows improved capacity and cycling stability and good rate performances.These improved performances of RAC-5 can be attributed to the abundant active sites from the highest surface area,the fast ionic transportation channels and shorten diffusion distance from the optimized hierarchical porous structure.Benefit from the high capacity,good rate and cycling performances of RH-derived Si/C anode material and AC cathode material,the designed Si/C||RAC LICs exhibit a high energy density of 227 W h kg-1 at a power density of 1146 W kg-1,and181 W h kg-1 can still be achieved even at the ultra-high power density of32595 W kg-1.Furthermore,the RH-derived LICs also show an improved cycling stability(16000 cycles at16800 W kg-1).(2)we have successfully developed a facile and sustainable method to transform the optical fibre wastes to yolk-shell Si@void@C nanospheres via magnesiothermic reaction.The ultra-small size of Si primary nanoparticles and the well-defined internal void spaces effectively accommodate the huge volume changes without rupturing carbon shell.Moreover,the carbon framework without SiC functions would not only act as a eletrolyte-blocking layer to keep a stable SEI on the outer surface,but also act as an effective conducting framework to address the issues of low electrical conductivity.As a result,the yolk-shell Si@void@C nanospheres have a capacity of more than 1400 mAh g-1 after 100 cycles at 0.4 A g-1,and a capacity of800 mAh g-1is still kept after 200 cycles at a high rate of 10 A g-1.In addition,Full cell Si@void@C||RAC LICs give a high energy density value of 231 W h kg-1.at a power density of 1237 W kg-1.Even at the ultra-high power density of34255 W kg-1,it could still provide an energy density of 189 W h kg-1.Furthermore,this LIC device also shows a great cycling stability(15000 cycles at 6.4 A g-1).(3)By combining the advantages of porous structure,hollow structure and yolk-shell structure,we proposed a new kind of hollow yolk-shell structure,porous Si@C ball-in-ball hollow structure,that is,carbon shells fully encapsulated porous Si hollow yolks.The porous and hollow structure of Si and the well-defined internal void spaces effectively accommodate the huge volume changes without rupturing carbon shell.Moreover,the carbon framework without SiC functions would not only act as a eletrolyte-blocking layer to keep a stable SEI on the outer surface,but also act as an effective conducting framework to address the issues of low electrical conductivity.As a result,porous Si@C ball-in-ball hollow spheres have a capacity of 1446 mAh g-1 after100 cycles at 1.0 A g-1,a capacity of586 mAh g-1 is still kept at a high rate of 32.0 A g-1,and a long cycling life(1000 cycles).In addition,Full cell Si@C ball-in-ball HSs||RAC LIC gives a high energy density value of 239 W h kg-1.at a power density of1376 W kg-1.Even at the ultra-high power density of 69600 W kg-1,it could still provide an energy density of154 W h kg-1.Furthermore,this LIC device also shows a great cycling stability(15000 cycles at 6.4 A g-1).(4)Porous Si microparticles with pore size of2.8 nm were successfully prepared by controlling the size of MgO via adjusting the ramp rate in magnesiothermic process.The surface area of porous Si microparticles could be significately reduced from 235.6m2 g-1 to 32.4 m2 g-1 by coating a layer of carbon as low as 5.8 wt%,in which the narrow external openings of pores on Si microparticle were easily closed to leave the interior pores unfilled.The rational designed structure could limit the formation of SEI layer,minimize the irreversibly Li trapping in amorphous carbon,and retain the internal void space for Si expansion.As a result,the initial Coulombic efficiency of the as-prepared Si/C microparticles reached to 87.5%with a high reversible capacity of 2242mAh g-1 at 0.4 A g-1 after 100 cycles.