Preparation And Performance Of Silicon/Carbon Nanofiber Anodes For Lithium-ion Battery

With the rapid development of electric equipment,it is urgent to develop lithium-ion batteries with high-capacity,high-rate and long-cycle performance.Silicon has been considered as one of most potential anode materials due to its high theoretical specific capacity(4200 m Ah·g^-1),low working potential（0.4 V vs.Li/Li⁺）and abundant reserves.However,there are some problems in silicon including large volume expansion and poor conductivity,which have limited its commercial application.Combining silicon and carbon fibers is considered as a promising strategy.It can not only solve the above problems,but also avoid the use of inactive materials such as binders and conductive additives.In order to develop high-performance silicon/carbon fiber composites,this thesis focuses on the structure design of silicon/carbon fiber and the modification of silicon materials and carbon fibers.The research content and results are as follows:（1）Using polyacrylonitrile and silicon nanoparticles as raw materials,three kinds of silicon@porous carbon nanofibers（Si@PCNF）,silicon@hollow porous carbon nanofiber（Si@HPCNF）and silicon/porous carbon nanofiber（Si/PCNF）composites with different silicon loadings were prepared by electrospinning.The effects of structures and silicon loadings on the morphology and electrochemical performance of different composites were studied.The results indicate that,as the silicon content increases,the electrochemical performance of all composites deteriorate.The composites with low silicon loading possess better electrochemical performance:Si@PCNF,Si@HPCNF and Si/PCNF exhibit reversible specific capacities of 842.1 m Ah·g^-1,709.6m Ah·g^-1 and 357.7 m Ah·g^-1 after 500 cycles at a current density of 0.5 A·g^-1,respectively.Si@PCNF electrodes possess the excellent performance due to two aspects:one is the interconnection of the inner carbon nanofibers and silicon nanoparticles,which is beneficial to the rapid transfer of lithium ions.The other is that the outer carbon nanofibers shell prevents the silicon nanoparticles from directly contacting the electrolyte directly,which is conductive to improving structural stability.（2）In order to relieve the volume expansion of silicon,the porous silicon（PSi）materials were prepared by the metal-induced chemical etching method,and the silicon@titanium dioxide（Si@TiO₂）composites were prepared by the sol-gel method.The PSi@PCNF and Si@TiO₂@PCNF composites were prepared using the same method as previous chapter.The results indicate that,PSi@PCNF electrodes possess high discharge specific capacity of 1196.5 m Ah·g^-1 after 500 cycles at 0.5 A·g^-1 and a reversible specific capacity of 672.7 m Ah·g^-1 after 1000 cycles at a high current density(2 A·g^-1),showing good high current charge and discharge capability.Additionally,PSi@PCNF electrodes exhibit pseudo-capacitance contribution rate of 72.87%at the scan rate of1.00 m V·s^-1,showing excellent ion transmission performance and rate capability.Such excellent performance is attributed to that PSi materials with high specific surface area and more active sites can both relieve the stress of its own volume expansion and improve the charge transfer speed.（3）Porous silicon@sulfur-doped porous carbon fiber（PSi@SPCF）porous silicon@boron-doped porous carbon fiber（PSi@BPCF）and porous silicon@sulfur-boron co-doped porous carbon fiber（PSi@SBPCF）were prepared by adding heteroatom using thiourea and boric acid as the sulfur source and boron source respectively.The results show that,PSi@SPCF electrodes exhibit excellent cycle performance with the discharge specific capacity of 1987.5 m Ah·g^-1 after 500 cycles at 0.5A·g^-1 and a reversible specific capacity of 1112.7 m Ah·g^-1 after 1000 cycles at 2 A·g^-1,which is attributed to the abundant defects induced by sulfur doping,increasing the active sites and improving the Li⁺adsorption energy.PSi@SBPCF electrodes possess outstanding ion transmission capacity and rate performance with the pseudo-capacitance contribution rate of 79.56%at 1.00m V·s^-1.And at the current densities of 0.1 A·g^-1,0.2 A·g^-1,0.5 A·g^-1,1.0 A·g^-1,2.0 A·g^-1,the first-turn discharge specific capacity of PSi@SBPCF electrodes are 5102.5 m Ah·g^-1,3183.3 m Ah·g^-1,2952.2 m Ah·g^-1,2622.4 m Ah·g^-1,2259.6 m Ah·g^-1,respectively;The electrical conductivity and rate performance of PSi@SBPCF electrodes are improved by the synergistic effect of sulfur and boron doping,where sulfur doping increases active sites and boron doping provides ion carriers.