Fabrication Of Nanostructural SnO₂ Based Composite Fibers For Li Storage Application

Lithium ion batteries?LIBs?are the most widely used rechargeable batteries.Every year,billions of LIBs are produced and applied to various electronic devices.Thus,increasing the performance of LIBs will significantly bring economic and social benefits.The current anodes of commercial LIBs are made of graphite.Its application in LIBs faces two big issues,low theoretical capacity and low Li-inserting potential,which lead to low energy density and potential safety hazard.As a promising anode material,SnO₂ possesses a high capacity about 780 mA h g^-1 calculated with traditional method.Some researchers believe that the potential theoretical capacity can reach 1493 mA h g^-1.However,the SnO₂ based anode materials have to overcome two problems: huge volume expansion and poor conductivity.The two problems will lead to electrode pulverization and rapid capacity fading.Moreover,the highest theoretical capacity is calculated based on the reversible conversion of SnO₂ to Sn.On the strength of desiging SnO₂ and carbon materials based composites,we have explored several nanostructural SnO₂/C based composite nanofibers.This thesis involves the engineering of nanostructural nanofibers,the optimization of electrospinning technology,polymerization and carbonization of dopamine,the etching of metal oxide,and so on.We evaluate the lithium storage performance of these nanostructural SnO₂ based composite fibers.1.Carbon coated SnO₂/Fe₃O₄ composite nanofibersThe carbon coated SnO₂/Fe₃O₄@ composite nanofibers were prepared via a facile two-step process.The inner core metal oxide nanofibers were first made by electrospinning,then the shells were conformally coated with chemical bath deposition and a subsequent carbonization process with polydopamine as carbon source.When applied as a binder-free self-supported anode for lithium ion batteries,the composite nanofibers displayed high reversible capacity of 850 mA h g^-1 after 80 cycles at a current density of 100 mA g^-1 and excellent rate capacity of 402 mA h g^-1 at 1600 mA g^-1.2.Voids-involved SnO₂/C composite nanofibersVoids-involved SnO₂/C composite nanofibers were fabricateded by etching Fe₃O₄ of prepared carbon coated SnO₂/Fe₃O₄@ composite nanofibers.The inner voids space buffer the stress induced by volume change of SnO₂,the inner carbon frameworks prevent the agglomeration and the outer carbon shells are beneficial to the formation of stable SEI layer.The resultant nanofibers electrode displays not only a high reversible capacity of 986 mA h g^-1 at 200 mA g^-1 after 200 cycles,but also a high initial coulombic efficiency of 73.5 %.It has been shown that such a rational design can efficiently reduces the side reactions and promotes the reversible conversion of Sn to SnO₂ for both half and full cells.3.Porous SnO₂/C composite nanofiberWith the combination of electrospinning with etching treatment,porous SnO₂/C composite nanofibers were obtained using PAN as the cabon source and TEOS as precursors of template.Electrochemical tests show that the SnO₂/C composite nanofibers deliver initial initial coulombic efficiency of 70.8 % and reversible capacity of 735 mA h g^-1 at 300 mA g^-1 after 130 cycles,the high initial coulombic efficiency and reversible capacity also originate from the reversible conversion of SnO₂ to Sn.4.Porous carbon nanofibersThe porous carbon nanofibers were prepared by removing the inner metal oxide of carbon coated SnO₂/Fe₃O₄ composite nanofibers.When evaluated as anode material for LIBs,the porous carbon nanofibers delivers reversible capacity of 1324 mAh g^-1 at 200 mA g^-1 after 150 cycles,which is much higher than that of most metal oxide electrode and higher than that of voids-involved SnO₂ and carbon based electrode.The high reversible capacity can be attributed to the N and O doping,which creates rich defects and functional groups,in addition,the abundant microcavities are beneficial for lithium ions storage.