Studies On Preparation Of Transitional Metal Oxides And Carbon-based Materials By Electrospinning As High Performance Sodium Storage Anodes

Sodium ion batteries?SIBs?are now emerging as the next generation rechargeable batteries.Due to the similar electrochemical properties with lithium ion batteries and relatively low cost,sodium ion batteries are considered to be potentially applied in the field of large-scale energy storage.Among various anode materials,transitional metal oxides have the advantages of high specific capacity,resource abundance and facile synthesis process,meanwhile carbon materials are of low cost,high stability and environmental friendliness.Therefore,both of them are regarded as promising anodes for sodium ion batteries.However,traditional metal oxides always exhibit poor cycling performance,and carbon materials display relatively low specific capacity,which limit their practical application for large-scale energy storage.Thus it's very urgent for us to improve the electrochemical properties of these two kinds of materials.This thesis aims at improving the specific capacity,cycling performance and rate performance of these materials by fabricating a unique nanofiber structure using a facile,cost-effective and scalable electrospinning method combined with other experimental techniques.The main results are summarized as follows:?1?We designed a unique structure of Co₃O₄ nanoparticles embedded in carbon nanofibers?Co₃O₄@CNFs?by electrospinning and effectively improved the cycling stability,which was highly affected by the large volume expansion of oxide during cycling.The results indicate that high-performance Co₃O₄@CNFs electrode is obtained by controlling the carbonization temperature and carbon content of the composite.The as-prepared Co₃O₄@CNFs show initial discharge and charge capacities of 764.8 mAh g^-1and 422.4 mAh g^-1 respectively at a current density of 50 mA g^-1.After 100 cycles,the composite still remains a reversible capacity of about 300 mAh g^-1.When a large current density of 500 mA g^-1 is conducted on the composite,it still remains 251.7 mAh g^-1 with a coulombic efficiency of 99.6%after 500 cycles.The excellent electrochemical performance of the composite is attributed to the special nanofiber structure with carbon coating,which not only buffers the volume variation of Co₃O₄ nanoparticles,but also serves as the electron and ion pathways to improve the conductivity of the composite.?2?We synthesized a unique structure of ZnO nanoparticles embedded in carbon nanofibers?ZnO@CNFs?by electrospinning.By optimizing the carbonization temperature and carbon content,the conductivity of the ZnO based composite is much improved.Results show that the obtained composite displays initial discharge and charge capacities of392 mAh g^-1 and 246.8 mAh g^-1 respectively at a current density of 50 mA g^-1.After 100cycles,a discharge specific capacity of 264.3 mAh g^-1 is maintained,exhibiting decent electrochemical performance.The improvement in the specific capacity is mainly resulted from the special carbon coated nanofiber structure,which improves the conductivity of the composite and greatly increases the interface between the electrolyte and composite,resulting in higher availability of the composite.?3?We fabricated ZnO-doped carbon nanofibers?ZCNFs?by electrospinning.By doping ZnO into the carbon materials and adjusting the doping content of ZnO,PAN-based carbon materials with excellent electrochemical performance was prepared.Results demonstrate that carbon nanofibers deliver the highest specific capacity when the doping content of ZnO is 21.4 wt%.The initial discharge and charge capacities of ZCNFs are 473.9mAh g^-1 and 295.5 mAh g^-1 respectively at a current density of 50 mA g^-1.After 100 cycles,a discharge capacity of 324 mAh g^-1 is still retained,exhibiting superior cycling stability.By further characterizing and analyzing Na⁺storage mechanisms,we conclude that the improvement in the specific capacity is owing to the activation effect of ZnO to carbon materials,which not only enlarges the specific surface area of carbon materials,thus increasing the active sites for Na⁺storage,but also reduces the charge transfer resistance,hence improving the electrode process kinetics.