Preparation And Sodium Storage Properties Of Free-Standing Nitrogen-Doped Carbon Nanofibers Composite Mats

The abundance of lithium is low on the earth,the widespread application of lithium-ion batteries will cause serious problem of the lithium resources shortage.Therefore,researchers are seeking actively alternatives to lithium-ion batteries.The working mechanism of sodium-ion batteries is similar to lithium-ion batteries?,and sodium resources are more abundant.Therefore,sodium-ion batteries have become a research hotspot.However,the electrochemical kinetics of sodium ion batteries is slower than lithium-ion batteries'due to the larger radius of Na⁺,thus the improving of the electrochemical kinetics performance of materials is crucial for the development of sodium ion batteries.Nitrogen-doped carbon nanofibers are used widely as anode materials of sodium-ion batteries.Our research was based on the PAN-based nitrogen-doped carbon nanofibers which were obtained by electrospinning of polyacrylonitrile?PAN?and carbonization of PAN nanofibers and aimed to enhance their electrochemical kinetics performance and sodium storage capacity by making them composite with other hard carbon materials.The paper mainly includes two parts:?1?Porous carbon is a material with abundant pore structure and good thermal stability.Its porous structure can reduce the diffusion resistance of Na⁺in the material,which is beneficial to improve the sodium storage property of the material.Therefore,in the first part,we composited PAN-based nitrogen-doped carbon nanofibers with porous carbon to achieve better sodium storage performance by increasing the diffusion rate of Na⁺.Therefore,we successfully composited PAN nanofibers?PAN-NFs-1?with porous Zn-MOF-74 by in-situ synthesis and electrospinning blending technology in this experiment.In the carbonization,PAN-NFs-1 was converted into nitrogen-doped carbon nanofibers and Zn-MOF-74 was decomposed into porous carbon in the fiber skeleton to obtain composite carbon nanofiber mats?PN-CNFs-T?of PAN-based nitrogen-doped carbon nanofibers and porous carbon.The results of electrochemical tests showed that the composite of porous carbon increased the diffusion rate of Na⁺in the electrochemical process and enhanced the sodium storage capacity of PAN-based nitrogen-doped carbon nanofibers.The obtained PN-CNFs-700 at the optimal carbonization temperature possessed good rate and cycle performance.It had a reversible capacity of about 210 mA h g^-1 at a current density of 0.05 A g^-1,and still had about 135 mA h g^-1 at a current density of 5 A g^-1.After 600 cycles at a current density of 0.2 A g^-1,PN-CNFs-700 still had a reversible capacity of 170.5 mA h g^-1.?2?Although the composite of porous carbon can increase the diffusion rate of Na⁺,it can also reduce the conductivity of the material to some extent.Therefore,in the second part,we used the method of improving conductivity to enhance the sodium storage performance of PAN-based nitrogen-doped carbon nanofibers.Graphene is a common two-dimensional carbon material and possesses good conductivity and enables fast electron transport.Therefore,graphene oxide?GO?and PAN nanofibers?PAN-NFs-2?were composited to GO/PAN-NFs-2 composite nanofibers by electrospinning blending technology in the experiment.We obtained the composite nanofibers of rGO and PAN-based nitrogen-doped carbon nanofibers?i.e.rGO-NCNFs-10?after carbonization of GO/PAN-NFs-2.The results of electrochemical test showed that the composite of rGO improved the conductivity of PAN-based nitrogen-doped carbon nanofibers,which made rGO-NCNFs-10 exert better sodium storage performance.At current density of 0.05 A g^-1,rGO-NCNFs-10 had a reversible capacity of about 300 mA h g^-1,at current density of 1 A g^-1,it had a reversible capacity of about 205 mA h g^-1,and the capacity retention efficiency was still 80.4%after1000 cycles at 1A g^-1.