Synthesis And Sodium Storage Properties Of High Performance Flexible Carbon Fibers And Carbon/Iron Sulfide

Lithium–ion batteries with high energy density and power density have been governing the market of batteries.The electric vehicles develope fast in recent years and promising to replace the current oil-driven vehicles in order to mitigate the problem of resource scarcity of petroleum and reduce the generation of greenhouse gases.However,a scarcity of lithium resources gets more serious.The sodium is abundant and widely distributed in the earth crust and similar to lithium in physical and chemical properties.Hence sodium-ion batteries?NIBs?are very promising to be used for large-scale energy storage.The preparation of high performance anode materials gives much significance to implement the application of NIBs which is also a hot area of research.In this paper,two anode materials with high performance for NIBs were developed through an electrospinning technology.1.The freestanding and flexible N,S co-doped carbon nanofibers?NSCNFs?was prepared by a method of electrospinning followed by thermal treatment.The as-prepared NSCNFs with interlaced structure showed good structural stability,this 3D conductive network could also provide short path for electron transfer.A high specific surface area of698.4 m² g^-1 was obtained for NSCNFs with a high pore volume of 0.295 cm³ which would increase the contact area between electrolyte and electrode,facilitate the Na⁺diffusion,provide more actives and improve the adsorption capacitance.The intrinsic N and extrinsic S-containing oxygen groups could introduce pseudocapacitance effect by reversibly react with Na⁺to increase specific capacity of electrode material.The NSCNFs exhibited high rate capability and good cycling performance when used as anode material for NIBs.It delivered high reversible capacity of 133 mA h g^-1 at 30 A g^-1 and retained a capacity of 164 mA h g^-1 at10 A g^-1 after 6000 cycles.2.The FeS nanocrystallites were uniformly distributed into carbon nanofibers?FeS@CNFs?to construct a freestanding and flexible electrode material through a strategy of electrospinning combined with thermal treatment.Benefit from the confining effect of carbon nanofibers,the dimension of FeS was reduced to nanoscale;the carbon nanofibers could buffer the volume change of FeS during charge/discharge;the problem of poor conductivity of FeS was solved by highly conductive carbon nanofibers.Hence,the FeS nanocrystallites exhibited good electrochemical activity.The porous carbon nanofibers were benefit for penetration of electrolyte and could shorten the ionic diffusion path.The doping N and S improved the electrochemical performance of carbon nanofibers and introduced pseudocapacitance effect into the carbon to increase specific capacity and it was the guarantee of electrode to work well at a high rate.It was identified that the sodium storage mechanism of FeS@CNFs was pseudocapacitance dominated which was consistent with high rate capability and good cycling performance.It delivered high capacities of 367 and 278 mA h g^-1at 10 and 30 A g^-1.A capacity of 364 mA h g^-1 could be retained after 700 cycles at 10 A g^-1corresponding to a retention ratio of 95.8%.