Electrospun Hard Carbon Nanofibers Prepared For Sodium Ion Storage

Nowadays,the sodium-ion batteries(SIBs)have gradually gained increasing attention due to the low cost and the abundant reserves of sodium over lithium.Because of the larger ionic radius of sodium-ion,which influences the ion transport and cycle stability,it is very difficult to simply employ anode materials for LIBs on SIBs.A lot of works have been done to design the anode materials with high performance for SIBs.On account of their large interlayer distance and disordered structure,hard carbon materials have been regarded as one of the most promising anode materials to accommodate sodium ions.However,there are still some problems which hinder the application of hard carbon in SIBs,for example:? the discharge/charge profiles of hard carbon delivered long low-voltage plateau,which is disadvantage to safety issue of SIBs;? the mechanism of sodium ions storage in hard carbon is still controversial;? with flexible energy storage devices emerging,the electrode is required to be flexible as well.Therefore,It's significant to developing free-standing hard carbon electrode for SIBs.In this work,we took polyvinylpyrrolidone(PVP)and Cu(NO3)2 as raw materials and fabricated a series of hard carbon nanofibers(CNFs)film with controlled architectures by electrospinning.The addition of Cu(NO3)2 in PVP can lead to formation of cross-linked structure of nanofibers.Remarkably,this is the first report that the Cu(NO3)2 can work as cross-link agent of nanofibers.The formation of cross-linked structure can be ascribed to strongly coordination of PVP with Cu(NO3)2.What's more,the Cu(NO3)2 can also manipulate the micro structure of final CNFs,it was found that addition of Cu(NO3)2 can increase the disorder of CNFs.We investigated the relationship between the micro structure of CNFs and their electrochemical performance.With the disordered degree of CNFs increasing,the proportion of the slope region in the discharge profiles gradually increase and that of low voltage plateau region reduce,the sodium ions storage behavior in CNFs changes from "adsorption-intercalation" to"adsorption" only.Besides,the CNFs with higher disordered structure demonstrate excellent electrochemical performance.Moreover,the security of SIBs has also improved.We further employed CNFs film as free-standing work electrode without any binders and conducting additives,and the CNFs film demonstrates excellent rate performance and long cycle stability.At 50 mA g-1,the specific capacity of CL-CNFs film can reach up to 449 mA h g-1.At 5 and 10 A g-1,the CL-CNFs film deliver the initial reversible capacity of 148 and 121 mAh g-1,respectively,and still maintain 126 and 111 mA h g-1 after 500 cycles without obvious capacity fading.The excellent electrochemical properties of the CNFs film are attributed to its unique cross-linked structure that endows the CL-CNFs film fast electron transfer and ion diffusion kinetics as well as robust structural stability to bear repeated impact of Na ion during discharge/charge process.This work is great significative to design high performance carbon nanofibers as anode for the development of SIBs.