Electrospun carbon nanofibers as anode materials for sodium ion batteries with excellent cycle performance

Abstract

A simple and scalable electrospinning process followed by thermal treatment was used to fabricate carbon nanofibers (CFs). The asprepared CFs were investigated as anode materials for sodium ion batteries (SIBs). Remarkably, due to their weakly ordered turbostratic structure and a large interlayer spacing between graphene sheets, the CFs exhibit a dominant adsorption/insertion sodium storage mechanism that shows high reversibility. As a result, the CFs show excellent electrochemical performance, especially cycle stability (97.7% capacity retention ratio over 200 cycles). Reversible capacities of 233 and 82 mA h g À1 are obtained for the CFs at a current density of 0.05 A g À1 and even a high current density of 2 A g À1 , respectively. The excellent cycle performance, high capacity and good rate capability make the CFs promising candidates for practical SIBs.

Is single layer graphene a promising anode for sodium-ion batteries?

“…Moreover, a large fraction of the sodium storage between the graphene layers was generally accompanied by a superior anodic rate performance of the material [19,[22][23][24]. In conclusion from these studies, it seems that graphite-like materials with large interlayer distance and low porosity are potentially the most suitable anodes to allow the development of the SIBs.…”

“…On the contrary, a large proportion of the capacity provided by other carbons was ascribed to the adsorption of sodium into the pores [7]. In any event, the former mechanism should be preferred since the Na + ion pore-filling potential is close to that of the metal itself which may cause sodium plating.Moreover, a large fraction of the sodium storage between the graphene layers was generally accompanied by a superior anodic rate performance of the material [19,[22][23][24]. In conclusion from these studies, it seems that graphite-like materials with large interlayer distance and low porosity are potentially the most suitable anodes to allow the development of the SIBs.…”

“…Thus, no voltage plateaux were observed in the electrochemical profiles of electrospun carbon nanofibers [19] and reduced graphene oxide [24], indicating that the capacity was mainly due to the sodium insertion into the graphene layers.…”

“…Therefore, efforts have been focused on the identification of other host carbon materials to achieve a successful reversible intercalation/insertion of the larger Na + anions. Thus, a variety of carbons having a certain degree of porosity, a low-ordered structure consisting of few-layers graphite nanocrystallites (graphite domains) and different morphologies, including hard and soft carbons [5,7,[11][12][13][14][15], carbon fibers and nanofibers [16][17][18][19][20], spherical carbons [21], hollow carbon nanowires [22] and nanospheres [23], reduced graphene oxide, [24] pyrolytic carbons from graphite oxide [25], as well as highly disordered carbon composites [26,27] have been investigated as anodes for SIBs. Good results as regards capacity and cycling stability were achieved with some of these carbons, the rate capability and particularly, the low coulombic efficiency during the first charge/discharge cycle due to the relatively large surface area (as compared with graphitic carbons) being the main drawbacks to overcome.…”

“…capacity and cycling stability were achieved with some of these carbons, the rate 4 capability and particularly, the low coulombic efficiency during the first charge/discharge cycle due to the relatively large surface area (as compared with graphitic carbons) being the main drawbacks to overcome. In these porous disordered carbons, the electrochemical storage of Na + ions occurs through two predominant mechanisms, insertion between the graphene layers of the small graphitic clusters in the higher potential region ( 1.5-0.1 V vs Na/Na + ) resulting in a sloping voltage profile, and/or adsorption by filling the porous structure as showed by the plateau at the lowest potentials (< 0.1 V vs Na/Na + ) [7,11,19,23].Thus, no voltage plateaux were observed in the electrochemical profiles of electrospun carbon nanofibers [19] and reduced graphene oxide [24], indicating that the capacity was mainly due to the sodium insertion into the graphene layers.A common feature for these carbons is the large interlayer spacing [19,23,24], all of them having values  0.37 nm, which has been determined from theoretical calculations to be the critical minimum value required to allow good reversible Na + ion insertion [22]. On the contrary, a large proportion of the capacity provided by other carbons was ascribed to the adsorption of sodium into the pores [7].…”