# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 3 Rechargeable Lithium-Ion and Sodium-Ion Batteries
### Sodium-Ion Batteries
As for anode materials consisting of electrospun TMO-fibers, the high aspect ratio fiber morphology is responsible for the interestingly high initial specific capacity (983 mAh/g) recently reported for spinel cobalt oxide (Co\({}_{3}\)O\({}_{4}\)) fibers [133]. However, after the first sodiation/de-sodiation  
Figure 11: **(a)** SEM image of the free-standing membrane consisting of Na\({}_{3}\)V\({}_{2}\)(PO\({}_{4}\))\({}_{3}\)/carbon fibers. **(b)** TEM and **(c)** high-resolution TEM images of the composite fibers (reproduced from reference [127] with permission. Copyright 2017, 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Elsevier). cycle, their capacity lowers down to 576 mAh/g as an effect of the formation of CoO as a stable intermediate. The CoO-based anode retains 71% of its initial capacity over 30 cycles [133]. Anodes based on electrospun iron(III) oxide fibers doped with silicon exhibit better stability [140]. They are able to deliver 350 mAh/g after 70 cycles retaining 85% of the initial capacity--a result never achieved for iron oxide-based electrode standard formulation. The doping with an aliovalent element induces changes in the crystalline phase of the oxide and in its morphology, as well. The undoped fibers consist of interconnected polycrystalline hematite (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) grains (Figure 12a), whereas the Si-doped fibers are constituted by elongated maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)) nanostructures developing mainly along the longitudinal fiber axis (Figure 12b). The latter change is responsible for the improved cyclability, while the formation of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), an intrinsically more conductive phase, accounts for the enhanced electrochemical performance with a specific capacity (400 mAh/g at a rate of C/20) higher by a factor of \(\sim\)4 compared to the \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)-based electrode [140]. Electrodes based on \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) NPs with standard formulation (Table S4) exhibit worse performance too, with lower stability and smaller delivered capacity (\(\sim\)150 mAh/g after 50 cycles) [136]. Instead, in free-standing \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)/C fibrous membranes, prepared by using PMMA as a sacrificial agent to generate pores in the nitrogen-doped PAN-derived carbon fibers (Figure 13), the electrochemical behavior benefits from \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) NPs being embedded in high aspect-ratio carbon-fibers [136], often reported as one-dimensional (1D) nanostructures. After 50 cycles at 0.1 C, the binder-free electrode still delivers 290 mAh/g. Figure 12: TEM images of (**a**) undoped \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and (**b**) Si-doped \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) fibers (reproduced from reference [140] with permission. Copyright 2018, Elsevier). Figure 13: (**a**) SEM and (**b**) TEM images of the composite \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)/C fibers, with elemental mapping at the bottom (reproduced from reference [136] with permission. Copyright 2018, Elsevier). By far, greater stability pertains to anodes based on electrospun carbon fibers containing anatase NPs (Table S4). TiO\({}_{2}\)/C fibers are able to deliver 240 mAh/g at 0.2 A/g with capacity retention close to 100% over 1000 cycles [134]. Over the same number of cycles, thanks to the synergistic interaction between anatase and tin NPs (Figure 14), TiO\({}_{2}\)-Sn/C fibers deliver a lower reversible capacity (191 mAh/g) at a higher current density (1 A/g) with comparable capacity retention (95.4%) [137]. Very recently, [PERSON] et al. [139] have proposed a four-step process to prepare composite tin disulfide/N- and S-doped carbon nanofibers. As schematically depicted in Figure 15a, the process consists of (i) electrospinning, followed by (ii) stabilization in the air, (iii) sulfidation, and (iv) carbonization in a vacuum.