# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 3 Rechargeable Lithium-Ion and Sodium-Ion Batteries
### Sodium-Ion Batteries
[123] are able to deliver a reversible capacity of 103 mAh/g at a rate of 1 C, and to retain 56 mAh/g even at a very high rate (50 C), further exhibiting a ultra-long cyclability with 96.5% capacity retention after 1000 cycles at 2 C. The shortened length of Na-ion transport associated to the very small size of the oxide particles, together with the fiber porosity, favoring the permeation of the electrolyte, and the material structure consisting of a three-dimensional network of interconnected fibers that results in improved ionic and electronic transport, is responsible for this behavior [123]. Figure 10: (**a**) Sketch of the shape evolution of the building blocks forming the electrospun Na\({}_{0.44}\)MnO\({}_{2}\) fibers upon the increase of the heat treatment temperature. (**b**,**c**) TEM images (insets: related SAED patterns) of the fibers obtained at (b) 600 \({}^{\circ}\)C and (c) 800 \({}^{\circ}\)C (reproduced from reference [124] with permission. Copyright 2016, Elsevier). Different from cathode materials, for which satisfactory results in terms of stability and charge retention have been achieved [105], anode materials still represent a challenge [128]. Graphite, the standard anode material for LIBs, exhibits poor performance in SIBs since Na\({}^{+}\) ions hardly intercalate between its layers because of the energetic instability of the Na-graphite intercalation compounds [105; 129]. Non-graphitic disordered anode materials, such as hard carbons, where the enlarged interlayer spacing and/or the smaller size of the parallel graphitic domains allows for the sodium insertion, are currently utilized. Recently, self-standing membranes consisting of fulvic acid-based electrospun hard-carbon nanofibers have been evaluated as binder- and collector-free anode materials [130]. They have shown a reversible sodium intercalation capacity of 248 mAh/g with a capacity retention ratio of 91% after 100 cycles at a current density of 100 mA/g. The proper interlayer distance, coupled with the abundance of oxygenated functional groups and benefits deriving from the high aspect-ratio morphology of the electrospun fibers (i.e., short ion diffusion distance and excellent stress tolerance) account for their good storage performance. However, since the production of hard carbons generally has great environmental impact [116], different, more eco-friendly anode materials have been recently investigated for rechargeable SIBs [117; 128; 129; 131]. Conversion materials, such as transition metal oxides (TMOs) and transition metal sulfide (TMSs), conversion-alloying materials, such as transition metal phosphide (TMPs), and alloying materials, such as elements of the 14 th and 15 th groups in the periodic table, have received particular attention [129; 131; 105]. The highest theoretical capacity (\(\sim\)2600 mAh/g) pertains to phosphorus. Nevertheless, the large volume changes (up to \(\sim\)400%) undergone by the alloying materials during the sodiation/de-sodiation process, with consequent electrode pulverization and rapid capacity fading, is still an unresolved problem, which hampers their use in practical applications [131; 105]. Conversely, TMSs and TMOs exhibit still high theoretical capacities (up to \(\sim\)900 and \(\sim\)1000 mAh/g, respectively) and the main limitations of their use as anode materials, namely structural damage leading to pulverization and poor electronic conductivity, can be effectively tackled with nano-sizing [132; 133], confinement in a carbonaceous matrix [134; 135; 136; 137; 138; 139], and doping with aliovalent elements [140]. Tables S3 and S4 respectively report the preparation conditions of some ENMs for SIB anodes and the main parameters describing their performance. As in the case of LIB anodes, PAN [130; 135; 136; 134; 137; 138; 139] and polyvinyl PVP [134; 137; 138; 139] are the polymers most frequently selected to prepare the spinnable solution, whereas polymethyl methacrylate (PMMA) generally acts as a sacrificial pore-forming agent [136; 141].