# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 3 Rechargeable Lithium-Ion and Sodium-Ion Batteries
### Sodium-Ion Batteries  
The use of sodium would remarkably reduce storage costs since its resources are more abundant and better distributed on Earth. A study based on a prospective life-cycle-assessment has demonstrated that sodium-ion batteries (SIBs), showing, per kWh of storage capacity, impacts at the lower end of the range reported for current LIBs [116], represent a more sustainable, cost-effective, and eco-friendly alternative to LIBs in the energy- and climate-related challenges of the 21 st century. Of course, many efforts are still required to meet the increasing demands of the market on zero-emission electric vehicles and on-grid energy storage and to fill the gap between LIB and SIB technology. The development of SIB technology requires the identification/design and preparation of new electrode materials with reversible Na\({}^{+}\) intercalation/conversion/alligation reaction to reduce the technological gap with LIBs. In principle, in spite of the higher weight of sodium compared to lithium (23 g/mol against 6.9 g/mol), its higher standard electrode potential (\(-\)2.71 V vs. SHE against \(-\)3.02 V vs. SHE for lithium) and the larger size of its ions (1.02 A against 0.76 A), the transition from LIBs to SIBs should not cause any consequence in terms of energy density, since the capacity primarily depends on the characteristics of the nanostructures acting as electrodes [105]. Various materials have been evaluated as cathode materials for SIBs obtaining interesting results, in terms of stability and charge retention [105; 117; 118; 119]. They include layered sodiated transition metal oxides (Na\({}_{1-x}\)MO\({}_{2}\), where M stands for Ti, V, Cr, Mn, Fe, Co and Ni, Ru) with O3- and P2-type structure (for x close to 0 or in the range 0.3-0.7, respectively) and their derivatives, layered transition metal oxides and fluorides, transition metal polyanion materials, phosphates, fluorophosphates, pyrophosphates, mixed phosphates, carbonophosphates, sulfates, fluorosulfates, cyanides, and organic  
Figure 9: SEM images of self-standing paper-like electrospun fibrous membranes prepared (**a**) without and (**b**) with post-spinning cold pressing. An open and loose structure is obtained in the former case, whereas in the latter, the membrane has a more compact and tight structure and is rigid. Bottom: elemental mapping of the Ge, O, C and N fiber components (readapted from ref. [57] with permission. Copyright 2017, Elsevier). compounds, with theoretical (measured) specific capacities ranging between 79 and 710 mAh/g (30 and 300 mAh/g) [105]. Table S1 reports some examples of ENMs for SIB cathodes and their preparation conditions; the main parameters describing their performance are reported in Table S2. Polyvinyl pyrrolone (PVP) [120; 121; 122; 123] and polyvinylacetate (PVAc) [124; 125] are frequently used as polymers for the preparation of the spinnable solution. The fibers of sodiated transition metal oxides, obtained after thermal removal of the organic component of the as-spun fibers, generally consist of interconnected building blocks, whose shape may evolve with changing the heating temperature (Figure 10) [120; 122; 124]. As in the case of LIBs, the preparation of the working electrode by utilizing these kinds of materials (in the form of powders) requires the use of binder and carbonaceous additive (Table S2). Besides, a current collector is necessary to complete the SIB assembly. On the contrary, paper-like flexible membranes, consisting of carbon fibers encapsulating very small-sized oxide particles (Figure 11), can be directly used as self-supporting working electrodes--they do not need any binder, conductive additive, and collector [123; 125; 126; 127], which improves the overall energy density of batteries. These cathode materials generally offer a variety of advantages and exhibit relevant electrochemical performance for Na-storage. For instance, the self-standing membranes formed by a non-woven network of composite NaVPO\({}_{4}\)F/C fibers synthesized by [PERSON] et al.