# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 3 Rechargeable Lithium-Ion and Sodium-Ion Batteries
### Lithium-Ion Batteries
Therefore, it has been increasingly and extensively utilized for the production of all the battery components--not only anodes [12; 56; 57; 58; 72; 74; 81; 82; 84; 85; 86; 87; 88; 89; 90; 91] and cathodes [92; 93; 94; 95; 12], but also electrolytes [96; 97; 12; 98] and separators [12; 99; 100], which makes it feasible to manufacture secondary batteries by assembling components produced exclusively by electrospinning (Figure 5). Figure 4: Schematic illustration of a traditional lithium-ion battery (LIB) cell [62; 63]. \(\mathrm{Li^{+}}\) ions migrate through the electrolyte, and electrons flow through the external circuit. De-lithiation reaction (charge process): \(\mathrm{C_{x}+LiMO_{2}\to Li_{(1-y)}MO_{2}+C_{x}Li_{y}}\); lithiation reaction (discharge process): \(\mathrm{Li_{(1-y)}MO_{2}+C_{x}Li_{y}\to C_{x}+LiMO_{2}}\), with \(\mathrm{x=6\ and\ y\simeq 0.5}\). Actually, the increasing number of published papers focused on this subject witnesses the growing interest of the scientific community for the use of this technique in the manufacturing of LIB components. Figure 6 displays the numbers relative to a single database. Several exhaustive review papers are available in the literature that report the outstanding electrochemical properties that suitably engineered ENMs exhibit compared to the commercial references (see f.i. [101, 102, 14, 15, 18, 100, 12], just to cite a few of them). Indeed, the LIB-technology exploiting ENMs continuously progresses thanks to the incessant efforts made to enhance their performance and to render easier and more efficient their production process. For instance, [PERSON] et al. [80] have recently proposed to utilize polyimide (PI), in place of the commonly selected polyacrylonitrile (PAN), to fabricate flexible self-standing paper-like electrospun carbon membranes. According to [PERSON] et al., the oxidative stabilization, through which PAN is converted to an infusible stable ladder polymer [102], is a slow and not easily controllable step, whereas PI, containing aromatic heterocyclic rings with semi-ladder structure, can be easily obtained through a thermal or chemical imidization process from the polyamic acid (PAA) precursor [80]. Flexible self-standing paper-like electrospun nitrogen-doped carbon fibrous membranes produced via the multi-step process schematically illustrated in Figure 7a, using PI with biphenyl and pyrimidine rings (Figure 7b), are able to deliver a reversible specific capacity of 695 mAh/g at a rate of 0.1 A/g. This value is higher than that reported, at the same rate, by [PERSON] et al. [78] for composite anodes consisting of PAN-derived carbon fibers enriched with thermally reduced graphene oxide. Flexible anodes derived from heterocyclic PI further exhibit good stability, retaining 245 mAh/g after 300 cycles at a rate of 1.5 A/g [80]. Figure 5: Sketch of LIB consisting exclusively of electrospun components, inspired to those proposed in [93]. Figure 6: Time distribution of papers dealing with lithium- and sodium-ion batteries (LIBs and SIBs) based on ENMs published in Scopus-indexed scientific journals from 2007 to January 2019. Owing to the very high theoretical specific capacity of silicon (4200 mAh/g), carbon fibers with embedded Si nanoparticles (NPs) have received great attention as a possible solution to the rapid capacity fading caused by the pulverization of the bare silicon electrode due to its mechanical failure upon repeated Li insertion and extraction. In particular, [PERSON] et al. [82] have designed flexible non-woven membranes consisting of electrospun silicon/carbon@carbon fibers (Si/C@CNFs). The fibers, fabricated by the use of a co-axial electrospinningning configuration (Figure 8a), exhibit a core-shell structure (Figure 8b-e), where the external shell is constituted by PAN-derived carbon, and the internal core consists of well-dispersed Si NPs wrapped to PVP-derived carbon.