# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 3 Rechargeable Lithium-Ion and Sodium-Ion Batteries
### Lithium-Ion Batteries  
As sketched in Figure 4, a LIB cell consists of anode (e.g., graphite), cathode (e.g., a Li-containing transition metal oxide, LiMO\({}_{2}\), with M standing for Mn, Fe, Co, or Ni), and a separator permeable to the ions and the electrolyte (e.g., LiPF\({}_{6}\) in an organic solvent). It is able to supply energy thanks to the spontaneous oxidation-reduction reactions occurring at the electrodes. During the charging process (de-lithiation), energy is supplied by an external source; Li\({}^{+}\) ions are extracted from the cathodic material, diffused in the electrolyte, and enter the anodic material (according to the  
Figure 3: Sketch of the main electrospun fiber typologies produced (**a**,**b**) without and with (**c**) single-step or (**d**–**f**) multi-step post spinning heat treatment. (**a**) Polymer fibers; (**b**) polymer fibers with conductive carbonaceous additives; (**c**) oxide fibers; (**d**) porous carbon fibers; (**e**) composite carbon fibers; (**f**) composite carbon fibers with conductive carbonaceous additives. reaction \(\mathrm{C_{x}+LiMO_{2}\to Li_{(1-y)}MO_{2}+C_{x}Li_{y}}\), in the case of a traditional LIB [62; 63]), while electrons are transferred to the positive electrode through the external circuit. In the discharge process (lithiation), the opposite process takes place (i.e., \(\mathrm{Li_{(1-y)}MO_{2}+C_{x}Li_{y}\to C_{x}+LiMO_{2}}\), in the considered example): \(\mathrm{Li^{+}}\) ions, extracted from the anodic material, are re-inserted into the lithiated cathodic material, and the cell provides energy. The gravimetric capacities of the commonly used electrodes (372 mAh/g for graphite and 274-286 mAh/g for LiMO\({}_{2}\)[64]) are inadequate to meet the ever-increasing requirements for ESS devices. The search for alternative highly performing, durable, low-cost, and eco-friendly electrode materials is continuously progressing. As for the anodes, the focus is mainly on transition metal oxides \(\mathrm{MO_{x}}\)[65; 66; 67; 68], which store \(\mathrm{Li^{+}}\) ions through the conversion reaction \(\mathrm{MO_{x}+2x\ Li^{+}+2x\ e^{-}\leftrightarrow M^{0}+x\ Li_{2}O}\), and IV group elements \(\mathrm{M^{0}}\)[69; 70; 71; 72; 73] that are able to store larger amount of \(\mathrm{Li^{+}}\) ions through the alloying reaction \(\mathrm{M^{0}+y\ Li^{+}+y\ e^{-}\leftrightarrow Li_{y}M}\)[74], whereas transition metal oxides with olivine structure seem to be promising materials for cathodes [75]. However, some problems limiting the performance and life of the new electrode materials and thus hampering their commercialization still need to be solved. Various strategies have been proposed to enhance electronic conductivity and to reduce pulverization of the active material, to limit the use of inactive battery components (binder and current collectors), as well as to meet the additional requirements imposed by the development of flexible electronics. It is widely recognized that carbon incorporation and nano-sizing improve the electrochemical and mechanical performance of the electrode materials [76; 72; 77; 78; 79], providing them with enhanced electronic conductivity and increased resistance against the large volume changes, from which they suffer during the lithiation/de-lithiation process. It has been further shown that the use of binder-free self-supporting flexible carbon-based membranes as electrode materials [56; 57; 58; 66; 80; 81; 82; 83; 84] translates into a beneficial reduction of the inactive weights with a consequent gain in the effective gravimetric and volumetric capacities of the batteries. These benefits are of crucial importance in manifold applications including transportation, aerospace, and portable and wearable electronics. Electrospinning is a viable route to simultaneously implement all these ameliorative strategies.