# Electrospinning-Based Carbon Nanofibers for Energy and Sensor Applications
## 4 Application of Carbon Nanofibers
### Energy Storage Applications
#### 4.1.2 Li-Ion Battery Anode  
Lithium-ion battery anode materials are one of the most popular applications of carbon-based materials. [PERSON] et al. studied an anode material made from PAN fibers decorated with MnO\({}_{2}\) (Figure 13a) [96]. The MnO\({}_{2}\) nanosheets were prepared using the wet chemical method with NaH\({}_{2}\)PO\({}_{2}\), PVP, and KMnO\({}_{4}\). After the electrospinning of the PAN precursor, a solution containing MnO\({}_{2}\) was sprayed on top of the fibers. Then, the materials underwent the carbonization process, in which the MnO\({}_{2}\) nanosheets were tightly deposited on top of the CNFs. Though the carbon anode can operate as both lithium-ion and sodium-ion storage, the application in lithium-ion batteries seems to be more promising with a much higher capacity.  
Figure 12: (**a**) Hollow, porous CNFs for supercapacitors from dual nozzles, with PMMA for the inner nozzle and PAN/PVP for the outer nozzle [90]. (**b**) Porous CNFs decorated with MnO\({}_{2}\) from the mixed precursor of PAN, CD surfactant graphene, and graphene for the supercapacitor [92]. (**c**) Multilayer CNFs, rGO, and PPy for the application of the supercapacitor [93]. (**d**) Supercapacitor from couple electrode Sn/SnO\({}_{2}\)/GQD/CNFs versus GQD/CNFs [95].  
Recently, MXene with its two-dimensional structure has become the ideal sheet material for electrochemical device applications. [PERSON] et al. mixed MXene with the PAN solution for the outer layer of a dual nozzle to produce hollow CNFs (Figure 13b) [97]. By using PMMA for the inner nozzle, the authors can create a hollow structure since PMMA can decompose and evaporate very well at high-temperature carbonization. The MXene in the hollow CNF structure can yield a high capacity, four times higher than the MXene paste electrode when applied to a lithium-ion battery.  
Though many studies based on CNFs decorated with metal or metal oxide have been conducted, they are usually embedded in the outer wall of the CNFs. This can limit the initial coulombic efficiency as the particles were exposed to the outside. [PERSON] et al. introduced hollow CNFs with Si nanoparticles inside the fibers (Figure 13c) [98]. Although the inner nozzle was fed with a solution containing PVP and Si nanoparticles, the outer nozzle was fed only with the PAN solution. Carbonization resulted in the hollow fiber structure that was filled with Si nanoparticles. Though the initial coulombic efficiency was improved, the fibers can break and become unstable when overloaded with particles.  
In addition, the capacity of the battery can be improved by modified particles. Urchin-like vanadium-based oxide (V\({}_{2}\)O\({}_{3}\)) particles were synthesized by [PERSON] et al. using V\({}_{2}\)O\({}_{5}\) as a material (Figure 13d) [99]. The particles were mixed with a PAN solution as a precursor for electrospinning. Although the urchin-like particles are large in size compared with the diameter of the fibers, the collected fibers were very uniform. Multiple valence states of vanadium oxide allow the reversible lithium-ion intercalation/deintercalation. As the particles were electrospun from the same solution with PAN, the material showed good stability due to the interaction between PAN and vanadium.