# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 4 Fiber-Based Energy-Storage Systems
### Fiber-Based Battery
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types** & **Main Features** & **Key Materials** & **Ref.** \\ \hline \multicolumn{4}{c}{Fiber-based lithium-ion batteries} \\ \hline MWCNT based battery & Twisted structure & MWCNT fiber, Li wire, MnO\({}_{2}\) & [139] \\ \hline Silicon-based battery & High energy density & twisted MWCNT/Si fiber & [48] \\ \hline Aqueous battery & Safety, environmentally friendly & polyimide/CNT fiber, LiMn\({}_{2}\)O\({}_{4}\)/CNT fiber, lithium sulfate aqueous solution & [140] \\ \hline All-solid-state battery & Safety & LiFePO\({}_{4}\), Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\), polyethylene oxide with a lithium LiTFSI & [141] \\ \hline \multicolumn{4}{c}{Fiber-based non-lithium-ion batteries} \\ \hline Zn-air battery & High power/energy density, safety, eco-friendly & Zn, gel polymer electrolyte & [142] \\ \hline Al-air battery & Abundance, high volumetric capacity, three-electron-redox properties & Al, CNT/Ag-nanoparticle hybrid sheets, PVA-based hydrogel electrolyte & [143] \\ \hline Ni-Fe battery & Safety, high ionic conductivity, low cost. & CoP@Ni(OH)\({}_{2}\) NWAs/CNT, S-\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) & [144] \\ \hline Sodium battery & Abundance, low cost, environmental sustainability & Sodium foil, NiS\({}_{2}\)\(\subset\)PCF & [145] \\ \hline \multicolumn{4}{c}{Fiber-based supercapacitors} \\ \hline Carbon-based supercapacitor & Electrochemical double-layer & GF@3D-G(Graphene based electrode), capacitor & H\({}_{2}\)SO\({}_{4}\)-PVA & [146] \\ \hline Metal oxide based supercapacitor & Pseudocapacitor & Elastic fiber, PVA-based hydrogel electrolyte, MnO\({}_{2}\) & [49] \\ \hline Thermally drawn supercapacitor & High productivity, the longest functional supercapacitor & Thermal drawable materials(PVDF, COC, CPE), CB, activated carbon & [53] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main features of fiber-based batteries and supercapacitors. [PERSON] et al. focused on aqueous fiber-based lithium-ion batteries (AFBLIBs) to mitigate safety and environmental issues (Figure 6c) [140]. They inserted paired two electrodes with a separator into a heat-shrinkable tube and injected an aqueous lithium sulfate (Li\({}_{2}\)SO\({}_{4}\)) solution (0.5 M). Conventional LIBs generally use flammable organic electrolytes, and therefore, have safety problems. However, AFBLIBs are non-flammable and have no risk of explosion. Furthermore, AFBLIBs are not toxic and environmentally friendly. Due to these advantages, AFBLIBs are being actively developed by many research groups. Besides AFBLIBs, another strategy to secure safety can be fiber-based all-solid-state batteries(FBASSBs). The electrodes and electrolytes of FBASSBs are all-solid-state instead of liquid-state or polymer gel-state as in the conventional FBLIBs. [PERSON] et al. reported an FBASSB with the coaxial structure using lithium iron phosphate(LiFePO\({}_{4}\)), lithium titanate(Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\)), and polyethylene oxide with a lithium trifluoromethanesulfonate as a cathode, anode, and electrolyte, respectively, as shown in Figure 6d [141]. Although LIBs are one of the most active research areas, some challenges, such as the high cost of LIBs, the combustibility of organic electrolytes, and the lack of sustainability of lithium resources, remain to be solved. To address these issues fundamentally, non-lithium-ion batteries based on alternative materials, including metal-air battery (MAB), lithium-sulfur battery (Li-S battery), and sodium (Na)-based battery, have been investigated. One of the promising types of non-lithium-ion batteries is a MAB [147]. By repeating oxygen oxidation and reduction reactions, it can store about 10 times more energy compared to conventional LIBs. Moreover, since MABs use oxygen (O\({}_{2}\)) or carbon dioxide (CO\({}_{2}\)) in the air as the cathode, it can be lighter than LIBs that use conventional electrodes.