# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 4 Fiber-Based Energy-Storage Systems
### Fiber-Based Battery
Over the past 10 years, a wide variety of fiber-based metal-air batteries (FBMABs) have been reported, including Zn [142; 148], Al [143], Li [149; 150; 151; 152; 153; 154], etc. Zinc has the advantages of high power/energy density, excellent safety, and eco-friendly characteristics. [PERSON] et al. proposed a fiber-based Zn-air battery by coating gelatin-based gel polymer electrolyte on the Zn electrode using the cellophane template to pour the gel solution (Figure 7a) [142]. On  
Figure 6: Schematic illustration of representative fiber-based lithium-ion batteries. (**a**) MWCNT-based battery fiber. Reproduced with permission from [139], Copyright (2013), Wiley. (**b**) Si-based battery fiber. Reproduced with permission from [48], Copyright (2014), Wiley. (**c**) Aqueous lithium-ion battery fiber. Reproduced with permission from [140], Copyright (2016), Royal Society of Chemistry. (**d**) All-solid-state battery fiber. Reproduced with permission from [141], Copyright (2019), American Chemical Society. the other hand, aluminum (Al) is one of the most abundant and sustainable metals in nature and has a high volumetric capacity, due to its low mass density and three-electron-redox properties [155]. [PERSON] et al. proposed a fiber-based Al-air battery by coating a gel electrolyte and wrapping cross-stacked CNT/silver-nanoparticle hybrid sheets (Figure 7b) [143]. However, current Al-air batteries have the problem of the passivation of Al, as well as the lower energy density. Other than LIBs and MABs, many other types of batteries have been reported to fabricate ideal batteries for wearable textiles. One of them is a Ni-metal battery. Ni-metal batteries, including Ni-Zn batteries [156], have received great attention, due to their energy density similar to LIBs, ionic conductivity, environmentally friendly characteristics, and low cost. Fiber-based Ni-Fe battery was reported recently (Figure 7c) [144]. The researchers in this study fabricated a Ni-based positive electrode and a Fe-based negative electrode by using a facile hydrothermal method, while twisting these two electrodes together with a post-annealing treatment. Unlike Ni-Zn batteries, Ni-Fe based batteries used Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\) as the anode materials. These oxides are abundant, inexpensive, non-toxic, and highly resistant to corrosion. Sodium is chemically similar to lithium as it belongs to the same group in the periodic table, hence Na-based battery was also simultaneously developed with LIB in the late 1980s. Many research groups reported flexible fiber-based sodium batteries as sodium has the advantages of abundance, low cost, and environmental  
Figure 7: Schematic illustration of representative fiber-based non-lithium-ion batteries. (**a**) Zn-based metal-air battery fiber. Reproduced with permission from [142], Copyright (2015), Wiley. (**b**) Al-based metal-air battery fiber. Reproduced with permission from [143], Copyright (2016), Wiley. (**c**) Ni-Fe based Ni-metal battery fiber. Reproduced with permission from [144], Copyright (2019), Royal Society of Chemistry. (**d**) Sodium-based metal-air battery fiber. Reproduced with permission from [145], Copyright (2018), Wiley. sustainability. [PERSON] et al. proposed a fiber-based sodium battery by inserting the yolk-shell NiS2 nanoparticles embedded in porous carbon fibers (NiS\({}_{2}\)\(\subset\)PCF), sodium foil, and the separator into a heat-shrinkable polyethylene tube with the sealing process using a heat gun and a hot melt adhesive (Figure 7d) [145].