Flexible Zinc‐Ion Hybrid Fiber Capacitors with Ultrahigh Energy Density and Long Cycling Life for Wearable Electronics

Abstract

Emerging wearable electronics require flexible energy storage devices with high volumetric energy and power densities. Fiber‐shaped capacitors (FCs) offer high power densities and excellent flexibility but low energy densities. Zn‐ion capacitors have high energy density and other advantages, such as low cost, nontoxicity, reversible Faradaic reaction, and broad operating voltage windows. However, Zn‐ion capacitors have not been applied in wearable electronics due to the use of liquid electrolytes. Here, the first quasisolid‐state Zn‐ion hybrid FC (ZnFC) based on three rationally designed components is demonstrated. First, hydrothermally assembled high surface area and conductive reduced graphene oxide/carbon nanotube composite fibers serve as capacitor‐type positive electrodes. Second, graphite fibers coated with a uniform Zn layer work as battery‐type negative electrodes. Third, a new neutral ZnSO4‐filled polyacrylic acid hydrogel act as the quasisolid‐state electrolyte, which offers high ionic conductivity and excellent stretchability. The assembled ZnFC delivers a high energy density of 48.5 mWh cm−3 at a power density of 179.9 mW cm−3. Further, Zn dendrite formation that commonly happens under high current density is efficiently suppressed on the fiber electrode, leading to superior cycling stability. Multiple ZnFCs are integrated as flexible energy storage units to power wearable devices under different deformation conditions.

Rechargeable Aqueous Zinc‐Ion Batteries with Mild Electrolytes: A Comprehensive Review

“…Comparing with the traditional aqueous electrolytes, the attractive features of the gel electrolytes lie in the following aspects: 1) the gel electrolytes can avoid electrolyte leakage, increasing the user friendliness; [225] 2) the gel electrolytes can maintain the structural integrity and electrochemical performances of the device, even under the application scenarios with dynamic external forces; [230] 3) the gel electrolytes simultaneously play the role of separators, remarkably simplifying the configurations of flexible ZIBs; [231] 4) the gel electrolytes can alleviate the dissolution of cathode active materials while impeding the dendrite growth and corrosion reaction on anode side. [232] Currently, various polymers, such as gelatin, [233] sodium alginate (SA), [234,235] xanthan gum (XG), [236] polyvinyl alcohol (PVA), [87] polyacrylamide (PAM), [237] poly(acrylonitrile) (PAN), [238] poly-(ethylene glycol) diacrylate (PEGDA) [239] and polyacrylic acid [144] have been studied to prepare gel electrolytes for ZIBs.…”

“…[229,250,251] The salt-tolerant polyacrylic acid (PAA) hydrogel can work in both alkaline and neutral electrolytes, and is therefore adoptable for both ZIBs and Zn-Air batteries. [144,252] For example, Pei et al [229] recently developed a solid state Zn-Air battery that can work at À 20°C, by rationally designing the anti-freezing polyacrylic acid (PAA) electrolyte and cold-tolerant noble-metal free catalyst. Moreover, Pei and Chen et al [144] further developed a flexible zinc-ion hybrid fiber capacitor based on a ZnSO 4 -filled PAA electrolyte, fibrous rGO/ CNT positive electrode and Zn/graphite negative electrode (Figure 18).…”

“…[144,252] For example, Pei et al [229] recently developed a solid state Zn-Air battery that can work at À 20°C, by rationally designing the anti-freezing polyacrylic acid (PAA) electrolyte and cold-tolerant noble-metal free catalyst. Moreover, Pei and Chen et al [144] further developed a flexible zinc-ion hybrid fiber capacitor based on a ZnSO 4 -filled PAA electrolyte, fibrous rGO/ CNT positive electrode and Zn/graphite negative electrode (Figure 18). The resulting high ionic conductive hydrogel enables efficient Zn 2 + adsorption/desorption on positive electrodes and the fast reversible Zn plating/striping on negative electrodes.…”

“…This Zn 2 + /Al + dual-cation device is attractive for large-scale stationary energy storage, since all its components are abundant, low-cost, and sustainable. Replacement of the expanded graphite nano-sheets with electric double layer electrode materials (e. g. activated carbons, [141][142][143] carbon nanotubes and granphenes [144] ) gives a zinc-ion hybrid supercapacitors. In this kind of devices, dual-cation electrolytes are unnecessary, thanks to the electric double-layer charge storage mechanism of the activated carbons.…”

“…Schematic illustration of the synthesis of rGO/CNT and Zn-coated graphite fibers for assembling a Zn-ion hybrid fiber capacitor and its energy storage mechanism. Reproduced with permission from Ref [144]…”