Fig 1

Schematic illustration for the applications of flexible metal-air batteries 3. Adapted with permission from Ref. 3. Copyright 2020, Elsevier B.V."

Fig 2

Schematic configuration of typical ZAB 4. Adapted with the permission from Ref. 4. Copyright 2020 American Chemical Society."

Fig 3

Preparation and characterizations of gel polymer electrolyte 7. (a) Photographs of synthetic process of hydrogel polymer electrolyte solution, scale bar: 1 cm; (b) photographs of free-standing hydrogel polymer electrolyte after crosslinking with initial state, bending and stretching, scale bar: 1 cm; (c) photographs of the hydrogel polymer electrolyte in a stretching process up to 300%, scale bar: 0.5 cm; (d) alternating current (AC) impedance spectra of the hydrogel polymer electrolyte at the frequency range from1000 kHz to 0.01 Hz. Adapted with permission from Ref. 7. Copyright 2020, Elsevier B.V."

Fig 4

Preparation of PVA-SiO2 and measurement of circulation ability of flexible zinc-air battery 12. (a) Schematic diagram of the flexible ZAB and preparation process of the porous PVA-based nanocomposite GPE along with its inner structure; (b) GCD testing with a duration of 20 min per cycle at 3 mA∙cm?2 of the ZABs based on various PVA-based GPEs. Adapted with permission from Ref. 12. Copyright 2020, Elsevier B.V."

Fig 5

Synthesis and mechanical properties of high and low temperature resistant hydrogels 15. (a) Preparation of the temperature-tolerant hydrogel; (b) schematic illustration of the hydrogen bonding interactions taking place between Gly and the PAM/PAA polymers, and water molecules; (c) bending tests at different bending angles and (d) charge and discharge polarization curves at ?20 ℃; (e, f) mechanical tests on the temperature-tolerant aqueous Zn-air battery under various deformation conditions. Adapted with the permission from Ref. 15. Copyright 2020, American Chemical Society."

Fig 6

Tensile ability test of sandwich and cable type zinc-air batteries16. (a) Galvanostatic discharge–charge cycling curves at a current density of 5 mA∙cm?2; (b) Cycling test of fiber-shaped zinc-air battery for rechargeability at a current density of 5 mA∙cm?2 under 500% strain. The inset is the photographs of the fiber-shaped zinc–air battery at a fully released state and 500% strain. Adapted with the permission from Ref. 16. Copyright 1999–2021, John Wiley & Sons, Inc."

Fig 7

Cross-linking principle of graphene and cellulose, microstructure and comparison of charge-discharge polarization curves and cycles between QAFCGO and commercial A201 20. (a) Schematic diagram of the overall preparation procedure for the QAFCGO membrane; (b) a SEM image (cross section) of the QAFCGO membrane; (c) a SEM image (surface view) of the QAFCGO membrane with an inset photograph of QAFCGO membrane showing flat and uniform GO surface; (d) a SEM image of the cellulose dense interwined network structure with an inset TEM image of cellulose fibers; (e) a photograph of the QAFCGO membrane showing flexibility; (f) charge and discharge polarization curves of the batteries using the QAFCGO and A201 membranes; (g) galvanostatic charge and discharge cycling of the batteries using the QAFCGO and A201 membranes at 1 mA∙cm?2 with a 20 min per cycle period. Adapted with the permission from Ref. 20. Copyright 1999–2021, John Wiley & Sons, Inc."

Table 1

Typical GPEs used in flexible ZABs."

Fig 8

Photographs of commercial copper foams and zinc coverings and mechanical properties testing of assembled flexible zinc-air batteries 27. (a) Photographs of the commercial Cu foams and the Zn-coated foams with 130, 40, and 25 ppi, respectively; (b) galvanostatic charge/discharge cycling tests of the ZABs with each cycle being 200 s tested under 0°, 45° and 90° bending conditions; (c) photographs of five red LEDs in parallel powered by the three as-prepared ZABs in series under bending state, respectively. Adapted with the permission from Ref. 27. Copyright 1999–2021, John Wiley & Sons, Inc."

Fig 9

Manufacturing process photographs and mechanical test of the extrudable and rechargeable all solid state ZAB 28. (a) Schematic illustration of the fabrication processes for the squeezable and rechargeable all-solid-state ZAB; (b) galvanostatic discharging/charging cycling curves. Adapted with the permission from Ref. 28. Copyright 2021, Elsevier B.V."

Fig 10

Zinc spring preparation process for ZABs and mechanical properties testing 7. (a)Schematic illustration to the fabrication of the fiber-shaped Zn-air battery; (b) photographs of a fiber-shaped Zn–air battery bent to increase angles. scale bar: 1 cm; (c) photographs of a fiber-shaped Zn-air battery before and after stretching by 10%, scale bar: 1 cm. Adapted with the permission from Ref. 7. Copyright 2021, Elsevier B.V."

Fig 11

Preparation process of NC-Co3O4 and SEM characterization 32. (a) Schematic illustration of the fabrication process for hierarchical NC-Co3O4 arrays on flexible carbon cloth; (b) and (c) SEM and (d) digital images of NC-Co3O4 nanoarrays on carbon cloth. Adapted with the permission from Ref. 32. Copyright 1999–2021, John Wiley & Sons, Inc."

Fig 12

Preparation principle and electrochemical properties and mechanical properties testing 34. (a) Schematic diagram of the flexible zinc-air battery. Pt/C+IrO2 and (Ni, Co)3O4@Ni-foam; (b) the polarization curve and corresponding power density plot; (c) the long-time galvanostatic discharge curves under different bending conditions at 5 mA∙cm?2; (d) the C–D cycling curves. Adapted with the permission from Ref. 34. Copyright 2021, Elsevier B. V."

Fig 13

Sandwich structure of a zinc-air battery 36. Adapted with the permission from Ref. 36. Copyright 2020, the Royal Society of Chemistry."

Fig 14

Manufacturing process, cross section and mechanical properties testing of all solid state cable zinc-air battery 40. (a) Schematic diagram of the all-solid-state cable-type flexible Zn–air battery assembly and (b) coating process of gelatin-based gel polymer electrolyte (GGPE) and 0.1 mol∙L?1 KOH on the surrounding spiral zinc anode. Adapted with the permission from Ref. 40. Copyright 1999–2021, John Wiley & Sons, Inc."

Fig 15

NiCo2O4@N-OCNT Preparation method, electrochemical and mechanical properties test 42. (a) Scheme showing the fabrication approach of NiCo2O4@N-OCNT film; (b) scalable approaches of NiCo2O4@N-OCNT air electrode (left) and the demonstration of flexible Zn-air battery (right). NiCo2O4@N-OCNT films all-in-one air electrode enabled high-performance flexible cable shaped Zn-air battery; (c) optical images of the cable-like flexible Zn-air battery using the NiCo2O4/N-OCNT composite film as the air electrode and zinc wire as the anode; (d) power density plots and charging-discharging polarization curves of Zn-air battery; (e) two 20-cm-long Zn-air batteries with straight, folded at 90°, wavy bended, and spiral shapes were connected in-series, illumining three parallel red LEDs. Adapted with the permission from Ref. 42. Copyright 1999–2021, John Wiley & Sons, Inc."

Fig 16

The challenges in zinc-air batteries."