Spinel oxides are generally expressed in a formula of AB
2O
4 (A: Mg, Ni, Fe, Co, Mn,
etc.; B: Al, Fe, Co, Cr, Mn, Ni,
etc.), in which oxygen anions are in the form of cubic close packing, 1/8 of the tetrahedral sites are occupied by A atom, whereas 1/2 of the octahedral sites are occupied by B atom (
Fig. 6a) [
107]. Spinel oxides with mixed valences exhibit electrical and semiconducting properties which allow them to be used directly as electrocatalysts, and the transfer of electrons can occur at relatively low activation energy [
107]. Spinel oxides as bifunctional oxygen electrocatalysts for Zn-air batteries have demonstrated excellent discharging/charging performance and stability [
56,
113]. For wearable Zn-air batteries, flexible air electrodes can be designed
via changing the construction and composition of spinel oxides. Di and Li [
49] utilized lightweight conductive nitrogen-doped carbon nanotubes as substrate for loading cobalt oxides nanoparticles (Co
3O
4/N-CNT), in order to provide more channel for gas diffusion and improve the electrocatalytic activity for ORR and OER. The solid-state Zn-air battery assembled with Co
3O
4/N-CNT aerogel films exhibited a long-term stable open-circuit potential, as revealed by the negligible change for charge transfer resistance upon a 90° bending. However, they did not discuss the battery performance regarding to the energy density and power density. Zhong and co-workers [
20] first developed an integrated electronic device, consisting of a flexible Zn-air battery integrated with a flexible display device. The assembled flexible Zn-air battery composed of Cu deposited Zn anode, spinel Co
3O
4/CC air electrode, and hydrogel electrolyte. The preparation procedure of the Co
3O
4/CC flexible air electrode is shown in
Fig. 6b. Co(OH)
2 was grown
in situ on the surface of carbon fibers in carbon cloth by electrodeposition, then Co
3O
4/CC was formed upon annealing at 400 ℃ in air. In this work, Zn-air batteries did not exhibit obviously changed galvanostatic charge/discharge behaviours under various bending radii of 0, 13, 28 and 51 mm (
Figs. 6c and
d), also no obvious change in the discharge performance before and after being repeatedly bent at a bending radius of 28 mm for 300 cycles either (
Fig. 6e). Such high mechanical stability and excellent rechargeable performance of Co
3O
4/CC air electrode could be ascribed to the strong adhesion of Co
3O
4 on conductive CC support. Ji
et al. [
32] introduced rich oxygen vacancies into Co
3O
4 particles by regulating the oxidation process under relatively low-temperature on the basis of Kirkendall effect. They also introduced a porous N-doped carbon structure to Co
3O
4 and obtained Co
3O
4-x HoNPs@HPNCS-60, to improve the electrocatalytic activity and flexibility of Co
3O
4. With tailored oxygen vacancies as well as the unique structure design, this electro-catalyst exhibited a higher discharge potential (
ca. 1.2 V) as well as a lower charge potential (
ca. 1.73 V) than those of commercial Pt/C+RuO
2 catalysts (
ca. 1.18 V and 1.90 V, respectively). This battery offered a high capacity of 779.36 mAh/g
zn with a power density of 94.1 mW/cm
2. The assembled Zn-air batteries could light 8 yellow lights and exhibited a stable cycle performance.