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Figure 6. (A) Schematic illustration of ordered arrays on nickel foam as self-supported for flexible Zn-air battery [96] Reproduced with
permission from Ref. [96] . Copyright 2019 John Wiley & Sons, Inc.; (B) Schematic presentation of nanocellulose membrane
functionalization and demonstration of flexible Zn-air battery [98] . Reproduced with permission from Ref. [98] . Copyright 2015 Royal
Society of Chemistry.

cycling stability, maintaining a capacity retention rate of 92% after 5000 cycles at a current density of 5C .
[100]
This work opens a direction for the development of long-lived flexible Zn-MnO .
2
PERSPECTIVES
This review describes the overall landscape of aqueous Zn-based batteries, including Zn-ion batteries, Zn-
air batteries, redox flow batteries, and flexible batteries. Within each classification, the basic working
principle and recent research progress are summarized. Currently, there are still many obstacles that need to
be removed on the road to the commercialization of Zn-based batteries. Therefore, two main perspectives
related to battery material design and intrinsic mechanism exploration are given as follows:

(1) Material design and optimization to improve electrochemical performance is still a challenge for various
types of Zn-based battery configurations. For Zn-ion batteries, the search for materials that can withstand
sustained multi-electron transfer while maintaining structural stability is strongly needed for both the
intercalation and conversion cathodes. Electrocatalysts, with sufficient active sites, high activity and
sufficient durability, are required in Zn-air batteries. The development of highly active cathode materials
and cost-effective, stable and ion-conducting membranes is crucial for redox flow batteries. For flexible
batteries, the mechanical flexibility and structural stability of battery materials are prerequisites for flexible
devices, while the hydroxyl conductivity, water-holding ability, and alkali tolerance of gel electrolytes
determine the performance of Zn-air batteries.

(2) Apart from material design, the electrochemical behavior of electrode materials is still not well
understood. Deeper mechanistic studies are desperately needed to further improve the electrochemical
performance of Zn-based batteries. Taking Zn-air batteries as an example, although metal-based materials
are widely used as bifunctional catalysts in Zn-air batteries, it was only recently revealed that their derived
metal hydroxides are the real active sites for OER. A metal hydroxide layer was found on the
Ba Sr Co Fe O -δ surface during the OER reaction. This phenomenon was also found when spinel
0.8
0.2
3
0.5
0.5
CoFe Al O was used as a catalyst. Therefore, further mechanistic understanding through in situ probing
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4
methods is strongly demanded to aid the search for better catalyst materials.