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Page 8 of 13 Xiao et al. Energy Mater 2023;3:300007 https://dx.doi.org/10.20517/energymater.2022.84

Figure 5. (A) Redox couples commonly adapted in flow batteries and corresponding standard reduction potentials; (B) schematic
illustration of the charging and discharging mechanisms in alkaline Zn-I flow batteries; (C) specific capacity and energy density at
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different current densities of alkaline Zn-I flow batteries [85] . Reproduced with permission from Ref. [85] . Copyright 2018 Royal Society of
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Chemistry.
which broadened the potential window of the cell by 0.497 V and obtained an improved energy density of
330.5 W h L -1[85] . For battery membranes, apart from the commercial cation-selective Nafion membranes,
polyolefin-based membranes have also been used in the Zn-I flow batteries [91,92] .
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Flexible batteries
The adoption of flexible Zn-based batteries for wearable devices has gained increased attention due to their
environmental friendliness and cost-effectiveness . Flexible Zn-based batteries usually consist of flexible
[18]
electrode materials and polymer electrolytes. Flexible cathode materials are usually integrated active
electrode materials on flexible substrates, such as carbon cloth, nickel foam, stainless-steel mesh, etc. [93-95] . As
shown in Figure 6, a typical fabrication process of flexible Zn-air battery cathodes was presented
[Figure 6A] . A chemical vapor deposition was employed with ZnCo-Hexamine (HMT) as the gas source
[96]
to obtain cobalt-decorated carbon arrays with a nickel foam substrate as the flexible cathode. Gel
electrolytes, usually synthesized by polymer membranes absorbing aqueous electrolytes, have been explored
and widely used in flexible Zn-air batteries. In general, the performance of gelled electrolytes is mainly
determined by the properties of the chosen gelling agent. Polyvinyl alcohol (PVA) is generally considered to
be a perfect gelling agent due to its abundant hydroxyl functional groups and good water solubility .
[96]
However, its relatively low ion conductivity severely limits its commercial application. To address this
challenge, a modified cellulose membrane was prepared as a promising alternative . Fu et al. used a
[97]
functionalized cellulose membrane with a rich hydrogen-bonded network structure by tetraammonium salt
treatment [Figure 6B], demonstrating excellent electrochemical performance in a flexible Zn-air battery .
[98]
Although the Zn-air battery has an ultra-high specific energy density, 6070 Wh L , it is not a suitable
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flexible battery system because its semi-open structure that ensures easy access of atmospheric oxygen to the
battery system would lead to continuous water loss in the electrolyte and then causes the battery to fail .
[11]
Therefore, seeking a new cathode material that also has the characteristics of high specific energy density,
but rarely suffers from the adverse effects of half-cells, may be the future direction. For example, Wang et al.
developed a Zn-Co O flexible battery with an energy density of 2807 Wh L -1[99] . In addition, Zn-ion batteries
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have also been adopted as flexible batteries due to their better rechargeability. For example, a Zn-MnO
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battery, with α-MnO nanofiber as the positive electrode, Zn sheets as the negative electrode and
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ZnSO /MnSO solution as the electrolyte, was successfully demonstrated. This battery exhibited excellent
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