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Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89 Page 3 of 31

Figure 1. Schematic outline of the review. This work will elaborate on the structures and mechanisms of bulk electrolytes and
Zn/electrolyte interfaces, introduce their characterization techniques, and summarize recent advances in electrolyte modifications.

-
Wang et al. developed a salt-concentrated aqueous electrolyte [1 m Zn(TFSI) + 20 m LiTFSI, TFSI :
2
bis(trifluoromethanesulfonyl)imide; m: mol kg ], which greatly suppressed HER . Their results suggested
-1
[23]
that Zn is closely coordinated with anions, excluding highly reductive solvated water and rendering the
2+
water in electrolytes inert. Reducing the amount of solvated water has become a pivotal criterion for
designing high-performance electrolytes for Zn batteries ever since. The solvation structure of Zn and its
2+
associated reactions that occur on the Zn anode surface are illustrated in Figure 2A. When the solvated Zn
2+
migrates to the Zn/electrolyte interface during charging, electrons at the Zn electrode surface are transferred
to the Zn , causing the Zn to desolvate and transform into Zn atom as well as being deposited on the
2+
2+
anode. Meanwhile, the remaining electrons are captured by solvated water, and then HER takes place.
Free water
As is known, water consumption occurs not only during the Zn deposition process
[4]
-
-
(2H O + 2e → 2OH + H ↑) but also when the battery is at rest . Because Zn is an amphoteric metal, it can
2
2
react with proton (H ) (Zn + 2H O → Zn(OH) + H ↑). In this case, no Zn salt is added to water solutions,
+
2
2
2
and hence no solvated water exists. Such a result poses a challenge to the viewpoint on governing HER
activity with the amount of solvated water. In 2020, Xie et al. proposed a new interpretation: Confining the
activity of free water can inhibit HER . By using polyethylene glycol (PEG) as a crowding agent, the
[24]
electrolyte’s cathodic potential is fully 1.1 V lower than it would be without PEG. They presented that a
decrease in the activity of water is due to the fact that the water, mostly free water, is confined in the
network of PEG via forming a hydrogen bond (H bond) with PEG. Inspired by this example, the ‘‘H bond-
anchored’’ electrolytes begin to emerge in Zn batteries. In general, these electrolytes abide by the basic