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Page 2 of 31 Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89

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huge natural abundance, low cost, and high theoretical capacity (820 mAh g and 5,854 mAh cm ) of
[1-3]
metallic Zn . In addition, compared with strongly acidic and alkaline electrolytes, using mildly acidic
aqueous electrolytes (pH ranging from 3 to 7) contributes to Zn batteries with excellent environmental
[4,5]
compatibility and inherent safety . However, the major obstacle to the implementation of Zn batteries is
the intrinsic thermodynamic instability of Zn metal anodes in mildly acidic aqueous electrolytes, which
involves H evolution, Zn corrosion, and dendritic growth . These undesirable results will degrade battery
[6-9]
2
performance or even result in battery failure.
In recent years, many strategies have been proposed to develop highly stable Zn anode systems, such as
surface coating [10-14] , host materials design , separator alteration [16,17] , and electrolyte optimization [18,19] .
[15]
Among them, the electrolyte optimization strategy is recognized as one of the most viable and effective
approaches to improve the electrochemical performance of the Zn anode due to the features of facile
preparation and cost effectiveness . In parallel to developing novel electrolytes, research into the influence
[20]
mechanisms of the structural compositions of electrolytes on the performance of Zn anodes is in full swing.
Researchers have realized that the fundamental structures in electrolytes directly determine the bulk and
interfacial electrolyte properties and hence the Zn battery performance. Understanding the complicated
structures and their correlations with electrolyte properties is of great significance for achieving the rational
[21]
design of Zn battery electrolytes beyond conventional trial-and-error approaches . Recently, some reviews
have introduced electrolytes and their development stages, which help to solve complicated chemical/
electrochemical issues of Zn anodes in aqueous media. However, a comprehensive and detailed review of
how electrolyte structures (in the bulk phase and at the Zn surface) affect the stability and reversibility of Zn
anodes has not been published so far .
[22]

In this review, we concentrate on investigating the microstructures of mildly acidic aqueous electrolytes and
their functional mechanisms in stabilizing Zn anodes. As shown in Figure 1, the principle models and
characterization techniques for the structures of bulk electrolytes and Zn/electrolyte interfaces are examined
in detail. We also present recent advances in mildly acidic aqueous electrolytes for Zn batteries. The effects
of electrolyte compositions, including the salt (with salt concentration), solvent, and additive, are
mentioned and critically evaluated. Perspectives on future research directions towards such electrolytes are
also outlined. We hope this review can provide a guideline for the rational design of advanced electrolytes
and boost the development of this field.

UNDERSTANDING THE MOLECULAR ORIGIN OF THE ENHANCED STABILITY OF WATER
IN BULK ELECTROLYTES
Two different water environments in controlling HER
Metallic Zn is a desirable anode material for aqueous batteries because of its high specific capacity, low
redox potential [-0.76 V vs. the standard hydrogen electrode (SHE)], low cost, good water compatibility,
and intrinsic safety . However, in the charging process, except for Zn ion (Zn ) deposition
[2,3]
2+
(Zn + 2e → Zn), hydrogen evolution reaction (HER) inevitably occurs at the metallic Zn surface in mildly
2+
-
acidic aqueous electrolytes, resulting in an increased local concentration of hydroxide ion (OH ) . This
- [6,7]
would promote Zn corrosion and the formation of zinc oxide (ZnO) passivation layer (Zn + 2OH →
-
-
-
Zn(OH) + 2e → ZnO + H O +2e ) and alkaline Zn salts, eventually inducing the Zn dendrite growth caused
2
2
by the tip effect and the uneven distribution of Zn flux [Figure 2]. Therefore, effective means are needed to
2+
suppress HER and accompanied side reactions.
Solvated water
HER can be inhibited by regulating the compositions and structures of electrolytes. For example,