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Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89 Page 23 of 31

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with an areal capacity of 15 mAh cm . In addition, the Zn||Zn symmetric battery can reach an ultra-long
[45]
lifetime of 8,000 h (333 days) thanks to the organic cosolvent-PEO400 (point 58) . To the best of our
knowledge, this holds the record for stabilizing the Zn anode with a mildly acidic aqueous electrolyte.

In addition to the cycle life of Zn anodes, another important metric for assessing the Zn anode performance
is the Zn plating/stripping CE. In order to attain a high energy density, the Zn mass should be kept to a
minimum, which calls for a high CE. The reported CE values for Zn plating/stripping obtained in the
above-mentioned electrolyte modifications from various test protocols and cell setups are summarized in
Figure 10B. We compare three test parameters for CE, including the cumulative capacity of the Zn plating
during short-circuit or excessive resistance formation, which is an important indicator of the battery life,
areal capacity, and CE value. Similar to Figure 10A for the cycle life of Zn anodes, the parameters chosen for
these CE measurements are highly dispersed and arbitrary. Two-thirds of newly published electrolytes claim
CEs > 99% in the literature. However, in terms of the cumulative plated capacity, less than a tenth of the
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electrolytes can exceed 1 Ah cm . To our knowledge, the salt-NH OAc (point 33) is the record holder for
[108]
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contributing to a cumulative plated capacity of 6.88 Ah cm . The additive-gamma butyrolactone (GBL,
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point 95) helps achieve the highest CE of 99.93% with a cumulative plated capacity of 3.1 Ah cm . When
[143]
contrasting various electrolyte alteration strategies for Zn reversibility, the strategy of modifying salt
concentrations also has no advantages, and the results of the other three strategies are difficult to
differentiate.
CONCLUSIONS AND OUTLOOK
Large-scale energy storage with Zn batteries has bright futures. However, the side reactions and dendritic
growth on the Zn anode surface seriously obstruct the commercialization of Zn batteries. Recently,
electrolyte modification has drawn a lot of interest as a potential and efficient remedy for the issues of Zn
anodes. In this review, we have a systematic summary of mildly acidic aqueous electrolytes used in Zn
batteries. We emphatically discussed the relationship between the electrolyte microstructures and the
performance of Zn anodes. The relevant principles and characterization techniques are mentioned and
evaluated. Thereafter, recent progress on the manipulation strategy of each composition of mildly acidic
aqueous electrolytes for Zn batteries is systematically summarized. Although the Zn anode performance has
been improved thanks to the electrolyte modification, further effort is still needed to make progress
[Figure 11].

The structure-function relationship between electrolyte structures and Zn anode performance
So far, most theories about the impact of the electrolyte structure on the Zn anode performance are still in
the infancy stage. This is mostly due to the fact that although the electrolyte compositions in mildly acidic
aqueous electrolytes are only Zn salt, solvent, and additives/cosolvents, their interactions and
microstructures are complicated. It has been confirmed that altering the Zn solvation structure and the
2+
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water H-bond network affects the probability of HER, but the effects of the Zn -outer, anion-solvation, and
additive/cosolvent-anion structures are unknown. This results in difficulty in establishing a correlation
between the electrolyte structure and the Zn anode performance. Meanwhile, different compositions
interact with each other, and a comprehensive consideration of the interactions between different
compositions contributes to the design of better electrolytes. In addition, it is well known that due to the
influence of the Zn metal anode, the electrolyte structure on the Zn surface is obviously different from that
in the bulk phase. However, there are few studies involving the electrolyte structure on the Zn surface. In
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fact, the interfacial electrolyte structure is worth studying, because it has an important influence on the Zn
desolvation process, which in turn affects the Zn nucleation process.