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Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89 Page 5 of 31

the free water structure has an important impact on their activity. In addition, electrolyte modification
2+
usually alters the Zn solvation structure and the H-bond network of free water simultaneously, so a more
trustworthy conclusion may require a combination of the two aforementioned mechanisms.

The correlation effects in bulk electrolytes
Understanding the underlying molecular interactions between electrolyte compositions provides a useful
framework for guiding the rational design of electrolytes with satisfactory performance. Here, we provided a
comprehensive survey of the interactions between each composition (including Zn salt (Zn and anion),
2+
water, and additives or cosolvents) in mildly acidic aqueous electrolytes. As shown in Figure 3A, these
interactions can be roughly categorized into three types at different levels of detail, including the cation-
anion interaction, the interaction between ion and water (cation-water and anion-water interaction), and
the interaction of additive/cosolvent with water or Zn salt (additive/cosolvent-water interaction or additive/
cosolvent-ion interaction).

The cation-anion interaction
For ions in solution, there are three aggregation states of ions, including solvent-separated ion pair (SSIP),
contact ion pair (CIP), and aggregate (AGG). As shown in Figure 3B, SSIP refers to the ion pairs which are
separated by more than one solvent. Each ion in the SSIP state holds its individual solvation shell and is
only slightly affected by the counterion. CIP is usually the closest-distance contact ion pair, which is formed
through the electrostatic forces between the cation and the anion. The AGG state corresponds to the
presence of ions in the form of clusters. Ionic aggregation states are influenced by the ion species, properties
of the solvent molecule, and the most influential ion concentrations . Most ion complexes in aqueous
[32]
-1
solutions are dominated by SSIP when the salt concentration is less than 1 mol L (M). As the salt
concentration increases, ions engage in each other’s solvation shells, resulting in CIP. AGG states are mostly
found in high-concentration water-in-salt electrolytes, where the solvation shell of cation/anion is entirely
surrounded by opposite-charged ions and contains almost no water. The occurrence of AGG has also been
observed for specific Zn salt with bulky anions, such as TFSI and trifluoromethanesulfonate (OTf ) .
-[23]
- [33]
The ion-water interaction
Salt dissolution can be considered as the process in which cations and anions in an ionic crystal overcome
the attraction force between each other, dissociate from the crystal lattice to become gas ions, and then enter
the water solution and combine with polar water molecules to form hydrated ions. As shown in Figure 3C,
2+
2+
the Zn solvation structure is usually composed of Zn and six water molecules. There are many different
2-
-
-
2-
kinds of anions (including TFSI , OTf , SO , Cl , ClO , etc.) being used in Zn batteries. Compared with
-
4
4
Zn , anions have different solvation structures due to their differences in sizes, shapes, and even charges. As
2+
one of the few empirical endeavors toward the anion effects in water, Collins et al. introduced the concepts
of kosmotropic and chaotropic solutes: Smaller anions like Cl are kosmotropes, meaning that they are water
-
-
structure makers ; larger ions like OTf and TFSI are chaotropes, which emphasizes that these species have
[34]
-
to be considered as water structure breakers [34,35] . This theory well explains the promoted stability of Zn
anodes using bulky anionic salts such as Zn(OTf) and Zn(TFSI) . These bulky anions, as water
[36]
[23]
2
2
structure breakers, strongly disrupt the H-bond network of water, inhibiting HER and stabilizing Zn
electrodes.
The interaction of additive/cosolvent with water or Zn salt
Except for Zn salt and water, a common aqueous electrolyte in Zn batteries contains additional
compositions in terms of additives or cosolvents. As shown in Figure 3D, the additive/cosolvent can interact
with ions, water, and itself in electrolytes. At present, three concepts - chelation, electron acceptor/donor