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Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89 Page 7 of 31

nitriles [47-49] , are widely utilized as additives/cosolvents in Zn batteries. Their introduction can not only
maintain the homogeneity of electrolytes but also restrict water activity by breaking the H-bond network of
water. Theoretically, hydrophobic solvents such as most esters [31,50] have a strong mutual repulsion with
water, which makes them potentially feasible to significantly disrupt the H-bond network of water when
hydrophobic-aqueous systems are realized. Besides, once the hydrophobic additive/cosolvent coordinates
2+
into the Zn solvation sheath, it would favorably diminish the solvation interaction between solvated water
and Zn . However, due to the immiscibility of the hydrophobic organic with water, it is intuitive that
2+
hydrophobic organics may not meet the demands of the homogeneity of electrolytes and are therefore
rarely used as cosolvents in aqueous electrolytes. Fortunately, Miao et al. successfully prepared a series of
[31]
hydrophobic carbonate-based hybrid aqueous electrolytes by coupling Zn(OTf) . Their results show that
2
-
the amphiprotic OTf anion with hydrophilic -SO and hydrophobic -CF groups acts as a linker
3
3
interplaying with both water and carbonate to make them miscible. This work emphasizes how important it
is to consider the interactions between different compositions when designing electrolytes.
Although mildly acidic aqueous electrolytes only contain Zn salt, water, and additives/cosolvents, electrolyte
microstructures are still intricate. This is primarily caused by the complicated interactions between ions and
molecules. These interactions are roughly divided into three types, including the ion-ion, ion-water, and
additive/cosolvent-ion (or water) interaction. Different ion-pair states, which can be roughly categorized as
SSIP, CIP, and AGG, are present in ion complexes. The occurrence of these states mainly depends on ion
concentrations. The proposed concepts of kosmotropic and chaotropic anions well elaborate on and
distinguish the effects of different anions on water. The molecular properties of additives/cosolvents,
including hydrophilicity and hydrophobicity, chelation, and AN/DN value, have significant influences on
electrolyte structural evolution. Although the universality of the effects above is still far from being achieved,
using these theories enables us to identify key principles and research guidelines for an enhancement of
electrolyte solutions for future Zn battery technologies.

Characterization techniques of bulk electrolyte structures
Computational methodologies
Computational simulations have been widely utilized to understand the principles behind electrolyte
modification strategies at the molecular level. According to previous reports, commonly used
computational techniques include DFT calculation, molecular dynamics (MD) simulation, high-throughput
virtual screening (HTVS), and machine learning (ML) [top of Figure 4]. DFT calculation is primarily used
to explore electronic structure properties and interactions between the compositions in electrolytes. For
example, the reducibility of chemicals can be discerned by computing the energy of molecular orbitals
(MOs) via DFT. According to MO theory [51,52] , the lower the LUMO energy level of a molecule, the easier it
is to capture electrons and demonstrate reducing activity. In addition, it was reported that Zn can induce a
2+
decrease in the LUMO energy of water . Thus, if the LUMO energy of Zn -X (X: the modifier in
[53]
2+
electrolyte regulation) is higher than that of Zn -H O, the modified electrolyte will have enhanced
2+
2
2+
2+
reduction stability. DFT calculation is also able to understand the Zn solvation process. As is known, Zn
2+
forms a solvation structure with a hydration layer composed of six water molecules, namely Zn(H O) . As
6
2
mentioned above, these water molecules are solvated by Zn and are highly reductive. Therefore, to expel
2+
2+
water from the Zn solvation sheath, the chemical used for electrolyte modification should be more
zincophilic than water; That is, it should have a higher binding energy to Zn , so as to construct an altered
2+
Zn solvation structure with the lower solvation energy than Zn(H O) 6 2+[54] . In addition, DFT calculation
2+
2
provides insight into the electronic structure properties of electrolytes. Methods of wave function analysis,
including molecular surface electrostatic potential (MSEP) and deformation charge density (DCD), can be
used to visually describe the evolution of electrons before and after the interactions between the
compositions in the electrolyte. In addition, DFT calculation can also identify molecule shapes, electrophilic