Page 38 - Read Online

Page 20 of 31 Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89

ethers [25,69,120,121] , esters [50,31,122-124] , sulfones [39,46] , nitriles [47-49] , and amides [125,126] are the currently known organic
cosolvents. Abundant carboxyl groups in alcohols can be seen as H-bond acceptors; these carboxyl groups
interact with water and break water H-bond networks, thereby reducing the activity of water. Different from
2+
the mechanism of inhibiting HER by alcohols, carbonyl groups in ethers have strong affinities to Zn ,
which allows ether molecules to participate in the Zn solvation sheath and inhibit HER by reducing the
2+
amount of solvated water. In addition, these ethers have the ability to adsorb on the Zn metal surface, which
helps to control random Zn diffusion and prevent the growth of Zn dendrites. Owing to the presence of
2+
hydrophobic methyl groups, most ester solvents are not water-soluble. Therefore, it is often necessary to
add amphoteric anions, such as OTf and TFSI , to make them soluble in water. These hydrophobic ester-
-
-
based aqueous electrolytes exhibited high inhibition of HER due to their superior ability to disrupt the H-
bond network of water. Sulfone solvents (with sulfonyl group), nitrile solvents (with nitrile group), and
amide solvents (with acyl group) all contain functional groups that can bind to Zn , so they solvate with
2+
2+
Zn and exclude water from the Zn solvation sheath, preventing the occurrence of water-induced side
2+
reactions. The hydrated eutectic electrolyte, which is another organic/aqueous hybrid electrolyte, has
recently been reported. This kind of electrolyte is a derivative of the deep eutectic electrolyte (DEE) [127-129] .
The DEE is a eutectic mixture characterized by a solidification temperature lower than that of its individual
component. Owing to the numerous appealing properties, e.g., high electrochemical stability, ease of
synthesis, and low cost, such an electrolyte has gained significant study interest in Zn batteries. But because
of being nonaqueous, they have much lower ionic conductivity and are hence less attractive. Unlike this
kind of electrolyte, a hydrated eutectic electrolyte not only maintains most eutectic properties but also has a
high level of ionic conductivity as a result of the addition of water [18,130-132] . Water molecules in the electrolyte
are mainly confined to the internal interaction network of eutectic solvents by H bonds, so this kind of
electrolyte has low water freezing points, enabling it to be used in low-temperature batteries . Overall,
[131]
although organic/aqueous hybrid electrolytes significantly improve the performance of Zn anodes, there are
still safety issues caused by the addition of flammable organic solvents. In addition, most organic solvents
are toxic and not environmental-friendly, thus limiting their further development.

Additive
In contrast, adding a small number of additives (with total mass ratios lower than 5 wt%) into aqueous
electrolytes has been experimentally developed to enhance battery safety and achieve high CEs of Zn
plating/stripping. The additives used in Zn batteries can be divided into inorganic additives [72-75] and organic
additives [37,70,71,83,84,87] . Organic additives have better water solubility than inorganic additives and are thus
widely studied. Organic additives can be further classified into organic small molecule additives [37,70,87] and
organic polymer additives [71,83,84,157,158] . Compared with organic polymer additives, organic small molecule
additives have been more intensively studied due to their diversity, simple structure, ease of synthesis, and
pro-environment. At present, adding organic small molecule additives into aqueous electrolytes is a
promising commercially viable electrolyte modification strategy for Zn batteries.

The mechanisms of different electrolyte compositions
Interest in Zn batteries has been increasing recently, and some inventive electrolyte design strategies have
appeared. In order to clearly understand the rationale behind each one, different electrolyte modification
2+
strategies are compared using a radar graph [Figure 9] which considers the Zn solvation structure, H-bond
network of water, Zn nucleation process, Zn anode surface texture, Zn deposition behavior, and
2+
construction of SEI. As depicted in Figure 9A, three mechanisms - constructing an SEI, modifying the Zn
solvation structure, and modulating the Zn flux at the Zn/electrolyte interface - are primarily used by the
2+
salt modification to impact Zn anodes. There is no instance where the idea of controlling the Zn nucleation
process has been used for the salt regulation strategy. The only instance of an effect on the Zn anode
through the surface texture is the report about OTf mentioned above . The impact of the salt
-
[81]