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Page 18 of 31 Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89

Salt
Recent advances in mildly acidic aqueous electrolyte modification during the past few years are summarized
in Figure 8. Based on the electrolyte compositions, they are classified into three categories: salt (with salt
concentration), cosolvents, and additives. As an important component of electrolytes, Zn salt serves as a
current carrier in batteries. In addition, since different Zn salts have different solvation structures and
compatibility with Zn metal, the type of Zn salt has an important effect on the performance of Zn
electrodes. Issues of Zn anodes can be solved by changing the kinds of Zn salt. Zn salt, including ZnSO ,
[94]
4
[96]
[54]
Zn(NO ) [95] , ZnCl , Zn(OTf) 2 [36] , Zn(ClO ) [96] , Zn(CH COO) (Zn(OAc) ) , Zn(TFSI) 2 [23] , and
2
2
2
3
3 2
4 2
Zn(H PO ) , have recently been reported. Among these Zn salt, because NO and ClO are strong
-
[92]
-
2
4 2
3
4
oxidants, Zn(NO ) and Zn(ClO ) are able to oxidize Zn foil, which would result in severe Zn corrosion.
3 2
4 2
The most water-soluble Zn salt in water is ZnCl (solubility of about 30 m), but it easily decomposes into
2
chlorine gas (Cl ) under high pressure, limiting its practical application. The ionic conductivity of Zn(OAc)
2
2
in water is relatively low, only a dozen mS cm , which is not suitable for high-rate batteries. Zn(TFSI) is not
-1
2
-1[2]
appropriate for large-scale applications due to its high cost of roughly 28,369 US$ kg . In contrast, ZnSO
4
is the most widely used Zn salt because of the stable SO and good compatibility with Zn anodes. However,
2-
4
it has a disadvantage: it is prone to produce detrimental byproducts like Zn (OH) SO ·xH O during cycling.
6
4
2
4
As an organic Zn salt, Zn(OTf) has been extensively employed in recent years. By coordinating the bulky
2
anion OTf with Zn and lowering the amount of solvated water, this salt has a strong ability to inhibit
-
2+
-
HER. Besides, OTf can change the Zn surface texture, which causes the Zn atom to nucleate and grow
towards the Zn (002) facet, thereby suppressing Zn dendrites. The use of Zn(H PO ) is mainly due to its
4 2
2
capacity to produce P-containing, uniform, and dense SEI on Zn anodes during the initial charge/discharge
process, preventing the interaction between Zn metal and water and reducing the possibility of side
reactions. In addition to altering anions in Zn salt, adding cations to the electrolyte can improve the
+ [97]
durability of Zn anodes. Some cations, such as the arginine cation (Arg ) , Ce 3+[98] , tetrabutylammonium
cation (TBA ) , benzyltrimethylammonium cation (TMBA ) , and Na , can give rise to uniform Zn
+ [99]
+[85]
+ [86]
deposition by electrostatic shielding. Inorganic cations like cholinium cation (Ch ) , Mg 2+[101] , and Sc 3+[102]
+ [100]
can form the solvation structure on their own, which affects the Zn solvation structure and the Zn
2+
2+
+ [60]
deposition process. Some organic cations, such as 1-ethyl-3-methylimidazolium (EMIm ) , trimethylethyl
+ [3]
) , can induce the formation
ammonium cation (Me EtN ) , and tributyl(2-methoxyethyl) cation (P 444(2O1) + [103]
3
of SEI to prevent side reactions between water and Zn anodes. Zn anodes can also be stabilized by adding
anions distinct from the original Zn salt into the electrolyte, such as citrate anion (Cit ) , I 3-[104] , OH -[105] ,
3- [59]
F , and etrakis (perfluoro-tert-butoxy) aluminate (TPFA ) . The primary function of these anions is to
- [106]
-[90]
help produce beneficial SEI layers. Additionally, the electrolyte has lately been supplemented with some
unusual salt, such as lanthanum nitrate [La(NO ) ] , NH OAc , 1-methyl-1-ethyl pyrrolidinium
[107]
[108]
4
3 3
[61]
bromide (MEP·Br) , NH I , sodium lauryl sulfate (SDS) , sodium lignosulfonate (SL) , and
[109]
[111]
[110]
4
TMACl . These salts cover the beneficial effects of the cations and the anions mentioned above,
[112]
protecting Zn anodes in multiple aspects.
Salt concentration
2+
Previous studies have suggested that altering salt concentrations can change the Zn solvation structure and
the SEI composition . Electrolytes with high-concentration salt are known as water-in-salt electrolytes.
[23]
2+
Anions coordinate with Zn instead of water molecules in such electrolytes [113,114] . With little solvated water,
water activity in such electrolytes is extremely low, which inhibits water-induced side reactions and results
in an excellent CE for Zn anodes . Besides, such electrolytes induce the formation of SEI composed of the
[115]
anion decomposition products at the Zn surface during the initial cycling [116,117] . Sheltered by the SEI layer,
the Zn anode is protected from corrosion by electrolytes. Water-in-salt electrolytes provide novel routes to
highly reversible Zn anodes. However, excessive voltage polarization is a common issue with such
electrolytes. The increase in salt concentrations results in decreased ionic conductivity and increased