Zinc (Zn) batteries have attracted more and more attention due to large volumetric capacity, abundance of Zn, and environmental friendliness. When the aqueous electrolytes are considered, Zn batteries provide a promising solution to safety hazards and economic challenges facing prevailing Li-ion batteries.
However, the currently available aqueous Zn electrolytes are far from ideal. Aqueous Zn batteries have been struggling with the rapid performance degradation arising from the poor reversibility of Zn anodes and the dissolution of cathodes.
A research team led by Prof. CUI Guanglei from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of the Chinese Academy of Sciences has proposed a new class of aqueous electrolytes, called hydrated eutectic electrolytes, to ensure better performance of aqueous Zn batteries. The study was published in Joule on July 1.
The new aqueous electrolyte was fabricated by coupling a hydrated Zn salt (Zn(ClO4)2·6H2O) exclusively with a neutral ligand (succinonitrile, SN).
"The aqua cationic Zn species and corresponding water molecules' coordination states are reorganized. SN enters the primary solvation shell of Zn2+, while all water molecules contribute to the formation of the eutectic structure and remain bound in the metal coordination sphere," said Dr. ZHAO Jingwen from QIBEBT, co-corresponding author of the study.
That’s why the electrochemical behaviors of the hydrated eutectic electrolytes were different from those of traditional aqueous electrolytes. Hydrated eutectic electrolytes were highly suitable for the Zn-organic batteries from both anode and cathode aspects.
"It is known that the perchlorate anions are reactive and susceptible to decomposition in aqueous solutions," CUI said. "However, due to the suppressed Zn2+-H2O interplay, the commonly accepted nonideal perchlorate anion can be stabilized in the eutectic network."
Owning to the rich intermolecular interactions in the hydrated eutectic electrolytes, stable low-temperature operation even at -20°C was also obtained.
The study offers a simple and promising way to tame the multivalent electrolyte structure toward creating long-life rechargeable aqueous batteries.
Fig.: Schematic diagrams of Zn2+ solvation structure and interfacial reactions in (a) traditional aqueous electrolyte and (b) hydrated eutectic electrolyte (Image by ZHAO Jingwen and YANG Wuhai)
