A novel high durability ZnCl2-NH4Cl threshold-WIS (water-in-salt) electrolyte for rechargeable zinc-air battery

Background

For rechargeable zinc-air battery (ZAB), KOH electrolyte exhibits poor zinc reversibility, carbonate formation, hydrogen evolution, ZnO passivation, dendrite formation, and high corrosion. Recently, 23.8M ZnCl₂ hydrate melt electrolyte was reported to be without these problems but suffers from high viscosity, low conductivity, and pure oxygen requirement. The authors realized that acidic environment enables all the durability advantages with the exception of anti-dendrite formation, which is due to the elimination of the concentration gradient at sufficiently high salt concentration.

Methods

As durability advantages of concentrated ZnCl₂ electrolyte has often been associated with the water-in-salt (WIS) condition, 10m ZnCl₂ at the threshold-WIS concentration was thought to be the minimum concentration to enable the durability benefits and was selected to be tested with NH₄Cl added for pH stability. 10 m ZnCl₂ was tested with varying NH₄Cl for changes in viscosity, conductivity, acidity, and density. Then Raman spectroscopy, chronoamperometry, linear sweep voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-and-discharge cycling were done to compare the ZnCl₂-NH₄Cl mixture to 6M KOH.

Significant findings

Results show 10 m ZnCl₂ w/wo 5 m NH₄Cl gave superior performance with ZAB cycling for >6000 cycles/>60 days at 1 mA/cm² and > 89% voltage efficiency without the durability problems associated with KOH.