Abstract
The use of room temperature ionic liquids (RTILs) as potential electrolytes for rechargeable Zn-anode batteries suffers from a disadvantage of insufficient conductivity. In this study, water added ionic liquids composed of pyrrolidinium/imidazolium cations and bis(trifluoromethanesulfonyl)imide anions were investigated with regard to their physicochemical behavior and their response to Zn/Zn(II) redox reactions. Peak conductivity was achieved by adding water within the range of 2.0–2.5 wt%. The best kinetic properties for Zn/Zn(II) redox, including decreased redox overpotential and on average three times the diffusion rate and exchange current density, were also achieved for this composition range. Density functional theory (DFT) calculations showed that water addition led to the weakening of Zn-TFSI bonds and, consequently, an increase in the concentration of Zn2+ dissolved in RTILs and involved in the redox reaction, which accounts for the improved Zn redox kinetics. Water also affected the morphology of Zn electrodeposited from RTILs. Zinc deposits grew from nanocrystalline to microcrystalline in scale with poorer adhesion in water added RTILs with pyrrolidinium cations. However, Zn deposits obtained from water added RTILs with imidazolium cations showed no obvious change in size and adhesion properties.
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