The state of water in 1-butly-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide and its effect on Zn/Zn(II) redox behavior

•The state of water was studied in hydrophobic RTIL, BMP-TFSI.•Conductivity of BMP-TFSI increases with water amount.•Bound water in BMP-TFSI improves Zn redox reversibility.•2.0 wt% is the optimal amount of water added in BMP-TFSI.

Water was added as a diluent into the hydrophobic room temperature ionic liquid (RTIL) 1-butly-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) with the purpose of improving the conductivity of the RTIL, which is being considered as a potential electrolyte for rechargeable Zn-air batteries. The behavior of water additions in BMP-TFSI and its effect on Zn/Zn(II) redox behavior in BMP-TFSI were studied. Two types of water, free or unbound water and bound water, were identified in BMP-TFSI through DSC and FTIR analysis and density functional theory (DFT) modeling. When the water content was less than ~2.0 wt%, water molecules interacted with [TFSI]− anions through hydrogen bonding. When the water amount was greater than 2.0 wt%, clusters of water molecules (free water) were observed. Bound water in BMP-TFSI improved Zn/Zn(II) redox reversibility by reducing the activation overpotential for electrochemical reactions. The optimal amount of water, in terms of the kinetic properties for Zn/Zn(II) redox in BMP-TFSI, was 2.0 wt%. Compared with water-free BMP-TFSI, both the diffusion coefficient for the Zn species and the exchange current density for Zn/Zn(II) redox reactions increased by approximately 70% in BMP-TFSI with 2.0 wt% water addition.