Electrochemical behavior of Zn/Zn(II) couples in aprotic ionic liquids based on pyrrolidinium and imidazolium cations and bis(trifluoromethanesulfonyl)imide and dicyanamide anions

Motivated by the potential of using room temperature ionic liquids (RTILs) as electrolytes to replace traditional aqueous electrolytes for Zn-anode secondary batteries, Zn/Zn(II) redox reactions have been studied in four aprotic RTILs based on pyrrolidinium ([Pyrr](+)) and imidazolium ([Im](+)) cations, and bis(trifluoromethanesulfonyl)imide ([TFSI](-)) and dicyanamide ([DCA](-)) anions. Cyclic voltammetry results suggest a smaller overpotential for Zn redox in [Im](+) cation based and [DCA](-) anion based RTILs than in [Pyrr](+) and [TFSI](-) based RTILs. Potentiodynamic polarization experiments indicate a strong dependence of the electrode reaction mechanism for the Zn species on the RTIL anions. In [TFSI](-) based RTILs, Zn2+ ions are the electroactive species, with the electrode reaction being a single-step, two-electron transfer process. In [DCA](-) based RTILs, two-step, single-electron reactions account for the electrode mechanism. The exchange current densities derived from Tafel analysis for the Zn species in the four RTILs are greater than 10(-3) mA/cm(2), with the [Im](+) cation based RTIL possessing the highest value of 9.9 x 10(-3) mA/cm(2). The results obtained will assist in obtaining a better understanding of the electrochemical behavior of Zn in RTILs, shedding light on the development of RTILs for Zn-anode secondary batteries. (C) 2012 Elsevier Ltd. All rights reserved.