For Zn anode rechargeable batteries, there are a number of shortcomings associated with using traditional KOH aqueous electrolytes. These include drying-out of the electrolyte due to water evaporation and dendrite formation at the Zn electrode during recharging, which severely impair battery performance (e.g., cycle life and capacity) and limit their application. In particular, to solve the problem of dendrite formation that could cause short-circuit issues, many attempts have been made to modify the Zn electrode and the electrolyte, as well as to choose a desirable and robust separator. However, no breakthrough has been achieved on the basis of conventional KOH aqueous electrolytes. It is, therefore, critical to either modify conventional KOH aqueous electrolytes or explore alternative electrolytes to eliminate these bottlenecks to the development of a feasible Zn anode rechargeable battery system. Room temperature ionic liquids (RTILs) in recent years have been increasingly recognized as potential electrolytes or electrolyte components for rechargeable batteries. Applying non-volatile RTILs as electrolytes provides potential benefits of achieving a longer service life, as drying out due to water evaporation is no longer a problem. Furthermore, RTILs demonstrate the capacity to modify metal deposit morphology, which may contribute greatly to preventing Zn dendrite formation and improving battery cycle life. On the other hand, compared with alkaline electrolytes, a simple electrolyte system composed of an RTIL as the sole component faces the challenge of enhancing its low conductivity (one to two orders of magnitude lower than aqueous electrolytes) before it can be practically applied in a battery.

With the purpose of developing electrolyte systems that can harness the benefits from both RTILs (Zn morphology control) and aqueous electrolytes (rapid Zn redox kinetics), two groups of electrolytes are investigated in this study. One is based on RTILs, composed of pyrrolidinium or imidazolium cations and bis(trifluoromethanesulfonyl)imide or dicyanamide anions, with the incorporation of diluents (water and/or dimethyl sulfoxide (DMSO)). Another one adopts RTILs as additives to modify conventional KOH aqueous electrolytes. A larger portion of this work was focused on the former group. By applying cyclic voltammetry (CV), potentiodynamic polarization and chronoamperometry (CA), the kinetics, reversibility and cyclability of Zn redox behavior is explored in the studied electrolytes. The morphology of Zn deposits is observed and analyzed using scanning electron microscopy (SEM). With respect to RTIL-based electrolytes, conductivity measurements, together with Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and gas-phase density functional theory (DFT) are performed to investigate water interaction with RTIL ions and to shed light on the mechanisms for improved Zn redox behavior with water addition.

For RTIL-based electrolytes, to balance the pros (improved electrolyte conductivity and Zn redox kinetic performance) and cons (reduced electrochemical stability of RTILs) of adding diluent(s) is of great importance in the development of workable electrolyte systems. Among six kinds of studied RTILs, i.e., 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI), 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (MPrP-TFSI), 1-methyl-1-pentylpyrrolidinium bis(trifluoromethanesulfonyl)imide (MPP-TFSI), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-butyl-1-methylpyrrolidinium dicyanamide (BMP-DCA) and 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA), an electrolyte system composed of EMI-DCA with the addition of both water and DMSO at a mole ratio of EMI-DCA:​H2O:​DMSO = 1:​1.1:​2.3 exhibits the best performance in terms of electrolyte conductivity, electrochemical properties for Zn redox reactions and Zn deposit morphology.

For conventional alkaline aqueous electrolytes, adding an appropriate RTIL as the electrolyte additive can effectively eliminate Zn dendrite formation during electrodeposition. It is worth noting that hydrophilic RTILs are better relative to hydrophobic RTILs when it comes to obtaining desirable Zn morphologies and preventing dendritic Zn formation. An electrolyte composed of 9.0 M KOH + 5.0 wt% ZnO with a hydrophilic RTIL, i.e., 0.5 wt% EMI-DCA, appears to be a promising electrolyte system. These results give insights into developing novel alkaline aqueous electrolytes, which are deliberately modified with hydrophilic RTILs, for Zn anode rechargeable batteries.