High Zn concentration pyrrolidinium-dicyanamide-based ionic liquid electrolytes for Zn2+/Zn0 electrochemistry in a flow environment

The cycling performance of N-butyl-N-methylpyrrolidinium dicyanamide [C 4 mpyr][dca] ionic liquid (IL) with H 2 O additive for application in a Zn 2+ /Zn 0 redox couple is reported for the first time under realistic flow conditions using a 3D printed flow half-cell prototype. This IL electrolyte displayed a superior performance at high current densities (3 mA cm -2 ) under the flow condition, in terms of cycling efficiency (60 ± 2% vs 45 ± 3%) and long-term cycling stability (>200 cycles) in contrast to similar experiments performed under a static or "no flow" condition. This is possibly due to different Zn 2+ speciation mechanisms and/or different structuring of the IL cation and anion at the electrode/electrolyte interface under static and dynamic conditions. Significantly, [C 4 mpyr][dca] IL allowed a high solubility of the Zn(dca) 2 salt, up to a ∼1:1 molar ratio, which is desirable for achieving a high energy density. This high concentration IL electrolyte composition, which has been studied here for the first time, displayed the steadiest long-term cycling stability (>100 cycles), a compact and dendrite-free Zn morphology, as well as a high volumetric capacity (ca. 1.6 Ah/L) at higher current density (3 mA cm -2 ). It was also revealed that H 2 O is essential in the electrolyte to achieve an improved cycling efficiency (65 ± 2%), and more than 1 wt % H 2 O is essential to attain a uniform well adhered Zn deposit. The dendrite-free Zn morphology, even at higher water contents (10 wt %), allows this system to work successfully in ambient atmospheric conditions. However, considering both the cycling efficiency and Zn deposition morphology, the optimized H 2 O content in the electrolyte was ∼3 wt %.