Zn Electrochemistry in 1‐Ethyl‐3‐Methylimidazolium and N‐Butyl‐N‐Methylpyrrolidinium Dicyanamides: Promising New Rechargeable Zn Battery Electrolytes

• Published in: ChemElectroChem Vol. 1; no. 10; pp. 1688 - 1697
• Main Authors: Simons, Tristan J., MacFarlane, Douglas R., Forsyth, Maria, Howlett, Patrick C.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY‐VCH Verlag 14.10.2014
• Subjects: batteries; electrolytes; ionic liquids; overpotential; zinc
• DOI: 10.1002/celc.201402177
• ISSN: 2196-0216

We have studied both 1‐ethyl‐3‐methylimidazolium (C2mim) and N‐butyl‐N‐methylpyrrolidinium (C4mpyr) dicyanamide (dca) ionic liquids (ILs) containing 3 wt % H2O and 9 mol % Zn(dca)2 salt for their ability to support Zn0/2+ electrochemistry in the context of a rechargeable Zn battery. Despite the similarities of the two IL electrolyte systems [identical H2O and Zn(dca)2 contents], the system based on [C2mim] supported much higher current densities for Zn electrochemistry at greatly reduced overpotentials [−0.23 V vs. Zn pseudo‐reference, 32 mA cm−2 (red) and 61 mA cm−2 (ox)] compared to the [C4mpyr]‐based electrolyte [−0.84 V vs. Zn pseudo‐reference, 8 mA cm−2 (red) and 15 mA cm−2 (ox)]. The overpotential for Zn deposition is reduced by 0.13 V on Zn metal surfaces compared to glassy carbon (GC), regardless of the electrolyte used. The morphologies of the Zn deposits on both GC and Zn surfaces were also studied. The Zn surfaces promote a deposition that displays a smooth morphology, resulting from an instantaneous nucleation mechanism demonstrated by chronoamperometric experiments. Finally, both [C2mim] and [C4mpyr] electrolytes were tested in symmetrical Zn|Zn cells, where it was determined that the [C2mim] system could sustain over 90 cycles at 0.1 mA cm−2, whereas the [C4mpyr] based system could only achieve 15 cycles at the more modest current density of 0.05 mA cm−2. Rechargable promises: Two ionic liquids based on the dicyanamide anion are studied for their ability to cycle zinc while providing smooth morphologies of deposited zinc. The imidazolium‐based ionic liquid is found to perform better than its pyrrolidinium analogue and shows promise for zinc‐based secondary battery applications.