Copper ionic liquids: Tunable ligand and anion chemistries to control electrochemistry and deposition morphology

• Copper ionic liquid displayed partially reversible plating and stripping.
• Ionic liquid reference electrode was evaluated with ferrocene and cobaltocene.
• Strongly coordinating ligands suppressed copper dendrite formation.
• Anions influenced copper particle size and morphology.

A multi-technique investigation was performed on three copper-based ionic liquids to elucidate the influence of coordinating ligands and charge-balancing anions on the electrochemical properties of the materials. Galvanostatic cycling of Cu(OHCH2CH2NH2)6(BF4)2 (Cu1) in 1-butyl-3-methyl-imidazolium hexafluorophosphate gave partially reversible plating of copper that was consistent with cyclic voltammetry data (collected using an ionic liquid-based reference electrode verified with measurements of ferrocene, cobaltocene, and lithium). Scanning electron microscopy also showed pitting in the copper-coated surface of the electrode that was consistent with the stripping wave observed by cyclic voltammetry. Potentiostatic deposition in neat Cu1 showed significant dendrite formation. The substitution of the OHCH2CH2NH2 ligands of Cu1 with stronger coordinating NH(CH2CH2OH)2 in Cu(NH(CH2CH2OH)2)6(BF4)2 (Cu2) resulted in the complete suppression of both copper stripping and dendrite formation. Substitution of the BF4- anions of Cu2 with CF3SO3- in Cu(NH(CH2CH2OH)2)6(CF3SO3)2 (Cu3) shifted the copper deposition 0.1 V more negative and produced slightly larger spherical particles (1.5 μm versus 5 μm). The results suggested that while the anion composition influenced particle size, and the metal–ligand bond strength helped control particle morphology, both factors affected the electrochemical properties including the plating and stripping of copper.

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