Tailoring lithium dicyanoimidazolide: structure-property relationships

Abstract

Taking lithium 2-trifluoromethyl-4,5-dicyanoimidazol-1-ide as a parent lithium salt for Li-ion batteries, a systematic property tuning in two series based on variously 2-and 4-substituted 4,5dicyanoimidazolide and 2-phenyl-4,5-dicyanoimidazolide scaffolds is demonstrated. A straightforward synthetic approach afforded fourteen desired derivatives with a systematically evaluated structure, whose properties were further investigated from various perspectives. Stabilization of the imidazolide anions via the substituent effects has been examined using dissociation constants, Hammett equation, 13 C NMR shifts and electronic absorption spectra. Solubility in dimethyl carbonate and 1,2-dimethoxyethane has further identified lithium salts with a potential for a practical application and their solutions were further investigated for aggregation phenomena. Compared to the current state-of-the-art and using absorption spectroscopy, a significantly more sensitive and straightforward methodology is presented, which allowed to identify perspective substituents hindering aggregation. The viscosity and density measurements further confirmed significant electrolyte's property tuning upon changing the structure of the lithium dicyanoimidazolide, which is in line with the subsequent electrochemical measurements. Based on the complete gathered data, an extension via the 1,4-phenylene moiety along with the peripheral (O)CF 3 -substitution proved to be useful strategy towards stabilized anions with a promising application in lithium-ion batteries.