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, systematic property tuning in two series based on variously 2- and 4-substituted 4,5-dicyanoimidazolide 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. The stabilization of the imidazolide anions via the substituent effects was examined using dissociation constants, the Hammett equation, ¹³C NMR shifts and electronic absorption spectra. Solubility in dimethyl carbonate and 1,2-dimethoxyethane further identified lithium salts with a potential for a practical application, and their solutions were further investigated for aggregation phenomena. Using absorption spectroscopy, a significantly more sensitive and straightforward methodology is presented, which allows the identification of perspective substituents hindering aggregation. The viscosity and density measurements further confirmed the significant property tuning of electrolytes upon changing the structure of lithium dicyanoimidazolide, which is in line with the subsequent electrochemical measurements. Based on the complete gathered data, extension via the 1,4-phenylene moiety along with peripheral (O)CF₃-substitution proved to be a useful strategy towards stabilized anions with a promising application in lithium-ion batteries.