Investigation of Imidazole-Based Lithium Conducting Materials
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Investigation of Imidazole-Based Lithium Conducting Materials A. R. Czardybon, K. Sivasubramaniam, G.R. Goward* McMaster University, Department of Chemistry, and The Brockhouse Institute for Materials Research 1280 Main St. W. Hamilton, ON, L8S 4M1 Canada ABSTRACT This study aims to develop novel polyelectrolytes including lithiated imidazole heterocycles for use in lithium ion rechargeable batteries. Lithium ion local mobility in these materials is characterized by 6,7Li solid-state NMR. By comparing these results with macroscopic ionic conductivity, measured by impedance spectroscopy, we will be able to develop a picture of the ionic conductivity at the microscopic level. Multinuclear solid state NMR provides information on microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Our research includes 6,7Li variable MAS NMR studies at intermediate spinning speeds, relaxation investigations to determine spin-lattice relaxation times (T1) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. A very long T1 (135 s at ambient temperature) and an activation energy Ea = 17.2 kJ/mol suggests rigid molecule structure and the absence of the ring reorientation of the model compound, lithium imidazolium (LiIm). We compare this to the behavior of LiIm doped with lithium methanesulfonate, which we show to form a new ionic complex with lower T1 and corresponding lower activation energy. With the goal of creating new polyelectrolytes, we have synthesized electrolytes incorporating lithiated imidazole rings, where lithium transport may be independent of polymer-backbone flexibility, and thus polymers with high Tg may be viable. Such materials are highly desirable for secondary lithium polymer battery applications. INTRODUCTION Imidazole and imidazole-based polymers have attracted attention recently as protonconducting materials with potential applications as membrane materials in polymer electrolyte membrane fuel cells [1,2,3,4,5] and lithium batteries [6,7]. Secondary lithium ion batteries supply power to portable electronics, including laptops and cell phones. Recent efforts in our group have focussed on the possibility of using lithiated imidazolebased materials as an alternative polyelectrolyte for lithium ion batteries. Lithium transport in novel materials may not depend as strongly on polymer-backbone flexibility or coordination of the Li+ ions and higher Tg polymers can be used. The objective of our study is to synthesize and characterize ionic mobility of these imidazole-based materials. We have chosen polymers in which lithiated imidazole rings are attached through long flexible chains, which will aid the ion transfer process. Investigations of the conductive properties of these materials include 6,7Li solidstate NMR and will be extended to impedance spectroscopy in our ongoing studies. We expect to achieve a better understanding of lithium local mobility mechanisms using the combination of these two m
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