Mechanisms of Mobile Ion Hopping in Polymerized Ionic Liquids

Polymerized ionic liquids (polyILs) present a subclass of solid polymer electrolytes, where the free ions are mobile while the counter ions are grafted to the polymer chains 1 . As a result of their unique chemical and physical properties, polyILs are a new class of materials with potential applications as the electrolytes for battery and fuel cells 2 . It is believed that ionic conductivity is dependent on the chemical structures of both the polymer and counter ions 3 , which enables us to tune the properties of polyILs. To systematically study the effects of chemical structure and the choice of ions, molecular dynamics (MD) simulations where undertaken to provide atomic-level information of the ion hopping mechanisms. Four types of hopping were identified as: (1) intrachain; (2) interchain; (3) non-associating free ions; and (4) rattling. To find out the effects of different cationic groups and the length of alkyl tails, simulations were performed for poly(n-alkyl VIm) and poly(n-alkyl VATMA), with the counter ion of TFSI¯. Subsequently, the anion was substituted by Br¯ and BF 4 ¯ for the poly(pentyl VIm) to investigate the effects of anions. Structural properties were analyzed from the results. The numbers of associating cations and chains to the anions were calculated, and the changing in the anion hopping types were found for the systems mentioned above. This quantitative analysis should guide the design of polyILs for electrochemical applications in the future. (1) Stacy, E. W.; Gainaru, C. P.; Gobet, M.; Wojnarowska, Z.; Bocharova, V.; Greenbaum, S. G.; Sokolov, A. P. Fundamental Limitations of Ionic Conductivity in Polymerized Ionic Liquids. Macromolecules 2018 , 51 (21), 8637–8645. (2) Green, O.; Grubjesic, S.; Lee, S.; Firestone, M. A. The Design of Polymeric Ionic Liquids for the Preparation of Functional Materials. Polym. Rev. 2009 , 49 (4), 339–360. (3) Mecerreyes, D. Polymeric Ionic Liquids: Broadening the Properties and Applications of Polyelectrolytes. Prog. Polym. Sci. 2011 , 36 (12), 1629–1648. Figure 1