title: Molecular origin of high free energy barriers for alkali metal ion transfer through ionic liquid-graphene electrode interfaces.
abstract: In this work we study mechanisms of solvent-mediated ion interactions with charged surfaces in ionic liquids by molecular dynamics simulations, in an attempt to reveal the main trends that determine ion-electrode interactions in ionic liquids. We compare the interfacial behaviour of Li(+) and K(+) at a charged graphene sheet in a room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, and its mixtures with lithium and potassium tetrafluoroborate salts. Our results show that there are dense interfacial solvation structures in these electrolytes that lead to the formation of high free energy barriers for these alkali metal cations between the bulk and direct contact with the negatively charged surface. We show that the stronger solvation of Li(+) in the ionic liquid leads to the formation of significantly higher interfacial free energy barriers for Li(+) than for K(+). The high free energy barriers observed in our simulations can explain the generally high interfacial resistance in electrochemical storage devices that use ionic liquid-based electrolytes. Overcoming these barriers is the rate-limiting step in the interfacial transport of alkali metal ions and, hence, appears to be a major drawback for a generalised application of ionic liquids in electrochemistry. Some plausible strategies for future theoretical and experimental work for tuning them are suggested.
authors.affiliation: Institute of Chemistry, University of Tartu, Ravila 14a, 50411, Tartu, Estonia.
authors.forename: Vladislav
authors.initials: V
authors.lastname: Ivaništšev
authors.affiliation: Departamento de Física da Materia Condensada, Universidade de Santiago de Compostela, Campus Vida s/n E-15782, Santiago de Compostela, Spain.
authors.forename: Trinidad
authors.initials: T
authors.lastname: Méndez-Morales
authors.affiliation: Department of Chemistry, University of Cambridge, Lensfield Road, CB2 1EW, Cambridge, UK.
authors.forename: Ruth M
authors.initials: RM
authors.lastname: Lynden-Bell
authors.affiliation: Facultade de Ciencias, Universidade da Coruña, Campus A Zapateira s/n E-15071, A Coruña, Spain.
authors.forename: Oscar
authors.initials: O
authors.lastname: Cabeza
authors.affiliation: Departamento de Física da Materia Condensada, Universidade de Santiago de Compostela, Campus Vida s/n E-15782, Santiago de Compostela, Spain.
authors.forename: Luis J
authors.initials: LJ
authors.lastname: Gallego
authors.affiliation: Departamento de Física da Materia Condensada, Universidade de Santiago de Compostela, Campus Vida s/n E-15782, Santiago de Compostela, Spain.
authors.forename: Luis M
authors.initials: LM
authors.lastname: Varela
authors.affiliation: Department of Physics, Scottish Universities Physics Alliance (SUPA), Strathclyde University, John Anderson Building, 107 Rottenrow East, G4 0NG, Glasgow, UK. maxim.fedorov@strath.ac.uk.
authors.forename: Maxim V
authors.initials: MV
authors.lastname: Fedorov