Borophene vs. graphene interfaces: Tuning the electric double layer in ionic liquids

Abstract

In this work we perform molecular dynamics simulations of mixtures of a prototypical protic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), with lithium tetrafluoroborate (LiBF4), confined between two borophene walls of three different surface charges, −1, 0 and +1 e/nm2, where e is the elementary charge. The properties of the system are analyzed by means of ionic density profiles, angular orientations of [BMIM]+ cations close to the wall and vibrational densities of states for the salt cations close to the walls. The lateral structure of the first layer close to the surface is also studied on one hand, calculating Minkowski parameters and the Shannon entropy of the patterns of the 2D density maps of the anions placed there and, on the other hand, computing the 2D-Fourier transform of the positions of these anions. Our results are compared with those obtained previously for the same mixtures confined between two graphene walls. Although similarities exist between both cases, interesting differences are observed in the lateral structure that the ionic liquid adopts near borophene interfaces due to their strong anisotropy. In particular, we have observed that borophene induces more markedly ordered 2D patterns in the innermost layer of the ionic liquid electric double layer, specially when they are charged. It is this feature that makes borophene a potential candidate for battery electrode applications with possibilities beyond those of graphene.