The Role of Energy Scales for the Structure of Ionic Liquids at Electrified Interfaces -- A Theory-Based Approach

Abstract

Ionic liquids offer unique bulk and interfacial characteristics as battery electrolytes. Our continuum approach naturally describes the electrolyte on a macroscale. An integral formulation for the molecular repulsion,which can be quantitatively determined by both experimental and theoretical methods, models the electrolyteon the nanoscale. In this article, we perform a systematic series expansion of this integral formulation, derive a description of chemical potentials in terms of higher-order concentration gradients, and rationalize the appearance of fourth-order derivative-operators in modified Poisson equations, recently proposed in this context. In this way, we formulate a rigorous multi-scale methodology from atomistic quantum chemistry calculations to phenomenologic continuum models. We apply our generalized framework to ionic liquids near electrified interfaces and perform analytic asymptotic analysis. Three energy scales describing electrostatic forces between ions, molecular repulsion, and thermal motion determine the shape and width of the long-ranging charged double layer. We classify the charge screening mechanisms dependent on the system parameters dielectricity, ionsize, interaction strength, and temperature. We find that the charge density of electrochemical double layers in ionic liquids either decays exponentially, for negligible molecular repulsion, or oscillates continuously. Charge ordering across several ion-diameters occurs if the repulsion between molecules is comparable with thermal energy and Coulomb interaction. Eventually, phase separation of the bulk electrolyte into ionic layers emerges once the molecular repulsion becomes dominant. Our framework predicts the exact phase boundaries between these three phases as function of temperature, dielectricity and ion-sizes.