Computational and Experimental Study of Li-Doped Ionic Liquids at Electrified Interfaces

Abstract

We evaluate the influence of Li-salt doping on the dynamics, capacitance, and structure of three ionic liquid electrolytes, [pyr14]­[TFSI], [pyr13]­[FSI], and [EMIM]­[BF4], using molecular dynamics and polarizable force fields. In this respect, our focus is on the properties of the electric double layer (EDL) formed by the electrolytes at the electrode surface as a function of surface potential (Ψ). The rates of EDL formation are found to be on the order of hundreds of picoseconds and only slightly influenced by the addition of Li salt. The EDLs of three electrolytes are shown to have different energy storage capacities, which we relate to the EDL formation free energy. The differential capacitance obtained from our computations exhibits asymmetry about the potential of zero charge and is consistent with the camel-like profiles noted from mean field theories and experiments on metallic electrodes. The introduction of Li salt reduces the noted asymmetry in the differential capacitance profile. Complementary experimental capacitance measurements have been made on our three electrolytes in their neat forms and with Li salt. The measurements, performed on glassy carbon electrodes, produce U-like profiles, and Li-salt doping is shown to strongly affect capacitance at high magnitudes of Ψ. Differences in the theoretical and experimental shapes and magnitudes of capacitance are rationalized in terms of the electrode surface and pseudocapacitive effects. In both neat and Li-doped liquids, the details of the computational capacitance profile are well described by Ψ-induced changes in the density and molecular orientation of ions in the molecular layer closest to the electrode. Our results suggest that the addition of Li+ induces disorder in the EDL, which originates from the strong binding of anions to Li+. An in-depth analysis of the distribution of Li+ in the EDL reveals that it does not readily enter the molecular layer at the electrode surface, preferring instead to be localized farther away from the surface in the second molecular layer. This behavior is validated through an analysis of the free energy of Li+ solvation as a function of distance from the electrode. Free energy wells are found to coincide with localized concentrations of Li+, the depths of which increase with Ψ and suggest a source of impedance for Li+ to reach the electrode. Finally, we make predictions of the specific energy at ideal graphite utilizing the computed capacitance and previously derived electrochemical windows of the liquids.

Adding salt to expand voltage window of humid ionic liquids

“…As the potential becomes more negative, the anions become fewer and further away from the electrode (see the peak height and location of anion distribution in Supplementary Fig. 5), so that Li + ions get reduced attraction from anions; meanwhile, owing to the stronger electrostatic interaction with the charged electrode, the Li + ions could overcome the energy barrier by IL ion layer 32 and then be attracted to the electrode surface (Fig. 1f).…”

“…5 and 7 ), since the intermolecular energy of ion pair, computed by interaction energies between cation and anions within its solvation shell 53 , reveals that the Li + -anion interaction (around −370 kJ mol −1 ) is stronger than that of [Pyr 13 ] + -[TFSI] − pair (about −220 kJ mol −1 ). Under positive polarization, the potential well shows a distinct valley at around 0.71 nm in the second ion layer, due to a balance between a strong repulsion to the positive electrode and sufficient binding from the first anion layer 32 , corresponding with the Li + number density profile.

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A molecular dynamics study of a fully zwitterionic copolymer/ionic liquid‐based electrolyte: Li+ transport mechanisms and ionic interactions

“…The formation of aggregates results in an extensive network composed of [Li] + and anions interacting with each other, leading to a slowing down effect of the ion dynamics. For example, Haskins and coworkers 82 interaction in Figure 7(A), it is possible to see peaks at distances lower than 1.0 nm, which clearly shows the formation of these aggregates.…”

Boosting Rechargeable Batteries R&D by Multiscale Modeling: Myth or Reality?

“…Red and blue bars indicate the (x,y) plane of the anode and cathode surfaces, respectively. Reproduced from ref (243). Copyright 2016 American Chemical Society.…”