Simulating the Effect of Anions on Spreading of Nanodroplets and the Monolayer Behavior of Quaternary Ammonium-Based Ionic Liquids on Li(100) and Li(110) Metal Facets

Choice of ionic liquids (ILs) with [NTf₂]⁻ versus [FSI]⁻ (stability-promoting and fluidity-promoting, respectively) anions have recently attracted attention in different applications for issues related to inertness, conductivity, viscosity, electrode stability, and environmental hazards. We used molecular dynamics (MD) simulation and studied the spreading behavior of nanodroplets of quaternary ammonium-based ILs, triethylpentylammoniumbis­(trifluoromethylsulfonyl)­imide ([N₂₂₂₅]­[NTf₂]) and triethylpentylammoniumbis­(fluorosulfonyl)­imide ([N₂₂₂₅]­[FSI]), on the electrochemically favorable Li metal substrate with (100) and (110) facets. The effect of the anion type and the facet type on the spreading and monolayer formation is well demonstrated by noting that only [N₂₂₂₅]­[FSI] on Li(110), among others, spreads and forms a monolayer at room temperature (298 K) spontaneously within 1/2 time of the (long-time) simulation performed. The fourfold symmetry of the surface structure of the Li(100) facet introduces a barrier to the spreading at 298 K as compared to the rectangular symmetry of Li(110) facet. Correlation functions and density profiles of the monolayers confirm that the major difference in the properties of the two anions is related to the differences in the tendency of each anion F and O atoms toward the Li atom facets. The results of natural bond orbital (NBO) analysis indicate a higher anion–cation charge transfer in [N₂₂₂₅]­[FSI]. The energy of binding of IL with the Li(110) surface is larger than Li(100) and is in the order of [N₂₂₂₅]­[NTf₂] > [N₂₂₂₅]­[FSI]. The lower surface density and higher surface roughness of the kinked Li(100) facet compared to Li(110) is responsible for this. Furthermore, the more negative partial charges (and higher electron occupancy) of F’s atoms in [FSI]⁻ then in [NTf₂]⁻ can make it potentially more reactive toward Li(100) upon spreading the IL, consistent with the observation in the literature. However, the less structural hindrance in [FSI]⁻ than in [NTf₂]⁻ may moderate such a reaction. According to simulations of the IL monolayer on (100) and (110) facets, the density of the layer immediate to the solid surface (resembling the Stern layer) is templated by the density and morphology of the solid surface, which especially makes the diffusion coefficient higher for [N₂₂₂₅]­[FSI] IL and the highest on the Li(110) substrate.