The interaction of ethylammonium tetrafluoroborate [EtNH3+][BF4−] ionic liquid on the Li(001) surface: towards understanding early SEI formation on Li metal

Abstract

The electrode cyclability of high energy density Li–metal batteries can be significantly improved with the use of ionic liquid (IL) based electrolytes, which can ameliorate device issues through the suppression of dendrite initiation and propagation. This enhancement is often attributed to the formation of a stable solid electrolyte interphase (SEI) layer between the electrode and the electrolyte. In this paper, we have modelled the adsorption of the IL ethylammonium tetrafluoroborate [EtNH₃⁺][BF₄⁻] on a Li(001) surface, using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations to capture the initial stages of the SEI layer formation, and gain a greater insight into the stability of [EtNH₃⁺][BF₄⁻] on a lithium surface. Eleven unique minimum energy structures of the [EtNH₃⁺][BF₄⁻] pair adsorbed on the Li(001) surface were found, having binding energies between −1.80 eV to −1.58 eV. The interface between the electrolyte molecules and electrode surface were stabilized by the formation of Li–F bonds between the anion and Li surface leading to formation of Li_x–BF₄ clusters, where x = 2–4. This was accompanied by a transfer of charge from the lithium surface to the cation and anion. The thermal stability of the IL was investigated via AIMD simulations, and the IL was found not to spontaneously dissociate on the surface at room temperature or at an elevated temperature of 157 °C within the examined simulation time of 4.64 ps, with Li_x–BF₄ clusters forming early into the simulations (<1 ps). These findings provide useful information for future development of Li–metal batteries.