Li-diffusion at the interface between Li-metal and [Pyr₁₄][TFSI]-ionic liquid: Ab initio molecular dynamics simulations

We previously reported comprehensive density functional theory-molecular dynamics (DFT-MD) at 400 K to determine the composition and structure of the solid electrolyte interface (SEI) between a Li anode and [Pyr₁₄][TFSI] ionic liquid. In this paper, we examined diffusion rates in both the Li-electrode region and SEI compact layer in smaller 83Li/2[TFSI] and larger 164Li/4[TFSI] systems. At 400 K, the Li-diffusion constant in the Li-region is 1.35 × 10^-10 m²/s for 83Li/2[TFSI] and 5.64 × 10^-10 m²/s for 164Li/4[TFSI], while for the SEI it is 0.33 × 10^-10 m²/s and 0.22 × 10^-10 m²/s, thus about one order slower in the SEI compared to the Li-region. This Li-diffusion is dominated by hopping from the neighbor shell of one F or O to the neighbor shell of another. Comparing the Li-diffusion at different temperatures, we find that the activation energy is 0.03 and 0.11 eV for the Li-region in the smaller and larger systems, respectively, while for the SEI it is 0.09 and 0.06 eV.