Solid Electrolyte Interphase Formation at the Ionic Liquid Electrolyte-Lithium-Metal Interface Using an Ab Initio Molecular Dynamics Approach

An interfacial study is performed using ab initio molecular dynamics (AIMD) simulations to elucidate the electrochemical interfacial stability of an ionic liquid (IL) as component of an electrolyte in contact with a Li-metal anode when an additive, vinylene carbonate (VC), is added to the electrolyte. The IL electrolyte is composed of 1-butyl-1-methylpyrrolidinium bis(fluoro-sulfonyl)-imide (PYR14+FSI-) with lithium bis(trifluoromethane-sulfonyl)imide salt (Li+TFSI-) in a 8:2 concentration ratio. Reactions observed at the interface electrolyte/Li-metal during the AIMD simulations are verified by electronic structure calculations using density functional theory calculations considering a continuum solvent model. We found that the addition of VC to the IL prevents further reduction of the counter-ion TFSI- and retards reduction of FSI- in comparison to the case when the additive VC is absent. The PYR14+ reduction is never observed with or without the additive. The 1st, 2nd, and 3rd electron affinities of the PYR14+ are -0.08, -0.36 and -3.65 eV, indicating little to large repulsive behavior against electrons. The opposite is observed for the two negative ions, FSI- and TFSI-, which yield 1st electron affinities of 1.73 and 1.83 eV, respectively. Reactions obtained under open circuit conditions. Dissociation mechanisms are presented for all IL components reduced at the IL/Li-metal interface.