First-Principles Study of EMIM-FAFSA Molecule Adsorption on a Li(100) Surface as a Model for Li-Ion Battery Electrodes

Abstract

The atomic and electronic structures of 1-ethyl-3-methyl imidazolium [C6H11N2]+ (EMIM) fluoroalkyl-fluorosulfonyl amid (FAFSA) molecule adsorption on a Li(100) surface were examined using periodic density functional theory calculations, as a model for a room-temperature ionic-liquid (RTIL) electrolyte/Li-anode interface in a Li-ion battery. First, we examined the nature of isolated FAFSA anions and EMIM-FAFSA pairs for bis(fluorosulfonyl) amid [(F–SO2)2N]− (FSA), fluorosulfonyl(trifluoromethylsulfonyl) amide [(F3C–SO2)N(SO2–F)]− (FTA), bis(trifluoromethylsulfonyl) amid [(F3C–SO2)2N]− (TFSA), and 1,2,3-dithiazolidine-4,4,5,5-tetrafluoro-1,1,3,3-tetraoxide [−((F2C–SO2)N(SO2–CF2))−]− (CTFSA). These FAFSA– molecules except for CTFSA– with a ring structure have both trans and cis conformers. Free EMIM-FAFSA pairs prefer to form trans conformers, while cis conformers become more stable when the pairs are adsorbed on a Li(100) surface. The ion-pair adsorption on a Li(100) surface generally reveals the following features, essentially similar to the EMIM-BF4/Li case in our previous studies: the surface Li atoms under the FAFSA anion are remarkably attracted toward the anion, leading to O–Li or F–Li bond formation, while valence electrons around the Li atoms are transferred to the EMIM cation, leading to substantial reduction of EMIM+. The EMIM-FSA, EMIM-FTA, and EMIM-TFSA systems show similar features with systematic variations depending on the fluoralkyl substituent, while the EMIM-CTFSA pair shows somewhat different features. We discussed the relation between the present theoretical results and the experimental electric transport properties at RTIL/electrode interfaces.