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

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  Hubert Valencia, Masanori Kohyama, Shingo Tanaka, and Hajime   Matsumoto, J. Phys. Chem. C, 116 (15), pp 8493–8509 (2012).


  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.

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