Hubert Valencia - Ab initio study of EMIM-BF4 crystal interaction with a Li (100) surface as a model for ionic liquid/Li interfaces in Li-ion batteries

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  Hubert Valencia,  Masanori Kohyama,  Shingo Tanaka, and  Hajime Matsumoto, J. Chem. Phys.   131(24),  244705 (2009).


  We examined the atomic and electronic structures of an interface   between a 1-ethyl-3-methyl imidazolium tetrafluoroborate   (EMIM-BF4)   ionic-liquid crystal and a Li(100) surface by periodic   density-functional calculations, as a model for a   room-temperature ionic-liquid (RTIL) electrolyte/Li interface at   a Li-ion battery electrode. Results are compared with our   previous theoretical study of the EMIM-BF4  molecular adsorption on Li surfaces [H. Valencia et al.,   Phys. Rev. B 78,   205402 (2008)]. For the EMIM-BF4  crystal structure, the present projector augmented wave scheme   with the generalized gradient approximation can reproduce rather   correct intramolecular structures as well as satisfactory   short-ranged intermolecular distances, while long-range   intermolecular distances are overestimated due to the lack of   correct description of long-range dispersive interactions. We   constructed a coherent crystal/crystal interface model where four   EMIM-BF4  pairs are stacked on a       p(4×3)  Li (100) surface cell so as to simulate RTIL-layer deposition on   a Li surface. We observed significant attraction of surface Li   ions toward contacting           BF4  anions, counterbalanced by electron transfer toward           EMIM+  cations near the interface, revealing the tendency of easy   ionization of Li and           LixBF4  cluster formation, coupled with the reduction of           EMIM+.   These features are similar to those observed in the   EMIM-BF4  molecular adsorption, while these have been proved to occur in   the crystal-layer adsorption. We examined the adhesive energy,   wetability, and detailed electronic structure at the   crystal/crystal interface.

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