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Title: The Importance of Ion Packing on the Dynamics of Ionic Liquids during Micropore Charging

There is an emerging concern that using room-temperature ionic liquids (RTILs) together with microporous electrodes may compromise supercapacitors power density in spite of their benefit for enhancing energy density due to possibly slow transport of ions inside narrow pores. Based on molecular simulations of the diffusion of EMIM+ and TFSI ions in slit-shaped micropores (width < 2 nm,) under conditions similar to those during pore charging, we show that, in pores that accommodate only a single layer of ions, the ions diffuse increasingly faster as the pore becomes charged, even faster than Na^+ ions in bulk water. However, this trend can be reversed when the pore becomes highly charged. In pores wide enough to fit more than one layer of ions, the ion diffusion is typically slower than in the bulk, and only changes modestly as the pore becomes charged. Analysis of these results revealed that the fast (or slow) diffusion of ions inside a micropore is correlated most strongly with the dense (or loose) ion packing inside the pore during charging. The molecular details of ions and the precise width of pores modify these trends relatively weakly, except when the pore size is so narrow that the conformation ofmore » ions is strongly constrained by the pore walls. Insight from these results should be useful for establishing guidelines for the design of RTILs and porous electrode materials for supercapacitors.« less
Authors:
 [1] ;  [1] ;  [2] ;  [3] ;  [2] ;  [4] ;  [4]
  1. Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)
  2. Univ. of Utah, Salt Lake City, UT (United States)
  3. Army Research Lab., Adelphi, MD (United States)
  4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Publication Date:
OSTI Identifier:
1235003
Grant/Contract Number:
AC05-00OR22725
Type:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry Letters
Additional Journal Information:
Journal Volume: 7; Journal Issue: 1; Journal ID: ISSN 1948-7185
Publisher:
American Chemical Society
Research Org:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States
Language:
English