  The lithium transport mechanism in ternary polymer electrolytes, consisting
of PEO/LiTFSI and various fractions of the ionic liquid
N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonimide, are
investigated by means of MD simulations. This is motivated by recent
experimental findings [Passerini et al., Electrochim. Acta 2012, 86, 330-338],
which demonstrated that these materials display an enhanced lithium mobility
relative to their binary counterpart PEO/LiTFSI. In order to grasp the
underlying microscopic scenario giving rise to these observations, we employ an
analytical, Rouse-based cation transport model [Maitra at al., PRL 2007, 98,
227802], which has originally been devised for conventional polymer
electrolytes. This model describes the cation transport via three different
mechanisms, each characterized by an individual time scale. It turns out that
also in the ternary electrolytes essentially all lithium ions are coordinated
by PEO chains, thus ruling out a transport mechanism enhanced by the presence
of ionic-liquid molecules. Rather, the plasticizing effect of the ionic liquid
contributes to the increased lithium mobility by enhancing the dynamics of the
PEO chains and consequently also the motion of the attached ions. Additional
focus is laid on the prediction of lithium diffusion coefficients from the
simulation data for various chain lengths and the comparison with experimental
data, thus demonstrating the broad applicability of our approach.
