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Abstract
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.