Diddo Diddens, Andreas Heuer
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, in press], 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 facilitated by the ionic-liquid molecules only. Rather, the reason for the enhanced lithium mobility can be found in the plasticizing effect of the ionic liquid, which enhances the dynamics of the PEO chains and thus 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.
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http://arxiv.org/abs/1211.3290
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