Diddo Diddens, Andreas Heuer
The lithium transport mechanism in ternary polymer electrolytes, consisting of poly(ethylene oxide) (PEO), lithium-bis(trifluoromethane)sulfonimide (LiTFSI) and the ionic liquid N-methyl-N-propylpyrrolidinium TFSI is analyzed vi molecular dynamics (MD) simulations. Starting from the classical, binary electrolyte PEO/LiTFSI, we focus on two different strategies by which the ternary electrolytes can be devised, namely by (a) adding the ionic liquid to the binary system, and (b) substituting the PEO chains in PEO/LiTFSI by the ionic liquid. In order to elucidate in how far the microscopic lithium transport differs between these two electrolyte classes, we employ an analytical, Rouse-based cation transport model [Maitra et al., PRL 2007, 98, 227802], which has originally been devised for the binary systems. Within this framework, the cation transport is characterized via three different mechanisms, each quantified by an individual time scale. Our analysis demonstrates that this model is also applicable to the ternary electrolytes, allowing us to express the effect of the ionic liquid in terms of the time scales of the transport model. Whereas the addition of the ionic liquid to PEO/LiTFSI plasticizes the polymer network and thus also increases the lithium ion mobility, no significant effect can be observed when substituting the PEO chains by the ionic liquid. This is due to the fact that in the latter case the amount of free, mobile ether oxygens reduces, since more lithium ions coordinate to the PEO backbone, which compensates with the plasticizing effect. Thus, the segmental mobility displays a decisive role in polymer electrolytes. In total, our findings are agreement with recent experimental observations [Passerini et al., Electrochim. Acta 2012, in press].
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http://arxiv.org/abs/1211.3413
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