  We present an extensive molecular dynamics (MD) simulation study of the
lithium ion transport in ternary polymer electrolytes consisting of
poly(ethylene oxide) (PEO), lithium-bis(trifluoromethane)sulfonimide (LiTFSI)
and the ionic liquid N-methyl-N-propylpyrrolidinium
bis(trifluoromethane)sulfonimide (PYR13TFSI). In particular, we focus on two
different strategies by which the ternary electrolytes can be devised, namely
by (a) adding the ionic liquid to PEO20LiTFSI, and (b) substituting the PEO
chains in PEO20LiTFSI by the ionic liquid. In order to grasp the changes of the
overall lithium transport mechanism, we employ an analytical, Rouse-based
cation transport model (Maitra et al., Phys. Rev. Lett., 2007, 98, 227802),
which has originally been devised for binary PEO-based electrolytes. This model
distinguishes three different microscopic transport mechanisms, each quantified
by an individual time scale. In the course of our analysis, we extend this
mathematical description to account for an entirely new transport mechanism,
namely the TFSI-supported diffusion of lithium ions decoupled from the PEO
chains, which emerges for certain stoichiometries. We find that the segmental
mobility plays a decisive role in PEO-based polymer electrolytes. That is,
whereas the addition of the ionic liquid to PEO20LiTFSI plasticizes the polymer
network and thus also increases the lithium diffusion, the amount of free,
mobile ether oxygens reduces when substituting the PEO chains by the ionic
liquid, which compensates the plasticizing effect. In total, our observations
allow us to formulate some general principles about the lithium ion transport
mechanism in ternary polymer electrolytes. Moreover, our insights also shed
light on recent experimental observations (Joost et al., Electrochim. Acta,
2012, 86, 330).
