Simulation Study of the Lithium Ion Transport Mechanism in Ternary Polymer Electrolytes: The Critical Role of the Segmental Mobility

Abstract

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. 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).

Improving the Lithium Ion Transport in Polymer Electrolytes by Functionalized Ionic-Liquid Additives: Simulations and Modeling

“…Similar issues have also been discussed in Ref. 18, where larger τ 1 -values have been found if the EO : Li ratio is lowered (here, values of up to τ 1 ≈ 300 ns have been found for EO : Li = 8 : 1 and N = 54, thus roughly falling into the same range as the values in Table I). …”

“…The slightly lower value for m = 4 can essentially be attributed to the implicit dependence of τ 1 on τ 3 due to the extrapolation. Consistently, n 2 (t) is basically identical for both m. Compared with the electrolytes from our previous publications 17,18,35 with EO : Li = 20 : 1 (τ 1 ≈ 130-150 ns), this mechanism is significantly hampered due to the reduced number of free monomers and the associated crowding of the PEO chains by lithium ions. Similar issues have also been discussed in Ref.…”

“…That is, although Raman measurements indicated an increased coordination between lithium ions and TFSI anions, 12 our previous molecular dynamics (MD) simulations 17,18 of PEO/LiTFSI electrolytes mixed with Nmethyl-N-propyl pyrrolidinium TFSI (Pyr 13 TFSI) revealed that TFSI coordinates lithium only partially, while the remainder of the lithium coordination sphere is comprised of PEO oxygens. Consequently, virtually all lithium ions remain coordinated to PEO.…”

“…17,18 This model is based on both Figure 2. Sketch of the microscopic lithium transport mechanisms taken into account by the analytical model.…”

“…29,34,35 Alternatively, the lithium ions can also be partly or entirely coordinated to the respective anions of the lithium salt, depending on the salt's tendency to form ion pairs or ion clusters, which in turn affects the lithium dynamics 36 (with TFSI being a weakly coordinating anion, as also reflected by our previous simulations 35 ). Due to the choice of the conventional pyrrolidinium ion with alkyl substituents in the ternary SPEs investigated so far experimentally [14][15][16] and numerically, 17,18 the IL cation is essentially non-coordinating. In the present case, however, the EOs from the side chain of Pyr 1mEO can also coordinate to the lithium ions, and possibly even decouple them from the PEO chains as motivated above.…”