Lithium Ion Transport Mechanism in Ternary Polymer Electrolyte-Ionic Liquid Mixtures: A Molecular Dynamics Simulation Study

Abstract

The lithium transport mechanism in ternary polymer electrolytes, consisting of PEO20LiTFSI and various fractions of the ionic liquid PYR13TFSI, is investigated by means of MD simulations. This is motivated by recent experimental findings 1 , which demonstrated that these materials display an enhanced lithium mobility relative to their binary counterpart PEO20LiTFSI. In order to grasp the underlying microscopic scenario giving rise to these observations, we employ an analytical, Rouse-based cation transport model 2 , 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.

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

“…We observe that τ 3 is about 20% shorter for m = 4 than for m = 1 due to the enhanced decoupling of lithium ions by Pyr 1mEO ions, resulting in both a slightly larger number of free PEO segments required to receive an ion as well as an increased decoupling frequency. It is worth noting that the τ 3 -values are significantly larger than in our previous study 17 (τ 3 ≈ 17 − 24 ns), which can be understood by the lower fraction of free EOs (required to receive a lithium ion) for a ratio of EO : Li = 10 : 1 (as opposed to EO : Li = 20 : 1 in Ref. 17).…”

“…This difference can essentially be related to the different EO : Li ratio (EO : Li = 20 : 1 in Ref. 17) resulting in fewer free EOs required to bridge two chains, as well as the stronger dilution of PEO in the present study, which both favors locally compact coordination shells at a single chain. Finally, about 25% (m = 1) and 30% (m = 4) of all lithium ions are at least partly coordinated by TFSI anions, either when being decoupled or attached to PEO.…”

“…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.…”