Abstract
We present a theoretical study combining molecular dynamics (MD) simulations with an analytical lithium ion transport model [Maitra and Heuer, Phys. Rev. Lett. 2007, 98, 227802] to highlight a novel strategy to increase the lithium mobility in polymer electrolytes based on poly(ethylene oxide) (PEO). This is achieved by using a pyrrolidinium-based ionic liquid (IL) where the cation has been chemically functionalized by a short oligoether side chain [von Zamory et al., Phys. Chem. Chem. Phys. 2016, 18(31), 21539] as an additive. Since the oligoether moieties at the pyrrolidinium cations form pronounced coordinations to the lithium ions for sufficiently long side chains, the ions can be detached from the PEO backbone. In this way, a fundamentally new lithium ion transport mechanism is established (shuttling mechanism), in which the lithium dynamics is decoupled from the polymer dynamics, the latter typically being slow under experimental conditions. Based on our simulations, we incorporate this novel mechanism into our existing model, which accurately reproduces the observed lithium dynamics. We demonstrate that the use of oligoether-functionalized IL additives significantly increases the lithium diffusivity. Finally, we show that for experimentally relevant electrolytes containing long polymer chains, an even stronger increase of the lithium mobility can be expected.
Highlights
This is achieved by a using a chemically functionalized ionic liquid (IL), in which the pyrrolidinium cation bears a short oligoether substituent,[20] which for a sufficient number of monomers can coordinate to the lithium ions and even detach them from the poly(ethylene oxide) (PEO) backbones
This contrasts the microscopic transport mechanism in conventional SPEs, where all ion transport processes occur at the PEO chains
It turned out that for a side-chain length of m = 1 monomer, no stable coordination between lithium ions and the IL cation can be established, while for m = 4, about 16% of the lithium ions were decoupled from PEO
Summary
Solid polymer electrolytes (SPEs) are promising candidates for lithium metal batteries, which are optimal energy storages to power electric vehicles due to their high specific capacity.[1,2] Apart from electromotive applications, high energy densities are beneficial for portable electronic devices. Three different microscopic transport mechanisms typically found for PEO-based polymer electrolytes[29,33,34,35] are taken into account, of which one mechanism is composed of two subcontributions To assess their individual impact on the overall lithium dynamics, each mechanism is quantified by a characteristic time scale (see sketch in Figure 2): First, the lithium ions diffuse along the backbone of the coordinating PEO chains. One would ideally determine D1 at t = τ3 to capture the total number of PEO monomers a given lithium ion has traveled while being coordinated to a particular chain This time scale exceeds the length of our simulations, which is why we extrapolated the scaling n2(t) ∝ t0.8 observed in Figure 4 to t = τ3 in order to estimate D1(t = τ3) and, correspondingly, τ1 (Table I). To extract the corresponding time scale τ2, the Rouse expression for the segmental MSD, g2 (t )
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