Unveiling the Ion Conduction Mechanism in Imidazolium-Based Poly(ionic liquids): A Comprehensive Investigation of the Structure-to-Transport Interplay

Abstract

Polymerized ionic liquids (poly­(ILs)) are considered highly promising for the realization of high-performance and intrinsically safer electrolytes for rechargeable batteries due to their high charge density. However, to date little is known about the ion conduction mechanism for this class of solid polymer electrolytes (SPEs). Herein, we performed an in-depth characterization of a homologous series of 1-alkyl-3-vinylimidazolium bis­(trifluoro­methane)­sulfonimide-derived homopolymers, i.e., p­(C n VIm-TSI) with n = 2, 4, 6, 8, and 10, serving as a model compound family. A particular focus was set on the interplay of the physicochemical properties, nanostructure, and ionic conductivity as well as on the impact of the additional incorporation of a lithium salt, LiTFSI. The results reveal that the nanostructure of these self-assembling poly­(ILs) plays a decisive role for the ion conduction mechanism, allowing for a (partial) decoupling of charge transport and segmental relaxation of the polymer backbone.

Decoupling segmental relaxation and ionic conductivity for lithium-ion polymer electrolytes

“…6). Following the earlier work on rigid polymers [73][74][75][76][77] as well as rather recent studies, 88,89 Delhorbe et al 87 assigned this finding to the frustration in polymer packing related to the bulky imidazolium cation being is closely bonded to the polymer backbone, which result in sufficient free volume for the anions to move. Moreover, they reported additional studies for such poly(IL)s including LiTFSI, showing that the addition of the lithium salt may not lead to substantial changes in the ion conduction mechanism.…”

“…While Winey and co-workers 86 suggested that this energy barrier increases with an increasing alkyl chain length, Delhorbe et al 87 investigated an extended set of C n VImTFSI poly(IL)s with n = 2, 4, 6, 8, and 10, showing that such "linear" conclusions have to be taken with care. According to their results, the ion conduction in such systems is governed by a rather complex interplay of physicochemical properties (including the T g ), the charge carrier concentration, and the eventual formation of nanostructures.…”

“…Walden plot (molar conductivity vs. fluidity) for different C n VIm-TFSI-derived poly(IL)s with n = 2, 4, 6, 8, and 10; the "ideal" Walden line with a slope of 1, obtained for 10 mM aqueous solution of LiCl serves as reference (reprinted with permission from Delhorbe et al87 Copyright 2017 American Chemical Society).…”

Influence of side chain linker length on ion‐transport properties of polymeric ionic liquids

“…1) for a similar imidazolium-based polyIL. 27,28 Together, such observations have raised the question, 29 "what are the mechanisms underlying the influence of the linker side-chain length on the transport properties of polyILs? "…”

Photopolymerization of Ionic Liquids – A Mutually Beneficial Approach for Materials Fabrication

“…While mostly imagined as solvents and/or electrolytes, [17,18] their use as a monomer is more recent. A variety of polyelectrolytes or "poly-ionic liquids" (PILs) have been synthesized from IL precursors and used as ion conductors, [19][20][21][22] binders, [23][24][25][26] metal stabilizers, [27,28] among others. [29] Their preparation is typical of other free-radical polymerizations and can be controlled to create block-copolymers, [30,31] dendrimers/branched poly-mers, [32,33] and others.…”