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

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Polyzwitterions (polyZIs), macromolecules with repeating ampholytic monomers, are a novel class of materials with attractive properties for battery electrolytes. In this study, we probe the ion transport characteristics and underlying mechanisms in two salt‐doped (Li+‐TFSI−) polyZIs of similar composition with contrasting zwitterion (ZI) ionic organization: pendant monomers organized via backbone‐anion‐cation (B‐ZI−‐ZI+, Motif B) and backbone‐cation‐anion (B‐ZI+‐ZI−, Motif C). Within both Motifs B and C, the counterion of the pendant‐end ZI moiety shows higher mobility. Similarly, when comparing Li+ or TFSI− across motifs, it is seen that the respective pendant‐end counterion possesses higher mobility than its backbone‐adjacent counterpart. Furthermore, when comparing counterions to same‐position ZI moieties, TFSI− is seen to possess higher mobility than Li+ in each case, a result rationalized by invoking the lower interaction strength between the TFSI− and ZI+. Analysis of ion‐transport mechanisms demonstrate that the mobility of countercharges to the pendant‐end ZI moiety correlates with the ion‐association relaxation timescale, similar to ideas noted in polymerized ionic liquids in past studies. However, the mobility of countercharges to the backbone‐adjacent ZI moiety is shown to be correlated with a cage relaxation time, which incorporates the combined effects of frustrated motion due to the presence of the polymer backbone and pendant‐end ZI moiety and the higher mobility in a population of lightly ZI‐coordinated ions. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 578–588

Section: Introductionsupporting

confidence: 91%

Section: Resultsmentioning

confidence: 97%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Ion transport mechanisms in salt‐doped polymerized zwitterionic electrolytes

Keith

Ganesan

2020

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Polarization of ionic liquid and polymer and its implications for polymerized ionic liquids: An overview towards a new theory and simulation

Gao

Itliong

Kumar

et al. 2021

Ionic liquid (IL)‐containing polymers garner attention for electrochemical applications. This article overviews recent experimental and theoretical studies of polymer electrolytes that would be likely to cultivate new theoretical and computational frameworks for IL‐containing polymers. The first two sections outline the uniqueness of ILs that differentiates them from inorganic salts in polymers and explore deviation from the concept of the metaphor “room‐temperature molten salt.” Such distinct properties include (1) large intrinsic dipole moment and electronic polarizability, (2) hydrogen bonding, (3) π‐interactions, (4) a broad distribution of charges over the entire ion, and (5) the anisotropy of the ions. Moreover, the complexity of these properties substantially increases when the ions are polymerized. Indeed, their exceptional features would overcome the hurdle due to a trade‐off between ionic conductivity and mechanical robustness in inorganic salt‐doped polymers. Given these facts, the rest of the article focuses on emerging trends in the study of the dielectric response, phase separation, ion conductivity, and mechanical robustness of the polymer electrolytes, highlighting outstanding observations in experiments that may inspire existing theory and simulation. Our discussion also includes improving computational complexity for IL‐containing polymers. To this end, recent machine learning studies that consider ILs and polymer liquids are presented.

Mechanisms of ion transport in lithium salt‐doped polymeric ionic liquid electrolytes at higher salt concentrations

Zhang

Lin

Ganesan

2021

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We used atomistic simulations to study the mechanisms of ion transport in salt‐doped polymeric ionic liquid systems at higher salt concentrations. Consistent with the experimental observations, our simulations indicate that at higher salt concentrations, the anion mobilities become lower than that of the lithium cations. Further, the anion mobilities become relatively insensitive to the salt concentration, while the mobilities of lithium increase with increasing salt concentration. We rationalize the results for the anion mobilities by considering the fractions of anions which are exclusively coordinated with the polycations (Type1); co‐coordinated with cations and lithium (Type2); and those exclusively coordinated with lithium (Type3). By considering the coordination characteristics of the different types of anions and their hopping motions, we demonstrate that the net anion mobilities results from a compensation effect of the salt concentration dependence of the mobilities of the different anions. With respect to the mobilities of the lithium ions, we demonstrate that the latter moves primarily by a structural diffusion mechanism involving refreshing of the solvation shell during hopping. Further, for the majority of the lithium ions, the solvation shell is shown to be comprised of co‐coordinated Type2 anions, and that the number of polycations and the unique polymer chains involved in such coordination decreases with increasing salt concentration. Such changes are shown to weaken the solvation shell around the lithium, thereby facilitating faster ion motion. Together, our results suggest that systems in which the anion which exhibits a stronger coordination to the polycation in comparison to that of the lithium can facilitate higher transference numbers without a concomitant reduction in the mechanical strength.