Ion transport mechanisms in salt‐doped polymerized zwitterionic electrolytes

Abstract

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

A molecular dynamics study of a fully zwitterionic copolymer/ionic liquid‐based electrolyte: Li+ transport mechanisms and ionic interactions

“…For mixtures containing ILs and lithium ions, Borodin and coworkers 87 found that the [Li] + transport is correlated to ion pair lifetime (or “residence time”) of the [Li] + − [Anion] − interaction, and that longer lifetimes lead to slower lithium diffusion. Similarly, for mixtures of PEO with ILs and Li‐salts, Mogurampelly and Ganesan 75 found that while the diffusion mechanism of the [Cation] + is correlated with the [Cation] + − [Anion] − lifetime, the [Li] + diffusion is more correlated to the oligomer segmental dynamics, agreeing with the structural organization of the system, in which the oligomer chains coordinate strongly to the lithium ions.…”

“…In ternary mixtures such as IL/Polymer/[Li] + , there is a strong interaction between the lithium ion and the polymer chains that leads to the decoupling of the IL anion –[Li] + interaction and consequently to changes in the ionic diffusion mechanisms as compared to the neat IL with salt. In a series of papers, 71–76 Ganesan and coworkers have shown this effect for different mixtures containing ILs and polymers. For a ternary system containing PEO/IL/[Li] + (IL = 1‐butyl‐3‐methylimidazolium hexafluorophosphate), 71 the authors observed that the increase in IL content enhances the ionic mobilities due to the weakening in the IL anion –[Li] + interaction.…”

“…Since the ZI gel is a complex system, composed by different ions, it is hard to characterize the lithium‐ion diffusion mechanism due to the several interactions between all the ions in the system. In this context, Ganesan and collaborators have done an extensive investigation of the ionic diffusivities in polymer/ILs‐based electrolytes, 71–76 in which they have found a variety of diffusion mechanisms and correlations between the ionic mobilities and the polymer dynamics. In the case of the PEO/[BMIM][PF 6 ]/[Li][PF 6 ] system, they observed that the addition of the IL leads to a larger enhancement of the IL ions mobilities than to the Li + ions, which is related to the stronger interactions between the lithium‐ion and the polymer chains.…”

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

“…The restrained electrostatic potential (RESP) was post‐processed through Multiwfn package 68,69 . The resulting partial charges were further scaled down by a factor of 0.8 for obtaining better agreement of dynamical properties with respect to experiments 47–49,70,71 …”

“…In a series of studies, we have used atomistic simulations to examine the mechanisms underlying ionic conductivity in a variety of polyIL based systems 33,43–49 . In this context, our most recent study considered the ion mobilities and transport mechanisms in salt‐doped polyILs 50 .…”