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

Abstract

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.

Influence of Polarizability on the Structure, Dynamic Characteristics, and Ion-Transport Mechanisms in Polymeric Ionic Liquids

“…In our previous studies, we have used nonpolarizable atomistic molecular dynamics simulations to study the ion-transport mechanisms in polyILs and related systems, such as the origin of the ion-transport mechanism; the influence of the polymer chain length, types of counterions, and morphology on the ion-transport mechanism; − the influence of the lithium salt concentration or monomeric ionic liquid concentration on the ion-transport mechanisms; − and the influence of ion correlations on the inverse Haven ratio . As has been common practice, the lack of polarizability effects was compensated for by a simple charge scaling of the charged groups. ,,,, Our studies demonstrated that ion transport in such systems is governed by a hopping process involving ion pair association/dissociation of the mobile ion associated with counterions from distinct polymer chains.…”

“…As has been common practice, the lack of polarizability effects was compensated for by a simple charge scaling of the charged groups. ,,,, Our studies demonstrated that ion transport in such systems is governed by a hopping process involving ion pair association/dissociation of the mobile ion associated with counterions from distinct polymer chains. Such simulations were used to rationalize a number of experimental observations relating to the behavior of conductivity in simple homopolymeric polyILs, − , diblock copolymeric polyILs, , and salt-doped polyILs. − …”

Polymer Architecture-Induced Trade-off between Conductivities and Transference Numbers in Salt-Doped Polymeric Ionic Liquids

“…Such cocoordinations were shown to speed up the polymer segmental dynamics, and as a result, the dynamics of cocoordinated anions and lithium ions. Such results were shown to explain the salt concentration dependencies of anion and lithium mobilities in PBVIM systems. , In SI Section S4, we demonstrate that for the salt concentrations probed here, the lithium ions in both PVBMIM and PBVIM systems exist almost entirely (more than 96%) in such cocoordinated structures. Hence, we focus on the movement of such cocoordinated ions to understand the origin of the results in Figure .…”

“…We speculate that such differences arise from the much higher temperatures (600 K) employed in our studies relative to the Having established qualitative agreement between our simulation results and the experimental observations, we now turn to probe the mechanisms underlying the differences between the PVBMIM and PBVIM systems. Our earlier studies have examined the structural features of salt-doped PBVIM systems 59,60 and demonstrated the formation of cation−anion−lithium−anion−cation cocoordinations, which replace the cation−anion−cation coordinations in pure polyILs. Such cocoordinations were shown to speed up the polymer segmental dynamics, and as a result, the dynamics of cocoordinated anions and lithium ions.…”

Impact of Ion–Ion Correlated Motion on Salt Transport in Solvated Ion Exchange Membranes

“…Simulations were performed with five different concentrations of NaCl in the membrane, ranging from 0.04 to 1.00 M (0.08 to 2.00 mol normalL of H 2 normalO ), chosen to probe a range of concentrations from relatively dilute to non-dilute conditions. A multistep equilibration procedure, − inspired by ref , was applied to equilibrate the glassy polymer, and 30 independent systems were simulated at each condition tested to limit potential bias due to the equilibrated structure. Herein, membrane salt concentration, C +– , refers to the 1:1 concentration of salt (i.e., excluding Na + counterions).…”