Mechanisms of Ion Transport in Lithium Salt-Doped Polymeric Ionic Liquid Electrolytes

Abstract

Recent experimental results have demonstrated that polymeric ionic liquids doped with Li salts exhibit enhanced ionic mobilities and lithium ion transference numbers with increasing salt concentrations. In this study, we used atomistic molecular dynamics simulations on a model system of lithium salt-doped 1-butyl-3-methyl-imidazolium bistriflimide ionic liquids and poly­(1-butyl-3-methyl-imidazolium bistriflimide) electrolytes to identify the molecular mechanisms underlying such findings. Our results mirror qualitatively the experimental results on the influence of salt doping on the ion mobilities. Further, a surprisingly stronger dependence (coupling) between the lithium ion mobilities and polymer segmental dynamics is observed relative to the coupling between the anion diffusivities and polymer dynamics. We present results for ion coordination and hopping characteristics to rationalize such behaviors and identify the mechanistic origins of the properties of this emerging class of polymer electrolytes.

Structure–Property Relationship of Polymerized Ionic Liquids for Solid-State Electrolyte Membranes

“…The molecular structure of the studied PIL with cationic polymer backbone excludes these oxygen-related ion-hopping mechanisms. Instead, TFSI related transport mechanisms of the lithium-ions are proposed in the literature [ 23 ]. Even through, it is not possible to distinguish between the anions and cations influence on the ionic conductivity in the performed EIS measurements, comparable PEO membranes appear to have superior overall ionic conductivities.…”

“…According to Zhang et al the movement of lithium-ions through a TFSI-PIL system is coupled to a coordination of the lithium cations to the TFSI anions in different simultaneously occurring transport mechanisms [ 23 ]. The lithium-ions can either move as a kind of naked cations from one TFSI-coordination shell to another (structural movement) or within a fixed TFSI-complex shell from one cationic PIL center to another (vehicular movement) [ 23 ]. With increasing lithium concentration the overall lithium mobility increases and the vehicular movement is getting more influence over the structural movement [ 23 ].…”

“…The lithium-ions can either move as a kind of naked cations from one TFSI-coordination shell to another (structural movement) or within a fixed TFSI-complex shell from one cationic PIL center to another (vehicular movement) [ 23 ]. With increasing lithium concentration the overall lithium mobility increases and the vehicular movement is getting more influence over the structural movement [ 23 ]. Within the studied systems, the ionic conductivity is most likely dominated by the TFSI anions, since there is a significant excess of anions within the free moving ions.…”

“…On the one hand, a higher anion density may lead to phenomena like electrostatic repulsion and therefore lead to lower ionic conductivity values in EIS. On the other hand, a higher LiTFSI content shifts the Li:TFSI ratio into direction of the lithium-ions, which may lead to a transport promoting coordination environment for the lithium-ions [ 23 ]. Therefore, a lower overall ionic conductivity but higher lithium mobility does not have to exclude each other.…”

Structure Code for Advanced Polymer Electrolyte in Lithium‐Ion Batteries

“…Ganesan and coworkers demonstrated in their atomistic molecular dynamics simulations that the vehicular mode is dominant in LiFSI‐doped poly(1‐butyl‐3‐methyl‐imidazolium bistriflimide) electrolytes. [ 57 ] They also suggested that polymer segmental dynamics is quite important in Li + mobility since the Li + is indirectly connected to the polymer chains through Li‐anion‐polycation bridges, indicating that the co‐coordination mode’ importance. Besides, Sokolov and coworkers provided with useful insights regarding ionic correlations in polyIL electrolytes.…”