Abstract
Polymerized ionic liquids (PILs) doped with lithium salts have recently attracted research interests as the polymer component in lithium-ion batteries because of their high ionic mobilities and lithium-ion transference numbers. To date, although the ion transport mechanism in lithium-doped PILs has been considerably studied, the role of lithium salts on the dynamics of PIL chains remains poorly understood. Herein, we examine the thermal and rheological behaviors of the mixture of poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide (PC4-TFSI)/lithium TFSI (LiTFSI) in order to clarify the effect of the addition of LiTFSI. We show that the glass transition temperature Tg and the entanglement density decrease with the increase in LiTFSI concentration . These results indicate that LiTFSI acts as a plasticizer for PC4-TFSI. Comparison of the frequency dependence of the complex modulus under the iso-frictional condition reveals that the addition of LiTFSI does not modify the stress relaxation mechanism of PC4-TFSI, including its characteristic time scale. This suggests that the doped LiTFSI, component that can be carrier ions, is not so firmly bound to the polymer chain as it modifies the chain dynamics. In addition, a broadening of the loss modulus spectrum in the glass region occurs at high . This change in the spectrum can be caused by the responses of free TFSI and/or coordination complexes of Li and TFSI. Our detailed rheological analysis can extract the information of the dynamical features for PIL/salt mixtures and may provide helpful knowledge for the control of mechanical properties and ion mobilities in PILs.
Highlights
Polymerized ionic liquids (PILs), a subclass of solid polymer electrolytes, have advantageous properties of ionic liquids, such as flame-retardant, wide electrochemical window, and chemical stabilities, and can serve as a promising material for next-generation solid-state batteries [1,2,3,4,5,6]
At weight fractions of LiTFSI (wLiTFSI) = 0.3, no phase-separated structure was observed in the bright-field micrograph, and the cross-polarized micrograph is completely dark, indicating that Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is homogeneously mixed with PC4 -TFSI
(5) Comparison of the viscoelastic spectra at the reference temperature Tr * determined by the Williams Landel Ferry (WLF) analysis did not give the iso-frictional condition for PC4 -TFSI, suggesting a failure of our assumption that the free volume expansion factor is the same, independent of wLiTFSI
Summary
Polymerized ionic liquids (PILs), a subclass of solid polymer electrolytes, have advantageous properties of ionic liquids, such as flame-retardant, wide electrochemical window, and chemical stabilities, and can serve as a promising material for next-generation solid-state batteries [1,2,3,4,5,6]. The improvement in ionic conductivity was attributed to the decrease in the glass transition temperature of PIL/Li salt mixtures with increasing salt concentrations, caused by the modification of coordinated structures between polyions, counterions, and Li-ions [18,19,20] These results indicate that the mixture of PILs and Li salts can be an appealing solid-state polymer electrolyte alternative to conventional polyether/Li-salt mixtures. Zhang et al performed MD simulations on a PIL/Li salt system and indicated that even the charge-delocalized TFSI anions could coordinate with polymer cations as schematically shown in Figure 2 [19] From their simulation result, we assume that the intramolecular coordination structures depicted in Figure 2 can increase intramolecular cooperativity and possibly emphasize the sub-Rouse mode. We believe that a deeper understanding of the PIL dynamics will provide insights for improving the ionic conductivity and mechanical properties of PIL/Li-salt materials
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