Ion Transport Mechanism of a Gel Electrolyte Comprising a Salt in Binary Plastic Crystalline Mixtures Confined inside a Polymer Network

Abstract

We discuss here the ion transport mechanism of a gel electrolyte comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solvated by two plastic crystalline solvents, one a solid (succinonitrile, abbreviated as SN) and another (a room temperature ionic liquid) (1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, (abbreviated as IL) confined inside a linear network of poly(methyl methacrylate) (PMMA). The concentration of the IL component (x) determines the physical properties of the unconfined electrolyte (i.e., SN1-xILx-LiTFSI) and when confined inside the polymer network (GPE-x). The extent of disorder in the SN1-xILx-LiTFSI and the GPE-x electrolytes is enhanced compared to both the bare SN-LiTFSI and IL-LiTFSI electrolytes. The enhanced disordering in the plastic phase alters both the local ion environment and viscosity. These changes strongly influence the ion mobility and nature of predominant charge carriers and thus the ion conduction mechanism in SN1-xILx-LiTFSI and GPE-x. The proposed SN1-xILx-LiTFSI and the GPE-x electrolytes show predominantly anion conduction (tTFSI ≈ 0.5); however, lithium transference number (tLi ≈ 0.2) is nearly an order higher than the IL-LiTFSI (tLi ≈ 0.02-0.06). The ionic conductivity of SN1-xILx-LiTFSI is much higher (especially for x ≈ 0.1) compared to both SN-LiTFSI and IL-LiTFSI. The ionic conductivity of the GPE-x, though lower than the unconfined SN1-xILx-LiTFSI electrolytes, is still very promising, displaying values of ∼10-3 Ω-1 cm-1. The GPE-x displayed compliable mechanical properties, stable Li-electrode/electrolyte interface, low rate of Al corrosion, and stable cyclability over several tens of charge-discharge cycles when assembled in a separator-free Li-graphite cell. The promising electrochemical performance further justifies the simple strategy of employing mixed physical state plasticizers to tune the physical properties of polymer electrolytes requisite for application in rechargeable batteries.