Ion transport, mechanical properties and relaxation dynamics in structural battery electrolytes consisting of an imidazolium protic ionic liquid confined into a methacrylate polymer

Abstract

The effect of confining a liquid electrolyte into a polymer matrix was studied by means of Raman spectroscopy, differential scanning calorimetry, temperature-modulated differential scanning calorimetry, dielectric spectroscopy, and rheology. The polymer matrix was obtained from thermal curing ethoxylated bisphenol A dimethacrylate while the liquid electrolyte consisted of a protic ionic liquid based on the ethyl-imidazolium cation [C2HIm] and the bis(trifluoromethanesulfonyl)imide [TFSI] anion, doped with LiTFSI salt. We report that the confined liquid phase exhibits the following characteristics: (i) a distinctly reduced degree of crystallinity; (ii) a broader distribution of relaxation times; (iii) reduced dielectric strength; (iv) a reduced cooperativity length scale at the liquid-to-glass transition temperature (T g); and (v) up-speeded local T g-related ion dynamics. The latter is indicative of weak interfacial interactions between the two nanophases and a strong geometrical confinement effect, which dictates both the ion dynamics and the coupled structural relaxation, hence lowering Tg by about 4 K. We also find that at room temperature, the ionic conductivity of the structural electrolyte achieves a value of 0.13 mS/cm, one decade lower than the corresponding bulk electrolyte. Three mobile ions (Im+, TFSI-, and Li+) contribute to the measured ionic conductivity, implicitly reducing the Li+ transference number. In addition, we report that the investigated solid polymer electrolytes exhibit the shear modulus needed for transferring the mechanical load to the carbon fibers in a structural battery. Based on these findings, we conclude that optimized microphase-separated polymer electrolytes, including a protic ionic liquid, are promising for the development of novel multifunctional electrolytes for use in future structural batteries.

Low-Temperature Structural Battery Electrolytes Produced by Polymerization-Induced Phase Separation

“…These SBE systems should simultaneously possess high ionic conductivity, high mechanical stiffness/strength, safe operation, and electrode compatibility. PIPS based on SBEs consist of two coexisting phases (bicontinuous): a solid phase that provides high mechanical stiffness and a liquid phase that provides high ionic conductivity. − The mass or volume fraction of the solid phase can be used to tune the ionic conductivity and mechanical properties in which a trade-off exists between the two properties. − , Mechanical stiffness implies rigid polymer chains while ionic conductivity is facilitated by polymer flexibility . One early example of a bicontinuous electrolyte was applied to supercapacitors; specifically, a commercial epoxy resin cured in the presence of lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) and an ionic liquid electrolyte resulted in a conductivity of 4.00 × 10 –5 to 1.30 × 10 –4 S/cm at 30 °C and a modulus of 0.19 to 0.15 GPa, depending on the relative mass fraction .…”