Vitrimer ionogels towards sustainable solid-state electrolytes

Abstract

The growing demand for flexible, stretchable, and wearable devices has boosted the development of ionogels used as polymer electrolytes. Developing healable ionogels based on vitrimer chemistry is a promising approach to improve their lifetimes as these materials are usually subjected to repeated deformation during functioning and are susceptible to damage. In this work, we reported in the first place the preparation of polythioether vitrimer networks based on the not extensively studied associative S-transalkylation exchange reaction using thiol-ene Michael addition. Thanks to the exchange reaction of sulfonium salts with thioether nucleophiles, these materials demonstrated vitrimer properties such as healing and stress relaxation. The fabrication of dynamic polythioether ionogels was then demonstrated by loading 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM triflate) within the polymer network. The resulting ionogels exhibited Young's modulus of 0.9 MPa and ionic conductivities in the order of 10^-4 S cm^-1 at room temperature. It has been found that adding ionic liquids (ILs) changes the dynamic properties of the systems, most likely due to a dilution effect of the dynamic functions by the IL but also due to a screening effect of the alkyl sulfonium OBrs-couple by the ions of the IL itself. To the best of our knowledge, these are the first vitrimer ionogels based on an S-transalkylation exchange reaction. While the addition of ILs resulted in less efficient dynamic healing at a given temperature, these ionogels can provide materials with more dimensional stability at application temperatures and can potentially pave the way for the development of tunable dynamic ionogels for flexible electronics with a longer lifespan.

Scheme 1. Chemical structures of the chemicals and illustrations of polythioether vitrimer PTE-BuOBrs and dynamic ionogels PTE-BuOBrs-IL.

Fig. 1. (a) Rheological properties of PTE-BuOBrs5 and PTE precursor mixtures as a function of time during photopolymerization; (b) soluble fractions of PTE-BuOBrs samples after thermal treatment at 140 °C for 1.5 h as a function of alkylation agent content; (c) DMA curves of PTE-BuOBrs series samples and PTE control sample; (d) Young's modulus, tensile strength at yield and elongation at break extracted from the stress–strain test as a function of BuOBrs mol% of PTE-BuOBrs samples.

Fig. 2. Healing behavior of PTE-BuOBrs samples and PTE control sample at 140 °C during 2 h.

Fig. 3. (a) Stress relaxation behaviors of PTE-BuOBrs samples with 2 and 5 mol% of alkylation agent content and their control sample PTE at 140 °C; (b) stress relaxation curves of PTE-BuOBrs2 sample at different temperatures (c) stress relaxation curves of PTE-BuOBrs5 sample at different temperatures; (d) Arrhenius linear fit of relaxation times of PTE-BuOBrs2 and PTE-BuOBrs5 samples plotted as a function of 1000/T.

Fig. 4. (a) DMA tests of PTE-BuOBrs-IL ionogels and PTE-BuOBrs2 sample; (b) ionic conductivity of PTE-BuOBrs2-TFSI50 and PTE-BuOBrs2-Trif50 samples at different temperatures. Solid lines: calculation according to VTF equation.

Fig. 5. (a) Stress relaxation tests of PTE-BuOBrs2-TFSI50 sample at various temperatures; (b) stress relaxation tests of PTE-BuOBrs2-Trif50 sample at various temperatures; (c) Arrhenius linear plot extracted from relaxation times of PTE-BuOBrs2-TFSI50, PTE-BuOBrs2-TFSI50 and PTE-BuOBrs2 samples at different temperatures.

Fig. 6. (a) Pictures and the ionic conductivity behavior of PTE-BuOBrs2-TFSI50 before and after stacked healing; (b) pictures and the ionic conductivity behavior of PTE-BuOBrs2-Trif50 before and after stacked healing; (c) pictures and the tensile testing curves of PTE-BuOBrs2-TFSI50 before and after interfacial healing; (d) pictures and the tensile testing curves of PTE-BuOBrs2-Trif50 before and after interfacial healing; SEM image of the cross-section of two pieces of (e) PTE-BuOBrs2-TFSI50 after stacked healing and of (f) PTE-BuOBrs2-Trif50 after stacked healing.