Designing Ionic Conductive Elastomers Using Hydrophobic Networks and Hydrophilic Salt Hydrates with Improved Stability in Air

Abstract

Demonstrated examples of soft ionotronic devices include ionic spider webs, [2] ionic touch pads, [3,4] ionotronic luminescence, [5] ionotronic fibers, [6] triboelectric nanogenerators, [7,8] etc. Intrinsically stretchable ionic conductors that can tolerate large deformations are highly desirable for the development of high-performance soft ionotronics. Based on the liquid content, there are two types of ionic conductors, namely, liquid-rich ionic conductors and liquid-free ionic conductors.Liquid-rich ionic conductors, such as hydrogels containing dissolved salts and ionogels swollen by ionic liquids (ILs), which often contain over 90 wt.% liquid electrolytes, have demonstrated unique characteristics of high ionic conductivity, excellent stretchability, and optical transparency. Broad applications have been demonstrated with liquid-rich ionic conductors in the area of bioelectronics, [9,10] biomedical engineering, [11] soft anti-fogging devices, [12] sensing, [13] energy harvesting, [14] and ionotronic luminescent devices. [15] Nevertheless, liquids in such materials tend to leak and evaporate, which hampers the stable operation of liquid-rich ionic conductors over extended periods of time. For Existing soft ionic conductors fall into two distinct categories: liquid-rich ionic conductors containing large amounts of liquid electrolytes (≈70-90 wt.% water for hydrogels and ≈20-80 wt.% ionic liquids for ionogels), and liquidfree ionic conductors that do not contain liquid components (e.g., ionic conductive elastomers). However, they are often plagued by dehydration, leakage of toxic ionic liquids, and air aging. Here, using hydrophobic polymer networks and hydrophilic salt hydrates, ionic conductive elastomers (s-ICEs for short) containing only a tiny amount of bound water (≈1-5 wt.% are synthesized). Thanks to the small embedded water content, the s-ICEs are advantageous over liquid-rich ionic conductors in terms of enhanced mechanical/electrical stabilities and safety; they also outperform previously reported liquid-free ionic conductors by avoiding air-aging issues. The s-ICEs introduced here also show excellent stretchability, good elasticity, high fracture resistance, desirable optical transparency and ionic conductivity, which are comparable to those of state-of-the-art liquid-rich and liquid-free ionic conductors. With all the above advantages, the s-ICE represents an ideal material for practical applications of soft ionotronics in ambient conditions.