Ambiently and Mechanically Stable Ionogels for Soft Ionotronics

Abstract

Stretchable ionic conductors such as hydrogels and ionic‐liquid‐based gels (aka ionogels) have garnered great attention as they enable the development of soft ionotronics. Notably, soft ionotronic devices inevitably operate in humid environments or under mechanical loads. However, many previously reported hydrogels and ionogels, however, are unstable in environments with varying humidity levels owing to hydrophilicity, and their liquid components (i.e., ionic liquid, water) may leak easily from polymer matrices under mechanical loads, causing deterioration of device performance. This work presents novel hydrophobic ionogels with strong ionic liquid retention capability. The ionogels are ambiently and mechanically stable, capable of not absorbing moisture in environments with high relative humidity and almost not losing liquid components during long periods of mechanical loading. Moreover, the ionogels exhibit desirable conductivity (10−4–10−5 S cm−1), large rupturing strain (>2000%), moderate fractocohesive length (0.51–1.03 mm), and wide working temperature range (−60 to 200 °C). An ionic skin is further designed by integrating the concept of sensory artificial skins and triboelectric nanogenerators, which can convert multiple stimuli into various types of signals, including resistance, capacitance, short‐circuit current, and open‐circuit voltage. This work may open new avenues for the development of soft ionotronics with stable performance.

Hydrogels as Soft Ionic Conductors in Flexible and Wearable Triboelectric Nanogenerators

“…In comparison with the single network hydrogels, double network hydrogels have gained more attention in recent years because of the advantageous mechanical and electric properties. Instead of focusing on these general two-polymer or copolymer structures, [77][78][79] our discussion focuses on identifying more complex/unique compositional examples. For instance, Panwar et al [51] have synthesized a carboxymethyl chitosan (CMCh) /carboxymethyl cellulose/PDA (Ch-CMC-PDA) hydrogel by combining CMCh and CMC-D-PDA based on physical and covalent interactions, as shown in Figure 5c.…”

“…In comparison with the single network hydrogels, double network hydrogels have gained more attention in recent years because of the advantageous mechanical and electric properties. Instead of focusing on these general two‐polymer or co‐polymer structures, [ 77–79 ] our discussion focuses on identifying more complex/unique compositional examples. For instance, Panwar et al.…”

“…Reproduced with permission. [ 79 ] Copyright 2021, Wiley‐VCH. g) Molecular structure of the PBA/PEGDA/LiTFSI gel, mechanism of ion transport, weight change of the hydrogel, and TENGs.…”

“…The same group have also used ionic liquids to prepare stable ionogels that have the potential to reach higher temperatures not available with volatile solvents due to the vapor pressure. [ 79 ] The detailed molecular structures of these materials are shown in Figure 7f. Results with high conductivity and a wide operating temperature window (−60–200 °C) have been observed in the area of solvent‐free ionogels.…”

“…f) Molecular structures of elements for preparing ionogels. Reproduced with permission [79]. Copyright 2021, Wiley-VCH.…”