Molecular Mechanism Underpinning Stable Mechanical Performance and Enhanced Conductivity of Air-Aged Ionic Conductive Elastomers

Abstract

Ionic conductive elastomers (ICEs), thanks to their liquid-free nature, have emerged as one of the most promising candidates for conductors in soft ionotronics. Notably, most ionotronic devices need to work in ambient environments where the presence of water molecules is ubiquitous. Thus far, the longterm impact of ambient water on the performances of ICEs remains virtually unexplored. Here, we show that air-aged ICEs absorb a very low amount of environmental water (∼0.3−0.6 wt % of the ICEs), which endows ICEs with stable mechanical performance and strongly boosted conductivity. We study the underlying molecular mechanism and clarify that the scission of lithium bonds between lithium ions and elastomer chains provoked by diffusing water molecules accounts for the observed changes in ICE properties. This work provides guidance for the practical mass application of ICE-based soft ionotronics in ambient environments.

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

“…As the concentration of LiClO 4 ·3H 2 O increases from 0.25 to 1 M, the ionic conductivity at room temperature of the s‐ICEs can reach 1.95 × 10 −3 S m −1 (Figure 2f), orders of magnitude higher than that of many existing liquid‐free ICEs and ionoelastomers, [ 26,29 ] because the existence of water molecules – though in very limited amount (i.e., 3 water molecules per lithium ion) – can effectively facilitate the motion of lithium ions. [ 34 ]…”

“…For comparison, we synthesize two types of liquid‐free ICEs – P(MEA‐ co ‐IBA) networks containing lithium salt LiTFSI and LiClO 4 , respectively – and measure their stress‐strain curves after storing them in ambient air for 24 h. The liquid‐free ICEs exhibit degraded mechanical performance after 24 h of storage due to moisture absorption (Figure S8, Supporting Information). More evidence of the air‐aging behavior of liquid‐free ICEs containing LiTFSI or LiClO 4 salt has been provided in the literature, [ 34 ] revealing that water molecule‐assisted lithium bond breakage is the underlying mechanism for the degradation of the mechanical properties of liquid‐free ICEs under ambient conditions. [ 34 ] To further showcase the s‐ICEs’ tolerance to extremely dry conditions, we store s‐ICE‐0.5 samples in a humidity‐free nitrogen environment (water content <<0.01 PPm, oxygen content <<0.01 PPm).…”

“…More evidence of the air‐aging behavior of liquid‐free ICEs containing LiTFSI or LiClO 4 salt has been provided in the literature, [ 34 ] revealing that water molecule‐assisted lithium bond breakage is the underlying mechanism for the degradation of the mechanical properties of liquid‐free ICEs under ambient conditions. [ 34 ] To further showcase the s‐ICEs’ tolerance to extremely dry conditions, we store s‐ICE‐0.5 samples in a humidity‐free nitrogen environment (water content <<0.01 PPm, oxygen content <<0.01 PPm). It is observed that, after 72 h of storage, the s‐ICE samples do maintain their mechanical properties (Figure 4a‐c, Figure S7b, Supporting Information), showing that the mechanical performance of the s‐ICEs is insensitive to extremely dry conditions, in conspicuous contrast to liquid‐rich ionic conductors, especially hydrogels.…”

“…[ 27 ] Unfortunately, such an excellent combination of properties is only stable in dry conditions. In other words, the hydrophilic nature of Li + ions in the electrolyte salt makes liquid‐free ICEs moisture sensitive, [ 34 ] resulted in an evolving (decreased) stiffness and stress at break over time due to the plasticizing effect of water. [ 34 ] Yet, stability in ambient air is essential for applications in the presence of oxygen and water.…”

“…In other words, the hydrophilic nature of Li + ions in the electrolyte salt makes liquid‐free ICEs moisture sensitive, [ 34 ] resulted in an evolving (decreased) stiffness and stress at break over time due to the plasticizing effect of water. [ 34 ] Yet, stability in ambient air is essential for applications in the presence of oxygen and water. As a potential solution to this stability problem we developed copolymer networks swollen with hydrophobic ionic liquids such as [C 2 mim][NTf 2 ] and [BMMIm][TFSI] resulting in ambiently stable ionogels [ 16 ] but exhibiting a maximum strength of only ≈0.7 MPa.…”