Burebi Yiming

Zhaoxin Zhang

Yuchen Lu

Xingang Liu

Costantino Creton
email: costantino.creton@espci.psl.eu

Shuze Zhu
email: shuzezhu@zju.edu.cn

Zheng Jia
email: zheng.jia@zju.edu.cn

Shaoxing Qu

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

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 long-term 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. LiClO 4 -1.0 stored in air for 24 h As-prepared LiClO 4 -1.0 LiClO 4 -1.0 LiClO 4 -0.5 LiTFSI-1.0 LiTFSI-0.5 Copolymer LiClO 4 -1.0 LiClO 4 -0.5 LiTFSI-1.0 LiTFSI-0.5 Copolymer (e) ambient air LiClO 4 -1.0 LiClO 4 -0.5 LiTFSI-1.0 LiTFSI-0.5

■ INTRODUCTION

The interdisciplinary field of soft ionotronics is advancing rapidly despite its recent origin 1 .

Significant achievements have been demonstrated with applications as diverse as biomimetics [START_REF] Hui | Ionic cable[END_REF][START_REF] Li | Fast-moving soft electronic fish[END_REF][START_REF] Lee | Ionic spiderwebs[END_REF][START_REF] Kim | Highly stretchable, transparent ionic touch panel[END_REF] , energy harvesting [START_REF] Xiong Pu | Ultrastretchable, transparent triboelectric nanogenerator as electronic skin for biomechanical energy harvesting and tactile sensing[END_REF][START_REF] Yongyuan Ren | Ionic liquid-based click-ionogels[END_REF][START_REF] Zhang | Transparent, and Thermally Stable Triboelectric Nanogenerators Based on Solvent-Free Ion-Conducting Elastomer Electrodes[END_REF] , multimodal sensing [START_REF] Gu | Integrated Soft Ionotronic Skin with Stretchable and Transparent Hydrogel-Elastomer Ionic Sensors for Hand-Motion Monitoring[END_REF][START_REF] Ying | An ambient-stable and stretchable ionic skin with multimodal sensation[END_REF][START_REF] Wang | A dual-trigger-mode ionic hydrogel sensor for contact or contactless motion recognition[END_REF][START_REF] Yiming | Ambiently and Mechanically Stable Ionogels for Soft Ionotronics[END_REF] , and other functional devices [START_REF] Kim | Highly stretchable, transparent ionic touch panel[END_REF][START_REF] Yang | Organic liquid-crystal devices based on ionic conductors[END_REF][START_REF] Lim | Ion-to-ion amplification through an open-junction ionic diode[END_REF][START_REF] Han | Dual pH-Responsive Hydrogel Actuator for Lipophilic Drug Delivery[END_REF][START_REF] Liu | Hydrogel Coating Enabling Mechanically Friendly, Step -Index, Functionalized Optical Fiber[END_REF][START_REF] Liu | Functional hydrogel coatings[END_REF] . As a key component of soft ionotronics, intrinsically stretchable ionic conductors are required for the development of high-performance soft ionotronic devices. The prevailing stretchable ionic conductors can be categorized into two types, namely, liquid-based (e.g., electrolyte water, ionic liquid) ionic conductors and liquid-free ionic conductors.

Hydrogels containing dissolved salts emerge as the prototypical example of liquid-based ionic conductors. Salt-containing hydrogels have demonstrated their advantages of high ionic conductivity, optical transparency, stretchability, and prominent biocompatibility, for a wide variety of applications such as skin-like ionotronic devices [18][START_REF] Lei | A Bioinspired Mineral Hydrogel as a Self-Healable, Mechanically Adaptable Ionic Skin for Highly Sensitive Pressure Sensing[END_REF][START_REF] Wang | Extremely stretchable and healable ionic conductive hydrogels fabricated by surface competitive coordination for human-motion detection[END_REF] , artificial axons 2 , triboelectric nanogenerators [START_REF] Xiong Pu | Ultrastretchable, transparent triboelectric nanogenerator as electronic skin for biomechanical energy harvesting and tactile sensing[END_REF] , and electroluminescent devices [START_REF] Larson | Highly stretchable electroluminescent skin for optical signaling and tactile sensing[END_REF][START_REF] Yang | Electroluminescence of Giant Stretchability[END_REF][START_REF] Yang | Ionotronic Luminescent Fibers, Fabrics, and Other Configurations[END_REF] . These devices, however, suffer from key shortcomings inherent to water-based electrolytes, which may easily evaporate or leak.

According to the present consensus, the introduction of salts or chemicals into hydrogels can only slow down but hardly prevent the evaporation of water, which are impediments to the stable operation of hydrogel-based ionotronic devices over long periods of time 1, [START_REF] Bai | Transparent hydrogel with enhanced water retention capacity by introducing highly hydratable salt[END_REF][START_REF] Yuk | Hydrogel bioelectronics[END_REF][START_REF] Liu | Hydrogel machines[END_REF] . In addition, ionotronic devices based on hydrogels are often plagued by weak adhesion of hydrogels to other functional materials, such as elastomers, metals, and ionogels, due to the fluid nature of water --the majority constituent of hydrogels --which changes neighbor readily and cannot transfer stresses. The last few years have witnessed transformative advances in achieving tough bonding of hydrogel to other materials, but the adhesion method still involves complicated and material-specific treatment processes [START_REF] Hyunwoo Yuk | Tough bonding of hydrogels to diverse non-porous surfaces[END_REF][START_REF] Takayuki Nonoyama | Double-Network Hydrogels Strongly Bondable to Bones by Spontaneous Osteogenesis Penetration[END_REF][START_REF] Liu | Bonding dissimilar polymer networks in various manufacturing processes[END_REF][START_REF] Gao | A Universal Strategy for Tough Adhesion of Wet Soft Material[END_REF][START_REF] Qi Ge | 3D printing of highly stretchable hydrogel with diverse UV curable polymers[END_REF] .

Another representative family of liquid-based ionic conductors is ionogels, which are polymeric gels swollen by ionic liquids (ILs). Most of the ILs are stable and nonvolatile, thereby endowing ionogels with high ionic conductivity, broader working temperature than hydrogels, wide electrochemical window, as well as good mechanical ruggedness [START_REF] Bideau | ionic liquid based hybrid materials[END_REF][START_REF] Wang | Ionic Gels and Their Applications in Stretchable Electronics[END_REF][34] . In this respect, various soft ionotronic devices that can work in harsh conditions have been demonstrated with ionogels. Examples include ionotronic sensors that can monitor mechanical motions in extreme environments (e.g., high vacuum and high humidity) [START_REF] Cao | Transparent, mechanically robust, and ultrastable ionogels enabled by hydrogen bonding between elastomers and ionic liquids[END_REF] , triboelectric nanogenerators that can operate at both high and low temperatures [START_REF] Yongyuan Ren | Ionic liquid-based click-ionogels[END_REF] , and ionic cables that can function under water [START_REF] Larson | Highly stretchable electroluminescent skin for optical signaling and tactile sensing[END_REF] .

However, one major problem faced by the ionogel system is the degradation of ionic conductivity under mechanical loads (such as squeezing) or after long-term exposure to open air, largely due to the leakage of IL [START_REF] Yiming | Ambiently and Mechanically Stable Ionogels for Soft Ionotronics[END_REF][START_REF] Lei | A highly transparent and ultra-stretchable conductor with stable conductivity during large deformation[END_REF][START_REF] Chen | Highly stretchable and transparent ionogels as nonvolatile conductors for dielectric elastomer transducers[END_REF] . Moreover, the electrical and mechanical properties of ionogels often conflictintroduction of IL, the key conducting constituent of ionogels, leads to enhanced ionic conductivity but diminished strength and modulus [START_REF] Kim | Highly stretchable, transparent ionic touch panel[END_REF][START_REF] Yiming | Ambiently and Mechanically Stable Ionogels for Soft Ionotronics[END_REF][START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF][START_REF] Kim | Ionoskins: Nonvolatile, Highly Transparent, Ultrastretchable Ionic Sensory Platforms for Wearable Electronics[END_REF][START_REF] Seo | Mechanically Robust, Highly Ionic Conductive Gels Based on Random Copolymers for Bending Durable Electrochemical Devices[END_REF] so that ionogels based on IL struggle to attain both properties.

In light of these challenges common to liquid-based ionic conductors, enormous efforts have been devoted to the development of liquid-free ionic conductors for soft ionotronics, by avoiding the incorporation of liquid components. Liquid-free ionoelastomers, in which either cations or anions are fixed to the elastomer network and the other ionic species are mobile, and ionic conductive elastomers (ICEs), which in contrast consist of elastomer networks and mobile cations and anions, have been reported in the last few years [START_REF] Seo | Mechanically Robust, Highly Ionic Conductive Gels Based on Random Copolymers for Bending Durable Electrochemical Devices[END_REF][START_REF] Shi | Highly stretchable and transparent ionic conducting elastomers[END_REF][START_REF] Hyeong Jun Kim | Ionoelastomer junctions between polymer networks of fixed anions and cations[END_REF][START_REF] Qu | Solid-state and liquid-free elastomeric ionic conductors with autonomous self-healing ability[END_REF][START_REF] Dang | Facile solvent-free synthesis of multifunctional and recyclable ionic conductive elastomers from small biomass molecules for green wearable electronics[END_REF][START_REF] Wang | Multifunctional Liquid-Free Ionic Conductive Elastomer Fabricated by Liquid Metal Induced Polymerization[END_REF][START_REF] Zhang | Dynamically Crosslinked Dry Ion-Conducting Elastomers for Soft Iontronics[END_REF] . These materials are liquid-free and thus do not suffer from limitations inherent to the liquid constituents. Very recently, we developed a new type of ICE, in which the elastomer networks host lithium cations and associated anions via lithium bonds and hydrogen bonds, respectively. The material outperforms other state-of-the-art ICEs in terms of mechanical properties, exhibiting high strength, fracture toughness, stretchability, self-healing ability, strong adhesion to many materials, and desirable 3D printability as well [START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF] . The discovery of mechanically robust ICEs paves the way for developing soft ionotronic devices with improved stability and durability relative to their liquid-based counterparts.

However, it is important to recognize that ionotronic devices inevitably work under ambient conditions in which the presence of water molecules is ubiquitous. ICEs rely predominantly on ions as charge carriers, and a wide variety of ions such as lithium cations (Li + ) are hygroscopic, capable of attracting water molecules from ambient environments and holding them among their polymer networks. For this reason, uncovering the effect of ambient water on the ICEs is essential for enabling ambient mass application of ICE-based ionotronic devices, and thus is of significant practical interest in the field of soft ionotronics. Yet while these new stretchable ionic conductors emerged around 2018 and have been thriving since 2020 [START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF][START_REF] Seo | Mechanically Robust, Highly Ionic Conductive Gels Based on Random Copolymers for Bending Durable Electrochemical Devices[END_REF][START_REF] Shi | Highly stretchable and transparent ionic conducting elastomers[END_REF][START_REF] Hyeong Jun Kim | Ionoelastomer junctions between polymer networks of fixed anions and cations[END_REF][START_REF] Qu | Solid-state and liquid-free elastomeric ionic conductors with autonomous self-healing ability[END_REF][START_REF] Dang | Facile solvent-free synthesis of multifunctional and recyclable ionic conductive elastomers from small biomass molecules for green wearable electronics[END_REF][START_REF] Wang | Multifunctional Liquid-Free Ionic Conductive Elastomer Fabricated by Liquid Metal Induced Polymerization[END_REF][START_REF] Zhang | Dynamically Crosslinked Dry Ion-Conducting Elastomers for Soft Iontronics[END_REF] , the influence of ambient water on ICEs in terms of their mechanical and electrical properties has not as yet been investigated. Here we demonstrate that ambient water molecules have a significant influence on the microstructure and performances of ICEs, endowing air-aged ICEs with stable mechanical properties and remarkably boosted conductivity. To decipher the underlying mechanism, we first prepare different types of ICEs and compare their mechanical and electrical performances before and after exposing them to ambient air for a prolonged time (up to 96 hours). In order to rationalize the observed changes in mechanical properties, we carried out atomistic-level simulations by LAMMPS to probe the effect of water molecules on intramolecular interactions between Li + , water molecules, and elastomer chains. Finally, we discuss the balance of properties of the ICE in comparison with existing other published studies.

■ MATERIALS AND METHODS

Materials.

Lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium perchlorate (LiClO 4 ), and Ethylene glycol methyl ether acrylate (MEA) were obtained from Shanghai Aladdin.

Isobornyl acrylate (IBA) and benzophenone (BP) were purchased from Shanghai Maklin and Nantong Feiyu, respectively. Synthesis of the Liquid-Free ICEs. We synthesize two groups of liquid-free ICEs using two representative lithium saltslithium bis(trifluoromethane)sulfonimide (LiTFSI) and lithium perchlorate (LiClO 4 ), respectively, by extending the method introduced previously [START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF] .

LiTFSI or LiClO 4 is first dissolved in liquid binary mixtures of ethylene glycol methyl ether acrylate (MEA) and isobornyl acrylate (IBA) (shown in Figure 1a). The molar fraction of IBA is fixed to be 0.2 in the binary mixtures and the molar concentration of lithium salts (i.e., LiTFSI and LiClO 4 ) is taken to be 0.5 M or 1 M in this work. 0.0052 M of benzophenone is added as the photo-initiator. The ICEs are then formed by ultraviolet-induced polymerization for 7.5 h in a nitrogen environment, without any added solvent. The obtained ICEs are named A-x, where A and x represent the name and molar concentration of the lithium salt contained in the ICEs, respectively; For example, LiClO 4 -1 refers to the ICE containing 1 M LiClO 4 . The resulting as-prepared ICEs are lightly crosslinked (due to chain transfer reactions) random copolymer networks of P(MEA-co-IBA) containing mobile cations and anions dispersed throughout the network, and contain no volatile components (Figure 1b). The composition of each sample is given in Table 1 with the molar and weight fraction of monomers and lithium salts. For all pure-shear samples, the width was 50 mm and the thickness was 2 mm, the distance between the two clamps was 5 mm. The pre-notched sample was prepared by using a razor and had a 25 mm-long single-edge notch. The tests were performed on a universal testing machine (Instron 3343) with a 50 N loading cell. A displacement rate of 100 mm/min was employed for all the tests, yielding a stretch rate of 0.14 s -1 for the uniaxial tension test and 0.33 s -1 for the pure shear tests.

Characterization of Water Uptake. As-prepared liquid-free samples (diameter: 25 mm; thickness: 2 mm) of the ICEs were stored in ambient conditions. Then the weight of the samples was measured at certain intervals (every 3.5 h for the first day, every 12 h for the second day, and every 24 h for the following days). The weight change of each sample was monitored for 4 days (i.e., 96 h).

Molecular Dynamics Simulations. Simulations are carried out using the LAMMPS package and visualization is performed through OVITO software. Recently developed REAX force field [START_REF] Liu | Effects of High and Low Salt Concentrations in Electrolytes at Lithium-Metal Anode Surfaces Using DFT-ReaxFF Hybrid Molecular Dynamics Method[END_REF] is used to simulate the interactions between lithium salts and polymer chains. We build a simulation box with periodic boundary conditions along the polymerization direction of MEA monomers (Figure S5). There are two polymer chains inside the simulation box, each is polymerized from 8 MEA monomers. The initial size of the simulation box is 1.9 ×1.8 ×1.6 nm 3 .

Then single cation, anion, or water molecule are added to the interface as needed to build corresponding models (e.g., Figure 3a). Such models have the molecular composition close to

ICEs used in experiments. The time step in MD is 0.25 fs. In the equilibrium stage, the system is equilibrated at 300 K for 50 ps in the NVT ensemble (Figure S6). In the deformation stage, a carbon atom is fixed on the lower MEA chain skeleton, and the carbon atoms on the upper MEA chain skeleton are displaced along the corresponding direction (Figure S16). For every interval of 0.01 nm in displacement, the energy of the system is minimized, after which the resultant total force of all atoms on the lower MEA chain along the displacement direction is calculated, so that a displacement-force curve is obtained. We change the initial relative position of the molecules (e.g., Li + , H 2 O) to establish different initial conditions and perform multiple simulations.

Electrical Characterization. A rectangular strip of ICE (dimensions: 40  5  2 mm 3 ) was prepared using a cutter. The bulk resistance of the samples was measured by a desktop multimeter (Keithley, DAQ 6510). The conductivity of the stretchable ICEs was then calculated by , where corresponds to the distance between the electrodes, denotes the cross-sectional area of the sample, and the bulk resistance measured by the multimeter. The conductivity of air-aged samples were measured every 3.5 h for the first day and every 12 h for the following days. The conductivity of each sample was monitored for four days (96 h).

■ RESULTS AND DISCUSSION

Basic properties of the as-prepared ICEs. Figure 1c shows the representative stress-stretch curves of the as-prepared ICEs and pure copolymer P (MEA-co-IBA) (i.e., the polymer backbone of ICEs) before being exposed to ambient air. Due to the low crosslinking level, the as-prepared

ICEs exhibit a very large stretch at break with . Interestingly, both groups of ICEs possess enhanced stiffness relative to the pure copolymer P (MEA-co-IBA) that contains no lithium salt as well as a pronounced strain hardening at large strain without losing the high stretch at break (Figure 1c, Figure S1 and Figure S2). This stiffening suggests the formation of additional physical bonds between polymer chains and dispersed ions in the ICEs. It is reasonable to hypothesize that strong interactions (namely, lithium bonds) form between Li + and carbonyl oxygens on the polymer backbone, and comparatively weaker hydrogen bonds form between anions (TFSI -or ClO 4 -

) and the chains (as illustrated in Figure 1b) [START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF] . Through these intramolecular interactions, Li Moreover, the ionic conductivity of as-prepared ICEs is lower than that of liquid-based ionic conductors such as ionogels by many orders of magnitude [START_REF] Yiming | Ambiently and Mechanically Stable Ionogels for Soft Ionotronics[END_REF][START_REF] Cao | Transparent, mechanically robust, and ultrastable ionogels enabled by hydrogen bonding between elastomers and ionic liquids[END_REF][START_REF] Shi | Shujiang Ding Highly Stretchable and Transparent Ionic Conductor with Novel Hydrophobicity and Extreme-Temperature Tolerance[END_REF]49 , mainly because of the lithium and hydrogen bonds formed between polymer chains and ions, which restrict both the motion of polymer chains and transport of ions and thus limit conductivity.

Mechanical properties of air-aged ICEs.

To explore the impact of ambient environments on the mechanical properties of the material, we expose the as-prepared ICEs as well as the pure copolymer P (MEA-co-IBA) to ambient air for 96 h and measure their stress-stretch curves every 24 h. As evident from Figure 2a, air-aged ICEs, which are stored in ambient environments for 24 h, soften as compared to their as-made counterparts (Figure S3). More specifically, Figure 2b and (3) After 24 h storage in ambient conditions, the mechanical properties of air-aged ICEs become rather stable, remaining at a comparable level to that of the pure copolymer. As a control experiment, we also age samples of as-made ICEs and copolymer in a dry nitrogen environment (water content<<0.01 ppm, oxygen content<<0.01 ppm), and found that, for the entire testing period of 96 h, the ICEs stored in the nitrogen environment maintain relatively stable mechanical properties (Figure S4). Despite the lower modulus and strength, all air-aged ICEs keep the same strain at break similar to ICE samples stored in nitrogen environments (Figure 2d) as summarized in Table 2. for LiClO 4 -1, and 0.35 wt% for LiClO 4 -0.5; The higher the content of lithium salt is, the more ambient water is absorbed (Figure 2e). Such a water absorption decreasing the mechanical properties is typically observed in semi-crystalline polymers containing hydrogen bonds or hydrophilic moieties and the best-known example is polyamides [START_REF] Silva | Study of the water absorption and its influence on the Young's modulus in a commercial polyamide[END_REF] . However, it is not commonly observed in elastomers to that extent with such a low amount of water absorption.

Table 2. Summary of mechanical and electrical properties of the air-aged ICEs and the as-prepared ICEs (liquid-free)

As-prepared Air-aged (24 h) LiTFSI-0.5 LiClO4-0. Atomistic origins of water-induced modification of air-aged ICEs. Since the high strength and modulus of liquid-free ICEs have been ascribed to dense arrays of physical bonds (namely, lithium bonds and hydrogen bonds) formed between polymer chains and ions [START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF] . We hypothesize that the observed changes in mechanical properties observed in air-aged ICEs are a consequence of physical bond scission due to the presence of water molecules. To ascertain our hypothesis and explore the atomistic origins of water-induced modification of the air-aged ICEs, we use molecular dynamics simulations to study the influence of lithium salt as well as water molecules on the interactions between neighboring polymer chains. The lithium salt considered is LiTFSI. We employ a REAX force field recently developed to simulate the effects of lithium salt concentrations in polymer electrolytes [START_REF] Liu | Effects of High and Low Salt Concentrations in Electrolytes at Lithium-Metal Anode Surfaces Using DFT-ReaxFF Hybrid Molecular Dynamics Method[END_REF] . The simulations are carried out in LAMMPS [START_REF] Plimpton | Fast Parallel Algorithms for Short-Range Molecular Dynamics[END_REF] . In order to construct models with molecular compositions close to ICEs in experiments, we convert the added amount of monomers and salt during synthesis, and the weight gain due to moisture absorption, into the molar ratio of each constituent component. We estimate that the molar ratio of monomers, LiTFSI, and water is roughly 16:1:1 for LiTFSI-0.5. We only consider the MEA monomers in the simulations because we expect interactions to occur with MEA which is also the majority component. To this end, our simulation is designed to qualitatively probe the inter-chain binding strength between two polymerized MEA chains, modulated by the existence of LiTFSI and water at the interface. To begin with, we build a simulation box with periodic boundary condition along the x-direction (Figure 3a), which is the polymerization direction (i.e., chain direction) of MEA monomers, so that effectively there are two polymer chains aligned in parallel.

Each polymer chain in the simulation box is polymerized from 8 MEA monomers (Figure S5).

Then, different interface models (Figure 3a) can be constructed depending on the content that is further added at the interface, while respecting the estimated molar ratio within the simulation box.

Since the ICEs are synthesized by dissolving lithium salts in the liquid mixture of monomers, it is expected that the cations and anions are spatially separated along polymer chains after polymerization. For this reason, we consider cations and anions separately in setting up the simulations.

The left panel of Figure 3a shows the Li + model in which a single lithium cation is added to the interface, corresponding to the molecular structure of as-prepared ICEs. The system is first equilibrated at 300K in the NVT ensemble in order to form conformed interfaces between the polymer chains and the lithium ion (Figure S6). Then we consider two deformation modes: the interface separation mode (mode 1 in Figure 3a), in which two chains are pulled apart in perpendicular to the backbone direction, and the interface shearing mode (mode 2 in Figure 3a), in which two chains are pulled apart parallel to the backbone direction. The middle panel of Figure 3a shows then the equivalent situation with the Li + /H 2 O model, in which a water molecule is added near the lithium ion. Finally, the right panel of Figure 3a shows the no-addition model, in which neither water molecule nor Li + is added. Note that the Li + /H 2 O model and no-addition model represent the microstructure of air-aged ICEs and pure copolymer, respectively. We apply the same simulation procedures for all three models and track the forces on the box along the pulling directions as a function of displacement.

Figure 3b shows representative displacement-force curves of the three models for mode 1.

All curves appear similar in the early stages when the pulling displacement is small (e.g., less than 0.1 nm). However, for the Li + model representing as-prepared ICEs, the force evolves to a sharp peak followed by a steep drop, and then decreases gradually to zero. This is in stark contrast with the Li + /H 2 O model (for air-aged ICEs) and the no-addition model (for pure copolymer), where the sharp peaks are not observed and the resultant forces remain relatively low (Figure 3b). To understand the atomistic origin of the sharp peak force in Li + model, we examine carefully the change in the molecular configuration of the interface before and after the occurrence of the sharp peak force (insets 1 and 2 of Figure 3b). We find that initially the lithium ion bonds simultaneously to both polymer chains via the carbonyl oxygens, forming two chain-connecting lithium bonds (two purple arrows in inset 1 of Figure 3b), which are maximally stretched as the force reaches the peak, until one chain-connecting lithium bond fractures (inset 2 of Figure 3b), giving rise to the sharp drop of the force. In contrast, in the Li + /H 2 O model, after the equilibration stage, we find that the water molecule bonds to the lithium ion (dashed lines in 1'-3' Figure 3b), leaving only one chain-h um b ( w 1'-3' in Figure 3b). In other words, in the presence of a nearby water molecule, the lithium ion is not able to simultaneously interact with both polymer chains. As a result, during chain separation, no lithium bonds are fractured and the resulting force is much smoother and lower than that of the Li + model (Figure 3b). Therefore, the formation of lithium ions that simultaneously bind to both polymer chains via chain-connecting lithium bonds is the key mechanism strengthening the interface as well as the entire as-prepared ICE. Due to the lack of such mechanism both in Li + /H 2 O model and in the no-addition model, the corresponding forces resisting deformation are much lower and broadly similar. This result is consistent the experimental observation in Figure 2b and 2c that as-prepared ICEs exhibit higher modulus and strength than air-aged ICEs and pure copolymer, and that the air-aged ICEs and pure copolymer have similar modulus and strength. We have done 10 groups of simulations with different initial conditions and all results support the above observation (Figure S7).

Figure 3c shows representative displacement-force curves of the three models for mode 2.

Two sharp peaks of force appear in the Li + model. The sharp peaks indicate the repeated scission and reforming of the chain-connecting lithium bonds, and can be seen from molecular configurations of the interface (insets 1-3 of Figure 3c). Initially the lithium ion bonds to both polymer chains, forming two chain-connecting lithium bonds (two purple arrows in inset 1 of Figure 3c), which are stretched to their critical state (the first peak) during polymer chain sliding, until one of the chain-connecting lithium bonds breaks (inset 2 of Figure 3c). As the sliding continues, when the lithium ion captures a new chain-connecting site, then a new chain-connecting lithium bond reforms (Figure 3c, inset 3), restoring a lithium ion that simultaneously links both polymer chains. The above scission and reforming processes repeats during the sliding process, giving rise to another sharp force peak (Figure 3c). A cascade of such lithium bond scission and reforming events can dissipate a significant amount of energy, and thus in high fracture toughness of as-prepared ICEs [START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF] . In contrast, for the Li + /H 2 O model, the lithium ion cannot simultaneously form lithium bonds with two polymer chains in the presence of nearby water molecule. Therefore, no lithium bonds are fractured during chain sliding (insets 1' 2' f Figure 3c), thereby giving rise to much smoother and lower resultant force. Likewise, due to the lack of energy dissipation mechanism (i.e., the scission and reforming of chain-connecting lithium bonds) in no-addition model, the resistant force is similar to that of the Li + /H 2 O model. 10 groups of simulations for mode 2 with different initial conditions have been carried out and all results agree well with the above observation (Figure S8).

For the case of TFSI -anions, the simulation results (Figures S9-S11) indicate that the weakening effect induced by the water molecule does not appear for the interface shearing mode (Figure S11), but remains present for the interface separation mode (Figure S10). In summary, results from molecular dynamics simulations strongly suggest that the molecular mechanism of water-induced modifications of the air-aged ICEs is mainly the scission of chain-connecting lithium bonds induced by water absorption. The dynamic lithium bonds between Li + and carbonyl oxygens on the polymer backbone prefer to dissociate when water molecules come nearby. Hence, the diffusing water molecules in the air-aged ICEs accelerate the exchange rate of the lithium bonds to a level far beyond the loading rate, thereby rendering the lithium bonds mechanically invisible and effectively diminishing the number of lithium bonds [START_REF] Mayumi | Fracture of dual crosslink gels with permanent and transient crosslinks[END_REF][START_REF] Kean | Increasing the maximum achievable strain of a covalent polymer gel through the addition of mechanically invisible cross-links[END_REF] , eventually giving rise to the water-induced modifications of the air-aged ICEs.

' 2' h w h h h -connecting lithium bond in the

Li + /H 2 O model remains stable, so that the energy dissipation mechanism due to bond breaking and reforming is absent.

Fracture resistance and elastic recovery of air-aged ICEs. Though affected by water molecules in ambient air, air-aged ICEs exhibit excellent fracture resistance and a highly elastic behavior with rapid recovery, an important property for practical applications of ICEs in ambient environments. We measure the fracture toughness of both as-prepared and air-aged ICEs using the pure-shear geometry [START_REF] Sun | Highly stretchable and tough hydrogels[END_REF] , as well as the work of fracture of the ICEs by the uniaxial tension test. Although the fracture toughness and work of fracture of the air-aged ICEs are smaller than that of as-made ICEs (Figure 4a and 4b), the air-aged ICEs still possess high fracture toughness.

For example, after absorbing environmental water molecules, the ICE LiClO 4 -1 exhibit a fracture toughness of ~7000 J/m 2 and work of fracture of ~10 MJ/m 3 (Figure 4a), exceeding that of most other state-of-the-art soft ionic conductors [START_REF] Yiming | Ambiently and Mechanically Stable Ionogels for Soft Ionotronics[END_REF][START_REF] Yang | Organic liquid-crystal devices based on ionic conductors[END_REF][START_REF] Shi | Shujiang Ding Highly Stretchable and Transparent Ionic Conductor with Novel Hydrophobicity and Extreme-Temperature Tolerance[END_REF][START_REF] Sun | Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity[END_REF][START_REF] Feng Luo | Oppositely Charged Polyelectrolytes Form Tough, Self-Healing, and Rebuildable Hydrogels[END_REF][START_REF] Hong | 3D Printing of Highly Stretchable and Tough Hydrogels into Complex, Cellularized Structures[END_REF] . We also evaluate the fractocohesive length of both as-prepared and air-aged ICEs, which is defined as the ratio of fracture toughness over work of fracture (i.e., ) [START_REF] Chen | Flaw sensitivity of highly stretchable materials[END_REF][START_REF] Yang | Polyacrylamide hydrogels. I. Network imperfection[END_REF] . The fractocohesive length characterizes the flaw sensitivity of materials: the stretchability of material is insensitive to any flaw of the size smaller than its fractocohesive length. LiTFSI-based ICEs present a fractocohesive length of ~0.46-0.48 mm, while the LiClO 4 -based ICEs have a fractocohesive length of ~0.4-0.7 mm and these values are comparable to those of the liquid-free as-prepared ICEs (Figure 4c).

To explore the elastic recovery of ICEs, we subject as-prepared and air-aged ICEs of all types to a cycle of loading and unloading with the maximum stretch of = 11 and a strain rate of 0.14 s -1 . The hysteresis of an ICE sample is represented by the dissipated energy , the area between the loading and unloading stress-stretch curves. Unlike as-prepared ICE samples, which exhibit hysteresis loops depending on both the type and concentration of lithium salt [START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF] , the air-aged ICEs exhibit almost identical hysteresis loops, regardless of the salt type and concentration (Figure 4d, Figure S12). This observation is consistent with the molecular dynamics simulation: As-prepared ICEs contain a large number of chain-connecting lithium bonds and hydrogen bonds connecting polymer chainsthe amount of which largely depends on the salt type and concentrationand the repeated breaking and reforming of these physical bonds dissipate energy [START_REF] Long | Time Dependent Behavior of a Dual Cross-Link Self-Healing Gel: Theory and Experiments[END_REF][START_REF] Guo | Mechanics of a Dual Cross-Link Gel with Dynamic Bonds: Steady State Kinetics and Large Deformation Effects[END_REF] . In contrast, relative to their as-prepared counterparts, air-aged ICEs effectively contain much fewer, if any, load-bearing lithium bonds linking adjacent chains due to the competition with water molecules. This difference also results in a lower residual stretches for air-aged ICEs at a given strain rate (Figure S13). The enhanced elastic recovery ratio may arise from the plasticizing effect of water molecules, which facilitate chain motion by effectively reducing chain-connecting lithium bonds, as informed by the molecular dynamics simulations.

To further investigate the elastic recovery behavior of air-aged ICEs under cyclic loadings, we cyclically stretch air-aged LiTFSI-0.5 to increasing maximum stretch levels ( = 3 to = 15). The ICE is highly elastic with fast self-recovery, having an ultimate recovery ratio of 93.2% and a corresponding residual stretch of only = 1.95 as released from = 15 (Figure 4e, Figure S14). The dissipated energy associated with each cycle increases almost linearly with (Figure 4f). The air-aged LiTFSI-0.5 also demonstrates low hysteresis and small residual strains when subjected to 100 successive loading-unloading cycles with = 6 and = 11 (Figure S15). These results indicate a highly elastic behavior of air-aged ICEs with rapid recovery and low hysteresis, a trait highly desirable for soft ionotronic devices. ambient waterno more than ~0.6 wt% for all types of ICEs tested in this work (Figure 2e), with the molar concentration of absorbed water molecules being comparable to that of Li cations in the ICEs. Notably, such a limited amount of ambient water uptake can boost the ionic conductivity of the air-aged ICEs by one to two orders of magnitude. For example, LiClO 4 -1 possesses an ionic conductivity of after 96 h exposure to ambient air, which is ~57 times larger than its conductivity measured in the as-prepared liquid-free state (Figure 5a, Table 2). The basic mechanism underpinning the enhanced conductivity can be understood as follows: In as-prepared ICEs, as revealed by the molecular dynamics simulation results, Li + acts as transient physical crosslinks to connect neighboring polymer chains via chain-connecting lithium bonds, restricting the motion of both the polymer chains and the ions. By contrast, in the presence of water molecules, Li + must exchange much faster with the carboxyl bonds due to the competition with water and effectively become much more mobile, thus leading to a significantly increased conductivity. The air-aged ICEs reported in this work exhibit a conductivity comparable to that of recently-reported ionogels [START_REF] Yiming | Ambiently and Mechanically Stable Ionogels for Soft Ionotronics[END_REF][START_REF] Cao | Transparent, mechanically robust, and ultrastable ionogels enabled by hydrogen bonding between elastomers and ionic liquids[END_REF][START_REF] Shi | Shujiang Ding Highly Stretchable and Transparent Ionic Conductor with Novel Hydrophobicity and Extreme-Temperature Tolerance[END_REF] , yet containing much less liquid content than ionogels do, thereby reducing the hassles of leaking liquid materials to a minimum. The conductivity of the air-aged ICEs and as-prepared ICEs are listed in Table 2 for reference.

Comparison of air-aged ICEs to other state-of-the-art soft ionic conductors. The development of ICEs started around 2018, and since then ICEs have been incorporated in many applications, including ionic skins, triboelectric nanogenerators, and touch screens [START_REF] Lei | A highly transparent and ultra-stretchable conductor with stable conductivity during large deformation[END_REF][START_REF] Yiming | A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer[END_REF][START_REF] Qu | Solid-state and liquid-free elastomeric ionic conductors with autonomous self-healing ability[END_REF][START_REF] Zhang | Dynamically Crosslinked Dry Ion-Conducting Elastomers for Soft Iontronics[END_REF][START_REF] Zhang | Transparent, and Thermally Stable Triboelectric Nanogenerators Based on Solvent -Free Ion -Conducting Elastomer Electrodes[END_REF] , owing to their liquid-free nature as well as ideal combination of mechanical and electrical characteristics. In this work, it is found that although the exposure to ambient water causes a reduction in modulus and strength of the ICEs, the stretchability remains the same, elastic recovery is slightly increased and the ionic conductivity is remarkably improved. In other words, the air-aged ICEs still present a desirable combination of stable mechanical properties and enhanced conductivity. To show this, we compare air-aged ICEs (namely, LiClO 4 -1) to other state-of-the-art ionic conductors, including liquid-free ICEs, ionoelastomers, and ionogels, in terms of Y u ' modulus, strength, stretchability, and conductivity. We found that the air-aged ICEs are a highly competitive candidate material for soft ionotronics since they possess an overall performance similar to the most advanced stretchable ionic conductors reported very recently (Figure 5b, Table S1). Thanks to the very limited amount of water absorbed from ambient air, the air-aged ICEs outperform most liquid-free ICEs and ionoelastomers in terms of conductivity and are superior to liquid-based ionic conductors (e.g., hydrogels and ionogels) for long-term applications as the latter inevitably suffer from liquid leakage and evaporation. ■ ASSOCIATED CONTENT

Supporting Information

Additional information and figures.

■ AUTHOR INFORMATION

Corresponding Author *

  + and associated anions act as transient physical crosslinks among polymer chains, ff v y h m ' stiffness without decreasing its extensibility. From this molecular picture, ICEs with higher molar concentration of lithium salts should be stiffer, in agreement with experimental measurements shown in Figure 1c. It is also found that the strength Y u ' m u u f L C O 4 -containing ICEs are roughly three times larger than that of their LiTFSI-containing counterparts with the same salt concentration (Figure S2), probably because ClO 4 -anions form a different clustering or are better dispersed in comparison with TSFI -anions.

Figure 1 .

 1 Figure 1. Molecular design and physical properties of the as-prepared liquid-free ICEs. (a)

Figure. 2c show

  Figure. 2c show how mechanical properties of the ICEs and copolymers evolve over time under

Figure 2 .

 2 Figure 2. Mechanical properties and weight change of air-aged ICEs. (a) Representative

Figure 3 .

 3 Figure 3. Atomistic origins of water-induced modification of air-aged ICEs. (a) Pre-equilibration

Figure 4 .

 4 Figure 4. Fracture resistance, hysteresis, and elastic recovery of air-aged ICEs. (a) Fracture

Figure 5 .

 5 Figure 5. Comparison of air-aged ICEs to other state-of-the-art soft ionic conductors. (a) The

Table 1 . Composition of materials synthesized in this work.

 1

		Weight fraction of the components(wt%)	Molar fraction of the components
	Sample name
		MEA	IBA	LiTFSI	LiClO4	MEA	IBA	LiTFSI	LiClO4
	Copolymer	71.42%	28.57%	0%	0%	80%	20%	0%	0%
	LiTFSI-0.5	62.5%	25%	12.5%	0%	74.60%	18.64%	6.76%	0%
	LiTFSI-1	55.55%	22.22%	22.23%	0%	69.87%	17.46%	12.67%	0%

E-mail: (C. Creton) costantino.creton@espci.psl.eu; (Z. Jia) zheng.jia@zju.edu.cn; (S.Z. Zhu) shuzezhu@zju.edu.cn

■ ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (Grant Nos. 11802269, 12072314, and 12002304), the 111 Project (Grant No. B21034), and the One-Hundred Talents Program of Zhejiang University.

Notes

The authors declare no competing financial interest.