Polymer Actuators Using Ion-Gel Electrolytes Prepared by Self-Assembly of ABA-Triblock Copolymers

Well-defined ABA-triblock copolymers, polystyrene-<i>block</i>-poly­(methyl methacrylate)-<i>block-</i>polystyrene (SMS), which have two different polystyrene (PSt) weight fractions (<i>f</i>_PSt), were synthesized by successive atom-transfer radical polymerizations. Ion gels consisting of SMS and an ionic liquid, (1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­amide [C₂mim]­[NTf₂]), were prepared using the cosolvent evaporation method with tetrahydrofuran. Atomic force microscope images of the ion gels indicated that PSt is phase-separated to form sphere domains that serve as physical cross-linking points because PSt is not compatible with [C₂mim]­[NTf₂], while a continuous poly­(methyl methacrylate) (PMMA) phase with dissolved [C₂mim]­[NTf₂] is formed to serve as ion conduction paths. Accordingly, the ion gels are formed by the self-assembly of SMS and the preferential dissolution of [C₂mim]­[NTf₂] into the PMMA phase. The viscoelastic properties of the gels can be easily controlled by changing <i>f</i>_PSt in SMS and [C₂mim]­[NTf₂] concentration in the ion gels. The ion gels that exhibit high ionic conductivities (>10^–3 S cm^–1) at room temperature were used as an electrolyte of an ionic polymer actuator, which has a trilaminar structure consisting of the ion-gel electrolyte sandwiched between two composite carbon electrodes containing high-surface-area activated carbon powders. By applying low voltages (<3.0 V) to the electrodes, the actuator exhibited a soft bending motion toward the anodic side

Driving Mechanisms of Ionic Polymer Actuators Having Electric Double Layer Capacitor Structures

Two solid polymer electrolytes, composed of a polyether-segmented polyurethaneurea (PEUU) and either a lithium salt (lithium bis­(trifluoromethanesulfonyl)­amide: Li­[NTf₂]) or a nonvolatile ionic liquid (1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­amide: [C₂mim]­[NTf₂]), were prepared in order to utilize them as ionic polymer actuators. These salts were preferentially dissolved in the polyether phases. The ionic transport mechanism of the polyethers was discussed in terms of the diffusion coefficients and ionic transference numbers of the incorporated ions, which were estimated by means of pulsed-field gradient spin–echo (PGSE) NMR. There was a distinct difference in the ionic transport properties of each polymer electrolyte owing to the difference in the magnitude of interactions between the cations and the polyether. The anionic diffusion coefficient was much faster than that of the cation in the polyether/Li­[NTf₂] electrolyte, whereas the cation diffused faster than the anion in the polyether/[C₂mim]­[NTf₂] electrolyte. Ionic polymer actuators, which have a solid-state electric-double-layer-capacitor (EDLC) structure, were prepared using these polymer electrolyte membranes and ubiquitous carbon materials such as activated carbon and acetylene black. On the basis of the difference in the motional direction of each actuator against applied voltages, a simple model of the actuation mechanisms was proposed by taking the difference in ionic transport properties into consideration. This model discriminated the behavior of the actuators in terms of the products of transference numbers and ionic volumes. The experimentally observed behavior of the actuators was successfully explained by this model

Ionic Liquid Electrolytes for Lithium–Sulfur Batteries

A variety of binary mixtures of aprotic ionic liquids (ILs) and lithium salts were thoroughly studied as electrolytes for rechargeable lithium–sulfur (Li–S) batteries. The saturation solubility of sulfur and lithium polysulfides (Li₂S_<i>m</i>), the active materials in the Li–S battery, in the electrolytes was quantitatively determined, and the performance of the Li–S battery using the electrolytes was also investigated. Although the solubility of nonionic sulfur was low in all of the electrolytes evaluated, the solubility of Li₂S_<i>m</i> in the IL-based electrolyte was strongly dependent on the anionic structure, and the difference in the solubility could be rationalized in terms of the donor ability of the IL solvent. Dissolution of Li₂S_<i>m</i> in the ILs with strong donor ability was comparable to that achieved with typical organic electrolytes; the strongly donating IL electrolyte did not prevent redox shuttle reaction in the Li–S cells. The battery performance was also influenced by unfavorable side reactions of the anions (such as tetrafluoroborate (BF₄^–) and bis­(fluorosulfonylamide) ([FSA]⁻)) with polysulfides and by slow mass transport in viscous ILs, even though the dissolution of Li₂S_<i>m</i> into the IL electrolyte was greatly suppressed. Among the IL-based electrolytes, the low-viscosity [TFSA]-based ILs facilitated stable charge/discharge of the Li–S batteries with high capacity and high Coulombic efficiency. The unique <i>solvent effect</i> of the ILs can thus be exploited in the Li–S battery by judicious selection of ILs that exhibit high lithium-ion-transport ability and electrochemical stability in the presence of Li₂S_<i>m</i>

Effect of Variation in Anion Type and Glyme Length on the Nanostructure of the Solvate Ionic Liquid/Graphite Interface as a Function of Potential

Atomic force microscope (AFM) force curves are used to probe the effect of anion species and glyme length on the nanostructure of the solvate ionic liquid (SIL)/highly ordered pyrolytic graphite (HOPG) interface as a function of applied potential. At all potentials, the lithium tetraglyme bis­(trifluoromethylsulfonyl)­imide (Li­(G4)­TFSI)/HOPG is more structured than lithium tetraglyme bis­(perfluoroethylsulfonyl)­imide (Li­(G4)­BETI)/HOPG because [BETI]⁻ has greater conformational flexibility. The Li­(G3) trifluoroacetate (TFA)/HOPG interface is even more disordered because [TFA]⁻ coordinates strongly to the lithium ion, leading to a high concentration of free glyme. The Li­(G3)­TFSI/HOPG interface is more structured than the Li­(G4)­TFSI/HOPG interface because the longer glyme increases the molecular flexibility of the complex cation. The Li­(G1)₂TFSI/HOPG interface has weak interfacial structure because monoglyme is poorly coordinating so the free glyme concentration is high. Despite Li­(G3)­TFSI, Li­(G4)­TFSI, and Li­(G4)­BETI being good SILs (meaning the free glyme concentration is low), application of a negative potential to the HOPG surface leads to the desolvation of Li⁺ from the glyme at the surface

Printable Polymer Actuators from Ionic Liquid, Soluble Polyimide, and Ubiquitous Carbon Materials

We present here printable high-performance polymer actuators comprising ionic liquid (IL), soluble polyimide, and ubiquitous carbon materials. Polymer electrolytes with high ionic conductivity and reliable mechanical strength are required for high-performance polymer actuators. The developed polymer electrolytes comprised a soluble sulfonated polyimide (SPI) and IL, 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­amide ([C₂mim]­[NTf₂]), and they exhibited acceptable ionic conductivity up to 1 × 10^–3 S cm^–1 and favorable mechanical properties (elastic modulus >1 × 10⁷ Pa). Polymer actuators based on SPI/[C₂mim]­[NTf₂] electrolytes were prepared using inexpensive activated carbon (AC) together with highly electron-conducting carbon such as acetylene black (AB), vapor grown carbon fiber (VGCF), and Ketjen black (KB). The resulting polymer actuators have a trilaminar electric double-layer capacitor structure, consisting of a polymer electrolyte layer sandwiched between carbon electrode layers. Displacement, response speed, and durability of the actuators depended on the combination of carbons. Especially the actuators with mixed AC/KB carbon electrodes exhibited relatively large displacement and high-speed response, and they kept 80% of the initial displacement even after more than 5000 cycles. The generated force of the actuators correlated with the elastic modulus of SPI/[C₂mim]­[NTf₂] electrolytes. The displacement of the actuators was proportional to the accumulated electric charge in the electrodes, regardless of carbon materials, and agreed well with the previously proposed displacement model

Solubility of Poly(methyl methacrylate) in Ionic Liquids in Relation to Solvent Parameters

The solubility of poly­(methyl methacrylate) (PMMA) in 1-alkyl-3-methylimidazolium ionic liquids (ILs) with different anionic structures has been explored. Nearly monodisperse PMMA-grafted silica nanoparticles (PMMA-<i>g</i>-NPs) were used as a measurement probe for evaluating the PMMA solubility in ILs. The hydrodynamic radius (<i>R</i>_h) of PMMA-<i>g</i>-NPs was measured in the ILs by dynamic light scattering (DLS). Changes in <i>R</i>_h and colloidal stability, that is, the PMMA-solubility change in the ILs, were observed depending on the ionic structures of the ILs. The solubility was mainly affected by the anionic structures of the ILs rather than by the alkyl chain length of the cationic structure. Solvent parameters, including Lewis basicity, solubility parameters, and a hydrophobicity parameter, were used to discuss the change in the PMMA solubility in ILs with different ionic structures. By considering the PMMA solubility in the ILs using these parameters, it was found that there is a good correlation between the PMMA solubility and the hydrophobicity parameter of the anions. Although the change in the PMMA solubility with different cationic structures was not remarkable, the hydrophobicity of the cations also played a role in the solvation of PMMA by providing a low-polarity environment adequate to dissolve PMMA

Thermally Reversible Ion Gels with Photohealing Properties Based on Triblock Copolymer Self-Assembly

We describe a functional soft material that can spontaneously repair damage by straightforward application of light illumination. The composite material is composed of a common ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim]­PF₆), and a well-defined ABA triblock copolymer consisting of the IL-compatible poly­(ethylene oxide) (PEO) middle block with thermo- and photosensitive random copolymers combining <i>N</i>-isopropylacrylamide (NIPAm) and 4-phenylazophenyl methacrylate (AzoMA) including azobenzene chromophore as terminal A blocks. The composite shows a sol–gel transition under UV light (366 nm, 8 mW cm^–2) irradiation at 47 °C, whereas that observed under visible light (437 nm, 4 mW cm^–2) is 55 °C, due to the difference in photochromic states of the azobenzene unit. The ABA triblock copolymer undergoes a reversible gel–sol–gel transition cycle at the bistable temperature (53 °C), with a reversible association/fragmentation of the polymer network resulting from the photoinduced self-assembly of the ABA triblock copolymer in [C₄mim]­PF₆. A damaged ABA ion gel shows a remarkable photohealing ability based on drastic changes in the fluidity of the polymer–IL composite triggered by light illumination. The damaged part is successfully repaired by shining UV light resulting in solubilization to fill the crack, followed by gelation to fix the crack triggered by visible light illumination. Tensile tests confirmed the excellent recovery efficiency of the resultant photohealed ABA ion gel, which was as high as 81% fracture energy relative to the original sample. The flexible, self-supported ABA ion gel is designed for various applications to exhibit not only photohealing ability to improve operating lifetime of the material but also specific functionalities imparted by the IL, such as high ion conductivity, thermal stability, and (electro)­chemical stability

Enhancing Li–S Battery Performance with Limiting Li[N(SO₂F)₂] Content in a Sulfolane-Based Sparingly Solvating Electrolyte

By enhancing the stability of the lithium metal anode and mitigating the formation of lithium dendrites through electrolyte design, it becomes feasible to extend the lifespan of lithium–sulfur (Li–S) batteries. One widely accepted approach involves the utilization of Li[N(SO2F)2] (Li[FSA]), which holds promise in stabilizing the lithium anode by facilitating the formation of an inorganic-dominant solid electrolyte interface (SEI) film. However, the use of Li[FSA] encounters limitations due to inevitable side reactions between lithium polysulfides (LiPSs) and [FSA] anions. In this study, our focus lies in precisely controlling the composition of the SEI film and the morphology of the deposited lithium, as these two critical factors profoundly influence lithium reversibility. Specifically, by subjecting an initial charging process to an elevated temperature, we have achieved a significant enhancement in lithium reversibility. This improvement is accomplished through the employment of a LiPS sparingly solvating electrolyte with a restricted Li[FSA] content. Notably, these optimized conditions have resulted in an enhanced cycling performance in practical Li–S pouch cells. Our findings underscore the potential for improving the cycling performance of Li–S batteries, even when confronted with challenging constraints in electrolyte design

Thermosensitive Phase Separation Behavior of Poly(benzyl methacrylate)/Solvate Ionic Liquid Solutions

We report a lower critical solution temperature (LCST) behavior of binary systems consisting of poly­(benzyl methacrylate) (PBnMA) and solvate ionic liquids: equimolar mixtures of triglyme (G3) or tetraglyme (G4) and lithium bis­(trifluoromethanesulfonyl)­amide. We evaluated the critical temperatures (<i>T</i>_cs) using transmittance measurements. The stability of the glyme–Li⁺ complex ([Li­(G3 or G4)]⁺) in the presence of PBnMA was confirmed using Raman spectroscopy, pulsed-field gradient spin–echo NMR (PGSE-NMR), and thermogravimetric analysis to demonstrate that the complex was not disrupted. The interaction between glyme–Li⁺ complex and PBnMA was investigated via ⁷Li NMR chemical shifts. Upfield shifts originating from the ring-current effect of the aromatic ring within PBnMA were observed with the addition of PBnMA, indicating localization of the glyme–Li⁺ complex above and below the benzyl group of PBnMA, which may be a reason for negative mixing entropy, a key requirement of the LCST