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Polymer gel electrolytes (PGE) have seen a renewed interest in their development because they have high ionic conductivities but low electrochemical degradation and flammability. PGEs are formed by mixing a liquid lithium-ion electrolyte with a polymer at a sufficiently large concentration to form a gel. PGEs have been extensively studied, but the direct connection between their microscopic structure and macroscopic properties remains controversial. For example, it is still unknown whether the polymer in the PGE acts as an inert, stabilizing scaffold for the electrolyte or it interacts with the ionic components. Here, a PGE composed of a prototypical lithium-carbonate electrolyte and polyacrylonitrile (PAN) is pursued at both microscopic and macroscopic levels. Specifically, this study focused on describing the microscopic and macroscopic changes in the PGE at different polymer concentrations. The results indicated that the polymer-ion and polymer–polymer interactions are strongly dependent on the concentration of the polymer and the lithium salt. In particular, the polymer interacts with itself at very high PAN concentrations (10% weight) resulting in a viscous gel. However, the conductivity and dynamics of the electrolyte liquid components are significantly less affected by the addition of the polymer. The observations are explained in terms of the PGE structure, which transitions from a polymer solution to a gel, containing a polymer matrix and disperse electrolyte, at low and high PAN concentrations, respectively. The results highlight the critical role that the polymer concentration plays in determining both the macroscopic properties of the system and the molecular structure of the PGE.
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Holding it together: noncovalent cross-linking strategies for ionogels and eutectogels
When a hydrogel simply won’t cut it – either because it dries out too quickly, or it does not tolerate more than roughly one volt when applied in an electrochemical device – where is the savvy materials researcher to turn? This is where two important classes of nonaqueous gel counterparts, known as ionogels and eutectogels, can truly shine. Replacing the aqueous liquid phase of a hydrogel with either an ionic liquid (IL) or a deep eutectic solvent (DES) allows one to realize an array of versatile gel electrolyte materials that offer outstanding nonvolatility, wider windows of electrochemical stability, reasonably high ionic conductivity, and nearly unlimited chemical design possibilities. In addition to choosing a specific IL or DES, there are a myriad of options when it comes to constructing a solid, three-dimensional, volume-spanning network (or scaffold) that will support the nonaqueous liquid phase of an ionogel or eutectogel. In this focused review, several recent approaches to forming these gels using noncovalent scaffold assembly and cross-linking are examined, and the primary noncovalent interactions responsible ( e.g. hydrogen bonding, solvophobicity, coulombic interactions) are identified. Noncovalent scaffold assembly in nonaqueous, ion-dense electrolytes often leads to supramolecular gel materials that can exhibit extreme stretchability, good toughness, and an ability to self-heal in many cases. After reviewing several strategies that have been recently employed for creating ionogels and eutectogels, a brief inspection of some motivating noncovalently cross-linked scaffolds reported for hydrogels is presented with the hopes that these may provide inspiration for the future design of novel ionogels and eutectogels by the materials research community.
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- Award ID(s):
- 1802729
- PAR ID:
- 10375758
- Date Published:
- Journal Name:
- Materials Advances
- ISSN:
- 2633-5409
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2MA00539E
