The development of advanced electrolytes compatible with lithium metal and lithium-ion batteries is crucial for meeting ever growing energy storage demands. One such class of materials, single-ion conducting polymer electrolytes (SIPEs), prevents the formation of ion concentration gradients and buildup of anions at the electrode surface, improving performance. One of the ongoing challenges for SIPEs is the development of materials that are conductive enough to compete with liquid electrolytes. Presented herein is a class of gel SIPEs based on crosslinked poly(tetrahydrofuran) diacrylate that present enhanced room temperature conductivities of 3.5 × 10–5 S/cm when gelled with lithium metal relevant 1,3-dioxolane/dimethoxyethane, 2.5 × 10–4 S/cm with carbonate solutions, and approaching 10–3 S/cm with dimethyl sulfoxide. Remarkably, these materials also demonstrate high ionic conductivity at low temperatures, 1.8 × 10–5 S/cm at −20 °C in certain solvents. Most importantly, however, when contrasted with identical SIPEs formulated with poly(ethylene glycol) diacrylate, the mechanisms responsible for the enhanced conductivity are elucidated: decreasing Li+–polymer interactions and gel solvent–polymer interactions leads to an increase in Li+ mobility. These findings are generalizable to various SIPE chemistries and can therefore be seen as an additional set of design parameters for developing future high conductivity SIPEs.
Solvent-free, single-ion conducting electrolytes are sought after for use in electrochemical energy storage devices. Here, we investigate the ionic conductivity and how this property is influenced by segmental mobility and conducting ion number in crosslinked single-ion conducting polyether-based electrolytes with varying tethered anion and counter-cation types. Crosslinked electrolytes are prepared by the polymerization of poly(ethylene glycol) diacrylate (PEGDA), poly(ethylene glycol) methyl ether acrylate, and ionic monomers. The ionic conductivity of the electrolytes is measured and interpreted in the context of differential scanning calorimetry and Raman spectroscopy measurements. A lithiated crosslinked electrolyte prepared with PEG 31 DA and (4-styrenesulfonyl)(trifluoromethanesulfonyl)imide (STFSI) monomers is found to have a lithium ion conductivity of 3.2 × 10 −6 and 1.8 × 10 −5 S/cm at 55 and 100 • C, respectively. The percentage of unpaired anions for this electrolyte was estimated at about 23% via Raman spectroscopy. Despite the large variances in metal cation-STFSI binding energies as predicted via density functional theory (DFT) and large variations in ionic conductivity, STFSI-based crosslinked electrolytes with the same charge density and varying cations (Li, Na, K, Mg, and Ca) were estimated to all have unpaired anion populations in the range of 19 to 29%.
Crystalline nanosheets formed by amphiphilic block copolypeptoids with halogenated phenyl side chains were imaged at the atomic-scale using cryogenic transmission electron microscopy (cryo-TEM). In general, the polypeptoid molecules adopt V-shaped configurations in the crystalline state, and adjacent molecules can pack with one another in either parallel or antiparallel arrangements, depending on the chemical composition. The halogen bond, which can have characteristic energies ranging from 1 to 5 kcal/mol, is commensurate with the parallel configuration. However, cryo-TEM images show that chains in the halogenated crystals were in the antiparallel configuration. Molecular dynamics (MD) simulations show that positively charged σ-holes, which are characteristic of halogen atoms covalently bonded to carbon atoms, play an important role in determining crystal geometry. Parallel and antiparallel configurations exhibited similar stability in simulations when standard force fields that only account for the electronegativity of halogen atoms were used. However, including the σ-hole in the simulations resulted in a destabilization of the parallel configuration. This combination of imaging and simulation, which has played an important role in structural biology, has the potential to improve our understanding of factors that govern noncovalent interactions in synthetic materials.
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