Modifying Li⁺ and Anion Diffusivities in Polyacetal Electrolytes: A Pulsed-Field-Gradient NMR Study of Ion Self-Diffusion

Abstract

Polyacetal electrolytes have been demonstrated as promising alternatives to liquid electrolytes and PEO for rechargeable lithium-ion batteries; however, the relationship between polymer structure and ion motion is difficult to characterize. Here, we study structure-property trends in ion diffusion with respect to polymer composition for a systematic series of five polyacetals with varying ratios of ethylene oxide (EO) to methylene oxide (MO) units, denoted P(xEO-yMO), and PEO. We first use 7 Li and 19 F pulsed-field-gradient NMR spectroscopy to measure cation and anion self-diffusion, respectively, in polymer/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt mixtures. At 90 °C, we observe modest changes in Li + diffusivity across all polymer compositions while anion (TFSI − ) self-diffusion coefficients decrease significantly with increasing MO content. At a given reduced temperature (T − Tg), all polyacetal electrolytes exhibit faster Li + self-diffusion than PEO. Intriguingly, P(EO-MO) and P(EO-2MO) also show slower TFSI − anion self-diffusion than PEO at a given reduced temperature. Molecular dynamics simulations reveal that shorter distances between acetal oxygen atoms (O-CH2-O) compared to ether oxygens (O-CH2-CH2-O) promote more diverse, often asymmetric, Li + coordination environments. Raman spectra reveal that anion-rich ion clusters in P(EO-MO) and P(EO-2MO) lead to decreased anion diffusivity, which along with increased cation diffusivity, elucidate the viability of polyacetals as high-performance polymer electrolytes.

Crosslinked xylose-based polyester as a bio-derived and degradable solid polymer electrolyte for Li+-ion conduction

“…Amongst the various polymer chemistries investigated for SPE application, those comprising of polyethylene oxide (PEO) with various lithium salts have undoubtedly received the most interest, however recent studies of similar polyethers and polyacetals have also been reported by Balsara and Coates. 9–13 Despite high ionic conductivity ( ca. 10 −3 S cm −1 at 70 °C) above the melting temperature of PEO, poor room temperature performance, high crystallinity and low lithium transference numbers (typically around 0.2) of PEO-based SPEs remain a limitation to their practical implementation.…”

Exploring the Ion Solvation Environments in Solid-State Polymer Electrolytes through Free-Energy Sampling

“…A series of such polyacetals with varying ratios of EO and MO in the repeat units, P(nEO-mMO), were synthesized 36 and systematically studied electrochemically 37 and through pulsed-field-gradient nuclear magnetic resonance spectroscopy measurements and MD simulations. 36 The polymers studied previously were P(EO-2MO), P(EO-MO), P(2EO-MO), P(3EO-MO), and P(4EO-MO) compared to PEO with LiTFSI salt. Polyacetal−LiTFSI SPEs were studied at similar reduced temperatures (T − T g ) as the glasstransition temperatures in the presence of LiTFSI varied significantly.…”

“…In our previous work, , the design strategy that was employed was to maintain a high oxygen-to-carbon ratio (p) in the backbone of the polymer to provide sufficient well-connected solvation sites for the Li cation by incorporating methylene oxide (O–CH 2 or MO) repeat units into the polymer backbone along with ethylene oxide (O–CH 2 –CH 2 or EO). A series of such polyacetals with varying ratios of EO and MO in the repeat units, P­( n EO- m MO), were synthesized and systematically studied electrochemically and through pulsed-field-gradient nuclear magnetic resonance spectroscopy measurements and MD simulations . The polymers studied previously were P­(EO-2MO), P­(EO-MO), P­(2EO-MO), P­(3EO-MO), and P­(4EO-MO) compared to PEO with LiTFSI salt.…”

Electric-Field-Induced Spatially Dynamic Heterogeneity of Solvent Motion and Cation Transference in Electrolytes

“…x-3 the ratio of Li + cations to O atoms within tetraglyme [30][31][32]. For these electrolytes, the cation, anion, and solvent may be selectively and unambiguously probed with 7 Li, 19 F, and 1 H NMR measurements, respectively.…”