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

DM Halat and RL Snyder and S Sundararaman and Y Choo and KW Gao and ZJ Hoffman and BA Abel and LS Grundy and MD Galluzzo and MP Gordon and H Celik and JJ Urban and D Prendergast and GW Coates and NP Balsara and JA Reimer, CHEMISTRY OF MATERIALS, 33, 4915-4926 (2021).

DOI: 10.1021/acs.chemmater.1c00339

Polyacetal electrolytes have been demonstrated as promising alternatives to liquid electrolytes and poly(ethylene oxide) (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 as P(xEO-yMO), and PEO. We first use Li-7 and F-19 pulsed-field-gradient NMR spectroscopy to measure cation and anion self-diffusion, respectively, in polymer/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt mixtures. At 90 degrees 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 - T-g), 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, support the viability of polyacetals as high-performance polymer electrolytes.

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