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

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journal contribution
posted on 29.06.2021, 19:35 by David M. Halat, Rachel L. Snyder, Siddharth Sundararaman, Youngwoo Choo, Kevin W. Gao, Zach J. Hoffman, Brooks A. Abel, Lorena S. Grundy, Michael D. Galluzzo, Madeleine P. Gordon, Hasan Celik, Jeffrey J. Urban, David Prendergast, Geoffrey W. Coates, Nitash P. Balsara, Jeffrey A. Reimer
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 7Li and 19F 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 (TTg), 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.