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Modifying Li+ and anion diffusivities in polyacetal electrolytes: A pulsed-field-gradient NMR study of Ion self-diffusion

  • Author(s): Halat, DM
  • Snyder, RL
  • Sundararaman, S
  • Choo, Y
  • Gao, KW
  • Hoffman, ZJ
  • Abel, BA
  • Grundy, LS
  • Galluzzo, MD
  • Gordon, MP
  • Celik, H
  • Urban, JJ
  • Prendergast, D
  • Coates, GW
  • Balsara, NP
  • Reimer, JA
  • et al.
Abstract

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 (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, support the viability of polyacetals as high-performance polymer electrolytes.

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