Optimizing Ion Transport in Polyether-Based Electrolytes for Lithium Batteries
Abstract
We report on the synthesis of poly(diethylene oxide-alt-oxymethylene), P(2EO-MO), via cationic ring-opening polymerization of the cyclic ether monomer, 1,3,6-trioxocane. We use a combined experimental and computational approach to study ion transport in electrolytes comprising mixtures of P(2EO-MO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. Mixtures of poly(ethylene oxide) (PEO) and LiTFSI are used as a baseline. The maximum ionic conductivities, σ, of P(2EO-MO) and PEO electrolytes at 90 °C are 1.1 × 10^(–3) and 1.5 × 10^(–3) S/cm, respectively. This difference is attributed to the T_g of P(2EO-MO)/LiTFSI (−12 °C), which is significantly higher than that of PEO/LiTFSI (−44 °C) at the same salt concentration. Self-diffusion coefficients measured using pulsed-field gradient NMR (PFG-NMR) show that both Li+ and TFSI– ions diffuse more rapidly in PEO than in P(2EO-MO). However, the NMR-based cation transference number in P(2EO-MO) (0.36) is approximately twice that in PEO (0.19). The transference number measured by the steady-state current technique, t_(+,ss), in P(2EO-MO) (0.20) is higher than in PEO (0.08) by a similar factor. We find that the product σt_(+,ss) is greater in P(2-EO-MO) electrolytes; thus, P(2EO-MO) is expected to sustain higher steady-state currents under dc polarization, making it a more efficacious electrolyte for battery applications. Molecular-level insight into the factors that govern ion transport in our electrolytes was obtained using MD simulations. These simulations show that the solvation structures around Li+ are similar in both polymers. The same is true for TFSI–. However, the density of Li+ solvation sites in P(2EO-MO) is double that in PEO. We posit that this is responsible for the observed differences in the experimentally determined transport properties of P(2EO-MO) and PEO electrolytes.
Additional Information
© 2018 American Chemical Society. Received: December 20, 2017; Revised: March 5, 2018; Publication Date (Web): April 3, 2018. The authors gratefully acknowledge Zhen-Gang Wang and Michael Webb for useful discussions. This research was supported by the National Science Foundation under DMREF Award NSF-CHE-1335486. DSC experiments were performed at the Molecular Foundry user facilities at Lawrence Berkeley National Laboratory supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. The authors declare no competing financial interest.Attached Files
Supplemental Material - ma7b02706_si_001.pdf
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Additional details
- Eprint ID
- 85580
- DOI
- 10.1021/acs.macromol.7b02706
- Resolver ID
- CaltechAUTHORS:20180403-132507860
- CHE-1335486
- NSF
- DE-AC02-05CH11231
- Department of Energy (DOE)
- Created
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2018-04-03Created from EPrint's datestamp field
- Updated
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2021-11-15Created from EPrint's last_modified field