Relationship between Conductivity, Ion Diffusion, and Transference Number in Perfluoropolyether Electrolytes
- Univ. of California, Berkeley, CA (United States). Dept. of Materials Science and Engineering; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Sciences Division
- Univ. of California, Berkeley, CA (United States). Dept. of Chemical and Biomolecular Engineering; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Sciences Division
- Univ. of North Carolina, Chapel Hill, NC (United States). Dept. of Chemistry
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Environmental Energy Technologies Division
- Univ. of North Carolina, Chapel Hill, NC (United States). Dept. of Chemistry; North Carolina State Univ., Raleigh, NC (United States). Dept. of Chemical and Biomolecular Engineering
- Univ. of California, Berkeley, CA (United States). Dept. of Chemical and Biomolecular Engineering; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Sciences Division and Environmental Energy Technologies Division
Connecting continuum-scale ion transport properties such as conductivity and cation transference number to microscopic transport properties such as ion dissociation and ion self-diffusivities is an unresolved challenge in characterizing polymer electrolytes. Better understanding of the relationship between microscopic and continuum scale transport properties would enable the rational design of improved electrolytes for applications such as lithium batteries. Here, we present measurements of continuum and microscopic ion transport properties of nonflammable liquid electrolytes consisting of binary mixtures of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and perfluoropolyethers (PFPE) with different end groups: diol, dimethyl carbonate, ethoxy-diol, and ethoxy-dimethyl carbonate. The continuum properties, conductivity and cation transference number, were measured by ac impedance spectroscopy and potentiostatic polarization, respectively. The ion self-diffusivities were measured by pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR), and a microscopic cation transference number was calculated from these measurements. The measured ion self-diffusivities did not reflect the measured conductivities; in some cases, samples with high diffusivities exhibited low conductivity. We introduce a nondimensional parameter, β, that combines microscopic diffusivities and conductivity. We show that β is a sensitive function of end-group chemistry. In the ethoxylated electrolytes, β is close to unity, the value expected for electrolytes that obey the Nernst-Einstein equation. In these cases, the microscopic and continuum transference numbers are in reasonable agreement. PFPE electrolytes devoid of ethoxy groups exhibit values of β that are significantly lower than unity. In these cases, there is significant deviation between microscopic and continuum transference numbers. We propose that this may be due to electrostatic coupling of the cation and anion or contributions to the NMR signal from neutral ion pairs.
- Research Organization:
- Energy Frontier Research Centers (EFRC) (United States). Center for Mesoscale Transport Properties (m2mt); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- DOE Contract Number:
- AC02-05CH11231; SC0012673
- OSTI ID:
- 1474938
- Journal Information:
- Macromolecules, Vol. 49, Issue 9; Related Information: © 2016 American Chemical Society.; ISSN 0024-9297
- Publisher:
- American Chemical Society
- Country of Publication:
- United States
- Language:
- English
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