Ion Transport in Block Copolymer Electrolyte Affected by Distortion of Lamellae Domains under DC Polarization

(a) Schematic representation of the simultaneous dc polarization and SAXS experiment. An SEO/LiTFSI electrolyte with randomly oriented grains is sandwiched between two lithium electrodes with current passing parallel to the x-axis. X-rays pass parallel to the z-axis. Scanning the beam along the x-axis allows for spatial resolution between the electrodes. (b) Illustration of lamellae domain spacing d increasing with salt concentration. Lamellae with interfaces oriented perpendicular to the flow of ionic current (LAM⊥) swell and contract to a greater degree than those with interfaces oriented parallel to the current direction (LAM||).

Scientific Achievement

We studied the structure of block copolymer electrolyte using small-angle X-ray scattering (SAXS) during direct current (dc) polarization and observed the distortion of the lamellae domain spacing d to accommodate the salt concentration gradient.

Significance and Impact

The results shed light on the nature of ion transport in nanostructured block copolymers under dc polarization and indicate that the process of block copolymer rearrangement limits the maximum current (ilim) that can be sustained by a block copolymer electrolyte.

Research Details

  • Polystyrene-blockpoly(ethylene oxide) (PS-b-PEO , or SEO) doped with bis-(trifluoromethylsulfonyl)amine lithium salt (LiTFSI) was studied.
  • The measured ilim is approximately a factor of two lower than that predicted from concentrated solution theory.
  • SAXS experiments show a net increase in the domain spacing of the block copolymer as the salt concentration gradient develops. LAM⊥ grains swell and contract to a greater extent compared to LAM||.
  • The observed limiting current originates from the distortion of lamellae domains to accommodate the salt concentration gradient, which is not accounted for by concentrated solution theory.

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DOI: 10.1021/acs.macromol.1c01295

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