ma9b01879_si_001.pdf (829.54 kB)
Enhanced Conductivity via Homopolymer-Rich Pathways in Block Polymer-Blended Electrolytes
journal contribution
posted on 2019-12-12, 13:07 authored by Melody
A. Morris, Seung Hyun Sung, Priyanka M. Ketkar, Joseph A. Dura, Ryan C. Nieuwendaal, Thomas H. EppsThe optimization of ionic conductivity and lithium-ion
battery stability can be achieved by independently tuning the ion
transport and mechanical robustness of block polymer (BP) electrolytes.
However, the ionic conductivity of BP electrolytes is inherently limited
by the covalent attachment of the ionically conductive block to the
mechanically robust block, among other factors. Herein, the BP electrolyte
polystyrene-block-poly(oligo-oxyethylene methacrylate)
[PS-b-POEM] was blended with POEM homopolymers of
varying molecular weights. The incorporation of a higher molecular
weight homopolymer additive (α > 1 state) promoted a “dry
brush-like” homopolymer distribution within the BP self-assembly
and led to higher lithium salt concentrations in the more mobile homopolymer-rich
region, increasing overall ionic conductivity relative to the “wet
brush-like” (α < 1 state) and unblended composites,
where α is the molecular weight ratio between the POEM homopolymer
and the POEM block in the copolymer. Neutron and X-ray reflectometry
(NR and XRR, respectively) provided additional details on the lithium
salt and polymer distributions. From XRR, the α > 1 blends
showed increased interfacial widths in comparison to their BP (unblended)
or α < 1 counterparts because of the more central distribution
of the homopolymer. This result, paired with NR data that suggested
even salt concentrations across the POEM domains, implied that there
was a higher salt concentration in the homopolymer POEM-rich regions
in the dry brush blend than in the wet brush blend. Furthermore, using 7Li solid-state nuclear magnetic resonance spectroscopy, we
found a temperature corresponding to a transition in lithium mobility
(TLi mobility) that was a function
of blend type. TLi mobility was found
to be 39 °C above Tg in all cases.
Interestingly, the ionic conductivity of the blended BPs was highest
in the α > 1 composites, even though these composites had
higher Tgs than the α < 1 composites,
demonstrating that homopolymer-rich conducting pathways formed in
the α > 1 assemblies had a larger influence on conductivity
than the greater lithium ion mobility in the α < 1 blends.