(180be) Molecular Weight Dependence of Ionic Conductivity for Low Molecular Weight Block Copolymer Electrolytes

Molecular
Weight Dependence of Ionic Conductivity for Low Molecular Weight Block
Copolymer Electrolytes

Rodger
Yuan,¹ Alex Teran,^2,3 Scott
Mullin,^2,3Nisita Wanakule,²Nitash Balsara^2,3,4

¹Department of Materials
Science and Engineering, University of California, Berkeley, CA, ²Department
of Chemical Engineering, University of California, Berkeley, CA, ³Environmental
Energy and Technologies Division, Lawrence Berkeley National Laboratory,
Berkeley, CA ⁴Materials Science Division, Lawrence Berkeley National
Laboratory, Berkeley, CA

Background: Poly(styrene-block-ethylene oxide) (SEO) copolymers
show promise as electrolytes for lithium batteries. Microphase-separation
of SEO allows for lithium ion conduction via pathways through the polyethylene
oxide (PEO) phase while the polystyrene (PS) phase provides mechanical support to
hinder lithium dendrite formation. Previous studies show a positive
relationship between molecular weight and ionic conductivity for mixtures of
SEO and lithium bis(trifluoromethanesulfone) imide (LiTFSI) with lamellar morphologies and total molecular
weights over 30 kg/mol. In this study, we determined the molecular weight
dependence of lamellar SEO/LiTFSI systems with
molecular weights below 30 kg/mol.

Results: Since the ionic
conductivity of PEO varies with molecular weight, we define a normalized
conductivity, σ_n(T), as

σ_n(T,M_PEO) = σ(T)/[(2/3)ϕ_{PEO/LiTFSIσPEO}(T,M_PEO)]

where σ_PEO(T) is the conductivity of homopolymer PEO with molecular weight, M, at temperature, T,
obtained from previous work and (2/3) is a morphology factor to account for
lamellar domains oriented parallel to the electrodes that cannot contribute to
the conductivity. This value represents the effective conductivity of the block
copolymer electrolyte relative to PEO of the same molecular weight. Results of
the conductivity study are summarized in Figure 1.

Figure 1: σ_n vs. MW PEO at 90^oC,
after annealing, of the combined data of low molecular weight copolymers
(black) and high molecular weight copolymers (red) from a previous study.

Conclusion: The results show that normalized ionic conductivity and
absolute ionic conductivity has a minimum around the MW PEO of 5500 g/mol. This
data contradicts the work of previous studies that show increasing conductivity
with increasing molecular weight of PEO. Current understanding of the ionic
conductivity of PEO-based block copolymer electrolytes is that PEO chains have
limited mobility at the PS-PEO interface so minimization of interface to volume
ratio via increase of domain spacing will increase ionic conductivity. This
study shows that there are additional factors that contribute to the ionic
conductivity of block copolymers. We suspect that these additional factors may
be associated with the glass transition temperature maximum of PEO at MW of 10⁴ g/mol or chain
movement at low molecular weights but further study is required to demonstrate
this.