279607 Molecular Weight Dependence of Ionic Conductivity for Low Molecular Weight Block Copolymer Electrolytes

Monday, October 29, 2012
Hall B (Convention Center )
Rodger Yuan, Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA

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

Rodger Yuan,1 Alex Teran,2,3 Scott Mullin,2,3 Nisita Wanakule,2 Nitash Balsara2,3,4

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, σn(T), as

σn(T,MPEO) = σ(T)/[(2/3)ϕPEO/LiTFSIσPEO(T,MPEO)]

where σPEO(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: σn vs. MW PEO at 90oC, 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.


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