Abstract
The transport properties of block copolymer-derived polymer electrolyte membranes (PEMs) are sensitive to microstructural disorder originating in the randomly oriented microdomains produced during uncontrolled self-assembly by microphase separation. This microstructural disorder can negatively impact performance due to the presence of conductivity-impeding grain boundaries and the resulting tortuosity of transport pathways. We use magnetic fields to control the orientational order of Li-doped lamellar polyethylene oxide (PEO) microdomains in a liquid crystalline diblock copolymer over large length scales (>3 mm). Microdomain alignment results in an increase in the conductivity of the membrane, but the improvement relative to non-aligned samples is modest, and limited to roughly 50% in the best cases. This limited increase is in stark contrast to the order of magnitude improvement observed for magnetically aligned cylindrical microdomains of PEO. Further, the temperature dependence of the conductivity of lamellar microdomains is seemingly insensitive to the order-disorder phase transition, again in marked contrast to the behavior of cylinder-forming materials. The data are confronted with theoretical predictions of the microstructural model developed by Sax and Ottino. The disparity between the conductivity enhancements obtained by domain alignment of cylindrical and lamellar systems is rationalized in terms of the comparative ease of percolation due to the intersection of randomly oriented lamellar domains (2D sheets) versus the quasi-1D cylindrical domains. These results have important implications for the development of methods to maximize PEM conductivity in electrochemical devices, including batteries.
Highlights
Self-assembly of block copolymers (BCPs) is a fascinating phenomenon that can be utilized to create composite materials with periodic nanostructure in a cost-effective manner [1]
While the crystalline polyethylene oxide (PEO) fraction in the neat BCP is estimated to be about 36% based on the magnitude of PEO melting enthalpy relative to that of pure highly crystalline PEO [26,27], the glass transition temperature for the neat material was not observed
Glass transitions were observed in the Li-containing samples, with an increase in Tg recorded with increasing Li-concentration (Figure 3b). These observations indicate a decrease in the crystallinity of PEO on Li inclusion, and are consistent with the expected complexation of lithium ions by ether oxygen atoms of the PEO
Summary
Self-assembly of block copolymers (BCPs) is a fascinating phenomenon that can be utilized to create composite materials with periodic nanostructure in a cost-effective manner [1]. Ion conducting diblock copolymers are compelling materials in the design of electrolyte membranes for rechargeable lithium batteries [12,13]. In this application, a polar polymer block acts as solid electrolyte facilitating lithium transport during the operation of the battery while the other block contributes to the mechanical. Compared to homopolymer-based or porous electrolyte-soaked composite materials, block copolymer membranes offer better structural stability, potentially longer battery lifetime and greater safety of use. BCP membranes are promising for their potential to mechanically restrict the highly undesirable dendritic lithium formation during the charging process [14]. It is notable that suppression of such dendrite formation has been observed recently in relatively low modulus materials as well [15]
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