AmericanChemicalSociety.com

Background Information

Polymer electrolytes have distinct advantages over traditional liquid electrolytes used in rechargeable lithium batteries.  Perhaps the most important advantage is enhanced consumer safety through the elimination of the flammable solvents.  Only a handful of systems, however, have been commercialized despite over 30 years of vigorous research in this area.  Most strategies to improve polymer electrolyte performance focus on highly disordered systems, relying on a large body of experimental evidence that suggests ion conduction is higher in amorphous systems above the glass transition temperature than analogous crystalline systems.  Nonetheless, there are a few recent examples of polymer electrolytes wherein organized polymer chains that possess higher ionic conductivities than the disordered analogs.  Observations of enhanced ionic conductivity in “organized” polymer electrolytes challenge the conventional wisdom for designing polymer electrolytes and raise fundamental questions concerning the role of polymer chain order on ionic conductivity.  This research provides crucial pieces of data to understanding the mechanism of ion conduction in oriented polymer electrolytes.

First Year Results

Degrees of chain orientation for three polymer electrolyte systems (PEO-LiCF₃SO₃, PEO-NaCF₃SO₃, and PEO-LiPF₆) were assessed with polarized FT-IR spectroscopy.  The PEO-LiCF₃SO₃ system has been fully analyzed and presented at the 12^th International Symposium on Polymer Electrolytes (ISPE 12).

For polymer systems that have uniaxial symmetry (or effectively uniaxial symmetry), such as the PEO and P(EO)₃LiCF₃SO₃ phases present in the PEO-LiCF₃SO₃ system, the average orientation can be characterized in terms of the average value of the angle between the polymer axis of the structural repeat unit and the elongation direction.  Fortunately, straightforward methods are available for assessing chain orientation with dichroic FT-IR experiments.  Results from a combination of two-dimensional FT-IR spectroscopy and polarized FT-IR spectroscopy show that the PEO and P(EO)₃LiCF₃SO₃ phases align at approximately the same rate as the polymer electrolytes are stretched.  Furthermore, a strain of 300% is sufficient to fully orient the chains in both phases.  The spectroscopic results are consistent with a stress-induced melt-recrystallization process for aligning the polymer chains.  Stretching the films pulls polymer chains from both PEO and P(EO)₃LiCF₃SO₃ phases, aligning the polymer helices along the stretch axis.  These chains then recrystallize such that the polymer helices of the newly formed PEO and P(EO)₃LiCF₃SO₃ crystalline domains are oriented in the stretching direction.

Quantitative measurements of polymer chain orientation for both PEO and P(EO)₃LiCF₃SO₃ phases provide some support for the hypothesis that ionic conductivity enhancement is due to the alignment of the polymer chain helices.  If lithium ion conduction in crystalline polymer electrolytes is viewed as consisting of two major components (facile intra-chain lithium ion conduction and slow helix-to-helix inter-grain hopping), then alignment of the polymer helices will affect the ion conduction pathways for these polymer electrolytes by reducing the number of inter-grain hops between P(EO)₃LiCF₃SO₃  domains required to migrate through the polymer electrolyte.  In a stretched polymer electrolyte, a significant number of the PEO and P(EO)₃LiCF₃SO₃ domains recrystallize with the polymer helices oriented along the direction of the stress field.  Thus, ions within the P(EO)₃LiCF₃SO₃ domains encounter fewer barriers to ion motion when an external electric field is oriented parallel to the stretch direction compared to an electrolyte with a random distribution of chain orientations.  Consequently, ionic conductivity is enhanced in the stretched samples along the stretch direction.  In contrast, ionic conductivity measured perpendicular to the stretch axis is not increased because there are a relatively large number of inter-grain boundaries in the stretched materials perpendicular to the stretch direction, consistent with published experimental results for this system.  Polarized FT-IR spectra have also been recorded for the PEO-LiPF₆ and PEO-NaCF₃SO₃ systems.  Although, the results have not been fully analyzed, the overall trends are very similar to the PEO-LiCF₃SO₃ system.

Impact of First Year Results

An initial estimate of the impact on the scientific community can be gauged by the receipt of the work at ISPE 12.  This is a seminal meeting in the battery electrolyte community, bringing together the major researchers in the field from around the world.  This work received one of only two best poster awards given at the meeting.  The results from the PEO-LiCF₃SO₃ system are currently in the form of a manuscript that is to be submitted to a special issue of Solid State Ionics, which is covering the proceedings of ISPE 12, by mid-October 2010. 

Impact of the ACS Grant on the PI and Participating Students

The ACS grant provided crucial financial support to develop Dr. Burba's research laboratory, and provided the necessary funds to attend ISPE 12.  Participation in the meeting will potentially result in new scientific collaborations.  Furthermore, Dr. Burba's undergraduate research students greatly benefited from working on this project.  A total of five students have participated in some way with this project (three students during this review period).  All students participated in disseminating the research results either at meetings or writing a manuscript.

Directions for the Second Year

In situ complex impedance measurements of stretched polymer electrolytes (parallel and perpendicular to the stretch direction) were attempted during the first year of research.  However, resistivities of the polymer electrolytes were much higher than anticipated and could not be reliably measured with the impedance cell currently available.  Alternative cell designs are being explored to facilitate this important part of the proposed research.

Polarized FT-IR spectroscopy of additional PEO-salt systems (viz., PEO-KCF₃SO₃ and PEO-LiClO₄) are currently underway to test the generality of the conclusions drawn from the PEO-LiCF₃SO₃ system.  In addition, studies to characterize relaxation rates of the oriented systems were initiated in August 2010.  Dichroic measurements of stretched polymer samples are recorded at fixed time intervals as a function of temperature to assess the persistence of the imposed order.

Collaboration with Dr. Spence Pilcher was initiated in August 2010 to synthesize amorphous, high molecular weight methylene-oxide-linked ethylene glycol materials.  These materials will test whether or not the conductivity enhancement in PEO-based systems requires the presence of crystalline PEO-salt domains.  The synthesis of the compound is expected to be finished by December 2010, followed by spectroscopic studies in spring 2011.