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Background Information

Electrolytes for lithium rechargeable batteries constitute a major area of active research interest. A considerable amount of research activity is focused on replacing the highly flammable, volatile components used in commercial lithium rechargeable batteries with polymer electrolytes or a non-volatile compound with low flammability, such as a room temperature ionic liquid. In both cases, understanding how the constituent components interact is critical to designing functional replacements for the “traditional” battery electrolytes. In the context of polymer electrolytes, only a handful of systems 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.

Project Results

Polymer Chain Orientation and Conductivity of Poly(ethylene Oxide)-based Electrolytes

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. For polymer systems that have uniaxial symmetry (or effectively uniaxial symmetry), 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, methods are available for assessing chain orientation through 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. 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 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.

Ion-ion Interactions in Room-Temperature Ionic Liquids At the beginning of the second year of funded support, an exciting collaboration with Dr. Roger Frech at the University of Oklahoma was developed to assess the degree of long-range charge ordering in ionic liquids in room-temperature ionic liquids (RTILs). The local environment about the ions composing an RTIL can be viewed as a perturbed crystal lattice with some degree of disorder introduced over the ion sites (i.e., the RTIL possess a quasicrystalline lattice similar to the solid-state crystal structure). Quasilattice structure was determined through a combination of transmission and ATR FT-IR spectroscopy. The method relies on applying two independent approaches to calculate the dipole moment derivative of select vibrational modes for an ion within a RTIL. The first method uses dipolar coupling theory to calculate the dipole moment derivative from the TO-LO split of various IR bands observed in a transmission IR spectrum. The second approach determines the dipole moment derivative from the optical constants of the RTIL, which are obtained from polarized ATR FT-IR spectra. In a perfect crystal, the two estimations of the dipole moment derivative are identical. Thus, any disparities may be interpreted as degradation in the quasilattice of the RTIL.   A preliminary study of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate found an 8-fold disparity between the dipole moment derivative for n_s(SO₃) calculated from dipolar coupling theory and that determined from the optical constants of the ionic liquid. Thus, the quasilattice of this material is not highly structured.

Impact of Research Results

The polymer chain orientation studies received a best poster award at the 12^th International Symposium on Polymer Electrolytes. In addition, the research results are published in Electrochimica Acta. Spectroscopic assessments of quasilattice structure for one RTIL were published in the Journal of Chemical Physics in 2011.

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 these projects. All students participated in disseminating the research results either at meetings or writing a manuscript.

Future Research Directions

Temperature-dependent studies of tensile-stretched polymer electrolytes are underway to assess relaxation processes in oriented polymer electrolytes. This work will provide crucial information about the persistence of imposed order and its impact on ionic conductivity. In addition, quantitative measurements of polymer chain alignment for electrolytes aligned with an external magnetic field are planned for spring 2012.

Assessing quasilattice structure in RTILs is expected to particularly fruitful, for the long-range correlated motion of ions in a RTIL directly affects the phenomenological properties of RTILs that make them attractive as solvents. Future research initiatives are to expand the RTILs investigated, focusing on thermal affects, dilution in molecular solvents, and determining the role of alkyl side-chain length for a family of 1-alkyl-3-methylimidazolium-based ionic liquids.