Dependence of transport properties and influence of substrates on structure and polymer dynamics in a composite polymer electrolyte.

摘要

Composite polymer electrolytes are comprised of a polymer, an appropriate salt, and a relatively inert third component. When configured into a battery, these systems are touted as alternative energy storage devices with improved properties. As a result, much research has been directed towards understanding fundamental aspects in structure and dynamics of these materials in hopes of constructing a versatile all-solid-state-battery. Using impedance spectroscopy, nuclear magnetic resonance, differential scanning calorimetry, Raman spectroscopy, and x-ray diffraction we have studied the properties of a novel composite polymer electrolyte (CPE) comprised of polyethylene oxide (PEO), lithium trifluoromethane sulfonate (LiTf), and an organic-inorganic composite (OIC). While the coupling of the cation (lithium) to polymer mobility is believed to promote conduction in polymer-based materials, we observe that in this CPE the PEO mobility and ionic conductivity exude contrasting trends. In order to understand the transport properties of this composite electrolyte, we have performed experiments on electrolytic and non-electrolytic systems. These studies led to the formulation of a free-ion model that produces good qualitative agreement between theoretical number densities and experimental conductivities. In addition, the model predicts diffusion coefficients that agree within an order of magnitude with experimental values, which further justifies our approach. Key aspects of this model include polymer-salt interactions and the stiffening of the polymer with increased OIC weight fraction. Furthermore, we extend the model to systems with high and low molecular weight PEO, various salt concentrations, chemically different inert components, and contrasting substrates. The latter part of the aforementioned study introduces a second aspect of this work, which is that glass and Teflon substrates significantly affect electrolyte structure and dynamics. Using the techniques described above, we explain this behavior in terms of solution-substrate surface tensions and interfacial bonding between PEO and the substrates.