(622e) Breaking the Compensation Effect within the Vogel-Tammann-Fulcher Equation for Polymer-Based Electrolytes
AIChE Annual Meeting
Wednesday, November 1, 2017 - 4:30pm to 4:45pm
Single-ion conducting polymer electrolytes have long suffered from poor conductivity at low to moderate temperatures, though their ability to eliminate deleterious concentration gradients in lithium ion batteries has continued to attract research interest. We have synthesized a new random copolymer of oligomeric poly(ethylene glycol) and ion containing sulfone monomers (PSf-co-PEG) that possesses a wide miscible and accessible composition window, thereby allowing careful elucidation of the interplay between the impacts of segmental motion and ion content on the final observed conductivity. Here we demonstrate those effects, and highlight the importance of optimized data fitting in the commonly used Vogel-Tammann-Fulcher (VTF) equation. Particularly, we show that the use of a fixed Vogel temperature 50 - 100°C below the glass transition may lead to misleading trends in the VTF prefactor and activation energy. Utilizing an optimized fitting method applied to the PSf-co-PEG system, we have found a strong positive correlation between the VTF equation prefactor and apparent activation energy for polymers in this electrolyte class. This relationship, termed the compensation effect, has been observed in many related activated processes governed by the Arrhenius equation, and we demonstrate the effect for several additional polymer electrolyte classes. Given that conductivity scales inversely and exponentially with apparent activation energy, maximum conductivity must be achieved in samples where the activation energy is small. For a system in which the compensation effect exists, low activation energies are only found at low values of the prefactor, clearly indicating the limiting nature of the compensation effect for any electrolyte. We find that blending of small molecules, in this case short chain poly(ethylene glycol), breaks the apparent compensation effect, lending new motivation for a common method to increase ionic conductivity and demonstrating a clear route to high Li+ transference number, high conductivity electrolytes.