Polymer Physics Seminar - Nitash Balsara

Nitash Balsara, professor of chemical and biomolecular engineering, The Charles W. Tobias Professor in Electrochemistry, University of California, Berkeley

“Negative Diffusion Coefficients and Concentration Hot-Spots Induced by Flow of Current in Polymer Electrolytes”

The need for creating safe electrolytes for lithium batteries is significant given the continued safety problems associated with current lithium-ion batteries.

Nonflammable polymer electrolytes offer a possible solution but the rate of lithium ion transport is too low for practical applications. In this talk, I will discuss some of the fundamental factors that limit ion transport in polymers.

In all electrolytes, the current generated at steady state is governed by the applied potential. This relationship, which one might call a modified Ohm’s Law, depends on Stefan-Maxwell diffusion coefficients. In the first part of my talk, we show experimental results indicating that these diffusion coefficients are negative over a substantial salt concentration range in polymer electrolytes. We then use these diffusion coefficients to analyze and predict battery performance. A crucial ingredient in the analysis is a “condition” that my PhD student Danielle Pesko arrived at. In the second part of my talk, I examine the passage of current through a nanostructured block copolymer.

Salt concentration gradients caused by the passage of current cause order-order and order-disorder phase transitions that are taken as indicators of the local salt concentration in the cell. It enables the detection of salt concentration hot spots, regions where the local salt concentration is much higher than that in the surrounding regions. We discuss the implications of this phenomenon in the context of transport limitations that may arise generally when current flows through heterogeneous media.