Current-Induced Motion of Polymer Chains and Relevance to Rechargeable Batteries

It is natural to focus on the movement of charged ions in electric fields. In this talk however, I will focus on the movement of neutral "solvent" molecules used to dissolve the ions inside batteries. Any energy used to move solvent molecules is wasted, and thus it is important to understand how and why they move. In our case, the solvent is a polymer; I believe that polymer electrolytes will play an essential role in enabling next-generation rechargeable batteries. The continuity equation provides the foundation for modeling mass transport. I will describe how the continuity equation changes in the presence of electric current. While ion velocities can be inferred from current-voltage measurements, there were no well-established approaches for measuring velocities of neutral molecules in batteries. My group (and others) have recently developed experimental approaches for accomplishing this. Surprisingly, the field-induced velocity of uncharged long polymer chains is comparable to that of the ions and is independent of chain length. Theoretical predictions will be compared with experimental results. In addition to continuum-scale experiments, I will describe the results of quasi-elastic neutron scattering experiments which enable the study of the motion of polymer segments in the vicinity of ions on the Angstrom length scale. These measurements and computer simulations on the same length scale provide insight into the bottlenecks that limit ion transport. I will conclude by describing the connection between our work and the continuing push to commercialize all-solid rechargeable batteries for electric vehicles and other clean-energy-related applications.