Impact of Ion Transport and Dissociation on Polymer Electrolyte Battery Rate Capability

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Solid, polymer electrolytes are needed to enable transformative increases in battery capacity for advanced electric and hybrid-electric transportation applications. However, the current polymer electrolytes suffer from low ion transport rates, most commonly characterized by conductivity. This fundamental engineering science project will evaluate the hypothesis that conductivity alone is not a good metric for battery rate capability and that an equally critical component is ion transport in the polymer electrolytes. Such understanding will allow intelligent design of the next generation of solid electrolytes for electric vehicle batteries. Novel polymer electrolytes will be studied with experimental techniques not typically applied to electrolytes. The techniques provide direct measurement of ion concentration in real time and ion speciation, both of which are important for accurate and complete characterization of polymer electrolyte performance in a battery. The investigators of this project combine complementary expertise in engineering and chemistry, respectively, which will be used to further the understanding of ion-transport and battery performance through cutting-edge, time-resolved measurements and new synthetic polyelectrolytes tailored to probe structure-property relationships in these systems. For educational and outreach activities, this project includes hands-on battery activities related to this research that will be presented at economically disadvantaged middle schools. Students involved in this research will be trained with skills needed in the synthetic development of new precision polyelectrolyte systems and commercial development of lithium batteries. The goal of this research is to develop a complete picture of transport in polymer electrolytes and how it connects to battery performance. The specific aims are (1) to fully characterize transport in a systematic set of polymer electrolytes using several complementary techniques, (2) to investigate the role of ion speciation and electrolyte structure on transport behavior, and (3) to determine limiting currents for polymer-electrolyte batteries and compare to rate predictions based on a complete understanding of transport. Physical insight into the underpinnings of transport will be achieved using surface-enhanced Raman spectroscopy to evaluate dissociation state and x-ray scattering to examine the connectivity of ionic structure. This work will be conducted on the current standard polymer electrolyte, poly(ethylene oxide) (PEO) containing lithium bis-trifluoromethanesulfonimide (LiTFSI) salt. The effect of nanostructure will be examined using a mechanically strong PEO-containing block copolymer with LiTFSI. Precision polyelectrolytes blended with PEO will be used to draw conclusions about the effect of anion connectivity on dissociation state and its commensurability with the cation structure. Finally, limiting currents of polymer electrolyte batteries will be measured and predicted with continuum-level simulations. This project encompasses a thorough evaluation of transport in polymer electrolytes for lithium batteries which could shift the paradigm in understanding what limits the discharge rate in these batteries. Knowledge will be gained of the physical underpinnings of transport using a combination of frontier techniques and novel materials. A validated polymer electrolyte battery model will be developed and made publicly available. It will assist the battery community in determining target transport properties for the next generation of solid electrolytes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.