Collaborative Research: Connection Between Electrical Double Layer and Interfacial Reactivity in WiSEs

About This Grant

Limited availability of raw materials including cobalt and lithium is a roadblock for widespread adoption of Li-ion batteries. One way to circumvent this roadblock is to develop metal-ion batteries beyond lithium. Water-in-salt electrolytes (WiSEs), which are highly concentrated solutions where the salt content is much higher than the water content, are promising electrolytes. These electrolytes offer improved safety, reduced flammability, efficiency, and performance, while also reducing the cost compared to other highly concentrated electrolytes. This collaborative project will use experiments and theoretical modeling to gain a fundamental understanding of the behavior of WiSEs at electrified interfaces. Discoveries in battery electrolytes have the potential to drive economic prosperity through economic growth and technological advancement, while simultaneously improving societal welfare by enhancing safety. The project will support training of two graduate students at the intersection of electrochemical systems and interfacial engineering and will provide opportunities for undergraduates to participate in the research. This project will advance fundamental knowledge of WiSEs at electrified interfaces, focusing on K- and Na-based WiSEs and model electrode materials. The research hypothesis is that tuning the WiSE electrical double layer (EDL) provides a pathway to modulate the heterogeneous electron transfer rate. This project will provide insight into how surface potential, ion specific effects, the amount of solvent and electrode material determine the EDL structure and cluster composition, as well as the screening of electrode charge. Task B will deliver how the species associations and voltage drop at the interface determine the activity and reactivity of each species, and thereby, modulate the interfacial electron transfer. In experiments, the EDL will be studied via attenuated total reflectance–surface enhanced infrared absorption spectroscopy, electrochemical impedance spectroscopy, and potential-dependent force spectroscopy by Atomic Force Microscopy. This will be combined with surface-force measurements using a surface forces apparatus and wide/small x-ray scattering to determine the 3D structure from the interface into the bulk. Mechanistic insight into electron transfer will be gained using in-situ voltammetry with an ultramicroelectrode. For the theory, the current EDL framework will be extended to account for surface effects, divalent cations and redox molecules, calculate species and clusters activities, and incorporate these predictions into an interfacial reactivity model based on the coupled ion-electron transfer theory. The theory will give access to information that is difficult to access experimentally, e.g., molecular configurations at the interfaces, association constants, cluster distribution; this insight will help test and revise hypotheses. The experiments will provide data to validate the 3D interfacial structures, to determine the accuracy of the models, and, in turn, will help improve and tune the theory and models. 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.