Nanoscale Insight into the Nucleation, Growth, and Properties of the Solid Electrolyte Interphase in Water-in-Salt Electrolytes

About This Grant

Part 1: Non-technical summary Electrochemical energy storage plays a central role in modern technologies. In recent years, highly concentrated electrolytes have gained growing attention in energy storage research, in large part because they can expand the operating range and improve the stability of rechargeable batteries. At the same time, commonly used lithium-based batteries are approaching fundamental performance limits and rely on materials that face cost and supply constraints. As a result, alternative battery chemistries based on more abundant elements, such as sodium and potassium, are increasingly important for future energy technologies. A major challenge limiting the development of these next-generation batteries is the incomplete understanding of how protective layers - known as the solid electrolyte interphase (SEI) - form at the electrode surface during battery operation. The SEI plays a critical role in determining battery safety, lifetime, and efficiency. An ideal SEI layer should be chemically stable, electronically insulating, and ionically conductive. Its structure and properties are sensitive to the chemical environment near the electrode, but details regarding how and why remain poorly understood, especially in highly concentrated electrolytes. With support from the Solid State and Materials Chemistry Program in the Mathematical and Physical Sciences Directorate, this project aims to uncover the fundamental mechanisms that control the formation, stability, and evolution of the SEI by combining laboratory experiments with modeling. With a primary focus on super-concentrated electrolytes, the project studies water-in-salt electrolytes (WiSEs) with sodium and potassium, as well as mixtures that incorporate ionic liquids (WiSILs). By identifying how electrolyte composition influences interfacial behavior, this work informs the design of more efficient, safer, and cost-effective materials for next-generation battery systems. Beyond advancing fundamental knowledge, this project serves the national interest by supporting economic growth and technological innovation, while improving societal welfare through enhanced public safety. Broader impacts include training two graduate students at the intersection of materials and interfacial science and electrochemistry, and the involvement of undergraduate students in the principal investigator’s laboratory, helping to strengthen the STEM workforce pipeline. Part 2: Technical summary With support from the Solid State and Materials Chemistry Program in the Mathematical and Physical Sciences Directorate, this project addresses fundamental gaps in the understanding of solid electrolyte interphase (SEI) formation in super-concentrated electrolytes beyond Li-based systems. The central hypothesis is that interfacial reactivity, SEI nucleation, and growth cannot be inferred solely from bulk electrolyte properties but are instead governed by ion aggregation and solvent organization within the electrical double layer (EDL) at the electrode-electrolyte interface. The research further explores how incorporating ionic liquids into water-in-salt electrolytes can introduce not only tunability but also the emergence of new fundamental phenomena that influence SEI formation. The project thus focuses on sodium- and potassium-based water-in-salt electrolytes (WiSEs) and related water–in–salt–in–ionic liquids (WiSILs), leveraging distinct nanostructures to test hypotheses and elucidating composition-structure-property relationships for SEIs in these super-concentrated electrolytes and establishing new pathways for controlling SEI properties. Experimental research is complemented by collaboration with experts in molecular dynamics simulations of highly concentrated electrolytes to provide molecular-level insight into experimentally observed phenomena. The results are expected to advance fundamental understanding of interfacial chemistry in complex electrolytes, with broad implications for solid-state and materials chemistry, electrochemistry, and the design of next-generation battery systems. 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.