CAREER: Probing Polysulfide Redox Chemistry in Tunable Metal-Organic Frameworks for Energy Storage

Johns Hopkins University, Baltimore MD

Investigators

Abstract

Part 1: NON-TECHNICAL SUMMARY Although lithium ion batteries have revolutionized the development of portable devices and electric vehicles, they are still limited by their low charge storage capacity and high cost. Lithium sulfur batteries, which utilizes inexpensive sulfur materials, are attractive energy storage devices with up to 10 times the storage ability of current lithium ion batteries. However, lithium sulfur batteries have short lifetimes, leading to frequent battery replacement. This CAREER award project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, elucidates the critical electrochemical processes that have so far hindered the adoption of lithium sulfur batteries by using metal-organic frameworks (MOFs) as a well-defined model system for evaluating the sulfur chemistry inside a battery. MOFs are porous materials that are composed of a network of connected metal ions and organic compounds arranged in an ordered fashion. The metal ions and organic linkers can be easily changed to generate a series of materials with different structures and chemical functionalities. In this way, MOFs can be manipulated to simulate battery environments for uncovering chemical mechanisms. The PI and her research group advance the fundamental knowledge on an important electrochemical challenge to Lithium sulfur batteries, and discovery of new battery materials. Alternative high-density energy storage devices, such as lithium sulfur batteries, may enable applications in smaller portable devices, long-range electric vehicles, and reliable electric grid management. Through this project the PI also promotes engagement between local elementary school students, graduate students, and postdoctoral fellows in scientific discourse, particularly in the area of energy. Additionally the PI designs a new interactive curriculum for incorporation into an existing afterschool program that serves inner-city elementary school students in Baltimore. This hands-on demonstration series aims to provoke 5th graders to evaluate their role in the advancement of energy technologies as consumers and future scientists. The scientific and educational merits of this project advance the frontiers of energy storage technologies, promote public engagement between researchers and young students, and align with the national interest of a sustainable energy future. Part 2: TECHNICAL SUMMARY Essentially all degradation processes in today's modern batteries involve poorly understood reactions at complex interfaces. This project uses molecular functionalization techniques on geometrically and atomically well-defined chemical systems to establish materials design criteria. Metal-organic frameworks (MOFs) are used to elucidate factors that influence charge transport in porous materials and develop new strategies to promote efficient polysulfide redox in lithium sulfur (Li-S) batteries. Li-S batteries can reach up to 10x the energy density of LiBs and sulfur is earth-abundant and affordable. However, the commercialization of Li-S batteries is challenged by unstable cycling performance due to polysulfide dissolution and incomplete sulfur utilization. This CAREER project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, takes advantage of the tunability and modularity of MOFs to decouple structural and chemical variables to acquire fundamental knowledge of the underlying charge transport and redox processes in Li-S batteries. Using Zr MOFs as robust model systems, the Thoi research group works to 1) identify structure-property-function relationships between pore structure, ion diffusion, and polysulfide redox properties, 2) incorporate redox-active components to control electron and ion transport, and 3) embed chemical anchors to trap polysulfides for obtaining mechanistic insights into their redox chemistry. Atomistic understanding of structure-property relationships provides design criteria for new functional sulfur electrodes. The fundamental knowledge acquired also enables the development of new charge conducting materials for electrochemical applications in the future, thus extending the application of MOFs beyond traditional gas storage and separation to electronic devices. This CAREER award project also promotes engagement between local elementary school students, graduate students, and postdoctoral fellows in scientific discourse, particularly in the area of energy. A demonstration series, entitled “EnergyNOW!” is developed for 5th grade students in an afterschool STEM program, with the objective of having students examine their relationship with energy. Each lesson includes a Q&A segment that stimulates scientific inquiry, a demonstration that involves role play and/or an experiment, and a hands-on activity that encourages problem-solving and decision-making. 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.