Excellence in Research: Designing Biodegradable Nonaqueous Electrolytes for Grid-Scale Energy Storage

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Rechargeable batteries have gained attention for energy storage applications due to environmental concerns over the use of fossil fuels. As a key component in a battery, an electrolyte serves as a medium for the transfer of charges (typically in the form of ions) between a pair of electrodes. One type of electrolytes is electrolyte solutions consisting of salts dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous). These electrolytes are typically in a liquid state during the battery operation. Despite the advantages of safety, non-toxicity, and low cost, aqueous electrolytes generally have a narrow electrochemical stability window (i.e., ~1.23 V), beyond which undesired water electrolysis occurs, triggering a series of problems for the battery electrolytes and electrodes. For nonaqueous electrolytes with organic solvents, one of the major issues is their high flammability, which might cause safety concerns during battery use and failure. This project addresses the research of novel nonaqueous electrolytes for next-generation Zinc (Zn)-ion batteries. This fundamental research will have a profound impact on efforts to further advance the technologies, enabling more cost-effective, safe, and durable batteries. The broad application and market of these technologies supports the increased use of energy generated from renewable sources, which contributes to the transition to a carbon net-zero future. The project also supports undergraduate and graduate researchers in a multi-disciplinary research experience with outreach to underrepresented groups in STEM education. The nonaqueous electrolytes under study are based on Zn salts dissolved in a deep eutectic solvent (DES), which forms a ternary system. The new system has several advantages including low cost, nonflammability, desired electrochemical performance, and environmental friendliness. This represents a new approach that can overcome the inherent issues associated with the above-mentioned aqueous and nonaqueous electrolytes. The objectives of this research are 1) to construct binary and ternary phase diagrams using differential scanning calorimetry (DSC), X-Ray Diffraction (XRD) and other techniques and determine melting points of various compositions in the individual binary systems and the ternary system, particularly the ternary eutectic point and other compositions in the isotropic liquid phase regions; 2) to obtain other physical and electrochemical properties of the selected compositions with lower melting points, including density, viscosity, ionic conductivity, and electrochemical stability window; 3) to understand the influence of Zn ion and the possible complexes on the molecular interactions and solvation structures in these compositions through spectroscopic characterization (e.g., NMR, Raman, and FTIR) and atomistic and molecular simulation; and 4) to understand anode/electrolyte interfacial behaviors (e.g., plating/stripping Coulombic efficiency, dendrite formation) and cathode/electrolyte interface (e.g., interface stability, electrochemical reaction mechanism) using operando optical microscopy, SEM, and other advanced techniques. The goal of this project is to provide fundamental insights into the relationships among structure, property, and performance for the DES system, and facilitate designing advanced battery electrolytes for grid-scale energy storage.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.