(211c) Advancing Lithium Battery Safety and Performance: Aqueous Electrolyte Design with Ternary Eutectic System

Lithium-ion batteries are widely used in modern electronics and portable power storage systems; however, safety concerns remain a significant consideration. Particularly, the risk of fire and explosion associated with non-aqueous electrolytes underscores the ongoing need for research and innovation. One solution to these issues involves the use of aqueous electrolytes. The recent concept of 'water-in-salt' (WISE) electrolytes has emerged, wherein the presence of numerous salt aggregates within the electrolyte prioritizes salt decomposition over solvent decomposition, forming robust interphase layers on electrodes and enhancing battery system stability. In the realm of aqueous electrolytes, efforts have been made to further expand the electrochemical stability window of WISE by reducing the number of water molecules in the lithium ion solvation shell. In this study, acetamide was introduced as a third component. Ternary eutectic electrolytes were formed by altering the ratio of lithium salt, acetamide, and water molecules, with the 2:3:1 ratio showing the most significant extension of the electrochemical stability window. Comparisons among the systems were observed through Molecular Dynamics (MD) and Density Functional Theory (DFT) analysis. In order to validate the micro-scopic bonding network, we computed the radial distribution functions (RDFs), and the results showed that the Li-TFSI interaction peaked at 2:3:1, with subsequent values occurring at 2:4:1 and 1:3:1. Li-TFSI-Li-TFSI-Li was also measured, and the distance between them was closest at 2:3:1. This demonstrated that the aggregation at 2:3:1 had a large number of solvated structures. The closest distance of 2.625Å of Li-H₂O in 2:3:1 indicates that 2:3:1 has more compact solvent shell than the other two compositions, although having the smallest RDF pick. The most solvated structures from each system were chosen based on the MD results. Through the use of DFT calculations, it was determined that the OH binding of H₂O strengthened in the 2:3:1 solvent structure, implying a weakening of the H₂O network. Electronic structure changes are shown by the projected density of states as a result of solvation structure modifications. The lowest unoccupied molecular orbital (LUMO) value of H₂O rises upward relative to the other two systems, the HOMO-LUMO gap of H₂O widens, and the HOMO-LUMO of TFSI increases. As the gap changes, H₂O breakdown is inhibited and the formation of a high-quality passive layer on the surface is facilitated. Moreover, an electrode/electrolyte interphase is produced during TFSI breakdown, which enhances electrochemical stability. Lastly, we computed the situation of raising the interfacial layer's thickness because water breakdown can still happen because of electron tunneling, even when the potential window is raised. As a result, DFT determined that as SEI thickness increases, the tunneling coefficient reduces, and MD verified that water decomposition decreases. This study provides this novel electrolyte design that greatly expands the electrochemical stability window while retaining the high safety of aqueous electrolytes by combining non-flammable asymmetric donor-acceptor molecules with aqueous solutions. This breakthrough goes beyond lithium-ion technology and advances the creation of high-energy-density, safe, and affordable aqueous batteries.