2022 Annual Meeting
Investigating Fluorinated Electrolytes and Guiding Electrolyte Engineering in Lithium Metal Batteries
To support a clean energy transition, and specifically to decarbonize the transportation sector, electric vehicles must become more competitive with internal combustion engine vehicles, specifically by increasing the range of EVs. This can be done by making higher energy density batteries, such as lithium metal batteries. Lithium metal could enable very energy dense devices due to its low density, low electrochemical potential, and high theoretical specific capacity, more than twice that of lithium-ion batteries.However, lithium metal batteries have yet to be commercialized due to insufficient coulombic efficiency, yet to reach 99.99%, and dendrite growth, a potential cause of shirt-circuiting batteries, a safety hazard. Electrolyte engineering can be used to solve each of these problems, specifically via the formation of an effective solid electrolyte interphase (SEI) which forms via reactions between the lithium and electrolyte and can conduct lithium ions while also blocking dendrites. Over the past decade, batteries made with fluorinated electrolytes have been found to yield improved performance, but the reasons for this are unclear and the mechanisms at play are yet to be uncovered. As such, to hasten the development of commercial lithium metal batteries and allow for more targeted electrolyte engineering, increased mechanistic understanding of what makes a good electrolyte is needed. To do this, Aurbach cycling on a wide variety of fluorinated electrolytes with a range of co-solvents has been done during this project. The selected co-solvents provide different beneficial electrolyte characteristics including increased oxidative stability, increased solubility, and decreased viscosity. These fluorinated electrolytes include a variety of chemistries and functional groups including aldehydes, ketones, cyclic carbonates, and borate-based ethers, among others. This work has found novel trends in the formulation of effective electrolytes. First, regardless of the co-solvent, fluoroethylene carbonate allows for stable cycling and high coulombic efficiency making this a promising electrolyte for further evaluation. Second, many fluorinated electrolytes are overly reactive and produce a wide variety of side reactions, leading to meaningless coulombic efficiency values. The breadth of results produced by this research has provided a comprehensive set of data which allows for further understanding of how to best engineer and evaluate novel electrolyte formulas, while beginning to illuminate the key mechanisms at play in these batteries.