FeF2-Li Batteries in Ionic Liquid Electrolytes

Abstract

The electrification of the aviation industry requires secondary batteries that meet strict technical specifications, which cannot be achieved by current lithium-ion cells. A step improvement in stack-level energy densities in addition to safer battery chemistries compatible with operations at temperatures up to 80°C are necessary to minimize water-cooling requirements and their adverse impact on energy density. [1]Lithiation of transition metal fluorides (TMFs) cathodes occurs via a two- or three-electron transfer conversion mechanism, enabling an increase in theoretical energy density by 200% to 300% compared to conventional intercalation compounds. [2]Ionic liquids (ILs) are characterized by their high thermal stability, little to no flammability, and low vapor pressure. [3]Combining TMF cathodes with a lithium metal anode and an IL electrolyte results in a battery with a high energy density, safe cell operation, and high-temperature compatibility suitable for applications in electric aviation. [4, 5]Here we present the progress we have made in the development of secondary FeF2-IL-lithium-metal batteries. Through a systematic investigation of active material particle morphology and carbon composite preparation techniques, we have been able to demonstrate stable capacity retention for over 500 cycles.A post-mortem characterization analysis elucidates the role of the IL in forming stable interphases with both anode and cathode materials and on their ageing mechanism at room and elevated (80°C) temperatures. Viswanathan, V. et al. The challenges and opportunities of battery-powered flight. Nature 601, 519–525 (2022).Wang, L. et al. Li-free Cathode Materials for High Energy Density Lithium Batteries. Joule 3, 2086–2102 (2019).Maton, C., De Vos, N. & Stevens, C. V. Ionic liquid thermal stabilities: Decomposition mechanisms and analysis tools. Chemical Society Reviews 42, 5963–5977 (2013).Xiao, A. W. et al. Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes. Nature Materials 19, 644–654 (2020).Olbrich, L. F., Xiao, A. W. & Pasta, M. Conversion-type fluoride cathodes: Current state of the art. Current Opinion in Electrochemistry 30, 100779 (2021). Figure 1