Iron fluoride-lithium metal batteries in bis(fluorosulfonyl)imide-based ionic liquid electrolytes

The aviation industry’s shift toward electrification demands greater energy density and enhanced cell safety compared to commercial lithium-ion batteries. Transition metal fluoride cathodes can store multiple lithium ions per metal center through a conversion reaction mechanism, resulting in a 3-fold increase in capacity compared to intercalation compounds. Additionally, fluoride cathodes exhibit remarkable thermal stability due to the ionic nature of the metal-fluoride bond. However, their practical implementation faces challenges due to their limited electronic and ionic conductivity. In this study, we conducted a comprehensive investigation of FeF2-Li metal cells in a lithium bis(fluorosulfonyl)imide N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid electrolyte. We explored the effects of FeF2 particle size, the distribution of conductive additives within the electrode, and the influence of the bis(fluorosulfonyl)imide anion on electrochemical behavior and its evolution throughout cycling. Our findings suggest that the rate requirements for electric aviation could be met at 80°C. [Display omitted] •Wet-milling conditions for homogeneous iron fluoride particle size distribution•Composite cathode with an active material content of 85% by novel mixing technique•FSI anion creates a protective interphase, prevents particle fusion, yet depletes with cycling•Reversible cycling using a metallic lithium anode up to 120°C Olbrich et al. explored the iron fluoride-lithium metal system with an FSI-based ionic liquid electrolyte, finding that FSI prevents particle agglomeration but is consumed in a degradation mechanism forming iron oxide. The authors examine its viability for electric flight applications.