Iron Fluoride-Based Conversion Cathodes for High-Energy Batteries—Tackling Efficiency Challenges Through Interface Engineering

The development of high-energy batteries for electric vehicles (EVs) hinges on intercalation-type materials, with limited interstitial sites to accommodate lithium. Alternatively, metal chalcogenides, oxides, and halides can storge more lithium through conversion mechanisms. For practical use as cathodes, iron fluorides (FeF_x: FeF₂ and FeF₃) and their substituted variants are some of the few conversion compounds capable of multi-electron redox with high reversibility at high potentials [1,2]. Recent studies show that conversion in FeF_x does not necessarily go through complete structural breakdown and rebuilding during lithiation/delithiation. Instead, it follows a topotactic displacement process, which resembles intercalation, involving the transport of both Li and Fe ions within the F-anion framework [3-6]. In addition, iron fluorides exhibit excellent thermal stability due to the ionic nature of Fe-F bond, making them particularly suited for cathode application. However, their poor electronic and ionic conductivity, also linked to the highly ionic Fe-F bond, results in sluggish reaction kinetics during reconversion, low initial Coulombic efficiency (ICE), and large voltage hysteresis, consequently limiting energy efficiency to <70% [3, 7].
In this presentation, we will share our research progress in addressing these challenges in FeFx-based conversion cathodes, through interface engineering guided by in situ characterization and computational modeling. Specific examples will be given to demonstrate our process design strategies for interface engineering to improve reconversion kinetics, reduce hysteresis, improve energy efficiency and working potentials. We will discuss the pathway forward for implementing FeF_x-based cathodes into high-energy cells for large-scale applications.
[1] Amatucci, G. G., Pereira, N., Fluoride Based Electrode Materials for Advanced Energy Storage Devices, J. Fluorine Chem. 128, 243 (2007).
[2] Wang F., et al., Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes, J. Am. Chem. Soc. 133, 18828 (2011);
[3] Wang, F., et al. "Ternary metal fluorides as high-energy cathodes with low cycling hysteresis." Nat. Commun. 6, 6668 (2015)
[4] Karki K., et al., Revisiting Conversion Reaction Mechanisms in Lithium Batteries, J. Am. Chem. Soc., 140, 17915 (2018).
[5] Xiao, A.W., et al. "Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes." Nat. Mater. 19, 644 (2020).
[6] Hua, X., et al. "Revisiting metal fluorides as lithium-ion battery cathodes." Nat. Mater. 20, 841 (2021).
[7] Li, L., et al. "Origins of large voltage hysteresis in high-energy-density metal fluoride lithium-ion battery conversion electrodes." J. Am. Chem. Soc., 138, 2838 (2016).

Acknowledgement Funding support by US Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office (VTO).