Surfactant-Assisted Synthesis of FeF3 • 0.33 H2o and Its Application As Cathodic Material for Electrochemical Energy Storage Devices: From Lithium to Fluoride-Ion Batteries

Abstract

New generation of electrochemical energy storage devices is needed to face the continuous increase in energy stored capacity demand at lower costs from human society. Moreover, due to the constant increase in number of batteries produced, the development of more sustainable devices without the consumption of limited resources at affordable costs is an actual research challenge. The conventional lithium batteries, the most widespread kind of electrochemical energy storage devices, made by an intercalation cathode (e.g., LiCoO2, LiFePO4) and a graphite anode suffer of limited capacity for high-performance application (such as in electric vehicles) and use limited mineral resources with adverse environmental consequences.Iron fluoride is a promising conversion cathode for positive electrodes in next-gen lithium batteries due to its low cost, great abundance, and ease of extraction1,2. The main problems of metal fluoride-based cathodes are the low conductivity and the structural changes during charge/discharge cycles that limit device capacity and lifespan. To effectively increase the storage capacity, several attempts to use metal lithium anodes have been made. However, this solution is now limited to primary batteries because of dendrites growth during metal plating that causes capacity fading and safety concerns.In the last decade, batteries based on fluoride-ion shuttling are drawing attention3,4 due to the possibility to use a high-capacity conversion cathode, exploiting a multi-electron transfer, and a metal anode without dendrites formation. This kind of devices, in the early stage of development, rely on electrochemical fluorination of an electropositive metal anode and the defluorination of a more “noble” metal fluoride without a metal plating-stripping process. Nowadays, electrolyte limitation is the main challenge for the development of a fluoride-ion battery that can compete with state-of-the-art lithium-ion batteries.In this work, a surfactant-assisted solvothermal route to obtain iron fluoride hydrate (FeF3 0.33 H2O) is presented. A hierarchical grape-like structure was obtained and it was tested as cathode active material in a 2025 coin-cell working in a metal-lithium battery configuration. In these tests it showed improved performance, compared to the same synthesis without surfactant. The synthetized material was also characterized by morphological and electrochemical point of view by means of Electrochemical Impedance Spectroscopy (EIS), galvanostatic charge discharge (GCD) and cyclic voltammetry (CV).The possibility to use the same cathodic material in a room-temperature fluoride-ion battery with a metal anode was assessed by using fluoride conducting liquid electrolytes5: encouraging results about the feasibility were achieved; however, further research in the field of liquid electrolytes resulted fundamental for bringing performances of this technology to the level of actual lithium batteries.