Interfacial Reactivity and Thermal Stability of Molten Salt Electrolytes with FeF₃ Cathode in Thermal Batteries

FeF3 is a promising cathode material for thermal batteries due to its high capacity and an operating voltage exceeding 3V. However, its susceptibility to hydration presents challenges in preserving its pristine form. Lithium halide molten salts are commonly employed as electrolytes in thermal batteries, but the interfacial reactions between FeF3 and these electrolytes under typical thermal battery conditions (above 500 °C) remain underexplored. This study focuses on examining these interfacial reactions through density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations.

We begin by investigating the interactions between molten LiBr and FeF3, hypothesizing the behavior of their interface. Our analysis shows that the atomic bonding between the electrolyte and cathode is influenced by the presence of moisture (H2O). When H2O is present, oxygen forms LiOx or FeOx bonds without decomposition of water, while other interactions remain largely unaffected. Bromine becomes isolated as Li-Br bonds break, with Li forming LiFx bonds with fluorine, and Fe either retains its FeF3 structure or forms additional bonds with fluorine. Charge analysis over time reveals electron transfer from LiBr to FeF3, where Br donates electrons to fluorine and becomes neutral Br. Radial distribution function (RDF) analysis indicates that moisture impacts Li-Br and Fe-Br bonds, but not Li-F or Fe-F bonds. Overall, moisture has a minor effect on interfacial reactions, slightly influencing atomistic configurations without significantly altering the overall behavior.

We also investigate the solid-solid interface stability between the molten salt electrolytes and the FeF3 cathode using DFT and AIMD, comparing the thermal stability of four different molten salts: LiBr, LiCl, KCl, and LiF. Preliminary results show that LiF exhibits the highest stability, maintaining its crystalline structure, while the other candidates demonstrate disordered interfaces. Ongoing analysis includes RDF, time-dependent coordination numbers, and charge distribution to further explore these interactions.

This work was supported by Agency for Defense Development of the Korean Government [grant number UI247005TG].