Role of the Fe-FeCl₂ Contact Interface in Promoting Redox Reversibility and Electrochemical Kinetics in Fe/FeCl₂-Graphite Molten Salt Battery

Abstract

The Fe/FeCl₂-Graphite battery is an intermediate-temperature molten salt electrochemical system. It employs the solid Fe/Fe2+ redox as the negative electrode reactions and graphite positive electrode based on the intercalation/de-intercalation reaction of AlCl- 4, offering high safety, low material cost and a moderate operating temperature range of 110-150 °C. However, the poor electronic conductivity and high nucleation barrier in the FeCl2 material result in limited electrochemical reversibility and reaction kinetics. Here, we demonstrate that the Fe-FeCl₂ contact interface effectively enhances the redox reversibility and electrode kinetics by providing favorable sites for Fe⁰ nucleation and improving electronic conductivity. The prepared Fe-FeCl₂-CR electrode delivers low voltage hysteresis of 0.09 V and exhibits excellent electrochemical reversibility in the Fe/FeCl₂-graphite molten salt battery, achieving a specific capacity of 162.6 mAh g⁻¹ at 3 mA cm⁻² and retaining 93% of its capacity after 200 cycles. Compared with the pure FeCl₂ electrode, the Fe-FeCl₂-CR electrode shows lower direct‑current pulse resistance and negligible nucleation overpotential, which are primarily attributed to the Fe‑philic nucleation interface. This mechanism is further confirmed by density functional theory (DFT) calculations, revealing that the (110) crystal plane of nano‑Fe possesses the highest binding energy (-55.94 eV) during charging, thereby serving as the potential dominant interface for Fe⁰ nucleation. Additional self‑discharge and capacity expansion tests further confirm the electrochemical reaction stability of Fe-FeCl₂-CR electrode in Fe/FeCl₂-Graphite molten salt battery.