Room Temperature Fluorination/Defluorination of FeFx Positive Electrode for All-Solid-State Fluoride Ion Batteries
Authors: Yano Akira , Yamamoto Hiroki , Maeyoshi Yuta , Fujinami So , Nakatani Tomotaka , Shimoda Keiji , Orikasa Yuki , Inada Yasuhiro , Shikano Masahiro , Sakaebe Hikari , Yoshii Kazuki 
Published: December 2024

Introduction
A fluoride battery is a new concept battery that uses fluoride ions as charge carriers. It is attracting attention as an innovative battery that surpasses lithium-ion batteries due to its high theoretical energy density. Furthermore, by making an all-solid-state battery using a solid electrolyte, it is possible to construct a kinetically advantageous battery. Iron fluoride (FeF3) is a positive electrode material with a high theoretical capacity of 712 mAh g-1. A charge/discharge capacity exceeding 500 mAh g-1 at 160 °C has been reported for a composite electrode using FeF3 powder.1 The main future challenge for FeFx positive electrode is to improve charge/discharge characteristics at low temperatures and to clarify the fluorination/defluorination mechanism. However, the currently available solid electrolytes have low conductivity, causing an increase in ohmic polarization of the composite electrode and making it difficult to evaluate the FeFx positive electrode at low temperatures. In this study, FeFx thin film electrodes were fabricated, in which the effect of ohmic polarization can be suppressed by reducing the current. Using the thin film samples, the charge/discharge characteristics and electrode reaction mechanism of FeFx positive electrodes at room temperature were investigated in detail.
Experimental
On a LaF3 single crystal substrate (solid electrolyte), a 10 nm thick FeF3 layer and C layer were sequentially deposited as a positive electrode and a current collector, respectively, by sputtering. A PbF2 layer and a Pb foil were laminated on the opposite side of the substrate as a counter electrode. The fabricated FeFx all-solid-state thin film battery was placed in a cell that could be evacuated and heated, and then charged/discharged at the desired temperature and current rate (1C was defined 712 mAh g-1, the theoretical capacity of FeF3). The discharge/charge cutoff-voltage was set to -1.5/1.5 V or -2/3 V (vs. PbF2/Pb). The FeFx electrode reaction was electrochemically analyzed by steady state polarization and AC impedance measurements. The FeFx thin film structure was investigated by SEM and XPS. The electronic state, coordination structure, and crystal structure of the FeFx during charging/discharging were analyzed by operando XAFS/XRD using synchrotron radiation (SPring-8/BL28XU).
Result and Discussion
The experimental results are summarized below.
(1) The reversible capacities at 25 °C and 0.1C were 380 and 629 mAh g-1 (53% and 88% of the theoretical capacity) for the voltage ranges of -1.5/1.5 V and -2/3 V, respectively. No capacity degradation was observed up to 30 cycles in the cycle test at 25 °C and 0.1C.
(2) The electrode reaction current was proportional to the exponent of the overpotential. This relationship is known as Tafel behavior and indicates that the fluorination/defluorination of the FeFx positive electrode is rate-limited by the charge transfer process.
(3) During discharging/charging, reversible changes in Fe valences (Fe3+, Fe2+, Fe0), atomic bonds (Fe–F, Fe–Fe), and crystal structure (FeF3, FeF2, Fe) were observed. The reaction capacity determined from the change in average Fe valence and the reversible capacity measured electrochemically were almost equal.
(4) These results indicate that the FeFx positive electrode can be charged/discharged (fluorinated/defluorinated) at room temperature with high capacities close to the theoretical value, at practical current rates, and with excellent cyclability.
Acknowledgement
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under RISING3 project (JPNP21006), Japan.

References
[1] A. Inoishi et al., Adv. Energy Sustainability Res. 2022, 2200131.