Fluoride-ion batteries, which can utilize multi-electron reactions between metal / metal fluorides and feature high theoretical energy densities, are anticipated to replace lithium-ion batteries as the next generation devices. La [ref. 1], Ce [ref. 2], and Mg [ref. 3] have been reported as negative electrode materials of fluoride-ion batteries. However, these electrodes suffer from poor cycle performances due to the substantial volumetric changes during fluorination (47, 64, and 40%, respectively). In light of this, new active materials with small volumetric change during fluorine-uptake reaction are highly desirable. We have previously reported Y₂C, which possesses a layered rock-salt structure like LiCoO₂ (space group: R-3m) and demonstrated a reversible fluorine intercalation reaction as well as high capacity corresponding to two-electron reaction, making it suitable for the negative electrode [ref. 4]. In this study, to search for materials with superior electrochemical properties, we investigated the electrochemical properties of titanium oxides (Ti_xO; x=2, 3, 6) [ref. 5-7], which belong to the Magnéli-phase and possess crystal structures similar to Y₂C.

Ti_xO were synthesized by arc melting of stoichiometric Ti and TiO₂, followed by sintering at 400 ℃. The working electrodes were prepared using ball milling of Ti_xO, Ca_0.5Ba_0.5F₂ electrolyte (ball-milled CaF₂ and BaF₂), and vapor-grown carbon fiber (VGCF) at a ratio of 30 : 60 : 10 wt% or 47.5: 47.5 : 5 wt%. The counter electrode were prepared using ball milling of PbF₂ with acetylene black at a ratio of 95 : 5 wt%. All-solid-state batteries were fabricated in the Ar-filled globe box by stacking Ca_0.5Ba_0.5F₂ electrolyte (separator), working electrode, and counter electrode followed by pressing them at 392 MPa. Charge-discharge tests were conducted at 200 ℃ and the current density of 0.05 mA cm-2. Ex-situ X-ray diffraction measurements are performed using laboratory (CuKa) and synchrotron XRD (λ=0.8 Å) at BL19B2 in SPring-8.

XRD measurements confirmed the successful synthesis of Ti₂O, Ti₃O, and Ti₆O from the stoichiometric starting materials. Among Ti_xO (x = 2, 3, 6), Ti₂O exhibited the largest capacity, thus, the characteristics of Ti₂O were further investigated in detail. During the first discharge (fluorination) of Ti₂O, the potential gradually increased with a plateau around -0.7 V, and a capacity of 720 mAh g^-1 was obtained up to -0.34 V. On the other hand, during the first charging process (defluorination), a plateau was observed below -1.3 V, and the capacity was 678 mAh g^-1. Ex-situ SXRD measurements were performed during first discharge and charge reactions. As the discharge progressed, the intensity of the peaks of Ti₂O decreased while those of the new (unknown) peaks increased, suggesting that two-phase reaction occurs involving fluorinated-Ti₂O (Ti₂OF_y) structure. The structure of Ti₂OF_y and the details of the fluorination reaction will be discussed at the meeting.