Fluoride-ion batteries, operating through multi-electron reactions between metal-metal fluorides, possess remarkably high theoretical energy density and are considered promising candidates for post lithium-ion batteries. In previous research, La [1], Ce [2], and Mg [3] have been reported as potential anode materials for fluoride-ion batteries. However, their fluorination reactions are conversion reactions associated with an approximate 50% volume change, necessitating the exploration of alternative anode materials with small volumetric changes. In this study, we focused on the intercalation reaction, previously reported for cathode materials in fluoride batteries [4], and investigated Y₂C, which shares structural similarities with LiCoO₂. First-principles calculations suggested that Y₂C undergoes an intercalation reaction with Y₂CF₂, resulting in a relatively small volume change of 8% [5,6]. Nevertheless, the (de)intercalation reaction has not been experimentally reported, and the charge-discharge behavior remains to be elucidated. Consequently, we fabricated an all-solid-state fluoride-ion battery to evaluate the electrochemical characteristics of Y₂C and to unravel the reaction mechanism.

Y₂C was synthesized from yttrium and carbon by the arc melting method, and Ca_0.5Ba_0.5F₂ electrolytes were synthesized by the ball milling using CaF₂ and BaF₂. All-solid-state fluoride-ion batteries were assembled by stacking the Y₂C working electrode (Y₂C + Ca_0.5Ba_0.5F₂ + acetylene black), the Ca_0.5Ba_0.5F₂ electrolyte and PbF₂ counter electrode (PbF₂ + acetylene black). Charge-discharge tests were performed at 200 °C with a current density of 10.5 mA g⁻¹.

The crystal structure of the synthesized Y₂C was identified as the low-temperature phase with a layered rock-salt structure [7]. In the electrode composites prepared by ball milling, the crystallinity of Y₂C was reduced, and a part of Y₂C transfromed to Y₂CF₂-type structure. The initial discharge, corresponding to the fluorination reaction, exhibited a two-stage plateau, followed by a gradual potential elevation, and achieved a capacity of 565 mAh g⁻¹, while the charging capacity was 432 mAh g⁻¹. Y₂C transformed into Y₂CF₂ at the first discharge plateau and then into a low-crystalline YF₃-like structure, which did not revert to Y₂C after charging. Ex-situ XRD measurements at the first discharge plateau revealed that the Y₂C and Y₂CF₂ phases involves in fluorination reactions via continuous lattice expansion and phase transition. When discharge was terminated at 282 mAh g⁻¹, where the formation of Y₂CF₂ was completed, the Y₂CF₂ structure could be reverted to the Y₂C structure upon recharging.  Electron energy loss spectroscopy exhibited a reversible change in the fluorine concentration in the active material during the discharge-charge reaction. These results demonstrated the reversible intercalation reaction between Y₂C and Y₂CF₂. First-principles calculations of Y₂CF_x (0 ≤ x ≤ 6) validated these crystallographic transitions and confirmed that the reaction involves the exchange of electrons in the interlayer of the Y₂C slabs and fluoride ions at 0 ≤ x ≤ 2. The intercalation reaction of Y₂C is anticipated to offer a guideline for mitigating volume change in anode materials for fluoride-ion batteries.