Fluorination/Defluorination Behavior of Y₂C in Fluoride-Ion Battery Anodes

Despite the high theoretical energy density of fluoride-ion batteries (FIBs), their practical applications are hindered by the large volume changes associated with the redox reactions (typically metal ↔ metal fluoride interconversions) of most of the corresponding anode materials. Consequently, FIB anode materials that react at low potentials with small expansion and shrinkage are desired. Inspired by the low theoretical volume change (8%) of the Y2C ↔ Y2CF2 interconversion, we herein evaluated Y2C as an FIB anode material and determined its initial discharge and charge capacities as 565 and 432 mAh g–1, respectively. The first fluorination was characterized by a capacity plateau equivalent to a two-electron reaction at −2 V vs Pb/PbF2. The first and second halves of this region corresponded to the Y2C → Y2CF2 intercalation reaction and Y2CF2 lattice expansion, respectively, whereas further fluorination led to a YF3-like structure. Y2CF2 formed at the end of the first plateau was reversibly defluorinated to Y2C upon charging. The reversible change in the shape of the C K-edge electron energy loss spectrum during charge–discharge indicated the contribution of carbon to the redox reaction. Thus, this paper presents, for the first time, an account of the reversible electrochemical intercalation of fluoride ions in FIB anode materials, paving the way for FIB commercialization

Fluorosulfide La2+xSr1−xF4+xS2 with Triple-fluorite Layer Enabling Interstitial Fluoride-ion Conduction

Fluoride-ion conducting solid materials are applicable as solid electrolytes for sensing devices and next generation rechargeable batteries. Most of the previously reported materials have limited to the single-anion compounds such as fluorite-type, tysonite-type, and perovskite-type structures. These are suffered from further improvements by crystal structure modification which derives a paradigm shift in the material tailoring. Fluoride and sulfide ions prefer respective coordination environments because of the different ionic radii and electronegativity. This feature implies that fluorosulfide mixed-anion compounds have potential to form anion-ordering crystal structures with new fluoride-ion conducting layers. Herein, we have found that the fluorosulfide La2+xSr1−xF4+xS2 exhibits fluoride ion conduction. The presence of multiple anions results in the formation of anion-ordering two-dimensional crystal lattice with triple fluorite layers, which cannot be realized for metal fluorides. Sulfide ions in the crystal structure increases the number of interstitial sites of fluoride ions, forming fluoride ion conduction pathway

Lithium Atom and A‑Site Vacancy Distributions in Lanthanum Lithium Titanate

Lanthanum lithium titanate (LLTO) is one of the most promising electrolyte materials for all-solid-state lithium-ion batteries. Despite numerous studies, the detailed crystal structure is still open to conjecture because of the difficulty of identifying precisely the positions of Li atoms and the distribution of intrinsic cation vacancies. Here we use subangstrom resolution scanning transmission electron microscopy (STEM) imaging methods and spatially resolved electron energy loss spectroscopy (EELS) analysis to examine the local atomic structure of LLTO. Direct annular bright-field (ABF) observations show Li locations on O4 window positions in Li-poor phase La_0.62Li_0.16TiO₃ and near to A-site positions in Li-rich phase La_0.56Li_0.33TiO₃. Local clustering of A-site vacancies results in aggregation of Li atoms, enhanced octahedral tilting and distortion, formation of O vacancies, and partial Ti⁴⁺ reduction. The results suggest local LLTO structures depend on a balance between the distribution of A-site vacancies and the need to maintain interlayer charge neutrality. The associated local clustering of A-site vacancies and aggregation of Li atoms is expected to affect the Li-ion migration pathways, which change from two-dimensional in Li-poor LLTO to three-dimensional in Li-rich LLTO. This study demonstrates how a combination of advanced STEM and EELS analysis can provide critical insights into the atomic structure and crystal chemistry of solid ionic conductors