5 research outputs found
Fluorination/Defluorination Behavior of Y<sub>2</sub>C 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
Reversible Charge/Discharge Reaction of a Ternary Metal Fluoride, Pb2CuF6: A Highly Conductive Cathode Material for Fluoride-Ion Batteries
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<sub>0.62</sub>Li<sub>0.16</sub>TiO<sub>3</sub> and near to A-site positions in Li-rich phase La<sub>0.56</sub>Li<sub>0.33</sub>TiO<sub>3</sub>. Local clustering of A-site vacancies results
in aggregation of Li atoms, enhanced octahedral tilting and distortion,
formation of O vacancies, and partial Ti<sup>4+</sup> 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