Stability of Zirconium-Substituted Face-Centered Cubic Yttrium Hydride.

The stability of a zirconium (Zr)-substituted face-centered cubic (FCC) yttrium (Y) hydride (Y1-xZrx hydride) phase was investigated experimentally and theoretically. Two possible sites for hydrogen atoms exist in the FCC structure, namely, T- and O-sites, where hydrogen is present at the center of the tetrahedron and the octahedron composed of Y and/or Zr metals. The P-C isotherms revealed that the hydrogen content per metal (H/M) with 33% Zr-substituted YH3-δ was 2.2-2.3, which was lower than the expected value calculated from the starting composition of YH3-33% ZrH2 (Y0.67Zr0.33H2.67, H/M = 2.67). Hydrogen at the O-site in Y1-xZrx hydride mainly reacted during hydrogen desorption/absorption. On the basis of theoretical analyses, the hydrogen atoms do not occupy the center of the octahedron, when at least two of the six vertices of the octahedron were composed of Zr. The O-sites, where more than two Zr atoms coordinate, nonlinearly increased with the Zr content, and when the Zr content was >50%, almost no hydrogen atoms occupy the O-sites. The theoretical discussion supported the experimental results, and the Zr substitution was confirmed to reduce the occupancy of H at the O-site in the FCC YH3 significantly.

Realizing the Single-Phase Spinel-Type Sodium Titanium Oxide with the Li4Ti5O12-like Structure for Building Stable Sodium-Ion Batteries.

Sodium titanium oxide with a spinel-type structure is suitable for the stable sodium-intercalation host for the negative electrode of sodium-ion batteries, such as the spinel-type lithium titanium oxide (Li4Ti5O12, LTO) material for lithium-ion batteries. Recently, this has been partly discovered as the Na3LiTi5O12 (NTO) phase in the LTO particle. However, the single-phase NTO material has never been obtained, preventing accurate material characterizations and applications. Here, we successfully realized the NTO material with the single-phase by the chemical sodium insertion-extraction process. The chemical sodium-inserted LTO material is well converted to the pure NTO phase in the single particle level, via following chemical oxidation by water. The purified material was about 97 mol % of NTO as the single-phase spinel structure with a = 8.746 Å. The basic lattice framework of the prepared NTO was confirmed to be the same as that of the LTO. The single-phase NTO electrode shows 0.8 V versus Na+/Na of the Na-insertion and extraction potential, and 99.4% of Na-insertion capacity with 99.7% of Coulombic efficiency during 200 cycles of the Na-ion half-cell experiment. Further, the Na2Fe2(SO4)3/NTO full-cell shows 3 V-class stable charge-discharge character during 100 cycles. This excellent stability of Na-insertion and extraction properties of single-phase NTO extends the range of constructing safe and stable high-voltage oxide-based sodium-ion battery cells for practical use.

Facile Synthesis of LiH-Stabilized Face-Centered-Cubic YH3 High-Pressure Phase by Ball Milling Process.

A face-centered-cubic (FCC) YH3 phase is known to be stable only under high pressure (HP) of more than gigapascal order, and it reverts to the hexagonal YH3 ambient-pressure phase when the pressure is released. We previously found that the FCC YH3 can be stabilized even at ambient pressure by substituting Y for 10 mol % Li (LiH-stabilized YH3, LSY). The LSY was synthesized by heat treatment under gigapascal HP, but this process is unfavorable for mass production; that is, only a few tens of milligrams of a sample can be obtained in a single batch. In this study, we overcame this problem by applying a ball milling (BM) process for synthesizing the LSY phase, and the yield by the BM process reached on the order of grams. We confirmed that the structure of the BM sample was the same as that of the HP sample by X-ray diffractometry, Raman spectroscopy, and neutron total scattering pair distribution function analyses. The crystallinity of the BM sample, however, was lower than that of the HP sample. The difference in the crystallinity affects the thermal stability of the LSY. The BM sample with a lower crystallinity released hydrogen at a lower temperature. The BM sample was found to reversibly desorb/absorb hydrogen maintaining its initial FCC structure when the rehydrogenation temperature was at 423 K. However, when the rehydrogenation temperature of BM sample was more than 573 K, the FCC structure changed to the hexagonal ambient pressure phase due to thermal instability of FCC phase for the BM sample.

Stabilization of Face-Centered Cubic High-Pressure Phase of REH3 (RE = Y, Gd, Dy) at Ambient Pressure by Alkali or Alkaline-Earth Substitution.

Heavy rare-earth trihydrides such as GdH3, DyH3, and yttrium trihydide (YH3) usually show a hexagonal crystal structure under ambient pressure. This structure is known to transform to a face-centered cubic (FCC) one at higher pressures (at the order of GPa), and the FCC one returns to the hexagonal structure when the applied pressure is released. In this study, we investigated the structure of alkaline or alkaline-earth (A)-substituted REH3 (RE = Y, Gd, Dy; A = Li, K, or Mg) using X-ray diffraction, and measured the phase transition pressure. We found that this FCC high-pressure phase can be stabilized by 10-33 mol % A substitution for RE in the REH3. The mechanism of phase stabilization is simply explained by the ionic radius ratio between the cation and anion ( rcat/ rani), as well as the stabilities of other ionic crystals such as perovskite materials. For all considered REH3 samples, the FCC phase becomes stable when rcat/ rani > 0.856, such as in the case of 10 mol % Li-substituted YH3.

Study of the interface between Na-rich and Li-rich phases in a Na-inserted spinel Li4Ti5O12 crystal for an electrode of a sodium-ion battery.

Spinel lithium titanate (LTO; Li4Ti5O12) is one of the promising materials for negative electrodes of sodium-ion batteries (SIBs). The stable charge-discharge performance of SIB cells using LTO electrodes depends on the reversible Na insertion-extraction mechanism of LTO, where the spinel lattice is expanded with Na insertion, and two phases, Na-inserted LTO (Na-LTO) and Li-inserted LTO (Li-LTO) phases, are generated. These phases are confirmed using X-ray diffraction (XRD), while the mechanism of the two-phase coexistence with different lattice volumes is yet unclear. Here, we investigate the detailed morphology of the coexisting Na-LTO and Li-LTO phases using in situ XRD measurements and high-resolution transmission electron microscopy (TEM) observation. Na-LTO (a = 8.74 Å) and Li-LTO (a = 8.36 Å) phases are confirmed in both the electrochemically formed Na-inserted LTO electrode and the single-crystalline LTO thin specimen. We observed that the Na-LTO/Li-LTO interface is parallel to the (001) plane, and contains an inevitable lattice mismatch along the interface, while the expansion of the Na-LTO phase can be partially relaxed normal to the interface. We observed that the Na-LTO/Li-LTO interface has interface layers of lattice disordering with a 1-2 nm width, relaxing the lattice mismatch, as opposed to results from the previous scanning TEM observation. How the different lattice volumes at the two-phase interface are relaxed should be the key issue in investigation of the mechanism of Na insertion and extraction in LTO electrodes.