Kazutaka Ikeda

Quantifying cation coordination is essential for understanding ion transport in solid polymer electrolytes, yet reliable experimental determination of coordination numbers remains challenging. Here, we establish a calibrated framework that integrates Fourier transform infrared spectroscopy with neutron total scattering data to quantify cation coordination environments relevant to polymer electrolytes. Building on FTIR measurements using acetonitrile as a vibrational probe, apparent cation–acetonitrile coordination numbers can be extracted but are systematically overestimated due to coordination-dependent absorbance effects. Here, neutron total scattering provides an independent structural reference that enables correction of these biases. The corrected coordination numbers are subsequently converted to cation–polymer coordination numbers through a competitive coordination framework based on a site-balance assumption. Using this approach, quantitative coordination numbers for Li+, Na+, and Mg2+ are determined for poly(ethylene oxide) in acetonitrile solution across a range of compositions. This approach yields quantitatively reliable coordination numbers across multiple cation chemistries and polymer concentrations, providing a robust and experimentally accessible basis for interpreting spectroscopic coordination data in polymer electrolyte-relevant environments.

Clathrate hydrates are rare inclusion compounds capable of forming both crystalline and amorphous states, making them ideal materials for studying the effects of environmental factors such as symmetry and volume on the motions of guest molecules. In this study, amorphous hydrogen hydrate (a-HH) was formed at 7 K by using a low-temperature vapor deposition (VD) method and its structure and dynamics were investigated using synchrotron radiation x-ray diffraction (XRD), neutron diffraction, and quasielastic neutron scattering (QENS). The as-deposited a-HH possesses local cage-like space that accommodates hydrogen molecules, giving a structure clearly distinct from amorphous ice (a-ice). Upon heating, hydrogen molecules are released from a-HH at around 60 K, accompanied by a rapid development of the hydrogen-bond network. At 120 K, the structure approaches that of a-ice. The QENS spectrum observed during the VD displays localized motions of hydrogen molecules confined within a space with a characteristic radius of ∼2.6 Å. This space dimension is consistent with the free volume of small cages in hydrogen hydrates. The relaxation time of the motion of H2 molecules in the space is ∼50 ps, which is about one order of magnitude longer than the residence time reported for H2 confined in crystalline HH (c-HH). Thus, it was found that the behavior of H2 molecules in a-HH differs significantly from that in c-HH.

Understanding the structure–property relationships in glasses remains challenging owing to their structural complexity, especially in multicomponent practical glasses. In this work, neutron diffraction with isotopic substitution was employed to investigate lithium coordination environments in halide (chloride and/or bromide)-doped borate glasses, which exhibit variations in lithium-ion conductivity associated with anion mixing. Lithium-specific neutron pair distribution function analysis reveals that the conductivity enhancement in oxide–halide mixed glasses originates from the breaking of one short Li–O bond and the formation of two longer Li–halogen bonds. The formation of LiO₃Cl₂ and LiO₃Br₂ units is observed in chloride- and bromide-doped glasses, respectively, whereas a glass containing equal amounts of chloride and bromide exhibits LiO₃ClBr units in addition to LiO₃Cl₂ and LiO₃Br₂ units. These results suggest that the coexistence of multiple lithium–oxygen–halogen polyhedral units hinders lithium-ion migration, leading to reduced ionic conductivity in halide–halide mixed glasses. This work provides new insights into the structure–property relationships in lithium-ion conducting glasses through lithium-specific structural analysis.

Hydrogen absorption and desorption in solids are pivotal reactions involved in batteries and hydrogen storage devices. However, conventional thermodynamic and electrochemical hydrogen storage using high-capacity materials suffers from high hydrogen-desorption temperatures and instability of electrolytes. In this work, we explored electrochemical hydride ion (H ^– )–driven hydrogen storage and developed a solid electrolyte, anti–α-AgI–type Ba _0.5 Ca _0.35 Na _0.15 H _1.85 , which exhibits excellent H ^– conductivity and electrochemical stability. This electrolyte is compatible with several metal-hydrogen electrodes, such as titanim hydride and magnesium hydride (MgH ₂ ), allowing for high-capacity, reversible hydrogen storage at low temperatures. Specifically, Mg–H ₂ cells operating as hydrogen storage devices (Mg + H ₂ ⇄ MgH ₂ ) achieved a reversible capacity of 2030 milliampere hours per gram at 90°C, offering safe and efficient hydrogen-electricity conversion and hydrogen storage devices.

Because ammonia borane (AB) exhibits a high hydrogen storage density derived from dihydrogen bonds, it has the potential to be a safe, lightweight, and compact hydrogen storage material. In the tetragonal phase at around room temperature, the hydrogen occupancy is low, and the atomic arrangement and electron density distribution have not been clarified. Therefore, the hydrogen storage properties, including the decomposition reaction mechanism, which proceeds from ∼373 K, also remain unclear. In this study, neutron/X-ray total scattering measurements were performed on AB above and below its phase-transition temperature of 225 K, and the disordered atomic arrangement of hydrogen in the tetragonal phase was clarified by reverse Monte Carlo modeling of the obtained pair distribution function. In addition, the charge of each atom was quantitatively investigated by first-principles calculations and Bader charge analysis. Hydrogen in AB was orderly arranged, forming N-H^δ+···H^δ--B dihydrogen bonds in the low-temperature orthorhombic phase; however, in the room-temperature tetragonal phase, where the hydrogen atoms were disorderly arranged, the distribution of negatively charged hydrido-like H^δ- atoms coordinated to boron extended toward neutral. This situation shows that the dihydrogen bonds are unstable, which may lead to the decomposition of AB.

Z-bond, a new weak interaction that couples H-bond and electrostatic interactions, plays an important role in ionic liquid and deep eutectic solvent (DES) formation. However, little direct experimental observation of the Z-bonds is available. In the present work, X-ray scattering (XRS) and isotope-substituted neutron scattering (ISNS) multi-data reverse driven all atomic modeling [empirical potential structure refinement (EPSR)] was employed to elucidate the microstructure of choline chloride (ChCl)/3H₂O DES. The results show that Z-bonds are the determinative driving force for Ch⁺ solvation, while H-bonds directly drive Cl^- solvation. Density functional theory (DFT) calculations confirm both bond motifs and quantify their strengths. H-bonds facilitate the formation of longer chains and larger rings, whereas Z-bonds predominantly result in the formation of medium-length chains and smaller rings. The size distribution of chains and rings formed by Z-bonds significantly surpasses that of H-bonds. Thus, the Z-bonds result in a lower diffusion coefficient of Ch⁺ [(0.0336 ± 0.0011) × 10^-5 cm²/s] than that of Cl^- [(0.0651 ± 0.0013) × 10^-5 cm²/s], emphasizing the efficacy of Z-bond structures in the modulation of transport properties.