Eiichiro Matsubara

Direct Observation of a Metastable Crystal Phase of LixFePO4 under Electrochemical Phase Transition

The phase transition between LiFePO4 and FePO4 during nonequilibrium battery operation was tracked in real time using time-resolved X-ray diffraction. In conjunction with increasing current density, a metastable crystal phase appears in addition to the thermodynamically stable LiFePO4 and FePO4 phases. The metastable phase gradually diminishes under open-circuit conditions following electrochemical cycling. We propose a phase transition path that passes through the metastable phase and posit the new phases role in decreasing the nucleation energy, accounting for the excellent rate capability of LiFePO4. This study is the first to report the measurement of a metastable crystal phase during the electrochemical phase transition of LixFePO4.

A concept of dual-salt polyvalent-metal storage battery

In this work, we propose and examine a battery system with a new design concept. The battery consists of a non-noble polyvalent metal (such as Ca, Mg, Al) combined with a positive electrode already well-established for lithium ion batteries (LIBs). The prototype demonstrated here is composed of a Mg negative electrode, LiFePO4 positive electrode, and tetrahydrofuran solution of two kinds of salts (LiBF4 and phenylmagnesium chloride) as an electrolyte. The LIB positive-electrode materials such as LiFePO4 can preferentially accommodate Li+ ions; i.e., they work as a “Li pass filter”. This characteristic enables us to construct a septum-free, Daniel-battery type dual-salt polyvalent-metal storage battery (PSB). The presented dual-salt PSB combines many advantages, e.g., fast diffusion of Li+ ions in the positive electrode, high cyclability, and a high specific capacity of lightweight polyvalent metals. The concept is expected to allow the design of many combinations of dual-salt PSBs having a high energy density and high rate capability.

Toward “rocking-chair type” Mg–Li dual-salt batteries

High energy-density rechargeable batteries are strongly demanded from the viewpoint of energy and environmental concern. This work is devoted to fundamental electrochemistry on a novel concept of rechargeable batteries, “rocking-chair type” Mg–Li dual-salt batteries (DSBs), where both Mg and Li cations are carrier ions. In this system, dangerous dendritic growth is drastically suppressed by co-electrodeposition of Mg and Li, and Mg–Li alloys can be used as anode materials with high electrical capacities. As a DSB cathode material that can accommodate both Mg and Li cations, we use a spinel oxide MgCo2O4, in which an eccentric insertion mechanism, the “intercalation & push-out” process, occurs. Mg insertion occurs at 2.9 V vs. Mg2+/Mg and Li insertion occurs at 3.1 V vs. Li+/Li, being consistent with ab initio calculations, and its capacity approximately amounts to 150–200 mA h g−1. In the combination of MgCo2O4 and Mg50Li50 alloys, the cell voltage during discharge is as high as about 2–3 V. The concept of rocking-chair type DSB systems provides a new strategy for future safe rechargeable batteries combining high energy/power densities.

Structural instability of metallic glasses under radio-frequency-ultrasonic perturbation and its correlation with glass-to-crystal transition of less-stable metallic glasses

It has been reported that the structural stability is significantly deteriorated under radio-frequency-ultrasonic perturbation at relatively low temperatures, e.g., near/below the glass transition temperature T(g), even for thermally stable metallic glasses. Here, we consider an underlying mechanism of the ultrasound-induced instability, i.e., crystallization, of a glass structure to grasp the nature of the glass-to-liquid transition of metallic glasses. Mechanical spectroscopy analysis indicates that the instability is caused by atomic motions resonant with the dynamic ultrasonic-strain field, i.e., atomic jumps associated with the beta relaxation that is usually observed for low frequencies of the order of 1 Hz at temperatures far below T(g). Such atomic motions at temperatures lower than the so-called kinetic freezing temperature T(g) originate from relatively weakly bonded (and/or low-density) regions in a nanoscale inhomogeneous microstructure of glass, which can be straightforwardly inferred from a partially crystallized microstructure obtained by annealing of a Pd-based metallic glass just below T(g) under ultrasonic perturbation. According to this nanoscale inhomogeneity concept, we can reasonably understand an intriguing characteristic feature of less-stable metallic glasses (fabricated only by rapid melt quenching) that the crystallization precedes the glass transition upon standard heating but the glass transition is observable at extremely high rates. Namely, in such less-stable metallic glasses, atomic motions are considerably active at some local regions even below the kinetic freezing temperature. Thus, the glass-to-crystal transition of less-stable metallic glasses is, in part, explained with the present nanoscale inhomogeneity concept.

Mechanical-energy influences to electrochemical phenomena in lithium-ion batteries

In lithium-ion batteries, Li ions usually infiltrate into the anode active material, which usually leads to the formation of Li compounds with expanding volumes. It is well known that the volume strain associated with dilatation/contraction at the intercalation/deintercalation cycles gradually deteriorates the electrode. The intention of this work devoting a simple Li/Sn battery system is to clearly show that such a mechanical strain accompanied by the formation of the Li–Sn compounds causes the following more fundamental phenomena: (i) the electrode potential tends to be lower than the value predicted from the chemical thermodynamics consideration, (ii) the kinetics rate of lithiation or delithiation is significantly retarded (i.e., much slower than expected from the diffusion of Li), and (iii) the electromotive force can be controlled by utilizing the elastic strain actively. Through this paper, we demonstrate the mechanical effects of such mechanical strain or energy on the electrochemical reaction with various experimental supports.

Geometry of Crystals

The origin of crystallography can be traced to the study for the external appearance of natural minerals, such as quartz, fluorite, pyrite, and corundum, which are regular in shape and clearly exhibit a good deal of symmetry. A large amount of data for such minerals have been systematized by applying geometry and group theory. “Crystallography” involves the general consideration of how crystals can be built from small units. This corresponds to the infinite repetition of identical structural units (frequently referred to as a unit cell) in space. In other words, the structure of all crystals can be described by a lattice, with a group of atoms allocated to every lattice point.

Intercalation and Push‐Out Process with Spinel‐to‐Rocksalt Transition on Mg Insertion into Spinel Oxides in Magnesium Batteries

On the basis of the similarity between spinel and rocksalt structures, it is shown that some spinel oxides (e.g., MgCo2O4, etc) can be cathode materials for Mg rechargeable batteries around 150 °C. The Mg insertion into spinel lattices occurs via “intercalation and push‐out” process to form a rocksalt phase in the spinel mother phase. For example, by utilizing the valence change from Co(III) to Co(II) in MgCo2O4, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg2+/Mg, and similarly it occurs around 2.3 V vs. Mg2+/Mg with the valence change from Mn(III) to Mn(II) in MgMn2O4, being comparable to the ab initio calculation. The feasibility of Mg insertion would depend on the phase stability of the counterpart rocksalt XO of MgO in Mg2X2O4 or MgX3O4 (X = Co, Fe, Mn, and Cr). In addition, the normal spinel MgMn2O4 and MgCr2O4 can be demagnesiated to some extent owing to the robust host structure of Mg1−xX2O4, where the Mg extraction/insertion potentials for MgMn2O4 and MgCr2O4 are both about 3.4 V vs. Mg2+/Mg. Especially, the former “intercalation and push‐out” process would provide a safe and stable design of cathode materials for polyvalent cations.

Approach for three-dimensional observation of mesoscopic precipitates in alloys by coherent x-ray diffraction microscopy

An approach for evaluating three-dimensional spatial distribution of precipitates buried within a bulk alloy by coherent x-ray diffraction microscopy (CXDM) is proposed. In this study, the outer shape and the internal structure of the micrometer-sized sample in a precipitation-hardened aluminum alloy are observed by CXDM. A high-electron-density region resulting from precipitates is clearly visualized, implying that the precipitates distribute inhomogeneously in the Al matrix at several tens of nanometer scale. This is the important result toward in situ observation of the formation of internal structures in metallic materials on the nanomesoscale by CXDM with x-ray free electron lasers.

XPS/GIXS studies of thin oxide films formed on FeCr alloys

Abstract X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray scattering (GIXS) have been used for characterizing thin oxide films formed on Fe-10 ∼ 90mass%Cr alloys by heating at 873 K in air. The XPS depth profiles indicate that the oxide film of Fe10mass%Cr alloy consists of mainly an iron oxide, and a chromium oxide is predominant in the oxide film formed on alloys with chromium more than 50mass%. In Fe30mass%Cr alloy, the oxide film consists of a two layered structure; iron rich oxide in the outer layer and chromium rich oxide in the inner layer. The thickness of the oxide films appears to be insensitive to the bulk chromium concentration in the range between 30 and 90 mass% under the present oxidation condition. The GIXS results identify the main crystallographic structure of the oxide film with corundum (Fe2O3 or Cr2O3) type structure, although it includes the preferential orientation and depends on the bulk chromium concentration.

A new aspect of Chevrel compounds as positive electrodes for magnesium batteries

Chevrel compounds are regarded as potential positive-electrode materials for magnesium rechargeable batteries, but their redox potential is only about 1.2 V vs. Mg/Mg2+. In this work, we show logically and experimentally that the redox potential of Chevrel compounds can be as high as about 2–3 V vs. Mg/Mg2+. A crucial basis for this is that Cu cations can be extracted from CuxMo6S8 at around 1.2–1.6 V vs. Mg/Mg2+ in the conventional electrolyte (Grignard-reagent/tetrahydrofuran) while the anodic dissolution of Cu metal can occur above about 1.7 V vs. Mg/Mg2+ in the same electrolyte, which means that the chemical potential of Cu in Chevrel compounds is higher than that in pure Cu metal. This thermodynamic conflict inevitably compels us to consider a certain interaction with the solvent, rather than simple deintercalation from the compound, which is discussed throughout the paper. With the use of large-molecule solvents or ionic liquids, we have observed an intriguing relaxation phenomenon, where the cations move to find more stable sites, which directly indicates that the Chevrel compounds have several sites for cations.