Hirofumi Tsukasaki

Covalency-reinforced oxygen evolution reaction catalyst.

The oxygen evolution reaction that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal–air batteries and direct solar water splitting. Cost-efficient ABO3 perovskites have been studied extensively because of their high activity for the oxygen evolution reaction; however, they lack stability, and an effective solution to this problem has not yet been demonstrated. Here we report that the Fe4+-based quadruple perovskite CaCu3Fe4O12 has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ and the gold standard RuO2. The covalent bonding network incorporating multiple Cu2+ and Fe4+ transition metal ions significantly enhances the structural stability of CaCu3Fe4O12, which is key to achieving highly active long-life catalysts.

Bifunctional Oxygen Reaction Catalysis of Quadruple Manganese Perovskites

Bifunctional electrocatalysts for oxygen evolution/reduction reaction (OER/ORR) are desirable for the development of energy conversion technologies. It is discovered that the manganese quadruple perovskites CaMn7 O12 and LaMn7 O12 show bifunctional catalysis in the OER/ORR. A possible origin of the high OER activity is the unique surface structure through corner-shared planar MnO4 and octahedral MnO6 units to promote direct OO bond formations.

A novel discharge–charge mechanism of a S–P2S5 composite electrode without electrolytes in all-solid-state Li/S batteries

All-solid-state Li/S cells with high safety and high capacity were fabricated using a sulfur composite electrode prepared by mechanically milling S, P2S5 and a conductive additive (Ketjen black). The cells with 50 wt% sulfur content in the composite electrode showed a high reversible capacity of 942 mA h (g-sulfur)−1 at a constant current density of 0.64 mA cm−2 (0.1C). The discharge–charge mechanism of the high-capacity sulfur composite electrode without electrolytes was investigated. XRD and NMR measurements showed that amorphous P2S5+x species, where sulfur chains bridged phosphorus atoms, were produced in the as-milled composite electrode. Mixing of the amorphous P2S5+x and Ketjen black in the submicron order was indicated from the FE-SEM observation and EDX mapping of the electrode. XRD and TEM measurements of the sulfur electrodes before and after the discharge–charge processes indicated that the compounds in the electrodes remained in the amorphous state during these processes. XPS measurement showed that cleavages and associations of the disulfide bonds occurred in the amorphous compounds during the discharge–charge processes. A novel discharge–charge mechanism with an atomic-level dispersion of a sulfur redox part in an ion conductive part was proposed for the high-capacity sulfur electrode.

Direct observation of a non-crystalline state of Li 2 S–P 2 S 5 solid electrolytes

There are two types of solid electrolytes which has been recently expected to be applied to all-solid-state batteries. One is the glasses characterized by an amorphous state. The other is the glass ceramics containing crystalline in an amorphous matrix. However, the non-crystalline state of glasses and glass ceramics is still an open question. It has been anticipated that sea-island and core-shell structures including crystalline nanoparticles have been proposed as candidate models for glass ceramics. Nevertheless, no direct observation has been conducted so far. Here we report the non-crystalline state of Li2S–P2S5 glasses and glass ceramics, and the crystallization behavior of the glasses during heating via direct transmission electron microscopy (TEM) observation. High resolution TEM images clearly revealed the presence of crystalline nanoparticles in an amorphous region. Eventually we suggest that the precipitation and connection of crystalline nanoparticles in an amorphous matrix are key to achieving high ionic conductivity.

Pair distribution function analysis of sulfide glassy electrolytes for all-solid-state batteries: Understanding the improvement of ionic conductivity under annealing condition

In general, the ionic conductivity of sulfide glasses decreases with their crystallization, although it increases for a few sulphide glasses owing to the crystallization of a highly conductive new phase (e.g., Li7P3S11: 70Li2S-30P2S5). We found that the ionic conductivity of 75Li2S-25P2S5 sulfide glass, which consists of glassy and crystalline phases, is improved by optimizing the conditions of the heat treatment, i.e., annealing. A different mechanism of high ionic conductivity from the conventional mechanism is expected in the glassy phase. Here, we report the glassy structure of 75Li2S-25P2S5 immediately before the crystallization by using the differential pair distribution function (d-PDF) analysis of high-energy X-ray diffraction. Even though the ionic conductivity increases during the optimum annealing, the d-PDF analysis indicated that the glassy structure undergoes no structural change in the sulfide glass-ceramic electrolyte at a crystallinity of 33.1%. We observed the formation of a nanocrystalline phase in the X-ray and electron diffraction patterns before the crystallization, which means that Bragg peaks were deformed. Thus, the ionic conductivity in the mixture of glassy and crystalline phases is improved by the coexistence of the nanocrystalline phase.

Structural changes and microstructures in stuffed tridymite-type compounds Ba1−xSrxAl2O4

Crystal structures and microstructures in Ba1−xSrxAl2O4 solid solutions between the end members of BaAl2O4 and SrAl2O4 have been carefully investigated by powder X-ray diffraction, electron diffraction and transmission electron microscopy (TEM) imaging experiments. With the help of fast Fourier transform (FFT) calculation, high-resolution TEM images suggested that diffuse streaks along three equivalent 〈110〉 directions in the (001) plane, which appear in the P63 structure of Ba1−xSrxAl2O4 for x = 0.4, originate from the large structural fluctuation of the AlO4 tetrahedral network. On the other hand, the monoclinic P21 structure in Ba1−xSrxAl2O4 with x = 0.7 was found to consist of a modulated structure with . The present experimental results reveal that a structural phase boundary exists at approximately x = 0.6 between the P63 structure with a large structural fluctuation and a monoclinic P21 phase with the single-q modulated structure.

A Fluctuating State in the Framework Compounds (Ba,Sr)Al2O4

The structural fluctuation in hexagonal Ba1−xSrxAl2O4 with a corner-sharing AlO4 tetrahedral network was characterized at various temperatures using transmission electron microscopy experiments. For x ≤ 0.05, soft modes of q ~ (1/2, 1/2, 0) and equivalent wave vectors condense at a transition temperature (TC) and form a superstructure with a cell volume of 2a × 2b × c. However, TC is largely suppressed by Sr-substitution, and disappears for x ≥ 0.1. Furthermore, the q ~ (1/2, 1/2, 0) soft mode deviates from the commensurate value as temperature decreases and survives in nanoscaled regions below ~200 K. These results strongly suggest the presence of a new quantum criticality induced by the soft mode. Two distinct soft modes were observed as honeycomb-type diffuse scatterings in the high-temperature region up to 800 K. This intrinsic structural instability is a unique characteristic of the framework compound and is responsible for this unusually fluctuating state.

Features of Ferroelectric States in the Simple-Perovskite Mixed-Oxide System (1−x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 with Lower Ti Contents

A mixed-oxide system (1 − x)Pb(Zn1/3Nb2/3)O3–xPbTiO3, which has a simple perovskite structure, has been reported to exhibit remarkable piezoelectric responses for 0 < x ≤ 0.08. To understand the fe...

Modulated structures and associated microstructures in the ferroelectric phase of Ba1−xSrxAl2O4 for 0.7 ≤ x ≤ 1.0

In order to understand the ferroelectric and ferroelastic phases in Ba1−xSrxAl2O4 for 0.7 ≤ x ≤ 1.0, we have investigated the crystal structures and their associated microstructures of the ferroelectric and ferroelastic phases mainly by transmission electron microscopy (TEM) and scanning transmission electron microscopy–high-angle angular dark-field (STEM–HAADF) experiments, combined with powder X-ray diffraction experiments. Electron diffraction experiments showed that the ferroelectric and ferroelastic phases of Ba1−xSrxAl2O4 for 0.7 ≤ x ≤ 1.0 should be characterized as a modulated structure with the modulation vector of , whose space group should be monoclinic P21. High-resolution TEM experiments revealed that the microstructures in the monoclinic phase can be characterized as twin structures and nanometer-sized planar defects due to the monoclinic structure with the modulated structures, which are responsible for anomalous elastic behaviors and mechanoelectro-optical properties. In addition, subatomic-resolution STEM–HAADF images clearly indicated that the displacement of Al3+ ions involved in the AlO4 tetrahedra should play a crucial role in the formation of the modulated structures and twin structures.

Crystallization behavior of the Li 2 S–P 2 S 5 glass electrolyte in the LiNi 1/3 Mn 1/3 Co 1/3 O 2 positive electrode layer

Sulfide-based all-solid-state lithium batteries are a next-generation power source composed of the inorganic solid electrolytes which are incombustible and have high ionic conductivity. Positive electrode composites comprising LiNi1/3Mn1/3Co1/3O2 (NMC) and 75Li2S·25P2S5 (LPS) glass electrolytes exhibit excellent charge–discharge cycle performance and are promising candidates for realizing all-solid-state batteries. The thermal stabilities of NMC–LPS composites have been investigated by transmission electron microscopy (TEM), which indicated that an exothermal reaction could be attributed to the crystallization of the LPS glass. To further understand the origin of the exothermic reaction, in this study, the precipitated crystalline phase of LPS glass in the NMC–LPS composite was examined. In situ TEM observations revealed that the β-Li3PS4 precipitated at approximately 200 °C, and then Li4P2S6 and Li2S precipitated at approximately 400 °C. Because the Li4P2S6 and Li2S crystalline phases do not precipitate in the single LPS glass, the interfacial contact between LPS and NMC has a significant influence on both the LPS crystallization behavior and the exothermal reaction in the NMC–LPS composites.