Yuki Orikasa

High energy density rechargeable magnesium battery using earth-abundant and non-toxic elements.

Rechargeable magnesium batteries are poised to be viable candidates for large-scale energy storage devices in smart grid communities and electric vehicles. However, the energy density of previously proposed rechargeable magnesium batteries is low, limited mainly by the cathode materials. Here, we present new design approaches for the cathode in order to realize a high-energy-density rechargeable magnesium battery system. Ion-exchanged MgFeSiO4 demonstrates a high reversible capacity exceeding 300 mAh·g−1 at a voltage of approximately 2.4 V vs. Mg. Further, the electronic and crystal structure of ion-exchanged MgFeSiO4 changes during the charging and discharging processes, which demonstrates the (de)insertion of magnesium in the host structure. The combination of ion-exchanged MgFeSiO4 with a magnesium bis(trifluoromethylsulfonyl)imide–triglyme electrolyte system proposed in this work provides a low-cost and practical rechargeable magnesium battery with high energy density, free from corrosion and safety problems.

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.

Layered perovskite oxide: a reversible air electrode for oxygen evolution/reduction in rechargeable metal-air batteries.

For the development of a rechargeable metal-air battery, which is expected to become one of the most widely used batteries in the future, slow kinetics of discharging and charging reactions at the air electrode, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively, are the most critical problems. Here we report that Ruddlesden-Popper-type layered perovskite, RP-LaSr3Fe3O10 (n = 3), functions as a reversible air electrode catalyst for both ORR and OER at an equilibrium potential of 1.23 V with almost no overpotentials. The function of RP-LaSr3Fe3O10 as an ORR catalyst was confirmed by using an alkaline fuel cell composed of Pd/LaSr3Fe3O10-2x(OH)2x·H2O/RP-LaSr3Fe3O10 as an open circuit voltage (OCV) of 1.23 V was obtained. RP-LaSr3Fe3O10 also catalyzed OER at an equilibrium potential of 1.23 V with almost no overpotentials. Reversible ORR and OER are achieved because of the easily removable oxygen present in RP-LaSr3Fe3O10. Thus, RP-LaSr3Fe3O10 minimizes efficiency losses caused by reactions during charging and discharging at the air electrode and can be considered to be the ORR/OER electrocatalyst for rechargeable metal-air batteries.

Catalytic Activity Enhancement for Oxygen Reduction on Epitaxial Perovskite Thin Films for Solid‐Oxide Fuel Cells

The active ingredient: La{sub 0.8}Sr{sub 0.2}CoO{sub 3-{delta}} (LSC) epitaxial thin films are prepared on (001)-oriented yttria-stabilized zirconia (YSZ) single crystals with a gadolinium-doped ceria (GDC) buffer layer. The LSC epitaxial films exhibit better oxygen reduction kinetics than bulk LSC. The enhanced activity is attributed in part to higher oxygen nonstoichiometry.

First In Situ Observation of the LiCoO2 Electrode/Electrolyte Interface by Total-Reflection X-ray Absorption Spectroscopy†

Rechargeable lithium-ion batteries (LIBs) are widely used as electrical energy storage devices for technologies such as portable electronics and electric and hybrid vehicles, and they are considered to be serious power storage candidates for smart-grid electricity systems. Traditionally, research in the field has focused on battery improvement through a selective use of new or existing materials for positive and negative electrodes, as the bulk properties of electrodes primarily limit charge capacity and power. However, the durability of LIBs is largely rooted in side reactions that occur at the electrode/ electrolyte interface, especially those at the positive electrode. Thus, controlling the chemical stability of any electrode material with respect to the operating liquid electrolyte medium, which requires a control of the electrode/electrolyte interface through surface chemistry, is as important as designing new materials. The scale of such an interfacial region is speculated to be on the order of a few nanometers, which shall be deemed as approximately the Debye length. This scale indicates that structural and chemical information should be tracked with a resolution of a few nanometers to reveal the phenomena of the electrode/ electrolyte interface. Previous research has focused on a detailed examination of the interfacial reactions at the positive electrode surface by using methods such as X-ray photoelectron spectroscopy (XPS) and surface X-ray diffraction (SXRD). However, characterization of the electrode surface at the nanoscale under conditions of an operating battery remains insufficient because of the lack of suitable observation techniques. A proposed degradation mechanism for electrodes has been extrapolated from indirect information obtained from analysis of disassembled, deteriorated electrodes. To obtain concise and meaningful surface data, a technique that enables high-resolution analysis of chemical information at the solid electrode surface is required. X-ray absorption spectroscopy (XAS), which makes it possible to identify the electronic and local structures of a certain atom, is a potent and versatile technique to resolve the chemical states of a lithium-ion electrode material independently of its crystallinity. To extract information about the interfacial phenomena by XAS, total-reflection fluorescence XAS (TRF-XAS), which integrates the fluorescence yield obtained under total reflection, can be applied. A recent study has shown that polycrystalline thin films are preferred relative to epitaxial thin films (that are strongly influenced by the substrate) to simulate the conditions of applied composite electrodes. We herein use polycrystalline LiCoO2 thin films prepared by pulsed laser deposition (PLD) as the model electrodes; these electrodes are flat at the nanoscale and have structural properties similar to those of the applied composite electrode (see section S1 in the Supporting Information). Figure 1 shows the charge/discharge cycle dependencies of cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS) of the LiCoO2 thin films used in this study (see section S1 in the Supporting Information). Typical CVs

Quantitating the lattice strain dependence of monolayer Pt shell activity toward oxygen reduction.

Lattice strain of Pt-based catalysts reflecting d-band status is the decisive factor of their catalytic activity toward oxygen reduction reaction (ORR). For the newly arisen monolayer Pt system, however, no general strategy to isolate the lattice strain has been achieved due to the short-range ordering structure of monolayer Pt shells on different facets of core nanoparticles. Herein, based on the extended X-ray absorption fine structure of monolayer Pt atoms on various single crystal facets, we propose an effective methodology for evaluating the lattice strain of monolayer Pt shells on core nanoparticles. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity.

Effects of oxygen gas pressure on structural, electrical, and thermoelectric properties of (ZnO)3In2O3 thin films deposited by rf magnetron sputtering

Zinc indium oxide films were deposited by the rf magnetron sputtering method using a (ZnO)3In2O3 target. The films were prepared at 573 K in various Ar/O2 sputtering gases (O2 content: 0%–25%). The effect of the oxygen gas content in the sputtering gas on the structural, optical, electrical, and thermoelectric properties of the films was investigated. The films had a c-axis oriented layer structure. The films deposited at 0%–3% oxygen gas contents exhibited a high electrical conductivity with a high carrier concentration, n≈1020 cm−3, while the conductivity of the films significantly decreased above the 3% oxygen gas content, having a carrier concentration below 1018 cm−3. From the optical transmission measurement, the band gap of the films was estimated to be 3.01 eV. The films deposited at 3%–8% oxygen gas contents showed a high Seebeck coefficient, −300 μV/K, while the maximum power factor, 4.78×10−5 W/m K2, was obtained at the 2% oxygen gas content. The Seebeck coefficient and the power factor were ca...

The origin of anomalous large reversible capacity for SnO2 conversion reaction

Single-nanocrystalline SnO2 (2–4 nm ϕ) particles completely encapsulated within hollow-structured carbon black structures (Ketjen Black (KB), typically 40 nm ϕ) were prepared using our original in situ ultracentrifugation (UC treatment) materials processing technology. Ultracentrifugation at 75000g induces an in situ sol–gel reaction that brings about optimized linking between limited-size SnO2 nanocrystals and microcrystalline graphitic carbons of KB. Efficient entanglement and nanonesting have been accomplished by simultaneous nanofabrication and nanohybridization in the UC treatment, specifically at a ratio of SnO2/KB = 45/55. This composite exhibited a reversible capacity of 837 mA h g−1 per composite, equivalent to 1444 mA h g−1 (per pure SnO2 after subtracting the capacity attributed to KB in the composite) for remarkably many cycles, over 1200. Such high performance in regard to both capacity and cyclability has never been attained so far for SnO2 anode materials. The reversibility of changes in the Sn valence state (defined as “formal valence state” in the manuscript) from Sn(2.9+) to Sn(4.4−) was demonstrated by in situ XAFS measurements during the lithiation–delithiation process. Peculiar nanodots of typically 2–4 nm that look like single-crystal SnO2/carbon core–shell structures were found for the optimized dose ratio (45/55) in the HRTEM observation. After 10 cycles, all the materials showed complete encapsulation of the same-sized nanoparticles, which were covered and nested within the KB matrix and an electrolyte-derived polymeric film. These results indicate that the initially prepared SnO2/KB composites were transformed into a new species, represented as LixSnO1.45 (x: 0–7.3), which shows perfect reversibility and cyclability. This species can exchange a total of 7.3 electrons, including 2.9 electrons for the conversion reaction (1–2 V) and 4.4 electrons for the subsequent alloying process (0–1 V).

Depth-resolved X-ray absorption spectroscopic study on nanoscale observation of the electrode–solid electrolyte interface for all solid state lithium ion batteries

Depth-resolved X-ray absorption spectroscopy (DR-XAS) measurements were performed for the direct observation of the chemical state and local structure at the LiCoO2 electrode–solid electrolyte model interface, which can contribute towards the enhancement of the power density in all solid-state lithium batteries. The charge transfer resistance, measured by AC impedance spectroscopy, of the LiCoO2 electrode–solid electrolyte interface decreased with the introduction of a NbO2 interlayer at the interface, while the resistance increased with ZrO2 and MoO2 interlayers. Using DR-XAS with a depth resolution of about 7 nm, the changes in electronic structure and local structure of the LiCoO2 electrode were clarified. The extended X-ray absorption fine structure of DR-XAS revealed that the introduction of the NbO2 layer is effective for restricting the large Co–O bond change at the interface during delithiation. This interlayer relieved the stress at the interface due to the volume change of LiCoO2 during delithiation and then decreased the activation energy for the charge transfer process.

Ionic Conduction in Lithium Ion Battery Composite Electrode Governs Cross-sectional Reaction Distribution

Composite electrodes containing active materials, carbon and binder are widely used in lithium-ion batteries. Since the electrode reaction occurs preferentially in regions with lower resistance, reaction distribution can be happened within composite electrodes. We investigate the relationship between the reaction distribution with depth direction and electronic/ionic conductivity in composite electrodes with changing electrode porosities. Two dimensional X-ray absorption spectroscopy shows that the reaction distribution is happened in lower porosity electrodes. Our developed 6-probe method can measure electronic/ionic conductivity in composite electrodes. The ionic conductivity is decreased for lower porosity electrodes, which governs the reaction distribution of composite electrodes and their performances.