Yoshiharu Uchimoto

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.

Reaction mechanisms of MnMoO4 for high capacity anode material of Li secondary battery

Crystalline MnMoO4 was synthesized using a conventional solid reaction method and investigated for its physical and electrochemical properties as an anode material for Li secondary battery. The reversible amount of Li insertion/removal of MnMoO4 anode during the first cycle was about 800 mA h/g, accompanied by irreversible structural transformation into amorphous material. The amorphization during the first Li insertion was investigated by structural analysis using XRD of electrode. The charge compensation during Li insertion/removal was examined by measurement of X-ray Absorption Near Edge Structure (XANES) spectroscopy. Despite its irreversible structural transformation to amorphous during the first lithiation, subsequent cycles showed a reasonable cyclability. This paper presents the electrochemical properties of MnMoO4 and discusses the mechanism underlying the Li insertion/removal process.

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

Polymer electrolyte plasticized with PEG-borate ester having high ionic conductivity and thermal stability

Abstract We have focused on the PEG-borate ester as a new type of plasticizer for solid polymer electrolyte composed of poly(ethyleneglycol) methacrylate (PEGMA) and lithium bis-trifluoromethanesulfonimide (LiTFSI). The PEG-borate ester shows good thermal stability and high flash point. Ionic conductivity of the polymer electrolyte increases with increasing amount of the PEG-borate ester and exhibits values greater than 10−4 S cm−1 at 30 °C and 10−3 S cm−1 at 60 °C. Furthermore, PEG-borate ester has three EO chains whose lengths are variable, and various ionic conductivities are expected to depend on EO chain length. As a result, polymer electrolyte containing the PEG-borate ester whose EO chain length is n=3 shows highest ionic conductivity. Furthermore, polymer electrolytes containing PEG-borate esters show excellent thermal and electrochemical stability. The electrolytes are thermally stable up to 300 °C and electrochemically up to 4.5 V vs. Li+/Li.

Crystal structure of Ga-doped Ba2In2O5 and its oxide ion conductivity

Abstract Crystal structures of Ba 2 (In 1− x Ga x ) 2 O 5 ( x =0.00, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45) were analyzed and electrical conductivity were measured. Rietveld analysis for Ba 2 (In 1− x Ga x ) 2 O 5 ( x =0.20, 0.25, 0.30, 0.35, 0.40, 0.45) revealed that these oxides are belonging to space group Pm3m (No. 221). This result indicates that the oxide ion vacancies distribute randomly in the cubic perovskite type structure. Fourier transform of In K-edge EXAFS of Ba 2 (In 1− x Ga x ) 2 O 5 ( x =0.00, 0.10, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45) showed a peak between 1.2 and 2.0 A attributed to the nearest oxide ions around In 3+ cation. The peak was back-Fourier transformed, and the structural parameters were refined by the least-square fitting. The coordination number of In 3+ cation increased with increasing Ga 3+ cation content. This result indicates that coordination number of Ga 3+ cation is 4. Electrical conductivity for pure Ba 2 In 2 O 5 rapidly increased at about 930°C due to the order–disorder transition of oxygen vacancy. The electrical conductivities of Ba 2 (In 1− x Ga x ) 2 O 5 ( x =0.25, 0.30, 0.35, 0.40, 0.45) did not show a sharp discontinuity in the conductivity because the disorder phase of defective perovskite type structure was stabilized by doping Ga 3+ cations at low temperature. The oxide ion transference number of the Ga-doped Ba 2 In 2 O 5 was determined by ion blocking method was 1.00.

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.

Enhancement of Rate Capability in Graphite Anode by Surface Modification with Zirconia

Department of Applied Chemistry, Graduate School of Science and Engineering,Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, JapanZirconia coating the graphite was examined with a view to enhance the cycling stability of the graphite anode in Li-ion batteries.A thin zirconia film was produced on the graphite surface using an alkoxide precursor solution, followed by the conversion of thewet film into an oxide film by thermal annealing. The anode consisting of zirconia-treated graphite, acetylene black, and the binder~8:1:1! exhibits a capacity exceeding the theoretical value and a pronounced stability upon cycling even at a charge-dischargecurrent rate as high as 3C. It is suggested that the combination of nanocrystalline-zirconia and thein situ formed surface filmsbetter protects graphite from destruction upon cycling.© 2002 The Electrochemical Society. @DOI: 10.1149/1.1516410# All rights reserved.Manuscript submitted May 20, 2002; revised manuscript received August 17, 2002. Available electronically October 4, 2002.

Thermally stable polymer electrolyte plasticized with PEG-borate ester for lithium secondary battery

Abstract A novel polymer electrolyte was prepared by employing poly(ethyleneglycol) (PEG)-borate ester as plasticizer to the electrolyte composed of poly(ethyleneglycol) methacrylate (PEGMA) and lithium bis-trifluoromethanesulfonimide (LiTFSI). The PEG-borate ester shows good thermal stability and high flash point. The ionic conductivity of the polymer electrolyte increases with increasing amount of the PEG-borate ester and exhibits greater value than 10 −4 S cm −1 at 30°C and 10 −3 S cm −1 at 60°C.