Kazunori Takada

Superconductivity in two-dimensional CoO2 layers

Since the discovery of high-transition-temperature (high-Tc) superconductivity in layered copper oxides, many researchers have searched for similar behaviour in other layered metal oxides involving 3d-transition metals, such as cobalt and nickel. Such attempts have so far failed, with the result that the copper oxide layer is thought to be essential for superconductivity. Here we report that NaxCoO2·yH2O (x ≈ 0.35, y ≈ 1.3) is a superconductor with a Tc of about 5 K. This compound consists of two-dimensional CoO2 layers separated by a thick insulating layer of Na+ ions and H2O molecules. There is a marked resemblance in superconducting properties between the present material and high-Tc copper oxides, suggesting that the two systems have similar underlying physics.

A sulphide lithium super ion conductor is superior to liquid ion conductors for use in rechargeable batteries

We report that a heat-treated Li2S–P2S5 glass-ceramic conductor has an extremely high ionic conductivity of 1.7 × 10−2 S cm−1 and the lowest conduction activation energy of 17 kJ mol−1 at room temperature among lithium-ion conductors reported to date. The optimum conditions of the heat treatment reduce the grain boundary resistance, and the influence of voids, to increase the Li+ ionic conductivity of the solid electrolyte so that it is greater than the conductivities of liquid electrolytes, when the transport number of lithium ions in the inorganic electrolyte is unity.

Solid state lithium battery with oxysulfide glass

Abstract Oxysulfide glass, Li3PO4-Li2S-SiS2, shows a high ionic conductivity of 1.4 × 103 S/cm at ambient temperature and a wide electrochemical window. It is chemically and electrochemically stable against lithium metal. LixCoO2 and In-Lix were found to be good candidates for the electrode materials in solid state batteries with the glass for their electrode reactions with high reversibility. The solid state battery constructed with the materials reveals excellent cycling property, high rate capability up to almost 1 mA/cm2, and extremely small residual current.

Topochemical synthesis of Co-Fe layered double hydroxides at varied Fe/Co ratios: unique intercalation of triiodide and its profound effect.

Co-Fe layered double hydroxides at different Fe/Co ratios were synthesized from brucite-like Co(2+)(1-x)Fe(2+)(x)(OH)(2) (0 ≤ x ≤ 1/3) via oxidative intercalation reaction using an excess amount of iodine as the oxidizing agent. A new redoxable species: triiodide (I(3)(-)), promoted the formation of single-phase Co-Fe LDHs. The results point to a general principle that LDHs with a characteristic ratio of total trivalent and divalent cations (M(3+)/M(2+)) at 1/2 may be the most stable in the oxidative intercalation procedure. At low Fe content, e.g., starting from Co(2+)(1-x)Fe(2+)(x)(OH)(2) (x < 1/3), partial oxidation of Co(2+) to Co(3+) takes place to reach the M(3+)/M(2+) threshold of 1/2 in as-transformed Co(2+)(2/3)-(Co(3+)(1/3-x)-Fe(3+)(x)) LDHs. Also discovered was the cointercalation of triiodide and iodide into the interlayer gallery of as-transformed LDH phase, which profoundly impacted the relative intensity ratio of basal Bragg peaks as a consequence of the significant X-ray scattering power of triiodide. In combination with XRD simulation, the LDH structure model was constructed by considering both the host layer composition/charge and the arrangement of interlayer triiodide/iodide. The work provides a clear understanding of the thermodynamic and kinetic factors associated with the oxidative intercalation reaction and is helpful in elucidating the formation of LDH structure in general.

Exfoliated Nanosheet Crystallite of Cesium Tungstate with 2D Pyrochlore Structure: Synthesis, Characterization, and Photochromic Properties

Layered cesium tungstate, Cs(6+x)W(11)O(36), with two-dimensional (2D) pyrochlore structure was exfoliated into colloidal unilamellar sheets through a soft-chemical process. Interlayer Cs ions were replaced with protons by acid exchange, and quaternary ammonium ions were subsequently intercalated under optimized conditions. X-ray diffraction (XRD) measurements on gluelike sediment recovered from the colloidal suspension by centrifugation showed a broad pattern of a pronounced wavy profile, which closely matched the square of calculated structure factor for the single host layer. This indicates the total delamination of the layered tungstate into nanosheets of Cs(4)W(11)O(36)(2-). Microscopic observations by transmission electron microscopy and atomic force microscopy clearly revealed the formation of unilamellar crystallites with a very high 2D anisotropy, a thickness of only approximately 2 nm versus lateral size up to several micrometers. In-plane XRD analysis confirmed that the 2D pyrochlore structure was retained. The colloidal cesium tungstate nanosheet showed strong absorption of UV light with sharp onset, suggesting a semiconducting nature. Analysis of the absorption profile provided 3.6 eV as indirect band gap energy, which is 0.8 eV larger than that of the bulk layered precursor, probably due to size quantization. The nanosheet exhibited highly efficient photochromic properties, showing reversible color change upon UV irradiation.

Robust High-κ Response in Molecularly Thin Perovskite Nanosheets

Size-induced suppression of permittivity in perovskite thin films is a fundamental problem that has remained unresolved for decades. This size-effect issue becomes increasingly important due to the integration of perovskite nanofilms into high-κ capacitors, as well as concerns that intrinsic size effects may limit their device performance. Here, we report a new approach to produce robust high-κ nanodielectrics using perovskite nanosheet (Ca2Nb3O10), a new class of nanomaterials that is derived from layered compounds by exfoliation. By a solution-based bottom-up approach using perovskite nanosheets, we have successfully fabricated multilayer nanofilms directly on SrRuO3 or Pt substrates without any interfacial dead layers. These nanofilms exhibit high dielectric constant (>200), the largest value seen so far in perovskite films with a thickness down to 10 nm. Furthermore, the superior high-κ properties are a size-effect-free characteristic with low leakage current density (<10(-7) A cm(-2)). Our work provides a key for understanding the size effect and also represents a step toward a bottom-up paradigm for future high-κ devices.

Solid-state lithium battery with graphite anode

Abstract Solid-state lithium batteries with a unique construction are reported in this paper. These batteries contain two kinds of lithium ion-conductive solid electrolytes, LiI–Li2S–P2S5 glass contacted with the anode material and Li3PO4–Li2S–SiS2 glass or Li2S–GeS2–P2S5 crystalline material contacted with the cathode. The former electrolyte was selected as that stable to electrochemical reduction, and the latter two to oxidation. This construction made it possible to use graphite as the anode and LiCoO2 as the cathode in the solid-state lithium battery. The energy density of the battery is 390 W h·l−1 and 160 W h·kg−1 per total volume and weight of the cathode and anode layers, respectively, which are comparable to those of commercialized Li-ion batteries.

Unconventional superconductivity and nearly ferromagnetic spin fluctuations in NaxCoO2.yH2O

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor Na x CoO 2 · y H 2 O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na x CoO 2 were examined, a SC bilayer-hydrate sample with T c ∼4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/ T 1 in the SC sample, it was found that the coherence peak is absent just below T c and that 1/ T 1 is proportional to temperature far below T c . These results, which are in qualitative agreement with the previous result by Fujimoto et al. , suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/ T 1 T of the SC sample shows a gradual increase below 100 K down to T c , whereas 1/ T 1 T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/ T 1 T and χ bulk in the SC sample, the increase in 1/ T 1 T is at...

Synthesis and electrochemical properties of lithium ion conductive glass, Li3PO4Li2SSiS2

Abstract As a continuation of our previous study, further experiments were performed on Li 3 PO 4 Li 2 S SiS 2 lithium ion conductive glass. In the present study, we employed a twin roller for quenching process instead of liquid nitrogen. We found that the glass forming region expands by twin roller technique and conductivity up to 1.5 × 10 −3 S/cm was achieved. Structural analysis on the glass revealed that Li 3 PO 4 doping changes the glass structure of Li 2 S SiS 2 , thereby enhancing the electrical conductivity.

Lithium ion conduction in LiTi2(PO4)3

Abstract Li + ion conduction was examined for a mixture of LiTi 2 (PO 4 ) 3 (LTP) and a glassy electrolyte, 0.01Li 3 PO 4 –0.63Li 2 S–0.36SiS 2 . The addition of LTP with 10 wt.% resulted in a significant decrease in activation energy for conduction and little influence on the Li + ion conductivity, although it reduced the conduction path in the glass. The 7 Li NMR spectra of LTP was quite similar to that of Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) with a high conductivity of 10 −3 S cm −1 . These results suggest that Li + ion conduction in LTP is as high as that in the glassy electrolyte and LATP.