Kazuyasu Tokiwa

Negative electrodes for Na-ion batteries

Research interest in Na-ion batteries has increased rapidly because of the environmental friendliness of sodium compared to lithium. Throughout this Perspective paper, we report and review recent scientific advances in the field of negative electrode materials used for Na-ion batteries. This paper sheds light on negative electrode materials for Na-ion batteries: carbonaceous materials, oxides/phosphates (as sodium insertion materials), sodium alloy/compounds and so on. These electrode materials have different reaction mechanisms for electrochemical sodiation/desodiation processes. Moreover, not only sodiation-active materials but also binders, current collectors, electrolytes and electrode/electrolyte interphase and its stabilization are essential for long cycle life Na-ion batteries. This paper also addresses the prospect of Na-ion batteries as low-cost and long-life batteries with relatively high-energy density as their potential competitive edge over the commercialized Li-ion batteries.

New High-Tc Superconductor Family of Cu-Based Cu1-xBa2Can-1CunO2n+4-δ with Tc>116 K

A new high-temperature and nontoxic superconductor family of Cu1-xBa2Can-1CunO2n+4-δ with critical temperature (Tc)>116 K was discovered. These materials were prepared by the high-pressure technique. A superconductor sample with the highest T c contains mainly a Cu1-x Ba2Ca3Cu4O12-δ phase. The T c value of the Cu1-x Ba2Ca3Cu4O12-δ phase is 116.4 K. The phase has two kinds of tetragonal structures: body-centered tetragonal (bct) and simple tetragonal. The lattice parameters of bct are a=7.7114 A and c=35.986 A. The disordering or full occupation of Cu-vacancy sites leads to the simple tetragonal phase with the space group of P4/mmm and the half values of the lattice constants (a=3.8557 A and c=17.993 A) of the bct phase. The relationship between the lattice constant c and the number of CuO2 layers n is c/2=8.4+3.2(n-1) ( A). The small CuO2-block spacing (8.4 A) of this family suggests a lower anisotropy and higher J c than in Hg- and Tl-based superconductor families.

How to make superconducting-anisotropy least in high-Tc cuprate superconductors

Abstract The guiding principles to reduce superconducting-anisotropy were derived for high-T c cuprate superconductors. Based on the principles, we have attained the least superconducting-anisotropy γ=1.6 for a Cu-based CuBa 2 Ca 3 Cu 4 O 12−y (Cu-1234) superconductor. Further lower superconducting-anisotropy has been proposed for Cu-1245 (γ=1.3) and Cu-1256 (γ≌1.0) Cu-based superconductors.

Single-crystal synthesis, structure refinement and electrical properties of Li0.5CoO2

Single crystals of Li0.5CoO2 were synthesized for the first time by means of a chemical lithium-ion deintercalation process using LiCoO2 single crystals as a parent compound. A single-crystal x-ray diffraction study revealed an ordered superstructure with monoclinic symmetry, that could be assigned to the space group P 2/m. The crystal structure, including anisotropic displacement parameters for each atom, was refined to the conventional values R 1 = 3.1%, wR 2 = 10.9%, and S = 1.16 using 2385 independent reflections in the range sinθ/λ<1.3 A−1. The lithium-ion and vacancy ordering structure in Li0.5CoO2 was clearly revealed in the electron density distribution analyzed by the maximum entropy method (MEM) using the single-crystal x-ray diffraction data. The in-plane electrical resistivity in Li0.5CoO2 measured using the single-crystal specimen was around 2 mΩ cm and approximately temperature independent, suggesting metallic conduction. These findings were very consistent with those obtained by the FLAPW calculations.

New High-Tc Superconductor Ag1-xCuxBa2Can-1CunO2n+3-δ Family with Tc> 117 K

A new high-temperature and nontoxic superconductor Ag1-x Cux Ba2Can -1Cun O2n +3-δ family with T c>117 K was discovered. These materials are prepared by the high-pressure technique. A superconductor sample with the highest T c contains mainly a Ag1-x Cux Ba2Ca3Cu4O11-δ (x=0.75) phase. The phase has a body-centered tetragonal structure with lattice constants of a=7.7270 A and c=36.222 A. The disordering of Ag sites leads to the space group of P4/mmm and the lattice constants of a=3.8635 A and c=18.111 A. The T c value of the Ag1-x Cux Ba2Ca3Cu4O11-δ phase is 117.1 K. The relationship between the lattice constant c and the number of CuO2 layers n is c/2=5.3+3.2n ( A). The small CuO2-block spacing (8.5 A) of this family suggests a lower anisotropy and higher J c than Hg- and Tl-based superconductor families.

Study on enhancement of Tc (≥130 K) in TlBa2Ca2Cu3Oy superconductors

A TlBa2Ca2Cu3Oy (Tl-1223) superconductor with an ideal composition has a much higher Tc than the established value (~120 K). The intrinsic Tc arises by suppression of Tl substitution for Ba and Ca sites, which inherently occurs in samples prepared by conventional methods. The substitution of Tl was suppressed by preserving the samples in a heavily over-doped state (Tc~100 K) which was realized by a high-pressure (solid and oxygen) synthesis method. In addition, we found several synthesis conditions for which we could obtain high-Tc samples with good reproducibility. Residual carbon in the precursor was found to be harmful for superconductivity, so that it had to be reduced as much as possible before the high-pressure synthesis. The synthesis temperature and starting concentration of Tl, which also affect Tc, had to be controlled properly to enhance Tc. Samples show that Tc≥130 K by post-annealing in a reducing atmosphere at 650-700 °C for 0.5 h to 12 h. The sample prepared with the starting composition of Tl0.75Ba2Ca2Cu3.25Oy had an almost ideal grain composition and showed a Tc of 133.5 K. This Tc is comparable with that of HgBa2Ca2Cu3Oy (Hg-1223) which is known to have the highest Tc in superconductors.

High-pressure synthesis of single-phase 1223 mercury-based superconductors

Abstract We report the synthesis, superconducting properties and X-ray diffraction of a single-phase “1223” superconductor, i.e. Hg0.75Ba2Ca2.05Cu3.2O8+δ having a Tc of 102 K (as-synthesized sample) and 133 K (oxygen-annealed sample). The X-ray diffraction patterns of the 102 K phase and the 133 K phase both indicate a nearly perfect single phase, corresponding to the crystal structural arrangement of the Hg-1223 compound. The as-synthesized sample has the lattice constants a=3.848(1) A and c=15.779(0) A . A slight contraction in the c-axis is observed for oxygen-annealed samples. The resulting oxygen nonstoichiometry appears to play an important role in the enhancement of Tc from 102 to 133 K.

Tc vs n Relationship for Multilayered High-Tc Superconductors

We have proposed a T c vs n relationship for multilayered high- T c superconductors from the analysis of HgBa 2 Ca n -1 Cu n O y grain-aligned samples containing n =6 to 16 phases. The sample showed a large and sharp superconducting transition at 105 K in its susceptibility vs temperature curve. This means that multilayered HgBa 2 Ca n -1 Cu n O y can maintain a high T c at least up to n ≈16, otherwise many transitions would have been observed. It is generally believed that the highest T c decreases with n for n ≥4–5 for multilayered superconductors. We conclude that the highest T c is almost constant above about n =5 if the factors reducing T c , such as disorder, are eliminated for multilayered superconductors. This behavior can be explained using an inhomogeneous charge distribution model in multilayered cuprates.

Cu1−xTlxBa2Ca3Cu4O12−y (Cu1−xTlx-1234) superconductor with Tc=126 K

Abstract The samples of Cu 1−x Tl x Ba 2 Ca 3 Cu 4 O 12−y (x=0∼0.5) system with low Tl concentration were prepared by a high pressure techniques. The Cu 1−x Tl x Ba 2 Ca 3 Cu 4 O 12−y (Cu 1−x Tl x -1234: x=0.4∼0.5) samples show Tc of 126 K and superconducting anisotropy of γ=4. Their Tc values are higher than those of parent materials of CuBa 2 Ca 3 Cu 4 O 12−y (Cu-1234) and TlBa 2 Ca 3 Cu 4 O 11 (Tl-1234) by 8 K and 4 K, respectively. The anisotropy of Cu 1−x Tl x -1234 system is close to the value (γ=3.5) of Tl-1234 and higher than that of Cu-1234 (γ=1.6.

High-pressure synthesis of TlBa2Can−1CunOy (n=3 and 4) with Tc=133.5 K (n=3) and 127 K (n=4)

Abstract TlBa 2 Ca n −1 Cu n O y ( n =3: Tl-1223 and n =4: Tl-1234) have been found to have comparable T c with Hg-system which is known to show the highest T c in superconductors. Samples were synthesized under a pressure of 3.5 GPa using precursors with small residual carbon concentrations. We found that starting composition of Tl and synthesis temperature also affect the T c . Polycrystalline samples showed T c of 133.5 (Tl-1223) and 127 K (Tl-1234) after the post-annealing in nitrogen flow, respectively. Composition analysis suggested that suppression of the substitution of Tl for Ba- and Ca-site enhances the T c .