[en] Layered Li(Ni_1/2Mn_1/2)O₂ was prepared by the solution and mixed hydroxide methods, characterised by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and studied by cyclic voltammetry (CV) and charge discharge cycling in CC and CCCV modes at room temperature (r.t.) and at 50 deg. C. The XPS studies show about 8% of Ni³⁺ and Mn³⁺ ions are present in Li(Ni²⁺_1/2Mn_1/2⁴⁺)O₂ due to valency-degeneracy. The compound prepared at 950 deg. C, 12 h, solution method gives a second cycle discharge capacity of 150 mA h g^-1 (2.5-4.4 V) at a specific current of 30 mA g^-1 and retains 137 mA h g^-1 at the end of 40 cycles. CV shows that the redox process at 3.7-4.0 V corresponds to Ni²⁺↔Ni⁴⁺ and clear indication of Mn^3+/4+ couple was noted at 4.2-4.5 V. The observed capacity-fading (2.5-4.4 V) is shown to be contributed by the polarisation at the end of charging. The cathodic capacity is stable up to 40 cycles in the voltage window, 2.5-4.2 V both at room temperature and 50 deg. C

[en] Layered Li(Ni_1/3Co_1/3Mn_1/3)O₂ was prepared by mixed hydroxide method and characterised by means of X-ray diffraction, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry and charge-discharge cycling. The hexagonal lattice parameters obtained for the compound are: a=2.864 and c=14.233 Angst. XPS studies show that the predominant oxidation states of Ni, Co and Mn in the compound are 2+, 3+ and 4+, respectively with small content of Ni³⁺ and Mn³⁺ ions. Initial discharge capacity of 160 mAh/g was obtained in the range 2.5-4.4 V and at a specific current of 30 mA/g of which 143 mAh/g was retained at the end of 40 charge-discharge cycles. At lower current (10 mA/g) and in the voltage window 2.5-4.7 V, discharge capacity of 215 mAh/g is obtainable. From the voltage profile and cyclic voltammetry, the redox processes occurring at ∼3.8 and ∼4.6 V are assigned to the Ni^2+/4+ and Co^3+/4+ couples, respectively

No abstract available

[en] The electrochemical performance of mixed oxides, Ca₂Fe₂O₅ and Ca₂Co₂O₅ for use in Li-ion batteries was studied with Li as the counter electrode. The compounds were prepared and characterized by X-ray diffraction and SEM. Ca₂Fe₂O₅ showed a reversible capacity of 226 mAh/g at the 14th cycle and retained 183 mAh/g at the end of 50 cycles at 60 mA/g in the voltage window 0.005-2.5 V. A reversible capacity in the range, 365-380 mAh/g, which is stable up to 50 charge-discharge cycles is exhibited by Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V and at 60 mA/g. This corresponds to recycleable moles of Li of 3.9±0.1 (theoretical: 4.0). Significant improvement in the cycling performance and attainable reversible capacity were noted for Ca₂Co₂O₅ on cycling to an upper cut-off voltage of 3.0 V as compared to 2.5 V. Coulombic efficiency for both compounds is >98%. Electrochemical impedance spectroscopy (EIS) data clearly indicate the reversible formation/decomposition of polymeric surface film on the electrode surface of Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V. Cyclic voltammetry results compliment the galvanostatic cycling data

[en] Compounds of the formula LiCo_1-yCr_yO₂ (0.0≤y≤0.20 and y=1.0) have been synthesized by high temperature solid-state reaction and were characterized by XRD and FT-IR. Hexagonal a and c lattice parameters increase with increasing y as expected from ionic size effects. Cyclic voltammograms reveal that the phase transformation occurring at x=0.5 in Li_1-x(Co_1-yCr_y)O₂ is suppressed for y=0.05 and 0.10. Low-current (0.01 C; 1 C=140 mA g^-1) galvanostatic charging curves show that the deintercalation voltage for y=0.05 and 0.10 decrease for a given x as compared to LiCoO₂. Galvanostatic charge-discharge cycling of the Li(Co_1-yCr_y)O₂ cathodes at 0.14 C and 2.7-4.3 V (vs. Li) show that increasing amount of chromium content in the LiCoO₂ lattice drastically reduces the amount of Li that can be reversibly cycled. Ex-situ XRD of the cycled cathodes show that slight cation-mixing occurs in the layered structure for y=0.05 and 0.10 and could be the reason for their poor electrochemical performance. Reversible Li intercalation/deintercalation is not possible in LiCrO₂ in the voltage range 2.7-4.3 V

[en] Synthesis of new ternary superconductors of the type, LnMo₆Ch₈ (Ln = rare earth or Y; Ch = S, Se), has recently led to new materials which exhibit a tendency towards both superconductivity and magnetic order when one of the constituent ions is a rare earth with large magnetic moment. Extensive studies have revealed that superconductivity and antiferromagnetism coexist in DyMo₆S₈ and TbMo₆S₈ below 0.4 and 1.0 K respectively. However, the development of a ferromagnetic order in a system at Tsub(M) such that Tsub(M) < Tsub(c) (superconduct. transition temp.) may result in complete destruction of superconductivity. Such a system is called reentrant superconductor. Careful studies by a variety of techniques have shown that HoMo₆S₈ and ErRh₄B₄ constitute the first examples of reentrant superconductors. The values of Tsub(c), Tsub(M) are 1.2, 0.64 and 8.5, 1.0 K respectively. It is possible that some systems (including ErRh₄B₄) show precursor effects in the form of fluctuations into a state of oscillatory magnetization near the on-set point Tsub(M). A detailed study shall reveal the influence of superconductivity on magnetic ordering. In this paper, a general experimental survey of the structure and property data on the known reentrant superconductors are presented and future propsects are discussed. (author)

[en] Complete series of solid solutions exist in the mixed oxide pyrochlore system, Ersub(2)sup(3+)(Vsub(1-x)sup(3+)Fesub(x)sup(3+))sub(4/3)Wsub(2/3)sup(6+)Osub(7), with the crystal structure changing from cubic to rhombohedral at x = 0.25. The compounds are semiconductors at 300 K exhibiting p- or n-type behaviour depending on the value of x. The observed resistivity and Seebeck coefficient data as a function of temperature have been interpreted in terms of a hopping mechanism. (auth.)

[en] A wide variety of electropositive elements of the periodic table can be inserted into the vacant sites in the host framework structure of hexagonal NbTiP₃O₁₂ (an analog of nasicon) to give rise to isostructural phases. Synthesis, characterization, and preliminary data on the structure, IR spectra, and electrical resistivity are presented. Possible areas for further exploration are delineated

[en] The Chevrel-phase compounds Mo⁶X⁶I² (X = S,Te) exhibit very interesting superconducting behvior. The transition temperatures dramatically increase in both Mo⁶X⁸ structures be replacing 2 of the X atoms with iodine atoms. This enhancement in superconducting behavior is closely related to the number of electrons on the Mo⁶ cluster in these compounds. The iodine atoms in these compounds preferentially occupy the 2c sites while the remaining X atoms occupy the 6f sites. We have performed the Moessbauer effect measurements using the 57.6-keV transition in ₁₂₇I to evaluate the charge on the iodine. Assuming the charge on S to be -2, the experimental value of nearly -1 charge on iodine leaves 22.3 electrons per Mo⁶ cluster which is close to that predicted in SnMo⁶S⁸). In Mo⁶Te⁶I², assuming Te to have nearly -1 charge, we obtain 28.3 electrons per Mo⁶ cluster. The observation of superconductivity in this compound then reflects a significantly distinct band structure of tellurides compared to those of sulphides. The quadrupole interactions at the odine nucleus in Mo⁶S⁶I² and Mo⁶Te⁶I² are small but have opposite signs signifying bonding differences between these compounds

No abstract available