RESUMEN
Li and Mn-rich Li1+xNiyCozMnwO2 (LMR-NMC, 0 < x < 0.2; w > 0.5) materials remain commercially relevant owing to their high specific capacity. Due to this stoichiometry, their synthesis forms always at least two phases: monoclinic Li2MnO3 and rhombohedral LiNiaCobMncO2 (a = b = c = 1) layered moieties. However, a complete understanding of their complex crystal structure has not yet been fully realized. The monoclinic phase may become electrochemically active only at high potentials (>4.6 V vs. Li). To complicate matters even more, it has been shown that the electrochemical performance of these materials, having formally the same stoichiometry, can vary with the chosen method of material synthesis. Identification of the chemical and/or structural reasons for these variations in performance is crucial to ensure the promotion of these important cathode materials towards a practical use. Yet most methods of analysis cannot distinguish the subtle, localized variations that account for such differences. Here, solid state 6,7Li NMR was found to be successful in identifying several distinctions between compounds with identical chemical formulae. Many distinctions can be made, and even suggested to account for some of the differences in the electrochemical behaviors noted for the differently prepared materials.
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Chevrel phases (CPs), M(x)Mo(6)T(8) (M = metal, T = S, Se) are unique materials, which allow for a fast and reversible insertion of various cations at room temperature. In spite of extensive studies of these materials, the origin of their high ionic mobility remained unclear. In a previous paper we presented for the first time a proper classification of the very complex transport behavior of different cations in the Mo(6)T(8) hosts: (i) apparent immobility of the large M cations such as Pb(2+), Sn(2+), Ag(+) in the ternary phases, MMo(6)T(8); (ii) coupled M+M' diffusion in the quaternary phases, M(x)M'(y)Mo(6)T(8), where both large and small cations can assist; (iii) cation trapping in the Mg-Mo(6)S(8), Cd-Mo(6)S(8), and Na-Mo(6)T(8) systems; (iv) a combination of low- and high-rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu-Mo(6)S(8), Mn-Mo(6)S(8), and Cd-Mo(6)Se(8) systems, and (v) a fast ionic transport for small cations such as Ni(2+), Zn(2+), and Li(+). It was shown that this behavior could be understood by a relatively simple crystallographic analysis (mapping of all the cation sites and estimations of their potential energy according to the distances of these sites from adjacent anions and cations) of the diffusion routes, which differ for different cations. For this analysis, it was necessary to complete our knowledge about the cation location in the crystal structure for several CPs with known ionic mobility. This article presents the results of a combined Rietveld analysis of powder X-ray and high-resolution neutron diffraction profiles for NaMo(6)T(8), ZnMo(6)T(8), CdMo(6)T(8), and MnMo(6)S(8). All seven compounds can be defined as classic CPs, where cation delocalization from the center of the largest cavity between the Mo(6)T(8) blocks decreases with the length of the ideal chemical bond, M-T. In addition, this work details the effect of the structural parameters on the ionic conductivity in CPs, namely, it shows how subtle changes in cation delocalization may be crucial for diffusion kinetics.
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This series of papers is devoted to unique cathode materials for Mg batteries, MgxMo6T8 (T = S, Se, x = 1 and 2) Chevrel phases (CPs). In this part, a combination of neutron and high-resolution synchrotron X-ray diffractions was used to study the crystal structure of Mg2Mo6Se8, which is triclinic at room temperature (space group P1, a = 6.868 A, b = 6.921 A, c = 6.880 A, alpha = 93.00 degrees , beta = 94.40 degrees , gamma = 96.22 degrees ). In contrast to other members of the MgxMo6T8 family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg(2+) ions per formula are located in the tetrahedral sites of the outer ring. This surprising cation location, predicted previously for Mg-containing CPs by ab initio calculations, provides the uniform distribution of the cation charge in the triclinic structure, which is similar to that of rhombohedral CPs. A mapping of the cation sites was widely used to demonstrate the variety of cation arrangement in CPs and the factors affecting this arrangement, as well as to clarify the origin of the exceptionally high mobility of the Mg(2+) ions in Mg2Mo6Se8.
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Combined CV studies and UV-Vis-NIR spectroelectrochemical investigations revealed an unusual stability of the p- and n-doped PMOThOD in the wide potential window of 4 V. The n-doping process occurs in this polymer down to -2.7 V (vs. Ag/Ag+) in a non-destructive way with the characteristic development of the omega3 transition as a function of the doping level. In situ electronic transport studies revealed a high conductivity of the n-doped polymer which implies high mobility of the negatively charged carriers in the freshly doped PMOThOD film electrodes. An increase in the cathodic polarization, long-term cycling of the film electrodes, especially of higher thickness, results in a growing contribution of the negatively charged carriers trapping to the redox properties of the PMOThOD. The trapping of the charged carriers reduces gradually the electronic conductance of the PMOThOD film, but its effect on the redox-capacity of the film (in a typical scan rates range up to 50 mV s(-1)) is only minor.
Asunto(s)
Oxadiazoles/química , Espectroscopía Infrarroja Corta/métodos , Tiofenos/química , Conductividad Eléctrica , Electroquímica , Electrodos , Estructura Molecular , Sensibilidad y Especificidad , Espectrofotometría Ultravioleta/métodos , Propiedades de SuperficieRESUMEN
This work presents, for the first time, a general mechanism of a rhombohedral (R)-triclinic (T) phase transition in Chevrel Phases (CPs) with small cations (radius<1 A), which was unclear in spite of intensive studies of these important materials in the past. In contrast to previous interpretation of the R<-->T transition in some CPs as cation ordering, T-distortion is regarded here as a particular case of general adaptation of the framework to cation insertion, which includes the deformations of the coordination polyhedra and their tilting. The research is based on a combination of experimental studies (in-situ neutron diffraction at different temperatures) for one model compound, MgMo6Se8, and structural analysis for a variety of known CPs. This analysis shows that the structure flexibility is fundamentally different for the R and T forms. As a result of the lower flexibility, in the R form, a strict correlation exists between the compression of the framework along the -3 symmetry axis and the cation position in the structure (the so-called 'delocalization'). The decreasing delocalization in the R-CPs, which occurs on cooling, leads to excessive repulsion within the cations pairs (R-Cu1.8Mo6S8 case) or undesirable asymmetry in the cation polyhedra (R-MgMo6Se8 case). The higher flexibility of the T framework allows for relaxation of these structural strains by increasing the cation-cation distances and forming a more symmetric cation environment, sometimes with higher coordination number (CN), like CN=5 in the T-Fe2Mo6S8 type. Thus, this work also proposes possible driving forces for T-distortion in CPs.
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We report on a discovery of fast cathode materials, ternary Chevrel phases (CPs), CuyMo6S8, for rechargeable magnesium batteries; the related electrochemical process displays a unique coupling between reversible Mg insertion, and Cu extrusion/ reinsertion; this coupling results in an entirely new intercalation mechanism which combines the total chemical reversibility of the electrochemical reaction of MgxCuyMo6S8 with irreversibility of its separate stages once Cu extrusion stage is reached (in MgxCuyMo6S8: 0.5x + y > 4).
RESUMEN
A sonochemical process has been developed for the insertion of tin nanoparticles into mesoporous carbon, giving a product which was used as a building block for the anode of a rechargeable Li battery; the electrode could deliver a reversible capacity of 400 mAh gr(-1)(C + Sn) at 100% cycling efficiency, which is higher than that of graphite electrodes.
RESUMEN
Electrochemical insertion of Mg ions into Mo6S8 Chevrel phase is a unique, model system for studying the nature of the energetic inhomogeneity of the host sites suitable for ions accommodation. We show that the two energy state model can be successfully used for describing the very specific Mg ions insertion kinetics into the host, in particular, as relates to a drastic increase of Mg ions mobility in the vicinity of the critical potential of 1.25 V (vs Mg). This is accompanied by very pronounced changes of the impedance spectra. On the other hand, similar behavior of impedance spectra could be obtained for geometrically nonhomogeneous intercalation electrodes, comprising a distribution of thicknesses. One can frequently meet both these cases in practice for a vast variety of intercalation electrodes (e.g., for lithiated graphite, composite Li(x)MO2, M = Mn, Ni, Co, etc.). In this paper, we developed a methodology aimed at a reliable distinction between the two alternatives, based on complex impedance and complex capacitance analysis.
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This paper deals with a study of the shape of the chronoamperometric response (current, I, vs time, t) and, eventually, the mechanism of Li-ions insertion and deinsertion to/from composite graphite electrodes obtained by a small-amplitude (incremental) technique, such as potentiostatic intermittent titration (PITT). The dependences of log I, the Cottrell parameter It(1/2), and the differential parameter d log I/d log t on the process duration (vs log t) were carefully examined both for single- and two-phase coexistence domains. log I vs log t curves for single-phase domains were characterized by a single monotonic curve with a gradually increasing slope. In contrast, the same curves for two-phase domains consist of two sequential downward concave lines. Both types of response were explained by using the cell-impedance-control model. To separate the contributions of solid-state diffusion, Ohmic drops, and slow interfacial charge-transfer kinetics to the chronoamperometric response, the data were presented in the form of the inverse Cottrell parameter, (It(1/2))(-1) vs t(-1/2), from which the chemical diffusion coefficient (D) could be obtained. Refined values of D for Li insertion into graphite obtained herein agree very well with values of the component diffusion coefficient obtained from quasielastic neutron scattering for Li insertion into HOPG, reported in the literature.
RESUMEN
The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, because it may provide a considerably higher energy density than the commonly used lead-acid and nickel-cadmium systems. Moreover, in contrast to lead and cadmium, magnesium is inexpensive, environmentally friendly and safe to handle. But the development of Mg batteries has been hindered by two problems. First, owing to the chemical activity of Mg, only solutions that neither donate nor accept protons are suitable as electrolytes; but most of these solutions allow the growth of passivating surface films, which inhibit any electrochemical reaction. Second, the choice of cathode materials has been limited by the difficulty of intercalating Mg ions in many hosts. Following previous studies of the electrochemistry of Mg electrodes in various non-aqueous solutions, and of a variety of intercalation electrodes, we have now developed rechargeable Mg battery systems that show promise for applications. The systems comprise electrolyte solutions based on Mg organohaloaluminate salts, and Mg(x)Mo3S4 cathodes, into which Mg ions can be intercalated reversibly, and with relatively fast kinetics. We expect that further improvements in the energy density will make these batteries a viable alternative to existing systems.
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AS101, a synthetic organotellurium compound, was found to have immunomodulating properties by initiation of cytokine production in vitro and in vivo. Phase I/II clinical trials currently in progress on AIDS and cancer patients treated with AS101 show significant increases in various immunological parameters, with minimal toxicity. Recently, AS101 and the protein kinase C (PKC) inducer, phorbol myristate acetate (PMA), were shown to synergize in the secretion of interleukin-2 (IL-2) and colony-stimulating factor (CSF) in vitro, by human and mouse lymphoid cells. The bryostatins, a group of natural macrocyclic lactones isolated from marine invertebrates (Bugula neritina) have been reported to be potent PKC activators with no tumour promoting activity. In this study, we investigated the synergistic effect of AS101 and a partially purified preparation of bryostatin on the production of several cytokines. Our data confirm the presence of synergism, which greatly enhances cell proliferation, IL-2, tumour necrosis factor (TNF) and interferon-gamma (IFN-gamma) secretion by human mononuclear cells (MNC) and the production of IL-2 and TNF by mouse cells. The absence of tumour-promoting activity of the bryostatins makes them particularly good candidates, in combination with AS101, for immunomodulation in vivo in clinically immunosuppressed conditions.