45 research outputs found
Toward Atomistic Understanding of Materials with the Conversion–Alloying Mechanism in Li-Ion Batteries
publishedVersio
Preparation, characterization, molecular and electronic structures, TDDFT, and TDDFT/PCM study of the solvatochromism in cyanovinylferrocenes
Cis and trans isomers of ferrocene-based donor-acceptor assemblies, Fc-C(I)=CH(I), Fc-C(I)=CH(CN), and Fc-C(CN)=CH(CN) (Fc is ferrocene), along with the Fc-C(CN)=C(CN)_2 complex have been prepared and characterized by ^1H, ^(13)C, gHMQC, and gHMBC NMR spectra, IR, UV-vis, and MCD spectroscopy methods, as well as elemental analyses. The oxidation potentials, investigated by cyclic voltammetry, of all donor-acceptor assemblies are in agreement with the electron-acceptor strength of the substituents attached to the ferrocene core. X-ray crystallography studies of cis and trans isomers of Fc-C(I)=CH(CN) and Fc-C(CN)=CH(CN) reveal a significant rotational flexibility of the cyanovinyl group, which was explained on the basis of semiempirical PM3 calculations. Electronic structures and solvatochromic properties of all complexes were investigated by UV-vis spectroscopy, density functional theory (DFT), time-dependent DFT (TDDFT), and polarized continuum model (PCM) TDDFT approaches. The calculated vertical excitation energies and magnitudes of solvatochromic effect are consistent with the experimental data and clearly suggest the dominance of metal-to-ligand charge-transfer bands in the visible region of the UV-vis spectra for all complexes
Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus
The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio
Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus
The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio
Mixed-valence states formation in conformationally flexible metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin
Metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin have been prepared and characterized by UV-Vis, MCD, ^1H, ^(13)C, and variable-temperature NMR, APCI- and ESI-MS, and Mössbauer spectroscopy, while their redox properties were investigated using electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation approaches. The electronic structure calculations at Density Functional Theory level reveal that both compounds adopt saddle conformations and the HOMOs in both complexes are predominantly metal-centered, while the LUMOs predominantly consist of porphyrin π* orbitals. In spite of the rotational freedom of ferrocenyl substituents at room temperature, both metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin are able to form mixed-valence states upon the successive ferrocene-based two- and one-electron oxidations, respectively, as confirmed by UV-Vis, MCD, Mössbauer, electro-, and spectroelectrochemical methods, and thus, the earlier suggested (Boyd et al. Chem. Commun., 1999, 637) requirements for the formation of mixed-valence states in ferrocene-containing porphyrins should be revised
Mixed-valence states formation in conformationally flexible metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin
Metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin have been prepared and characterized by UV-Vis, MCD, ^1H, ^(13)C, and variable-temperature NMR, APCI- and ESI-MS, and Mössbauer spectroscopy, while their redox properties were investigated using electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation approaches. The electronic structure calculations at Density Functional Theory level reveal that both compounds adopt saddle conformations and the HOMOs in both complexes are predominantly metal-centered, while the LUMOs predominantly consist of porphyrin π* orbitals. In spite of the rotational freedom of ferrocenyl substituents at room temperature, both metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin are able to form mixed-valence states upon the successive ferrocene-based two- and one-electron oxidations, respectively, as confirmed by UV-Vis, MCD, Mössbauer, electro-, and spectroelectrochemical methods, and thus, the earlier suggested (Boyd et al. Chem. Commun., 1999, 637) requirements for the formation of mixed-valence states in ferrocene-containing porphyrins should be revised
Constraints on decaying Dark Matter from XMM-Newton observations of M31
We derive constraints on parameters of the radiatively decaying Dark Matter
(DM) particles, using XMM-Newton EPIC spectra of the Andromeda galaxy (M31).
Using the observations of the outer (5'-13') parts of M31 we improve the
existing constraints. For the case of sterile neutrino DM, combining our
constraints with the latest computation of abundances of sterile neutrino in
the Dodelson-Widrow (DW) scenario, we obtain the lower mass limit m_s < 4 keV,
which is stronger than the previous one m_s < 6 kev, obtained recently by Asaka
et al. (2007) [hep-ph/0612182]. Comparing this limit with the most recent
results on Lyman-alpha forest analysis of Viel et al. (2007) [arXiv:0709.0131]
(m_s > 5.6 kev), we argue that the scenario in which all the DM is produced via
DW mechanism is ruled out. We discuss however other production mechanisms and
note that the sterile neutrino remains a viable candidate of Dark Matter,
either warm or cold.Comment: 13 pages, 12 figure
Constraining DM properties with SPI
Using the high-resolution spectrometer SPI on board the International
Gamma-Ray Astrophysics Laboratory (INTEGRAL), we search for a spectral line
produced by a dark matter(DM) particle with a mass in the range 40keV < M_DM <
14MeV, decaying in the DM halo of the Milky Way. To distinguish the DM decay
line from numerous instrumental lines found in the SPI background spectrum, we
study the dependence of the intensity of the line signal on the offset of the
SPI pointing from the direction toward the Galactic Centre. After a critical
analysis of the uncertainties of the DM density profile in the inner Galaxy, we
find that the intensity of the DM decay line should decrease by at least a
factor of 3 when the offset from the Galactic Centre increases from 0 to 180
degrees. We find that such a pronounced variation of the line flux across the
sky is not observed for any line, detected with a significance higher than 3
sigma in the SPI background spectrum. Possible DM decay origin is not ruled out
only for the unidentified spectral lines, having low (~3 sigma) significance or
coinciding in position with the instrumental ones. In the energy interval from
20 keV to 7 MeV, we derive restrictions on the DM decay line flux, implied by
the (non-)detection of the DM decay line. For a particular DM candidate, the
sterile neutrino of mass MDM, we derive a bound on the mixing angle.Comment: Minor changes; v.2 - Final version appeared in MNRA
A lower bound on the mass of Dark Matter particles
We discuss the bounds on the mass of Dark Matter (DM) particles, coming from
the analysis of DM phase-space distribution in dwarf spheroidal galaxies
(dSphs). After reviewing the existing approaches, we choose two methods to
derive such a bound. The first one depends on the information about the current
phase space distribution of DM particles only, while the second one uses both
the initial and final distributions. We discuss the recent data on dSphs as
well as astronomical uncertainties in relevant parameters. As an application,
we present lower bounds on the mass of DM particles, coming from various dSphs,
using both methods. The model-independent bound holds for any type of fermionic
DM. Stronger, model-dependent bounds are quoted for several DM models (thermal
relics, non-resonantly and resonantly produced sterile neutrinos, etc.). The
latter bounds rely on the assumption that baryonic feedback cannot
significantly increase the maximum of a distribution function of DM particles.
For the scenario in which all the DM is made of sterile neutrinos produced via
non-resonant mixing with the active neutrinos (NRP) this gives m_nrp > 1.7 keV.
Combining these results in their most conservative form with the X-ray bounds
of DM decay lines, we conclude that the NRP scenario remains allowed in a very
narrow parameter window only. This conclusion is independent of the results of
the Lyman-alpha analysis. The DM model in which sterile neutrinos are
resonantly produced in the presence of lepton asymmetry remains viable. Within
the minimal neutrino extension of the Standard Model (the nuMSM), both mass and
the mixing angle of the DM sterile neutrino are bounded from above and below,
which suggests the possibility for its experimental search.Comment: 20 pages, published in JCA