77 research outputs found
Bulk and Surface Stabilization Process of Metastable Li-Rich Disordered Rocksalt Oxyfluorides as Efficient Cathode Materials
Manganese based disordered rocksalt systems have attracted attention as Co-free and high capacity cathode materials for Li-ion batteries. However, for a practical application these materials are considered as metastable and exhibit too limited cyclability. In order to improve the structural stability of the disordered rocksalt LiMnTiOF (0 ≤ x ≤ 1) system during cycling, we have introduced a mild temperature heat treatment process under reducing atmosphere, which is intended to overcome the structural anomalies formed during the mechanochemical synthesis. The heat-treated samples presented better electrochemical properties, which are ascribed to a structural defect mitigation process both at the surface and in the bulk, resulting in improved crystal structure stability. In addition, the optimized particle size and the smaller BET surface area induced by the recrystallization contributes to the observed enhanced performance. Among the studied compositions, the heat treated LiMnTiOF sample displayed better electrochemical performance with a discharge capacity of 165 mAh g after 100 cycles at 0.1 C (∼80% of the initial capacity), when combined with further conditioning of the cells. The results point explicitly towards a guided stabilization approach, which could have a beneficial effect regarding the application of DRS oxyfluoride materials for sustainable LIBs
Observation of a new phase transition between fully and partially polarized quantum Hall states with charge and spin gaps at
The average electron spin-polarization of two-dimensional electron
gas confined in multiple quantum-wells was measured by
nuclear magnetic resonance (NMR) near the fractional quantum Hall state with
filling factor . Above this filling factor (), a strong depolarization is observed corresponding to two spin flips per
additional flux quantum. The most remarkable behavior of the polarization is
observed at , where a quantum phase transition from a partially
polarized () to a fully polarized ()
state can be driven by increasing the ratio between the Zeeman and the Coulomb
energy above a critical value .Comment: 4 pages including 4 figure
Toward Better Stability and Reversibility of the Mn/MnDouble Redox Activity in Disordered Rocksalt Oxyfluoride Cathode Materials
Cation-disordered rocksalt (DRS) materials have shown good initial reversibility and facile Liinsertion and extraction in the structure at high rates. However, all of the Li-rich oxyfluorides introduced so far suffer from short cycle lifetimes and severe capacity fading. In the current study, we combine the strategy of using high-valent cations with partial substitution of oxygen anions by fluorine ions to achieve the optimal Mn/Mn double redox reaction in the composition system LiMnTiOF (0 ≤ x ≤ 2/3). While Ti-rich compositions correlate to an O-oxidation plateau and a partial Mn–Mn redox process at high voltages, owing to the presence of Ti3+ in the structure, a new composition LiMnTiOF with a lower amount of Ti shows better electrochemical performance with an initial high discharge capacity of 227 mAh g (1.5–4.3 V window) and a Coulombic efficiency of 82% after 200 cycles with a capacity of 136 mAh g (>462 Wh kg). The structural characteristics, oxidation states, and charge-transfer mechanism have been examined as a function of composition and state of charge. The results indicate a double redox mechanism of Mn/Mn in agreement with Mn–Ti structural charge compensation. The findings point to a way for designing high-capacity DRS materials with multi-electron redox reactions
Structural and Electrochemical Insights from the Fluorination of Disordered Mn-Based Rock Salt Cathode Materials
Quantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures
We present a detailed quantitative magneto-optical imaging study of several
superconductor/ferromagnet hybrid structures, including Nb deposited on top of
thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and
tailored magnetic landscapes imprinted in the permalloy layer. The
magneto-optical imaging data is complemented with and compared to scanning Hall
probe microscopy measurements. Comprehensive protocols have been developed for
calibrating, testing, and converting Faraday rotation data to magnetic field
maps. Applied to the acquired data, they reveal the comparatively weaker
magnetic response of the superconductor from the background of larger fields
and field gradients generated by the magnetic layer.Comment: 21 pages, including 2 pages of supplementary materia
NMR Determination of 2D Electron Spin Polarization at
Using a `standard' NMR spin-echo technique we determined the spin
polarization of two-dimensional electrons, confined to GaAs quantum wells, from
the hyperfine shift of Ga nuclei in the wells. Concentrating on the temperature
and magnetic field dependencies of spin polarization at Landau level filling
factor , we find that the results are described well by a simple
model of non-interacting composite fermions, although some inconsistencies
remain when the two-dimensional electron system is tilted in the magnetic
field.Comment: 4 pages (REVTEX) AND 4 figures (PS
Preparation of intergrown P/O-type biphasic layered oxides as high-performance cathodes for sodium ion batteries
This study reports on the solid-state synthesis and characterization of novel quaternary P/O intergrown biphasic NaMnyNiFeTiO (y = 0.6, 0.55, 0.5, 0.45) cathode materials. Electrochemical tests reveal superior performance of the P/O biphasic materials in a sodium ion battery compared to the single P2 or O3 phases, proving the beneficial effect of the intergrowth of P2 and O3 materials. The nature of the P/O interface was studied by transmission electron microscopy. The analysis shows a semi-coherent interface grown along the a/b and c axes with local differences in the transition metal concentration along the interface between the two phases. EDX and EELS characterization revealed a charge compensation mechanism across the phase boundary based on variation of the transition element distribution, balancing the different sodium contents in the P and O phases. The results reported in this study provide a better understanding of P/O biphasic materials
Critical Behavior of Nuclear-Spin Diffusion in GaAs/AlGaAs Heterostructures near Landau Level Filling \nu=1
Thermal measurements on a GaAs/AlGaAs heterostructure reveal that the state
of the confined two-dimensional electrons dramatically affects the nuclear-spin
diffusion near Landau level filling factor \nu=1. The experiments provide
quantitative evidence that the sharp peak in the temperature dependence of heat
capacity near \nu=1 is due to an enhanced nuclear-spin diffusion from the GaAs
quantum wells into the AlGaAs barriers. We discuss the physical origin of this
enhancement in terms the possible Skyrme solid-liquid phase transition.Comment: 1 LateX file, 3 figures, submitte
Masses of composite fermions carrying two and four flux quanta: Differences and similarities
This study provides a theoretical rationalization for the intriguing
experimental observation regarding the equality of the normalized masses of
composite fermions carrying two and four flux quanta, and also demonstrates
that the mass of the latter type of composite fermion has a substantial filling
factor dependence in the filling factor range , in agreement
with experiment, originating from the relatively strong inter-composite fermion
interactions here.Comment: 5 pages, 2 figure
A 3D insight on the catalytic nanostructuration of few-layer graphene
The catalytic cutting of few-layer graphene is nowadays a hot topic in materials research due to its potential applications in the catalysis field and the graphene nanoribbons fabrication. We show here a 3D analysis of the nanostructuration of few-layer graphene by iron-based nanoparticles under hydrogen flow. The nanoparticles located at the edges or attached to the steps on the FLG sheets create trenches and tunnels with orientations, lengths and morphologies defined by the crystallography and the topography of the carbon substrate. The cross-sectional analysis of the 3D volumes highlights the role of the active nanoparticle identity on the trench size and shape, with emphasis on the topographical stability of the basal planes within the resulting trenches and channels, no matter the obstacle encountered. The actual study gives a deep insight on the impact of nanoparticles morphology and support topography on the 3D character of nanostructures built up by catalytic cutting
- …