Sergio Brutti

Magnesium hydride as a high capacity negative electrode for lithium ion batteries

Conversion reactions in lithium batteries have been proved for several classes of materials, such as oxides, fluorides, sulphides, nitrides, phosphides and recently for hydrides. Metal hydrides can be electrochemically reduced to a highly conductive composite material consisting of nanometric metallic particles dispersed in an amorphous LiH matrix. Magnesium hydride undergoes a reversible conversion reaction and it has very good theoretical performances, i.e. a theoretical specific capacity of 2038 mA h g−1 and a working potential of 0.5 V vs. Li+/Li. The purpose of our study is to investigate the MgH2 redox activity by evaluating the effect of ball milling pre-treatments and by studying the conversion reaction mechanism. Three materials, prepared by submitting bulk MgH2 to different ball milling procedures, are investigated. By coupling electrochemical tests, ex situ X-ray powder diffraction and transmission electron microscopy, we prove that the lithium incorporation does not follow a simple direct conversion path as it follows at least a sequence of four consecutive processes: (a) the hydride conversion reaction of MgH2 to give Mg and LiH, (b) the alloying of Li in hcp Mg and (c and d) the formation and lithium enrichment of a bcc Li–Mg solid solution. Furthermore some experimental clues suggest that the mechanism is probably even more complex as it can imply the formation of other unknown intermediate Li–Mg–H phases. Moreover large morphological changes occur upon lithium incorporation in the electrodes: in particular an extended sintering of the metal nanoparticles occurs upon cycling. This effect leads to electrode pulverization and capacity fading. On the other hand MgH2 shows a very limited potential hysteresis between discharge and charge and very promising kinetics at high current.

Vaporization thermodynamics of MgB2 and MgB4

The vaporization behavior of MgB2 and MgB4 under thermodynamic conditions has been studied by the Knudsen effusion-mass spectrometry technique. In the temperature range explored (883–1154 K), magnesium borides are observed to decompose by loss of gaseous Mg only. The equilibrium pressures of Mg(g) have been measured during high-temperature decompositions involving MgB2/MgB4 and MgB4/MgB7 two-phase mixtures and the corresponding standard reaction enthalpies were determined. The decomposition temperatures for MgB2 and MgB4 were also inferred by the relevant Van’t Hoff plots.

Thermodynamic stabilities of intermediate phases in the Ca–Si system

Abstract Vaporization thermodynamics in the binary system calcium–silicon has been studied by Knudsen effusion-mass spectrometry and vacuum microbalance techniques. The equilibrium partial pressure of Ca(g) over the two-phase regions in the composition range 20–75at.% Si has been measured and the standard enthalpy changes for the appropriate vaporization reactions were determined from the temperature dependence of the measured vapor pressures. The standard reaction enthalpy changes were also evaluated by the third-law method using the pressure data in conjunction with estimated Gibbs energy functions. Standard enthalpies of formation of the calcium silicides were derived from the standard reaction enthalpy values at room temperature. The results obtained for ΔfH°298 were the following: Ca2Si=−56.1±3.1, Ca5Si3=−55.3±3.5, CaSi=−49.6±2.2, Ca3Si4=−40.6±1.5, Ca14Si19=−44.4±2.3, CaSi2=−37.8±1.6 all in kJ/mol atoms. The results for Ca2Si, CaSi and CaSi2 may be compared with previous measurements, all other results are first determinations.

Preparation and Characterization of Nanocomposite Polymer Membranes Containing Functionalized SnO2 Additives

In the research of new nanocomposite proton-conducting membranes, SnO2 ceramic powders with surface functionalization have been synthesized and adopted as additives in Nafion-based polymer systems. Different synthetic routes have been explored to obtain suitable, nanometer-sized sulphated tin oxide particles. Structural and morphological characteristics, as well as surface and bulk properties of the obtained oxide powders, have been determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) and Raman spectroscopies, N2 adsorption, and thermal gravimetric analysis (TGA). In addition, dynamic mechanical analysis (DMA), atomic force microscopy (AFM), thermal investigations, water uptake (WU) measurements, and ionic exchange capacity (IEC) tests have been used as characterization tools for the nanocomposite membranes. The nature of the tin oxide precursor, as well as the synthesis procedure, were found to play an important role in determining the morphology and the particle size distribution of the ceramic powder, this affecting the effective functionalization of the oxides. The incorporation of such particles, having sulphate groups on their surface, altered some peculiar properties of the resulting composite membrane, such as water content, thermo-mechanical, and morphological characteristics.

High Voltage Li-Ion Battery Using Exfoliated Graphite/Graphene Nanosheets Anode

The achievement of a new generation of lithium-ion battery, suitable for a continuously growing consumer electronic and sustainable electric vehicle markets, requires the development of new, low-cost, and highly performing materials. Herein, we propose a new and efficient lithium-ion battery obtained by coupling exfoliated graphite/graphene nanosheets (EGNs) anode and high-voltage, spinel-structure cathode. The anode shows a capacity exceeding by 40% that ascribed to commercial graphite in lithium half-cell, at very high C-rate, due to its particular structure and morphology as demonstrated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The Li-ion battery reveals excellent efficiency and cycle life, extending up to 150 cycles, as well as an estimated practical energy density of about 260 Wh kg(-1), that is, a value well exceeding the one associated with the present-state Li-ion battery.

Synchrotron powder diffraction Rietveld refinement of MgB20 crystal structure

Abstract The crystal structure of MgB 20 has been investigated by X-ray and synchrotron powder diffraction at room temperature. As it happens in other metal-boron systems, the Rietveld refinement of this magnesium boron-rich phase shows a structure derived directly from β-boron. The substantially unaltered B–B framework hosts magnesium atoms in three different atomic positions (D, E and F holes); the occupancy of these sites is partial. The resulting unit cell is trigonal with space group R 3 m , the refined hexagonal axes a =1.09830(4) and c =2.41561(15) nm.

Thermodynamics of the Ni-Yb system

The experimental description of the Ni-rare earth systems is incomplete, with many gaps and uncertainties remaining in both the phase diagram and the thermochemical information. No thermodynamic information is available for the Ni-Yb system, except for an estimation of the enthalpy of formation of the Ni5Yb compound. In this work, we investigated the thermochemistry of the Ni-Yb intermediate phases by means of tensimetric measurements and, on a few compositions, calorimetric measurements. The equilibrium vapor pressures over two solidphase regions were measured by Knudsen effusion-mass spectrometry and Knudsen effusion-weight loss, and the data were analyzed by the second- and the third-law methods in order to derive the enthalpy changes for the HT decomposition reactions. Furthermore, the enthalpy of formation of the NiYb compound was preliminarily determined by direct reaction calorimetry (DRC). The heats of formation of the five intermediate phases was finally obtained by a combined analysis of all the collected data. The values are as follows: Ni17Yb2, −13.9±0.3 kJ/mol atoms; Ni5Yb, −20.9±4.4 kJ/mol atoms; Ni3Yb, −26.0±4.8 kJ/mol atoms; Ni2Yb, −32.0±4.6 kJ/mol atoms; and NiYb, −28.0 ± 2 kJ/mol atoms. The results are compared with those estimated by the Miedema’s model.

Surface Reactivity of a Carbonaceous Cathode in a Lithium Triflate/Ether Electrolyte-Based Li–O2 Cell

Li-O2 batteries are currently one of the most advanced and challenging electrochemical systems with the potential to largely overcome the performances of any existing technology for energy storage and conversion. However, these optimistic expectations are frustrated by the still inadequate understanding of the fundamentals of the electrochemical/chemical reactions occurring at the cathode side, as well as within the electrolyte and at the three-phase interface. In this work, we illustrate the evolution of the morphology and composition of a carbonaceous cathode in the first discharge/charge in a Li-O2 cell with an ether-based electrolyte by X-ray photoemission spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy. Experiments have been carried out ex situ on electrodes recuperated from electrochemical cells stopped at various stages of galvanostatic discharge and charge. Apparently, a reversible accumulation and decomposition of organic and inorganic precipitates occurs upon discharge and charge, respectively. These precipitations and decompositions are likely driven by electrochemical and chemical parasitic processes due to the reactivity of the cathode carbonaceous matrix.

Thermochemistry of ytterbium silicides

Abstract The results of the investigation of the high temperature decomposition reactions in vacuum under equilibrium conditions of ytterbium silicides in the whole composition range are reported. By means of the Knudsen Effusion–Mass Spectrometry (KE–MS) and the Knudsen Effusion–Weight Loss (KE–WL) techniques, the Yb(g) vapour pressures in equilibrium over the various high temperature and low temperature biphasic regions were measured in the temperature range 781–1395 K and the reaction enthalpies for the respective decompositions were derived. From this set of experimental data we derived for the first time the heats of formation of all the six known Si–Yb intermediate phases. The following values ΔfH°298 are recommended: Si3Yb5=−48.3±3.6, Si4Yb5=−53.2±4.6, SiYb=−51.1±5.1, Si4Yb3=−48.0±3.1, Si5Yb3=−41.3±2.6, Si1.74Yb=−37.4±0.9, all in kJ/mol atoms.

1,2-Dimethoxyethane Degradation Thermodynamics in Li−O2 Redox Environments

The reaction thermodynamics of the 1,2-dimethoxyethane (DME), a model solvent molecule commonly used in electrolytes for Li-O2 rechargeable batteries, has been studied by first-principles methods to predict its degradation processes in highly oxidizing environments. In particular, the reactivity of DME towards the superoxide anion O2- in oxygen-poor or oxygen-rich environments is studied by density functional calculations. Solvation effects are considered by employing a self-consistent reaction field in a continuum solvation model. The degradation of DME occurs through competitive thermodynamically driven reaction paths that end with the formation of partially oxidized final products such as formaldehyde and methoxyethene in oxygen-poor environments and methyl oxalate, methyl formate, 1-formate methyl acetate, methoxy ethanoic methanoic anhydride, and ethylene glycol diformate in oxygen-rich environments. This chemical reactivity indirectly behaves as an electroactive parasitic process and therefore wastes part of the charge exchanged in Li-O2 cells upon discharge. This study is the first complete rationale to be reported about the degradation chemistry of DME due to direct interaction with O2- /O2 molecules. These findings pave the way for a rational development of new solvent molecules for Li-O2 electrolytes.