Transmission electron microscopy studies on carbon materials prepared by mechanical milling

The effects of mechanical milling on the multiscale organization (structure and microtexture) of various carbon materials were investigated by means of Transmission Electron Microscopy. We show that mechanical grinding generates an increasing amount of disordered carbon at a rate depending on the type of grinding mode used (shear- or shock-type grinding). When the shock-type grinding is used, the triperiodic structure and the lamellar microtexture of the graphite completely break down to give microporous and turbostratic carbons made of misoriented nanometric Basic Structural Units (BSUs). Graphite grinding permits the elaboration of disordered carbons. The involved mechanism is different from a simple reverse graphitization, since not only structure but also microtexture are strongly modified by the grinding. After heat treatment at 2800°C, the graphite organization is not recovered, and a mesoporous turbostratic carbon is mainly obtained. All the carbon precursors studied, submitted to strong grinding, leads to similar microporous carbons. Shear grinding is less effective since remnants of graphitic carbon are still present within the disordered carbon

Effect of carbon additives on the electrochemical properties of AB5 graphite composite electrodes prepared by mechanical milling

The effect of mechanical milling on powder mixtures consisting of graphite and AB5 alloys, prepared either by mechanical alloying or by a high-temperature melting process, has been investigated. The resulting hydride-forming composite electrodes show a 10 and 40% capacity enhancement for arc-melted and mechanically prepared AB5 alloys, respectively. Such an increase in capacity is suggested to be the result of several cumulative effects: (1) a mechanically induced reducing role of graphite which eliminates the AB5 particles of oxide coatings, enabling a better hydrogen adsorption/absorption and diffusion into the insertion sites of the alloy, (2) the appearance of an increasingly important double-layer capacitance on each particle with increased milling time that adds to the faradic component, and (3) the improved electronic conductivity between the active AB5 material and the graphite that allows a better utilization of the alloy

Unique effect of mechanical milling on the lithium intercalation properties of different carbons

The effect of the carbon precursors morphology during a mechanical grinding experiment on the physical/electrochemical properties of the resulting ball-milled carbonaceous powders was studied. These properties strongly depend on the type of grinding mode used (shear or shock-type grinding); however, for each grinding mode, they were found to be totally independent of the nature of the precursor used (graphite, carbon, coke) and/or of its morphology (layers, microbeads and fibres). Eighty h of shock-grinding was found to produce carbonaceous milled powders able to reversibly intercalate two lithiums per six carbons 'Li2C6' while still having an irreversible capacity of 0.8 Li. The transparency of the precursor morphology and nature to mechanical grinding with carbon materials is ascribed to the strong 2D character of the graphite structure. Finally, the observed large reversible capacity in our hydrogen-free carbonaceous milled samples is explained on the basis of the previously proposed 'house of cards' model

Electrochemical properties of AB5-type hydride-forming compounds prepared by mechanical alloying

Two types of intermetallic compounds, LaNi4M with M = Mn, Co, Cu and Al and polysubstituted alloys Mm(NiAlMnCo)5 with Mm = Mischmetal were synthesized by mechanical alloying. In most cases, 10 h grinding was sufficient to obtain single phase alloys with crystallites of about 6 nm and particles ranging between 5 and 80 mm in diameter. The polysubstituted MmNi3.6Al0.35Mn0.25Co0.8 alloys, once removed from the grinding container, were found to present an electrochemical capacity of 159 mA h/g. Such a capacity was increased to 216.3, 241.7 and 273.5 mA h/g by a post-anneal step under vacuum to temperatures of 673, 873 and 1073 K, respectively. The various electrochemical capacity depends on both surface states and internal strains in the sample so that the heat treatments release these strains and enhance the capacity

Doubling the capacity of lithium manganese oxide spinel by a flexible skinny graphitic layer

By coating nanoparticular lithium manganese oxide (LMO) spinel with a few layers of graphitic basal planes, the capacity of the material reached up to 220 mAhg-1 at a cutoff voltage of 2.5 V. The graphitic layers 1) provided a facile electron-transfer highway without hindering ion access and, more interestingly, 2) stabilized the structural distortion at the 3 V region reaction. The gain was won by a simple method in which microsized LMO was ball-milled in the presence of graphite with high energy. Vibratory ball milling pulverized the LMO into the nanoscale, exfoliated graphite of less than 10 layers and combined them together with an extremely intimate contact. Ab initio calculations show that the intrinsically very low electrical conductivity of the tetragonal phase of the LMO is responsible for the poor electrochemical performance in the 3 V region and could be overcome by the graphitic skin strategy proposed.close0

Physical characterization of carbonaceous materials prepared by mechanical grinding

By means of mechanical grinding, we recently reported the ability to prepare tailor-made carbon materials able to reversibly intercalate two lithiums per six carbons (e.g., Li2C6) while having irreversible capacities of 320 mA h/g. A schematic model involving two different types of surface area was previously proposed to account for the reversible and irreversible capacities measured vs. Li with these powders. We experimentally verified this model by means of differential scanning calorimetry (DSC) measurements. Transmission Electronic Microscopy (TEM), which is a powerful tool for the direct imaging of poorly organized materials at the atomic scale has been used, together with Raman Spectroscopy, to follow the disorganization generated by mechanical grinding. © 1999 Elsevier Science S.A. All rights reserved

Raman study on carbonaceous materials prepared by mechanical milling

Through Raman spectroscopy studies, we show that mechanical grinding generates an increasing amount of unorganized carbon at a rate depending on the type of grinding mode used (Shear and Shock-type grinding). The first-order Raman spectrum for pure unground graphite has a well-known G sharp band at 1579 cm -1, which corresponds to the E 2g vibration while the ground samples present a broadened G band accompanied by new components at about 1610 cm -1 (D′), 1510 cm -1 (D″) and 1348 cm -1 (D), usually explained as arising from disorder and defects 1. Shock-type grinding produces a faster disorder increase than shear-type grinding. The latter preserves part of the graphitic character. The general effect of mechanical milling remains however opposite to that of Thermal Treatment (Graphitization). © 1998 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license under the Gordon and Breach Science Publishers imprint