Luc Aymard

Metal hydrides for lithium-ion batteries

Classical electrodes for Li-ion technology operate via an insertion/de-insertion process. Recently, conversion electrodes have shown the capability of greater capacity, but have so far suffered from a marked hysteresis in voltage between charge and discharge, leading to poor energy efficiency and voltages. Here, we present the electrochemical reactivity of MgH(2) with Li that constitutes the first use of a metal-hydride electrode for Li-ion batteries. The MgH(2) electrode shows a large, reversible capacity of 1,480 mAh g(-1) at an average voltage of 0.5 V versus Li(+)/Li(o) which is suitable for the negative electrode. In addition, it shows the lowest polarization for conversion electrodes. The electrochemical reaction results in formation of a composite containing Mg embedded in a LiH matrix, which on charging converts back to MgH(2). Furthermore, the reaction is not specific to MgH(2), as other metal or intermetallic hydrides show similar reactivity towards Li. Equally promising, the reaction produces nanosized Mg and MgH(2), which show enhanced hydrogen sorption/desorption kinetics. We hope that such findings can pave the way for designing nanoscale active metal elements with applications in hydrogen storage and lithium-ion batteries.

Production of CoNi alloys by mechanical-alloying

Abstract Co Ni solid solutions, such as Co10Ni20, Co40Ni40, Co50Ni50, Co60Ni40 and Co70Ni30 alloys, have been produced by mechanical-alloying using Co and Ni powders prepared in our laboratory by the polyol process as metallic precursors. The grinding transforms the hexagonal Co phase into a cubic Co phase by stacking fault creation and permits a possible mutual diffusion of Co and Ni species in the two f.c.c. phases. The resulting powder alloys are nanocrystalline with a high atomic-level of internal strain. Magnetic data on these powders shows behaviour typical of a soft ferromagnetic alloy. A strong influence of oxygen contamination on magnetic properties of these alloys was observed, mainly on the saturation magnetization which decreases with increasing oxygen content.

XAS investigations on nanocrystalline Mg2FeH6 used as a negative electrode of Li-ion batteries

In the frame of research on new metallic hydrides as conversion reaction materials for negative electrodes of Li-ion batteries, the MgFe2H6 complex hydride has been investigated in and ex situ using XRD, XAS and magnetic measurements at different states: initially ball milled complex hydride (CH), electrochemically desorbed (ED) and thermally desorbed (TD). Fe–H bonding is clearly evidenced by EXAFS in the CH sample. It is also observed that the electrochemical reaction leads to a nanocrystalline state that needs local probe analyses to be fully investigated and interpreted. From the XAS and magnetic data, the different routes (ED and TD) for dehydrogenation of the complex hydride are compared. For both electrochemically and thermally driven reactions, the hydrogen depletion from Mg2FeH6 hydride leads to decomposition into its constituting elements Mg and Fe. However, different Fe structures are observed: bulk α-Fe and amorphous Fe nanoparticles for TD and ED samples, respectively.

MgH2–TiH2 mixture as an anode for lithium-ion batteries: synergic enhancement of the conversion electrode electrochemical performance

A 0.7MgH2 + 0.3TiH2 mixture was prepared by reactive grinding of Mg and Ti powders under hydrogen and tested as a conversion electrode for lithium-ion batteries. This composite presents superior electrochemical properties compared to MgH2 or TiH2 based electrodes, either in terms of reversible capacities or polarization versus applied current rate. A substantial reversible capacity of 1540 mA h g−1 is measured at a suitable potential of 0.52 V vs. Li+/Li0 (current rate 0.1Li h−1). For the same current rate, electrode polarization is limited to 0.2 V and stabilizes at 0.46 V for 1.0Li h−1 while a continuous polarization increase is observed for MgH2 based electrodes. In situ XRD analyses of this composite demonstrate that Mg and Ti hydrides are both electrochemically active and contribute together to the capacity. TiH2 improves MgH2 conversion process kinetics while MgH2 enables a reversible conversion reaction for TiH2. The three consecutive reactions are: (1) MgH2 + 2Li+ + 2e− ⇌ Mg + 2LiH; (2) δ-TiH2 + xLi+ + xe− ⇌ δ-TiH2−x + xLiH (x ≤ 0.5) and (3) δ-TiH2−x + (2 − x)Li+ + (2 − x)e− ⇌ α-Ti + (2 − x)LiH (x = 0.5). This is, to our knowledge, the first example of a reversible lithium conversion process with TiH2 powder.

Formation mechanism of a crystalline Ag50Pd50 solid solution by impact ball–materials interaction

The formation of the crystalline Ag50Pd50 solid solution during mechanical alloying under shock interaction of Ag and Pd metallic powders has been studied using electron microscopy and X-ray diffraction. The process involved for impact interaction occurs via the incorporation of the small Pd particles into the Ag matrix in which, under the action of mechanical strain, the mutual diffusion of Ag and Pd leads to the formation of two solid solutions, one rich in Ag and the other rich in Pd. Further grinding allows the homogenization of the two Ag and Pd solid solutions whose composition changes to finally reach the initial Ag50Pd50 composition. The defects as dislocations generated by the impact conditions may be responsible for the reaction mechanism.

Influence of size and shape of metallic silver and palladium powders on the Ag70Pd30 solid solution formation by mechanical alloying

Silver-palladium solid solutions of composition Ag70Pd30 have been produced by mechanical alloying using, as metallic precursors, silver and palladium powders of different particle sizes and shapes prepared in our laboratory by the polyol process. The reaction kinetics as well as the morphologies of the final alloys are strongly dependent on the initial particle size and shape of both the silver and palladium powders. This observation can be explained by considering that the initial stage of mixing is governed by an important particle-size effect.