L. Aymard

Mechanochemical synthesis of Li–Mn–O spinels: positive electrode for lithium batteries

Li-Mn-O oxides were synthesized by mechanochemistry from a stoichiometric mixture of Li 2 O and MnO 2 using various grinding times (0 < t milling < 15 h). X-ray diffraction patterns of the ground samples (t milling < 10 h) exhibit the same features as LiMn 2 O 4 spinel structure (SG: Fd3m) with, however, a slight discrepancy in the lattice parameter (a cub ) suggesting a non-stoichiometry of the Li-Mn-O oxides, a cub increases with milling time to reach for 8 h of grinding a value of 8.24 A similar to stoichiometric LiMn 2 O 4 . As a matter of fact, after 8 h of milling, mechanosynthesized Li-Mn-O spinel-type oxide shows quasi-identical electrochemical performances as high temperature LiMn 2 O 4 ground for 1 h.

Preparation of Mg2Ni base alloy by the combination of mechanical alloying and heat treatment at low temperature

Abstract The synthesis of Mg 2 Ni and Mg 2 Ni 0.7 Co 0.3 alloys has been carried out through the combination of mechanical-alloying and a subsequent heat treatment at low temperature. These alloys consist of a two-phase mixture Mg 2 Ni 1− x M x +M containing iron impurities.

Influence of grinding on the catalytic properties of oxides

The influence of grinding on the structure and catalytic performances of two families of oxides, namely the perovskite-type La0.8Sr0.2MnO3±λ and the spinel-type Li–Mn–O, was investigated. Ball-milling of the well-known La0.8Sr0.2MnO3±λ prepared either by a solid state reaction or by a sol-gel method led to (i) a decrease in carbon black combustion temperature of 100 °C (TC=540 °C) and of 40 °C (TC=505 °C) for ceramic and sol-gel ground 5-h samples, respectively, and (ii) to faster kinetics and higher rates of toluene conversion. A greater enhancement of the catalytic performances was obtained by using mechanical milled lithiated manganese oxides that are reported as promising catalyst candidates for the first time. Li–Mn–O catalysts were synthesized by room temperature mechanochemistry of a stoichiometric mixture of Li2O and MnO2 using various milling times (0<tmilling<15 h). The nonstoichiometry, large surface area and disorder nature of the ground samples were of great benefit regarding catalytic applications. A remarkable decrease in the carbon black combustion temperature of 200 °C (from 650 to 450 °C) was obtained when using a mixture of Li2O and MnO2 ground for 3 or 4 h. This low TC value favorably compares with the TC of 500 °C of ceramic LiMn2O4, which shows, however, better catalytic performances than most of the perovskite-type oxides. The grinding proves to be efficient as well for volatile organic compounds (VOCs) combustion. The inactive ceramic LiMn2O4 exhibits a 100% toluene conversion rate for a temperature lower than 200 °C when ground 5 h.

Effect of carbon additives on the electrochemical properties of AB{sub 5} graphite composite electrodes prepared by mechanical milling

The effect of mechanical milling on powder mixtures consisting of graphite and AB{sub 5} 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 AB{sub 5} 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 AB{sub 5} 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 AB{sub 5} material and the graphite that allows a better utilization of the alloy.

Enhanced hydrogen sorption capacities and kinetics of Mg2Ni alloys by ball-milling with carbon and Pd coating

Solid-state hydrogen storage alloys are becoming a practical method to transport and utilize hydrogen as fuel for various technologies. In this paper, the kinetics and capacity of hydrogen desorption from Mg-based alloys have markedly been enhanced by tuning the surface composition of alloy particles. Mg 2 Ni-C 1 . x composites (where t refers to the pregrinding time and x to the Brunauer-Emmet-Teller specific surface area) were prepared by ball-milling the alloy in the presence of preground graphite, and Pd-coated Mg 2 Ni alloy powders were obtained by controlled chemical deposition of Pd on the alloy surface. We have found that the optimization of the pregrinding step of carbon is a determinant factor in enhancing the hydrogen desorption capacity of the Mg 2 Ni-10 wt.% C 1 0 . 3 2 0 composites to 2.6 wt.% at 150 °C, the maximum performance so far reported on desorption for Mg-based alloys. Such value can even be raised to 2.8 wt.% by applying Pd deposition on the composite.

Metal hydrides: an innovative and challenging conversion reaction anode for lithium-ion batteries.

Summary The state of the art of conversion reactions of metal hydrides (MH) with lithium is presented and discussed in this review with regard to the use of these hydrides as anode materials for lithium-ion batteries. A focus on the gravimetric and volumetric storage capacities for different examples from binary, ternary and complex hydrides is presented, with a comparison between thermodynamic prediction and experimental results. MgH2 constitutes one of the most attractive metal hydrides with a reversible capacity of 1480 mA·h·g−1 at a suitable potential (0.5 V vs Li+/Li0) and the lowest electrode polarization (<0.2 V) for conversion materials. Conversion process reaction mechanisms with lithium are subsequently detailed for MgH2, TiH2, complex hydrides Mg2MHx and other Mg-based hydrides. The reversible conversion reaction mechanism of MgH2, which is lithium-controlled, can be extended to others hydrides as: MHx + xLi+ + xe− in equilibrium with M + xLiH. Other reaction paths—involving solid solutions, metastable distorted phases, and phases with low hydrogen content—were recently reported for TiH2 and Mg2FeH6, Mg2CoH5 and Mg2NiH4. The importance of fundamental aspects to overcome technological difficulties is discussed with a focus on conversion reaction limitations in the case of MgH2. The influence of MgH2 particle size, mechanical grinding, hydrogen sorption cycles, grinding with carbon, reactive milling under hydrogen, and metal and catalyst addition to the MgH2/carbon composite on kinetics improvement and reversibility is presented. Drastic technological improvement in order to the enhance conversion process efficiencies is needed for practical applications. The main goals are minimizing the impact of electrode volume variation during lithium extraction and overcoming the poor electronic conductivity of LiH. To use polymer binders to improve the cycle life of the hydride-based electrode and to synthesize nanoscale composite hydride can be helpful to address these drawbacks. The development of high-capacity hydride anodes should be inspired by the emergent nano-research prospects which share the knowledge of both hydrogen-storage and lithium-anode communities.

Influence of oxygen and hydrogen milling atmospheres on the electrochemical properties of ballmilled graphite

The effect of the grinding oxygen or hydrogen atmosphere on the physical/electrochemical properties of ballmilled graphite was studied. These properties strongly depend on the atmosphere and pressure of the milling container of the oxygen and the hydrogen contents of the samples. For instance, grinding under a 10 bars of H 2 atmosphere leads to a highly disordered carbon, while 10 bars under O 2 gives a heterogeneous carbon having a well and poorly crystallized graphite component. In both cases, the reversible (x rev ) and irreversible (x irrev ) capacities and the double-layer capacitance increase with milling time. The graphite ground 10 h under a p O2 = 10 bars strongly differs from the other samples. We ascribed such behavior to the presence of a large amount of oxygenated functional groups in relation with the formation of nanoporosity and/or the predominance of edge carbon upon grinding. In contrast, under p O2 = 0.2 bar or p H2 = 10 bars, the variation of the Brunauer-Emmett-Teller (BET) surface area with milling time is correlated with the irreversible capacity. Differences in the voltage composition curves are also noticed with a low oxygen pressure (p O2 = 10 -6 mbar) strongly reducing the polarization of the charge/discharge cycling curve while leading to x rev = 1.7 Li for Li x C 6 . Finally, 10 h of milling (R = 24) under p O2 = 0.2 har were shown to produce a carbon with an x rev = 1.6 Li and x irrev = 0.5 Li for Li x C 6 . while 80 h (R = 8) under p O2 = 10 bars led to a carbon having a double-layer capacitance of 57 F/g in Et 4 N BF 4 .

A new Mg0.9Y0.1Ni hydride forming composition obtained by mechanical grinding

Abstract We report on the synthesis of an electrochemically active Mg0.9Y0.1Ni polynanocrystalline phase by mechanical alloying. This alloy presents an initial capacity of 323 mAh/g that decreases upon cycling to reach a stable value of 220 mAh/g after 30 complete charge/discharge cycles (e.g. 62% capacity retention). Such an increase in capacity retention with respect to pure MgNi alloy (62% instead of 24%) is due to the addition of yttrium that enhances the resistance of the alloy against corrosion in concentrated alkaline media.

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

Abstract Two types of intermetallic compounds, LaNi 4 M 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 MmNi 3.6 Al 0.35 Mn 0.25 Co 0.8 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.

Study of the formation reactions of silver-palladium alloys by grinding and post-milling isothermal annealing

Abstract Silver-palladium alloys in the whole composition range have been produced by mechanical alloying using, as metallic precursor, silver and palladium powders prepared in our laboratory by the polyol process. The influence of milling conditions and isothermal annealing on the Ag + Pd mixture was investigated. Two types of mixer-mill, operating either by impact or friction, were used. We established that the reaction mechanism as well as the textural features of the final alloys are strongly dependent upon both the nature of the interactions (impact or friction) and the energy of mechanical strain. Moreover, it has been observed that for high impact energy interactions, the crystalline alloy transforms into an amorphous alloy. These observations can be explained in terms of an equilibrium between the density of defects and heat recovery during the grinding process, with the friction process generating more heat than the impact process.