Nanocrystalline materials for Ni–MH batteries

Abstract

Nanocrystalline metal hydrides are a new class of materials in which outstanding performance may be obtained by proper engineering of the microstructure. In this work, we have studied experimentally the structure, electrochemical and electronic properties of nanocrystalline and polycrystalline TiFe-, LaNi 5- and Mg 2Ni-type phases. These materials were prepared either by mechanical alloying (MA) followed by annealing or by induction melting method, respectively. The properties of hydrogen host materials can be modified substantially by alloying to obtain the desired storage characteristics. For example, it was found that the respective replacement of Fe in TiFe by Ni and/or by Mg, Cr, Mn, Co, Mo, Zr improved not only the discharge capacity but also the cycle life of these electrodes. In the nanocrystalline TiFe 0.125Mg 0.125Ni 0.75 powder, discharge capacity of up to 158 mA h g −1 was measured. Independently, it was found that cobalt substituting nickel in LaNi 4− x Mn 0.75Al 0.25Co x alloy greatly improved the discharge capacity and cycle life of LaNi 5 material. In nanocrystalline LaNi 3.75Mn 0.75Al 0.25Co 0.25 powder, discharge capacities up to 260 mA h g −1 were measured. On the other hand, the Mg 2Ni electrode, mechanically alloyed and annealed, displayed the maximum discharge capacity (100 mA h g −1) at the 1st cycle but degraded strongly with cycling. In nanocrystalline Mg 1.5Mn 0.5Ni alloy discharge capacities up to 241 mA h g −1 were measured. Finally, the electronic properties of nanocrystalline alloys are compared to that of polycrystalline samples. The studies show, that electrochemical properties of Ni–MH batteries are the function of the microstructure and the chemical composition and of used electrode materials.