Effect of particle size on the electrode performance of MgNi hydrogen storage alloy

Amorphous MgNi material has been synthesized by mechanical alloying and sieved into three particle size fractions: >75, 20–75, and <20 μm. The influence of the MgNi particle size on its electrochemical behavior as negative electrode for Ni–metal hydride (MH) batteries has been investigated. The initial discharge capacity is higher as the MgNi particle size increases, i.e. the initial discharge capacity of particles larger than 75 μm is 439 mAh/g compared to 396 and 328 mAh/g for 20–75 and <20 μm particles, respectively. In addition, the cycle life of the MgNi electrode is improved by increasing the particle size. That is, the >75 μm particles electrode retains 41% of its initial capacity after 15 cycles compared to 35 and 23% for 20–75 μm and <20 μm particles electrodes, respectively. The rate dischargeability is also improved by increasing the MgNi particle size. For example, at 400 mA/g, the >75 μm particles electrode delivers 33% of its capacity measured at 20 mA/g compared to 25 and 19% for 20–75 μm and <20 μm powders, respectively. These results can be explained by the lower specific surface area of the electrode constituted of large particles which limits the formation of Mg(OH)2 resulting from the MgNi oxidation by the electrolyte. Moreover, on the basis of the evolution of the cycling discharge capacities with increasing charge input for the different powder fractions, it appears that the electrode resistance to pulverization is improved by increasing the MgNi particle size.