Improvement of electrode performances of Mg 2Ni by mechanical alloying

Abstract

Nanocrystalline and amorphous Mg 2Ni-based hydrogen storage alloys for Ni–MH batteries had been synthesized by mechanical alloying. The surface modification and Zr addition had also been carried out for improvement of its electrode performance. In comparison with the arc-melted polycrystalline one, the nanocystalline Mg 2Ni phase showed a higher discharge capacity. By increasing milling time from 120 to 160 h, the grain size of Mg 2Ni phase was more refined and the discharge capacity was raised from 180 to 370 mAh g −1. The discharge capacity of the 160-h milled Mg–Ni–Zr amorphous alloys also reached 530 mAh g −1. XPS (X-ray photoelectron spectroscopy) analysis showed that Mg2p spectra were shifted to lower binding energy implying the enhancement of the hydrogen diffusion and charge transfer reaction, and resulted in increasing the discharge capacity in amorphous alloys. To prevent the rapid degradation, the alloy powders were also coated with Ni and graphite by additional ball milling. The Ni and graphite protected the Mg from oxidation, and the coated powders showed a better cyclic stability. After 50 cycles, the degradation of bare electrode was 94% of maximum capacity, but that of coated electrode with Ni and graphite was 45 and 76% of maximum capacity, respectively.