Research and Development of Mg-Based Hydrogen Storage Alloys for Intended Use in Nickel-Metal Hydride Batteries

Abstract

The physicochemical and electrochemical characteristics of Mg-based hydrogen storage alloys modified by various methods are discussed for intended use in nickel-metal hydride batteries. A Mg₂Ni-Ni composite prepared by ball-milling of Mg₂Ni alloy with Ni (Mg₂Ni/Ni=1/1.28 in mole ratio) has a homogeneous amorphous structure and exhibits much improved hydriding-dehydriding characteristics and a very high discharge capacity at room temperature. Surface modification of amorphous MgNi alloys by ball-milling with graphite led to an improvement in the hydriding-dehydriding and charge-discharge characteristics. It is suggested that both a charge-transfer reaction between graphite and Mg and an increase in the Ni/Mg ratio on the alloy surface may be responsible for the enhanced characteristics of the MgNi-graphite composite. By using carbon materials with lower crystallinity for the surface modification, the ball-milling time to reach a maximum discharge capacity was shortened. Moreover, it was found that the combination of partial substitution with Ti and V and the subsequent surface modification with graphite promoted hydrogen desorbability and improved charge-discharge cycle performance and high rate dischargeability. The combination of bulk and surface modifications is very effective in improving the physicochemical and electrochemical characteristics of Mg-based alloys.