RECHARGEABLE HYDROGEN BATTERIES USING RARE-EARTH-BASED HYDROGEN STORAGE ALLOYS

Electrochemical properties of low cost MmNi5-based-hydrogen storage alloys (Mm = mischmetal) were extensively examined. The alloy MmNi3.5Co0.7Al0.8 showed a very long cycle life with reasonable discharge capacity (250 mA h g-1) and rate capability. An ingot with a very high endurance and low lattice strain was obtained under controlled conditions, including high rate cooling in the casting process for obtaining a columnar structure, no heat treatment and prevention of stoichiometric deviation. A columnar structure was formed so that the c axis of the hexagonal structure was oriented parallel to the cooling plane. This alloy was significantly distinguished in crystal growth from a manganese-containing alloy (MmNi3.5Co0.8Al0.3Mn0.4) which had an equiaxial structure with considerable lattice strain, which needed conventional heat treatment. Deviation from stoichiometric composition to the nickel-rich side caused a significant decrease in capacity and cycle life owing to the precipitation of AlNi3 at grain boundaries. The decay in capacity of the MH electrode using MmNi3.5Co0.7Al0.8 was only 10% after 2000 cycles. The cylindrical sealed cell also showed a very long cycle life (a capacity decay of 6% after 2000 cycles). The high capacity sealed cell had a 1.5-2 times higher energy density (210 W h dm-3; 65 W h kg-1) with a longer cycle life and better rate capability than the high capacity Ni-Cd battery.