Rechargeable hydrogen batteries using rare-earth-based hydrogen storage alloys

Abstract

Electrochemical properties of low cost MmNi 5-based-hydrogen storage alloys (Mm ≡ mischmetal) were extensively examined. The alloy MmNi 3.5Co 0.7Al 0.8 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 (MmNl 3.5Co 0.5Al 0.3Mn 0.4) 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 AlNi 3 at grain boundaries. The decay in capacity of the MH electrode using MmNi 3.6Co 0.7Al 0.8 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.