Studies of Sn, Co, Al, and Fe additives in C14/C15 Laves alloys for NiMH battery application by orthogonal arrays

The effects of Sn, Co, Al, and Fe as additives to C14/C15 Laves phase alloy in nickel metal hydride battery are studied with modified L-16 orthogonal arrays. Sixteen alloys with the general formula Ti9Zr2-wV5Ni38Cr5-xMn16-y-zSnwCoxAlyFez were prepared and made into sealed cells. The main components of these alloys are C14 and C15 structured AB2 phases with significant difference in Ni-content. Other minor phases found are Zr2Ni2Sn, ZrNi, Zr7Ni10, and Zr9Ni11. In the gas phase study, while the variations in pressure-concentration isotherm hysteresis and slope factor are small, the detrimental effects to storage capacity from Sn and Al are more evident than those from Co and Fe. Al lowers the equilibrium pressure plateau while the other three additives raise it. In the electrochemical study, a higher level of Sn promotes a greater content of the Zr2Ni2Sn phase which is less stable in an alkaline environment and has a detrimental effect on both charge retention and cycle life performance. Sn is also not desirable for applications with a demanding request of high-rate and low-temperature performance. The addition of Sn is to ease the activation and to reduce raw material cost by allowing the use of less expensive zirconium source. Adequate amounts of Co and Al addition improve activation, low temperature, and specific power of the battery. Addition of Fe makes activation more difficult and decreases specific power, but improves both charge retention and cycle life. We propose two recipes; one level composed of 1.5% Co and 0.4% Al of additives for power applications and the other with 1% Co and 0.4% Fe for a good balance among other battery properties. (C) 2009 Elsevier B.V. All rights reserved.