Nickel alloys, mischmetal

A series of mischmetal nickel alloys are known as typical hydrogen-absorbing materials (see Chapter 6.3) and are used as the negative electrode in Ni-MH rechargeable batteries (see Chapter 8.4). By the fluorination treatment of mischmetal nickel alloys the electric conductivity is significantly improved by the replacement of La-oxide and La-hydroxide layers on the Ni-emiched Lap2 surface. The layer was also found to protect the surface firom KOH alkaline solution as electrolyte and to improve the long-term durability. 

Hysteresis, the pressure difference between absorption and desorption, should be small for most practical applications of hydrides. Hysteresis varies markedly from alloy to alloy in a manner not understood by researchers in the field. However, an empirical body of knowledge now exists to show a number of systems where hysteresis can be made quite small. The nickel-aluminum-mischmetal system (e.g., Figure 2) is a good example of one with small hysteresis. 

Sandrock, G. D., A New Family of Hydrogen Storage Alloys Based on the System Nickel-Mischmetal-Calcium, "Proc. 12th IECEC", 1977, 1, 951, Washington, DC. 

Cobalt is used in various types of rechargeable batteries. In the Ni-Cd batteries, cobalt represents 1-5% of the weight of nickel hydroxide at the cathode. In the nickel metal hydride battery (Ni/MH), cobalt is moreover employed at the anode as a hydrogen storage alloy such as mischmetal (NiCoAlMn (5-15% Co). Finally, in the Li-ion batteries, the cathode could be LiCo02 where cobalt could represent 50% of the weight (6, 7). 

AB5 alloys are now mainly used for nickel/metal hydride batteries. Mischmetals, which are unrefined mixtures of rare earths, are mostly substituted instead of lanthanum, mainly because they are less expensive than pure La. But such substitutions also are required to stabilize the alloy and prevent its premature degradation. 

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