As is known in the art, certain alloy materials are capable of absorbing and desorbing hydrogen. These materials can be used as hydrogen storage media and/or as electrode materials for metal hydride batteries, fuel cells, metal hydride air battery systems, and the like. Such materials are known generally as metal hydride materials.
One particular class of metal hydride materials having utility in metal hydride battery systems is known as the ABx class of materials with reference to the crystalline sites that its members component elements occupy. ABx type materials are disclosed for example in U.S. Pat. No. 5,536,591 and U.S. Pat. No. 6,210,498, the disclosures of both of which are incorporated herein by reference. Presently, there is significant interest in utilizing a particular group of ABx materials referred to as AB2 alloys in metal hydride battery systems. This is due to the fact that AB2 type materials, unlike many of the presently used AB5 alloy materials, generally do not incorporate significant amounts of expensive, rare earth elements. Furthermore, batteries incorporating AB2 materials generally exhibit high charge storage capacities.
As is recognized in the art, before being placed into service, metal hydride batteries must be subjected to an activation process. This process typically comprises putting the batteries through one or more particularly profiled charge/discharge cycles, and is necessary in order to assure that the battery manifest its maximum charge storage capacity. High-rate capability is another significant parameter of any battery system and is a measure of the battery's ability to deliver energy at a high rate. High-rate capability is a very important parameter in high power applications such as electric vehicles, power tools, and the like wherein large amounts of power must be supplied rapidly.
Heretofore, the utility of AB2 materials in metal hydride battery systems has been limited by the fact that they tend to be hard to activate and show low high-discharge rate capabilities, as compared to AB5 materials and the like. As will be explained in detail hereinbelow, the present invention recognizes that the activation and high-rate properties of AB2 metal hydride alloys are significantly improved if those alloys include a catalytic secondary phase. Further in accord with the present invention, it has been found that these phases can be formed by the introduction of a modifier element into the alloy material.
Materials of the present invention include at least a first and a second phase. The first phase (which may comprise a group of phases) has an AB2 type crystalline structure, and is also referred to as the “main phase”. The second phase includes concentrations of the modifier element which are significantly higher than concentrations in the first phase. The materials of the present invention are relatively low in cost, and when incorporated into metal hydride battery systems manifest large charge storage capacities together with significantly improved activation and high-rate discharge properties. These and other advantages of the present invention will be apparent from the drawings, discussion, and description which follow.