Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1998-04-22
2001-04-03
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S577000, C148S675000
Reexamination Certificate
active
06210498
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to improved hydrogen storage alloys and improved methods for making such alloys and to improved rechargeable nickel metal hydride (NiMH) electrodes and batteries employing such alloys and methods. More specifically, the instant invention relates to improved hydrogen storage materials which electrochemically store and release hydrogen and which function as the negative electrode materials for NiMH batteries
BACKGROUND OF THE INVENTION
Rechargeable NiMH batteries have become the batteries of choice for many applications ranging from consumer applications such as laptop computers and cell telephones to large scale applications such as electric vehicle (EV) propulsion and hybrid electric vehicle (HEV) propulsion systems.
For example, Ni—MH batteries, such as those described in copending U.S. patent application Ser. No. 07/934,976 now U.S. Pat. No. 5,277,299 to Ovshinsky and Fetcenko, the disclosure of which is incorporated herein by reference, have high energy and power densities, contain no toxic materials, can be rapidly recharged and have become the batteries of choice for both EV and HEV applications as well as consumer product applications. The fundamental principles of the technology incorporated in such batteries is disclosed in U.S. Pat. No. 4,623,597—Ovshinsky et al.
Some highly efficient electrochemical hydrogen storage materials have been formulated based on the principles set forth in the aforementioned U.S. Pat. No. 4,523,597. The present invention is based on fundamentally new discoveries relating to the compositions, structures, electron concentrations and local order and environment of such hydride materials having particular application as materials for the negative electrodes of NiMH batteries. The new hydride materials of the present invention provide an even higher density of accessible, reversible hydrogen storage sites in a preferred spectrum with superior kinetics and other properties and exhibit substantially improved and superior performance characteristics in NiMH batteries.
SUMMARY OF THE INVENTION
We have discovered that the hydrogen storage and related properties of the hydride materials of the invention are affected by certain parameters which can be controlled as will be disclosed in detail below to attain the desired properties.
One parameter which is relevant to materials which store hydrogen atoms/protons is the electronic environment of the storage sites therefor. Here we discuss hydrogen atoms and protons interchangeably because, as is apparent, a hydrogen ion is a proton, and once the moving proton comes to rest in an thermodynamically/electronically favored position, it becomes a hydrogen atom. Therefore, while the proton itself is physically very small, the electronic influence it has is large, and once it occupies a storage site, it becomes a hydrogen atom, having the characteristic atomic diameter of a hydrogen atom. Therefore, one way in which the storage sites can be increased is to form a more favorable electronic environment for the entering protons.
Increased hydrogen storage can be accomplished by increasing the theoretical number of available storage sites and by increasing the utilization of the theoretical storage sites. Increasing the number of theoretical storage sites involves changing the materials composition and its crystallographic structure. Increasing the utilization of the theoretical storage sites is a somewhat different problem. This problem is the one that involves improving the electronic environment of the storage sites. That is, by selecting the proper atoms (size, electron configuration, outer shell electrons, electronegativity, etc.) one can increase or decrease the electronic/thermodynamic desirability of the theoretically available storage sites, and therefore make these sites more or less likely to actually be favorable to storage of the hydrogen. For instance, if the atoms surrounding the theoretical storage site are too large, or do not have readily available outer shell electrons, etc., then the site will be less favorable for hydrogen storage. However, if the atoms are smaller and there is available electrons to hold the proton, the site will be more favorable for hydrogen storage.
Another aspect of this electronic environment problem is the effect of coulombic repulsion. This occurs when the hydrogen atoms stored in neighboring sites repel the surrounding atoms, which in turn reduces the electronic favorableness of nearby storage sites. That is, as the surrounding atoms are repelled from the previously stored hydrogen, they crowd the unfilled hydrogen storage sites nearby, increasing the coulombic repulsion of any protons attempting to enter the site. Reduction of the size of the neighboring atoms and thus the positive charge of the nucleus thereof, reduces this coulombic repulsion effect.
While the hydrogen storage density of a storage material is an important property thereof, it is not the only consideration. An additional property of interest is the “bond strength” of the stored hydrogen. The term “bond strength” is somewhat misleading in connection with hydrogen storage in that not all hydrogen storage materials “bond” the hydrogen in a conventional sense. Some materials, like magnesium, do bond hydrogen in a conventional sense. That is, the hydrogen forms covalent bonds with magnesium. These covalent bonds hold the hydrogen very strongly and it is therefore very difficult to get the hydrogen back out of the magnesium hydride. Other hydrogen storage alloy systems intercalate the hydrogen into thermodynamically favorable storage sites. These thermodynamically favorable sites, while not “bonds” in the conventional sense, act as such. Typically these intercalation sites have “bond energies” which are of much lower energy than, for instance, a covalent bond. This lower “bond energy” makes these materials more useful for hydrogen storage, in that the hydrogen can more easily be retrieved from the storage material.
While specific elements may form covalent bonds with the stored hydrogen, it is possible to form a spectrum of “bond energies” by adding elements together to form a hydrogen storage alloy. Addition of materials with d-orbital electrons to such an alloy may provide spectrum of “bond energies” required to make the materials useful for hydrogen storage. Additionally elements which contain f-orbital electrons may also be useful. Thus, while an element like magnesium, alone, may form covalent bonds with hydrogen which are hard to break, formation of an alloy by the addition of more elements can create a spectrum of bonding energies, thereby rendering the alloy suitable for “real world” hydrogen storage use. That is, this spectrum of bonding energies creates electronic pathways for utilization of the hydrogen under useful “real world” conditions.
Another property for consideration is the macrostructure of the storage material. By macrostructure we mean the topology/physical structure of the material and the crystallite size/shape. Topology/physical structure relate to the physical form that the material exists in, such as powder, flake, ribbon, plate, ingot, thin-film, etc. This parameter is important in that small particulate or thin ribbons/films allow the hydrogen easier access to the interior of the structure. Thus, there is less external material which could block utilization of the internal material. Additionally, the larger surface area of such a material allows for better access by the electrolyte of the battery. Also, the materials in the exterior of such a material have fewer crystallographic constraints. As for crystallite size, generally a smaller size will be more advantageous because, as with the topology, there will be less outside material which could block utilization of the interior material, and there will be fewer crystallographic limitations on the outer material. For instance the coulombic repulsion of the outer storage sites will be lower because there are fewer surrounding atoms. Thus, by making the crystallites sma
Chao Benjamin
Ovshinsky Stanford R.
Young Rosa T.
Energy Conversion Devices Inc.
Jenkins Daniel J.
Schumaker David W.
Siskind Marvin S.
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