WO1993022800A1 - Silver metal-hydride cells having energy density and low self-discharge - Google Patents

Silver metal-hydride cells having energy density and low self-discharge Download PDF

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WO1993022800A1
WO1993022800A1 PCT/US1993/004019 US9304019W WO9322800A1 WO 1993022800 A1 WO1993022800 A1 WO 1993022800A1 US 9304019 W US9304019 W US 9304019W WO 9322800 A1 WO9322800 A1 WO 9322800A1
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alloy
hydrogen storage
percent
storage cell
electrochemical hydrogen
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PCT/US1993/004019
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French (fr)
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Srinivasan Venkatesan
Michael A. Fetcenko
Stanford R. Ovshinsky
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Ovonic Battery Company, Inc.
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M2/00Constructional details or processes of manufacture of the non-active parts
    • H01M2/14Separators; Membranes; Diaphragms; Spacing elements
    • H01M2/16Separators; Membranes; Diaphragms; Spacing elements characterised by the material
    • H01M2/164Separators; Membranes; Diaphragms; Spacing elements characterised by the material comprising non-fibrous material
    • H01M2/1653Organic non-fibrous material
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M4/00Electrodes
    • H01M4/02Electrodes composed of or comprising active material
    • H01M4/36Selection of substances as active materials, active masses, active liquids
    • H01M4/38Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/383Hydrogen absorbing alloys
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M10/00Secondary cells; Manufacture thereof
    • H01M10/34Gastight accumulators
    • H01M10/345Gastight metal hydride accumulators
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M2300/00Electrolytes
    • H01M2300/0002Aqueous electrolytes
    • H01M2300/0014Alkaline electrolytes

Abstract

An electrochemical hydrogen storage cell comprising: a negative electrode composed of an alloy having the following composition: (Vy'-yNiyTix'-xZrxCrz)aM'bM''cMdiiiMeiv where x' is between 1.8 and 2.2; x is between 0 and 1.5; y' is between 3.6 and 4.4; y is between 0.6 and 3.5; z is between 0.00 and 1.44; a designates that the V-Ni-Ti-Zr-Cr component, (V¿y'-y?NiyTix'-xZrxCrz), as a group, is at least 70 atomic percent of the alloy; M', M'', M?iii, and Miv¿ are modifiers chosen from the group consisting of Al, Mn, Mo, Cu, W, Fe, Co, and combinations thereof; and b, c, d, and e are modifier concentrations in said alloy; b + c + d + e > 0; and a + b + c + d + e = 100 atomic percent; and a positive electrode composed of silver.

Description

SILVER METAL-HYDRIDE CELLS HAVING ENERGY DENSITY AND LOW SELF-DISCHARGE

> This application is a continuation in part of U.S. Patent No. 07 746,015, filed August 14, 1991. 5 Field of the Invention

The present invention relates to rechargeable electrochemical cells. More particularly, the invention relates to rechargeable cells and batteries having negative electrodes formed of multicomponent, electrochemical hydrogen storage alloys and positive electrodes formed of silver oxide. Cells and batteries which use the negative 10 and positive electrodes of the present invention are characterized by increased energy density and longer cycle life. Background of the Invention

Silver positive electrodes are superior to other types of positive electrodes such as those composed of nickel hydroxide because the reaction at the silver electrode effectively 15 translates into higher operating voltages and greater energy density. For example at a silver electrode, on charge, discharge, and overcharge the following reactions occur: charge

2Ag + 2OH- < > Ag2O + H2O + 2e discharge

20 chMge

2Ag2O + 4OH- < > 4AgO + 2H2O + 4e discharge

A variety of disadvantages offset these superior characteristics of silver electrodes.

The higher oxide, AgO, is unstable and must be stabilized. Further, the Ag2O is soluble 25 in aqueous solutions and will migrate through the separator and form dendrites that ultimately short the cell. Finally, silver remains an expensive element.

To an extent, some of these disadvantages have been mitigated. AgO stability has been increased by using relatively high concentration electrolytes and dendrite formation is avoided by using separators such as cellophane or microporous polypropylene that block 30 the migration of Ag2O. Until the present invention, however, silver positive electrodes have been paired with negative electrodes to produce cells whose performance could be considered highly desirable, but only for special applications. Silver hydrogen cells were developed for aerospace and military applications that required batteries that could withstand repeated deep discharge. Charkey and Klien, in 23 Secondary Batteries 23-1 (1982), describe a high pressure silver hydrogen battery using a silver positive electrode and a hydrogen negative electrode. The hydrogen negative

Figure imgf000004_0001
electrode is formed from Teflon-bonded platinum black or platinized carbon bonded to nickel collectors. Thes& high pressure hydrogen cells use absorbers of asbestos and potassium titanate; either an "inorganic" separator or an "-inorganic/organic" separator; and an aqueous solution of 25-30% KOH as the electrolyte. They are constructed by alternately stacking silver electrodes, absorbers, membranes, and hydrogen electrodes until the desired capacity is reached. Plastic gas diffusion screens are placed between adjacent hydrogen electrodes to allow gas access to the active catalytic sites. Plastic end plates and a centrally located tie rod support the active pack. Such cells are reported to operate at 1.05 to 1.115N, and have an energy density of 75 to 85 Wh/kg and an estimated cycle life of 500-3000 cycles. Such cycle lives are only possible, of course, at about 20% depth of discharge ("DoD").

Schultze and Antoine, in Application of Hydrides to Silver Hydrogen System, SAFT1 Aerospace Department Publication, SAS-224/82-JPS/MD, Issue 2, 20 April 1982, describe high pressure silver-hydrogen hydride batteries that have many similarities to the high pressure cells Charkey and Klien describe. like Charkey and Klien, Schultze and Antoine describe a high pressure stacked assembly of electrodes along a central tie rod. The negative electrodes are described as preferably composed of LaΝi47, LaNi-Cu, Lao gCeoΛNi5, or Lao 6Ceα4N--5. The separator is a combination of grafted polyethylene (serving as a barrier to silver ions) and non-woven polyamide felt (for maintaining irrigation of the electrodes). The resulting silver-hydrogen common pressure vessel prototype battery described had a 90Ah capacity; and an energy density of 60 Wh/kg. These cells are extremely large, about 30kg, and the hydrogen gas evolved during charging was contained in a pressure vessel rather than being absorbed in the hydride negative electrode. Compared to silver hydrogen cells, such silver hydride cells are inferior in terms of energy density and cycle life. Currently, zinc negative electrodes are used with silver positive electrodes

Societe des Accumulateurs Fixes et de Traction. whenever higher energy density cells are needed and cost is not a factor. Silver zinc cells yield between 70 to 120 Wh/kg depending on the rate of discharge and the size.

However, silver zinc cells generally have an extremely poor cycle life, usually between 10 and 150 cycles depending on the depth of discharge, and the zinc electrode experiences

5 a shape change due to volume expansion of the electrode over the life of the cell.

Even if cost were not a factor, the silver hydrogen, silver hydride, and zinc silver cells each have only a single outstanding performance parameter. At best, these are special purpose cells whose general usefulness is extremely limited.

Rechargeable cells that use a nickel hydroxide positive electrode and a metal 0 hydride hydrogen storage negative electrode ("metal hydride cells") are known in art.

When an electrical potential is applied across a metal hydride electrode in a cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical generation of a hydroxyl ion; upon discharge, the stored hydrogen is released to form a water molecule and release an electron: 5 charge

M + H2O + e < > M-H + Off discharge

Most metal hydride cells use a nickel hydroxide positive electrode. The charge 0 and discharge reactions that take place at the nickel hydroxide electrode involve only a single step, in contrast to a silver electrode as described above. More specifically, the following reactions take place at the nickel hydroxide electrode: charge

Ni(OH)2 + OH" < > NiOOH + H2O + e -r discharge

In a cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode (a " TMH" cell), the electrodes are separated by a non-woven, felted, nylon or polypropylene separator, and the electrolyte is usually an alkaline electrolyte, for example, 20 to 45 weight percent potassium hydroxide. 0 In general, rechargeable Ni-MH cells offer significantly higher specific charge capacities than commercially available cells that use cadmium negative electrode and a nickel hydroxide positive electrode. However, the Ni-MH cells that use a nickel hydroxide positive electrode do not have energy densities approaching those of the silver positive electrode cells discussed above.

The operational lifespan, that is the available number of charge and discharge cycles of a cell, typically deteπnines the types of applications for which a cell can be used. In general, Ni-MH cells have a longer lifespan than the silver positive electrode cells described above.

A variety of hydrogen storage alloys, both electrochemical and thermal, are known in the art The term "Ovonic alloy" is frequently used to refer to all AB2 type materials in deference to their development from amorphous thin film materials discovered by Stanford Ovshinsky. Some of the most efficient of these Ovonic alloys are Ti-V-Zr-Ni type active materials. These materials are disclosed in U.S. Patent No. 4,551,400 (hereinafter the '400 Patent) to Sapru, Hong, Fetcenko, and Venkatesan, the contents of which are incorporated by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti-N-Zr-Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain one or more AB2 phases with C14 and C15 type structures.

Other Ti-N-Zr-Ni Ovonic alloys may also be used for a rechargeable hydrogen storage negative electrode. One such family of materials are those described in U.S. Patent No. 4,728,586 (the "c586 Patent") to Venkatesan, Reichman, and Fetcenko for "Enhanced Charge Retention Electrochemical Hydrogen Storage Alloys and an Enhanced Charge Retention Electrochemical Cell," the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of the Ti-V-Ni-Zr hydrogen storage alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. Particularly preferred Ovonic alloys of the '586 Patent may be viewed stoichiometrically as comprising 80 atomic percent of an V-Ti-Zr-Ni moiety and up to 20 atomic percent Cr, where the ratio of (Ti + Zr + Cr + optional modifiers) to (Ni + V + optional modifiers) is between 0.40 to 0.67. The '586 patent, while mentioning the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, is silent as to specific additives and modifiers, the amounts and interactions of the modifiers, and the particular benefits that could be expected from the modifiers.

The V-Ti-Zr-Ni family of Ovonic alloys described in the c586 Patent has an inherently higher discharge rate capability than those of the '400 patent This is the result of a substantially higher surface areas for the N-Ti-Zr-Ni materials, and the metal/electrolyte interface. Measured in surface roughness factor (total surface area divided by geometric surface area), the V-Ti-Zr-Ni materials can have roughness factors of about 10,000. This very high surface area plays an important role in the inherently high rate capability of these materials.

The metal/electrolyte interface also has a characteristic surface roughness. The characteristic surface roughness for a given negative electrode hydrogen storage material is important because of the interaction of the physical and chemical properties of the host metals, as well as of the alloys and crystallographic phases of the alloys, in an alkaline environment The microscopic chemical, physical, and crystallographic parameters of the individual phases within the hydrogen storage alloy material are believed to be important in determining the macroscopic electrochemical characteristics of the hydrogen storage material. Since all of the elements, as well as many alloys and phases thereof, are present throughout the metal, they are also represented at the surfaces and at cracks which form the metal/electrolyte interface.

In addition to the physical nature of the roughened surface, it has been observed that the V-Ti-Zr-Ni materials tend to reach a steady state surface condition and particle size. This steady state surface condition is characterized by a relatively high concentration of metallic nickel. These observations are consistent with a relatively high rate of removal of the oxides of titanium and zirconium from the surface and a much lower rate of nickel solubilization. The resultant surface seems to have a higher concentration of nickel than would be expected from the bulk composition of the negative hydrogen storage electrode. Nickel in the metallic state is electrically conductive and catalytic, imparting these properties to the surface. As a result, the surface of the negative hydrogen storage electrode is more catalytic and conductive than if the surface contained a higher concentration of insulating oxides.

The surface, having a conductive and catalytic component, e.g., the metallic nickel, appears to interact with chromium alloys, in catalyzing various hydriding and dehydriding reaction steps. To a large extent, many electrode processes, including competing electrode processes, are controlled by the presence of chromium in the hydrogen storage alloy material, as disclosed in the '586 Patent U.S. Patent No. 5,096,667 ("the '667 patent") and U.S. Patent No. 5,104,617 ("the

'617 patent"), the contents of which are incorporated by reference, describe V-Ti-Zr-Ni-Cr

Ovonic alloys where the base elements are individually or collectively replaced with one or more specific modifiers. These references discuss various surface area and oxidation effects and how they are effected by specific modifiers.

An alternative class of hydrogen storage alloys is the AB5 hydrogen storage alloys. These alloys differ in chemistry, microstructure, and electrochemistry from the AB2 and N-Ti-Zr-Ni-Cr types of electrochemical hydrogen storage alloys. Rechargeable batteries utilizing AB5 negative electrodes, most of which are lanthanum-nickel alloys, are described, for example, in (i) U.S. Patent No. 3,874,928; (ϋ) U.S. Patent No. 4,214,043; (ϋi) U.S. Patent No. 4,107,395; (iv) U.S. Patent No. 4,107,405; (v) U.S. Patent No. 4,112,199; (vi) U.S. Patent No. 4,125,688; (vϋ) U.S. Patent No. 4,214,043; (viii) U.S. Patent No. 4-216,274; (ix) U.S. Patent No. 4,487,817; (x) U.S. Patent No. 4,605,603; (xii) U.S. Patent No. 4,696,873; and (xiii) U.S. Patent No. 4,699,856. These references are discussed extensively in U.S. Patent No. 5,096,667 and this discussion is specifically incorporated by reference.

It is clear form the documents cited above, that the AB5 type alloys are a distinct and specific class of materials. Extensive work on processing techniques and electrode cell design demonstrate that AB5 alloys represents a separate field of inventive effort from the AB2 and V-Ti-Zr-Ni-Cr classes of alloys. In particular, modification of AB5 type alloys must be viewed as practical only within the specific AB5 structure. This is due to the unique metallurgical, electrochemical, and oxidation characteristics of the AB5 class of alloys, especially regarding the use of lanthanum and other rare earths for electrochemical applications. Further, there is no prior teaching or suggestion regarding the selection and role of modifiers generally for the AB5 alloys or regarding specific performances that might result from specific modifiers. Thus, even if the prior art did consider the problems that result when AB5 materials are combined with a silver positive electrode, such a teaching would not teach or suggest that the AB2 and/or V-Ti-Zr-Ni-Cr classes of alloys could be used as the negative electrode with a silver positive electrode. As described in detail below, the cells of the present invention consist of an

Ovonic alloy negative electrode and a silver positive electrode and exhibit improved cycle life and discharge rate over cells of the prior art These and other advantages of the present invention are readily apparent from the drawings, discussion, and description below. Summary of the Invention

Thus, one object of the present invention is a rechargeable metal hydride cell having a silver positive electrode that is also a sealed pressure cell.

Another object of the present invention is a silver metal hydride cell in standard commercial sizes.

Yet another object of the present invention is a silver metal hydride cell having a long cycle life. These and other objects of the present invention are satisfied by an electrochemical cell comprising a negative electrode having the composition (Ny-.yNiyTi,--. xZrxCrz)aM'bM"cMd ]V--e iv; where x' is between 1.8 and 2.2; x is between 0 and 1.5; y' is between 3.6 and 4.4; y is between 0.6 and 3.5; z is between 0.00 and 1.44; a designates that the N-Ni-Ti-Zr-Cr component, (Ny.-yNiyTix-_xZrxCrz), as a group, is at least 70 atomic percent of the alloy; M', M", M , and Mlv are modifiers chosen from the group consisting of Al, Mn, Mo, Cu, W, Fe, Co, and combinations thereof; b, c, d, and e are modifier concentrations in the alloy, where b + c + d + e > 0; and a + b + c + d + e = 100 atomic percent; and a silver positive electrode.

Other objects of the present invention are satisfied by an electrochemical hydrogen storage cell comprising: a negative electrode composed of an alloy having a heterogeneous, disordered microstructure resulting from changes in the mutual solubility of the elements of said alloy, wherein hydrogen in a particular phase is not easily discharged even through low surface area, or an oxide of limited porosity or catalytic property; and a positive electrode composed of silver. Still other objects of the present invention are satisfied by an electrochemical cell comprising a negative electrode formed of a hydrogen storage alloy which comprises a plurality of hydrogen storing elements chosen from the group consisting of 14 to 22% vanadium; 28 to 39% nickel; 7 to 15% titanium; 15 to 34% zirconium; present in a ratio that optimizes hydrogen overpressure during operation of said electrochemical cell and at least one modifying element selected from the group consisting of 0.01 to 7% chromium; 0.01 to 7% cobalt; 0.01 to 7% iron; 0.01 to 3.6% manganese; 0.01 to 2.7% aluminum; selected to optimize desirable operational parameters of said electrochemical cell; and a positive electrode composed of silver. Brief Description of the Drawings

Figure 1 shows a typical two step discharge curve of a silver metal hydride cell.

Figure 2 shows the cycle life data obtained for silver metal hydride cells of the present invention.

Figure 3 shows the coulombic efficiency for silver metal hydride cells of the present invention.

Figures 4-9 show capacity v. cycle life data for different embodiments of the present invention. Detailed Description of the Invention

In accordance with the present invention, it has been found that electrochemical hydrogen storage alloys employed in cells using silver positive electrodes have a high- energy density, good cycle life, and high charge capacity.

The best Ovonic alloy negative electrodes have the following composition: (Vy,yNiyTir,xZrxCr2).M'bM"cMd ffiMe iv where x' is between 1.8 and 2.2;x is between 0 and 1.5; y' is between 3.6 and 4.4; y is between 0.6 and 3.5; z is between 0.00 and 1.44; a designates that the V-Ni-Ti-Zr-Cr component, (Vy^yN--yT--x-.x-Z-rxCrz), as a group, is at least 70 atomic percent of the alloy; M\ M", M21, and M* are modifiers chosen from the group consisting of Al, Mn, Mo, Cu, W, Fe, Co, and combinations thereof; and b, c, d, and e are modifier concentrations that in the alloy provide optimum performance; b + c + d + e > 0; and a + b + c + d + e = 100 atomic percent

More specifically, negative electrodes made of an alloy that comprises, on an atomic scale, 14 to 22% vanadium, 28 to 39% nickel, 7 to 15% titanium, 15 to 34% z-rconium, and one or more of the following elements in the stated percentages: up to 7% chromium; up to 7% cobalt; up to 7% iron; up to 3.6% manganese; and up to 2.7% aluminum also yield satisfactory results when combined with silver positive electrodes according to the present invention.

The Ovonic alloys of the present invention can also be classified as having a heterogeneous, disordered microstructure resulting from changes in the mutual solubility of the elements of the alloy, wherein hydrogen in a particular phase is not easily discharged even through low surface area, or an oxide of limited porosity or catalytic property.

Figure 1 shows a typical discharge curve of a silver metal hydride cell according to the present invention. The two-step discharge curve represents the two oxides of silver that are formed during charging. Midpoint voltage of the cell is between 1.2 and 1.25 volts. The energy density of the cell is 10bcl.2 .17 = 70.6Wh kg.

Normally, silver zinc cells of this size are rated at about 12Ah. The capacity of the cells of the present invention ranges from 11 Ah to 14Ah.

Figure 2 shows the cycle life data obtained for silver metal hydride cells of the present invention. The cells were tested through 89 cycles at 100% DoD before testing was suspended. Silver zinc cells of the same size give between 10 and 40 cycles, depending on the depth of discharge, where a higher DoD results in low cycle numbers.

Figure 3 shows the coulombic efficiency of cells according to the invention to be 85 to 90% throughout the cycling period. Although the silver electrode can evolve oxygen towards the end of its charge, it is not necessary to use 140% charge as required with a nickel electrode. The pressure developed in silver metal hydride cells is about 40 psi. Preparation of Negative Electrode Materials

The hydrogen storage alloy materials shown in Table 1 were prepared and fabricated as described below into negative electrode materials.

Figure imgf000011_0001

The alloys of Table 1 were prepared by weighing and mixing powders of the component elements into a graphite crucible. The crucible and its contents were placed in a vacuum furnace which was evacuated and then pressurized with approximately one atmosphere of argon. The crucible contents were melted by high frequency induction heating while under the argon atmosphere. The melting was carried out at a temperature between 1500 C - 1700 C until a uniform melt was obtained. At that time, the heating was terminated and the melt was allowed to solidify under a blanket inert atmosphere.

The ingot of alloy material was then reduced in size in a multi-step process. The first step involved a hydrid g/dehydriding process substantially as described in U.S. Patent No. 4,983,756 entitled Hydride Reactor Apparatus for Hydrogen Comminution of Metal Hydride Hydrogen Storage Alloy Material, the disclosure of which is specifically incorporated by reference. In this first step, the alloy was reduced in size to less than 100 mesh. Subsequently, the material obtained from the hydride/dehydride process was further reduced in size by an impact milling process in which the particles were tangentially and radially accelerated against an impact block. This process is described in U.S. Patent No. 4,915,898 entitled Improved Method for the Continuous Fabrication of Comminuted Hydrogen Storage Alloy Negative ■ Electrode Material, the disclosure of which is specifically incorporated by reference.

A fraction of the alloy material having a particle size of less than 200 mesh and a mass average particle size of about 400 mesh (38 microns) was recovered from the impact milling process and bonded to a nickel screen current collector by a process which involves disposing a layer of alloy material onto the current collector and compacting the powder and collector. Compacting was carried out under an inert atmosphere with two separate compaction steps, each at a pressure of about 16 tons per square inch. After compaction, the current collector and the powder adhered to it were sintered in an atmosphere of about 2 atomic percent hydrogen with the balance argon to form negative electrode materials.

These negative electrode materials were activated using the alkaline etch treatment described in U.S. Patent No. 4,716,088, the disclosure of which is specifically incorporated herein by reference. Example 1

Negative electrode material prepared from hydrogen storage alloy No. 2 as described above was trimmed to size and placed in a single layer pellon bag. Silver oxide positive electrodes (obtained from BST Systems, Inc., Plainfield, Conn.) were placed in a single layer pellon bag, and this pellon bag was then placed in an additional bag composed of five layers of cellophane. Thus prepared negative and positive electrodes were assembled into vented, prismatic, electrochemical cells filled with a 45% KOH electrolyte.

The analysis of the resulting cells is presented in Figures 1-3. Figure 1 shows a voltage of 1.0 V after about 5 hours. Figure 2 shows a peak capacity of about lOAh after 90 cycles. Figure 3 shows an efficiency of about 90 % after 90 cycles. Example 2

Cells were prepared and subjected to charging and discharging conditions as described above in Example 1, except alloy No. 1 was used.

The data obtained from these tests is set forth in Table 2, below.

Figure imgf000013_0001

Example 3

Cells were prepared in Example 1 as described above except alloy No. 2 was used. The resulting cells were subjected to charging and discharging conditions. The analysis of the resulting cells is presented in Figures 4-6. Figure 4 shows a peak capacity of about 14Ah and a cycle life of about 45 cycles. Figure 5 also shows a peak capacity of about 12Ah and a cycle life of about 45 cycles. Figure 6 shows a peak capacity of about 14Ah and a cycle life of greater than 145. Example 4

Cells were prepared as described in Example 1 above, except alloy No. 3 was used. The results of this analysis are presented in Figures 7-9.

Fig. 7 shows a peak capacity of about 11 Ah and a cycle life of 58. Fig. 8 shows a peak capacity of about 14Ah and a cycle life of 50. Fig. 9 shows a peak capacity of about 11 Ah and a cycle life of about 81. Analysis

The cycle life of these cells is particularly significant because they were discharged to 100% DoD. As is well known in the art, many rechargeable batteries do not perform well at 100% DoD conditions. With silver oxide zinc cells of the prior art, this is even more of a problem. However, by combining a silver positive electrode with a metal hydride negative electrode according to the present invention, the excellent properties of the positive electrode and the long life of the negative electrode are both preserved.

Cells formulated according to the present invention exhibit a uniform improvement in operational parameters compared to previously described silver electrodes cells that offered an improvement in one area at the expense of poor performance in others. For example, the cells of the present invention are all comparatively small, comparatively low pressure, sealed cells that have long lifespans, as well as excellent charge capacities and energy densities. The cells of the present invention, thus have more potential applications than previously known cells using silver positive electrodes. In view of the above, it is obvious to those skilled in the art that the present invention identifies and encompasses a range of alloy compositions which, when incorporated as a negative electrode in electrochemical cells having a silver positive electrode result in cells and batteries having improved performance characteristics.

Further, various operational parameters of the components of these cells may be independently controlled to optimize the cells and batteries of this invention in addition to optimizing the characteristics discussed above.

Further, it is obvious to those skilled in the art that the invention may be prepared by additional methods, using additional compositions, and different configurations (such as the "jelly roll" configuration of common "AA", "C" and "D" cells) without departing from its spirit and scope.

The drawings, discussion, descriptions, and examples of this specification are merely illustrative of particular embodiments of the invention and are not meant as limitations upon its practice. It is the following claims, including all equivalents, that define the scope of the invention. What is claimed is:

Claims

1. An electrochemical hydrogen storage cell comprising: a negative electrode composed of an alloy having the following composition:
(Vy,yNiyTix,xZrxCrz).M'bM"cMd iiiMe iv 5 where x' is between 1.8 and 2.2; x is between 0 and 1.5; y' is between 3.6 and 4.4; y is between 0.6 and 3.5; 10 z is between 0.00 and 1.44; a designates that the V-Ni-Ti-Zr-Cr component, (Vy.-yNiyTij.. xZrxCrz), as a group, is at least 70 atomic percent of the alloy;
M', M", Λ-T", and Miv are modifiers chosen from the group consisting of Al, Mn, Mo, Cu, W, Fe, Co, and combinations thereof; 15 b, c, d, and e are modifier concentrations in said alloy; b + c + d + e > 0; and a + b + c + d + e = 100 atomic percent; and a positive electrode composed of silver.
2. The electrochemical hydrogen storage cell of claim 1, wherein said negative 20 electrode alloy, on an atomic percent basis, comprises:
14 to 22 percent vanadium; 28 to 39 percent nickel;
7 to 15 percent titanium;
15 to 34 percent zirconium; and
25 at least one member selected from the group consisting of
0.01 to 7 percent chromium,
0.01 to 7 percent cobalt,
0.01 to 7 percent iron,
0.01 to 3.6 percent manganese, and
- 30 0.01 to 2.7 percent aluminum.
3. The electrochemical hydrogen storage cell of claim 1, wherein said negative electrode comprises an alloy having an heterogeneous, disordered microstructure resulting from changes in the mutual solubility of the elements of said alloy, wherein hydrogen in a particular phase is not easily discharged even through low surface area, or an oxide of limited porosity or catalytic property.
4. The electrochemical hydrogen storage cell claimed in claim 1, wherein said alloy used to form said negative electrode is subjected to an alkaline etch while in powder form.
5. The electrochemical hydrogen storage cell of claim 1, wherein said negative electrode comprises an alloy having the following composition:
V20.6Ti15Zr15Ni30Cr&6Co6.6Mn3.6Al2.7.
6. The electrochemical hydrogen storage cell of claim 1, wherein said negative electrode comprises an alloy having the following composition:
Figure imgf000016_0001
7. The electrochemical hydrogen storage cell of claim 1, wherein said negative electrode comprises an alloy having the following composition:
Figure imgf000016_0002
8. The electrochemical hydrogen storage cell of claim 1, further comprising a pellon separator.
9. The electrochemical hydrogen storage cell of claim 1, further comprising enclosing said positive electrode in a separator bag composed of layers of pellon and cellophane.
10. The electrochemical hydrogen storage cell of claim 10, wherein said separator bag comprises one layer of pellon and five layers of cellophane.
PCT/US1993/004019 1992-04-30 1993-04-29 Silver metal-hydride cells having energy density and low self-discharge WO1993022800A1 (en)

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US4728586A (en) * 1986-12-29 1988-03-01 Energy Conversion Devices, Inc. Enhanced charge retention electrochemical hydrogen storage alloys and an enhanced charge retention electrochemical cell
US5096667A (en) * 1989-11-24 1992-03-17 Energy Conversion Devices, Inc. Catalytic hydrogen storage electrode materials for use in electrochemical cells and electrochemical cells incorporating the materials

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