JPH0438106B2 - - Google Patents

Description

Claim 1: A sealed housing defining a cell space;
an electrochemical cell element comprised in the cell space and consisting of a zinc electrode with a current collector and an active substance, a nickel electrode, and a barrier between them; a predetermined amount of alkaline electrolyte contained in the cell housing; a positive terminal electrically connected to the positive electrode; a negative terminal electrically connected to the zinc electrode; and a means for oxidizing hydrogen generated during use to maintain the internal pressure of the cell at a desired low level. , wherein the zinc electrode is substantially free of metallic zinc in the active material and in close contact with 1. A sealed nickel-zinc cell comprising a carboxylated styrene-butadiene binder in an amount sufficient to form a structural zinc electrode.

2. The battery of claim 1, wherein the zinc electrode comprises a mixture of zinc oxide and a cadmium source in an amount to provide at least 20% of the ampere-hour capacity of the nickel electrode.

3. the binder is present in an amount from about 3.8% to about 5%, based on the weight of the zinc electrode active material;
A battery according to claim 1.

4. The battery according to claim 2, wherein the negative electrode contains bismuth oxide.

5. A battery according to claim 1, wherein the cell space is cylindrical and the electrochemical cell element has a winding roll with an axial core space.

6. The means for oxidizing the hydrogen generated during use includes a hydrogen recombination catalyst, which catalyst is located in the core space of the winding roll and is not electrically connected to the electrochemical cell elements. , Claim No. 5
Batteries listed in section.

7. The battery according to claim 6, wherein the hydrogen recombination catalyst has carbon pieces supporting a platinum catalyst.

8. The battery according to claim 1, wherein the partition wall has at least two layers of microporous polypropylene film.

9. The battery according to claim 1, wherein the alkaline electrolyte is an aqueous potassium hydroxide solution.

10. The battery of claim 1, wherein the electrochemical cell element includes an electrolyte absorbent core layer disposed between the positive electrode and the septum.

11. The battery of claim 10, wherein the core layer comprises a non-woven polypropylene material.

BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to electrochemical cells;
More specifically, it relates to sealed rechargeable nickel-zinc batteries.

2 Explanation of Conventional Law A wide variety of portable devices, such as tape and video recorders, power supplies, calculators, radios, shavers, television sets, pagers, mobile telephones, microcomputers, etc., have been developed over the last few years. They were developed over time, and their uses began to expand accordingly. In alternative situations, primary batteries have been utilized as a power source. However, although primary batteries offer the advantage of relatively high energy density, they are relatively expensive because they constantly need to be replaced.

For this reason, it has been useful to use rechargeable batteries as a power source. Various lead-acid batteries have been utilized, particularly where cost is a primary consideration. However, this type of battery has a relatively low energy density, a low cycle life capability, and at the same time requires a relatively long time to charge.

The unsuitability of primary and lead-acid batteries has led to the use of nickel-cadmium in certain applications. This type of battery is relatively expensive compared to lead-acid batteries, but provides relatively high energy density and extremely long cycle life. Moreover, this type of battery can be used at relatively high charge/discharge rates.

Despite the advantages offered by nickel-cadmium batteries, there continues to be a need for power source applications that can achieve higher energy densities and operate at higher working voltages. This situation has led to research into nickel-zinc rechargeable batteries for these applications. Nickel-zinc systems are well known and, at least potentially, offer substantial advantages. Compared to nickel-cadmium batteries, nickel-zinc batteries have higher working or operating voltages (ie, about 1.65 volts) and can potentially provide significantly higher energy density. US Pat. Nos. 3,951,687 and 4,037,033 disclose arrangements for nickel-zinc cells.

Despite this promise, the commercial use of nickel-zinc batteries in the portable applications described herein has been extremely limited. This is primarily due to the inability to adequately handle the relatively high internal pressure inherent in this system. For example, the cadmium electrode in a nickel-cadmium battery is only marginally thermally stable with respect to reduction of the aqueous alkaline battery electrolyte, with consequent evolution of hydrogen gas;
Zinc electrodes in the nickel-zinc system are unstable. Therefore, nickel-zinc systems tend to generate hydrogen gas under all conditions of use: charging, discharging, overcharging, and leaving open circuit.

To be commercially useful, a sealed nickel-zinc battery should therefore possess the ability to offset evolved hydrogen gas, ensuring that the battery does not vent under conditions of use. should. If the battery leaks, loss of electrolyte can result in reduced performance. Additionally, discharge of alkaline electrolyte into the environment can be harmful to electronics or other components where the battery is used. The permissible internal pressure depends on the strength of the container used. about
450 to 500p.sig (approximately 31.5 to 35Kg/cm ² gauge)
For small cylindrical batteries that typically use a container material that ruptures at such pressures (at which the top of the container typically detaches from the container), safety considerations dictate that approximately 250 p.sig (approx.
It is recommended to use a means of evacuation that will cause leakage at internal pressures of the order of cm ² gauge). For prismatic batteries, the plastic container typically used is such that this type of battery has a significantly lower internal pressure, approximately 25 p.sig (approximately 1.75 kg/cm ² gauge).
Requires leakage to some extent.

The internal pressure developed in a sealed nickel-zinc cell under service conditions is 250 p.sig (17.5
Kg/cm ²・gauge) can be easily exceeded.
Considerable effort has been expended in developing nickel-zinc systems that can operate at satisfactorily low internal pressures.

Moreover, there are many other problems involved in developing commercially practical sealed nickel-zinc cells. For example, it has been found that the problem of undue pressure build-up occurs during storage, regardless of the means used to offset the generated hydrogen gas. Thus, it has been discovered that when a nickel-zinc battery is left in a fully discharged state for an extended period of time, internal pressure builds up to a level that causes the battery to leak.

Another problem lies in the relatively high impedance values of conventional nickel-zinc batteries. For whatever reason, conventional nickel-zinc batteries appear to be characterized by an impedance level that limits the available current level.

Additionally, conventional batteries of this type appear to have suboptimal tolerance to overcharge conditions.
Although not fully understood, this suboptimal tolerance is based on previously utilized septum structures. Also, in this regard, the cycle life of nickel-zinc cells appears to be less than optimal, especially at high discharge rates.

Finally, conventional cells of this type appear to result in zinc passivation. this is,
Of course, this will have a negative impact on capacity.

Therefore, despite previous efforts to provide a commercially viable sealed nickel-zinc battery, this type of battery is not suitable as an alternative to the various power sources currently employed for portable device applications. has made very few, if any, advances. There is certainly a need to provide a commercially attractive and cost effective sealed nickel-zinc battery that satisfactorily avoids the problems described above.

OBJECTS OF THE INVENTION Accordingly, it is a principal object of the present invention to provide a sealed nickel-zinc battery that provides improved cycle life and electrical performance in use.

Another object is to provide a battery of the above type that is simple in construction and economical to manufacture.

Yet another object of the invention is to provide a battery of the above type that is capable of operating at relatively high current levels.

Another object of the present invention is to maintain the discharge state for a long time.
It is an object of the present invention to provide a battery of the above type that can be left unused without undue internal pressure accumulation.

Another object of the invention is to provide a cell of the above type in which zinc passivation is minimized.

Yet another object is to provide a battery of the aforementioned type possessing improved tolerance to overcharge conditions.

Other objects and advantages of the invention will become apparent from the following detailed description and drawings.

FIG. 1 is a side view of a nickel-zinc battery embodying the invention, with portions cut away to show the internal structure.

FIG. 2 is a cross-sectional view taken generally along line 2--2 of FIG. 1, further illustrating the internal structure of a battery according to the present invention.

FIG. 3 is a graph depicting the cycle life performance of batteries of the present invention as compared to conventional batteries.

FIG. 4 is a graph depicting the hydrogen pressure developed within a nickel-zinc cell with and without a hydrogen recombination catalyst utilized in accordance with one aspect of the present invention.

Although the present invention has various modifications and variations,
Preferred embodiments are shown in the drawings and described in detail below. However, it is not intended to limit the invention to the particular forms disclosed. On the contrary, the intention is to cover all modifications and variations falling within the spirit and scope of the invention as expressed in the appended claims.

SUMMARY OF THE INVENTION In general, the present invention provides a nickel-zinc battery with improved electrical performance characteristics, a mixture utilized in the negative electrode, a specific binder used in the negative electrode,
and by selecting the barrier system utilized. Each of these factors individually confers improved performance to the battery. Optimal performance is provided by utilizing all of the features described herein.

Additionally, for certain applications, a subfeature of the present invention provides a particular means of dealing with internal pressure build-up due to hydrogen generation.

DETAILED DESCRIPTION OF THE INVENTION Returning now to an illustrative embodiment, the drawings depict a rechargeable sealed nickel-zinc battery incorporating the present invention, which battery is generally is indicated by the symbol 10. This particular construction of the cell is merely exemplary and may be modified as desired. Battery 10 is cell 1
4 and an outer housing 12 defining the outer housing 12. The cup-shaped housing 12 has an open end 16 which is closed by a lid 18 to seal over the open end 16 by an annular insulator 20. An apertured disc 22 is secured to the open end 16 of the outer housing 12 by an annular retainer 24 to hold the lid 18 when the lid is forced outward, such as by the buildup of internal pressure inside a sealed cell. A piercing tab 26 adapted for piercing is provided. Reclosable vents can be used if desired, and vent constructions of this type are known.

As best seen in FIG. 2, the cell elements shown generally at 28 are cell 14.
contained therein in the form of a take-up roll, a negative electrode layer 30, a positive electrode layer 32, and a partition generally indicated at 34 intermediate these electrode layers;
Consists of.

According to one aspect of the invention, a core layer is preferably provided for electrolyte absorption. For example, including a core layer on the side of the septum that contacts the positive electrode layer imparts a longer cycle life to the cell, especially when service conditions include relatively high discharge rates (e.g., about 2C or more). It was discovered that.

Any alkali-resistant material capable of absorbing the electrolyte can be used. Generally, a nonwoven fabric made of synthetic resin such as polypropylene may be used. One illustrative example of a suitable polypropylene core sheet is "Wevryl 1488" nonwoven (Kendall Company) having a thickness of about 3 mils (0.075 mm).

Thus, as best seen in FIG. 2, an electrolyte-absorbing core layer 36 is provided on the surface of the septum 34 in contact with the positive electrode layer 32. A core layer could also be provided in contact with the negative electrode layer 30, if desired. However, although the use of a core layer in contact with the negative electrode offers advantages over not including a core layer in contact with either electrode, superior performance is achieved when the core layer is in contact with the positive electrode. It was discovered that this can be achieved when it exists only on the partition wall surface.

As best seen in FIG. 1, apertured disk 22 cooperates with lid 18 in defining the negative terminal of the housing. More specifically, first coupling means tab 38 is electrically coupled to negative electrode layer 30 and extends outwardly from the roll to electrically couple with lid 18 .

The outer housing 12 suitably comprises a metal can and defines the positive terminal of the battery. In this manner, the second connecting tab means 40 is electrically connected to the positive electrode layer 32 and the housing 12, as illustrated in FIG.

As shown for the illustrated embodiment, when used in a cylindrical battery, the positive and negative electrodes and septum should be sufficiently flexible to provide a single rolled element. Manufacturing techniques for providing suitable positive and negative electrode layers with suitable flexibility are well known. Suitable techniques are described in co-pending patent applications to Menard et al., identified herein.

The negative zinc electrode may thus be constructed using conventional techniques. By way of example, a powder mixture of the desired materials and binder can be roller coated onto a suitable current collector, such as copper wire gauze.

Although such mixtures have traditionally used both zinc oxide and zinc, it has been discovered that the presence of zinc metal tends to create undesirable pressures when the battery is left fully discharged for an extended period of time. It was done. This pressure build-up can reach levels that cause the cell to leak. For this reason, according to a major aspect of the invention, the mixtures utilized incorporate little or no zinc metal.

Various binder materials with zinc polarity are known. Typically, the binder material used is inert in the battery environment and is incorporated in just enough amount to hold the mixture together and also provide positive binding to the current collector.

Traditionally utilized binder materials, such as polytetrafluoroethylene, require the use of relatively large quantities to achieve the desired cohesive structure for the negative electrode, and are often weighed based on the weight of the mixture used. reaches about 10%. Such relatively large amounts of binder result in cells with relatively high impedance values. This limits the current level available when using the battery.

Accordingly, another major aspect of the invention consists of utilizing an elastomeric, self-curing, carboxylated styrene-butadiene latex as the binder material. This binder is preferably about 3.8 g based on the total weight of the negative electrode mixture.
It has been found satisfactory to use amounts ranging from % to about 5%. It was found that this level of amount provided a suitable adhesion structure for the negative electrode.
Moreover, and importantly, this results in a battery that is characterized by a relatively low impedance compared to conventional cells, and thus can be utilized at significantly higher current levels. Amounts in excess of 5% by weight may certainly be used, but such amounts tend to provide little benefit and increase impedance. Specific examples of suitable binders are AMSCO RES4150 and 4816, manufactured by the AMSCO Division of Union Oil Company.

If desired, the negative electrode may contain other components, some of which are known. Moreover, according to another aspect of the invention, it has been discovered that it is useful to include a small but significant amount of cadmium. This is believed to act to stabilize the negative electrode against shape changes while simultaneously reducing the rate of hydrogen evolution. In fact, the cadmium present is electrochemically inert until the operating cell voltage is reduced to about 1.30 volts or so. Under these reduced conditions, the inclusion of cadmium appears to serve to minimize the zinc passivation that would otherwise occur.

The amount of cadmium utilized should be such as to provide 20% of the ampere-hour capacity of the positive active material. Amounts above this minimum level may certainly be used, and the upper limit appears to be constrained by economic considerations. An amount of cadmium in the range of about 5 to 6%, based on the total weight of the negative electrode mixture, should be adequate to provide such a minimum level.

The cadmium component may be used in the mixture as cadmium oxide. However, it is preferred to use cadmium metal, as described in the co-pending Gibbard application. The use of cadmium oxide therefore results in some loss in capacity.

It has also been found useful to include bismuth oxide, Bi ₂ O ₃ , in the negative electrode mixture in an amount of about 7 to 8% based on the weight of the negative electrode mixture. It is believed that it functions as a corrosion inhibitor.

Prior art has used calcium hydroxide as another component of the negative electrode mixture. The negative electrode mixture described herein provides satisfactory performance and therefore does not require the inclusion of calcium hydroxide. It is therefore preferred that the negative electrode mixture is essentially free of calcium.

As is well known, nickel-based electrolytes using a conventional aqueous solution such as potassium hydroxide as an electrolyte
In the zinc system, the zinc species(s) formed during the discharge are soluble in the electrolyte to a significant extent. Some of the active zinc material thus enters the electrolyte during discharge of the system as well as during leaving the system in the discharged condition. When recharging the battery system, the zinc species(s) in the electrolyte return to the zinc electrode, but can change the structure of the electrode. This active zinc material can thus migrate from the edges or periphery of the electrode structure and collect in the central region of the electrode, resulting in an irreversible loss of capacity. This phenomenon has often been called "shape change."

Because of this phenomenon, the cell elements used in the present invention should be placed in the cell in a manner that at least minimizes shape changes. It has been found that when cylindrical cells are involved, it is satisfactory to simply roll the element so that it is under compression while in the cell. This helps minimize problematic shape changes.

As is also well known, this replating or redeposition of zinc often occurs in the form of dendritic or branched crystals (dendritic crystals) with sharp points, which are located between plates or electrodes of opposite polarity. easily crosslinks, thereby causing short circuits and cell destruction. Therefore, the material used as the partition wall should be a membrane with a relatively fine, uniform-diameter pore structure that allows the electrolyte to pass through, but prevents resinous crystals from entering. Furthermore, the materials used should be chemically stable in the cell environment. Additionally, materials that are suitable should have sufficient flexibility and strength properties to adequately withstand any shape changes and/or electrode expansion that may occur during use. A large number of materials have been proposed for use and are well known, as are methods of their manufacture.

As one illustrative example, the septum may be comprised of commercially available "Celgard" polypropylene film (Celanese Fiber Company). Two layers of this type of material (each layer approximately 1 mil (0.025 mm) thick)
It has been found to be particularly desirable to form the barrier layer 34 using a diaphragm layer 34, the individual layers being indicated generally at 42 and 44 (FIG. 2).
The use of two layers ensures that the pores in each layer do not line up with each other due to defects created during and after manufacture, and a single layer or more than two layers may be used as well if desired.

Conventional methods have used either films such as polyvinyl alcohol on the septum or cement to bond the individual layers together to form a composite unitary structure. However, it has been discovered that such techniques substantially reduce the cell's tolerance to overcharge conditions. According to yet another aspect of the invention, it is preferred to use a septum structure without any film on the septum layer or septum layers and without cement or other adhesives when multiple septum layers are used. In this manner, it has been discovered that the resulting cells possess excellent tolerance to overcharging.

Any conventional alkaline electrolyte used in the case of nickel-zinc systems may be used. As an example, it is satisfactory to use an aqueous potassium hydroxide solution containing about 25% by weight potassium hydroxide. It is desirable to initially use an electrolyte saturated with Zn(OH) ₂ to prevent initial dissolution of zinc oxide into the electrolyte. As is known in the sealed battery art, electrolyte usage should be sufficiently limited to provide an effective oxygen recombination reaction. In an illustrative embodiment, the required electrolyte can be added to the open space in the core of the rolled cell element before sealing the cell.

As for the first connecting tab 38, it should be made of a conductive material with a hydrogen evolution overpotential at least approximately as large as that of zinc. An illustrative example is a nickel element plated with copper and then with silver. The lid 18 may suitably consist of a sheet of steel plated with nickel, which is then coated with a copper plating and then with a silver plating. The second connecting tab 40 may, for example, consist of a nickel element, which is electrically connected to the nickel plating 46 of the outer housing 12.

A wide variety of materials are known for connection tabs and housings for nickel-zinc systems, and these materials can be used in the nickel-zinc system of the present invention.
May be used in zinc batteries. The specific materials of construction can therefore vary widely.

Additionally, if desired, a water sealant coating may be applied to the metal or other surfaces in the cell, as is known. A suitable sealant is the styrene-butadiene material described herein as a binder for the negative electrode mixture. As shown in FIG. 1, coating 48 was applied to the exposed surfaces of lid 18 and first connecting tab 38. It may be applied by brushing to a thickness, for example about 1 mil (0.025 mm).

Additionally, an insulator 5 is provided to ensure adequate insulation between the cell element 28 and the terminals.
0.50' may be included if desired. Although shown spatially removed from cell element 28 for simplicity of illustration, insulator 50 is shown in bulkhead 4.
2, 44, which preferably terminate slightly above the upper end of the electrode.

The nickel-zinc cells of the present invention may be used in either prismatic or cylindrical designs as desired in the particular application. Similarly, the capacity of the battery can vary within wide limits, and its dimensions are dictated by the requirements of the specific end use. By way of example, a cylindrical, sub-C sized cell for use in a cordless or portable power device may be suitable with a capacity of, for example, 1.2 Amp-hours.

The cell of the present invention must also incorporate means for oxidizing the hydrogen generated during use and maintaining a satisfactorily low internal pressure in the cell. A variety of catalytic means are known and may be used.

In this regard, one sub-aspect of the invention provides a hydrogen recombination catalyst placed in the cell as a source of hydrogen oxidation, which does not include electrical connections to the cell components. Any suitable fuel cell cathode may be used. By way of illustration, the recombination catalyst suitably comprises a cloth with about 1% platinum by weight on carbon particles adhered to the carbon cloth by a hydrophobic binder such as polytetrafluoroethylene. Suitable recombination catalysts, such as this illustrative embodiment, are commercially available. As seen in the figures, the hydrogen recombination catalyst 52 is located within the axial core space of the cell element of the winding roll and does not include electrical connections to the cell electrodes. This method facilitates cell assembly.

It has been found that by using a hydrogen recombination catalyst, the internal pressure generated under cyclic conditions is substantially reduced.
Although performance during long-term storage and fast charging/discharging is improved, in both cases the internal pressure increases to the extent that the cell leaks.

For this reason, it is preferable to utilize the invention in the co-pending patent application of Gitbard et al. as a means of dealing with hydrogen generation. More specifically, it is preferred to use a positive electrode containing a catalyst such as silver for hydrogen oxidation. Sealed nickel-zinc batteries utilizing this approach are characterized by relatively low internal pressure during use and storage, and operate under relatively large rates of charging and discharging without building up pressure that would cause leakage. It is possible.

FIG. 3 shows the increased cycle life of cells according to the invention at high discharge rates. Curves A and C in FIG. 3 represent the discharge curves of a cell according to the invention at two-hour and half-hour rates, respectively. Curves B and D are the discharge curves for the aforementioned prior art nickel-zinc cells at discharge rates of 2.5 hours and 1 hour, respectively. The discharge rate conditions of curves B and D are less severe than the discharge rates used in the cells of the present invention. As seen in Figure 3,
The discharge capacity of the discharge cell remains substantially higher than that of conventional nickel-zinc for up to 200 cycles or more.

FIG. 4 depicts the cell performance achieved using hydrogen recombination catalyst 52. Curve E depicts the hydrogen pressure that occurs when recombination catalyst 52 is not present, and curve F depicts the hydrogen pressure in the cell when recombination electrode 52 is provided. The substantial reduction in pressure generated by including the recombination catalyst is evident.

An example of parameters that are suitable for providing a 1.2 amp-hour, sub-C sized cell according to the present invention is as follows. The negative electrode layer consists of a first mixture of zinc, zinc oxide, cadmium oxide, bismuth oxide and a styrene-butadiene binder rolled onto a copper wire mesh. Based on the weight of this mixture, zinc is present in an amount of 4.5%, cadmium oxide in an amount of 5.8%, bismuth oxide in an amount of 7.5%,
The binder was present in an amount of about 4%, the remainder being zinc oxide. The hydrogen recombination catalyst consisted of a 0.1 inch by 1 inch (2.5 mm by 25 mm) piece of carbon cloth containing 1% by weight platinum. Negative electrode dimensions are 0.016" x 1.31" x 9" (0.04 x 3.3 x 22.9cm)
and the positive electrode dimensions are 0.028 inch x 1.2 inch x
It was 7 inches (0.07 x 3.0 x 17.8 cm).

The negative electrode may initially contain a zinc material charged in an amount of about 35% of the total theoretical ampere-hour capacity; It should then be converted to zinc oxide. The resulting cadmium should represent approximately 25% of the 1.2 amp hour battery capacity. The amount of zinc oxide initially present is approximately 425% of the ampere-hour capacity of the cell, with the actual capacity being limited by the positive electrode.