The present invention relates to electrochemical cells, for example alkaline electrochemical cells and their component parts. Certain aspects of the invention can be applicable to any electrochemical system that currently requires a certain anode:cathode capacity balance for reasons of performance and/or reliability. A conventional cylindrical alkaline electrochemical cell is illustrated in
Referring to
The separator 32 is substantially cylindrical, and includes an ionically permeable material and is interposed between the anode 26 and the inner peripheral sidewalls of the cathode rings 24 to prevent electrical contact between the anode 26 and the cathode 24 while permitting ionic transport between the anode 26 and the cathode 24. The separator 32 further extends radially across the flat surface of the cell 18, proximal the positive terminal end and between the inner surface of the can 22 and the anode 26. This portion of the separator 32 may be integral to the cylindrical separator 32, or, as is common in the art, may be in the form of a separate “bottom cup” comprising similar but often thicker material as illustrated in
The cathode 24 includes a cathode active material, which can be manganese dioxide. The manganese dioxide can be electrolytic manganese dioxide (EMD). Accordingly, as EMD is added to the cathode mix, the discharge capacity of the cell 18 is correspondingly increased. It should be appreciated that chemical manganese dioxide (CMD) or natural manganese dioxide (NMD) may be alternatively used instead of or concurrently with EMD. Accordingly, the term manganese dioxide as used throughout this disclosure refers to EMD, CMD, NMD, or a combination thereof. It should further be appreciated that the manganese dioxide may be purified if desired as is conventional, to minimize impurities which can cause excessive anode gassing.
In a broad embodiment, the electrochemical cell 18 includes an electroactive extender material different from the primary cathode active material. Because the extender material is disposed at a location in the cell 18 opposite the anode 26 with respect to the separator 32, and because the extender improves the cell discharge characteristics, the extender material can also be referred to as a cathode extender that can be physically separate from the cathode 24 (
Non-limiting examples of suitable cathode extender materials include single and mixed-metal oxides, sulfides, hydroxides or salts such as CuO, CuS, Cu(OH)2, Cu2O, CuF2, Cu(IO3)2, silver oxides, nickel oxyhydroxides and complexes such as copper iodate, copper oxyphosphate or any stable metal complex including those available from mineral sources directly or as synthesized complexes.
Additional non-limiting examples of suitable cathode extender materials in accordance with certain aspects of the present invention are identified generally by the formula MxCuyOz, where M is any suitable element, as noted, while 1≦x≦5, 1≦y≦5 and 1≦z≦20. Compounds having AMxCuyOz as general formula (where A can be selected from among, for example, Li, Na, K, Rb, Cs, Ca, Mg, Sr and Ba and mixtures thereof) can also be designed for use as cathode active materials. In certain aspects of the invention, at least one of CuO, Cu(OH)2− and MxCuyOz are used as the cathode extender.
One example of a process for preparing a mixed oxide cathode extender material involves chemically reducing a mixed solution of metal salts together with a complexing agent and a reducing agent (for example sodium tetra-borohydride (NaBH4), sodium formate, formic acid, formaldehyde, fumaric acid or hydrazine) to produce a compound containing the metals. A complex compound of the form AwMxCuy can also be prepared upon addition of a third metal salt as a precursor in this reduction step. The resulting product can be oxidized under acidic conditions with an oxidizing agent (for example hydrogen peroxide, potassium permanganate, potassium persulfate or potassium chlorate) to form a copper based mixed oxide.
For instance, Cu/Mn compounds prepared in this maimer were shown by X-ray diffraction (XRD) analysis to include a mixed copper manganese oxide phase. Although, no ASTM card corresponds to this oxide, its diffraction pattern is similar to that of Cu2Mn3O8. Other compounds such as Cu2Mn2O5 alone or in combination with CuO were also detected when the pH during the oxidation reaction was lowered (i.e., made more acidic) during the oxidation process. Controlling the oxidation conditions can be used to change the structure of the resulting copper based mixed oxide materials. Final composition and degree of crystallinity of these products may be efficaciously controlled. In addition to the mixed oxide phase, the product of the synthesis may also contain other phases including manganese oxides and copper oxides. As is known to one skilled in the art, such low to medium temperature solution based synthesis methods may produce amorphous mixed metal oxide products.
It is also envisioned that oxidation of the Cu/Mn compounds can be carried out in, for example, an alkaline solution or a solution having a neutral pH. Organic or inorganic acid (or base) can be used to adjust the pH of the oxidation solution. Also, the compounds can be first heat treated prior to chemical oxidation. Furthermore, the synthesized mixed copper metal oxide compounds can be heat-treated prior to being mixed with conducting material to form the cathode.
The mixed oxide compounds can also be prepared by known mechanical alloying methods using a high-energy ball mill or by direct high-temperature synthetic methods in a furnace using various starting material like carbonates, nitrates, acetates, and the like. Such metastasis reactions may be readily designed by one skilled in the art to produce high yield reactions with the desired purity for use as an electrochemical cell component. It is further envisioned that MxCuyOz− or AMxCuyOz-copper based mixed oxide materials can alternatively be made by co-precipitating a mixture of a mixed metal salt solution followed by heating the precipitate under appropriate conditions.
It should be appreciated that the above-mentioned materials can be provided as either the primary active material or as the extender material to the extent that the discharge voltage of the extender material is lower than the initial discharge voltage of the primary active material. It should also be noted, as one skilled in the art would appreciate, that although copper oxide is denoted here by the common formula CuO, such materials do not inherently have perfect stoichiometry. In other words, copper and oxygen in CuO are not exactly in a 1:1 ratio, but rather the Cu:O ratio typically ranges from about 0.9:1 to about 1.1:1. It is common to find that such materials are obtained over a range of stoichiometric ratios and also prove viable as useful electrode materials over such ranges. This holds true for other electrode materials disclosed herein as well.
One aspect of the invention provides the extender material in the cell in an amount no greater than that of the primary active material. For example, in the case of an alkaline Zn/MnO2 cell, in place of a cathode in which the active material is 100% EMD, the cathode has substantially the same total weight of active material wherein more than 50% of the material is EMD, the balance being a cathode extender material. The remainder of the cathode components can be those conventional in an alkaline Zn/MnO2 battery, although one skilled in the art will readily recognize that the proportions may vary depending on the amount and conductive and binding properties of the extender material.
Another aspect of the invention provides a cathode extender material that exhibits a discharge voltage lower than an initial discharge voltage of the primary cathode active material. In the case of an alkaline Zn/MnO2 cell, the cathode extender material desirably discharges at a voltage lower than the 1st electron of the manganese dioxide reduction.
Another aspect of the present invention provides an extender material that has a high specific discharge energy density (at least as high as that of the primary cathode active material). As known to one having ordinary skill in the art, energy density can be defined as capacity per unit mass (gravimetric energy density), or ampere-hours (Ah) per unit volume (volumetric energy density) with units of mAh/g or Ah/cc respectively. For example, in the case of an alkaline cell with a manganese oxide cathode, the extender material has an energy density of at least about 300 mAh/g or at least 1.5 Ah/cc, such as for example, CuO (674 mAh/g, 4.26 Ah/cc for a two electron discharge), Cu2O (337 mAh/g) or Cu(IO3)2 (902 mAh/g). A high volumetric and a high gravimetric density material is desirable in certain aspects of the invention, since this permits a small amount of the extender to have a desired impact on the discharge behavior and capacity without occupying too much volume within the cell. For example, the extender occupies less than about 30% of the cathode volume in accordance with certain aspects of the invention.
Another aspect of the present invention provides a cell including an extender material that achieves an anode/primary cathode capacity ratio greater than 1:1. The extender material has a substantially flat and stable discharge voltage profile as in the case of a copper oxide or copper hydroxide extender.
Step 1: 2CuO+2e+H2O→Cu2O+2OH−
Step 2: Cu2O+2e+H2O→2Cu+2OH−
Furthermore, an electrochemical cell including an extender, in accordance with further aspects of the present invention, has an anode capacity/cell internal volume ratio within a range defined at its lower end by 0.5 Ah/cc or, alternatively 0.55 Ah/cc, and at its upper end by 0.9 Ah/cc or, alternatively 1.0 Ah/cc. The cathode extender material allows the cathode to continue to discharge until substantially all of the excess anode and electrolyte is consumed, such that insufficient residual electrolyte remains to cause excessive gassing. Current commercial alkaline cells are restricted to an anode capacity/internal cell volume ratio of approximately 0.5 Ah/cc based on a zinc capacity of 820 mAh/g and an MnO2 capacity of 400 mAh/g assuming a 1.33 electron reduction of MnO2.
In the case of a Zinc/manganese dioxide cell, the cathode extender allows significant new design flexibility where the anode to primary cathode capacity ratio is within a range defined at its lower end that is between and includes 0.98:1, 1:1, 1.03:1, 1.05:1 and, alternatively 1.1:1, and at its upper end by 1.5:1 thereby significantly increasing the discharge capacity of such cells in the usable discharge voltage range of many devices (i.e., above 0.8V) and extending the discharge at discharge voltages below about 0.8V (depending on discharge current), thereby preventing the hydrogen evolution that would normally occur upon over-discharge in conventional cells (without extender) if an anode:cathode capacity ratio greater than 1:1 were to be used, as calculated using the Zinc and MnO2 capacity values detailed above in this invention. An electrochemical cell including the extender has a cell discharge capacity greater than that of an otherwise identical cell containing primary active material in place of the extender.
Furthermore, the extender prevents excessive gassing that would be typically encountered in a cell with an anode to primary cathode electrochemical balance ratio of greater than 1:1 during over-discharge and when the cell goes into voltage reversal. Reduced gassing improves reliability of cells in a series string, and reduces the likelihood of voltage reversal and decrimping in the event of premature failure of a battery in the string.
Use of the cathode extender material allows use of an increased anode-primary cathode capacity ratio compared to conventional cells, thus increasing the electrolyte amount in the cell (and hence water) available to the MnO2 in the cathode. This can significantly improve the discharge efficiency of MnO2, as compared to conventional cells, without the disadvantage of cell gassing. It will be appreciated that as the discharge efficiency of the primary cathode active material increases, the amount of cathode required decreases, thereby freeing up space inside the cell for additional active material or extender material as desired.
Relatedly, if the total anode volume is increased, then the cathode volume (hence, mass) is reduced to fit in the available cell volume. In a standard bobbin design round cell, the cathode inner diameter will then necessarily have to be larger, creating a higher cathode active surface area (due to the increased diameter). This also benefits the cathode discharge efficiency by reducing the cathode current density during discharge.
Referring to
In general, the extender may be located anywhere in the so long as it is in electronic contact with the positive terminal or the primary cathode material. It may therefore be blended with the primary cathode material, or be separated from it. In some instances, it may be desirable to keep it separate from the primary active material. By way of example, in a standard Zn/MnO2 cell, the MnO2 has a density of 4.5 g/cc, consumes two moles of water per mole of MnO2, and incorporates protons into its structure to yield MnOOH (a poor electronic conductor and a material of lower density than the MnO2). The need for extra water in the cell for the cathode reaction limits the amount of zinc that can be used, resulting in relatively low volumetric energy density. The EMD also has a sloping discharge curve.
On the other hand, copper oxide (CuO) which has a density of approximately 6.3 g/cc, consumes half the water in the first electron discharge compared to MnO2, shows less volume expansion, has a relatively flat discharge curve, and provides high volumetric energy density in a cell. In a cathode that comprises a suitable percentage of EMD (say, 80-90% of total cathode active material by weight) and 10-20% CuO extender by weight, the EMD, which has an initially high operating voltage but a rather sloping discharge curve, discharges first, followed by the CuO, with a relatively sharp transition between them. In a cathode containing a physical mixture of the two, performance of the CuO portion of the cathode deteriorates as MnO2 content increases, presumably for the following reasons. In such a cathode, the CuO discharge reaction takes over after the MnO2 discharges its first electron. However, insufficient electrolyte is available to the CuO at this stage, for efficient reaction, causing mass transfer polarization. The MnO2 volume expansion can also separate the CuO particles from themselves and from the conducting material (graphites) that is usually provided in the cathode. The conducting material can be natural or synthetic graphite, and further can include expanded graphite as appreciated by one having ordinary skill in the art. The effect of the initial MnO2 discharge reaction is an increase of the ohmic resistance in the cathode, resulting in a further loss in voltage. The net effect of these processes is that the CuO material operates at a significantly lower voltage than it otherwise would when discharged by itself.
Certain aspects of the present invention therefore seek to mitigate the detrimental effects of dissimilar discharge behaviors by optionally providing in the cell a primary cathode and an extender in separate layers or tablets (or in separate layers that can comprise mixtures of oxides), or in a separate location in the cell, such that the extender material is able to discharge efficiently, as close as possible to its inherent reduction potential.
In a flat (prismatic) battery where the cathode may be in disk form, the active materials can be in stacked annular layers one over the other, concentric rings, or as adjacent arcuate segments (e.g., semicircular segments) one within the other as shown in
For a cylindrical battery configuration, which uses an annular cylindrical cathode in a can, either pressed externally and inserted as multiple hollow cylinders, also referred to as “tablets,” or fabricated in-situ in the can, the same concept can be used to keep the materials separated as shown in
Further related cell configurations having advantage in the manufacturing process are also contemplated. For example, in the case of an alkaline Zn/MnO2 cell, an extender is included in the cell at a location separate from the primary cathode material (i.e., the extender does not form part of the cathode), such that the weight of the EMD is greater than the weight of the extender material as illustrated in
Referring to
When the extender comprises a material that can corrode and generate anode-fouling species which, if allowed to travel to the anode, can negatively impact battery performance, a separate barrier material 35 can be provided, that can effectively limit the migration of anode-fouling soluble species. Suitable barrier materials include cellulosic films like cellophane, polyvinyl alcohol (PVA_) films, modified or cross-linked PVA films, laminated combinations, or suitable hybrids of such films and the like. Another such polymer is ethyl vinyl acetate (EVA) emulsion, that contains vinyl acetate monomers, vinyl acetate-ethylene copolymers and vinyl acetate polymers that can be used as films, or coated on non-woven separator materials to effectively limit migration of anode fouling soluble species. The barrier material 35 isolates the cathode extender from the anode and thus minimizes anode fouling. If the extender material is located as shown in
Alternatively, referring to
Alternatively, referring now to
In a typical manufacturing process, separate toroidal cathode tablets are fabricated in tabletting equipment before being inserted into the battery can. Multiple tablets are inserted into the cans until the required height is obtained. Such a process lends itself very well to the use of separate tablets of different materials. Accordingly, it is envisioned that the ratio of materials A to B can be varied depending on the amount of extender desired. Likewise, the number of such tablets can also be varied depending on the application.
Several approaches to increasing the anode:primary cathode capacity ratio of, and rebalancing, an alkaline cell are contemplated. In a first embodiment, wherein the anode and cathode volumes are fixed (and can be conventional), the alkaline cell includes an anode having more zinc mass per unit of anode volume than in conventional cells, thereby providing greater electrochemical discharge capacity to the cell over a wide range of discharge rates. The other components of the gelled zinc anode can be conventional and can comprise electrolyte, gelling agents, surfactants, and the like.
In a second embodiment, the anode:primary cathode capacity ratio can be increased from the current industry standard of below approximately 1:1, to as high as about 1.5:1 by increasing volume available for the anode in accordance with certain aspects of the present invention. In this second embodiment, the increased anode capacity and the resulting increased water:primary cathode molar ratio combine to achieve a greater anode and cathode discharge efficiency and hence, cell capacity. Without intending to be limited by theory, it is believed that the presence of more electrolyte (and hence more water) in the cell enabled by the higher anode amount favors primary cathode discharge by improving mass transport, thereby increasing primary cathode discharge efficiency as shown in
If desired, the barrier separator 35 disposed between the extender material 33 and the anode 26 can effectively limit the migration of the generated anode-fouling soluble species, such as silver species, copper species, and/or sulfur species, from the extender 33 into the anode compartment 28 while permitting migration of hydroxyl ions and water. Further, the cathode 24 or extender 33 or both can include an agent that reduces or prevents ionic species from migrating from the cathode toward the anode. Agents such as polyvinyl alcohol, activated carbon, various clays, and silicates such as Laponite and the like have shown an ability to adsorb or block ionic species.
Step 1: 2CuO+2e+H2O→Cu2O+2OH−
Step 2: Cu2O+2e+H2O→2Cu+2OH−
The cell, as illustrated in
In view of the above, it will be seen that the several advantages of the invention are achieved and other advantageous results attained. As various changes could be made in the above processes and composites without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
This application claims the benefit of U.S. Provisional Patent Application No. 60/528,414 filed Dec. 10, 2003, the disclosure of which is incorporated by reference as if set forth in its entirety herein.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US04/41484 | 12/10/2004 | WO | 00 | 5/9/2007 |
Number | Date | Country | |
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60528414 | Dec 2003 | US |