Battery cathode

BATTERY CATHODE

CROSS REFERENCE TO RELATED
APPLICATION

[0001] This application is a continuation-in-part of U.S. Ser. No. 09/001,822, filed on Dec. 31, 1997.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to batteries.

[0003] Batteries, such as alkaline batteries, are commonly used as energy sources. Generally, alkaline batteries have a cathode, an anode, a separator and an electrolytic solution. The cathode is typically formed of manganese dioxide, carbon particles and a binder. The anode can be formed of a gel including zinc particles. The separator is usually disposed between the cathode and the anode. The electrolytic solution, which is dispersed throughout the battery, can be a hydroxide solution.

SUMMARY OF THE INVENTION

[0004] The invention relates to batteries, such as alkaline batteries, having cathodes that include manganese dioxide and relatively small nonsynthetic, nonexpanded graphite particles. These batteries have good performance characteristics. For example, the batteries can exhibit high energy output at a high discharge rate, such as a discharge rate equal to at least the battery's capacity (in units of Ampere-hours) discharged in one hour. The batteries can have various industry standard sizes, such as AA, AAA, AAAA, C or D.

[0005] "Nonsynthetic graphite particles" refer to graphite particles that are prepared without using an industrial or laboratory graphitization process.

[0006] "Nonexpanded graphite particles" refer to graphite particles that are prepared without any industrial or laboratory particle expansion process.

[0007] In one aspect, the invention features a cathode that includes manganese dioxide and nonsynthetic, nonexpanded graphite particles having an average particle size of less than about 20 microns.

[0008] The particle size is measured using a Sympatec HELIOS analyzer. For a given sample of graphite particles, the average particle size is the particle size for which half the volume of the sample has a smaller particle size.

[0009] In another aspect, the invention features an electrochemical cell including a cathode, an anode and a separator disposed between the cathode and the anode. The cathode includes manganese dioxide and nonsynthetic, nonexpanded graphite particles having an average particle size of less than about 20 microns.

[0010] In some embodiments, the separator includes a nonwoven, non-membrane material and a second nonwoven, non-membrane material disposed along a surface of the first material. In these embodiments, the separator can be devoid of a membrane layer or an adhesive layer disposed between the nonwoven, non-membrane materials. A membrane material refers to a material having an average pore size of less than about 0.5 micron, whereas a non-membrane material refers to a material having an average pore size of at least about 5 microns.

[0011] The cathode can have a porosity of from about 21% to about 28%. The porosity of the cathode is the relative volume of the cathode that is not taken up by solid material, such as, for example, manganese dioxide, graphite particles and binder.

[0012] The anode can have a porosity of from about 2 grams of zinc particles to about 2.45 grams of zinc particles per cubic centimeter of anode volume that is taken up by liquid or solid material.

[0013] The battery can have a relatively small amount of manganese dioxide and/or zinc particles compared to the amount of electrolytic solution. For example, the weight ratio of manganese dioxide to electrolytic solution can be from about 2.2 to about 2.9, and the weight ratio of zinc particles to electrolytic solution can be from about 0.9 to about 1.25. This is calculated based on the amount of electrolytic solution dispersed throughout the cathode, the anode and the separator.

[0014] The batteries can be AA or AAA batteries that demonstrate good results when tested according to the cc photo test, the 1 Watt continuous test, the half Watt continuous test, the pulsed test, the half Watt rm test and/or the quarter Watt rm test. These tests are described below.

[0015] Other features and advantages of the invention will be apparent from the description of the preferred embodiments thereof and the claims.

BRIEF DESCRIPTION OF THE DRAWING [0016] The FIGURE is a cross-sectional view of a battery.

DESCRIPTION OF THE PREFERRED
EMBODIMENTS

[0017] The preferred batteries are alkaline batteries that have a cathode formed of manganese dioxide, relatively small, nonsynthetic, nonexpanded graphite particles and optionally a binder.

[0018] Referring to the FIGURE, a battery 10 is shown that has a cathode 12, an anode 14, a separator 16, an outer wall 18 that contacts the outer diameter of cathode 12 and insulating layer 26. Battery 10 further includes an anode collector 20 that passes through a seal member 22 and into anode 14. The upper end of anode collector 20 is connected to a negative end cap 24 which serves as the negative external terminal of battery 10. Layer 26 can be formed of an electrically nonconducting material, such as a heat shrinkable plastic. In addition, an electrolytic solution is dispersed throughout battery 10.

[0019] If the graphite particles disposed within cathode 12 are too large, the conductivity of cathode 12 may not be sufficiently low. However, if the graphite particles are too small, cathode 12 may be comparatively dense, reducing the amount of electrolytic solution in cathode 12 and decreasing the efficiency of battery 10. Therefore, the graphite particles in cathode 12 preferably have an average particle size of at most 20 microns, more preferably from about 2 microns to about 12 microns and most preferably from about 5 microns to about 9 microns as measured using a Sympatec HELICS analyzer. In some embodiments, the graphite particles are nonexpanded, nonsynthetic graphite particles having an average particle size of about 7 microns as measured by this method. Nonsynthetic, nonexpanded graphite particles are available from, for example, Brazilian Nacional de Grafite (Itapecirica, MG Brazil).

[0020] The amount of graphite particles disposed within cathode 12 should be enough to improve the overall conductivity of cathode 12 while having minimal impact on the energy capacity of battery 10. Preferably, cathode 12 is from about 4 weight percent to about 10 weight percent graphite particles, more preferably from about 5 weight percent to about 9 weight percent graphite particles, and most preferably from about 6 weight percent to about 8 weight percent graphite particles. These weight percentage ranges correspond to when the electrolytic solution is not dispersed within cathode 12.

[0021] Cathode 12 can be a single pellet of material. Alternatively, cathode 12 can be formed of a number of cathode pellets that are stacked on top of each other. In either case, the cathode pellets can be made by first mixing the manganese dioxide, graphite particles and optionally the binder. For embodiments in which more than one pellet is used, the mixture can be pressed to form the pellets. The pellet(s) are fit within battery 10 using standard processes. For example, in one process, a core rod is placed in the central cavity of battery 10, and a punch is then used to pressurize the top most pellet. When using this process, the interior of wall 18 can have one or more vertical ridges that are spaced circumferentially around wall 18. These ridges can assist in holding cathode 12 in place within battery 10.

[0022] In embodiments in which cathode 12 is formed of a single pellet, the powder can be placed directly within battery 10. A retaining ring is set in place, and an extrusion rod passes through the ring, densifying the powder and forming cathode 12.

[0023] In certain embodiments, a layer of conductive material can be disposed between wall 18 and cathode 12. This layer may be disposed along the inner surface of wall 18, along the outer circumference of cathode 12 or both. Typically, this conductive layer is formed of a carbonaceous material. Such materials include LB1000 (Timcal), Eccocoat 257 (W.R. Grace & Co.), Electrodag 109 (Acheson Industries, Inc.), Electrodag 112 (Acheson) and EB005 (Acheson). Methods of applying the conductive layer are disclosed in, for example, Canadian Patent No. 1,263,697, which is hereby incorporated by reference.

[0024] Using a conductive layer, especially Electrodag 109 or EB005, between wall 18 and cathode 12 can reduce the pressure used when forming cathode 12 within battery 10. Thus, the porosity of cathode 12 can be made relatively high without causing the pellet(s) to be crushed or crack when forming cathode 12 within battery 10. However, if the porosity of cathode 12 is too low, an insufficient amount of electrolytic solution can be dispersed within cathode 12, reducing the efficiency of battery 10. Thus, in certain embodiments, cathode 12 has a porosity of from about 21% to about 28%, more preferably from about 25% to about 27%, and most preferably about 26%.

[0025] Within cathode 12, any of the conventional forms of manganese dioxide for batteries can be used. Distributors of such manganese dioxide include Kerr McGee, Co., Broken Hill Proprietary, Chem Metals, Co., Tosoh, Delta Manganese, Mitsui Chemicals and JMC.

[0026] In certain embodiments, cathode 12 can have from about 8.9 grams of manganese dioxide to about 9.8 grams of manganese dioxide. In these embodiments, cathode 12 preferably includes from about 9.3 grams to about 9.8 grams of manganese dioxide, more preferably from about 9.4 grams to about 9.65 grams of manganese dioxide, and most preferably from about 9.45 grams of manganese dioxide to about 9.6 grams of manganese dioxide.

[0027] In other embodiments, cathode 12 preferably includes from about 4 grams to about 4.3 grams of manganese dioxide, more preferably from about 4.05 grams to about 4.25 grams of manganese dioxide, and most preferably from about 4.1 grams to about 4.2 grams of manganese dioxide.

[0028] In some embodiments, cathode 12 may further include a binder. Examples of binders for cathode 12 include polyethylene powders, polyacrlyamides, Portland cement and fluorocarbon resins, such as PVDF and PTFE. In certain embodiments, cathode 12 includes a polyethylene binder sold under the tradename coathylene HA-1681 (Hoescht). When cathode 12 includes a binder, the binder preferably makes up less than about 1 weight percent of cathode 12, more preferably from about 0.1 weight percent to about 0.5 weight percent of cathode 12, and most preferably about 0.3 weight percent of cathode 12. These weight percentages correspond to when the electrolytic solution is not dispersed within cathode 12.

[0029] Cathode 12 can include other additives. Examples of these additives are disclosed in U.S. Pat. No. 5,342,712, which is hereby incorporated by reference. In some embodiments, cathode 12 preferably includes from about 0.2 weight percent to about 2 weight percent Ti02, more preferably about 0.8 weight percent Ti02.

[0030] Anode 14 can be formed of any of the standard zinc materials used in battery anodes. Often, anode 14 is formed of a zinc gel that includes zinc metal particles, a gelling agent and minor amounts of additives, such as gassing inhibitors.

[0031] If the porosity of anode 14 is too high, the amount of zinc within battery 10 is reduced which decreases the energy capacity of battery 10. However, if the porosity of anode 14 is too low, an insufficient amount of electrolytic solution can be dispersed within anode 14. Therefore, in some embodiments, anode 14 preferably has from about 2 grams to about 2.45 grams of zinc particles per cubic centimeter of anode volume, more preferably from about 2.1 grams to about 2.35 grams of zinc particles per cubic centimeter of anode volume, and most preferably from about 2.15 grams to about 2.3 grams of zinc particles per cubic centimeter of anode volume.

[0032] In certain embodiments, anode 14 preferably has from about 3.7 grams to about 4.25 grams of zinc particles, more preferably from about 3.8 to about 4.15 grams of zinc particles, and most preferably from about 3.9 grams to about 4.05 grams of zinc particles.

[0033] In other embodiments, anode 14 preferably has from about 1.5 grams to about 1.9 grams of zinc particles, more preferably from about 1.55 to about 1.85 grams of zinc particles, and most preferably from about 1.65 grams to about 1.75 grams of zinc particles.