This invention relates to an aqueous alkaline cell with a cathode mixture comprising silver copper oxide AgCuO2 or Ag2Cu2O3 or mixtures thereof.
Conventional alkaline electrochemical cells have an anode comprising zinc and a cathode comprising manganese dioxide. The cell is typically formed of a cylindrical casing. The casing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing at the closed end forms the positive terminal.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, polymeric binders, and other additives, such as titanium-containing compounds can be added to the cathode.
The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. The resistivity of EMD is fairly low. An electrically conductive material is added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. The resistivity of graphites such as flaky natural or expanded graphites can typically be between about 3xc3x9710xe2x88x923 ohm-cm and 4xc3x9710xe2x88x923 ohm-cm.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life). Since commercial cell sizes have been fixed, it is known that the useful service life of a cell can be enhanced by packing greater amounts of the electrode active materials into the cell. However, such approach has practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the rates of electrochemical reactions during cell discharge can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits the mobility of ions within both the electrolyte and the electrodes, which in turn degrades cell performance and service life. Although the amount of active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode. Thus, the total active cathode material is effectively limited by the amount of conductive additive required to provide an adequate level of conductivity.
It is desirable that the cell have high service life under normal drain rates, for example, between about 50 milliAmp and 500 milliAmp and also perform well in higher power application, at current rates between about 0.5 and 2.0 Amp, for example, between about 0.5 Amp and 1.5 Amp. Such high power application corresponds to a power output between about 0.5 and 1.5 Watt or even higher up to about 2.0 Watt. In conventional zinc/MnO2 cells the utilization of anode/cathode active materials falls off as the current drain or power output requirements move into the high power regime.
Although such alkaline cells are in widespread commercial use there is a need to improve the cell or develop a new type of cell that exhibits reliable performance and longer service life for normal applications such as flashlight, radio, audio recorders and portable CD players and desirably also performs even better than conventional zinc/MnO2 cells in high power applications.
The invention is directed to a primary (nonrechargeable) electrochemical alkaline cell having an anode comprising zinc and a cathode mixture comprising silver copper oxide selected from the compounds AgCuO2 or Ag2Cu2O3 or any mixture of AgCUO2 and Ag2Cu2O3. The term xe2x80x9csilver copper oxidexe2x80x9d as used herein, unless otherwise specified shall be understood to mean the compounds AgCuO2, Ag2Cu2O3 or mixtures thereof. The anode and cathode include an aqueous alkaline solution, preferably aqueous KOH solution. Such cell of the invention can be conveniently referenced herein as a Zn/Silver copper oxide alkaline cell. The silver copper oxide is preferably in the form of a powder having an average particle size between about 1 and 100 micron. The cathode mixture includes a conductive material such as flaky crystalline natural graphite or flaky crystalline synthetic graphite including expanded graphite and graphitic carbon nanofibers. The term graphitic carbon nanofibers as used herein shall mean graphitic carbon fibers having a mean average diameter less than 1000 nanometers (less than 1000xc3x9710xe2x88x929 meter) (The term xe2x80x9caveragexe2x80x9d or xe2x80x9cmean averagexe2x80x9d as used herein shall mean the xe2x80x9carithmetic mean averagexe2x80x9d unless otherwise specified.) Preferably, the graphitic carbon nanofibers have a mean average diameter less than 500 nanometer, more preferably less than 300 nanometers. Desirably the graphitic carbon nanofibers have a mean average diameter between about 50 and 300 nanometers, typically between about 50 and 250 nanometers. The cathode mixture includes an aqueous KOH solution, desirably having a concentration of between about 30 and 40 percent by weight, preferably between 35 and 45 percent weight KOH in water.
It has been determined that in the Zn/Silver copper oxide alkaline cell of the invention the anode can comprise conventional gelled zinc anode compositions as in commercial use in conventional zinc/MnO2 alkaline cells. By way of an example, not intended to be restrictive, the cathode can comprise the same composition as conventional cathode comprising MnO2 as used in commercial zinc/MnO2 alkaline cells, except that the MnO2 can be replaced in whole or in part by the AgCuO2 or Ag2Cu2O3 compounds, or any mixture of AgCuO2 and Ag2Cu2O3 compounds herein disclosed. The AgCuO2 has been determined to have advantages when used as a cathode in alkaline cells. The copper in AgCuO2 has a +3 valence and the silver a +1 valence. The Cu+3 and Ag+1 are available for reduction to copper metal and silver metal during discharge. As a result the AgCuO2 has a high theoretical specific capacity, namely, 526 milliAmp-hour/g. This is much higher than the theoretical specific capacity of MnO2, which is 308 milliAmp-hour/g and higher than the theoretical specific capacity of AgO, which is 436 milliAmp-hour/g or Ag2O, which is 117 milliAmp-hour/g. Additionally, the presence of silver (Ag+1) and (Cu+3) in the AgCuO2 compound causes an elevation in a Zn/AgCuO2 alkaline cell""s running voltage profile as compared to a Zn/MnO2 or Zn/CuO alkaline cell. Ag2Cu2O3 has copper at valence at +2 (Cu+2) which is lower than copper at valence +3 (Cu+3) in the AgCuO2 compound, and therefore, has a theoretical specific capacity of 412 milliAmp-hour/g, which is lower than the specific capacity of 526 milliamp-hour/g for the AgCuO2.
The Zn/Silver copper oxide alkaline cell exhibits a higher running voltage profile and longer service life than Zn/MnO2 cells in normal applications, e.g. at drains rates between about 50 and 600 milliamp. It also exhibits a high rate capability for high power applications, for example, at current drains between about 0.5 and 1.5 Amp or power applications between about 0.5 and 1.5 Watt. For example, at a drain rate of about 1 Amp, between about 75 and 80 percent of the theoretical capacity of the AgCuO2 can be utilized in a Zn/AgCuO2 alkaline cell. The AgCuO2, which has a copper valence of +3 (or Ag2Cu2O3) is nevertheless sufficiently stable in water or aqueous KOH electrolyte solution. With respect to AgCuO2, this compound does not react in water or aqueous KOH electrolyte during normal cell storage at room temperature as well as ambient temperatures between about xe2x88x9229xc2x0 C. and 46xc2x0 C. (xe2x88x9220xc2x0 F. and 115xc2x0 F.) to cause any significant degradation of the Cu+3 valence.
In a preferred embodiment, the silver copper oxide (AgCuO2, or Ag2Cu2O3 or mixtures thereof) comprises between about 82 and 90 percent by weight of the cathode mixture. In such embodiment the silver copper oxide can replace all of the manganese dioxide in the cathode. The performance of conventional Zn/MnO2 alkaline cells can also be improved if the silver copper oxide is used to replace a portion of the amount of MnO2 conventionally used in the alkaline cell cathode. In such case the silver copper oxide can be mixed into the cathode with MnO2. In either case, whether the AgCuO2 is used alone or in admixture with MnO2, the graphitic conductive material in the cathode, desirably comprises between about 2 and 10 percent by weight of the cathode, preferably between about 4 and 10 percent by weight of the cathode. The graphitic conductive material desirably comprises expanded graphite or natural graphite alone or in any mixtures thereof. In such case the graphitic conductive material comprises between about 3 and 10 percent by weight of the cathode, desirably between about 4 and 10 percent by weight of the cathode. The graphitic conductive material can contain only expanded graphite or only natural graphite or only graphitic carbon nanofibers, but can also contain natural graphite, expanded graphite and graphitic carbon nanofibers in any combination or mixture thereof. In such case the graphitic conductive material desirably comprises between about 4 and about 10 percent by weight of the cathode. The aqueous KOH solution desirably comprises between about 5 and 10 percent by weight of the cathode mixture. The aqueous KOH solution itself desirably comprises between about 30 and 40 percent by weight KOH, preferably between about 35 and 40 percent by weight KOH and about 2 percent by weight zinc oxide.
In one aspect the alkaline cell of the invention has an anode comprising-zinc and a cathode mixture comprising silver copper oxide in the form AgCuO2. Such cell, for example, an AA size cell, exhibits high capacity (mAmp-hrs) and high energy output (mWatt-hours) under discharge rates between about 500 and 1000 mAmp when compared to same size conventional alkaline cell having an anode comprising zinc and cathode comprising manganese dioxide. This advantage would also apply to other size cylindrical cells, for example, AAAA, AAA, C and D size cells as well as AA cells.
In another aspect the alkaline cell of the invention has an anode comprising zinc and a cathode mixture comprising silver copper oxide in the form of Ag2Cu2O3. Such cell, for example, an AA size cell, exhibits high capacity (mAmp-hrs) and high energy output (mWatt-hours) under discharge rates between about 500 and 1000 mAmp when compared to same size conventional alkaline cell having an anode comprising zinc and cathode comprising manganese dioxide. This advantage would also apply to other size cylindrical cells, for example, AAAA, AAA, C and D size cells as well as AA cells.
Although the embodiments herein are directed to a nonrechargeble cell, it may nevertheless be possible to produce a rechargeable cell with modified cathodes comprising AgCuO2 designed to partially discharge to Ag2Cu2O3, which in turn could be recharged back to AgCuO2. Such cell, for example, could be an alkaline cell with a zinc anode discharging to zinc oxide, rechargeable back to zinc.