REDUCED LEAKAGE METAL-AIR ELECTROCHEMICAL CELL

Abstract: The invention relates to a metal-air electrochemical cell which exhibits good discharge performance under relatively high temperature, high humidity conditions. The cell has improved discharge performance at a temperature between about 25 DEG C and about 38 DEG C and a relative humidity external of the cell of between about 45 and about 95 percent. Leakage from the cell also can be reduced.

Description: REDUCED LEAKAGE METAL-AIR ELECTROCHEMICAL CELL The invention generally relates to metal-air electrochemical cells. Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material. In order to prevent direct reaction of the anode material and the cathode material, the anode and the cathode are electrically isolated from each other by a sheet-like layer, typically called the separator. When a battery is used as an electrical energy source in a device, such as in a hearing aid, electrical contact is made to the anode and the cathode, allowing electrons flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge. One configuration of a battery is a button cell, which has the approximate size and cylindrical shape of a button. In a button cell, the container for the anode and the cathode includes a lower cup-like structure, called the cathode can, and an upper cup-like structure retained within the cathode can, called the anode can. The anode can and cathode can be separated by an insulator, such as an insulating gasket or seal. The anode can and the cathode can are crimped together to form the container. In a metal-air electrochemical cell, oxygen is reduced at the cathode, and a metal is oxidized at the anode. Oxygen is supplied to the cathode from the atmospheric air external to the cell through an air access port in the container. When the electrolyte in the cell is aqueous, hydrogen gas can be produced in the anode of the cell. Gas generation can lead to pressure build up in the cell, ultimately resulting in leakage or structural failure of the cell. The metal of a metal-air electrochemical cell can be zinc. Typically, when zinc is used in a metal-air battery, the zinc is alloyed with mercury (e.g., about 3 percent) to reduce hydrogen gas evolution. In general, the invention relates to a metal-air electrochemical cell which exhibits good discharge performance under relatively high temperature, high humidity conditions. Leakage from the cell can be reduced. Relatively high temperature, high humidity conditions are temperature between about 25 °C and about 38°C (e.g., 30°C) and a relative humidity external of the cell of between about 45 and about 95 percent (e.g., between 70 and 90 percent). The high temperature, high humidity conditions are similar to the ambient conditions the cell is exposed to during use (e.g., about 30°C and 90 percent relative humidity). For example, a zinc-air cell can be used in a high temperature and high humidity geographic location, such as the Far East, or in an hearing aid which is not hermetically sealed. The hearing aid is placed in an ear canal, which has a high relative humidity and a high temperature. It is important that the cell have good discharge performance and resists leakage under these operating conditions. In one aspect, the invention features a metal-air electrochemical cell including an anode, a cathode, and a separator electronically separating the anode and the cathode. The anode includes an anode can containing an anode gel. The anode gel includes an electrolyte. The cathode includes a cathode can having at least one air access port and containing a cathode structure. An insulator can be located between the anode can and the cathode can. The cathode structure can include a catalyst mixture and a current collector in electrical contact with the cathode can. The cell can further include an air disperser positioned between the air access port and the cathode structure. The anode can and cathode can are assembled (e.g., crimped together) to form a cell. In the anode, the anode volume is the volume within the cell contained between the inner surface of the anode can and the separator. The anode gel occupies most of the anode volume. The portion of the anode volume that is not filled with the anode gel is the void volume. The void volume of the cell after discharge is between about 7.5 percent and about 15 percent of the anode volume. Preferably, the void volume is between about 8 percent and about 12 percent (e.g., about 10 percent) of anode volume. Overall cell height and diameter dimensions are specified by the International Electrotechnical Commission (IEC). A cell can have one of five sizes: a 675 cell (IEC designation "PR44") has a diameter between about 11.25 and 11.60 millimeters and a height between about 5.0 and 5.4 millimeters; a 13 cell (IEC designation "PR48") has a diameter between about 7.55 and 7.9 millimeters and a height between about 5.0 and 5.4 millimeters; a 312 cell (IEC designation "PR41") has a diameter between about 7.55 and 7.9 millimeters and a height of between about 3.3 and 3.6 millimeters; and a 10 cell (IEC designation "PR70") has a diameter between about 5.55 and 5.80 millimeters and a height between about 3.30 and 3.60 millimeters. A 5 cell has a diameter between about 5.55 and 5.80 millimeters and a height between about 2.03 and 2.16 millimeters. The cell can have an anode can thickness of about 0.1016 mm. The cell can have an cathode can thickness of about 0.1016 mm. The metal-air electrochemical cell can be a 675 cell. The 675 cell can have a discharge performance of between about 700 mAh and about 480 mAh at a temperature between about 25°C and about 38°C (e.g., 30°C) and a relative humidity external of the cell of between about 45 and about 95 percent. Preferably, the discharge performance of the 675 cell is between about 680 mAh and about 510 mAh, more preferably between about 660 mAh and about 550 mAh (e.g., about 600 mAh). The metal-air electrochemical cell can be a 13 cell. The 13 cell can have a discharge performance of between about 295 mAh and about 200 mAh cell at a temperature between about 25°C and about 38°C (e.g., 30°C) and a relative humidity external of the cell of between about 45 and about 95 percent. Preferably, the discharge performance of the cell is between about 290 mAh and about 220 mAh, more preferably between about 280 mAh and about 230 mAh (e.g., about 260 mAh). The metal-air electrochemical cell can be a 312 cell. The 312 cell can have a discharge performance of between about 155 mAh and about 110 mAh for a 312 cell at a temperature between about 25°C and about 38°C (e.g., 30°C) and a relative humidity external of the cell of between about 45 and about 95 percent. Preferably, the discharge performance of the 312 cell is between about 152 mAh and about 115 mAh, more preferably between about 150 mAh and about 120 mAh (e.g., about 135 mAh). The metal-air electrochemical cell can be a 10 cell. The 10 cell can have a discharge performance of between about 85 mAh and about 50 mAh at a temperature between about 25°C and about 38°C (e.g., 30°C) and a relative humidity external of the cell of between about 45 and about 95 percent. Preferably, the discharge performance of the 10 cell is between about 84 mAh and about 55 mAh, more preferably between about 82 mAh and about 60 mAh (e.g., about 70 mAh). The metal-air electrochemical cell can be a 5 cell. The 5 cell can have a discharge performance of between about 45 mAh and about 40 mAh (e.g., about 43 mAh) at a temperature between about 25°C and about 38°C (e.g., 30°C) and a relative humidity external of the cell of between about 45 and about 95 percent. In another aspect, the invention features a method of manufacturing a metal-air electrochemical cell. The method includes assembling an anode and a cathode to form a cell having a void volume after discharge of between 7.5 percent and about 15 percent of the anode volume. In another aspect, the invention features a method of reducing leakage of electrolyte from a metal-air electrochemical cell. The method includes assembling an anode and a cathode to form a cell. The anode is assembled to have a void volume after discharge being between about 7.5 percent and about 15 percent of the anode volume. The metal-air electrochemical cells of the invention can have improved discharge performance under conditions of high temperature and high humidity relative to 20°C and 50 percent relative humidity. The cells have a reduced tendency of leaking relative to low void volume cells. Under high temperature and high humidity conditions, moisture can enter the cell, building up hydrostatic pressure in the cell. The hydrostatic pressure can lead to flooding of the cathode with electrolyte and, ultimately, disabling of the cell. The higher void volume of the cell increases the tolerance of the cell to take up atmospheric moisture. As void volume is increased, the capacity of the cell decreases. The void volume can be selected to reduce leakage and improve discharge performance while maintaining adequate cell capacity. FIG. 1 depicts a cross-sectional view of a metal-air cell. The metal-air electrochemical cells can be zinc-air cells having a relatively high void volume. The zinc-air batteries exhibit good discharge performance under relatively high temperature, high humidity conditions, such as at 30°C and 90% relative humidity. By including a void volume of greater than 7.5 percent and less than 15 percent in the cell, significant gains in cell performance can be achieved at relevant temperatures and humidities. In addition, the likelihood of leakage can be reduced. A zinc-air cell can be a button cell. Referring to FIG. 1 , a button cell includes anode 2 and cathode 4. Anode 2 includes anode can 10 and anode gel 60. Cathode 4 includes cathode can 20 and cathode structure 40. Insulator 30 is located between anode can 10 and cathode can 20. Separator 70 is located between cathode structure 40 and anode gel 60, preventing electrical contact between these two components. Air access port 80, located in cathode can 20, allows air to exchange into and out of the cell. Air disperser 50 is located between air access port 80 and cathode structure 40. Anode can 10 and cathode can 20 are crimped together to form the cell container, which has an internal volume, or cell volume. Together, inner surface 82 of anode can 10 and separator 70 form anode volume 84. Anode volume 84 contains anode gel 60. The remainder of anode volume 84 is void volume 90. A zinc-air cell uses zinc as the electrochemically active anode material. The anode gel contains a mixture of zinc and electrolyte. The mixture of zinc and electrolyte can include a gelling agent that can help prevent leakage of the electrolyte from the cell and helps suspend the particles of zinc within the anode. The cathode structure contains a material (e.g., a manganese compound) that can catalyze the reduction of oxygen which enters the cell as a component of atmospheric air passing through access ports in the bottom of the cathode can. The overall electrochemical reaction within the cell results in zinc metal being oxidized to zinc ions and oxygen from air being reduced to hydroxyl ions. Ultimately, zinc oxide, or zincate, is formed in the anode. While these chemical reactions are taking place, electrons are transferred from the anode to the cathode, providing power to the device. Noid volume is determined after discharge of the cell. The anode volume of the cell is established by the geometry of the cell and the dimensions of the components. The amount of the anode volume occupied by the anode gel is determined by the volume of anode gel added to the cell. As a zinc-air cell is discharged, the zinc of the anode gel is oxidized to zinc oxide. The oxidation of the zinc increases the volume occupied by the anode gel, since the density of zinc is higher than the density of zinc oxide. The volume of the anode gel expands during discharge due to the larger volume occupied by the oxidized zinc. The amount of expansion of the anode gel after discharge can be calculated from the zinc content of the gel and the change in density of the zinc component. The anode gel volume after discharge can then be calculated by adding the volume of expansion to the anode gel volume before discharge. Accordingly, the void volume after discharge can be calculated by taking the difference between the anode volume and the anode gel volume after discharge. Void volume 90 after discharge can be between about 7.5 percent and 15 percent. The increased void volume can assist in reducing leakage of electrolyte from the cell. The cathode structure has a side facing the anode gel and a side facing the air access ports. The side of the cathode structure facing the anode gel is covered by a separator. The separator can be a porous, electrically insulating polymer, such as polypropylene, that allows the electrolyte to contact the air cathode. The side of the cathode structure facing the air access ports is typically covered by a polytetrafluoroethylene (PTFE) membrane that can help prevent drying of the anode gel and leakage of electrolyte from the cell. Cells can also include an air disperser, or blotter material, between the PTFE membrane and the air access ports. The air disperser is a porous or fibrous material that helps maintain an air diffusion space between the PTFE membrane and the cathode can. The air disperser can be hydrophilic and absorbent, allowing it to soak up any moisture in the cathode side of the cell. The disperser can also limit damage done by electrolyte solution if it penetrates the cathode, which can occur under higher temperature and humidity conditions, particularly when the void volume of the cell is not large enough to compensate for gas generation in the anode. The cathode structure includes a current collector, such as a wire mesh, upon which is deposited a cathode mixture. The wire mesh makes electrical contact with the cathode can. The cathode mixture includes a catalyst for reducing oxygen, such as a manganese compound. The catalyst mixture is composed of a mixture of a binder (e.g., PTFE particles), carbon particles, and manganese compounds. The catalyst mixture can be prepared, for example, by heating manganese nitrate or by reducing potassium permanganate to produce manganese oxides, such as Mn 2 O 3 , Mn 3 O 4 and MnO 2 . The catalyst mixture can include between about 15 and 45 percent polytetrafluoroethylene by weight. For example, the cathode structure can include about 40 percent PTFE, which can make the structure more moisture resistant, reducing the likelihood of electrolyte leakage from the cell due to moisture uptake from the atmosphere. The electrochemical cell includes an anode formed of an anode gel. The anode gel includes an electrolyte, a zinc material, and a gelling agent. In certain embodiments, the mercury content of the zinc of the anode can be less than 3 weight percent of the zinc. In other embodiments, the mercury content of the zinc of the anode can contain less than 2 weight percent mercury. The zinc material can be a zinc alloy powder that includes less than 2 percent mercury. The zinc alloy can include, for example, lead, indium, or aluminum. Suitable zinc materials include zinc available from Union Miniere (Overpelt, Belgium), Duracell (USA), Noranda (USA), Grillo (Germany), or Toho Zinc (Japan), or zinc materials described in U.S. Ser. No. 08/905,254, filed August 1, 1997, and U.S. Ser. No. 09/115,867, filed July 15, 1998, each of which is incorporated herein by reference. Zinc-air anode materials are loaded into a cell in the following manner. A gelling agent (about 0.33 weight percent) and zinc powder are mixed to form a dry anode blend. The blend is then poured into the anode can and electrolyte is dispensed onto the dry anode blend to form the anode gel. The gelling agent can be a polyacrylate, such as a sodium polyacrylate. The gelling agent can be an absorbent polyacrylate. The anode gel includes less than 1 percent by weight of the gelling agent in the anode mixture weight. Preferably, the gelling agent content of the anode mixture is between about 0.2 and 0.8 percent by weight, more preferably between about 0.3 and 0.6 percent by weight, and most preferably about 0.33 percent by weight. The anode gel can include a surfactant or other additives. The electrolyte can be an aqueous solution of potassium hydroxide. The electrolyte can include between about 30 and 40 percent of potassium hydroxide. The electrolyte concentration can affect the rate at which a cell takes up water. The higher the electrolyte concentration, the more water it tends to absorb from the atmosphere. The electrolyte can also include between about 1 and 2 percent of zinc oxide. The anode can be composed of stainless steel having a copper layer on the inner surface of the can and a nickel layer on the outer surface of the can. The cathode can be composed of cold-rolled steel having inner and outer layers of nickel. The insulator, such as an insulating gasket, pressure-fit between the anode can and cathode can. The insulator can be an insulating polymeric material, such as nylon, polypropylene, or polyethylene. The can configuration can be a straight wall design, in which the anode can is straight, or a foldover design, in which the clip-off edge of the anode can, generated during stamping of the can, is placed on the top, outside of can, away from the interior of the cell. A straight wall design can be used in conjunction with an L- or J-shaped insulator, preferably L-shaped, that can bury the clip-off edge into the insulator foot. During storage, the air access ports are typically covered by a removable sheet, commonly known as the seal tab, that is provided on the bottom of the cathode can to cover the air access ports to restrict the flow of air between the interior and exterior of the button cell. The user peels the seal tab from the cathode can prior to use to allow oxygen from air to enter the interior of the button cell from the external environment. EXAMPLES The discharge capacities of five sizes of cells were calculated for three different void volumes in Examples 1-5. Each of the cells had an anode can thickness of about 0.152 mm and a cathode can thickness of about 0.203 mm. The discharge capacities are listed in Table I. The cell sizes correspond to the IEC designated cell sizes. The discharge capacities are based on discharge of zinc-air cells at 20°C and 50 percent relative humidity. The discharge capacities of the 5% void volume cells and 20% void volume cells are based on discharge measurements of fabricated cells. The reported discharge capacities at 10% void volume are nominal capacities based on the anode weight required to achieve a 10% void volume, minus an estimate of anode efficiency losses. The anode efficiency losses were estimated from those observed for 5% void volume and 20% void volume cells. TABLE I Void Volume 5% 10% 20% Example Cell Size Capacity Capacity Capacity (mAh) (mAh) (mAh) 1 675 610 560 480 2 13 260 240 200 3 312 135 125 110 4 10 70 65 50 5 5 45 43 40 The discharge capacities of five sizes of cells having thinner can wall thicknesses than in Examples 1-5 were calculated for three different void volumes in Examples 6-10. Each of the cells had anode can thickness of about 0.1016 mm and a cathode can thickness of about 0.1016 mm. The outer dimensions of the cells fell within the IEC designated ranges for each cell size. The discharge capacities are listed in Table II. TABLE II Void Volume 5% 10% 20% Example Cell Size Capacity Capacity Capacity (mAh) (mAh) (mAh) 6 675 700 645 585 7 13 295 275 225 8 312 155 145 130 9 10 85 80 62 10 5 45 43 40

Claims: C L A I M S 1. A metal-air electrochemical cell comprising: an anode including an anode can and an anode gel; a cathode, the cathode including a cathode can having at least one air access port and containing a cathode structure, the anode can and cathode can being assembled to form a cell; and a separator electronically separating the anode and the cathode positioned between the anode gel and the cathode structure; wherein the anode has an anode volume contained between an inner surface of the anode can and the separator, the anode volume containing an anode gel and a void volume, the void volume after discharge being between about 7.5 percent and about 15 percent of the anode volume. 2. The metal-air electrochemical cell of claim 1, wherein the void volume after discharge is between about 8 percent and about 12 percent. 3. The metal-air electrochemical cell of claim 1, wherein the void volume after discharge is between about 10 percent. 4. The electrochemical cell of claim 1, wherein the metal-air electrochemical cell has a discharge capacity of between about 700 mAh and about 480 mAh for a 675 cell at a temperature between about 25°C and about 38°C and a relative humidity external of the cell of between about 45 and about 95 percent. 5. The electrochemical cell of claim 1, wherein the metal-air electrochemical cell has a discharge capacity of between about 295 mAh and about 200 mAh for a 13 cell at a temperature between about 25°C and about 38°C and a relative humidity external of the cell of between about 45 and about 95 percent. 6. The electrochemical cell of claim 1, wherein the metal-air electrochemical cell has a discharge capacity of between about 155 mAh and about 110 mAh for a 312 cell at a temperature between about 25°C and about 38°C and a relative humidity external of the cell of between about 45 and about 95 percent. 7. The electrochemical cell of claim 1, wherein the metal-air electrochemical cell has a discharge capacity of between 85 mAh and about 50 mAh for a 10 cell at a temperature between about 25°C and about 38°C and a relative humidity external of the cell of between about 45 and about 95 percent. 8. The electrochemical cell of claim 1, wherein the metal-air electrochemical cell has a discharge capacity of between 45 mAh and about 40 mAh for a 5 cell at a temperature between about 25°C and about 38°C and a relative humidity external of the cell of between about 45 and about 95 percent. 9. The metal-air electrochemical cell of claim 4, wherein the temperature is about 30°C. 10. The metal-air electrochemical cell of claim 9, wherein the relative humidity is between about 70 and about 90 percent. 11. The metal-air electrochemical cell of claim 4, wherein the temperature is about 30°C and the relative humidity is about 90 percent. 12. The metal-air electrochemical cell of claim 5, wherein the temperature is about 30°C and the relative humidity is about 90 percent. 13. The metal-air electrochemical cell of claim 6, wherein the temperature is about 30°C and the relative humidity is about 90 percent. 14. The metal-air electrochemical cell of claim 7, wherein the temperature is about 30°C and the relative humidity is about 90 percent. 15. The metal-air electrochemical cell of claim 8, wherein the temperature is about 30°C and the relative humidity is about 90 percent. 16. The metal-air electrochemical cell of claim 1, wherein the anode gel includes zinc particles. 17. The metal-air electrochemical cell of claim 16, wherein the zinc particles comprise less than 2 weight percent mercury. 18. The metal-air electrochemical cell of claim 1, wherein the anode gel includes a sodium polyacrylate. 19. The metal-air electrochemical cell of claim 1, wherein the cathode structure includes a catalyst mixture and a current collector in electrical contact with the cathode can. 20. The metal-air electrochemical cell of claim 1, wherein the catalyst mixture includes between about 25 and 45 percent polytetrafluoroethylene by weight. 21. The metal-air electrochemical cell of claim 1, further comprising an air disperser positioned between the air access port and the cathode structure, the air disperser including an absorbent material. 22. The metal-air electrochemical cell of claim 1, wherein the anode can has a thickness of about 0.1016 mm. 23. The metal-air electrochemical cell of claim 1, wherein the cathode can has a thickness of about 0.1016 mm. 24. A method of manufacturing a metal-air electrochemical cell comprising: assembling an anode and a cathode to form a cell, the anode including an anode and an anode gel, the cathode including a cathode can having at least one air access port and containing a cathode structure, and the cell including a separator electronically separating the anode and the cathode positioned between the anode gel and the cathode structure; wherein the anode has an anode volume contained between an inner surface of the anode can and the separator, the anode volume containing an anode gel and a void volume, the void volume after discharge being between about 7.5 percent and about 15 percent of the anode volume. 25. The method of claim 24, wherein the void volume after discharge is between about 8 percent and about 12 percent. 26. The method of claim 24, wherein the anode gel includes zinc particles comprising less than 2 weight percent mercury. 27. The method of claim 24, wherein the catalyst mixture includes between about 25 and 45 percent polytetrafluoroethylene by weight. 28. The method of claim 24, wherein the cell further comprises an air disperser positioned between the air access port and the cathode structure, the air disperser including an absorbent material. 29. A method of reducing leakage of electrolyte from a metal-air electrochemical cell comprising: assembling an anode and a cathode to form a cell, the anode including an anode and an anode gel, the cathode including a cathode can having at least one air access port and containing a cathode structure, and the cell including a separator electronically separating the anode and the cathode positioned between the anode gel and the cathode structure; wherein the anode is assembled to have an anode volume contained between an inner surface of the anode can and the separator, the anode volume containing an anode gel and a void volume, the void volume after discharge being between about 7.5 percent and about 15 percent of the anode volume.

Inventors: LACEY NEVILLE, LACEY, NEVILLE
Applicants: DURACELL INC, LACEY NEVILLE, DURACELL INC., LACEY, NEVILLE
Application year: 1999
Main classification: H01M12
IPC codes: H01M4, H01M12