Batteries, such as alkaline and metal-air batteries, are commonly used as energy sources.

Generally, alkaline batteries include a cathode, an anode, a separator, and an electrolytic solution. The cathode can include, for example, manganese dioxide particles as the active material, carbon particles that enhance the conductivity of the cathode, and a binder. The anode may be, for example, a gel including zinc particles as the active material. The separator is disposed between the cathode and the anode. The electrolytic solution can be, for example, a hydroxide solution that is dispersed throughout the battery.

When a battery is used as an electrical energy source in a device, such as a hearing aid, a flashlight, or a cellular telephone, electrical contact is made to the anode and the cathode, allowing electrons to 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.

In a metal air battery, oxygen is reduced at the cathode, and a metal, such as zinc, is oxidized at the anode. Oxygen is supplied to the cathode from the atmospheric air external to the cell through air access ports in the battery housing.

Metal oxide, such as zinc oxide or zincate, is formed in the anode. Thus, the overall electrochemical reaction within a zinc-air electrochemical cell results in zinc metal being oxidized to zinc ions and oxygen from the air being reduced to hydroxyl ions. While these chemical reactions are taking place, electrons are transferred from the anode to the cathode thereby providing power to the device.

The zinc can also react directly with the electrolyte which results in the consumption of the zinc and the production hydrogen gas. Surfactants, mercury, and other metals such as lead and cadmium often are added to the anode to reduce the levels of hydrogen that are produced.

In general, the invention relates to a hydrogen permeable membrane for electrochemical cells. The hydrogen permeable membrane within the cell

permits hydrogen gas to exit the cell. As a result, electrochemical cells including a hydrogen permeable membrane generally have less internal pressure from hydrogen gas and less leakage.

In one aspect, the invention features an electrochemical cell, such as an alkaline or metal-air battery, including a cathode; an anode; a separator; a housing containing the cathode, the anode, and the separator, and defining an outlet; and a hydrogen selective membrane associated with the outlet of the housing. The hydrogen selective membrane is associated with the battery outlet, for example, by positioning and securing the membrane into or adjacent to the housing outlet. The hydrogen selectively permeable membrane includes a substrate layer and a hydrogen transportation layer, such as a metal-based material. A hydrogen selective membrane exhibits a selective permeability of hydrogen (H2) relative to carbon dioxide (CO2), water (H2O), and oxygen °2-Preferably, the selective permeability of the membrane is 10 times, more preferably, 100 times, and most preferably 1000 times more permeable for H2 than CO2. The selective permeability of the membrane can also be 10 times, more preferably, 100 times, and most preferably 1000 times more permeable for H2 than H2O.

In another aspect, the invention features an electrochemical cell, such as a metal-air battery, including a cathode; a cathode membrane; an anode; a separator; a housing containing the cathode, the cathode membrane, the anode, and the separator, and defining an outlet; and a membrane associated with the outlet.

The membrane has a permeance of H2 at a rate between about 10 to about 10,000 times less than the penneance of H2 through the cathode membrane. The membrane can be a hydrogen selective membrane or a non-selective membrane such as microporous polyethylene.

Embodiments of the invention may have one or more of the following advantages. Batteries including hydrogen selective membranes allow hydrogen gas to exit the housing without altering the levels of H2O and CO2 within the electrochemical cell. The membrane also reduces the likelihood of damage to the cathode by reducing the internal pressure of the battery. The membrane reduces the internal battery pressure of hydrogen gas and the voltage losses in a cell due to reaction of hydrogen gas with the positive electrode materials. Additionally,

electrolyte leakage is reduced or eliminated. The mechanical constraints of the housing, such as the rupturing pressure of a seal between anode and cathode portions of the housing, can also be reduced as a result of reduced internal pressure.

The difference in permeance of H2 through the vent membrane versus permeance through the cathode membrane permits release of hydrogen gas without drying or flooding the electrochemical cell.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

FIG. 1 is a cross-sectional view of an electrochemical cell; and FIG. 2 is a cross-sectional view of section A of FIG 1.

Referring to FIG. 1, a metal-air button cell battery 1 includes an anode 2, and a cathode 4. Anode 2 includes anode can 10 and anode gel 60. A hydrogen permeable membrane 6 is associated with an outlet 8 of anode can 10 via an adhesive 7. 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.

Anode gel 60, separator 70, and cathode structure 40, in combination with void volume 90, fill the cell volume. Void volume 90 can vary, for example, between 5 and 20 percent. The increased void volume can assist in reducing leakage of electrolyte, such as an aqueous solution of KOH, from the cell and reduce pressure build-up due to gas generation in the anode compartment.

Suitable adhesives which may be used to associate hydrogen permeable membrane 6 into or adjacent to the outlet include materials that are

chemically compatible with the materials of the electrochemical cell, i. e., anode and cathode materials, and electrolyte, and are capable of forming a gas tight seal between the membrane and the battery housing. Examples include, but are not limited to, polyamides, asphalt adhesives, and waxes. A suitable adhesive can be obtained, for example, from Jingxin Adhesive Co. as adhesive J-43. The hydrogen permeable membrane also may be attached into or adjacent to the outlet by mechanical means such as with a castle nut and an o-ring or a welded washer. The nut compresses the membrane against the o-ring which forms a gas tight seal between the housing and membrane.

Outlet 8 has a diameter, for example, between about 0.1 mm and about 1 mm. Although shown with one outlet and hydrogen permeable membrane, anode can 10 may include a plurality of outlets and membranes.

Referring to Fig. 2, an example of a hydrogen permeable membrane 6 includes a hydrogen transportation layer 100 sandwiched between a support layer 90 and a protective layer 110. Support layer 90 provides structural support for hydrogen transportation layer 100 and includes a support member 92 and a planarizing member 94 which levels unevenness in surface 93 of support member 92. Preferably, the permeance of hydrogen transmitted through hydrogen permeable membrane is greater than about 1 X 10-5 cm3/ (cm2. sec cmHg). The hydrogen permeable membrane exhibits a selective permeability of H2 relative to CO2 and H2O. Preferably, the selective permeability of the membrane is 10 times, more preferably, 100 times, and most preferably 1000 times more permeable for H2 than CO2. The membrane can also be selectively permeable for H2 relative too2' Although shown with the protective layer adjacent to the anode can, the hydrogen permeable membrane can be attached to the outlet of anode can with either side of the membrane, the protective layer or the support layer, adjacent the anode can provided that the layer exposed to the inside of the battery is chemically compatible with the materials of the electrochemical cell, i. e., anode and cathode materials.

Hydrogen transportation layer 100 is, typically, a metal film.

Suitable metal films, for example, include Pt, Pd, Ta, Nb, Rh, V, Zr, Ag, ABs misch metals, AB2 misch metals, and alloys thereof. The metal film may include an alloy of Pd and Ag atoms in a ratio (Pd: Ag), for example, between about 100: 1 and

about 1: 1, between about 10: 1 and about 1: 1, or between about 5: 1 and about 2: 1.

The transportation layer also can be alloyed with rare earth metals such as yttrium.

The thickness of layer 100 is adjusted to provide a metal film free of defects or pinholes and thereby reduce the permeability of gases such as CO, CO2, 02, and H20through the membrane. The exact thickness of the membrane required for a pin-hole or defect free surface depends upon the quality of the planarization layer.

Typically, the thickness of layer 100, for example, is between about 50 A and about 10,000 A. A preferred hydrogen transportation layer has a thickness less than about 1, 000 A.

Suitable materials for support member 92 include, but not limited to, polytetrafluoroethylene, polyimides, polyamides, styrene-butadiene and styrene polyisoprene block co-polymers, and polyolefins such as polypropylene, poly- sulfone, polydimethylsiloxane, and polytrimethylsilylpropyne. Support layer has a thickness, for example, between about 25 and about 300 jum. The diameter of pores in the support layer may be between about 10 A and about 2,000 A. Polypropylene support layers, such as Celgard, may be purchased from Hoechet Celanese Corporation, in Charlotte, N. C.

Planarizing materials include amorphous polymers. Examples include silicone, urethane, acrylic polymers, polyimides, polytetrafluoroethylene, polydimethylsiloxane, and polytrimethylsilylpropyne. Planarizing materials are available from Membrane Technologies and Research, Inc., located in Menlo Park, CA. The thickness of planarizing material is adjusted to level the unevenness of the support surface and thereby provide a flat surface on which the hydrogen transportation layer may be applied. Preferably, the planarizing material has a thickness less than about 10,000 A.

The protective layer may be, for example, any gas permeable polymer coating. Examples include, but are not limited to, polyimides, polyamides, styrene- butadiene or styrene polyisoprene block co-polymers, and polydimethylsiloxane.

The hydrogen permeable membrane can be formed via a combination of a number of techniques for sequentially depositing a planarizing layer, a hydrogen transportation layer, and a protective layer onto a support layer. For instance, the planarizing layer may be applied by spin-coating and the metal layer

by vacuum sputter-deposition. Examples of membranes produced by vacuum sputter-deposition can be found in"Metal Composite Membranes for Hydrogen Separation,"by Athayde et al. in the Journal of Membrane Science, 94 299 (1994), which is incorporated by reference in its entirety.

The anode can may include a tri-clad or bi-clad material. The bi-clad material can be stainless steel with an inner surface of copper. The tri-clad material is composed of stainless steel having a copper layer on the inner surface and a nickel layer on the outer surface of the can. The anode can also include a metallic coating such as tin on the inner surface. Preferably, the tin coating is on the inside surface of anode can that makes contact with zinc anode and electrolyte. The tin coating may be a layer on the inner surface of the can. The tin layer can be a plated layer having a thickness between about 1 and 12 microns, preferably between about 2 and 7 microns, and more preferably about 4 microns. The tin coating can be pre-plated on the metal strip or post-plated on the anode can. For example, the tin coating can be deposited by immersion plating (e. g., using a plating solution available from Atotech). The plated layer can have a bright finish or a matte finish.

A low porosity layer can exhibit less gassing in a low mercury metal-air electrochemical cell. The coating can include silver or gold compounds.

The cathode can is composed of cold-rolled steel having inner and outer layers of nickel. There is an insulator, such as an insulating gasket, pressure- fit between the anode can and cathode can. The gasket can be thinned to increase the capacity of the cell.

The anode can may be configured with a straight wall design, in which the side wall is straight, or a foldover design in thinner-walled cans (e. g., about 4 mils thickness). In a foldover design, the clip-off edge of the anode can which is generated during stamping of the can is bent away from the interior of the cell. The foldover design can reduce potential gas generation by decreasing the possibility of the anode materials making contact with exposed stainless steel at the anode can clip-off edge. A straight wall design can be used in conjunction with an L-or J-shaped insulator, preferably J-shaped, that can bury the clip-off edge into the insulator foot. When a foldover design is used, the insulator can be L-shaped.

The preferred anode material is zinc. Alternatively the anode

material can be a zinc alloy, in which the alloying elements can include, but are not limited to, In, Pb, Bi, or mixtures thereof. The anode gel may contain, for example, a mixture of zinc and electrolyte. The mixture of zinc and electrolyte can include a gelling agent, such as an absorbent polyacrylate, that can help prevent leakage of the electrolyte from the cell and helps suspend the particles of zinc within the anode. Suitable gelling agents are described, for example, in U. S. Patent No.

4,541,871, U. S. Patent No. 4,590,227, or U. S. Patent No. 4,507,438. The cathode structure contains carbon and 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-containing 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. The zinc material can be air blown or spun zinc.

Suitable zinc particles are described, for example, in U. S. S. N. 09/156,915, filed September 18,1998, U. S. S. N. 08/905,254, filed August 1,1997, and U. S. S. N.

09/115,867, filed July 15,1998, each of which is incorporated by reference in its entirety. The zinc can be a powder. The particles of the zinc can be spherical or nonspherical. For example, the zinc particles can be acicular in shape (having an aspect ratio of at least two).

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 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 Mn203, Mon304, and MnO2.

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.

Other embodiments are within the claims. For example, the hydrogen permeable membrane can be attached to the outside of the anode can. Additionally, the hydrogen permeable membrane may also be formed integrally to the anode can by depositing, such as by chemical deposition, the membrane materials into the outlet. In some embodiments, the hydrogen transportation layer may be a carbon- based molecular sieve which preferentially permeates hydrogen relative to other gases. Examples of carbon-based molecular sieves can be found, for example, in New Technology Japan April 1998.

In an alternate embodiment, a metal-air button cell battery includes a cathode membrane and an anode membrane that transmits H2 at a rate lower than the rate of H2 transmitted tllrough the cathode. Typically, the anode membrane is selected so that it transmits H2 at a rate about 10 to about 10,000 times less than the rate of H2 transmitted through the cathode membrane. The anode membrane can be a hydrogen permeable membrane having a selective permeability to hydrogen, as described above, or any non-selective membrane such as microporous polymers provided that membrane materials are chemically compatible with the materials of the electrochemical cell, i. e., anode and cathode materials, and electrolyte. In general, non-selective membranes, such as microporous polyethylene and PTFE, transmit hydrogen at rates comparable to the rates at which the membrane transmits

water. As a result, the permeance of hydrogen can be used to gauge the permeance of water vapor. For instance, a non-selective anode membrane which transmits hydrogen at a rate about 10 to about 10,000 times less than the rate of hydrogen transmitted through the cathode membrane will also transmit water vapor at a rate about 10 to about 10,000 times less than the rate of water vapor transmitted through the cathode.

In an electrochemical cell, the build-up of excess hydrogen gas creates a pressure differential of approximately 0.1 to 2 atmospheres across the anode membrane which causes the anode membrane to transmit hydrogen from the cell at a rate higher than the rate at which it transmits water. By selecting an anode membrane having a lower hydrogen permeability relative to the cathode membrane, the anode membrane will be less permeable to water vapor than the cathode membrane and will vent excess hydrogen without drying or flooding the electrochemical cell. A standard PTFE cathode membrane has a permeance to water and hydrogen between about 1 x 10-2 to about 1 x 10-4 cm3/ (cm2 sec cmHg).

Non-selective anode membranes, e. g., a microporous polyethylene membrane from Tonen, Inc. (Japan), have a permeance to hydrogen and water between about 1 x 10-3 to about 1 x 10-6 cm3/ (cm2 sec cmHg).

Additionally, the hydrogen permeable and anode membranes may be used in alkaline electrochemical cells such as AA, AAA, AAAA, C, or D alkaline batteries. Examples of alkaline batteries are described, for example, in U. S. Patent Nos. 5,283,139 and 5,856,040, each of which is incorporated by reference in its entirety. In cylindrically-shaped alkaline batteries, a hydrogen permeable membrane may be associated with an outlet formed in a negative metal cap. In an alkaline button-shaped cell, the membrane may be associated with an outlet formed in the anode can. The membrane may also be used with prismatic electrochemical cells having a thickness less than 10 mm, and preferably less than 4 mm. Examples of prismatic electrochemical cells are described, for example, in U. S. Patent Nos.

5,958,088 and 6,001,504, each of which is incorporated by reference in its entirety.

Moreover, although shown in the anode can, the outlet may be formed in any portion of the battery housing. The hydrogen permeable membrane may be associated with any outlet. For example, the hydrogen permeable membrane may be associated with an outlet formed in the cathode side of the battery housing.