Patent Publication Number: US-2004048146-A1

Title: Electrochemical cells and systems

Description:
TECHNICAL FIELD  
       [0001] The invention relates to electrochemical cells and systems.  
       BACKGROUND  
       [0002] Electrochemical cells are commonly used electrical power sources. A cell typically contains a negative electrode, a positive electrode, and an electrolyte. The negative electrode includes an active material that can be oxidized; the positive electrode contains or consumes an active material that can be reduced. The negative electrode active material is capable of reducing the positive electrode active material. In some embodiments, to prevent direct reaction of the positive electrode material and the negative electrode material, the negative electrode and the positive electrode are electrically isolated from each other by a separator.  
       [0003] When a cell is used as an electrical energy source in a device, electrical contact is made to the negative electrode (anode) and the positive electrode (cathode), allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. The electrolyte, for example, potassium hydroxide, in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to complete circuit and maintain charge balance throughout the cell during discharge.  
       [0004] In a metal-air electrochemical cell, the positive electrode active material is oxygen, which is supplied to the cathode from the atmospheric air external to the cell through one or more air hole(s) in the cell can. Oxygen is reduced at the cathode and a metal is oxidized at the anode.  
       [0005] In a metal-air electrochemical cell, such as a zinc-air cell, high energy density can be achieved because the cathode active material is not stored within the cell. However, this energy density can be decreased when the cell is open to the air because of carbonation of the electrolyte from atmospheric CO2 and water vapor ingress or egress. To prolong battery life, it is desirable that the cathode be isolated from the air when not in use (e.g., to reduce carbonation), but exposed to the air when in use. During use, it is desirable to provide uniform and sufficient air access to the cathode to provide, for example, uniform discharge of the active materials and/or a relatively high and steady running voltage. Systems, sometimes called air managers, can be used to provide air to the metal-air cell(s) when the cell(s) are needed, and to reduce exposure of the cell(s) to the environment (the air) when there is no load on the cell(s).  
       SUMMARY  
       [0006] The invention relates to electrochemical cells and systems, such as, for example, those having metal-air cells.  
       [0007] When used in a device, the systems provide good air management according to the power requirements of the device. Generally, the system exposes cell(s) in the system to air when the device is on, and limits air flow to the cell(s) when the device is off, thereby prolonging the life of the cell(s).  
       [0008] In one aspect, the invention features an electrochemical cell system including a cartridge having an air inlet channel and an air outlet channel, wherein during use air flows through the inlet channel in a substantially opposite direction than air flowing through the outlet channel, an air mover in the cartridge configured to move air through the channels, and a control circuit in the cartridge configured to control operation of the air mover.  
       [0009] Embodiments may include one or more of the following features. The air inlet channel includes an air inlet and the air outlet channel having an air outlet, and the air inlet and outlet are located at the same end of the cartridge. The inlet channel and the outlet channel are located on opposing sides of the cartridge. The air outlet channel extends substantially an entire length of the cartridge. The air outlet channel has a differential cross sectional area along its length. The air mover includes an impeller and a motor. The control circuit activates the air mover at a selected threshold current.  
       [0010] The system may include one or more metal-air cells in the cartridge. The metal-air cell may include a polymeric layer or air membrane, e.g., one including polytetrafluoroethylene, polypropylene, and/or Mylar®, surrounding a portion of the cell and defining an exterior portion of the cell. During use, air can first contact the polymeric layer of the metal-air cell at a portion between the ends of the cell.  
       [0011] The cartridge can be sized to fit into a battery compartment of an electronic device. The battery compartment can be sized to accommodate a plurality of batteries or cartridges. The cartridge may include a housing, and during use, air can be introduced into the housing between the ends of the housing. The cartridge may include a housing, and a portion of the outlet channel that is external to the housing.  
       [0012] In another aspect, the invention features an electrochemical cell system including a cartridge having a first end and an opposing second end, the cartridge further including an air inlet and an air outlet located at the same end, and an air mover in the cartridge, the air mover configured to move air from the inlet to the outlet.  
       [0013] Embodiments may include one or more of the following features. The cartridge includes a channel including the air outlet, the channel extending along substantially an entire length of the cartridge. The cartridge includes a housing, and a portion of the channel is external to the housing. The cartridge has a housing, and the air inlet and outlet are external to the housing. The air mover includes a fan (motor and impeller). The system has a control circuit in the cartridge configured to control operation of the air mover. The system includes one or more metal-air cells in the cartridge. The cartridge is sized to fit into a battery compartment of an electronic device. The battery compartment is sized to accommodate a plurality of batteries or cartridges.  
       [0014] In another aspect, the invention features an electrochemical cell system including a cartridge having an internal volume, and two metal-air cells removably placed in the cartridge, wherein the cells occupy at least 50% of the internal volume of the cartridge. The cells can occupy at least 60% of the internal volume of the cartridge.  
       [0015] In another aspect, the invention features an electrochemical cell system including a cartridge having an internal volume, an air mover in the cartridge, and a control circuit in the cartridge, wherein the air mover and the control circuit occupy less than about 2% of the internal volume of the cartridge. The air mover and the control circuit can occupy less than about 1.6% of the internal volume of the cartridge.  
       [0016] In another aspect, the invention features an electrochemical cell system including a cartridge, and two metal-air cells removably placed in the cartridge, wherein the system has an energy density greater than about 400 Wh/L. The system can have an energy density greater than about 420 Wh/L.  
       [0017] In another aspect, the invention features an electrochemical cell system including a cartridge, and two metal-air cells removably placed in the cartridge, wherein the system has a capacity greater than about 5.4 Ah/cell, e.g., greater than about 5.6 Ah/cell or about 5.8 Ah/cell.  
       [0018] In another aspect, the invention features a metal-air cell including an anode, a polymer layer having openings through the polymer layer, a separator between the anode and the polymer layer, and a cathode between the separator and the polymer layer. The polymer layer is a mesh and/or a resilient tubular sleeve.  
       [0019] In another aspect, the invention features a metal-air cell including an anode, an outer layer having interlocking opposing edges, a separator between the anode and the outer layer, and a cathode between the separator and the outer layer.  
       [0020] Embodiments may include one or more of the following. One edge of the outer layer is configured as dovetails. The outer layer includes a metal. The outer layer includes a polymer. The outer layer includes an air access opening and/or a slit.  
       [0021] In another aspect, the invention features a metal-air cell including a cathode current collector, and a cathode terminal having an integral portion extending radially from the terminal, the integral portion being attached to the cathode current collector.  
       [0022] Embodiments may include one or more of the following. The cathode terminal includes a plurality of discrete, integral portions extending radially from the terminal, the plurality of integral portions being attached to the cathode current collector. The plurality of integral portions are equally spaced about the cathode terminal. The current collector is welded to the integral portion. The cell further includes a polymer seal surrounding a portion of the cathode terminal.  
       [0023] In another aspect, the invention features a method of operating an electrochemical cell system. The method includes introducing air through an air inlet channel of a cartridge in a first direction, and introducing air through an air outlet channel of the cartridge in a second direction opposite to the first direction, wherein air is introduced through the channels by an air mover in the cartridge.  
       [0024] Embodiments may include one or more of the following. The method further includes contacting a metal-air cell in the cartridge with air at a portion between the ends of the cell. The method further includes replacing the metal-air cell with a second metal-air cell. The method further includes activating the air mover according to a preselected threshold current.  
       [0025] In yet another aspect, the invention features a method of operating an electrochemical cell system including providing a cartridge comprising an air inlet and an air outlet located at one end of the cartridge, introducing air into the air inlet, and flowing air through the air outlet and out the cartridge, wherein air is introduced through the air inlet by an air mover in the cartridge.  
       [0026] Embodiments may include one or more of the following. The method further includes contacting a metal-air cell in the cartridge with air at a portion between the ends of the cell. The method further includes replacing the metal-air cell with a second metal-air cell.  
       [0027] Embodiments may have one or more of the following advantages. The system provides a simple and functional system for managing air flow into a metal-air battery. The system can be formed in a variety of shapes to suit different devices, and the system can be produced at a low cost. Operation of the system is simple. In some embodiments, operation of the system is transparent to the user.  
       [0028] Other aspects, features, and advantages of the invention will be apparent from the description of the preferred embodiments thereof and from the claims. 
     
    
    
     DESCRIPTION OF DRAWINGS  
     [0029]FIG. 1 is a top perspective view of an embodiment of a cell cartridge.  
     [0030]FIG. 2 is a bottom perspective view of the cell cartridge of FIG. 1.  
     [0031]FIG. 3 is a perspective view of embodiments of metal-air cells and the cell cartridge of FIG. 1.  
     [0032]FIG. 4 is a partially exploded perspective view of the embodiments of FIG. 3.  
     [0033]FIG. 5 is a circuit diagram.  
     [0034]FIGS. 6 and 7 are perspective views of an embodiment of an end cap.  
     [0035]FIG. 8 is a perspective view of an embodiment of a contact end assembly.  
     [0036]FIG. 9 is a cross sectional view of the cell cartridge of FIG. 1.  
     [0037]FIGS. 10, 11,  12 ,  12 A,  12 B, and  13  show an embodiment of a metal-air cell and a method of making the cell.  
     [0038]FIGS. 14, 14A,  14 B,  14 C,  15 ,  15 A, and  15 B show an embodiment of a metal-air cell.  
     [0039]FIG. 16 is a schematic view of an embodiment of a cell and a cartridge.  
     [0040]FIG. 17 is a schematic view of an embodiment of a cell system.  
     [0041]FIGS. 18, 19,  20 ,  21 , and  22  are perspective, partially exploded views of an embodiment of a cell cartridge.  
     [0042]FIGS. 23 and 23A show an embodiment of a metal-air cell.  
     [0043]FIGS. 24A, 24B,  24 C,  24 D, and  24 E show an outer layer of a metal-air cell.  
     [0044]FIGS. 25A, 25B, and  25 C show an example of a cell system including exemplary dimensions. 
    
    
     DETAILED DESCRIPTION  
     [0045] Referring to FIGS.  1 - 4 , an electrochemical cell system  20  includes a re-useable cartridge  22  and one or more replaceable metal-air cells  24  (here, two) that can be placed in the cartridge. Cartridge  22  is designed to provide cells  24  with uniform and sufficient air flow during use. When not in use, the cartridge reduces exposure of the cells to air, thereby extending the life of the cells.  
     [0046] Cartridge  22  is generally sized and configured to fit into a battery compartment of an electronic device. Examples of devices include a camera, a camcorder, a cellular telephone, a toy, a CD player, or a flashlight. Cartridge  22  includes a housing  26  sized and configured to receive one or more cells  24 . Housing  26  can be, for example, made of a metal or a plastic, e.g., by molding or extrusion. As shown, housing  26  has a configuration sized to receive, e.g., slidably, two cylindrical cells  24  placed side-by-side. The configuration defines two nooks  56  and  58  on the sides of housing  26 . Nook  56  includes a cutaway portion  60 , and nook  58  includes a cutaway portion  62  (FIG. 9).  
     [0047] On the sides of housing  26 , cartridge  22  includes features for directing air flow through the housing to contact cells  24 . On a first side of housing  26 , cartridge  22  includes a plate assembly  32 , an air mover  34 , including, for example, a DC motor  35  available from Kot&#39;l JinLong Machinery, Wenzhou, China PR, an impeller  37 , a control circuit  36 , and a plate  38  (e.g., a thin member or label made of a plastic such as Mylar®). Plate assembly  32  is configured to engage with cutaway portion  60 . Plate assembly  32  includes a first cylindrical portion  40  that receives air mover  34 , a second cylindrical portion  42  spaced from the first cylindrical portion, and two grooves  44  (FIGS. 1 and 4). Grooves  44  extend from an end of second cylindrical portion  42  to first cylindrical portion  40 , and are in fluid communication with the spacing between portions  40  and  42  through an opening  45  in cylindrical portion  40 . When plate  38  covers plate assembly  32  (FIG. 3), the plate and grooves  44  form two air inlet channels  46  in fluid communication with the interior of housing  26  via opening  45  and first cylindrical portion  40 .  
     [0048] In other embodiments, air mover  34  can be a diaphragm pump and its variations, or a peristaltic pump. Examples of pumps are described in U.S. Pat. No. 6,274,261; WO 00/36696; WO 01/97317; WO 01/97318; WO 01/97319; and WO 02/31906, all of which are hereby incorporated by reference in their entirety.  
     [0049] Control circuit  36  is configured to control air mover  34  according to a preselected mode of operation. For example, in some embodiments, control circuit  36  can be designed to activate air mover  34  when the control circuit detects a certain voltage or current, e.g., a threshold current of about 30 milliamps. When the detected voltage or current changes, e.g., in the opposite direction, beyond a certain value, e.g., the threshold current, control circuit  36  can deactivate air mover  34 . An example of control circuit  36  is shown schematically in FIG. 5.  
     [0050] On a second side of housing  26  opposite the first side, cartridge  22  includes a cavity key  48  that defines a groove  50  on one surface. As shown, cavity key  48  extends the entire length of housing  26 , although in other embodiments, the cavity key can extend only a portion of the length of the housing. Cavity key  48  includes a relatively narrow portion  52  and a relatively wide portion  54 . When cavity key  48  is nested within nook  58  (e.g., by gluing), the cavity key and housing  26  define an air outlet channel  64  in which narrow portion  52  is adjacent to cutaway portion  62 . The cross sectional area of channel  64  along wide portion  54  is greater than the cross sectional area of the channel along narrow portion  52 .  
     [0051] At the ends of housing  26 , cartridge  22  includes a contact end assembly  28  attached to the housing and an end cap  30  removably attached to the housing. Referring to FIGS. 6 and 7, end cap  30  is configured to engage with cells  24  and an end of housing  26 . End cap  30  includes a lock pin  68 , a spring contact  70 , and a pull tab  72 . Lock pin  68  is a resilient member configured to engage with cylindrical portion  42  of plate assembly  32 . Lock pin  68  has a cone-shaped or mushroom-shaped tip portion that, when end cap  30  is fully engaged with housing  26 , extends past cylindrical portion  42  and secures the end cap to the housing (FIG. 9). Lock pin  68  can be made, for example, of a deformable urethane or a latex material. Spring contact  70  is attached to the interior surface of end cap  30  and contacts the terminals of cells  24  to electrically connect the cells in series. Pull tab  72 , e.g., a piece of fabric attached to the exterior surface of end cap  30 , is used to remove end cap  30  from housing  26 , e.g., to replace the cells. End cap  30  further includes tangs (here, two)  65  and two notches  74 . Tangs  65  position cells  24  within housing  26  such that there is a spacing or plenum between the exterior surface of the cells and the interior surface of the housing. Notches  74  align with grooves  44  of plate assembly  32  to define air inlets to housing  26 . Other mechanisms for attaching end cap  30  are possible. For example, end cap  30  can be hingeably attached to cartridge  22 . End cap  30  can be permanently attached to, e.g., integrally formed with, cells  24 .  
     [0052] Referring to FIG. 8, contact end assembly  28  includes a negative contact  76 , a positive contact  78 , and spring contacts  80  that contact the terminals of cells  24 . Negative contact  76  includes an integral shunt resistor for current sensing (FIG. 5). Negative contact  76  and positive contact  78  are connected to shunt and control leads  82  that connect to control circuit  36  (FIG. 5). Contact end assembly  28  further includes alignment tangs  84  (here, four) that position cells  24  within housing  26  to define a plenum between the exterior surface of the cells and the interior surface of the housing.  
     [0053] In one mode of operation, air is supplied to cells  24  when system  20 , specifically, control circuit  36 , detects a predetermined threshold current demand from the device in which the system is used. When control circuit  36  detects the threshold current, the circuit activates air mover  34 . Referring to FIG. 9, as a result, air is drawn through the air inlets and inlet channels  46 , through opening  45 , through impeller  37 , and through the spacing between cylindrical portions  40  and  42 , where the air contacts the exterior surface of cells  24  (see black arrows in FIG. 9). The air first contacts portions of cells  24  between the ends of the cells. Blown by the force of air mover  34  (motor  35  and impeller  37 ), air flows along the length of cells  24  at a rate sufficient for the oxygen in the air to be used for the electrochemical reactions of the cells to meet the power requirements of the device. Air then flows to contact end assembly  28  and reverses in direction. Air flows through cutaway portion  62 , through the relatively narrow portion of outlet channel  64 , through the relatively wide portion of the outlet channel, and out the air outlet. Other threshold currents or sensing mechanisms can be use to activate air mover  34 . The threshold current can be dependent, for example, on the size of the plenum between cells  24  and housing  26 , the rate of discharge, and/or the size of inlet and outlet channels  46  and  64 .  
     [0054] In other embodiments, impeller  37  can be configured to rotate such that air can be drawn into or sucked from an exterior of cartridge  22 , through outlet channel  64 , and blown out of inlet channel  46 , i.e., air flow is reversed from the air flow described above. Variable fan speed, e.g., for variable current requirements, can be used. For example, control circuit  36  may include an analog transistor rather than a switching transistor.  
     [0055] When control circuit  36  detects a current below the threshold value, e.g., when the device is turned off, the control circuit deactivates air mover  34 . As a result, air mover  34  stops urging air through system  20 , and air flow past cells  24  is reduced (e.g., relative to when the air mover is activated), thereby reducing possible carbonation and moisture transpiration, and extending the life of the cells.  
     [0056] Cells  24  and a method of manufacturing the cells will now be described.  
     [0057] Cells  24  are cylindrical metal-air electrochemical cells configured to be placed inside housing  26 . Referring to FIGS.  10 - 13 , cell  24  includes a cathode assembly  100 , a cathode seal  102  connected to one end of the cathode assembly, and an anode seal  104  connected to the other end of the cathode assembly. Cathode assembly  100  includes a cathode  106  formed on a current collector  108 . Only the exposed portion of current collector  108  is shown. Other portions of current collector  108  are surrounded by cathode material. A separator (not shown) is attached, e.g., glued, to an interior side of the cathode. An air membrane (not shown), an optional blotter or air diffusion layer  110  and a spacer layer  112  are wrapped around an exterior side of cathode  106 . Cathode assembly  100  is formed to define a cavity  114  in which an anode material  115  is placed. When cell  24  is fully assembled, spacer layer  112  defines an exterior surface of the cell. In some embodiments, the thickness of spacer layer  112  defines a minimum plenum of cell  24 . Spacer layer  112  can also be excluded so that the air membrane defines an exterior surface of the cell.  
     [0058] Cathode  106  includes an active cathode mixture formed on at least a portion of current collector  108 . The cathode mixture includes a blend of a catalyst, such as a manganese compound, carbon particles, and a binder. Useful catalysts include manganese oxides, such as Mn 2 O 3 , Mn 3 O 4 , MnO 2 , and combinations thereof, that can be prepared, for example, by admixing, heating manganese nitrate or by reducing potassium permanganate. Cathode  106  may include between about 1% and about 20%, such as between about 3% and about 5% of catalyst by weight.  
     [0059] The carbon particles are not limited to any particular type of carbon. Examples of carbon include Black Pearls 2000 and Vulcan XC-72 (Cabot Corp., Billerica, Mass.), Shawinigan Black (Chevron, San Francisco, Calif.), Printex (Degussa A G, Frankfurt, Germany), Ketjen Black (Akzo Nobel, Chicago, Ill.), and Calgon PWA (Calgon Carbon, Pittsburgh, Pa.). The cathode mixture may include between about 30% and about 70%, such as between about 50% and about 60%, of total carbon by weight.  
     [0060] Examples of binders include polyethylene powders, polyacrylamides, Portland cement and fluorocarbon resins, such as polyvinylidene fluoride and polytetrafluoroethylene. An example of a polyethylene binder is sold under the tradename Coathylene HA-1681 (Hoechst). A preferred binder includes polytetrafluoroethylene (PTFE) particles, e.g., T-30 (DuPont). The cathode mixture may include between about 10% and 40%, such as between about 30% and about 40%, of binder by weight. The cathode mixture can be formed by blending the catalyst, carbon particles and binder. In other embodiments, gas diffusion electrodes, such as those available from Alupower, Inc. or ETEK, can be used.  
     [0061] The blended cathode mixture is applied to cathode current collector  108 , such as a grid mesh, parallelogram metal, or expanded metal mesh screen formed into a cylinder, to form cathode  106 . Methods of making a cathode may include, for example, injection molding or extrusion, and are described in commonly-assigned U.S. Ser. No. 09/416,799, filed Oct. 13, 1999, hereby incorporated by reference in its entirety.  
     [0062] Current collector  108  is then attached (e.g., welded) to a cathode terminal cup  116 , which forms the pip of cell  24  (FIG. 11). As shown in FIGS. 12A and 15B, cathode terminal cup  116  includes an extended portion or arm  117  that is attached to an exposed portion of current collector  108 . In some embodiments, cathode terminal cup  116  includes more than one, e.g., two, three, four or more, extended portions  117  that are attached to current collector  108 . The multiple extended portions  117  can be equally spaced apart to help center cathode terminal cup  116  with the longitudinal axis of cell  24 . Attaching multiple extended portions  117  of cathode terminal cup  116  to current collector  108  can also provide a rigid and stable attachment. In other embodiments, current collector  108  (e.g., a mesh) can be integrally formed, e.g., by metal casting, with cathode terminal cup  116  having one or more extended portions  117 . A current collector and cathode terminal cup assembly can be placed into a mold or cavity, and a blended cathode mixture can be injected into the mold or cavity to form cathode  106 .  
     [0063] On the interior side of cathode  106 , a separator is attached (e.g., glued) to the cathode. The separator can be a porous, electrically insulating polymer, such as polypropylene, that allows electrolyte (described below) to contact cathode  106 . In some embodiments, the separator can be applied from solution and be formed on cathode  106  when the solution dries, as described in commonly-assigned U.S. Ser. No. 09/568,819, filed May 10, 2000, hereby incorporated by reference in its entirety.  
     [0064] In some embodiments, the air membrane is attached to the exterior side of cathode  106  by pressure, heat, and/or an adhesive. The membrane can be a porous, electrically insulating polymer, such as polytetrafluoroethylene (PTFE), that allows air to permeate from the exterior side of the cell to active sites for oxygen reduction. The membrane can also prevent liquid water from the electrolyte from passing to the exterior of the cell.  
     [0065] Cathode  106  (attached to cathode terminal  116 ) and the separator are then placed into an insert mold cavity. An insulating disk  118 , e.g., a sheet of Mylar® with an adhesive, is placed on the interior side of cathode terminal  116 , and electronically separates cathode terminal  116  from anode material  115  in cavity  114 . Cathode seal  102  and anode seal  104  are then formed, for example, by conventional insert molding techniques (e.g., using a Nissei NC-9000 G 11 System), such that cathode  106  is securely attached between the seals (FIGS. 12A and 12B). Seals  102  and  104  can be made of materials that are electrically-insulating, relatively resistant to an alkaline electrolyte (such as KOH), and/or relatively high melting (e.g., about 320° C.). Examples of materials for seals  102  and  104  include acrylonitrile-butadiene-styrene (ABS), polysulfones, nylons, polyethylene, and polypropylene. Cathode seal  102  surrounds the exposed portion of current collector  108 , portions of cathode terminal  116 , and portions of the cathode body. Cathode seal  102  becomes an integral part of cathode  106 , and insulates terminal  116  from contact with anode material  115 .  
     [0066] Cavity  114  is then filled with anode material  115 . Anode material  115  contains a mixture of zinc and electrolyte. The mixture of zinc and electrolyte can include a gelling agent that can help retain moisture within the cell, prevent leakage of the electrolyte from the cell, and/or suspend the particles of zinc within the anode.  
     [0067] The zinc material can be a zinc powder that is alloyed with lead, indium, aluminum, or bismuth. For example, the zinc can be alloyed with between about 400 and 600 ppm (e.g., 500 ppm) of lead, between 400 and 600 ppm (e.g., 500 ppm) of indium, or between about 50 and 90 ppm (e.g., 70 ppm) aluminum. Preferably, the zinc material can include indium, aluminum, and/or bismuth. The zinc material can be air blown or spun zinc. Suitable zinc particles are described, for example, in U.S. Ser. No. 09/156,915, filed Sep. 18, 1998, U.S. Ser. No. 08/905,254, filed Aug. 1, 1997, and U.S. Ser. No. 09/115,867, filed Jul. 15, 1998, each of which is incorporated by reference in its entirety.  
     [0068] 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 zinc material includes a majority of particles having sizes between 60 mesh and 325 mesh. For example, the zinc material can have the following particle size distribution:  
     [0069] 0-3 wt % on 60 mesh screen;  
     [0070] 40-60 on 100 mesh screen;  
     [0071] 30-50 wt % on 200 mesh screen;  
     [0072] 0-3 wt % on 325 mesh screen; and  
     [0073] 0-0.5 wt % on pan.  
     [0074] Suitable zinc materials include zinc available from Union Miniere (Overpelt, Belgium), Duracell (USA), Noranda (Canada), Grillo (Germany), or Toho Zinc (Japan).  
     [0075] The gelling agent is an absorbent polyacrylate. The absorbent polyacrylate has an absorbency envelope of less than about 30 grams of saline per gram of gelling agent, measured as described in U.S. Pat. No. 4,541,871, incorporated herein by reference. The anode gel includes less than 1 percent of the gelling agent by dry weight of zinc in the anode mixture. Preferably the gelling agent content 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 absorbent polyacrylate can be a sodium polyacrylate made by suspension polymerization. Suitable sodium polyacrylates have an average particle size between about 105 and 180 microns and a pH of about 7.5. Suitable gelling agents are described, for example, in U.S. Pat. No. 4,541,871, U.S. Pat. No. 4,590,227, or U.S. Pat. No. 4,507,438.  
     [0076] In certain embodiments, the anode material can include a non-ionic surfactant. The surfactant can be a non-ionic phosphate surfactant, such as a non-ionic alkyl phosphate or a non-ionic aryl phosphate (e.g., RA600 or RM510, available from Rohm &amp; Haas), which may be coated on the zinc surface. The anode material can include between about 20 and 100 ppm of the surfactant coated onto the surface of the zinc material. The surfactant can serve as a gassing inhibitor.  
     [0077] The electrolyte can be an aqueous solution of potassium hydroxide. The electrolyte can include between about 30 and 40 percent by weight, such as between 35 and 40 percent of potassium hydroxide. The electrolyte can also include between about 1 and 2 percent of zinc oxide.  
     [0078] Referring to FIG. 13, cell  24  is then sealed using an anode current collector assembly  120 . Assembly  120  includes an anode current collector  122  (e.g., a tin-plated brass rod or nail) attached (e.g., welded) to an anode cap  124  that forms an anode terminal. Current collector  122  and anode cap  124  (e.g., made from nickel plated  1110  cold-rolled steel) are attached to an electronically insulating seal member  126  (e.g., made of ABS). Seal member  126  can be attached to collector  122  and cap  124 , e.g., by over insert molding. Anode current collector assembly  120  can be attached to anode seal  104  by ultrasonic welding to seal cell  24 .  
     [0079] Blotter layer  110  is then wrapped around the exterior side of cathode  106 . Blotter layer  110  is used to distribute air and/or to absorb material, e.g., electrolyte that may leak through cathode  106 . Blotter layer  110  can be a woven or nonwoven material that is air-permeable, absorbent, and/or stable to the electrolyte, such as KOH. Blotter layer  110  can be, for example, Whatman paper, or Pelon (e.g., P3, P5, P12, or P28, nonwoven, uncalendered polyamides fabrics available from Freudenberg Nonwovens Technical Products Division, Lowell, Mass.).  
     [0080] In some embodiments, blotter layer  110  includes a material, such as potassium hydroxide, capable of reacting with carbon dioxide to reduce the occurrence of carbonation of the electrolyte. Carbonation of electrolyte can reduce the amount of electrolyte available to cell  24 . Carbonation can also increase leakage of electrolyte from cell  24  by forming potassium carbonate, which can increase the porosity of the air membrane and allow the electrolyte to leak through the membrane. In some embodiments, a paste of KOH can be applied to blotter layer  110  and dried. Since blotter layer  110  is placed on the exterior of cathode  106 , the KOH on the blotter layer can react with carbon dioxide before the carbon dioxide can react with electrolyte in the cathode. As a result, the amount of carbon dioxide that can react with electrolyte in the cathode and leakage of electrolyte can be reduced. The paste of KOH can be applied to blotter layer  110  in a pattern, such as grid or a series of stripes.  
     [0081] Spacer layer  112  is then wrapped around blotter layer  110 . Spacer layer  112  can be a non-conductive (e.g., a polymer such as polypropylene, polyethylene, nylon, polyurethane, or silicone rubber) mesh sleeve or grid that protects the exterior surface of cell  24 . The sleeve or grid can be relatively flexible and resilient, e.g., like an elastic band, to help keep blotter layer  110  in close contact with cathode  106 . The mesh can have openings that are, e.g., about one-eighth inch in width or diameter. Blotter layer  110  and spacer layer  112  can be glued to each other.  
     [0082] In some embodiments, cell  24  can include a can or outer housing, e.g., a cylindrical metal, plastic, or rubber housing. The outer housing or shroud may contain one or more air access ports. Blotter layer  110  and/or spacer layer  112  can help to define an air plenum between the interior surface of the can and the exterior surface of cathode  106 . For example, referring to FIGS. 23 and 23A, cell  24  can include an outer layer  500  having interlocking opposing edges  502 . As shown, edges  502  have pins and tails that mate with each other and lock to hold layer  500  into a generally cylindrical shape. Referring to FIGS.  24 A- 24 E, outer layer  500  can be formed by cutting, e.g., laser cutting, a flat blank  504  of material such as a metal or a plastic (FIG. 24A). Blank  504  can also be cut to include slits  506 , louvers, and/or air access openings. The formed blank can then be wrapped around a mandrel to bring and lock edges  502  together. Other configurations for edges  502  are possible. For example, edges  502  can be formed to include arrowhead shaped configurations, T-shaped configurations, or any other configurations that allow the edges to lock together.  
     [0083] In other embodiments, blotter layer  110  and/or spacer layer  112  can be complemented with or replaced by an air-permeable and liquid-impermeable barrier layer, e.g., a PTFE (available from Saint-Gobain Performance Plastics) or Mylar® membrane, that helps to maintain a consistent humidity level in cell  24 . The barrier layer can also help to restrict the electrolyte from leaking out of cell  24  and CO 2  from entering into the cell, and reduce physical damage to the cell. In some embodiments, the can may include louvers, as described in U.S. Pat. No. 6,232,007, hereby incorporated by reference in its entirety.  
     [0084] During storage, cell  24  can be covered with a removable sheet, for example, an oxygen semi-impermeable and hydrogen permeable sheet, that restricts the flow of air between the interior and exterior of the cell. A user can peel the sheet from cell  24  prior to placing the cell into housing  26  to allow oxygen to enter the interior of the cell. Cell  24  can also be stored in a sealed metal bag, and the user can remove the cell from the bag before use.  
     [0085] Cell  104  can be formed in numerous sizes, such as, for example, AA, AAA, AAAA, C, or D. In other embodiments, depending, for example, on the configuration of the housing and/or the battery compartment of the device, cell  24  can have a non-circular cross section, such as oval, elliptical, or polygonal, e.g., having 3, 4, 5, 6, 7, or 8 or more sides. The cross section can be irregular or regular.  
     Other Embodiments  
     [0086] In other embodiments, cell  201  includes a seamed cathode. Referring to FIGS.  14 - 15 , in which features similar to the features described above are designated with the same reference numbers, a seamed cathode  200  is formed by applying the cathode mixture to current collector  108  but leaving section of the current collector exposed (e.g., by masking), i.e., not coated with the cathode mixture. The exposed section can correspond to opposing edge portions of a cylinder that is formed from a flat sheet. To define cavity  114 , cathode  200  is wrapped (e.g., around a mandrel), and the exposed edge portions of current collector  108  are joined together (e.g., by welding), which forms a seam (not shown) extending along the length of cathode  200 . During formation of cathode seal  102  and anode seal  104 , e.g., by insert molding, the seam is covered or sealed by forming an internal seam  202  and an external seam  204 . Seams  202  and  204  can be polymeric (e.g., ABS) or rubber strips that are formed with seals  102  and  104  by insert molding. Cell  201  can be completed as described above for cell  24 . A seamless cathode can provide more active area (e.g., exposed portions) than a seamed cathode, e.g., one having seams  202  and  204 .  
     [0087] In some embodiments, system  20  can accommodate one cell or more than two cells, e.g., four, six, or eight or more. FIGS. 16 and 17 shows a cell system  300  having one cell  302  and a cartridge  304  configured to receive the cell. Cell  302  is generally as described above. Cartridge  304  includes an air mover  306 , as generally described above, located at one end of the cartridge. During use, air is drawn into one end of cartridge  304  (e.g., through an inlet, not shown), blown through the annular plenum defined between cell  302  and the cartridge, and out through the other end of the cartridge (e.g., through an outlet, not shown).  
     [0088] FIGS.  18 - 22  show a cell system  400  having four cells  402  and a cartridge  403 . Cartridge  403  includes a housing  404  configured to receive cells  402  and having two integrally-formed inlet channels  406 . As shown, cartridge  403  further includes two assemblies  420  generally including a plate assembly, an air mover, a control circuit, and a plate, as described above. At one end, cartridge  403  includes a first end cap  408 , which can be removable, and a cover  410 . End cap  408  has a plurality (here, four) of openings  413  and two outlets  415  in fluid communication with the openings. Cover  410  has two inlet ports  411 . At the other end, cartridge  403  includes a second end cap  412 , which can be fixed to the cartridge. Second end cap  412  includes a contact circuit board  414  as generally described above, an air mover housing  416 , and an air mover  418  in the air mover housing.  
     [0089] During use, air is drawn by air mover  418  through inlet ports  411  and flows through inlet channels  406  from end cap  408  to end cap  412 . Air then flows through openings  422  of end cap  412  and through air mover housing  416 . When air exits housing  416 , air flows through housing  404  and contacts cells  402 . Air then flows through openings  413  of end cap  408 , through outlets  415  and out of system  400 . In some embodiments, air can flow through assemblies  420 , as described above for system  20 . In other embodiments, air mover  418  can reverse the air flow through system  400 .  
     [0090] In other embodiments, one or more portions of the barrier layer or air membrane are modified relative to other portion(s) of the barrier layer to adjust the rate of flow of materials, such as oxygen, water, and carbon dioxide, through the barrier layer. For example, portion(s) of the barrier layer that are closer to an inlet channel or an outlet channel (i.e., shorter diffusion paths) may have higher transport resistance than other portion(s) of the barrier layer farther from the channel(s) (i.e., longer diffusion paths) to enhance (e.g., maximize) uniform oxygen access and/or to enhance (e.g., minimize) water transport. Portion(s) of the barrier layer can have different mass transport resistance or permeability to selected material(s) than other portion(s) of the barrier layer. Portion(s) of the barrier layer can have different porosity than other portion(s) of the barrier layer. Portion(s) of the barrier layer can have different apparent density than other portion(s) of the barrier layer. In these and other embodiments, one or more portions of the barrier layer are not uniform around the cells.  
     [0091] Numerous methods can be used to modify, e.g., increase or decrease, properties of the barrier layer, such as the mass transport resistance of a material, such as water vapor and/or oxygen, through the layer. In some embodiments, the barrier layer is modified by mechanical work. In other embodiments, the thickness of the barrier layer is modified. Other methods of modifying the barrier layer are described in commonly assigned U.S. Ser. No. 10/060,701, entitled “Batteries and Battery Systems” and filed Jan. 30, 2002, and U.S. Ser. No. ______ [Attorney Docket 08935-263001], entitled “Electrochemical Cells and Systems, filed on the same day as this application, hereby incorporated by reference in its entirety.  
     [0092] The cells can be placed symmetrically or asymmetrically. The cells can be placed end-to-end and/or side-by-side. The cells can be placed in series and/or in parallel within a cartridge.  
     [0093] In other embodiments, other types of electrochemical cells, e.g., air-assisted cells, can be used. Air-assisted cells are described, for example, in U.S. Pat. No. 6,372,370, hereby incorporated by reference in its entirety. Other types of metal-air cells, such as magnesium-air cells or aluminum-air cells, can also be used.  
     [0094] Other methods of making cells are described, for example, in commonly-assigned U.S. Ser. No. 10/060,701, filed Jan. 30, 2002, hereby incorporated by reference in its entirety. Other metal-air cells and methods of making them are described in U.S. Ser. No. 09/374,277, filed Aug. 13, 1999; U.S. Ser. No. 09/374,278, filed Aug. 13, 1999; U.S. Ser. No. 09/416,799, filed Oct. 13, 1999; U.S. Ser. No. 09/427,371, filed Oct. 26, 1999; and U.S. Ser. No. 09/494,586, filed Jan. 31, 2000, all of which are hereby incorporated by reference in their entirety.  
     [0095] The following example is illustrative and not intended to be limiting.  
     EXAMPLE  
     [0096] Referring to FIGS. 25A, 25B, and  25 C, illustrative dimensions (in mm) of an electrochemical cell system are shown. Based on the configuration shown and standard, median cells, the system has a volume of about 18.6 cc, of which about 1.5 cc (or 8.1%) is occupied by an air management system, which includes the air manager and the control circuit. The available cell volume is 5.8 cc for each cell. Therefore, for a two-cell system, about 62.0% of the cell system is available for the cells. The cell volume is based on a 5.9 Ah seamless cell with a 1.0 mm air plenum between the cells and the cartridge.  
     [0097] Based on an internal cell void volume of 10% (for zinc expansion) and a cell efficiency of 63% with an average operating voltage of 2.2 V, the following performance is projected:  
                               TABLE 1                                      Cathode Area   13.7   cm 2             Active Cathode   12.3   cm 2             Current Density at 1.0 A   81   mA/cm 2             Fill Weight   10.3   g           Capacity   5.9   Ah/cell           Energy   8.2   Wh           Cartridge Volume   18.6   cc           Energy Density   440   Wh/L                      
 
     [0098] The electrochemical cell system can operate with relatively high efficiency. For example, the system can be operated with at a relatively low air stoichiometric rate factor, such as about 2.5 to about 4, at a flow rate of about 2.5 to 9 L/hr, and an air plenum of about 0.5-1.2 mm, e.g., about 1.0 mm. The flow rate can be achieved using an air mover having a 3.8 mm diameter impeller operating at between 18,000 and 29,000 RPM. The system can also have relatively low flow losses, e.g., having a flow efficiency of 80% or greater based on the stoichiometric amount of air required divided by the minimum air flow the system would operate on at 1 Amp. It is believed that the efficiency of the system is due at least in part to the close proximity of fresh reactant air to the cathode. In some embodiments, the distance between the air mover exit and the cathode is between about two and four millimeters.  
     [0099] All publications and patents mentioned in this application are herein incorporated by reference to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated by reference.  
     [0100] Other embodiments are within the claims.