Abstract:
A method for fabricating cylindrical and prismatic rechargeable metal-air batteries is devised. The method includes using micro fans to control air flowing through the batteries via air pathways between the packs of electrodes and separator sheet. The air pathways are created by protrusions printed or molded on plastic spacer film. The air is used by the positive electrode for generating electricity when the metal-air battery is discharged. By conjunction of a second positive electrode and an energy storage device, the micro fans can be actuated as soon as the metal-air battery is demanded by a load. The in-cell air management can not only supply air for reactions but also shut the system to preserve materials when they are not in service.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for fabricating cylindrical and prismatic rechargeable metal-air batteries and their structure, which depends on micro pumps to create air draft passing the gaps between packs of electrodes and separators, and the micro pumps are turned on and off by an in-cell actuating system. 
     2. Related Art 
     The flourishing telecommunications have wired people around the world. In the near future, cellular phones will become daily tools rather than luxuries. As internet proliferates and prevails at every level, more and more people will connect the web via portable electronic devices such as cellular, notebook and PDA (personal digital assistant). All the activities in conjunction with increasing power need of the devices, more than ever, depend heavily on rechargeable batteries. 
     The fashion-trend of the portable electronic devices is an incessant reduction in size and weight. It imposes great challenges on the battery manufactures. They have to produce smaller and thinner batteries, and yet the capacity and other performances of the batteries are not to be compromised. In designing a battery, developers must consider its energy density (Wh/l) and specific energy density (Wh/Kg). The former has more impact than the latter, as the reduction in size is easier to observe than that in weight. Currently, rechargeable lithium ion (Li+) and nickel-metal hydride (Ni-MH) batteries are the two principal energy sources for many portable electronic devices. However, their dominance might be replaced by lithium polymer batteries someday. The new batteries hold some advantages including size and weight. 
     ZAB (Zn-air battery) should be another promising candidate for the rechargeable battery of next generation. Firstly, ZAB is very economical as Zn is an essential, cheap metal and the cathode of ZAB reacts on oxygen that is abundant and free. Secondly, ZAB is a safe and environmentally friendly battery. Thirdly, comparing with Li+ and Ni-MH batteries, ZAB is superior in energy densities: Z-A (200 Wh/Kg), Li+ (115 Wh/Kg) and Ni-MH (70 Wh/Kg); Z-A (276 Wh/l), Li+ (270 Wh/l) and Ni-MH (240 Wh/l). These numbers certify ZAB as a legitimate contender as the portable energy source. 
     Nevertheless, a rechargeable ZAB acceptable to the portable electronic industry is overdue. There are many technical hurdles need to be crossed before a viable secondary ZAB for 3C (communication, computer and consumer) applications is realized. Most of the technical difficulties are studied and addressed with feasible solutions, except one major problem is unsolved. The problem is the form factor that prevents ZAB from being made in cylindrical and prismatic forms as the commonly seen Li+ and Ni-MH batteries. The form factor is related to the air supply to the cathodes of ZABs when they are in cylindrical or prismatic form. 
     U.S. Pat. No. 5,569,551 discloses an electrically rechargeable ZAB using a Zn anode sandwiched between two air cathodes. While U.S. Pat. No. 5,554,452 reveals another electrically rechargeable ZAB module consisted of six-pack of batteries. Both works arranged the cathodes adjacent to air inlets. Because of the orientation, only two pairs of electrodes are allowed in one battery. In order to enhance the energy density as well as the power density, the resulted batteries are often bulky and heavy. U.S. Pat. No. 4,885,217 taught the use of a polymer web or net for constructing air cathodes. The net is sealed within the cathode to form an air pocket. Nevertheless, the spacers are rigid and the design can not generate cylindrical batteries. 
     The management of air of ZAB for a portable electronic device can be seen in U.S. Pat. No. 5,888,664. The air pathway is installed outside the batteries, and the module is bulky. The present invention devises an in-cell air management. When ZAB is configured in cylindrical or prismatic form, the air conduit can be constructed inside the battery similar to the water channel in U.S. Pat. No. 2,988,587. &#39;587 taught a water-activated magnesium reserve battery. The water channel for providing water to the cathodes is created by means of glass beads embedded on the cathodes. The present invention utilizes spacer sheets containing protrusions on forming air pathway during the fabrication of batteries. Micro fans are implemented in the middle of both end caps of batteries to create air draft in the pathway. The in-cell air management can effectively direct air to all layers of cathode. 
     SUMMARY OF THE INVENTION 
     The present invention provides an in-cell air management for metal-air batteries using the combination of: 
     1) spacer sheets with protrusions to form air pathway for the cathodes; and 
     2) micro pumps (fans) to create air draft in the pathway. 
     With the in-cell air management, metal-air batteries can be fabricated in cylindrical and prismatic forms. The gap-forming spacer sheets can be prepared via screen printing method or injection molding method of protrusion pattern on porous and thin substrates such as polypropylene, polyamide, polyethylene oxide, polyethylene terephthalate, polyacrylamide and polyurethane. As known to those skilled in the art, the protrusion materials may include epoxy, acetal, acrylic and urethane. If screen printing method is used, the protrusions are printed on the substrates and are subsequently cured by heat, radiation or moisture. While injection molding method can integrate the protrusions and spacer sheets in one step. 
     The spacer sheet is concentrically winded with other sheets of cell component such as electrodes and separator sheets to form the cylindrical batteries. The spacer sheet is placed with the protrusions (dots) against the cathode sheet. In making prismatic ZAB, the spacer sheet and cell-components sheets of the desired dimension are stacked in sequence with the protrusions against the cathode sheets. Because of the presence of protrusions, air pathway is formed by each layer of the cathode sheet. 
     Henceforth, it is thus an object of the present invention to provide air pathway for the cathodes of metal-air battery in close environments. The air pathway allows more pairs of electrodes per cell. The utilization of battery space is greatly enhanced. The energy density, power density, and working voltage of the resulted batteries are consequently improved. 
     It is another object of the present invention to impose micro pumps at both ends of metal-air batteries for creating air draft inside the batteries. Air is used by the cathodes of metal-air battery to generate electricity during discharge. The air pathway and micro pumps form the in-cell air management. 
     It is a further object of this invention to use the in-cell air management to fabricate metal-air battery in cylindrical and prismatic forms. The aforementioned shapes are the two most efficient configurations of batteries for portable electronic devices. With the in-cell air management, the form factor limitation on metal-air battery is lifted. 
     It is another further object of this invention to use the in-cell air management to control airflow in the batteries. As the air supply to the cathode is ceased, the chemical reactions of metal-air battery will stop. In theory, the battery can be preserved indefinitely. In other words, the in-cell air management is used to prolong the shelf life of metal-air battery. 
     It is a yet further object of this invention to devise an actuating system for the actuation of micro pumps. The actuating system includes a second cathode, which does not react on air, and an energy storage device such as supercapacitor. When a load is in request, metal anode works with the second cathode to provide nominal current to the supercapacitor. The latter amplifies the energy output to power the micro pumps to turn on metal-air battery. The in-cell actuating system is devised to impart metal-air battery a self-sustained breathing battery. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a schematic view of in-cell air management for cylindrical rechargeable ZAB in accordance with the instant invention; 
     FIG. 2 illustrates a cross-sectional side view of the cell core of FIG. 1; 
     FIG. 3 is a cross-sectional side view of the cell core according to another embodiment; 
     FIG. 4 is a schematic view of in-cell air management for prismatic rechargeable ZAB in accordance with the instant invention; and 
     FIG. 5 illustrates a cross-sectional side view of the cell core of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Cylindrical Metal-Air Battery 
     Referring now to FIG.  1  and FIG. 2, there is illustrated a preferred embodiment of cylindrical rechargeable metal-air battery according to this invention and a cross-sectional side view of the cell core  11  of FIG.  1 . The cylindrical metal-air battery  10  comprises: 
     a positive electrode  14  and a negative electrode (not shown) as the outputs of the battery  10 ; 
     a body, which includes: 
     a laminated compound material  12  including in the order of an insulating sheet  124 , a metal anode sheet  123 , a separator sheet  122 , a spacer sheet  121  having a plurality of protrusions such as dots  1211 , and a cathode sheet  125 , wherein the thickness of every sheet is preferably at ≦1 mm except the insulating sheet  124  which can be much thinner; 
     a cell core  11  formed by a roll of the laminated compound material  12  wrapped concentrically around the positive electrode  14 ; 
     a plurality of air pathways  13  are formed by the dots pressed against the cathode sheet  125 , and air becomes a cathode reactant which connects to the positive electrode  14  and the metal anode connects to the negative electrode; 
     a housing  17  made of metal or plastic for containing the cell core  11 ; 
     a first cap  18   a  and a second cap  18   b  which are on the opposite ends of the housing  17  axis for connecting the housing  17  to snugly seal the cell core  11 ; on the surface of both caps  18   a ,  18   b , air inlets  16  are provided for the passage of air and the positive electrode contact  19  of the battery  10  is placed at the center of the first cap  18   a;    
     a plurality of micro fans  15  for creating air draft in the air pathways  13  of the metal-air battery  10 , wherein the micro fans  15  are installed in the middle region of the first and second cap  18   a ,  18   b , respectively, of the battery  10 ; and 
     an in-cell actuating system (not shown) for activating the micro fans  15 . 
     For clearance of illustration, the in-cell actuating system, air filter and provision to prevent leakage of electrolyte is not shown. 
     The metal anode sheet  123  and cathode sheet  125  can be made by coating homogeneous pastes of the corresponding active materials with Teflon binders and solvents on current collectors such as perforated aluminum foil, perforated copper coil, copper mesh or nickel mesh. After the application of coatings, the sheets  123 , 125  can be dried in an oven to drive off the solvents. The sheets  123 ,  125  are then cut to the desired dimensions for assembly. The material of metal is applicable to zinc, aluminum, lithium, magnesium and iron. 
     The protrusions on the spacer sheet  121  can be prepared by screen printing method or injection molding method and the shape of protrusions can be dotted shape or rib  1212  (refer to FIG.  3 ). Protrusions and the spacer sheet  121  can be the same material or compatible materials. The materials of spacer sheets  121  may include polypropylene, polyamide, polyethylene oxide, polyethylene terephthalate, polyacrylamide and polyurethane. The materials of protrusions may include epoxy, acetal, acrylic and urethane. If screen printing method is used, the protrusions are printed on the spacer sheets  121  and are subsequently cured by heat, radiation or moisture. While injection molding method can integrate the protrusions and spacer sheets  121  in one step. 
     The size of protrusions effects the airflow and the energy capacity of battery  10 . The height of protrusions is preferably from 0.1 to 0.5 mm decided by a compromise between airflow and battery  10  capacity. The spacer sheets  121  can be purchased or prepared in-house to a thickness between 10 and 20 mil. Both protrusions and spacer sheets  121  should be inert towards the alkaline electrolyte used. The protrusions should also have adequate mechanical strength. 
     The function of the separator sheet  122  is to isolate the metal anode sheet  123  and the air cathode, and it also provides ionic conduction between the electrodes. The separator sheet  122  should hold in the electrolyte for long-term use. To those skilled in the art, the materials for the separator sheets  122  may include polyvinyl alcohol, cotton wool, cellulose and cellophane. The thickness of the separator sheet  122  is preferably ≦1 mm. 
     Lastly, the insulating sheet  124  is generally a PTFE film of 10-20 mil thick, and it is placed adjacent to the metal anode sheet  123  to prevent electric short. The positive electrode  14  such as metal lead is the contact for the cathode sheet to the outside circuit, while the contact for the metal anode sheet  123  is through a tab spot (not shown) welded to the metal anode sheet  123 . 
     Referring now to FIG. 3, is a cross-sectional side view of the cell core according to another embodiment. The laminated compound material  12   a  includes in the order of an insulating sheet  124   a , a metal anode sheet  123   a , a separator sheet  122   a , a spacer sheet  121   a  having a plurality of protrusions, and a cathode sheet  125   a  and the cell core  11   a  is formed by a roll of the laminated compound material  12   a  wrapped concentrically around the current collector  14   a . The difference between this embodiment and that of FIG. 2 is the shape of protrusions is rib  1212  and air pathways  13   a  are formed by the ribs  1212  and the cathode sheet  125   a.    
     Micro Pumps (Fans) 
     The micro pumps proposed in the instant invention can be fabricated by LIGA (German acronym for Lithographe, Galvanoformung, und Abformung) technique. The pumps may be composed of a bending element and at least one force element. The bending element may include single crystal silicon (such as silicon wafer) or an electroactive polymer (EAP) diaphragm. While the force element may include a piezoelectric crystal (such as zinc oxide), a magonestrictive alloy (such as terbium-dysprosium-iron), or a thermally dependent film (such as aluminum). The force element is attached to the bending element. When a voltage is applied to the force element, it will induce a shape change such as length of the element. The shape change of the force element will cause the bending diaphragm to flex inward or outward depending on the location of the force element. The flexing motion of the bending element will develop a flow of fluid such as air through a check valve of the micro pumps into a device which the pumps are attached. The intake of air depends on the voltage applied to the force element. If there is sufficient space inside a battery, a micro rotary fan or blower can be used as micro fans  15  to replace the bending diaphragm for drawing air into the battery  10 . Regardless of which design is selected, the mechanical unit is secured in the middle region of the first cap  18   a  and the second cap  18   b  of battery  10 . Furthermore, the micro fans  15  at the opposite sides of battery  10  are arranged to flex or spin in opposite direction so that air draft is created in the air pathways  13 . The flow rate of air is preferably greater than 10 cc. per minute, and the power consumption of micro fans  15  is preferably smaller than 0.5 W. 
     In-Cell Actuating System 
     To impart self-sustenance to metal-air battery  10  with the in-cell air management, an actuating system is devised inside the battery  10 . The in-cell actuating system is composed a power source, an energy converter, and a control network. Before air is drawn into metal-air battery  10 , the power for activating the micro fans  15  comes from cell reactions involving metal anode sheet  123  and a second cathode such as NiOOH, MnO 2  or AgO. The reaction of the second cathode does not require air, and it can be configured to work with metal anode sheet  123  to generate electricity repeatedly. In fabricating cylindrical metal-air battery  10 , a small section of the second cathode chosen can be integrated with the cathode sheet on the same sheet. On the other hand, a sheet of the second cathode can replace one of the cathode sheets in the stacks for forming prismatic metal-air battery  20  (refer to FIG.  4 ). In the hybrid battery, prior to the initiation of metal/air cathode, metal/second cathode can discharge independently to provide electricity required for powering the micro fans  15 . 
     The energy converter is a design using an energy storage device such as supercapacitor (also known as electric double layer capacitor, EDLC, or ultracapacitor). The preparation of supercapacitor is similar to that of battery. The active material for superacapacitor includes active carbon and metal oxide (such as RuO 2 , IrO 2 , Rh 2 O 3 , T a2 O 5 , V 2 O 5 , M o O 3 , TiO 2 , SnO 2 , NiO, F e2 O 3  or CoO x ). The supercapacitor depends on large specific surface area of the electrodes to store a large amount of charge (up to thousands Farad) which is classified as double layer capacitance or the so-called pseudo-capacitance. Because of the capability of quick release of all its stored charge, the supercapacitor has a much greater power density than batteries. AS a mater of fact, the supercapacitor is often used as load leveler for batteries. We discover that the supercapacitor can amplify the power of batteries by several times. During the fabrication of metal-air battery, one or several pairs of thin electrodes of the supercapacitor are enclosed. The same electrolyte, KOH, is used for both metal-air battery and supercapacitor. In this configuration, the battery electrodes (metal/second cathode) provide current to charge the supercapacitor through a control network. The on-board control network includes a sensor circuit, which monitors the demand of load, and an electronic on/off switch. When a load is in demand, the sensor circuit will issue a signal to turn on the switch, which then orders the charge of supercapacitor (charge can be completed in less than a second) and the delivery of power to activate the micro pumps (fans). As soon as a load is ceased, the sensor will terminate the current flow from the supercapacitor to the micro pumps (fans), and the battery is closed from atmosphere. 
     Prismatic Metal-Air Battery 
     FIG. 4 shows a preferred arrangement of in-cell air management for a prismatic rechargeable metal-air battery according to this invention. FIG. 5 illustrates a cross-sectional side view of the cell core  21  of FIG.  4 . The arrangement of cell components in FIG. 4 is slightly different from that in FIG.  1 . There are only four sheets of laminated compound material  22  to form stacks in FIG. 4 where the insulating sheet  124  is not required. However, a two-sided dotted spacer sheet  221  is sandwiched between two sheets of the cathode sheet  224 . Then, atop every cathode sheet  224  is the separator sheet  222  which is superimposed by the metal anode sheet  223 . The dots  2211  on the spacer sheets  221  are also pressed against the cathode sheets  224  to form air pathway  23 . Similar to that in FIG. 1, micro fans  24  are also installed in the middle region of the first and second caps  26   a ,  26   b  of battery  20  of FIG.  4 . When micro fans  24  are in motion, air will be drawn through air inlets  25  into the air pathways  23 . 
     To best illustrate the in-cell air management in prismatic metal-air batteries  20 , the body of the battery housing is not shown. Neither air filter, the in-cell actuating system for activating the micro fans  24 , nor provisions for preventing leakage of electrolyte are shown. Many sheets of the laminated compound material  22  can be stacked in the aforementioned sequence to form a prismatic metal-air battery  20  with multiple-pair of electrodes as shown in FIG.  4 . Each pair of +/− electrodes forms a unit cell. Depending on the need of applications, the cells can be connected internally in-series or in-parallel to construct the desired batteries. If a series connection is employed, the nominal working voltage of the resulted battery is the sum of voltage of each unit cell (about 1.22 V). On the other hand, if the cells are connected in-parallel, the resulted battery should have a total capacity equal to the sum of individual capacity. Therefore, the in-cell air management allows metal-air battery to be fabricated in cylindrical and prismatic forms, and multi-electrode metal-air battery is achieved. With more pairs of electrodes contained in a given battery package, both energy density and power density of the battery are enhanced. 
     Electrically Rechargeable Electrodes of ZAB 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Anode Formulation 
               
             
          
           
               
                   
                 Ingredients 
                 Weight (%) 
               
               
                   
                   
               
               
                   
                 ZnO 
                 40 
               
               
                   
                 Ca(OH) 2   
                 40 
               
               
                   
                 Polyvinyl alcohol 
                 10 
               
               
                   
                 H 2 O 
                 10 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Cathode Formulation 
               
             
          
           
               
                   
                 Ingredients 
                 Weight (%) 
               
               
                   
                   
               
               
                   
                 CaMnO 3   
                 67 
               
               
                   
                 Acetylene black 
                 21 
               
               
                   
                 Polytetraethylene solution 
                 12 
               
               
                   
                   
               
             
          
         
       
     
     TABLE I contains a preferred embodiment of electrically rechargeable formulation for the anode, while TABLE II for the cathode. The preparation process of the two electrodes is identical. First, the materials according to the formulation is charged and stirred in a homogenizing mixer for more than an hour. After mixing, the anode paste is roller-coated on a copper foil, while the cathode paste is on a copper mesh. After drying in an oven, the other side of each substrate is coated with the same anode or cathode paste. The coatings are also dried in oven. A dotted spacer sheet is made by screen printing of an UV-curable acrylic formulation on a thin polypropylene film. The openings of the screen ranges from 0.1 to 0.5 mm. The dots are then cured by UV radiation using a mercury lamp. A carboxylmethyl cellulose film of 1 mm thick is prepared as the separator sheet. The above sheets of cell components are cut to the desired dimensions, and cylindrical and prismatic batteries are prepared. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.