Abstract:
A metal-air battery is disclosed in one embodiment of the invention as including a cathode to reduce oxygen molecules and an alkali-metal-containing anode to oxidize the alkali metal (e.g., Li, Na, and K) contained therein to produce alkali-metal ions. An aqueous catholyte is placed in ionic communication with the cathode to store reaction products generated by reacting the alkali-metal ions with the oxygen containing anions. These reaction products are stored as solutes dissolved in the aqueous catholyte. An ion-selective membrane is interposed between the alkali-metal containing anode and the aqueous catholyte. The ion-selective membrane is designed to be conductive to the alkali-metal ions while being impermeable to the aqueous catholyte.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of to U.S. Provisional Patent No. 60/829,499 filed on Oct. 13, 2006 and entitled ADVANCED LITHIUM BATTERIES BASED ON CERAMIC MEMBRANE ELECTROLYTE. 
     
    
     U.S. GOVERNMENT INTEREST 
       [0002]    This invention was made with government support under Contract No. W91 QX-06-C-0092 awarded by the U.S. Army. This invention was also made with government support under Contract No. W9QX-06-C-0058 awarded by the U.S. Army. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates to batteries and more particularly to apparatus and methods for improving the performance of metal-air batteries. 
       DESCRIPTION OF THE RELATED ART 
       [0004]    Our society has come to rely on batteries to power a myriad of devices, including computers, cell phones, portable music players, lighting devices, as well as many other electronic components. Nevertheless, there is an ongoing need for further advances in battery technology. For example, there is currently a tremendous push to develop economical batteries that can be used by automobiles and other vehicles to reduce reliance on fossil fuels and reduce the output of carbon emissions. Furthermore, the “information age” increasingly demands portable energy sources that provide lighter weight, higher energy, longer discharge times, and smaller customized designs. To achieve these advances, technologists continue to work to develop batteries with higher and higher specific energies and energy densities while still providing acceptable safety, power densities, cost, and other needed characteristics. 
         [0005]    Metal-air batteries have many applications and advantages that give them a potential edge over many other types of batteries. Theoretically, for example, a lithium-air battery has a higher energy density than virtually all other types of practical battery technologies, including lithium-ion batteries. If lithium-air batteries can be improved significantly, they may provide dramatically improved specific energy, energy density and cost compared to many primary and secondary battery technologies, including lithium-ion secondary batteries. An improved lithium-air battery would be highly beneficial in military, consumer, and many other battery applications. 
         [0006]    Advantages of lithium-air batteries include their lower material and manufacturing costs compared to their lithium-ion counterparts. Lithium-air batteries may also be lighter weight, store more energy, have longer discharge times, and have a reduced size compared to conventional lithium-ion batteries. Because of these characteristics, lithium-air batteries (and other metal-air batteries) may have a unique opportunity in many battery markets. 
         [0007]    Conventional lithium-air batteries typically include a porous membrane (i.e., a lithium-ion conductive solid polymer electrolyte membrane) interposed between a lithium metal foil anode and a thin carbon composite cathode. The porous membrane permits a non-aqueous electrolyte (e.g., ethylene carbonate, propylene carbonate, etc.) to pass through the membrane while preventing the flow of electrons through the membrane. Oxygen, the electro-active cathode material in the battery, is reduced on the carbon composite cathode during discharge of the battery. Upon discharge, lithium ions and peroxide ions in the battery combine together to generate one or more reaction products (lithium hydroxide, lithium peroxide, etc.) at or near the cathode of the battery. 
         [0008]    Nevertheless, conventional lithium-air batteries, like many other battery technologies, are hindered by various limitations. The reaction products (i.e., lithium hydroxide, lithium peroxide, etc.) tend to clog up the pores of the cathode and limit the movement of reactants, significantly limiting the life of the battery. 
         [0009]    Unlike the non-aqueous air cathode that is not capable of attaining high current density, an air cathode in communication with an aqueous catholyte is capable of very high current density. For example, Gordon et al., in U.S. Pat. No. 7,259,126 disclosed an excellent oxygen cathode which when exposed to 50% NaOH and oxygen partial pressure of 0.85 atmospheres was able to attain 500 mA/cm 2  with only 350 mV polarization. Likewise, the manufacturer, Electric-Fuel shows on their website polarization data for their air cathode in 8.5M KOH and air where the air cathode had less than 600 mV polarization at 300 mA/cm 2 . 
         [0010]    In view of the foregoing, what is needed is an improved metal-air battery that is capable of overcoming one or more of the previously mentioned limitations. Specifically, a membrane is needed that will enable the utilization of an aqueous air cathode that will enable the utilization of a metal-air battery that will reduce or prevent the cathode from becoming clogged with various reaction products. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available metal-air batteries. 
         [0012]    Consistent with the foregoing and in accordance with the invention as embodied and broadly described herein, a metal-air battery is disclosed in one embodiment of the invention as including a cathode to reduce oxygen molecules and an anode to oxidize an alkali metal (e.g., Li, Na, or K) contained therein to produce alkali-metal ions. An aqueous catholyte is placed in electrical communication with the cathode. An ion-selective membrane is interposed between the alkali-metal-containing anode and the aqueous catholyte. The ion-selective membrane is designed to be conductive to the alkali-metal ions while being impermeable to the aqueous catholyte. 
         [0013]    In selected embodiments, the battery further includes an anolyte interposed between the alkali-metal-containing anode and the ion-selective membrane to conduct the alkali-metal ions from the anode to the ion-selective membrane. In certain embodiments, the aqueous catholyte is a separator material soaked in an aqueous catholyte solution. The aqueous catholyte may be used to store reaction products, such as alkali-metal hydroxides or oxides, and their respective hydrates produced at or near the cathode or in the catholyte. 
         [0014]    In certain embodiments, the ion-selective membrane may include a monolith of dense layer, a dense layer sandwiched between porous layers. The dense layer may be fabricated from an ionically conductive material, such as LiSICON, NaSICON, or other suitable ceramic appropriate to the anode material that is also impermeable to the aqueous catholyte. The porous layers, in selected embodiments, may provide structural integrity and rigidity to the dense layer. In certain embodiments, these porous layers may be infiltrated with the anolyte and catholyte, respectively, in order to provide ionic conductivity between the dense layer, and the anolyte and catholyte, respectively. 
         [0015]    In selected embodiments, the cathode may be a multi-layer structure. For example, in certain embodiments, the cathode may include a gas-diffusion layer that is hydrophobic but is permeable to oxygen. This may allow oxygen to enter the battery while preventing the aqueous catholyte from leaking out of the battery. A reaction layer may be placed adjacent to the gas-diffusion layer to reduce the oxygen molecules passing through the gas-diffusion layer. In selected embodiments, the reaction layer may contain or be infiltrated with a catalyst to aid in reducing the oxygen. In certain embodiments, this catalyst may include a perovskite-type compound. In other embodiments, this catalyst may include a manganese, cobalt and oxides thereof. In other embodiments, the reaction layer may also include an oxygen evolution catalyst to make the battery rechargeable. 
         [0016]    In another aspect of the invention, a method in accordance with the invention includes generating alkali-metal ions at an anode and transporting the alkali-metal ions through an ion-selective membrane that is impermeable to water. The method may further include transporting the alkali-metal ions through an aqueous catholyte solution. These alkali-metal ions may be reacted with oxygen or hydroxyl ions generated at a cathode to produce a reaction product, such as an alkali-metal hydroxide or oxide. These reaction products may then be stored as solutes dissolved in the aqueous catholyte solution and later precipitated over the course of discharge in the catholyte contained between separator and cathode. 
         [0017]    In yet another aspect of the invention, a metal-air battery in accordance with the invention includes a cathode to reduce oxygen molecules to produce peroxide or hydroxyl ions and an alkali-metal-containing anode to oxidize the alkali metal contained therein to produce alkali-metal ions. A catholyte solution is placed in electrical communication with the cathode to conduct the alkali-metal ions. An ion-selective membrane is interposed between the alkali-metal containing anode and the catholyte solution. This ion-selective membrane is conductive to the alkali-metal ions while being impermeable to the catholyte solution. The catholyte solution is designed to store, in the form of a dissolved solute, reaction products formed by reacting the peroxide or hydroxyl ions and the alkali-metal ions. 
         [0018]    The present invention provides an improved metal-air battery that overcomes various limitations of conventional metal-air batteries. The features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by practice of the invention as set forth hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through use of the accompanying drawings in which: 
           [0020]      FIG. 1  is a high-level block diagram showing one embodiment of a metal-air battery in accordance with the invention; 
           [0021]      FIG. 2  is a high-level block diagram showing one embodiment of a lithium-air battery in accordance with the invention; 
           [0022]      FIG. 3  is a high-level block diagram showing one embodiment of a sodium-air battery in accordance with the invention; 
           [0023]      FIG. 4  is a high-level block diagram showing one embodiment of a physical implementation of a metal-air battery in accordance with the invention; 
           [0024]      FIG. 5  is a top view of one embodiment of a metal-air battery in accordance with the invention in the form of a pouch cell; 
           [0025]      FIG. 6  is a side view of the pouch cell illustrated in  FIG. 5 ; 
           [0026]      FIG. 7  is a cutaway side view of the pouch cell illustrated in  FIG. 5 ; 
           [0027]      FIG. 8  is a graph showing the operating voltage of a sodium-air battery in accordance with the invention as a function of current density; 
           [0028]      FIG. 9  is a graph comparing the energy density of a 100 W-hr battery pack using a foil pouch cell design and a 100 W-hr battery pack using a titanium can cell design; and 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of certain examples of presently contemplated embodiments in accordance with the invention. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
         [0030]    Referring to  FIG. 1 , in general, a metal-air battery  100  in accordance with the invention that overcomes various problems of the prior art may include an anode  102 , a cathode  104 , an electrolyte separator/membrane  106 , and an aqueous catholyte layer  108  interposed between the electrolyte membrane  106  and the cathode  104 . Optionally, the battery  100  may also include a non-aqueous, organic anolyte layer  110  interposed between the anode  102  and the electrolyte membrane  106 . In other embodiments, the anolyte layer  110  may be omitted by placing the anode  102  in direct contact with the electrolyte membrane  106 . Current collectors  111 ,  113 , such as metal screens or meshes, may be placed in contact with or be embedded within the anode  102  and cathode  104 , respectively, to conduct electrical current to and from the anode  102  and cathode  104 . 
         [0031]    The anode  102  may contain an alkali metal (or alloys or compounds thereof) such as lithium, sodium, potassium, or the like, where it may be oxidized to produce alkali-metal ions upon discharge of the battery  100 . These alkali-metal ions may travel through the electrolyte separator/membrane  106  and the aqueous catholyte layer  108  until they reach the cathode  104 . At or near the cathode  104 , or in the catholyte  108 , the alkali-metal ions may combine with peroxide or hydroxyl ions, generated at the cathode  104 , where they may combine to form one or more reaction products, such as alkali-metal oxides or hydroxides. To prevent these reaction products from precipitating, building up, and clogging the pores of the reaction layer  114 , the aqueous catholyte  108  may be used to store the reaction products as solutes dissolved in the aqueous catholyte  108 . This is one significant advantage of using an aqueous catholyte  108  in combination with the impermeable electrolyte membrane  106 . By contrast, conventional metal-air batteries typically utilize non-aqueous organic electrolytes in which the reaction products are insoluble. Although the much higher solubility of these reaction products in aqueous electrolyte is beneficial over the prior art, there is nothing to prevent allowing the discharge of the anode to take place to the point where hydrates and precipitates of alkali metal oxides and hydroxides form in the catholyte. This may be desirable to achieve higher specific energy and energy density. 
         [0032]    In certain embodiments, the cathode  104  may include a gas-diffusion layer  112  and a reaction layer  114 . The gas-diffusion layer  112  may be gas-permeable but hydrophobic. Thus, the gas-diffusion layer  112  may allow oxygen gas to diffuse through the layer  112  in a direction  116 , while preventing the aqueous catholyte  108  from diffusing through the layer  112  in the opposite direction. The reaction layer  114 , on the other hand, may be permeable to the aqueous catholyte  108  (i.e., the layer  114  is hydrophilic) as well as oxygen passing through the gas-diffusion layer  112 . In selected embodiments, the reaction layer  114  may contain or be infiltrated with a catalyst material, such as a perovskite compound, or other material known by those skilled in the art to serve such purpose such as manganese, cobalt and oxides thereof, to aid in reducing the oxygen and/or forming hydroxyl ions. In other embodiments, the reaction layer  114  may also include an oxygen evolution catalyst to make the battery  100  rechargeable. 
         [0033]    Because the alkali metal of the anode  102  may react violently with water, and therefore be destroyed by water, the electrolyte membrane  106  may be a structure that is conductive to the alkali-metal ions but impermeable to the water of the catholyte layer  108 . In selected embodiments, the electrolyte membrane  106  may also be a multi-layer structure. For example, in certain embodiments, the electrolyte membrane  106  may include a thin (e.g., 20 to 300 μm) dense inner layer  118 , sandwiched between porous, lower density outer layers  120 . In certain embodiments, the dense layer  118  may form the actual membrane  106  that is ionically conductive but water impermeable. The porous layers  120 , on the other hand, may provide structural strength and rigidity to the dense layer  118  to keep it from breaking or cracking. In certain embodiments, the porous layers  120  may be infiltrated with the aqueous catholyte  108  and non-aqueous anolyte  110 , respectively, to provide ionic conductivity between the dense layer  118  and the catholyte  108  and anolyte  110  respectively. 
         [0034]    As mentioned, an alkali metal such as lithium or sodium metal may be used as the anode  102  with a metal current collector  113 , such as a copper mesh or screen, connected to or embedded within the anode  102 . In certain embodiments, the alkali-metal anode  102  may be placed in direct contact with the solid electrolyte membrane  106 . However, this may require verifying that the electrolyte membrane  106  is stable when in contact with the alkali-metal anode  102 . 
         [0035]    Alternatively, an ionically conductive liquid electrolyte (i.e., anolyte  110 ) which is conductive to the alkali metal at room temperature may be placed between the alkali metal anode  102  and the solid electrolyte membrane  106 . Such electrolytes may be made, for example, by mixing Lewis acid AlCl 3  with CH 3 SO 2 Cl/LiCl or with Cl 3 P═NPOCl 2 /LiCl, for a lithium-air battery, or with CH 3 SO 2 Cl/NaCl or with Cl 3 P═NPOCl 2 /NaCl for a sodium-air battery. These electrolytes typically have room temperature conductivities of approximately 1.0 to 2.5 Siemens/cm and exhibit electrochemical stability windows of 4.5 to 5.0 volts vs. Li+/Li or Na+/Na respectively. In the case of Li metal, greater than 60 cycles above 80 percent expected capacity has been demonstrated with a Li/LiMn 2 O 4  battery using this electrolyte. 
         [0036]    In other embodiments, the anolyte  110  may include organic liquids such as propylene carbonate, dioxolane, ethylene carbonate tetraethyleneglycol dimethylether (Tetraglyme), or room-temperature ionic-liquid, 1-hexyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide(C 6 mimBeti) or mixtures thereof. The anolyte  110  may also include various lithium or sodium salt(s) and a solvent such as dimethyl carbonate (DMC), diethylcarbonate (DEC), dipropylcarbonate (DPC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, lactones, esters, glymes, sulfoxides, sulfolanes, lithium perchlorate, polyethylene oxide (PEO) or polyacrylnitrile (PAN) with lithium trifluoromethylsulfonate, lithium hexafluorophosphate, or mixtures thereof. 
         [0037]    As mentioned, the cathode  104  may include a gas-diffusion layer  112  and a reaction layer  114 . One such cathode  104 , for example, is described in U.S. Pat. No. 7,259,126 entitled “Gas Diffusion Electrode and Catalyst for Electrochemical Oxygen Reduction and Method of Dispersing the Catalyst,” which is herein incorporated by reference. In certain embodiments, the reaction layer  114  may include Teflon, an oxygen reduction perovskite electrocatalyst, and hydrophilic carbon. In certain embodiments, the electrocatalyst of the reaction layer  114  may include a NdCaMnFeO 3  or La 0.6 Ca 0.4 CoO 3  perovskite. These electrocatalysts enable facile one-step reduction of oxygen to hydroxyl ions and exhibit negligible overpotential even at high cathodic current densities. The gas-diffusion layer  112 , on the other hand, may contain a higher amount of Teflon and hydrophobic carbon. In certain embodiments, the gas-diffusion layer  112  may be produced by hot pressing Teflon and hydrophobic carbon powders onto a metal screen, such as a nickel screen. 
         [0038]    In certain embodiments, the catholyte layer  108  may include a separator material, such as a cellulose separator material, soaked in an aqueous electrolyte that is conductive to the alkali metal ions produced at the anode  102 . For example, the catholyte layer  108  of a sodium-air battery may include a cellulose separator material soaked in aqueous NaOH. Similarly, the catholyte layer  108  of a lithium-air battery may include a cellulose separator material soaked in aqueous LiOH. In general, the catholyte layer  108  may include absorbent materials soaked in aqueous solutions containing alkali-metal salts such as dissolved alkali-metal hydroxide, chloride, carbonate, perchlorate, hexafluorophosphate, trifluoromethylsulfonate, nitrate, or the like. The catholyte solvent, in selected embodiments, may include aqueous mixtures of ethanol, methanol, propanol, isopropanol, and the like. 
         [0039]    As mentioned, the electrolyte membrane  106  may, in certain embodiments, include a NaSICON or LiSICON solid electrolyte membrane  106  (depending on the anode material) that is ionically conductive but impermeable to water. Most types of NaSICON and LiSICON are impermeable to water if they are fabricated with sufficient density. Ideally, the NaSICON or LiSICON is selected to maximize ionic conductivity. In certain embodiments, a suitable NaSICON membrane  106  may be produced by Ceramatec, Inc. of Salt Lake City, Utah. The Ceramatec NaSICON membrane  106  is based on a sodium-metal-phosphate structure (Na 1+x Zr 2 X y (PO 4 ) 3 ) wherein x is between 0 and 3 and X is a proprietary dopant. The Ceramatec NaSICON offers excellent moderate temperature Na +  ion conductivity of approximately 9×10 −2  Siemens/cm at 60° C. These NaSICON membranes  106  are excellent sodium-ion conductive and chemically stable and were originally developed for salt splitting and electrodialysis applications. 
         [0040]    The NaSICON membrane  106  described above can be formed with a thickness of less than 10 μm using a tape casting approach, which is much thinner than conventional polymer or liquid-filled porous separator membranes. An ultra-thin electrolyte membrane  106  may provide considerable savings in terms of volume and mass of the battery  100 , if of course the reduced thickness is not offset by a need for a thick inactive support material. Another advantage of using this type of membrane  106  is that the materials used to produce solid inorganic electrolytes are generally single ionic conductors. This may reduce or eliminate anionic concentration gradient across the electrolyte membrane  106  and may help to suppress undesirable side reactions or decomposition of the electrolyte. This may also expand the available choices for electrode materials and permit the battery  100  to operate at higher voltages. 
         [0041]    A solid electrolyte membrane  106  further has the advantage that it provides a dense, hard surface that can maintain its integrity and inhibit roughening of the microstructure. This may reduce mossy or dendritic deposits that may increase the resistivity of the membrane  106  over time. A NaSICON membrane  106  also is not influenced by scaling or precipitation of divalent, trivalent, tetravalent ions or dissolved solids and does not degrade in the presence of corrosive elements like sodium. The NaSICON membrane  106  can is best operated over pH range from neutral to caustic (e.g., 7 to 14) and selectively transport sodium ions in the presence of other ions at very high electrical efficiency (greater than 95 percent). 
         [0042]    Where a lithium ion conductor is required to produce a lithium-air battery, the electrolyte membrane  106  may include a material having the formula Li 1+x M x Ti 2−x (PO 4 ) 3  where 0≦x≦1.5 and where M is Al, Zr, Sn, Hf, Y, or mixtures thereof. In other embodiments, the electrolyte membrane  106  may include a material having the formula Li 1+x M x M″ 2−x (PO 4 ) 3−y (M′O 4 ) y  where 0≦x≦1.5; 0≦y≦3; M is Al, Y, Ti, or mixtures thereof; M″ is Zr, Sn, Si, Ti, Hf; and M′ is Si, Nb, or Ge, or mixtures thereof. In other embodiments, an ionically conductive ceramic membrane  106  may include a material having the formula Li 1+x Zr 2 Si x P 3−x O 12  where 0≦x≦3. In yet other embodiments, an ionically conductive ceramic membrane  106  may include a non-stoichiometric lithium-deficient material having the formula (Li 5 RESi 4 O 12 ) 1−δ (RE 2 O 3 2SiO 2 ) δ , where RE is Y, Nd, Dy, Sm, or a mixture thereof and where δ is the measure of deviation from stoichiometry, which may vary between about 0 and 1. 
         [0043]    Referring to  FIG. 2 , one example of a metal-air battery  100  using lithium as the anode material is illustrated. As shown, lithium may be reduced at the anode  102  to produce lithium ions. These lithium ions may travel through the anolyte  110 , electrolyte membrane  106 , and catholyte  108 , until they reach the cathode  104 . Meanwhile, at the cathode  104 , oxygen may pass through the gas-diffusion layer  112  to the reaction layer  114 , where electrons, oxygen, and water may react to generate peroxide and hydroxyl ions. At the reaction layer  114 , or in the catholyte  108 , the peroxide and hydroxyl ions may react with the lithium ions to form one or more of lithium hydroxide (as illustrated by the chemical reactions on the left) and lithium peroxide (as illustrated by the chemical reactions on the right). 
         [0044]    In general, lithium hydroxide may be generated in the lithium-air battery  100  according to the following equations: 
         [0000]      Anode/organic anolyte: Li→Li +   +e   −   (1) 
         [0000]      Cathode/aqueous catholyte: 0.5H 2 O+0.25O 2   +e   − →  (2) 
         [0000]      Cathode/aqueous catholyte: Li + OH − →LiOH  (3) 
         [0045]    Similarly, lithium peroxide may be generated in the lithium-air battery  100  according to the following equations: 
         [0000]      Anode/organic anolyte: Li→Li +   +e   −   (1) 
         [0000]      Cathode/aqueous catholyte: O 2 +2 e   − →O 2   2−   (2) 
         [0000]      Cathode/aqueous catholyte: 2Li + +2O 2   2− →Li 2 O 2   (3) 
         [0046]    As mentioned above, because the metal-air battery  100  uses an aqueous catholyte  108 , the lithium hydroxide and lithium peroxide generated according to the above equations may be stored as solutes dissolved in the aqueous catholyte  108  to a large extent before starting to form hydrates. 
         [0047]    Referring to  FIG. 3 , another example of a metal-air battery  100  using sodium as the anode material is illustrated. As shown, sodium in the anode  102  may be reduced to produce sodium ions. These ions may be conducted through the anolyte  110 , electrolyte membrane  106 , and catholyte  108 , until they reach the cathode  104 . Meanwhile, at the cathode  104 , oxygen may pass through the gas-diffusion layer  112  to the reaction layer  114 , where electrons, oxygen, and water may react to form peroxide and/or hydroxyl ions. At the reaction layer  114 , or in the catholyte  108 , the peroxide and/or hydroxyl ions may react with the sodium ions to form sodium hydroxide. 
         [0048]    In general, sodium hydroxide may be generated in the sodium-air battery  100  according to the following equations: 
         [0000]      Anode/organic anolyte: Na→Na +   +e   −   (1) 
         [0000]      Cathode/aqueous catholyte: 0.5H 2 O+0.25O 2   +e   − →OH −   (2) 
         [0000]      Cathode/aqueous catholyte: Na + +OH − NaOH  (3) 
         [0049]    The sodium hydroxide generated above may be stored as a solute dissolved in the aqueous catholyte  108  to a large extent prior to forming hydrates. 
         [0050]    Although lithium may theoretically produce a battery  100  with significantly higher energy density than sodium, sodium may become a desirable choice because of its higher solubility in water. For example, as indicated by Table I below, lithium may have a free energy of reaction and theoretical voltage that is higher than sodium. Because lithium is much lighter than sodium, lithium by itself has a much higher energy density than sodium (13308 W-hr/kg for lithium compared to 3632 W-hr/kg for sodium, a difference of 9676 W-hr/kg). 
         [0051]    Nevertheless, when considering the water that is consumed in the reaction, and then needed to maintain performance of the cathode, the overall performance of the sodium-air battery begins to approach or exceed the performance of the lithium-air battery. For example, once water is taken into account, the specific energy of the sodium-air battery is 2609 W-hr/kg and the specific energy of the lithium-air battery is 5792 W-hr/kg, a difference of 3183 W-hr/kg. When the oxygen consumed by the reaction is considered, the specific energy of the sodium-air battery is 2088 W-hr/kg and the specific energy of the lithium-air battery is 3857 W-hr/kg, a difference of only 1769 W-hr/kg. 
         [0052]    The sodium-air battery significantly outperforms the lithium-air battery when the water needed to dissolve the reaction products is taken into account. As shown in Table I, the solubility of sodium hydroxide (i.e., 103) in water is far greater than the solubility of lithium hydroxide (i.e., 12.8). As a result, significantly less water is needed in the aqueous catholyte  108  of the sodium-air battery compared to the lithium-air battery to dissolve the reaction products. When considering the added weight of the water, the energy density of the sodium-air battery (1304 W-hr/kg) is significantly greater than the energy density of the lithium-air battery (455 W-hr/kg). 
         [0053]    In practice, both systems can be operated to higher specific energies by allowing some alkali hydroxide hydrate to form which can be tolerated to some extent while still maintaining cathode performance. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Expected Energy Density Calculations for Li/O 2  and Na/O 2  batteries 
               
             
          
           
               
                 Characteristics 
                 Li/O 2  Battery 
                 Na/O 2  Battery 
               
               
                   
               
             
          
           
               
                 Free energy of reaction (ΔGr) 
                 −79.5 
                 −71.8 
               
               
                 Number of electrons (n) = 
                 1 
                 1 
               
               
                 Voltage (V) = 
                 3.4 
                 3.1 
               
               
                 M (W-hr/kg) 
                 13308 
                 3632 
               
               
                 M + 0.5H 2 O (W-hr/kg) 
                 5792 
                 2609 
               
               
                 M + 0.5H 2 O + 0.25O 2  (W-hr/kg) 
                 3857 
                 2088 
               
               
                 Solubility of MOH in water at 
                 12.8 
                 103 
               
               
                 20 C (g/100 cc) 
               
               
                 M + 0.5H 2 O + 0.25O 2  + Water 
                 438 
                 1172 
               
               
                 for Dissolution (W-hr/kg) 
               
               
                 M + 0.5H 2 O + Water for Dissolution 
                 455 
                 1304 
               
               
                 (W-hr/kg) 
               
               
                   
               
             
          
         
       
     
         [0054]    Referring to  FIG. 4 , in certain embodiments, a physical implementation of a metal-air battery  100  in accordance with the invention may include an electrically conductive housing  400   a ,  400   b  divided into two electrically isolated halves  400   a ,  400   b , such as stainless steel halves  400   a ,  400   b . One half  400   b  may contain the alkali-metal anode  102 , a current collector  113  (e.g., a copper screen) connected to or embedded within the anode  102 , and the anolyte layer  110 . The other half  400   a  may contain the cathode  104 , a current collector  111  (e.g., a nickel screen) connected to or embedded within the cathode  104 , and the catholyte layer  108 . In selected embodiments, the anolyte layer  110  is a separator material, such as a polypropylene non-woven separator, soaked in an anolyte solution. Similarly, the catholyte layer  108  may include a separator soaked in a catholyte solution, such as a cellulose separator soaked in a metal hydroxide solution. 
         [0055]    In certain embodiments, the electrolyte membrane  106  may be sandwiched between the two halves  400   a ,  400   b  to seal and isolate the anolyte and catholyte compartments and to electrically isolate the first half  400   a  from the second half  400   b . In certain embodiments, a plastic or elastomeric grommet or other suitable material may be used to seal the two halves  400   a ,  400   b  to the electrolyte membrane  106 . An electrically insulating clamping device  404 , such as a clip, band, crimp, or the like, may be used to clamp the halves  400   a ,  400   b  to the membrane layer  106  and hold the halves  400   a ,  400   b  together. In certain embodiments, openings  402  may be formed in the half  400   a  to allow oxygen to flow to the cathode  104  from the surrounding environment. 
         [0056]    Referring to  FIGS. 5 through 7 , in selected embodiments in accordance with the invention, the battery  100  may be designed in the form of a pouch cell  500 . The pouch cell  500  may achieve higher energy densities than ordinary cells by efficiently using space. The pouch cell  500  may also facilitate stacking and/or wiring the cells  500  in serial or parallel configurations to provide a battery  100  with desired voltage and current characteristics. Although the illustrated pouch cell  500  has a circular shape, the pouch cell  500  may also be designed to have a rectangular shape to maximize space utilization in a rectangular housing. 
         [0057]    As shown, in selected embodiments, the pouch cell  500  may include an electrically insulating outer shell or housing  501   a ,  501   b  such as a polyethylene housing  501   a ,  501   b . Like the previous example, the housing  501   a ,  501   b  may, in selected embodiments, be divided into two halves  501   a ,  501   b , with one half  501   a  housing the catholyte  108  and the other half  501   b  housing the anode  102  and the anolyte layer  110 . The electrolyte membrane  106 , which in this example includes a dense layer  118  sandwiched between two porous layers  120 , may separate the catholyte and anolyte compartments  108 ,  110 . In certain embodiments, a series of standoffs  512  may be used to create space between the anode  102  and the membrane  106  to accommodate the anolyte  110 . 
         [0058]    In selected embodiments, an electrically insulating support ring  502 , or clamp  502 , such as a polyethylene ring, may be bonded and sealed to an outer circumference of the membrane  106 . This support ring  502  may then be clamped, bonded, and sealed to flanges  504   a ,  504   b  of the housing  501   a ,  501   b  to provide an effective seal with the membrane  106  and seal the catholyte and anolyte compartments  108 ,  110 . Similarly, in selected embodiments, a support ring  506 , or clamp  506 , such as a polyethylene ring, may be positioned and bonded to an outer circumference of the cathode  104 . This ring  506  may also be bonded to the top of the housing  501   a.    
         [0059]    A bottom side of the cathode  104  may communicate with the catholyte  108  through an opening  508  in the housing  501   a  and a top side of the cathode  104  may communicate with an oxygen source. Electrically conductive tabs  510   a ,  510   b  may be electrically connected to current collectors  111 ,  113  (not shown) which may be connected to or embedded within the anode  102  and cathode  104 , respectively. 
         [0060]    Referring to  FIG. 8 , a polarization curve  800  showing the operating voltage of one embodiment of a non-optimized, sodium-air pouch cell, as a function of current density, is illustrated. In this example, the cell utilizes a dense NaSICON electrolyte membrane  118  with a thickness of approximately 380 μm. Ideally, the thickness of the membrane  118  and the volume of the catholyte  108  will be further reduced to decrease the polarization of the cell. This will flatten out the curve  800  and provide improved current/voltage characteristics. 
         [0061]    As shown, the polarization curve  800  of the sodium-air cell  100  exhibits a predominantly linear region when the current density is between about 0 and 4 mA/cm 2 , while delivering between about 2.7 and 1.5 volts. The voltage drops off significantly after the current density exceeds 4 mA/cm 2 . Thus, the cell is only viable at current densities that are less than about 4 mA/cm 2 ; however, with further optimization, much higher current densities are expected to be viable and with desirable voltage potential. 
         [0062]    Referring to  FIG. 9 , a graph comparing the energy density of a 100 W-hr battery pack using a foil pouch cell and a 100 W-hr battery pack using a titanium can cell is illustrated. The energy density of each type of battery is calculated as a function of the number of cells in the battery pack. As shown, the battery pack that uses the pouch cells has a higher energy density than the battery pack that uses the titanium cans for any number of cells in the battery pack. This is due to the pouch cell&#39;s more efficient use of space and materials. Nevertheless, the energy density differential may increase significantly as the number of cells in the battery pack increases. For example, the energy density of the battery pack using the pouch cells exceeds the energy density of the battery pack using the titanium cans by only about 20 percent where there are few cells (e.g., 0 to 10 cells). However, the energy density of the battery pack using the pouch cells exceeds the energy density of the battery pack using the titanium cans by approximately 50 percent when the number of cells approaches 50 cells. Thus, it becomes increasingly advantageous to use a foil pouch cell design when multiple cells are wired together. 
         [0063]    The present invention may be embodied in other specific forms without departing from its basic principles or essential characteristics. The described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.