Abstract:
An electrode structure and an electrochemical cell including the electrode structure are provided. The electrode structure includes a porous three-dimensional (3D) outer net including an interconnected plurality of outer metal lines that define a plurality of outer holes between adjacent ones of the outer metal lines. The outer metal lines include a porous 3D inner net, a first layer coating the inner net, and a second layer coating the first layer. The inner net includes an interconnected plurality of inner metal lines that define a plurality of inner holes between adjacent ones of the inner metal lines. The inner metal lines include a first metal. The first layer includes a second metal. The second layer includes a third metal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/442,654, filed on Feb. 14, 2011, in the United States Patent and Trademark Office, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Aspects of embodiments of the present invention relate to an electrode structure and an electrochemical cell using the electrode structure. 
         [0004]    2. Description of Related Art 
         [0005]    Research into sodium-based electrochemical cells for storing electric power that is generated for household use, electric power that is generated by solar power, and electric power that is generated by wind power and for supplying electric power to electric vehicles is continuing. Electrochemical cells, such as a sodium-chloride nickel cell and a sodium-sulfur (NaS) cell, are large-capacity cells that store a few kilowatts (kW) to a few megawatts (MW) of electric power and have high-energy densities and a long lifetime (life span). Due to the characteristics, they are used in a wide range of applications. 
       SUMMARY 
       [0006]    Aspects of embodiments according to the present invention are directed toward an electrode structure and an electrochemical cell using the electrode structure. 
         [0007]    In an exemplary embodiment according to the present invention, an electrode structure is provided. The electrode structure includes a porous three-dimensional (3D) outer net including an interconnected plurality of outer metal lines that define a plurality of outer holes between adjacent ones of the outer metal lines. The outer metal lines include a porous 3D inner net, a first layer coating the inner net, and a second layer coating the first layer. The inner net includes an interconnected plurality of inner metal lines that define a plurality of inner holes between adjacent ones of the inner metal lines. The inner metal lines include a first metal. The first layer includes a second metal. The second layer includes a third metal. 
         [0008]    The outer holes may average about 300 μm or smaller in diameter. 
         [0009]    The inner holes may average about 400 μm or larger in diameter. 
         [0010]    The electrode structure may further include a current collector for moving electrons between the outer net and an external circuit. 
         [0011]    The current collector may be sintered to the inner net. 
         [0012]    Each of the current collector and the first metal may include copper (Cu). 
         [0013]    The first metal may include copper (Cu) or iron (Fe). 
         [0014]    The second metal may have a lower standard electric potential than that of the third metal. 
         [0015]    The second metal may have a higher ionization tendency than that of the first metal. 
         [0016]    The second metal may have a higher ionization tendency than that of the first metal. 
         [0017]    The second metal may include zinc (Zn), tin (Sn), titanium (Ti), or chromium (Cr). 
         [0018]    The second metal may include Zn. 
         [0019]    The third metal may include nickel (Ni). 
         [0020]    The third metal may further include iron (Fe). 
         [0021]    The third metal may further include about 40% to about 70% Ni by weight of the third metal. 
         [0022]    The first metal may include copper (Cu) and the second metal may include zinc (Zn). 
         [0023]    In another exemplary embodiment according to the present invention, an electrochemical cell is provided. The electrochemical cell includes a housing, a first chamber in the housing and including an electrode material, a second chamber in the housing and including an electrode structure, and a solid electrolyte separating the first chamber from the second chamber. The electrode structure includes a porous three-dimensional (3D) outer net. The porous 3D outer net includes an interconnected plurality of outer metal lines that define a plurality of outer holes between adjacent ones of the outer metal lines. The outer metal lines include a porous 3D inner net, a first layer coating the inner net, and a second layer coating the first layer. The porous 3D inner net includes an interconnected plurality of inner metal lines that define a plurality of inner holes between adjacent ones of the inner metal lines. The inner metal lines include a first metal. The first layer includes a second metal. The second layer includes a third metal. 
         [0024]    The electrochemical cell may further include an electron conductor between the electrode structure and the solid electrolyte. 
         [0025]    The second metal may have a lower standard electric potential than that of the third metal and a higher ionization tendency than that of the first metal. 
         [0026]    The first metal may include copper (Cu), and the third metal may include nickel (Ni). 
         [0027]    The electrode structure may substantially fill the second chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a schematic vertical sectional view of an electrochemical cell according to an embodiment of the present invention; 
           [0029]      FIG. 2  is a side perspective view of a cathode structure, an electron conductor, and a solid electrolyte of the electrochemical cell of  FIG. 1 ; 
           [0030]      FIG. 3  is a schematic vertical sectional view of an electrochemical cell according to another embodiment of the present invention; 
           [0031]      FIG. 4  is an enlarged scanning electron microscope (SEM) photo of the IV region of  FIG. 1 ; 
           [0032]      FIGS. 5A and 5B  are cross-sectional views taken along the line V-V of  FIG. 4 ; 
           [0033]      FIG. 6  is a flowchart illustrating a method of manufacturing a cathode structure according to an embodiment of the present invention; 
           [0034]      FIG. 7A  is a perspective view of a first metal structure having a three-dimensional (3D) net structure of operation S 610  of  FIG. 6  according to an embodiment of the present invention; 
           [0035]      FIG. 7B  is a perspective view of a first metal structure having a 3D net structure of operation S 610  of  FIG. 6  according to another embodiment of the present invention; 
           [0036]      FIG. 7C  is an enlarged SEM photo of a part of a first metal of  FIGS. 7A and 7B ; 
           [0037]      FIG. 8  is a schematic cross-sectional view of an electrochemical cell according to another embodiment of the present invention; and 
           [0038]      FIG. 9  is a graph of a relationship between a charge and discharge cycle and a resistance of an electrochemical cell in a simulation result of the electrochemical cell according to an embodiment of the present invention. 
       
    
    
     EXPLANATION OF REFERENCE NUMERALS OF SOME OF THE ELEMENTS OF THE DRAWINGS 
       [0039]      
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 1, 1′: electrochemical cell 
                 110, 810: housing 
               
               
                 111, 811 anode material 
                 115, 815: wick 
               
               
                 120, 820: solid electrolyte 
               
               
                 135, 835: liquid electrolyte 
                 130, 830: cathode structure 
               
               
                 140, 140′: current collector 
                 141: lead line 
               
               
                 150, 860: electron conductor (carbon felt) 
               
               
                 160, 860: insulator 
                 170, 870: glass frit 
               
               
                 300: metal lines included in a cathode 
               
               
                 structure (outer net) 
               
               
                 310: first metal (inner net) 
               
               
                 320: second metal layer 
                 330: third metal layer 
               
               
                 C1, C1′: first chamber (anode chamber) 
               
               
                 C2, C2′: second chamber (cathode chamber) 
               
               
                 S: first metal structure 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
       [0040]    Aspects and characteristics of the present invention, and methods for accomplishing them may be apparent to one of ordinary skill in the art in view of embodiments described in detail with reference to the attached drawings. However, the present invention is not limited to the following embodiments, and may have different forms and should not be construed as being limited to the descriptions set forth herein. While this invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. 
         [0041]    As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated elements, steps, operations, and/or devices, but do not preclude the presence or addition of one or more other elements, steps, operations, and/or devices. It will also be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Hereinafter, for ease of understanding, like elements are denoted by like reference numerals. 
         [0042]      FIG. 1  is a schematic vertical sectional view of an electrochemical cell  1  according to an embodiment of the present invention.  FIG. 2  is a side perspective view of a cathode structure  130 , an electron conductor  150 , and a solid electrolyte  120  of the electrochemical cell  1  of  FIG. 1 . 
         [0043]    Referring to  FIG. 1 , the electrochemical cell  1  includes a housing  110 , the solid electrolyte  120  for dividing an inner portion of the housing  110  into a first chamber C 1  and a second chamber C 2 , an anode material (or anode electrode material)  111  included in the first chamber C 1 , and the porous cathode structure  130  included in the second chamber C 2 . 
         [0044]    The first chamber C 1  may be an anode chamber and may include the anode material  111 . The anode material  111  may be an alkali metal such as sodium. The sodium may be dissolved and thus present in a liquid phase. Besides sodium, the anode material may also be any other suitable alkali metal such as lithium or potassium. 
         [0045]    The first chamber C 1  may include a wick  115 . The wick  115  is disposed on an outer surface of the solid electrolyte  120  and induces a capillary phenomenon. Therefore, even when the first chamber C 1  is not completely filled with sodium, the outer surface of the solid electrolyte  120  may be surrounded by sodium according to the capillary phenomenon. 
         [0046]    When the first chamber C 1  is the anode chamber, the second chamber C 2  is a cathode chamber and may include the cathode structure  130 . The cathode structure  130  may include nickel (Ni) that is a cathode material. The cathode structure  130  is a porous metal body and has a three-dimensional (3D) net structure. For example, the cathode structure  130  may include a first metal such as copper (Cu), a second metal layer coated on the first metal, and a third metal layer coated on the second metal layer. The second metal layer and the third metal layer may be uniformly coated on the first metal. 
         [0047]    The first metal may use (or be made of) copper (Cu) that is relatively inexpensive and has excellent electron conductivity. In other embodiments, the first metal may use (or be made of) iron (Fe) or iron (Fe) in addition to copper (Cu). 
         [0048]    The third metal layer includes a cathode material of the electrochemical cell  1 . The third metal layer may include nickel (Ni). For example, the third metal layer may include (or be made of) nickel (Ni) or an alloy of nickel (Ni) and iron (Fe). 
         [0049]    The second metal layer includes a metal having a low or lower standard electric potential compared to the third metal layer and having a high or higher ionization tendency compared to the first metal. For example, when the third metal layer includes nickel (Ni), the second metal layer may use (or be made of) a metal, such as zinc (Zn), tin (Sn), titanium (Ti), or chromium (Cr), which has a discharge electric potential lower than that of nickel (Ni) in a liquid electrolyte, or a compound thereof. In addition, ionization tendencies of these metals are higher than that of copper (Cu) as the first metal. 
         [0050]    According to an embodiment of the present invention, the first metal may use copper (Cu), the second metal layer may use titanium (Ti), and the third metal layer may use nickel (Ni). In this case, titanium (Ti) may have a thickness of about 2 μm to about 10 μm (or of 2 μm to 10 μm), and nickel (Ni) may have a thickness of about 5 μm to about 50 μm (or of 5 μm to 50 μm). According to another embodiment of the present invention, the first metal may use copper (Cu), the second metal layer may use zinc (Zn), and the third metal layer may use an alloy of iron (Fe) and nickel (Ni). In this case, zinc (Zn) may have a thickness of about 2 μm to about 10 μm (or of 2 μm to 10 μm), and the alloy of iron (Fe) and nickel (Ni) may have a thickness of about 10 μm to about 80 μm (or of 10 μm to 80 μm). Nickel (Ni) that is a cathode material may occupy about 40% to about 70% (or occupy 40% to 70%) by weight of the alloy of iron (Fe) and nickel (Ni). According to another embodiment of the present invention, the first metal may use copper (Cu), the second metal layer may use tungsten (W), and the third metal layer may use nickel (Ni). 
         [0051]    The cathode structure  130  will be described in more detail with reference to  FIGS. 4 and 5  below. 
         [0052]    The second chamber C 2  may include a liquid electrolyte  135  such as NaAlCl 4 , in addition to the cathode structure  130 . The cathode structure  130  may be deposited in the liquid electrolyte  135 . 
         [0053]    A current collector  140  is included in the second chamber C 2  and allows electrons generated in charge and discharge operations to easily move. For example, the current collector  140  may allow electrons to easily move between the electrochemical cell  1  and an external circuit. The current collector  140  helps electrons to move from the external circuit to the second chamber C 2  during charging of the electrochemical cell  1 , and helps electrons to move from the second chamber C 2  to the external circuit during discharging of the electrochemical cell  1 . 
         [0054]    The current collector  140  includes an electric conductive material such as copper (Cu). For example, one end  140   a  of the current collector  140  may be disposed penetrating the center of the cathode structure  130 , and another end  140   b  thereof may be exposed outside the electrochemical cell  1 . 
         [0055]    The electron conductor  150  may be included in the solid electrolyte  120 . For example, the electron conductor  150  may be disposed between the cathode structure  130  and the inner surface of the solid electrolyte  120 . The electron conductor  150  allows electrons to easily move in the second chamber C 2 . The electron conductor  150  may include a carbon-based material. For example, the electron conductor  150  may be formed of carbon felt. 
         [0056]    The solid electrolyte  120  may allow ions to flow therethrough. Alkali ions that are generated during charging and discharging (for example, sodium ions) may flow from the first chamber C 1  to the second chamber C 2  through the solid electrolyte  120 , or from the second chamber C 2  to the first chamber C 1  through the solid electrolyte  120 . The solid electrolyte  120  may have a tube-shape having an open side (or end) and a closed side (or end), and may be disposed inside the housing  110 . 
         [0057]    The solid electrolyte  120  may include a β-alumina based material. For example, the solid electrolyte  120  may include β-alumina or β″-alumina. 
         [0058]    The insulator  160  may electrically insulate the first chamber C 1  from the second chamber C 2 . The insulator  160  may be joined to a side of the solid electrolyte  120  via an adhesive material, for example, a glass frit  170 . The insulator  160  may include α-alumina. 
         [0059]    In the present embodiment, the first chamber C 1  that is the anode chamber is included in the electrochemical cell  1 , and the second chamber C 2  that is the cathode chamber is disposed inside the first chamber C 1 . However, the present invention is not limited thereto. For example, the first chamber C 1  may be disposed inside the second chamber C 2 , and the second chamber C 2  may be included in the electrochemical cell  1 . 
         [0060]      FIG. 3  is a schematic vertical sectional view of the electrochemical cell  1  according to another embodiment of the present invention. 
         [0061]    Referring to  FIG. 3 , the electrochemical cell  1  includes the housing  110 , the solid electrolyte  120  for dividing an inner portion of the housing  110  into the first chamber C 1  and the second chamber C 2 , the anode material  111  included in the first chamber C 1 , and the porous cathode structure  130  included in the second chamber C 2 . 
         [0062]    The cathode structure  130  is a porous metal body and has a 3D net structure. The cathode structure  130  may include a first metal such as copper, a second metal layer coated on the first metal, and a third metal layer coated on the second metal layer. The second metal layer and the third metal layer may be uniformly coated on the first metal. 
         [0063]    The first metal may use (or be made of) copper (Cu) that is relatively inexpensive and has excellent electron conductivity. The third metal layer includes a cathode material of the electrochemical cell  1 . For example, the third metal layer may include (or be made of) nickel (Ni). Alternatively, the third metal layer may include an alloy of nickel (Ni) of about 40% to about 70% (or of 40% to 70%) iron (Fe) by weight of the third metal layer. 
         [0064]    The second metal layer includes a metal having a low standard electric potential compared to the third metal layer. For example, when the third metal layer includes nickel (Ni), the second metal layer may use a metal, such as zinc (Zn), titanium (Ti), or chromium (Cr), or a compound thereof. The ionization tendency of the second metal layer is higher than that of the first metal. If the first metal is exposed to a liquid electrolyte, performance of the electrochemical cell  1  deteriorates. To prevent or reduce such deterioration of performance of the electrochemical cell  1 , the ionization tendency of the second metal layer is higher than that of the first metal. The electrochemical cell  1  of the present embodiment is different from the electrochemical cell  1  described with reference to  FIGS. 1 and 2  in terms of a structure of the current collector  140 ′ coupled to the cathode structure  130 . The differences therebetween will now be described. 
         [0065]    The current collector  140 ′ may have a pole shape. One end  140   a  of the current collector  140 ′ may penetrate the center of the cathode structure  130 , and another end  140   b ′ thereof may not be exposed outside the electrochemical cell  1 . The other end  140   b ′ of the current collector  140 ′ is shorter than the other end  140   b  shown in  FIG. 1 , and thus the current collector  140 ′ may include a lead line  141  for an electrical connection to an external circuit. 
         [0066]    For example, the lead line  141  includes an electric conductive material. The lead line  141  may be coupled to a groove formed in the other end  140   b ′ of the current collector  140 ′. Alternatively, the lead line  141  may be coupled to the groove by using various methods like welding, adhering, etc. 
         [0067]      FIG. 4  is an enlarged scanning electron microscope (SEM) photo of the IV region of  FIG. 1 .  FIG. 5A  is a cross-sectional view of metal lines  300  included in the cathode structure  130  taken along the line V-V of  FIG. 4  according to an embodiment of the present invention.  FIG. 5B  is a cross-sectional view of the metal lines  300  included in the cathode structure  130  taken along the line V-V of  FIG. 4  according to another embodiment of the present invention. 
         [0068]    Referring to  FIG. 4 , the cathode structure  130  has a 3D net structure. For example, the metal lines  300  are 3-dimensionally connected to one another (i.e., interconnected outer metal lines) to form the 3D net structure (that is, an outer net). Meanwhile, a diameter t 1  of a hole (that is, an outer hole) formed in the porous cathode structure  130  may be smaller than about 300 μm (or about 300 μm or smaller). In the present embodiment, a first metal may use copper (Cu), a second metal layer may use zinc (Zn), and a third metal layer may use nickel (Ni). 
         [0069]    Referring to  FIG. 5A , each of the metal lines  300  included in the cathode structure  130  may be formed by sequentially coating a second metal layer  320  and a third metal layer  330  on a first metal  310 . That is, the second metal layer  320  and the third metal layer  330  may be disposed to surround the first metal  310  that is interposed between the second metal layer  320  and the third metal layer  330 . The second metal layer  320  and the third metal layer  330  may be uniformly coated on the first metal  310 . 
         [0070]    Referring to  FIGS. 5A and 5B , the metal lines  300  may have circular or triangular cross-sections. However, such shapes are exemplary, and the present invention is not limited to the cross-sectional shapes of the metal lines  300 . 
         [0071]      FIG. 6  is a flowchart illustrating a method of manufacturing the cathode structure  130  according to an embodiment of the present invention.  FIGS. 7A and 7B  are schematic perspective views of a first metal structure S having a 3D net structure. 
         [0072]    Referring to  FIG. 6 , in operation S 610 , the first metal structure S having the 3D net structure is prepared. For descriptive convenience, the first metal structure S having the 3D net structure is referred to as the first metal structure S. The first metal structure S may use copper (Cu) that is relatively inexpensive and has excellent electron conductivity. Referring to  FIG. 7A , the first metal structure S may have an approximately cylindrical shape. 
         [0073]    Referring to  FIG. 7B , the first metal structure S may be coupled to the current collector  140  in operation S 610 . For example, the first metal structure S and the current collector  140  may be coupled by thermally treating and sintering the first metal structure S and the current collector  140  in an N 2  atmosphere at a high temperature of about 800° C. to about 1000° C. 
         [0074]    The current collector  140  may be disposed in the center of the first metal structure S. In subsequent operations S 620  and S 630  of coating the second metal layer  320  and the third metal layer  330 , the current collector  140  acts as a handle. The current collector  140  may use copper (Cu) that is same as the first metal. 
         [0075]    The first metal structure S of  FIG. 7A  does not include the current collector  140 . The first metal structure S of  FIG. 7A  may be coupled to the current collector  140  after the cathode structure  130  is completely manufactured according to the present embodiment. 
         [0076]      FIG. 7C  is a partially enlarged SEM photo of the first metal structure S of  FIGS. 7A and 7B . 
         [0077]    Referring to  FIG. 7C , the first metal structure S has a 3D net structure. For example, metal lines  310  are 3-dimensionally connected to one another (i.e., interconnected inner metal lines) to form the 3D net structure (that is, an inner net). Meanwhile, a diameter t 2  of a hole (that is, an inner hole) formed in the first metal structure S may be at least about 400 μm (or about 400 μm or larger). 
         [0078]    Although the first metal structure S has a cylindrical shape in the present embodiment, the present invention is not limited thereto. According to other embodiments, the cathode structure  130  that is manufactured according to the present invention may have other shapes that wholly fill (or substantially fill) the second chamber C 2 , or whose volumes or cross-sectional areas are substantially the same as those of the second chamber C 2 . For example, if the solid electrolyte  120  of the electrochemical cell  1  of  FIGS. 1 and 3  has an approximately hexagonal shape, a first metal  310  may have a hexagonal (cross-sectional) shape. Meanwhile, if positions of the cathode chamber C 2  and the anode chamber C 1  exchange in the electrochemical cell  1  of  FIGS. 1 and 3 , the first metal  310  may have a hollow tubular shape. 
         [0079]    In operation S 620 , the second metal layer  320  is coated on the metal lines  310  included in the first metal  310 . The second metal layer  320  may use a metal, such as zinc (Zn), tin (Sn), titanium (Ti), or chromium (Cr), or a compound thereof. The second metal layer  320  may be coated on the metal lines  310  in a thickness of several μm to several tens of μm by using electric plating, electroless plating, physical deposition, chemical deposition, etc. 
         [0080]    In operation S 630 , the third metal layer  330  is coated on the second metal layer  320 . The third metal layer  330  may include nickel (Ni) as a cathode material. For example, the third metal layer  330  may include nickel (Ni) or an alloy of nickel (Ni). The alloy of nickel (Ni) may be an alloy of nickel (Ni) and iron (Fe). The third metal layer  330  may be coated on the second metal layer  320  in a thickness of several μm to several hundred μm by using electric plating, electroless plating, physical deposition, chemical deposition, etc. 
         [0081]      FIG. 7C  illustrates the first metal structure S before a coating process of operations S 620  and S 630  is performed.  FIG. 4  illustrates the cathode structure  130  completely manufactured according to the coating process. 
         [0082]    Referring to  FIGS. 7C and 4 , distances t 2  between the metal lines  310  included in the first metal structure S including copper (Cu) were each initially about 400 μm. Thereafter, if the cathode structure  130  is manufactured by coating the second and third metal layers  320  and  330  on the metal lines  310 , distances t 1  between the metal lines  300  included in the cathode structure  130  are each about 300 μm. The distances t 1  between the metal lines  300  included in the cathode structure  130  may vary according to thicknesses of the second and third metal layers  320  and  330 . 
         [0083]    A method of coating the cathode structure  130  of the present invention by coating zinc (Zn) that is the second coating layer  320  and nickel (Ni) that is the third coating layer  330  by using electroless plating will now be described below. The embodiment below is exemplary and the scope of the present invention is not limited. A method of coating zinc (Zn) that is the second coating layer  320  is first described. 
         [0084]    The first metal structure S as shown in  FIG. 7A  or  7 B is cleansed. The first metal structure S may be cleansed by soaking the first metal structure S in a weak alkali solution of about 2% of KOH and applying an ultrasonic wave thereto for about 5 minutes. Thereafter, the first metal structure S may be cleansed in distilled water three times for about 3 minutes each time. 
         [0085]    Thereafter, a solution containing a zinc precursor is manufactured. For example, about 20 g of NaOH and 100 ml of H 2 O are sufficiently mixed and are heated up to a temperature of 100° C. If about 5 g of Zn powder is added to the boiling solution, the solution containing the zinc precursor may be manufactured by a reaction of NaOH and Zn. 
         [0086]    The first metal structure S is added to the solution and the ultrasonic wave is applied thereto. Then, zinc (Zn) is coated on the metal lines  310  included in the first metal structure S. If the ultrasonic wave is applied to the solution containing the first metal structure S, zinc (Zn) may have a thickness of about 0.5 μm to about 1.5 μm. 
         [0087]    The first metal structure S coated with zinc (Zn) is taken from the solution and is cleansed. If the first metal structure S is coupled to the current collector  140  as shown in  FIG. 7A , the first metal structure S coated with zinc (Zn) may be easily taken from the solution. After cleansing the first metal structure S, if the first metal structure S is thermally treated at a temperature of about 150° C. for about 20 minutes, zinc (Zn) may be tightly coupled to the metal lines  310  of the first metal structure S. 
         [0088]    Next, a method of coating nickel (Ni) that is the third coating layer  330  is described. First, the structure in which zinc (Zn) is coated on copper (Cu) is cleansed. For example, the structure may be cleansed by soaking the structure in a weak alkali solution of about 2% of KOH and applying an ultrasonic wave thereto for about 5 minutes. Further, the structure may be cleansed after processing the structure with a palladium (Pd) catalyst and soaking the structure in a sulfuric acid solution of 6.5 wt % for about 1 minute. 
         [0089]    An electroless nickel-plating solution is manufactured. The electroless nickel-plating solution may include a nickel precursor, a deoxidizing agent, a pH-adjusting agent, and a complexing agent. Further, the electroless nickel-plating solution may include a small amount of an accelerator, a stabilizer, a surfactant, etc. 
         [0090]    The nickel precursor may use soluble nickel sulfate, nickel chloride, etc. The deoxidizing agent may use sodium hypophosphite, sodium borohydride, hydrazine, etc. Meanwhile, the pH-adjusting agent may use sodium hydroxide, ammonium hydroxide, etc. The complexing agent may allow a stable supply of nickel. The complexing agent may use two or more materials selected from the group consisting of lactic acid, glycolic acid, and malic acid. The accelerator may use citric acid soda, acetic acid soda, etc. The whole weight of the electroless nickel-plating solution may include nickel sulfate (NiSO 4 .6H 2 O) of about 5 wt %, sodium hypophosphite (NaH 2 PO 2 .6H 2 O) that is the deoxidizing agent of about 3.5 wt %, and the complexing agent of about 5.0 wt %. 
         [0091]    The electroless nickel-plating solution is heated at a temperature of about 80° C., and then the structure in which zinc (Zn) is coated on copper (Cu) is put into the electroless nickel-plating solution. The electroless nickel-plating solution is mixed in a direction so that nickel is uniformly coated. In this regard, pH is about 4.5. If such mixing is performed for about 15 minutes, a thickness of coated nickel may be about 10 μm to about 12 μm. The completely coated nickel is cleansed by H 2 O and is dried at a temperature of about 80° C. about 45 minutes. 
         [0092]      FIG. 8  is a schematic cross-sectional view of the electrochemical cell  1 ′ according to another embodiment of the present invention. The electrochemical cell  1 ′ of the present embodiment has a flat panel shape that is different from the tubular electrochemical cell  1  described with reference to  FIGS. 1 through 4 . 
         [0093]    Referring to  FIG. 8 , the electrochemical cell  1 ′ includes a housing  810 , a solid electrolyte  820  for dividing an inner portion of the housing  810  into the first chamber C 1 ′ and the second chamber C 2 ′, an anode material  811  included in the first chamber C 1 ′, and the porous cathode structure  830  included in the second chamber C 2 ′. 
         [0094]    The solid electrolyte  820  may allow ions to flow therethrough. The solid electrolyte  820  may include a β-alumina based material. For example, the solid electrolyte  820  may include β-alumina or β″-alumina. 
         [0095]    The insulator  860  may electrically insulate the first chamber C 1 ′ from the second chamber C 2 ′. The insulator  860  may be joined to a side of the solid electrolyte  820  via an adhesive material, for example, a glass frit  870 . The insulator  860  may include α-alumina. 
         [0096]    The first chamber C 1 ′ may be an anode chamber and may include an anode material  811 . The anode material may be an alkali metal such as sodium. The sodium may be present in a liquid phase. Besides sodium, the anode material may also be any other suitable alkali metal such as lithium or potassium. 
         [0097]    The first chamber C 1 ′ may include a wick  815 . The wick  815  is spaced apart from the solid electrolyte  820  by a set or predetermined space and induces a capillary phenomenon, as described above. 
         [0098]    When the first chamber C 1 ′ is the anode chamber, the second chamber C 2 ′ is a cathode chamber and may include a cathode structure  830 . The cathode structure  830  may include a first metal such as copper, a second metal layer coated on the first metal, and a third metal layer coated on the second metal layer. In this regard, the first metal may use copper (Cu) that is relatively inexpensive and has excellent electron conductivity. The third metal layer includes a cathode material such as nickel. 
         [0099]    The second metal layer includes a metal having a low standard electric potential compared to the third metal layer, and the ionization tendency of the second metal layer is higher than that of the first metal. For example, when the third metal layer includes nickel (Ni), the second metal layer may use a metal, such as zinc (Zn), titanium (Ti), or chromium (Cr), or a compound thereof. The specific shape and construction of the cathode structure  830 , and a method of manufacturing the cathode structure  830  are the same as described above. 
         [0100]    The cathode structure  830  has an approximately hexahedral shape and is contained in the flat panel type electrochemical cell  1 ′. However, the present invention is not limited to this shape of the cathode structure  830 . In other embodiments, the cathode structure  830  may have different shapes and may wholly fill (or substantially fill) the second chamber C 2 ′, or may have volumes or cross-sectional areas that are substantially the same as those of the second chamber C 2 ′. 
         [0101]    The second chamber C 2 ′ may include a liquid electrolyte  835  such as NaAlCl 4 , in addition to the cathode structure  830 . The cathode structure  830  may be deposited in the liquid electrolyte  835 . 
         [0102]      FIG. 9  is a simulation graph of a resistance of the electrochemical cell  1  having a capacity of about 40 Ah and a cathode structure at a temperature of about 95° C. according to an embodiment of the present invention. 
         [0103]    Referring to  FIG. 9 , the graph shows that the electrochemical cell  1  maintains a constant resistance value in spite of a repetition of charging and discharging. Generally, an anode and an electrolyte of a sodium-based electrochemical cell have relatively constant resistance values. However, a resistance value of a cathode of the sodium-based electrochemical cell is a very critical factor to performance thereof. The electrochemical cell  1  including the cathode structure according to the present embodiment may prevent or protect an electrode from degenerating or deforming in charging and discharging operations, and may maintain a constant resistance. A resistance value of a cathode of the electrochemical cell  1  including the cathode structure is maintained constant (or substantially constant), thereby obtaining high output characteristics and increasing cell lifetime. 
         [0104]    Furthermore, the electrochemical cell  1  according to the present embodiment may be realized by using a minimum amount of a cathode material such as nickel, thereby reducing manufacturing cost and maximizing cell efficiency. 
         [0105]    It should be understood that the exemplary embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims, and equivalents thereof.