Abstract:
The present invention relates to a redox flow secondary battery. The redox flow secondary battery of the present invention comprises a unit cell including a pair of electrodes made of a porous metal, wherein the surface of the porous metal is coated with carbon. According to the present invention, a redox flow secondary battery using porous metal electrodes uniformly coated with carbon is provided, thus improving conductivity of the electrodes, and the electrodes have surfaces uniformly coated with a carbon layer having a wide specific surface area, thus improving reactivity. As a result, capacity of the redox flow secondary battery and energy efficiency can be improved and resistance of a cell can be effectively reduced. Further, the electrodes are uniformly coated with a carbon layer, thus also improving corrosion resistance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2012-0013470 filed in the Korean Intellectual Property Office on Feb. 9, 2012 respectively, the entire contents of which are incorporated herein by reference. 
       TECHNICAL FIELD  
       [0002]    The present invention relates to a secondary battery, and more particularly, to a redox flow secondary battery that uses an electrode in which a porous metal is coated with carbon. 
       BACKGROUND  
       [0003]    Electricity storage technologies are important technologies for efficiently maximizing performance in areas such as efficient use of electricity, improvement of ability or reliability of a power supply system, expansion of introducing renewable energy in which a range of changes depending on time is large, energy recuperation of a moving object, and the like, throughout an entire energy industry, and their development possibilities to meet demands for social contribution are being gradually increased. 
         [0004]    In order to adjust a supply-demand balance of a semi-autonomous local power supply system such as a microgrid, appropriately distribute non-uniform output of development of the renewable energy such as wind power or solar energy generation, and control an influence of voltage and frequency changes generated by a difference from a conventional electric power system, studies on secondary batteries are being actively conducted, and expectations with respect to the utilization of the secondary batteries are being increased in these fields. 
         [0005]    Referring to characteristics required for a secondary battery to be used for storing of high-capacity power, the secondary battery should have a high energy storage density, and thus a redox flow secondary battery is being spotlighted as the secondary battery having a high capacity and high efficiency, which is the most appropriate to these characteristics. 
         [0006]    The redox flow secondary battery is formed so that a cell frame forms an outline of an entire cell, a center of the cell is divided by an ion exchange layer, and an anode and a cathode are located at both sides of the ion exchange layer. 
         [0007]    Further, the redox flow secondary battery is formed to include a bipolar plate and a current collector for externally conducting electricity from each of the electrodes provided, an anode tank and a cathode tank, which store electrolytes, an inlet in which the electrolytes flow in, and an outlet in which the electrolytes flow out. 
         [0008]    Various studies are being conducted on such the redox flow secondary battery to develop to an increase in both output and energy efficiency. Recently, a non-aqueous electrolyte rather than an aqueous electrolyte has been mainly used. 
         [0009]    As described above, in order to develop the redox flow secondary battery to which the non-aqueous electrolyte is applied, use of the electrode in which affinity with the non-aqueous electrolyte is high and having excellent electrical conductivity is required, and thus research and development of the electrode in which these requirements are satisfied are urgently needed. 
         [0010]    In the case of a carbon-based material used for an energy electrode material of a commercial redox flow secondary battery, since affinity with the non-aqueous electrolyte is very low as well as conductivity is significantly reduced compared to a metal electrode, improvement in energy efficiency is limited when applied to a non-aqueous redox flow secondary battery. 
         [0011]    Various studies for development of the metal electrodes are being conducted to improve an electrochemical characteristic of the non-aqueous redox flow secondary battery. However, there is a limit on increase of a specific surface area of the metal electrode in a manufacturing process, and thus these studies are not proposing a fundamental solution to an improvement of energy efficiency of the non-aqueous redox flow secondary battery. 
       SUMMARY  
       [0012]    The present invention is directed to providing a redox flow secondary battery capable of ensuring conductivity of an electrode using a porous metal having excellent conductivity. 
         [0013]    The present invention is also directed to providing a redox flow secondary battery using a porous metal electrode uniformly coated with carbon having a large specific surface area to improve energy efficiency. 
         [0014]    The present invention is also directed to providing a redox flow secondary battery in which a porous metal electrode is coated with carbon having a large specific surface area and thus reactivity is improved. 
         [0015]    One aspect of the present invention provides a redox flow secondary battery including a unit cell, a pair of current collectors, and a pair of cell frames. The unit cell is formed of a porous metal, and includes a pair of electrodes formed at a surface of the porous metal coated with carbon. The pair of current collectors are bonded to both outer surfaces of the unit cell. The pair of cell frames are attached to each outer surface of the current collectors. 
         [0016]    In the redox flow secondary battery according to the present invention, the amount of carbon coated on the surface of the porous metal may be 50 wt % or less compared to a weight of the porous metal. 
         [0017]    In the redox flow secondary battery according to the present invention, the porous metal may be any one selected from nickel (Ni), copper (Cu), iron (Fe), molybdenum (Mo), titanium (Ti), platinum (Pt), and iridium (Ir). 
         [0018]    In the redox flow secondary battery according to the present invention, the coating may be performed using any one selected from a dip coating method and a spray coating method. 
         [0019]    In the redox flow secondary battery according to the present invention, a carbon content of coating slurry for the coating may be 50 wt % or more. 
         [0020]    In the redox flow secondary battery according to the present invention, the unit cell includes an ion exchange layer, the pair of electrodes each bonded to both surfaces of the ion exchange layer and including an anode and a cathode, and a pair of plates in which one surface is bonded to an outer surface of each of the pair of electrodes and the other surface is bonded to the current collector. 
         [0021]    In the redox flow secondary battery according to the present invention, the unit cell generates electricity according to an oxidation-reduction reaction through the ion exchange layer between the electrodes. 
         [0022]    The redox flow secondary battery according to the present invention may further include anode and cathode tanks, pumps, inlets, and outlets. The anode and cathode tanks are disposed at left and right sides of cell frame, respectively, and configured to store an electrolyte to flow the electrolyte. The pumps each are connected to the anode and cathode tanks, and supplies the electrolyte. The inlet connects the pump to the cell frame so that the electrolyte flows into the unit cell through the cell frame. The outlet connects to the cell frame so that the electrolyte flowed out from the unit cell flows into the anode and cathode tanks. 
         [0023]    Another aspect of the present invention provides a redox flow secondary battery including at least one unit cell having at least one pair of electrodes formed of a porous metal coated with carbon. 
         [0024]    Still another aspect of the present invention provides a redox flow secondary battery including a pair of cell frames formed to face and to be spaced apart from each other, a pair of current collectors each attached to inner surfaces of the pair of cell frames, and at least two unit cells formed between the pair of current collectors, wherein the unit cell includes at least one pair of electrodes formed of a porous metal coated with carbon. 
         [0025]    According to the present invention, a redox flow secondary battery using a porous metal electrode uniformly coated with carbon is provided, and thus conductivity of the electrode is improved. 
         [0026]    Further, a surface of the porous metal electrode is uniformly coated with a carbon layer having a large specific surface area, and thus reactivity can be improved. 
         [0027]    Therefore, capacity of the redox flow secondary battery and energy efficiency can be improved and resistance of a cell can be effectively reduced. Further, the electrode is uniformly coated with the carbon layer, and thus corrosion resistance can also be improved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0028]      FIGS. 1 and 2  are views for describing a redox flow secondary battery according to an embodiment of the present invention. 
           [0029]      FIG. 3  is images for comparing the morphology of an electrode according to the present invention with comparative examples. 
           [0030]      FIGS. 4 and 5  are graphs for comparing cyclic voltammetry (CV) characteristics of embodiments of the electrode according to the present invention with comparative examples. 
           [0031]      FIG. 6  is a graph for comparing energy efficiency of the embodiment of the electrode according to the present invention with the comparative example. 
           [0032]      FIGS. 7 and 8  are views for describing a redox flow secondary battery according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0033]    Before detailed description of embodiments of the present invention, terms and words used in this specification and claims should not be interpreted as limited to commonly used meanings or meanings in dictionaries and should be interpreted with meanings and concepts which are consistent within the technological scope of the invention based on the principle that the inventors have appropriately defined concepts of terms in order to describe the invention in the best way. Therefore, since the embodiments described in this specification and configurations illustrated in drawings are only exemplary embodiments and do not represent the overall technological scope of the invention, it is understood that the present invention covers various equivalents, modifications, and substitutions at the time of the filing of this application. 
         [0034]    Hereinafter, exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings. The same reference numbers will be used throughout this specification to refer to the same or like parts. However, detailed descriptions of well-known functions or configurations that unnecessarily obscure the gist of the invention in the following explanations and accompanying drawings will be omitted. For the same reason, some components are exaggerated, omitted or schematically shown in the drawings, and a size of each component is not entirely reflected as an actual size. 
         [0035]    Hereinafter, exemplary embodiments of the inventive concept will be described in detail with reference to the accompanying drawings. 
         [0036]    First, a redox flow secondary battery according to an embodiment of the present invention will be described.  FIGS. 1 and 2  are views for describing the redox flow secondary battery according to the embodiment of the present invention. Here,  FIG. 1  is an exploded view showing disassembled components of the redox flow secondary battery according to the embodiment of the present invention, and  FIG. 2  is a cross-sectional view showing a cross section of the redox flow secondary battery according to the embodiment of the present invention. 
         [0037]    Referring to  FIGS. 1 and 2 , the redox flow secondary battery according to the embodiment of the present invention is a secondary battery charged or discharged using an oxidation-reduction reaction of a metal ion in which valency is changed. Further, the redox flow secondary battery according to the embodiment of the present invention may be driven in a voltage range of 0 to 3.0 V. 
         [0038]    The redox flow secondary battery according to the embodiment of the present invention may be formed to have a unit cell  100  having a multi-layer structure in a plate shape, a pair of current collectors  40  bonded to both outer surfaces of the unit cell  100  and formed in a plate shape, and cell frames  50  attached to outer surfaces of the current collectors  40 , respectively and formed in a plate shape. 
         [0039]    Here, the unit cell  100  includes an ion exchange layer  10 , electrodes  20 , and bipolar plates  30  (hereinafter, abbreviated as “plates”), of which each have a plate shape, and has a structure in which the electrodes  20 , in which an anode is opposite a cathode as a pair, each are bonded to both surfaces of the ion exchange layer  10  based on a center of the ion exchange layer  10 , and the plates  30  each are bonded to outer surfaces of the anode and the cathode of the electrodes  20 . Meanwhile, although not shown, a gasket may be selectively interposed between the electrode  20  and the ion exchange layer  10 . 
         [0040]    As described above, the ion exchange layer  10 , the electrodes  20 , and the plates  30 , of which each have a plate shape, form a unit cell  100  in a multi-layer structure. 
         [0041]    The oxidation-reduction reaction of the metal ion in which valency is changed occurs in the unit cell  100 . In this case, the oxidation-reduction reaction occurs between the anode and the cathode of the electrodes  20  through the ion exchange layer  10 , and thus electricity is generated by the oxidation-reduction reaction. 
         [0042]    When the electricity is generated at the anode and the cathode of the electrodes  20  of the unit cell  100 , the plates  30  and the current collectors  40  collect the generated electricity. The cell frames  50  maintains and supports a shape of the ion exchange layer  10 , the pair of electrodes  20 , the pair of plates  30 , and the pair of current collectors  40  described above. 
         [0043]    Further, the redox flow secondary battery according to the embodiment of the present invention may further include an anode tank  60 , a cathode tank  70 , pumps  61  and  71 , inlets  63  and  73 , and outlets  65  and  75 . 
         [0044]    The anode tank  60  and the cathode tank  70  store an anodic electrolyte and a cathodic electrolyte, respectively, to flow when required. It is preferably that the anode tank  60  and the cathode tank  70  respectively use non-aqueous electrolytes as the anodic electrolyte and the cathodic electrolyte, however, aqueous electrolytes may also be used. Such the anode tank  60  and the cathode tank  70  each are disposed on the both outer surfaces of the unit cell  100  corresponding to the anode and the cathode of the electrode  20  of the unit cell  100  described above. 
         [0045]    Further, the anode tank  60  and the cathode tank  70  are connected to the cell frames  50  through the inlets  63  and  73  and the outlets  65  and  75 , respectively. The inlets  63  and  73  are paths through which the electrolytes of the anode tank  60  and the cathode tank  70  flow into the unit cell  100 , and the outlets  65  and  75  are paths through which the electrolytes flow from the unit cell  100 . Further, the pumps  61  and  71  are provided to flow the electrolytes from the anode tank  60  and the cathode tank  70  and supply the electrolytes to the unit cell  100 , and are interposed between the anode tank  60  and the inlet  63  and between the cathode tank  70  and the inlet  73 , respectively. 
         [0046]    Therefore, the electrolytes flowed out from the anode tank  60  and the cathode tank  70  may be supplied to the unit cell  100  through the pumps  61  and  71 , the inlets  63  and  73 , the cell frames  50 , and the current collectors  40 , respectively and in the reverse order, flowed and stored in the anode tank  60  and the cathode tank  70 . 
         [0047]    In the redox flow secondary battery configured according to the embodiment of the present as described above, the ion exchange layer  10  may be formed of Nafion. Further, the plates  30  may be formed of graphite. 
         [0048]    As described above, the electrodes  20  are bonded to inner surfaces of the plates  30 , respectively. Such the electrodes  20  each have a characteristic in which a surface of a porous metal is uniformly coated with a carbon layer. According to the embodiment of the present invention, the electrodes  20  are formed at the porous metal thereof is uniformly coated with carbon. 
         [0049]    Here, the porous metal may be any one selected from nickel (Ni), copper (Cu), iron (Fe), molybdenum (Mo), titanium (Ti), platinum (Pt), and iridium (Ir). 
         [0050]    Further, it is preferable that the porous metal is coated so that the amount of carbon coated on the surface of the porous metal is 50 wt % or less compared to a weight of the porous metal. Further, it is preferable that a dip coating method or a spray coating method is used as a coating method. When a carbon coating slurry for the coating is manufactured, the coating slurry is manufactured to have a carbon content of 50 wt % or more. 
         [0051]    As described above, the porous metal electrode uniformly coated with carbon is used on surfaces of an aqueous or a non-aqueous redox flow secondary battery and a stacked type battery that will be described below, and thus capacity of the non-aqueous redox flow secondary battery and energy efficiency may be enhanced, and a corrosion characteristic may be improved. 
         [0052]    Next, a morphology of the electrode according to embodiments of the electrode of the present invention will be compared with comparative examples.  FIG. 3  is images for comparing the morphology of the electrode according to the present invention, and field emission scanning electron microscope (FESEM) images of the comparative examples and the embodiments of the electrode of the present invention are disclosed. 
         [0053]    Referring to  FIG. 3 , it may be determined that the electrodes  20  of the embodiments in which the surfaces of the porous metals are uniformly coated with carbon. Details of the comparative examples and the embodiments are as the following [Table 1]. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Amount of Carbon 
               
               
                   
                   
                 Metal type 
                 Pore Size 
                 Coating 
               
               
                   
                   
               
             
             
               
                   
                 Comparative 
                 Ni 
                 800 
                 0 wt % 
               
               
                   
                 Example 1 
                   
                   
                   
               
               
                   
                 Comparative 
                 Cu 
                 800 
                 0 wt % 
               
               
                   
                 Example 2 
                   
                   
                   
               
               
                   
                 Embodiment 1 
                 Ni 
                 800 
                 5 wt % 
               
               
                   
                 Embodiment 2 
                 Cu 
                 800 
                 5 wt % 
               
               
                   
                   
               
             
          
         
       
     
         [0054]    The electrodes  20  of Embodiments 1 and 2 of the present invention are coated using a spray coating method, after slurry having a composition of Super-P:binder:N-methylpyrrolidinone (NMP)=2.5:2.5:95 is manufactured, when the surface of the porous metal is coated with carbon. The amount of coated carbon (amount of carbon coating) was measured as a weight ratio of the coating before and after. 
         [0055]    A cyclic voltammetry (CV) characteristic of the electrode of the present invention will be compared through the embodiment of the electrode of the present invention and the comparative examples.  FIGS. 4 and 5  are graphs for comparing CV characteristics of the embodiments of the electrodes according to the present invention with the comparative examples. Here, the CV characteristic evaluation was performed on Comparative examples 1 and 2 of [Table 1] described above and the embodiments of the porous metal electrode coated with carbon using a spray coating method of a propylene carbonate (PC)-based organic electrolyte. 
         [0056]    In  FIGS. 4 and 5 , in order to evaluate an electrochemical characteristic of the porous metal electrode coated with carbon, a CV measurement was performed in various non-aqueous electrolytes. In this case, the measurement was performed under a condition of a scan rate of 1 mV/s in a potential area in a range of −1.8 to 0.0 V compared to Ag/Ag + .  FIG. 4  is a graph showing the CV characteristics of the comparative examples and the embodiments in a Co(bpy) + PC electrolyte, and  FIG. 5  is a graph showing the CV characteristics of the comparative examples and the embodiments in an Ni(bpy) + PC electrolyte. 
         [0057]    As shown in  FIGS. 4 and 5 , referring to CV results of the comparative examples and the embodiments, when copper (Cu) and nickel (Ni) porous metal electrode coated with carbon is applied in various PC-based non-aqueous electrolytes, it may be determined that reactivity of the embodiments is significantly increased compared to that of the comparative examples. That is, it may be determined that a current value to be used for oxidation of the ion was increased. The increase of the current value is due to improvement of conductivity of the electrode using the porous metal, and also because the carbon coated on the surface of the porous metal efficiently provides a redox reaction site. 
         [0058]    Next, energy efficiency characteristics of the electrode of the present invention will be compared through the embodiment of the electrode of the present invention and a comparative example.  FIG. 6  is a graph for comparing energy efficiency of the embodiment of the electrode according to the present invention with a comparative example. Here, energy efficiency of cells to which Comparative examples 1 and 2 are applied as an anode and a cathode, and energy efficiency of cells to which Embodiments 1 and 2 are applied as an anode and a cathode was compared. 
         [0059]    Referring to  FIG. 6 , it may be determined that the cells which are coated with carbon according to applications of Embodiments 1 and 2 show enhanced Coulomb efficiency and energy efficiency. In the case of the applied embodiments, the initial energy efficiency is 82% which is a better characteristic than 77% of the energy efficiency of the applied comparative examples. Further, the Coulomb efficiency was increased from 93% to 95% through carbon coating on the surface of the porous metal electrode. 
         [0060]      FIGS. 7 and 8  are views for describing a redox flow secondary battery according to another embodiment of the present invention. Here,  FIG. 7  is an exploded view showing disassembled components of the redox flow secondary battery according to another embodiment of the present invention.  FIG. 8  is a cross-sectional view showing a cross section of the redox flow secondary battery according to another embodiment of the present invention. 
         [0061]    Referring to  FIGS. 7 and 8 , the redox flow secondary battery according to another embodiment of the present invention is a secondary battery charged or discharged using oxidation-reduction reaction of a metal ion in which valency is changed. Further, the redox flow secondary battery according to another embodiment of the present invention may be driven in a voltage range of 1.5 to 3.0 V. 
         [0062]    The redox flow secondary battery according to another embodiment of the present invention includes a pair of cell frames  50 , a pair of current collectors  40 , and a plurality of unit cells  100 . 
         [0063]    The pair of cell frames  50  are spaced apart from each other by a predetermined distance and opposite each other. As described above, the pair of current collectors  40  are bonded to inner surfaces of the pair of cell frames  50  facing each other, respectively. The plurality of unit cells  100  are interposed between the pair of current collectors  40 . As described above, the plurality of unit cells  100  each include an ion exchange layer  10 , electrodes  20  including an anode and a cathode, and plates  30 . As shown, the plurality of unit cells  100  are connected to each other in series and share the plates  30  connected to each other. For example, the redox flow secondary battery in which three unit cells  100  are formed is shown in  FIGS. 7 and 8 . As shown, since the three unit cells  100  share the two connected plates  30 , there are four plates  30 . Such the redox flow secondary battery according to another embodiment of the present invention is a stacked type battery in which the three unit cells  100  are stacked in series. 
         [0064]    As described above, in a structure in which the plurality of unit cells  100  are connected to each other in series, the electrodes  20  each have a characteristic in that a surface of a porous metal thereof is uniformly coated with a carbon layer as disclosed in  FIG. 3 . Since each electrode  20  has the same configuration as the electrode of the redox flow secondary battery according to the embodiment of the present invention, detail descriptions will be omitted. 
         [0065]    Further, although not shown in  FIGS. 7 and 8 , the redox flow secondary battery according to another embodiment of the present invention further includes an anode tank  60 , a cathode tank  70 , pumps  61  and  71 , inlets  63  and  73 , and outlets  65  and  75  as the same as the redox flow secondary battery according to the embodiment of the present invention. 
         [0066]    The anode tank  60  and the cathode tank  70  store an anodic electrolyte and a cathodic electrolyte, respectively, to be flowed when required, and use a non-aqueous electrolyte as the anode and cathode electrolytes. Such the anode tank  60  and the cathode tank  70  each are disposed at left and right sides of the unit cell  100  corresponding to the anode and the cathode of the electrode  20  of the unit cell  100  described above. Further, the anode tank  60  and the cathode tank  70  are connected to the cell frames  50  through the inlet  63  and  73  and the outlet  65  and  75 , respectively. Further, the pumps  61  and  71  are provided to flow the electrolytes from the anode tank  60  and the cathode tank  70  and supply the electrolytes to the unit cells  100 , and are interposed between the anode tank  60  and the inlet  63  and between the cathode tank  70  and the inlet  73 , respectively. That is, the electrolytes flowed out from the anode tank  60  and the cathode tank  70  may be supplied to the unit cells  100  through the pumps  61  and  71 , the inlets  63  and  73 , the cell frames  50 , and the current collectors  40 , respectively and in the reverse order, flowed and stored in the anode tank  60  and the cathode tank  70 . 
         [0067]    Meanwhile, in the above-described embodiments, it was described for coating on the porous metal using only a dip coating method or a spray coating method. However, the embodiment of the present invention is not limited thereto. That is, various methods such as a vapor deposition method, a sputtering method, a chemical vapor deposition method, and the like may be selectively or complexly used if necessary. 
         [0068]    Further, in the above-described embodiments, it was described in an example for a case in which the electrode is formed of a porous metal. However, the embodiment of the present invention is not limited thereto, and the metal may be formed in a mesh shape. Further, a type in a flat plate shape, such as a conventional type, may also be possible when appropriately coated with carbon. 
         [0069]    Further, in the above-described embodiments, it was described in an example for a case in which the coating layer is formed on the electrode of the non-aqueous redox flow secondary battery. However, the embodiment of the present invention is not limited thereto, and may also be applied to the electrode of the aqueous redox flow secondary battery. 
         [0070]    In addition, in the above-described embodiments, it was described as an example as to the electrode provided in the redox flow secondary battery. However, the embodiment of the present invention is not limited thereto; the electrode may be widely applied to a battery including the electrode accommodated in an electrolyte, and specifically, to a stacked type battery. 
         [0071]    In this specification, exemplary embodiments of the present invention have been classified into the first, second and third exemplary embodiments and described for conciseness. However, respective steps or functions of an exemplary embodiment may be combined with those of another exemplary embodiment to implement still another exemplary embodiment of the present invention.