Abstract:
A separator for use in a molten salt battery has the problem that due to usage specific to the molten salt battery, the separator is placed under mechanical, thermal and chemical stress, so that cracking or rupture easily occurs, leading to a degradation in battery performance such as an internal short-circuit. The molten salt battery of the present invention includes a separator containing a metal oxide, particularly aluminum oxide and/or zirconium oxide in an amount of 75% or more. The separator improves mechanical, thermal and chemical resistance, and thus an internal short-circuit ascribable to the separator is hard to occur, so that the molten salt battery can be stably operated for a long period of time. The separator has high heat stability, so that the safety of the molten salt battery can be improved.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a molten salt battery using a molten salt as an electrolyte. 
       BACKGROUND ART 
       [0002]    In recent years, use of natural energy such as sunlight and wind power has been promoted. When electric power is generated using natural energy, the amount of electric power is easily varied, and therefore leveling of electric power supply by charge/discharge using a storage battery is necessary for stably supplying generated electric power. Accordingly, a storage battery for storage of electric power, which has a high energy density/high efficiency, is absolutely necessary for promoting use of natural energy. Such storage batteries for storage of electric power include a molten salt battery in addition to the sodium-sulfur battery and lead acid battery described in Patent Literature 1. 
         [0003]    The molten salt battery described in Patent Literature 2 is a battery using a molten salt for an electrolyte, and operates as the molten salt is melted. As the molten salt, for example, NaFSA with the sodium ion as a cation and FSA (bisfluorosulfonylamide; (FSO2)2N—) as an anion is used. The melting point of the molten salt is equal to or higher than room temperature, and a molten salt battery using the molten salt operates at a temperature higher than room temperature, for example around 100° C. In addition to the electrolyte, a positive electrode, a negative electrode and a separator are included in the molten salt battery. These members are required to have heat resistance. 
         [0004]    The separator is a sheet-shaped member for isolating the positive electrode and the negative electrode from each other, and retains therein a molten salt containing ions of an active material. In a conventional molten salt battery, a separator made of glass, which uses a glass cloth, or a separator of polyolefin resin which is used in a lithium ion secondary battery or the like is used. 
       CITATION LIST 
     Patent Literatures 
       [0005]    Patent Literature 1: Japanese Unexamined Patent Publication No. 2007-273297 
         [0006]    Patent Literature 2: Japanese Unexamined Patent Publication No. 2011-192474 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    A generally well known lithium ion secondary battery, nickel-metal hydride storage battery, or the like can be used at ordinary temperature, but a molten salt battery is a battery which operates at a temperature higher than ordinary temperature, i.e., about 80 to 100° C. Therefore, when the battery is operated, heating is required to elevate the temperature of the whole of the battery to a temperature equal to or higher than the melting point of a molten salt as an electrolyte so that the electrolyte of the battery is brought into a liquid state. At the time of stopping the battery, the electrolyte is solidified and thus can no longer function as an electrolyte when heating of the battery is stopped to decrease the temperature of the electrolyte to a melting point or lower. 
         [0008]    In this way, in the molten salt battery, the electrolyte alternates between a liquid state and a solid state as the battery is alternately operated and stopped. Due to this solid-liquid state change of the electrolyte, a conventional separator material is placed under mechanical stress, heat and chemical stress especially by the electrolyte existing in the separator. As a result, there is the problem that cracking and rupture easily occur, leading to a degradation battery performance such as an internal short-circuit. 
         [0009]    The present invention has been devised in view of the situation described above, and an object thereof is to provide a molten salt battery which stably functions even when an electrolyte alternates between a liquid state and a solid state as the battery is alternately operated and stopped. 
       Solution to Problem 
       [0010]    The present invention provides a molten salt battery using a molten salt as an electrolyte, the molten salt battery comprising a positive electrode, a negative electrode and a separator existing between the positive electrode and the negative electrode to isolate both the electrodes from each other, the separator containing a metal oxide material in an amount of 75% by mass or more. 
         [0011]    For example, a separator containing aluminum oxide and/or zirconium oxide in an amount of 75% by mass or more is preferred. 
         [0012]    On the other hand, for example, a remainder of components of the separator may include other metal oxides with substantially no organic compound contained. Alternatively, the reminder of components of the separator may include an organic compound. As the organic compound, for example, polyolefin or polyamide is preferred. 
         [0013]    In recent years, for example, various kinds of heat-resistant separators to improve safety, the separator including an inorganic fine particle layer on a microporous film made of polyolefin, have been proposed as a separator for a lithium ion secondary battery using an organic electrolyte. In contrast, the aforementioned specific operation method is used for a molten salt battery using a molten salt as an electrolyte. Therefore, a material specific to the molten salt battery is used for the separator, and the separator is required to have an unprecedented function for satisfactorily operating the battery. 
         [0014]    The separator in the molten salt battery of the present invention contains a metal oxide, particularly aluminum oxide and/or zirconium oxide in an amount of 75% by mass or more. Consequently, a function of a mechanically and chemically stable separator can be maintained even when the electrolyte alternates between a liquid state and a solid state as the battery is alternately operated and stopped. The separator containing aluminum oxide and/or zirconium oxide in an amount of 75% by mass or more not only has high mechanical strength in a heat cycle, but also has high chemical stability to the electrolyte even at a relatively high operating temperature. 
         [0015]    Further, the molten salt battery of the present invention has such a characteristic of high safety that the risk of heat generation/ignition or the like is extremely low even if an abnormal situation such as an internal short-circuit is encountered because an incombustible electrolyte material is used. The reason for this is that the incombustible electrolyte material exists throughout electric power generation element parts such as the positive electrode, the negative electrode and the separator of the molten salt battery. That is, even if water or the like enters from outside the battery due to an accident or the like, or a short-circuit occurs in the battery, the incombustible electrolyte material protects a part where an abnormality may occur, so that an onset of local abnormal heat generation is prevented. This is the reason why the molten salt battery can ensure high safety. 
       Advantageous Effects of Invention 
       [0016]    According to the present invention, a mechanically and chemically stable function can be maintained. Therefore, there can be provided a molten salt battery which stably functions even when an electrolyte alternates between a liquid state and a solid state as the battery is alternately operated and stopped, i.e., the problem of conventional molten salt batteries can be solved. 
         [0017]    In addition, there can be provided a molten salt battery with high safety in which the risk of heat generation/ignition or the like is extremely low even if an abnormal situation such as an internal short-circuit is encountered. 
     
    
     
       BRIEF DESCRIPTION OF DRAWING 
         [0018]      FIG. 1  is a schematic sectional view showing an example of a configuration of a molten salt battery of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    Hereinbelow, the present invention will be specifically described with reference to the drawings showing embodiments thereof. 
         [0020]      FIG. 1  is a schematic sectional view showing an example of a configuration of a molten salt battery of the present invention.  FIG. 1  shows a schematic sectional view of the molten salt battery when cut in a longitudinally direction. The molten salt battery is formed by arranging a positive electrode  1 , a separator  3  and a negative electrode  2  side by side in a rectangular-parallelepiped box-shaped battery container  51  which is opened at an upper surface, and firmly fixing a lid portion  52  to the battery container  51 . The battery container  51  and the lid portion  52  are formed of aluminum. 
         [0021]    The positive electrode  1  and the negative electrode  2  are formed in a rectangular flat shape, and the separator  3  is formed in a sheet shape. The separator  3  is interposed between the positive electrode  1  and the negative electrode  2 , and isolates the positive electrode  1  and the negative electrode  2  from each other so as not be short-circuited. The positive electrode  1 , the separator  3  and the negative electrode  2  are stacked on one another, and normally arranged longitudinally with respect to the bottom surface of the battery container  51 . 
         [0022]    A spring  41  formed of a corrugated plate-shaped metal is placed between the negative electrode  2  and the inner side wall of the battery container  51 . A flat plate-shaped presser plate  42  is formed of an aluminum alloy and has non-flexibility. The spring  41  energizes the presser plate  42  to press the negative electrode  2  to the separator  3  and positive electrode  1  side. By a counterforce to the pressing force, the positive electrode  1  is pressed to the separator  3  and negative electrode  2  side from the inner side wall on a side opposite to the spring  41 . The spring  41  is not limited to a metallic spring or the like, and may be an elastic body such as rubber. When the positive electrode  1  or the negative electrode  2  is expanded or contracted due to charge-discharge, a change in volume of the positive electrode  1  or the negative electrode  2  is absorbed by expansion and contraction of the spring  41 . 
         [0023]    The positive electrode  1  is formed by applying a positive electrode material  12 , which contains a positive electrode active material such as NaCrO 2  and a binder, onto a rectangular plate-shaped current collector of positive electrode  11  formed of aluminum. The positive electrode active material is not limited to NaCrO 2 . The negative electrode  2  is formed by applying a negative electrode material  22 , which contains a negative electrode active material such as tin, onto a rectangular plate-shaped current collector of negative electrode  21  formed of aluminum using plating, a deposition method or the like. When the current collector of negative electrode  21  is plated thereon with the negative electrode material  22 , a substrate is plated with zinc, and then plated with tin as a zincate treatment. The negative electrode active material is not limited to tin, and for example, tin may be replaced by metal sodium, carbon, silicon or indium. The negative electrode material  22  may be formed by, for example, including a binder in a powder of the negative electrode active material, and applying the powder onto the current collector of negative electrode  21 . Details of the separator  3  will be described later. 
         [0024]    In the battery container  51 , the positive electrode material  12  of the positive electrode  1  and the negative electrode material  22  of the negative electrode  2  are made to face each other, and the separator  3  is interposed between the positive electrode  1  and the negative electrode  2 . The positive electrode  1 , the negative electrode  2  and the separator  3  are impregnated with an electrolyte formed of a molten salt. The inner surface of the battery container  51  is structured to have an electrically insulating property by a method of coating the surface with a resin having an electrically insulating property in order to prevent a short-circuit between the positive electrode  1  and the negative electrode  2 . 
         [0025]    On the outer side of the lid portion  52 , a positive electrode terminal  53  and a negative electrode terminal  54  for establishing a connection to the outside are provided. The positive electrode terminal  53  and the negative electrode terminal  54  are insulated from each other, and a part of the lid portion  52  which faces the inside of the battery container  51  is also insulated by an insulating film or the like. One end of the current collector of positive electrode  11  is connected to the positive electrode terminal  53  by a lead wire  55 , while one end of the current collector of negative electrode  21  is connected to the negative electrode terminal  54  by a lead wire  56 . The lead wire  55  and the lead wire  56  are insulated from the lid portion  52 . The lid portion  52  is firmly fixed to the battery container  51  by welding. 
         [0026]    The electrolyte of the molten salt battery is a molten salt which is a conductive liquid in a molten state. At a temperature equal to or higher than the melting point of the molten salt, the molten salt is melted into an electrolytic solution, and the molten salt battery operates as a secondary battery. It is desirable that in the electrolyte, a plurality of molten salts be mixed for lowering the melting point. For example, the electrolyte is a mixed salt of NaFSA with the sodium ion as a cation and FSA as an anion, and KFSA with the potassium ion as a cation and FSA as an anion. 
         [0027]    The molten salt as an electrolyte may contain other anions such as TFSA (bistrifluoromethylsulfonylamide) or FTA (fluorotrifluoromethylsulfonylamide), and may contain other cations such as organic ions. In this form, sodium ions are carriers of charges in the electrolyte. 
         [0028]    The configuration of the molten salt battery shown in  FIG. 1  is a schematic configuration, and other components (not shown) such as a heater for heating the inside or a temperature sensor may be included in the molten salt battery.  FIG. 1  shows a principal configuration in which a pair of the positive electrode  1  and the negative electrode  2  is provided, but a more practical molten salt battery of the present invention may have a configuration in which a plurality of positive electrodes  1  and negative electrodes  2  are alternately arranged, and mutually superimposed with the separator  3  sandwiched between both the adjacent electrodes. 
         [0029]    Next, details of the separator  3  will be described. The separator  3  is a sheet-shaped member having Al 2 O 3  (aluminum oxide) or ZrO 2  (zirconium oxide) as main components. The separator  3  is impregnated therein with the molten salt, and has a porous structure so that carriers of charges are transferred between the positive electrode  1  and the negative electrode  2 . 
         [0030]    For example, the separator  3  is a woven fabric or nonwoven fabric formed of fibers having Al 2 O 3  as a main component, or a woven fabric or nonwoven fabric formed of fibers having ZrO 2  as a main component. The separator  3  may be configured to have both Al 2 O 3  and ZrO 2 , such as a nonwoven fabric formed by mixing fibers having Al 2 O 3  as a main component and fibers having ZrO 2  as a main component. The separator  3  may be non-fibrous sheet having Al 2 O 3  or ZrO 2  as a main component. 
         [0031]    The content of Al 2 O 3  and/or ZrO 2  contained in the separator  3  in the present invention is 75% or more in terms of % by mass. In the separator  3 , the content of Al 2 O 3  alone may be 75% by mass or more, or the content of ZrO 2  alone may be 75% by mass or more. The content of the total of Al 2 O 3  and ZrO 2  may be 75% by mass or more. 
         [0032]    Since the content of Al 2 O 3  and/or ZrO 2  is 75% by mass or more, the separator  3  can maintain a function of a mechanically and chemically stable separator, and there can be provided a molten salt battery which stably functions even when an electrolyte alternates between a liquid state and a solid state as the battery is alternately operated and stopped, i.e., the problem of conventional molten salt batteries can be solved. 
         [0033]    In addition, there can be provided a molten salt battery with high safety in which the risk of heat generation/ignition or the like is extremely low even if an abnormal situation such as an internal short-circuit is encountered. 
         [0034]    It is desirable that the content of Al 2 O 3  and/or ZrO 2  contained in the separator  3  be 100% by mass in principle. However, other metal oxides other than Al 2 O 3  and ZrO 2  and an organic compound may be contained as components (remainders) of the separator  3  in an amount of 25% by mass or less from the viewpoint of cost reduction and shape retention of the separator. Also in this case, an effect similar to that of a separator formed of pure Al 2 O 3  and/or ZrO 2  can be exerted. 
         [0035]    As the other metal oxides other than Al 2 O 3  and ZrO 2 , silica (SiO 2 ), yttria (Y 2 O 3 ), titania (TiO 2 ), ceria (CeO 2 ) and the like can be used. 
         [0036]    As the organic compound, one that is frequently used as a separator for a lithium ion secondary battery can be used. This is, for example, an organic compound of a polyolefin such as polyethylene (PE) or polypropylene (PP), a polyamide such as nylon or aramid, or the like. 
         [0037]    Examples of the separator having Al 2 O 3  and/or ZrO 2  as a main component include those formed by integrating fibers of these metal oxides into a sheet such as a felt (nonwoven fabric) or a cloth (woven fabric), and those formed by integrating powders (particles) of these metal oxides into a sheet. The other metal oxides other than Al 2 O 3  and ZrO 2  act as a binder for integrating them into a sheet. 
         [0038]    On the other hand, the organic compound such as a polyolefin or a polyamide similarly acts as a binder for ceramic fibers or powders. Using the organic compound as a material, a nonwoven fabric or a porous organic material sheet having a microporous membrane may be formed like the separator for a lithium ion secondary battery. In this case, two layers of the organic material sheet and a ceramic sheet of an Al 2 O 3  and/or ZrO 2  material or the like may be laminated and integrated to form a separator for this molten salt battery. The lamination of these two layers can be performed by thermocompression bonding. 
         [0039]    These separators of the present invention are excellent in mechanical, thermal and chemical resistance among electrolytes that are used around 100° C. As a result of being excellent in mechanical, thermal and chemical resistance, the probability of occurrence of an internal short-circuit ascribable to the separator is extremely low. The separator having Al 2 O 3  and/or ZrO 2  as a main component has such a characteristic of high safety that the risk of heat generation/ignition or the like is extremely low even if an abnormal situation such as an internal short-circuit is encountered. 
         [0040]    It is desirable that the thickness of the separator  3  be 0.02 to 0.5 mm. If the thickness of the separator  3  is less than 0.02 mm, the strength of the separator  3  having Al 2 O 3  or ZrO 2  as a main component is reduced, so that the separator is easily damaged. If the thickness of the separator  3  is more than 0.5 mm, the internal resistance of the molten salt battery is increased, and the volume energy density of the molten salt battery is reduced. It is desirable that the porosity of the separator  3  be 20 to 80%. If the porosity of the separator  3  is less than 20%, the internal resistance of the molten salt battery is increased. If the porosity of the separator  3  is more than 80%, the risk of occurrence of a short-circuit by direct contact of the positive electrode  1  and the negative electrode  2  is increased. 
         [0041]    In the above embodiment, the current collector of positive electrode  11  and the current collector of negative electrode  21  are made of aluminum, but may be made of any other electric conductor. The shape of the molten salt battery is not limited to the shape of rectangular parallelepiped, and may be any other shape. For example, the shape of the molten salt battery may be a cylindrical shape. 
       EXAMPLES 
       [0042]    Next, the present invention will be described further in detail based on Examples. However, Examples are not intended to limit the scope of the present invention. 
         [0043]    Durability as a separator for a molten salt battery was evaluated for separators as Examples of the present invention and previously known various separators as Comparative Examples. The configurations of components of separators subjected to the evaluation are shown in Table 1. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Major configuration of separator 
                   
               
               
                 Battery 
                 (composition: % by mass) 
                 Category 
               
               
                   
               
             
             
               
                 A 
                 Al 2 O 3 , 100% 
                 Present invention 
               
               
                 B 
                 ZrO 2 , 100% 
                 Present invention 
               
               
                 C 
                 Al 2 O 3 , 60% ZrO 2 , 40% 
                 Present invention 
               
               
                 D 
                 Al 2 O 3 , 75% SiO 2 , 25% 
                 Present invention 
               
               
                 E 
                 ZrO 2 , 92% Y 2 O 3 , 8% 
                 Present invention 
               
               
                 F 
                 Al 2 O 3 , 80% Polyolefin, 20% 
                 Present invention 
               
               
                 G 
                 Al 2 O 3 , 76% Aramid, 24% 
                 Present invention 
               
               
                 H 
                 Polyolefin, 100% 
                 Comparative Example 
               
               
                 I 
                 Aramid, 100% 
                 Comparative Example 
               
               
                 J 
                 Polyolefin, 70% Al 2 O 3 , 30% 
                 Comparative Example 
               
               
                   
               
             
          
         
       
     
         [0044]    Molten salt batteries used for evaluation of durability each have a configuration similar to that in  FIG. 1 , and all have identical electrolytes, positive electrodes and negative electrodes. Ten molten salt batteries of A to J shown in Table 1, which are only different in separators, were prepared. Batteries prepared using separators of A to G each contain Al 2 O 3  and/or ZrO 2  as a metal oxide material in an amount of 75% by mass or more, and belongs to the present invention. On the other hand, batteries prepared using separators of H to J are previously well known batteries of Comparative Examples, and the separators of H and I each contain neither Al 2 O 3  nor ZrO 2 . The separator of J contains Al 2 O 3 , but the ratio thereof is only 30%. 
         [0045]    The molten salt batteries of A to J prepared using the separators of A to J were evaluated as follows. Specifically, the prepared batteries of A to J were heated to 90° C. Charge and discharge were repeated at 90° C. to confirm that all the batteries of A to J normally had initial battery characteristics. Thereafter, heating of the batteries was stopped, and the batteries were allowed to cool to room temperature. 
         [0046]    From this state, the following temperature cycle test was started.
   (1) The battery is heated from room temperature to 90° C. for 5 hours.   (2) Constant-voltage charge to 3.5 V at a 90° C.-5 hour rate (0.2 C) and charge-and-rest for 1 hour are performed.   (3) A charge-discharge test of constant-current discharge to a final voltage of 2.8 V at a 5 hour rate (0.2 C) and discharge-and-rest for 1 hour is conducted.   (4) At the time of completion of the above test, heating of the battery was stopped, and the battery was allowed to cool to room temperature.   
 
         [0051]    Charge-discharge characteristic data were observed while the temperature cycle test of repeating a series of step of temperature elevation, charge-discharge and temperature lowering in (1) to (4) above was conducted. 
         [0052]    As a result, in the batteries of H to J of Comparative Examples, a degradation in performance of the battery was observed with a relatively small number of temperature cycle tests as compared to the batteries of A to G. The specific degradation in performance includes a reduction in discharge capacity of the battery, a reduction in open-circuit voltage (OCV) at a leaving time after charging/discharging, and the like. A battery, for which a degradation in performance was observed once, was accelerately degraded in performance in subsequent temperature cycle tests. 
         [0053]    In contrast, the batteries of A to G of the present invention maintained stable performance over a time period about two times as long as that for the batteries of H to J of Comparative Examples in terms of the number of temperature cycle tests. That is, according to the present invention, a function of a mechanically and chemically stable separator can be maintained, so that there can be provided a molten salt battery which stably functions even when an electrolyte alternates between a liquid state and a solid state as the battery is alternately operated and stopped. 
         [0054]    After the test, the separators of the batteries of H to J, which were degraded in performance, were disassembled and observed to find a situation in which the separator was partially cracked, and even ruptured in a severe case. That is, such a degradation in performance of the battery is ascribable to an internal short-circuit of a battery with a separator interposed therein. 
       INDUSTRIAL APPLICABILITY 
       [0055]    A molten salt battery of the present invention is capable of being used not only for storage of electric power but also as a general-purpose secondary battery. 
       REFERENCE SIGNS LIST 
       [0056]      1 : POSITIVE ELECTRODE 
         [0057]      11 : CURRENT COLLECTOR OF POSITIVE ELECTRODE 
         [0058]      12 : POSITIVE ELECTRODE MATERIAL 
         [0059]      2 : NEGATIVE ELECTRODE 
         [0060]      21 : CURRENT COLLECTOR OF NEGATIVE ELECTRODE 
         [0061]      22 : NEGATIVE ELECTRODE MATERIAL 
         [0062]      3 : SEPARATOR