Abstract:
A separator ( 3 ) of a molten salt battery is impregnated with a molten salt that serves as the electrolyte. The molten salt contains, as cations, at least one kind of ions selected from among quaternary ammonium ions, imidazolium ions, imidazolinium ions, pyridinium ions, pyrrolidinium ions, piperidinium ions, morpholinium ions, phosphonium ions, piperazinium ions and sulfonium ions in addition to sodium ions. These cations do not have adverse effects on a positive electrode ( 1 ). In addition, the melting point of the molten salt, which contains sodium ions and the above-mentioned cations, is significantly lower than the operating temperature of sodium-sulfur batteries, said operating temperature being 280-360 DEG C. Consequently, the molten salt battery is capable of operating at lower temperatures than sodium-sulfur batteries.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a molten-salt battery using a molten salt as an electrolyte. 
       BACKGROUND OF THE INVENTION 
       [0002]    Utilization of natural energy such as solar power or wind power has been recently promoted. In power generation by natural energy, the amount of power generation is likely to be changed due to the effects of weather conditions or the like. For this reason, the power supply should be leveled by charge/discharge using a storage battery for supplying generated power. That is, a storage battery having high energy density and high efficiency is essential in promoting utilization of natural energy. Such storage batteries include a sodium-sulfur battery disclosed in Patent Document 1. In the sodium-sulfur battery, sodium ions are used as conducting ions. Other storage batteries having high energy density and high efficiency include a molten-salt battery. 
         [0003]    Molten-salt batteries are batteries using molten salts as electrolytes and operate in a state where molten salts are molten. As a molten-salt battery, a battery using sodium ions for the conducting ions is known. In such a molten-salt battery, molten salts that contain sodium ions are used as electrolytes. A sodium-sulfur battery should operate at a temperature as high as 280-360° C. Also, a molten-salt battery should operate at the melting point of the molten salt or higher. For this reason, development of a molten-salt battery that operates at a lower temperature has been desired. 
         [0004]    The melting point of a molten salt as an electrolyte should be lowered to lower the operating temperature of the molten-salt battery. In general, when two salts are mixed, the melting point is lowered. Thus, it has been considered that a mixed salt in which a sodium salt and another cation salt are mixed is used for a molten-salt battery using a sodium ion as the conducting ion. The mixed salts may include, for example, a mixed salt of sodium salt and potassium salt, a mixed salt of sodium salt and cesium salt or the like. However, when the mixed salt of sodium salt and potassium salt is used, potassium ions enter into a positive-electrode active material in the molten-salt battery. Thereby, the crystal structure of the positive-electrode active material is changed, and the positive electrode may be deteriorated. When a mixed salt of sodium salt and cesium salt is used, the cesium ion may also cause deterioration of the positive electrode. In addition, because cesium is expensive due to its scarcity, the use of cesium increases the cost of the molten-salt battery. 
       PRIOR ART DOCUMENT 
     Patent Document 
       [0005]    Patent document 1: Japanese Published Unexamined Application No. 2007-273297 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the present invention is to provide a molten-salt battery that can lower the operating temperature without deterioration of a positive electrode by using, as an electrolyte, a cation-containing molten salt that does not cause adverse effects on the positive-electrode active material. 
         [0007]    In order to solve the problems, according to the first aspect of the present invention, the molten-salt battery using the molten salt comprising the sodium ion as cation for the electrolyte is provided. The molten salt comprises, as anion, an ion, the general chemical structural formula of which is represented by the following formula (1) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein X 1  and X 2  are the same or different from each other, and each of them is a fluoro group or a fluoroalkyl group), and comprises, as cation, the sodium ion as well as at least one organic cation included in an organic cation group consisting of; a quaternary ammonium ion, the chemical structural formula of which is represented by the following formula (2) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 1 , R 2 , R 3  and R 4  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms or an alkyloxyalkyl group having 1-8 carbon atoms); an imidazolium ion, the chemical structural formula of which is represented by the following formula (3) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 5  and R 6  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms); an imidazolinium ion, the chemical structural formula of which is represented by the following formula (4) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 7  and R 8  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms); a pyridinium ion, the chemical structural formula of which is represented by the following formula (5) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 9  is an alkyl group having 1-8 carbon atoms); a pyrrolidinium ion, the chemical structural formula of which is represented by the following formula (6) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 10  and R 11  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms); a piperidinium ion, the chemical structural formula of which is represented by the following formula (7) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 12  and R 13  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms); a morpholinium ion, the chemical structural formula of which is represented by the following formula (8) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 14  and R 15  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms); a phosphonium ion, the chemical structural formula of which is represented by the following formula (9) 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 16 , R 17 , R 18  and R 19  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms, an alkyloxyalkyl group having 1-8 carbon atoms or a phenyl group); a piperazinium ion, the chemical structural formula of which is represented by the following formula 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 20 , R 21 , R 22  and R 23  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms); and a sulfonium ion, the chemical structural formula of which is represented by the following formula 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein R 24 , R 25  and R 26  are the same or different from each other, and each of them is an alkyl group having 1-8 carbon atoms). 
         [0008]    In accordance with the construction above, the molten salt to be used as the electrolyte in the molten-salt battery comprises, as cation, the sodium ion as well as at least one of the quaternary ammonium ion, the imidazolium ion, the imidazolinium ion, the pyridinium ion, the pyrrolidinium ion, the piperidinium ion, the morpholinium ion, the phosphonium ion, the piperazinium ion and the sulfonium ion. Thereby, the melting point of the molten salt is considerably lower than 280-360° C. where the sodium-sulfur battery operates. 
         [0009]    In the molten-salt battery, the molten salt preferably comprises, as cation, the sodium ion as well as the quaternary ammonium ion in which R 1 , R 2 , R 3  and R 4  of formula (2) are the same or different from each other, and each of them is the alkyl group having 1-6 carbon atoms. 
         [0010]    In the molten-salt battery, the molten salt preferably comprises, as cation, the sodium ion as well as the imidazolium ion in which one of R 5  and R 6  of formula (3) is the methyl group, and the other one is the alkyl group having 1-6 carbon atoms. 
         [0011]    In the molten-salt battery, the molten salt preferably comprises, as cation, the sodium ion as well as the pyrrolidinium ion in which one of R 10  and R 11  of formula (6) is the methyl group, and the other one is the alkyl group having 1-6 carbon atoms. 
         [0012]    In the molten-salt battery, the molten salt preferably comprises, as cation, the sodium ion as well as the piperidinium ion in which one of R 12  and R 13  of formula (7) is the methyl group, and the other one is the alkyl group having 1-6 carbon atoms. 
         [0013]    In the molten-salt battery, the molten salt preferably comprises no potassium ion. 
         [0014]    In accordance with the construction above, the molten salt to be used as the electrolyte in the molten-salt battery comprises no potassium ion. Thereby, the positive electrode of the molten-salt battery is not deteriorated by the potassium ion. 
         [0015]    The molten-salt battery preferably comprises the positive electrode that contains NaCrO 2  as the positive-electrode active material, and the negative electrode that contains tin, sodium or a carbon material as the negative-electrode active material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional view of the molten-salt battery according to the present invention. 
           [0017]      FIG. 2  is a table that represents molar ratios in mixed salts of TMHA-FSA and NaFSA, and states of each mixed salt in each molar ratio at room temperature. 
           [0018]      FIG. 3  is a table that represents molar ratios in a mixed salt of EMI-FSA and NaFSA, and states of each mixed salt in each molar ratio at room temperature. 
           [0019]      FIG. 4  is a table that represents molar ratios in a mixed salt of P13-FSA and NaFSA, and states of each mixed salt in each molar ratio at room temperature. 
           [0020]      FIG. 5  is a graph that represents charge/discharge properties of the molten-salt battery using the mixed salt of P13-FSA and NaFSA as an electrolyte. 
           [0021]      FIG. 6  is a characteristic chart that represents the results of a charging test of the molten-salt battery using Na 2/3 Fe 1/3 Mn 2/3 O 2  for the positive-electrode active material. 
           [0022]      FIG. 7  is a characteristic chart that represents the results of a discharging test of the molten-salt battery using Na 2/3 Fe 1/3 Mn 2/3 O 2  for the positive-electrode active material. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    A molten-salt battery according to one embodiment of the present invention will hereinafter be specifically described with reference to  FIGS. 1 to 7 . 
         [0024]    As shown in  FIG. 1 , the molten-salt battery comprises a rectangular parallelepiped box-like battery container  51 . An opening is formed at the top of the battery container  51 . In the battery container  51 , a positive electrode  1 , a separator  3  and a negative electrode  2  are arranged. A lid  52  is attached to the battery container  51  so as to close the opening. The positive electrode  1  and the negative electrode  2  are formed in a rectangular plate-like shape. The separator  3  is formed in a sheet-like shape. The separator  3  is inserted between the positive electrode  1  and the negative electrode  2 . The positive electrode  1 , the separator  3  and the negative electrode  2  are laminated. In addition, the positive electrode  1 , the separator  3  and the negative electrode  2  are disposed in a direction perpendicular to the bottom of the battery container  51 . 
         [0025]    A spring  41  and a presser plate  42  are disposed between the negative electrode  2  and the inner wall of the battery container  51 . The spring  41  is made of an aluminum alloy and formed in a corrugated sheet-like shape. The presser plate  42  is inflexible and formed in a plate-like shape. The spring  41  urges the presser plate  42  to press the negative electrode  2  toward the separator  3  and the positive electrode  1 . The positive electrode  1  is counteracted by the spring  41 . That is, the positive electrode  1  is pressed from the inner wall of the battery container  51  on opposite side of the spring  41  toward the separator  3  and the negative electrode  2 . The spring  41  is not limited to metal springs or the like, and for example may be an elastic body like a rubber. When the positive electrode  1  or the negative electrode  2  swells or contracts by charge/discharge, volume change of the positive electrode  1  or the negative electrode  2  is absorbed by expansion and contraction of the spring  41 . 
         [0026]    The positive electrode  1  is formed by applying a positive-electrode material  12  on a positive-electrode current collector  11 . The positive-electrode current collector  11  is made of aluminum and formed in a rectangular plate-like shape. The positive-electrode material  12  comprises the positive-electrode active material like NaCrO 2  and a binder. It should be noted that the positive-electrode active material is not limited to NaCrO 2 . The negative electrode  2  is formed by plating a negative-electrode material  22  on a negative-electrode current collector  21 . The negative-electrode current collector  21  is made of aluminum and formed in a rectangular plate-like shape. A negative-electrode material  22  comprises the negative-electrode active material like tin. When the negative-electrode material  22  is plated on the negative-electrode current collector  21 , zincate treatment is conducted. In detail, it is plated with zinc, followed by tin as a foundation. The negative-electrode active material is not limited to tin and for example may be a metallic sodium, a carbon material, silicon or indium. The negative-electrode material  22  is formed by applying for example, a negative-electrode active material powder that contains the binder on negative-electrode current collector  21 . Preferably, the positive-electrode active material is NaCrO 2 , and the negative-electrode active material is tin, a metallic sodium or a carbon material. The carbon material is mainly composed of carbon, preferably a hard carbon. The positive-electrode current collector  11  and the negative-electrode current collector  21  are not limited to aluminum, and for example may be stainless steel or nickel. The separator  3  is composed of an insulating material such as a silica glass or a resin. The separator  3  comprises the electrolyte inside and is formed in a form that the sodium ion can pass through. The separator  3  is made of, for example, a glass fabrics or a porous resin. 
         [0027]    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  face each other. The separator  3  is inserted between the positive electrode  1  and the negative electrode  2 . The separator  3  is impregnated with the molten salt as the electrolyte. The molten salt in the separator  3  is in contact with both the positive-electrode material  12  of the positive electrode  1  and the negative-electrode material  22  of the negative electrode  2 . The inner face of the battery container  51  is coated with an insulating resin to prevent short-circuiting between the positive electrode  1  and the negative electrode  2 . On the outer side of the lid  52 , a positive terminal  53  and a negative terminal  54 , which are connected to an external terminal, are installed. The positive terminal  53  and the negative terminal  54  are insulated from each other. Also, the inner side of the lid  52  is insulated by an insulating coat or the like. The upper end portion of the positive-electrode current collector  11  is connected to the positive terminal  53  through the lead wire. The upper end portion of the negative-electrode current collector  21  is connected to the negative terminal  54  through the lead wire. The lead wire is insulated from the lid  52 . The lid  52  is attached to the battery container  51 . 
         [0028]    The molten salt infiltrating in the separator  3  is an ionic salt composed of sodium ion-containing cations and anions. The composition of the molten salt will be described later. The molten salt is molten at a temperature of its melting point or higher and becomes a conductive liquid containing the sodium ion. The molten-salt battery can operate as a secondary battery within a temperature range where the molten salt is molten. At this time, for the molten-salt battery, a molten salt containing the sodium ion is used as an electrolytic solution. During discharge, the sodium ion transfers from the negative electrode  2  to the positive electrode  1  in the electrolytic solution and is absorbed in the positive-electrode active material. 
         [0029]    Next, the composition of the molten salt will be described. 
         [0030]    The general chemical structural formula of the anion in the molten salt is represented by formula (1) mentioned above. In the formula (1), each of X 1  and X 2  is the fluoro group or the fluoroalkyl group. X 1  and X 2  may be the same or different from each other. In the anion represented by formula (1), each of X 1  and X 2  is preferably the fluoro group or the fluoroalkyl group having 1-8 carbon atoms. More preferably, the anion is an anion in which both X 1  and X 2  are the fluoro group, an anion in which both X 1  and X 2  are the fluoromethyl group, or an anion in which one of X 1  and X 2  is the fluoro group and the other one is the fluoromethyl group. When both X 1  and X 2  are the fluoro group, the anion is an FSA (bis-fluoro-sulfonylamide) ion. The chemical structural formula of the FSA ion is represented by the following formula (12). The FSA ion has two fluoro groups. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0031]    In formula (1), when both X 1  and X 2  are trifluoromethyl groups, the anion is a TFSA (bis-trifluoro-methylsulfonylamide) ion. The chemical structural formula of the TFSA ion is represented by the following formula (13). The TFSA ion has two trifluoromethyl groups. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0032]    In formula (1) mentioned above, when one of X 1  and X 2  is the fluoro group and the other one is the trifluoromethyl group, the anion is an FTA (fluoro-trifluoro-methylsulfonylamide) ion. The chemical structural formula of the FTA ion is represented by the following formula (14). The FTA ion has the fluoro group and the trifluoromethyl group. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0033]    The molten salt comprises, for example, the FSA ion, the TFSA ion or the FTA ion as anion. In addition, the anion may be an anion that has a fluoroalkyl group other than trifluoromethyl groups. 
         [0034]    In addition, the molten salt contains the sodium ion as cation, and further at least one organic cation included in an organic cation group consisting of the quaternary ammonium ion, the imidazolium ion, the imidazolinium ion, the pyridinium ion, the pyrrolidinium ion, the piperidinium ion, the morpholinium ion, the phosphonium ion, the piperazinium ion and the sulfonium ion. 
         [0035]    The general chemical structural formula of the quaternary ammonium ion is represented by formula (2) described above. In formula (2), R 1 , R 2 , R 3  and R 4  are each the alkyl group having 1-8 carbon atoms or the alkyloxyalkyl group having 1-8 carbon atoms. R 1 , R 2 , R 3  and R 4  may be the same or different from each other. In the quaternary ammonium ion, each of R 1 , R 2 , R 3  and R 4  is preferably the alkyl group having 1-6 carbon atoms. Since the molten salt containing the quaternary ammonium ion in which each of R 1 , R 2 , R 3  and R 4  is the alkyl group having 1-6 carbon atoms is excellent in resistance to reduction, it can stably coexist with sodium metals. This molten salt expresses excellent durability when used as the electrolyte for the molten-salt battery. The specific preferable examples include a trimethyl-n-hexylammonium ion, a trimethyl-n-octylammonium ion, an ethyldimethylpropylammonium ion and a methyl (2-methoxyethyl)dimethylammonium ion. For example, the chemical structural formula of the TMHA (trimethyl-n-hexylammonium) ion is represented by the following formula (15). The TMHA ion has three methyl groups and one hexyl group. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0036]    The molten salt using the TMHA ion is a mixed salt of a salt that contains the TMHA ion as cation and a salt that contains the sodium ion as cation. For example, the molten salt is a mixed salt of a TMHA-FSA salt, which contains the TMHA ion as cation and the FSA as anion, and of a NaFSA salt, which contains the sodium ion as cation and the FSA as anion. In addition, the quaternary ammonium ion to be used in the present invention may have other alkyl groups. 
         [0037]    The general chemical structural formula of the imidazolium ion is represented by formula (3) described above. In formula (3), each of R 5  and R 6  is the alkyl group having 1-8 carbon atoms. R 5  and R 6  may be the same or different from each other. In the imidazolium ion represented by formula (3), an imidazolium ion in which one of the R 5  and R 6  in formula (3) is the methyl group and the other one is the alkyl group having 1-6 carbon atoms is preferable. Since such an imidazolium ion-containing molten salt is excellent in resistance to reduction, it can stably coexist with sodium metals and express excellent durability when used as the electrolyte for the molten-salt battery. In addition, since the molten salt tends to show a particularly low melting point, the molten-salt battery can be operated from a low temperature. The specific preferable examples include a 1-ethyl-3-methylimidazolium ion, a 1-propyl-3-methylimidazolium ion, a 1-butyl-3-methylimidazolium ion, a 1-hexyl-3-methylimidazolium ion and a 1,3-dimethylimidazolium ion. The chemical structural formula of the EMI (1-ethyl-3-methylimidazolium) ion is represented by the following formula (16). In the EMI ion represented by formula (3) described above, R 5  is the ethyl group and R 6  is the methyl group. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0038]    In addition, the chemical structural formula of the BMI (1-butyl-3-methylimidazolium) ion is represented by the following formula (17). In the BMI ion represented by formula (3) described above, R 5  is the butyl group and R 6  is the methyl group. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0039]    The molten salt using the imidazolium ion is a mixed salt of a salt that contains the imidazolium ion as cation and a salt that contains the sodium ion as cation. For example, the molten salt is a mixed salt of an EMI-FSA salt that contains the EMI ion as cation and the FSA as anion and of NaFSA. In addition, the imidazolium ion may have other alkyl groups. 
         [0040]    The general chemical structural formula of the imidazolinium ion is represented by formula (4) described above. In formula (4), each of R 7  and R 8  is the alkyl group having 1-8 carbon atoms. R 7  and R 8  may be the same or different from each other. 
         [0041]    The general chemical structural formula of the pyridinium ion is represented by formula (5) described above. In formula (5), R 9  is the alkyl group having 1-8 carbon atoms. The preferable examples of the pyridinium ion include a 1-methylpyridinium ion, a 1-ethylpyridinium ion, a 1-propylpyridinium ion and a 1-butylpyridinium ion. The chemical structural formula of the BPy (1-butylpyridinium) ion is represented by the following formula (18). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0042]    In the BPy ion represented by formula (5) described above, R 9  is the butyl group. In addition, the pyridinium ion represented by formula (5) may have other alkyl groups. 
         [0043]    The general chemical structural formula of the pyrrolidinium ion is represented by formula (6) described above. In formula (6), each of R 10  and R 11  is the alkyl group having 1-8 carbon atoms. R 10  and R 1l  may be the same or different from each other. In the pyrrolidinium ion, preferably, one of R 10  and R 11  is the methyl group, and the other one is the alkyl group having 1-6 carbon atoms. Since the molten salt containing the pyrrolidinium ion in which one of R 10  and R 11  is the methyl group and the other one is the alkyl group having 1-6 carbon atoms is excellent in resistance to reduction, it can stably coexist with sodium metals. This molten salt expresses excellent durability when used as the electrolyte for the molten-salt battery. In addition, since the molten salt tends to show a particularly low melting point, the molten-salt battery can be operated from a low temperature. The specific preferable examples include a 1-methyl-1-ethylpyrrolidinium ion, a 1-methyl-1-propylpyrrolidinium ion and a 1-methyl-1-butylpyrrolidinium ion. The chemical structural formula of the 1-methyl-1-butylpyrrolidinium ion is represented by the following formula (19). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0044]    In the 1-methyl-1-butylpyrrolidinium ion represented by formula (6) described above, R 10  is the methyl group and R 11  is the butyl group. Additionally, in the P13 (1-methyl-1-propylpyrrolidinium) ion represented by formula (6) described above, R 10  is the methyl group and R 11  is the propyl group. The molten salt using the pyrrolidinium ion is a mixed salt of a salt that contains the pyrrolidinium ion as cation and of a salt that contains the sodium ion as cation. For example, the molten salt is a mixed salt of a P13-FSA salt that contains the P13 ion as cation and the FSA as anion and of NaFSA. In addition, the pyrrolidinium ion may have other alkyl groups. 
         [0045]    The general chemical structural formula of the piperidinium ion is represented by formula (7) described above. In formula (7), each of R 12  and R 13  is the alkyl group having 1-8 carbon atoms. R 12  and R 13  may be the same or different from each other. In the piperidinium ion, preferably, one of R 12  and R 13  is the methyl group, and the other one is the alkyl group having 1-6 carbon atoms. Since the molten salt containing the piperidinium ion in which one of R 12  and R 13  is the methyl group and the other one is the alkyl group having 1-6 carbon atoms is excellent in resistance to reduction, it can stably coexist with sodium metals. This molten salt expresses excellent durability when used as the electrolyte for the molten-salt battery. In addition, since the molten salt tends to show a particularly low melting point, the molten-salt battery can be operated from a low temperature. The specific preferable examples include a 1,1-dimethylpiperidinium ion, a 1-methyl-1-ethylpiperidinium ion and a 1-methyl-1-propylpiperidinium ion. 
         [0046]    The general chemical structural formula of the morpholinium ion is represented by formula (8) described above. In formula (8), each of R 14  and R 15  is the alkyl group having 1-8 carbon atoms. R 14  and R 15  may be the same or different from each other. The preferable examples of the morpholinium ion include a 1,1-dimethylmorpholinium ion, a 1-methyl-1-ethylmorpholinium ion, a 1-methyl-1-propylmorpholinium ion and a 1-methyl-1-butylmorpholinium ion. 
         [0047]    The general chemical structural formula of the phosphonium ion is represented by formula (9) described above. In formula (9), each of R 16 , R 17 , R 18  and R 19  is the alkyl group having 1-8 carbon atoms, the alkyloxyalkyl group having 1-8 carbon atoms or the phenyl group. R 16 , R 17 , R 18  and R 19  may be the same or different from each other. The preferable examples of the phosphonium ion include a triethyl(methoxyethyl) phosphonium ion and a methyltriphenylphosphonium ion. 
         [0048]    The general chemical structural formula of the piperazinium ion is represented by formula (10) described above. In formula (10), each of R 20 , R 21 , R 22  and R 23  is the alkyl group having 1-8 carbon atoms. R 20 , R 21 , R 22  and R 23  may be the same or different from each other. The preferable examples of the piperazinium ion include a 1,1,4,4-tetramethylpiperazinium ion and a 1,1-dimethyl-4,4-diethylpiperazinium ion. 
         [0049]    The general chemical structural formula of the sulfonium ion is represented by formula (11) described above. In formula (11), each of R 24 , R 25  and R 26  is the alkyl group having 1-8 carbon atoms. R 24 , R 25  and R 26  may be the same or different from each other. The preferable examples of the sulfonium ion include a trimethylsulfonium ion and a triethylsulfonium ion, a methyldiethylsulfonium ion and a methyldipropylsulfonium ion. 
         [0050]    As stated above, the molten salt used for the molten-salt battery of the present invention comprises, as cation, the sodium ion as well as at least one organic cation included in an organic cation group consisting of the quaternary ammonium ion, the imidazolium ion, the imidazolinium ion, the pyridinium ion, the pyrrolidinium ion, the piperidinium ion, the morpholinium ion, the phosphonium ion, the piperazinium ion and the sulfonium ion. That is, the molten salt is a mixture of a salt that contains the sodium ion as cation and of one or more salts that contains the quaternary ammonium ion, the imidazolium ion, the imidazolinium ion, the pyridinium ion, the pyrrolidinium ion, the piperidinium ion, the morpholinium ion, the phosphonium ion, the piperazinium ion or the sulfonium ion as cation. Previous studies have demonstrated that the melting point of the molten salt comprising the anion that has the chemical structural formula shown in formula (1), and the cation, which is the quaternary ammonium ion, the imidazolium ion, the imidazolinium ion, the pyridinium ion, the pyrrolidinium ion, the piperidinium ion, the morpholinium ion, the phosphonium ion, the piperazinium ion or the sulfonium ion is considerably lower than 280-360° C. where the sodium-sulfur battery operates. In addition, the molten salt to be used for the molten-salt battery of the present invention is a mixture of various salts. Thus, the melting point of the molten salt is lower compared to a molten salt consisting of one kind of salt. Consequently, the melting point of the molten salt to be used for the molten-salt battery of the present invention is considerably lower than 280-360° C. where the sodium-sulfur battery operates. For these reasons, the operating temperature of the molten-salt battery of the present invention can be considerably lowered than that of the sodium-sulfur battery. 
         [0051]    Additionally, the molten salt to be used for the molten-salt battery of the present invention contains no potassium ion. The potassium ion enters into the positive-electrode active material in the positive-electrode material  12 . Also, the potassium ion changes the crystal structure of the positive-electrode active material and causes deterioration of the positive electrode  1 . Neither does the molten salt to be used for the molten-salt battery of the present invention contain the cesium ion. Like the potassium ion, the cesium ion also causes deterioration of the positive electrode  1 . Thus, since the molten salt of the present invention contains neither the potassium ion nor the cesium ion, the positive electrode  1  of the molten-salt battery is neither deteriorated by the potassium ion nor the cesium ion. In addition, since the quaternary ammonium ion, the imidazolium ion, the imidazolinium ion, the pyridinium ion, the pyrrolidinium ion, the piperidinium ion, the morpholinium ion, the phosphonium ion, the piperazinium ion or the sulfonium ion does not enter the positive-electrode active material in the positive-electrode material  12 , the positive electrode  1  is not deteriorated. Thus, the molten salt of the present invention contains no component that deteriorates the positive electrode  1 . Thereby, the operating temperature of the molten-salt battery can be considerably lowered than that of the sodium-sulfur battery, while a decrease in the volume of the molten-salt battery is prevented. Furthermore, the molten salt does not comprise expensive cesium ions. Thereby, an increase in the cost of the molten-salt battery can also be prevented. 
       EMBODIMENTS 
       [0052]    Subsequently, embodiments will be more specifically explained with reference to the following first to fifth embodiments. 
       First Embodiment 
       [0053]    As a molten salt, a mixed salt of TMHA-FSA and NaFSA was prepared. Then states of the mixed salt at room temperature were investigated in relation to the molar ratios of the TMHA-FSA and the NaFSA in the mixed salt. First, a TMHA-Br produced by Wako Pure Chemical Industries, Ltd. and a KFSA produced by Mitsubishi Materials Electronic Chemicals Co., Ltd. were mixed in an equimolar ratio in water for preparing a TMHA-FSA. Then a resulting precipitate was filtrated and washed with water repeatedly several times. Subsequently, the TMHA-FSA was prepared by vacuum drying at 80° C. It should be noted that Br is bromine and K is potassium. The prepared TMHA-FSA and a NaFSA produced by Mitsubishi Materials Electronic Chemicals Co., Ltd. were mixed in various molar ratios in a glove box under an argon atmosphere to investigate its melting behavior at room temperature. 
         [0054]      FIG. 2  is a table that represents molar ratios in the mixed salts of TMHA-FSA and NaFSA, and states of the mixed salts in each molar ratio at room temperature. As shown in  FIG. 2 , seven mixed salts that the molar ratios of the TMHA-FSA and the NaFSA (TMHA-FSA:NaFSA) were respectively 8:2, 7:3, 6:4, 5:5, 4:6, 3:7 and 2:8 were prepared. Any mixed salts were liquid at room temperature. These results demonstrate that melting points of respective mixed salts are lower than 280-360° C. where the sodium-sulfur battery operates. 
       Second Embodiment 
       [0055]    As a molten salt, a mixed salt of EMI-FSA and NaFSA was prepared. Then states of the mixed salt at room temperature were investigated in relation to the molar ratios of the EMI-FSA and the NaFSA in the mixed salt. The EMI-FSA was obtained from Tokyo Chemical Industry Co., Ltd. The EMI-FSA and a NaFSA produced by Mitsubishi Materials Electronic Chemicals Co., Ltd. were mixed in various molar ratios in the glove box under the argon atmosphere to investigate its melting behavior at room temperature. 
         [0056]      FIG. 3  is a table that represents molar ratios in the mixed salts of EMI-FSA and NaFSA, and states of the mixed salts in each molar ratio at room temperature. As shown in  FIG. 3 , seven mixed salts that the molar ratios of the EMI-FSA and the NaFSA (EMI-FSA:NaFSA) were respectively 8:2, 7:3, 6:4, 5:5, 4:6, 3:7 and 2:8 were prepared. The mixed salts that the molar ratios were respectively 8:2 and 7:3 were liquid at room temperature. In addition, the mixed salts of other molar ratios were in a state where liquid and solid were mixed at room temperature. These results demonstrate that any melting points of the mixed salts are lower than 280-360° C. where the sodium-sulfur battery operates. 
       Third Embodiment 
       [0057]    As a molten salt, a mixed salt of P13-FSA and NaFSA was prepared. Then states of the mixed salt at room temperature were investigated in relation to the molar ratios of the P13-FSA and the NaFSA in the mixed salt. The P13-FSA was obtained from Tokyo Chemical Industry Co., Ltd. The P13-FSA and the NaFSA produced by Mitsubishi Materials Electronic Chemicals Co., Ltd. were mixed in various molar ratios in the glove box under the argon atmosphere to investigate its melting behavior at room temperature. 
         [0058]      FIG. 4  is a table that represents molar ratios in the mixed salts of P13-FSA and NaFSA, and states of the mixed salt in each molar ratio at room temperature. As shown in  FIG. 4 , seven mixed salts that the molar ratios of the P13-FSA and the NaFSA (P13-FSA:NaFSA) were respectively 8:2, 7:3, 6:4, 5:5, 4:6, 3:7 and 2:8 were prepared. Each mixed salt that the molar ratio was respectively 8:2, 7:3, 6:4, 5:5 and 4:6 was liquid at room temperature. In addition, the molten salts of other molar ratios were in a state where liquid and solid were mixed at room temperature. These results demonstrate that any melting points of the mixed salts are lower than 280-360° C. where the sodium-sulfur battery operates. 
       Fourth Embodiment 
       [0059]    The charge/discharge properties of the molten-salt battery using the mixed salt of P13-FSA and NaFSA as the electrolyte were investigated. First, NaCO 3  produced by Wako Pure Chemical Industries, Ltd. and a CrO 2  produced by Wako Pure Chemical Industries, Ltd. were mixed in a molar ratio of 1:1 for preparing NaCrO 2 . Next, the mixture of NaCO 3  and CrO 2  was pelletized, and the resulting product was burnt under an argon stream at 1223K for 5 hours, resulting in NaCrO 2 . Then, NaCrO 2 , acetylene black and PTFE (polytetrafluoroethylene) were kneaded in a volume ratio of 80:15:5 to produce the positive-electrode material  12 . Subsequently, an aluminum mesh as the positive-electrode current collector  11  was prepared, on which the positive-electrode material  12  was bonded by pressure to produce the positive electrode  1 . In addition, the P13-FSA and the NaFSA were mixed in a molar ratio of 1:1 in the glove box under the argon atmosphere to prepare a mixed salt as the electrolyte. Then a glass mesh was immersed in the prepared mixed salt to produce the separator  3 . In addition, the negative-electrode current collector  21  made of aluminum was prepared, on which tin as the negative-electrode active material was plated to produce the negative electrode  2 . Then a lower plate made of stainless steel was prepared, on which the positive electrode  1  was disposed with the positive-electrode material  12  up. Then the separator  3  was disposed on the positive electrode  1 , and the negative electrode  2  was disposed on the separator  3 . Furthermore, an upper cover made of stainless steel was disposed on the negative electrode  2 . Eventually, the upper cover was fixed to the lower plate by a bolt and a nut to produce a battery to be used for fourth embodiment. 
         [0060]      FIG. 5  is a graph that represents the charge/discharge properties of the molten-salt battery using the mixed salt of P13-FSA and NaFSA as the electrolyte. In this embodiment, a four-cycle charge/discharge test for the produced battery was carried out. In this test, an operating temperature was set to room temperature, a voltage in starting the charging was set to 2.5 V, and a voltage in starting the discharging was set to 3 V. Additionally, a charging/discharging rate was set to 0.1 C. In  FIG. 5 , the horizontal axis represents the capacity, and the vertical axis represents the voltage of the molten-salt battery. The upward-sloping curves shown in  FIG. 5  represent the charge properties, and the downward-sloping curves represent the discharge properties. In  FIG. 5 , the properties of the second charging/discharging were represented by continuous lines, the properties of the third charging/discharging were represented by dashed-dotted lines, and the properties of the fourth charging/discharging were represented by dashed lines. As shown in  FIG. 5 , even when the operating temperature was room temperature, the molten-salt battery could charge and discharge, and the charging and discharging could be repeated with the approximately-same properties. From these results, it could be confirmed that the molten-salt battery of the present invention could operate at a lower temperature than 280-360° C. where the sodium-sulfur battery operated. 
       Fifth Embodiment 
       [0061]    An embodiment using a material other than NaCrO 2  as the positive-electrode active material will be explained. A mixed salt of 1-methyl-1-propylpyrrolidinium-FSA and NaFSA was used as the electrolyte. The Na 2/3 Fe 1/3 Mn 2/3 O 2  was used for the positive-electrode active material. The molten-salt battery having thus obtained positive electrode  1  was used to investigate the charge/discharge properties. The molten salt used for the electrode was adjusted by mixing the 1-methyl-1-propylpyrrolidinium-FSA and the NaFSA in a molar ratio of 1:1. The battery used for the experiment was a half cell that comprised a reference electrode using a metallic sodium and a positive electrode  1  using the Na 2/3 Fe 1/3 Mn 2/3 O 2  as the positive-electrode active material. The positive electrode  1  was formed by applying the positive-electrode material  12  on the rectangular plate-like positive-electrode current collector  11  made of aluminum. The positive-electrode material  12  comprises the Na 2/3 Fe 1/3 Mn 2/3 O 2  and the binder. In this embodiment, a constant current was applied while the temperature of the battery was set at 353K (80° C.) for charge and discharge of the battery. In this case, the current value per unit mass of the positive-electrode active material in the positive electrode  1  was set to 5 mA/g. 
         [0062]      FIG. 6  represents the results of the charging test of the molten-salt battery using the Na 2/3 Fe 1/3 Mn 2/3 O 2  for the positive-electrode active material. The horizontal axis in  FIG. 6  represents the battery capacity during charging. The vertical axis in  FIG. 6  represents the voltage generated between the positive electrode  1  and the reference electrode during charging. The capacity is represented as the value per unit mass of the positive-electrode active material in the positive electrode  1 .  FIG. 6  represents a charging curve obtained by the experiment. As shown in  FIG. 6 , the experiment resulted in 103 mAh/g of the charging capacity. 
         [0063]      FIG. 7  represents the results of the discharging test of the molten-salt battery using the Na 2/3 Fe 1/3 Mn 2/3 O 2  for the positive-electrode active material. The horizontal axis in  FIG. 7  represents the battery capacity during discharging. The vertical axis in  FIG. 7  represents the voltage generated between the positive electrode  1  and the reference electrode during discharging.  FIG. 7  represents the discharging curve obtained by the experiment. As shown in  FIG. 7 , the experiment resulted in 98.7 mAh/g of the discharging capacity. Consequently, the coulombic efficiency of the battery used for experiment was 96%. As apparent from the  FIG. 6  and  FIG. 7 , the sodium ion transferred, which allowed charge and discharge, even in the case of the molten-salt battery in which the mixed salt of 1-methyl-1-propylpyrrolidinium-FSA and NaFSA was used as the electrolyte and the Na 2/3 Fe 1/3 Mn 2/3 O 2  was used for the positive-electrode active material. 
         [0064]    As stated above, the molten-salt battery of the present invention can operate at a considerably lower temperature than that of the sodium-sulfur battery without a decrease in the capacity. Since the molten-salt battery operates at a low temperature, energy supplied for operating the molten-salt battery is reduced, and energy efficiency of the molten-salt battery is improved. Also, the safety of the molten-salt battery is improved due to the lowered operating temperature. Additionally, time and trouble for heating the molten-salt battery to the operating temperature can be saved. Hence, convenience of the molten-salt battery is improved. Consequently, utilization of the molten-salt battery of the present invention can realize an electric storage device having high energy density, high efficiency, and excellent safety and convenience. Also, the molten salt to be used for the molten-salt battery of the present invention is non-volatile and non-combustible. Thereby, the electric storage device with excellent safety can be realized. In addition, the molten salt to be used for the molten-salt battery of the present invention has high concentration of the sodium ion. Thereby, the sodium ion adjacent to the active material is hardly to be lost during charging and discharging, allowing the charge and discharge to be fast. 
         [0065]    In addition, the molten-salt battery of the present invention may be formed in any form other than a rectangular parallelepiped shape. For example, the molten-salt battery may be formed in a circular cylindrical shape by forming the negative electrode  2  into a circular cylindrical shape and by disposing the separator  3  in a cylindrical shape and the positive electrode  1  around the negative electrode  2 .