Patent Publication Number: US-2022238876-A1

Title: Fluoride ion secondary battery

Description:
This application is based on and claims the benefit of priority from Japanese Patent Application No. 2021-010584, filed on 26 Jan. 2021, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fluoride ion secondary battery. 
     Related Art 
     In the conventional art, fluoride ion secondary batteries are proposed using fluoride ions as carriers. Fluoride ion secondary batteries are expected to be superior in performance to lithium-ion secondary batteries, and have been studied in various ways in recent years. 
     For example, a fluoride ion secondary battery proposed in the conventional art includes a positive electrode; a negative electrode including at least one of metal and a fluoride each including at least La; an ion conducting medium for conducting fluoride ions; and a housing that accommodates the positive electrode, the negative electrode, and the ion conducting medium in a sealed manner and has an interior oxygen concentration of 2 ppm or less (see, for example, Patent Document 1).
     Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2017-84506   

     SUMMARY OF THE INVENTION 
     Unfortunately, the conventional fluoride ion secondary battery as disclosed in Patent Document 1 can operate only at high temperature, such as 150° C., and will decrease by half in charging and discharging efficiency during three cycles of charge and discharge, which is significant degradation during the cycles. Thus, a need exists to develop a fluoride ion secondary battery having high charging and discharging efficiency at room temperature. 
     The present invention has been made in light of the circumstances mentioned above, and an object of the present invention is to provide a fluoride ion secondary battery having high charging and discharging efficiency at room temperature. 
     (1) An aspect of the present invention is to provide a fluoride ion secondary battery including: a positive electrode material layer including Ag; a negative electrode material layer including at least one of CeF 3  and PbF 2 ; and a solid electrolyte layer including LaF 3  and disposed between the positive electrode material layer and the negative electrode material layer. 
     (2) In the fluoride ion secondary battery according to aspect (1), the negative electrode material layer may include CeF 3 , and the fluoride ion secondary battery according to aspect (1) may further include a negative electrode current collector layer including carbon and disposed on an outer side of the negative electrode material layer. 
     (3) In the fluoride ion secondary battery according to aspect (1), the negative electrode material layer may include PbF 2 , and the fluoride ion secondary battery according aspect (1) may further include a negative electrode current collector layer including a Pb foil and disposed on an outer side of the negative electrode material layer. 
     (4) The fluoride ion secondary battery according to any one of aspects (1) to (3) may further include a positive electrode current collector layer including carbon and disposed on an outer side of the positive electrode material layer. 
     (5) In the fluoride ion secondary battery according to any one of aspects (1) to (4), the positive electrode material layer may have a thickness of 10 nm or more and less than 120 nm. 
     (6) In the fluoride ion secondary battery according to any one of aspects (1) to (5), the positive electrode material layer may have a thickness of 10 nm or more and 60 nm or less. 
     (7) In the fluoride ion secondary battery according to any one of aspects (1) to (6), the positive electrode material layer may have a thickness of 10 nm or more and 30 nm or less. 
     (8) In the fluoride ion secondary battery according to any one of aspects (1) to (7), the negative electrode material layer may have a thickness of 10 nm or more and less than 200 nm. 
     (9) In the fluoride ion secondary battery according to any one of aspects (1) to (8), the negative electrode material layer may have a thickness of 10 nm or more and 50 nm or less. 
     The present invention makes it possible to provide a fluoride ion secondary battery having high charging and discharging efficiency at room temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the structure of a fluoride ion secondary battery according to a first embodiment of the present invention; 
         FIG. 2  is a view showing an exemplary method for producing the fluoride ion secondary battery according to the first embodiment; 
         FIG. 3  is a graph showing the charging and discharging curves of a fluoride ion secondary battery according to Example 1; 
         FIG. 4  is a graph showing the relationship between the capacity and the square root of the number of cycles with respect to the fluoride ion secondary battery according to Example 1; 
         FIG. 5  is a graph showing the relationship between the charging and discharging efficiency and the square root of the number of cycles with respect to the fluoride ion secondary battery according to Example 1; 
         FIG. 6  is a view showing the structure of a fluoride ion secondary battery according to Example 2; 
         FIG. 7  is a view showing the structure of a fluoride ion secondary battery according to Comparative Examples 1 and 2; 
         FIG. 8  is a graph showing the intermittent charging and discharging curves of the fluoride ion secondary batteries according to Example 2 and Comparative Examples 1 and 2; 
         FIG. 9  is a graph showing the charging and discharging curves of a fluoride ion secondary battery according to Comparative Examples 1 to 6; 
         FIG. 10  is a view showing the structure of a fluoride ion secondary battery according to Example 3; 
         FIG. 11  is a graph showing the charging and discharging curves of the fluoride ion secondary battery according to Example 3; 
         FIG. 12  is a graph showing the relationship between the capacity and the square root of the number of cycles with respect to the fluoride ion secondary battery according to Example 3; 
         FIG. 13  is a graph showing the relationship between the charging and discharging efficiency and the square root of the number of cycles with respect to the fluoride ion secondary battery according to Example 3; and 
         FIG. 14  is a graph showing the relationship between the discharging capacity retention and the square root of the number of cycles with respect to the fluoride ion secondary batteries according to Examples 1 and 3. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
       FIG. 1  is a view showing the structure of a fluoride ion secondary battery  1  according to a first embodiment of the present invention. As shown in  FIG. 1 , the fluoride ion secondary battery  1  according to the embodiment includes a positive electrode material layer  11 , a negative electrode material layer  13 , and a solid electrolyte layer  15  disposed between the positive electrode material layer  11  and the negative electrode material layer  13 . 
     The positive electrode material layer  11  includes Ag. Preferably, the positive electrode material layer  11  includes only Ag or consists of Ag. The positive electrode material layer  11  releases fluoride ions F −  during discharging and stores fluoride ions F −  during charging. The positive electrode material layer  11  including Ag is characterized by being less likely to cause overvoltage at room temperature. The positive electrode material layer  11  including Ag may be formed by, for example, sputtering as described later. 
     The positive electrode material layer  11  preferably has a thickness of 10 nm or more and less than 120 nm. The positive electrode material layer  11  with a thickness of less than 10 nm may fail to be formed due to particle- or island-like deposition, or may cause undesirable uneven reaction even if successfully formed. The positive electrode material layer  11  with a thickness of more than 120 nm may cause an increase in overvoltage and a decrease in charging and discharging efficiency, which is not desirable. The positive electrode material layer  11  more preferably has a thickness of 10 nm or more and 60 nm or less, even more preferably 10 nm or more and 30 nm or less.  FIG. 1  shows an example in which the positive electrode material layer  11  has a thickness of 30 nm. 
     The negative electrode material layer  13  includes at least one of CeF 3  and PbF 2 . The negative electrode material layer  13  preferably includes CeF 3 . The negative electrode material layer  13  stores fluoride ions F −  during discharging and releases fluoride ions F −  during charging. The negative electrode material layer  13  including at least one of CeF 3  and PbF 2  is characterized by being less likely to cause overvoltage at room temperature. The negative electrode material layer  13  including at least one of CeF 3  and PbF 2  may be formed by, for example, sputtering as described later. 
     The negative electrode material layer  13  preferably has a thickness of 10 nm or more and less than 200 nm. The negative electrode material layer  13  with a thickness of less than 10 nm may fail to be formed due to particle- or island-like deposition, or may cause undesirable uneven reaction even if successfully formed. The negative electrode material layer  13  with a thickness of more than 200 nm may cause an increase in overvoltage and a decrease in charging and discharging efficiency, which is not desirable. The negative electrode material layer  13  more preferably has a thickness of 10 nm or more and 100 nm or less, even more preferably 10 nm or more and 50 nm or less.  FIG. 1  shows an example in which the negative electrode material layer  13  has a thickness of 50 nm. 
     The negative electrode material layer  13  may further include a solid electrolyte including, for example, a fluoride ion-conducting fluoride, and a conductive aid. The negative electrode material layer  13  may further include additional components, such as a binder, as long as such components do not impair the advantageous effects of the embodiment. 
     The fluoride ion-conducting fluoride may be any fluoride having fluoride ion conductivity. Examples include CeBaF x , BaLaF y , and other fluoride ion-conducting fluorides. The negative electrode material layer containing such a fluoride ion-conducting fluoride has improved fluoride ion conductivity. 
     The conductive aid may be any type having electron conductivity, such as carbon black. Examples of the carbon black include furnace black, Ketjen black, and acetylene black. The negative electrode material layer containing such a conductive aid has improved electron conductivity. 
     The solid electrolyte layer  15  includes LaF 3 . The solid electrolyte layer  15  including LaF 3  has excellent fluoride ion F −  conductivity. The solid electrolyte layer  15  including LaF 3  may be a commercially available LaF 3  substrate. The thickness of the solid electrolyte layer  15  is typically, but not limited to, 0.1 mm to 0.5 mm.  FIG. 1  shows an example in which the solid electrolyte layer  15  has a thickness of 0.5 mm. 
     The fluoride ion secondary battery  1  according to the embodiment preferably further includes a positive electrode current collector layer  12  disposed on the outer side of the positive electrode material layer  11 . The positive electrode current collector layer  12  may be any type having electron conductivity. For example, the positive electrode current collector layer  12  preferably includes carbon. The positive electrode current collector layer  12  may have any thickness, while  FIG. 1  shows an example in which the positive electrode current collector layer  12  has a thickness of 30 nm. 
     The fluoride ion secondary battery  1  according to the embodiment preferably further includes a negative electrode current collector layer  14  disposed on the outer side of the negative electrode material layer  13 . The negative electrode current collector layer  14  may be any type having electron conductivity. For example, when the negative electrode material layer  13  includes CeF 3 , the negative electrode current collector  14  preferably includes carbon. When the negative electrode material layer includes PbF 2 , the negative electrode current collector layer preferably includes a Pb foil. The negative electrode current collector layer  14  may have any thickness, while  FIG. 1  shows an example in which the negative electrode current collector layer  14  has a thickness of 30 nm. 
     The fluoride ion secondary battery  1  according to the embodiment having the features described above preferably further includes a case, such as a coin cell case or a laminate cell case. 
     The fluoride ion secondary battery  1  according to the embodiment may have any shape or size.  FIG. 1  shows a cylindrical example. In the example shown in  FIG. 1 , the positive electrode material layer  11 , the positive electrode current collector layer  12 , the negative electrode material layer  13 , and the negative electrode current collector layer  14  each have a diameter φ of 8 mm, and the solid electrolyte layer  15  has a diameter φ of 10 mm. The solid electrolyte layer  15  with such a larger diameter prevents short circuit between the positive and negative electrodes. 
     Next, a method for producing the fluoride ion secondary battery according to the embodiment will be described in detail with reference to  FIG. 2 . 
       FIG. 2  is a view showing an exemplary method for producing a fluoride ion secondary battery according to the embodiment. As shown in  FIG. 2 , first, a LaF 3  substrate as a solid electrolyte layer  15  is placed in a predetermined jig (not shown) not exposed to the air (for example, in an environment with a dew point of 80° C. or less and an oxygen concentration of 1 ppm or less). This step may be performed, for example, in a glove box manufactured by UNICO Ltd., in which the environment mentioned above is provided. 
     The LaF 3  substrate may be a commercially available LaF 3  substrate, such as one manufactured by Pier Optics Co., Ltd. The size of the LaF 3  substrate is, for example, but not limited to, 10 mm in diameter φ and 0.5 mm in thickness. The LaF 3  substrate to be used preferably has a mirror-finished surface. 
     Subsequently, the LaF 3  substrate set in the jig is transferred into the chamber of a sputtering system while the environment mentioned above is maintained. The sputtering system may be a commercially available sputtering system, such as, Sputtering Equipment EB1000 manufactured by Canon Anelva Corporation. The degree of vacuum in the chamber is, for example, 5×10 −4  Pa or less before the deposition. 
     Subsequently, DC sputtering is performed using the sputtering system to deposit a positive electrode material layer  11  including Ag on one surface of the LaF 3  substrate as the solid electrolyte layer  15 . After the deposition, intermission for a certain period of time is provided. 
     Subsequently, DC sputtering is performed using the sputtering system to deposit a positive electrode current collector layer  12  including carbon on the deposited positive electrode material layer  11 . 
     Subsequently, the LaF 3  substrate as the solid electrolyte layer  15  with the positive electrode material layer  11  and the positive electrode current collector layer  12  deposited in order on one surface thereof is transferred from the chamber of the sputtering system into the glove box while the environment mentioned above is maintained. After the transfer, the LaF 3  substrate is taken out of the jig, turned upside down, and placed again in the jig with the other surface of the substrate facing upward. 
     Subsequently, the LaF 3  substrate turned upside down and set again in the jig is transferred into the chamber of the sputtering system while the environment mentioned above is maintained. After the transfer, RF sputtering is performed using the sputtering system to deposit a negative electrode material layer  13  including CeF 3  on the other surface of the LaF 3  substrate as the solid electrolyte layer  15 . A similar operation may be used to deposit a negative electrode material layer including PbF 2 . 
     Subsequently, DC spurring is performed using the sputtering system to deposit a negative electrode current collector layer  14  including carbon on the deposited negative electrode material layer  13 . A similar operation may be used to deposit a negative electrode current collector layer including a Pb foil. 
     Subsequently, the LaF 3  substrate as the solid electrolyte layer  15  with the negative electrode material layer  13  and the negative electrode current collector layer  14  deposited in order on the other surface thereof is transferred from the chamber of the sputtering system into the glove box while the environment mentioned above is maintained. After the transfer, the product is subjected to an assembling step including installing it in a case, such as a coin cell case or a laminate cell case, to form the fluoride ion secondary battery  1  according to the embodiment. 
     For the deposition of each layer, each deposition rate may be evaluated in advance, and the sputtering time may be adjusted based on the evaluated deposition rates, so that the thickness of each layer may be controlled. Specifically, each sputtering deposition may be performed under certain conditions to form a layer on, for example, a quartz plate partially masked with a masking tape, such as a Kapton tape, attached thereto. Subsequently, the masking tape may be removed, and then the difference in level between the unmasked portion and the portion from which the masking tape has been removed (corresponding to the thickness of the film) is measured with a profile meter. The measurement may be performed under different conditions for each sputtering deposition, and the results of the measurement may be used to make a calibration curve, from which the deposition rate may be determined. This process makes it possible to control each deposition thickness as desired. 
     Next, the cell capacity of the fluoride ion secondary battery  1  according to the embodiment will be described. A positive electrode half cell and a negative electrode half cell may each have a capacity as shown below with respect to an example of the fluoride ion secondary battery  1  according to the embodiment shown in  FIG. 1 . 
     Positive Electrode Half Cell 
     Theoretical capacity: 248 mAh/g
 
Density: 10.49 g/cm 3  
 
     Thickness: 30 nm 
     Electrode area: 0.5 cm 2  
 
Battery capacity: 3.92 μAh
 
     Negative Electrode Half Cell 
     Theoretical capacity: 408 mAh/g
 
Density: 6.77 g/cm 3  
 
     Thickness: 50 nm 
     Electrode area: 0.5 cm 2  
 
Battery capacity: 6.9 μAh
 
     Regarding the example of the fluoride ion secondary battery  1  according to the embodiment, the cell capacity is determined by the theoretical capacity of the positive electrode, which is lower than that of the negative electrode as shown above. Specifically, the cell capacity of the example of the fluoride ion secondary battery according to the embodiment shown in  FIG. 1  is determined to be 3.92 μAh (the capacity of the positive electrode), and the N/P ratio is calculated to be 3.92/6.9=1.76. 
     The fluoride ion secondary battery  1  according to the embodiment having the features described above has advantageous effects as shown below. The fluoride ion secondary battery  1  according to the embodiment includes a positive electrode material layer  11  including Ag; a negative electrode material layer  13  including at least one of CeF 3  and PbF 2 ; and a solid electrolyte layer  15  including LaF 3  and disposed between the positive electrode material layer  11  and the negative electrode material layer  13 . The fluoride ion secondary battery  1  according to the embodiment is constructed using materials that are selected so as to be less likely to cause overvoltage even during room temperature operation of the corresponding half cell in a charging and discharging test and an intermittent charging and discharging test. Even when operated at room temperature, therefore, the fluoride ion secondary battery according to the embodiment can be prevented from suffering from high overvoltage and prevented from degrading during charging and discharging cycles, and can exhibit high charging and discharging efficiency. 
     The embodiments described above are not intended to limit the present invention and may be altered or modified within the scope of the present invention where the objects of the present invention can be achieved. 
     EXAMPLES 
     Next, examples of the present invention will be described, which are not intended to limit the scope of the present invention. 
     Example 1 
     In Example 1, a fluoride ion secondary battery according to the first embodiment shown in  FIG. 1  was prepared according to the production method described above. The prepared fluoride ion secondary battery of Example 1 was subjected to a constant-current (CC) charging and discharging test under the conditions shown below. 
     The CC charging and discharging test was performed under vacuum (1×10 −4  Pa or less) at room temperature 25° C. The cell capacity of the fluoride ion secondary battery of Example 1 is determined by the theoretical capacity (248 mAh/g) of the positive electrode, which includes Ag with a theoretical capacity lower than that of CeF 3  in the negative electrode. Thus, the current load was 200 nA, which corresponds to 1/20 C of the theoretical capacity (248 mAh/g) of the positive electrode including Ag. The current load corresponds to about 0.4 μA/cm 2  per unit area since the area of the cell of Example 1 is about 0.5 cm 2 . The cut-off condition was a cut-off voltage of 1.0 to 4.2 V or a cut-off time of 20 hours. Intermission for 10 minutes was provided at switching between charging and discharging. 
     The charging and discharging test procedure was as follows. A rate test was performed during the first 10 cycles. Specifically, the rate was gradually increased such that the rate was 1/20 C during 1st to 3rd cycles, 1/10 C during 4th to 6th cycles, 1 C during 7th and 8th cycles, and 5 C during 9th and 10th cycles. After the rate test, a 100-cycle charging and discharging test was performed at a constant rate of 1 C. In the middle of the test, the rate was changed to 1/20 C for the check of the charging and discharging capacity. The results of the charging and discharging test are shown in  FIGS. 3 to 5 . 
       FIG. 3  is a graph showing the charging and discharging curves of the fluoride ion secondary battery of Example 1.  FIG. 4  is a graph showing the relationship between the capacity and the square root of the number of cycles with respect to the fluoride ion secondary battery of Example 1.  FIG. 5  is a graph showing the relationship between the charging and discharging efficiency and the square root of the number of cycles with respect to the fluoride ion secondary battery of Example 1.  FIG. 3  shows the charging and discharging curves obtained during the first 10-cycle rate test.  FIGS. 4 and 5  show the charging and discharging capacity and the charging and discharging efficiency obtained in the rate test and in the 100-cycle charging and discharging test after the rate test. 
       FIGS. 3 to 5  indicate that the fluoride ion secondary battery of Example 1 can operate at room temperature. At room temperature, the initial charging and discharging efficiency is 90%, and 80% of the discharging capacity at 0.05 C remains at 5 C, which show good rate characteristics. At room temperature, the charging and discharging efficiency during the charging and discharging cycles at 1 C was found to be 99%, and the capacity retention after the 100 cycles of charging and discharging was found to be 50%. Those results show that the fluoride ion secondary battery of Example 1 has high charging and discharging efficiency at room temperature. 
     Example 2 
       FIG. 6  is a view showing the structure of a fluoride ion secondary battery  1   a  according to Example 2. In Example 2, the fluoride ion secondary battery  1   a  shown in  FIG. 6  was prepared according to the production method described above. The fluoride ion secondary battery  1   a  of Example 2 may also be called a Ag positive electrode half cell, in which a negative electrode material layer  13   a  including PbF 2  and having a thickness of 200 nm is provided in place of the negative electrode material layer  13  including CeF 3  and having a thickness of 50 nm in the fluoride ion secondary battery of Example 1 and in which a negative electrode current collector layer  14   a  including a 0.2 mm-thick Pb foil is provided in place of the negative electrode current collector  14  including carbon and having a thickness of 30 nm. The prepared fluoride ion secondary battery of Example 2 was subjected to an intermittent charging and discharging test under the conditions shown below. 
     The intermittent charging and discharging test was performed under vacuum (1×10 −5  Pa) at room temperature 25° C. Specifically, the intermittent charging and discharging test included repeating CC charging for 1 hour and intermission for 5 hours and repeating CC discharging for 1 hour and intermission for 5 hours. The current load was 400 nA, which corresponds to 1/10 C with respect to the designed capacity of the positive electrode, and the voltage was in the range of 0 to 1.7 V.  FIG. 8  shows the results of the intermittent charging and discharging test. 
     Comparative Examples 1 to 6 
       FIG. 7  is a view showing the structure of a fluoride ion secondary battery  2  according to Comparative Examples 1 to 6. In Comparative Examples 1 to 6, the fluoride ion secondary battery  2  shown in  FIG. 7  was prepared according to the production method described above. The fluoride ion secondary battery  2  according to Comparative Examples 1 to 6 may also be called a Cu positive electrode half cell, in which a positive material layer  21  including Cu and having a thickness of 3 nm is provided in place of the positive electrode material layer  11  including Ag and having a thickness of 30 nm in the fluoride ion secondary battery  1   a  of Example 2 and in which a positive electrode current collector layer  22  is provided with a thickness different from that of the positive electrode current collector layer  12  and a negative electrode material layer  23  including PbF 2  is provided with a thickness different from that of the negative electrode material layer  13   a  including PbF 2 . The fluoride ion secondary battery shown in  FIG. 7  was subjected to an intermittent charging and discharging test under the conditions shown below. The fluoride ion secondary battery shown in  FIG. 7  was also subjected to a CC charging and discharging test under the same conditions as those in Example 1. More specifically, the CC charging and discharging test was performed at room temperature 25° C. in Comparative Example 1, at 140° C. in Comparative Example 2, at 60° C. in Comparative Example 3, at 80° C. in Comparative Example 4, at 100° C. in Comparative Example 5, and at 120° C. in Comparative Example 6. 
     The intermittent charging and discharging test was performed under vacuum (1×10 −5  Pa) at room temperature 25° C. in Comparative Example 1 and under vacuum (1×10 −5  Pa) at 140° C. in Comparative Example 2. In each of Comparative Examples 1 and 2, the intermittent charging and discharging test included repeating CC charging for 1 hour and intermission for 3 hours and repeating CC discharging for 0.5 hours and intermission for 3 hours. The current load was 123 nA, which corresponds to 1/10 C with respect to the designed capacity of the positive electrode, and the voltage was in the range of 0.1 to 1.3 V.  FIG. 8  shows the results of the intermittent charging and discharging test. 
       FIG. 8  is a graph showing the intermittent charging and discharging curves of the fluoride ion secondary batteries  1   a  and  2  according to Example 2 and Comparative Examples 1 and 2. In  FIG. 8 , the height of peaks observed intermittently at the start of charging, namely the difference between OCV and CCV, represents the magnitude of overvoltage. In all of Example 2 and Comparative Examples 1 and 2, the peak switched to minus in the middle of the test, which means switching to discharging occurred after the set charging voltage was reached. The results in  FIG. 8  show that the set charging voltage was reached earlier in Comparative Examples 1 and 2 than in Example 2. 
       FIG. 8  also indicates that when subjected to the intermittent charging and discharging test at room temperature, the Cu positive electrode half cell of Comparative Example 1 shows very high overvoltage and further increases in overvoltage as charging proceeds as compared to the Ag positive electrode half cell of Example 2. This was presumed to be due to the ionic conductivity of CuF 2  produced in the Cu positive electrode material layer. 
     On the other hand, the Cu positive electrode half cell of Comparative Example 2 being subjected to the intermittent charging and discharging test at 140° C. shows a similar level of overvoltage to that of the Ag positive electrode half cell of Example 2 being subjected to the intermittent charging and discharging test at room temperature, which suggests that charging and discharging should be possible at a capacity close to the theoretical capacity of the Cu positive electrode material layer. The results show that reductions in DC component resistance (e.g., electron, ionic conductivity, charge transfer), which is a cause of overvoltage, are important for the operation at room temperature. 
       FIG. 9  is a graph showing the charging and discharging curves of the fluoride ion secondary battery  2  according to Comparative Examples 1 to 6.  FIG. 9  shows the charging and discharging curves obtained in a CC charging and discharging test which was performed at room temperature 25° C. in Comparative Example 1, at 140° C. in Comparative Example 2, at 60° C. in Comparative Example 3, at 80° C. in Comparative Example 4, at 100° C. in Comparative Example 5, and at 120° C. in Comparative Example 6. The charging and discharging curves show that the charging and discharging capacity decreases with decreasing temperature. The results also indicate that charging and discharging at room temperature is difficult when the Cu positive electrode material layer is used. 
     Example 3 
       FIG. 10  is a view showing the structure of a fluoride ion secondary battery  1   b  according to Example 3. In Example 3, the fluoride ion secondary battery  1   b  shown in  FIG. 10  was prepared according to the production method described above. The fluoride ion secondary battery  1   b  according to Example 3 has a positive electrode material layer  11  including Ag and having a thickness of 120 nm, in place of the positive electrode layer  11  including Ag and having a thickness of 30 nm in the fluoride ion secondary battery of Example 1, and has a negative electrode material layer  13  including CeF 3  and having a thickness of 200 nm, in place of the negative electrode material layer  13  including CeF 3  and having a thickness of 50 nm in the fluoride ion secondary battery of Example 1. 
     The prepared fluoride ion secondary battery  1   b  according to Example 3 was subjected to a CC charging and discharging test under the same conditions as those in Example 1. Specifically, first, a rate test was performed in which the rate was gradually increased, and then a 100-cycle charging and discharging test was performed at a constant rate of 1 C. In the rate test of Example 3, the rate was 1/20 C during 1st to 3rd cycles, 1/10 C during 4th and 5th cycles, 1 C during 6th and 7th cycles, and 2 C during 8th and 9th cycles. The results of the charging and discharging test are shown in  FIGS. 11 to 14 . 
       FIG. 11  is a graph showing the charging and discharging curves of the fluoride ion secondary battery  1   b  according to Example 3.  FIG. 12  is a graph showing the relationship between the capacity and the square root of the number of cycles with respect to the fluoride ion secondary battery  1   b  according to Example 3.  FIG. 13  is a graph showing the relationship between the charging and discharging efficiency and the square root of the number of cycles with respect to the fluoride ion secondary battery  1   b  according to Example 3.  FIG. 14  is a graph showing the relationship between the discharging capacity retention and the square root of the number of cycles with respect to the fluoride ion secondary batteries  1  and  1   b  according to Examples 1 and 3. 
     A comparison between the charging and discharging curves of the fluoride ion secondary batteries  1  and  1   b  according to Examples 1 and 3 shown in  FIG. 11  shows that, in Example 3, the charging and discharging capacity more decreased as the rate was increased to 1 C and 2 C, which shows an increase in overvoltage. The results suggest that the thickness of the positive electrode material layer including Ag should preferably have an upper limit of less than 120 nm and that the thickness of the negative electrode material layer including CeF 3  should preferably have an upper limit of less than 200 nm. 
     The relationship shown in  FIGS. 12 and 13  between the capacity, the charging and discharging efficiency, and the square root of the number of cycles with respect to the fluoride ion secondary battery  1   b  according to Example 3 was compared with the relationship shown in  FIGS. 4 and 5  between the capacity, the charging and discharging efficiency, and the square root of the number of cycles with respect to the fluoride ion secondary battery  1  according to Example 1. The comparison indicates that, in the initial rate test, the charging and discharging capacity and the charging and discharging efficiency decrease more in Example 3. In Example 3, the charging and discharging efficiency during the 100 cycles of charging and discharging at a rate of 1 C is around 98%, which has been found to be slightly lower than that in Example 1. The cycle durability has also been found to deteriorate more in Example 3. This was presumed to be due to, for example, volume change-induced delamination of each layer. Those results also suggest that the thickness of the positive electrode material layer including Ag should preferably have an upper limit of less than 120 nm and that the thickness of the negative electrode material layer including CeF 3  should preferably have an upper limit of less than 200 nm. 
     The relationships shown in  FIG. 14  between the discharging capacity retention and the square root of the number of cycles with respect to the fluoride ion secondary batteries  1  and  1   b  according to Examples 1 and 3 indicate that, in the cycle durability test at 1 C, the battery  1  of Example 3 deteriorates more than the battery  1   b  of Example 1. It has been found, however, that, in Example 3, the discharging capacity retention recovers during the check of the capacity at a rate of 1/20 C in the middle of the test. Those results also suggest that the thickness of the positive electrode material layer including Ag should preferably have an upper limit of less than 120 nm and that the thickness of the negative electrode material layer including CeF 3  should preferably have an upper limit of less than 200 nm. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
               1 ,  1   a ,  1   b : Fluoride ion secondary battery 
               11 : Positive electrode material layer 
               12 : Positive electrode current collector layer 
               13 ,  13   a : Negative electrode material layer 
               14 ,  14   a : Negative electrode current collector layer 
               15 : Solid electrolyte layer