Patent Publication Number: US-2022238877-A1

Title: Negative electrode for use in fluoride ion secondary battery and fluoride ion secondary battery including same

Description:
This application is based on and claims the benefit of priority from Japanese Patent Application No. 2021-010554, 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 negative electrode for use in a fluoride ion secondary battery and to a fluoride ion secondary battery including such a negative electrode. 
     Related Art 
     In the conventional art, fluoride ion secondary batteries are proposed using fluoride ions as carriers (see, for example, Patent Documents 1 to 6). 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, aluminum-based materials have been studied as candidates for the negative electrode active material in fluoride ion secondary batteries. In particular, aluminum fluoride has been studied for use in fluoride ion secondary batteries. Unfortunately, aluminum fluoride has a problem in that it is relatively less prone to electrochemical reactions due to its electrical insulating properties.
     Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2019-87403   Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2017-50113   Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2019-29206   Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2018-206755   Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2018-198130   Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2018-92863   

     SUMMARY OF THE INVENTION 
     Thus, a fluoride ion secondary battery has been provided including, as a negative electrode active material, an aluminum fluoride material doped with lithium metal. At present, however, such a fluoride ion secondary battery is required to have further improved characteristics. Specifically, the lithium metal-doped aluminum fluoride active material has relatively low ionic conductivity, and the concentration of the negative electrode active material in the negative electrode cannot be increased sufficiently, which makes it not easy to provide a battery with a large capacity. 
     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 a capacity larger than that of the conventional one. 
     (1) An aspect of the present invention is to provide a negative electrode for use in a fluoride ion secondary battery, the negative electrode including a negative electrode active material including zirconium fluoride. 
     (2) In the negative electrode according to aspect (1) for use in a fluoride ion secondary battery, the zirconium fluoride may be in the form of particles with an average particle size of 100 nm or less. 
     (3) The negative electrode according to aspect (1) or (2) for use in a fluoride ion secondary battery may have a zirconium fluoride content of less than 50% by mass. 
     (4) In the negative electrode according to any one of aspects (1) to (3) for use in a fluoride ion secondary battery, the negative electrode active material may further include metallic zirconium. 
     (5) In the negative electrode according to aspect (4) for use in a fluoride ion secondary battery, the metallic zirconium may be in the form of particles with an average particle size of 75 μm or less. 
     (6) The negative electrode according to aspect (4) or (5) for use in a fluoride ion secondary battery may have a metallic zirconium content of 8% by mass or less. 
     (7) Another aspect of the present invention is to provide a fluoride ion secondary battery including the negative electrode according to any one of aspects (1) to (6). 
     The present invention makes it possible to provide a fluoride ion secondary battery having a capacity larger than that of the conventional one. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the properties of aluminum fluoride and zirconium fluoride; 
         FIG. 2  is a diagram showing an exemplary method for producing a negative electrode according to an embodiment of the present invention for use in a fluoride ion secondary battery; 
         FIG. 3  is a diagram showing another exemplary method for producing the negative electrode according to the embodiment for use in a fluoride ion secondary battery; 
         FIG. 4  is an electron micrograph of zirconium fluoride in the form of microparticles to be subjected to ball milling; 
         FIG. 5  is an electron micrograph of zirconium fluoride in the form of microparticles resulting from ball milling; 
         FIG. 6  is an electron micrograph of zirconium fluoride in the form of nanoparticles resulting from ball milling; 
         FIG. 7  is a graph showing the charging and discharging curves of the negative electrode half cells of Example 4 and Comparative Example 1 for fluoride ion secondary batteries; 
         FIG. 8  is a graph showing the charging and discharging curves of the negative electrode half cells of Example 1 and Comparative Example 2 for fluoride ion secondary batteries; 
         FIG. 9  is a graph showing the charging and discharging curves of the negative electrode half cells of Examples 1 to 3 for fluoride ion secondary batteries; 
         FIG. 10  is a graph showing the relationship between the capacity and the zirconium fluoride concentration of the negative electrode half cells of Examples 1 and 4 to 6 for fluoride ion secondary batteries; 
         FIG. 11  is a graph showing the charging and discharging curves of the negative electrode half cells of Examples 1 and 8 for fluoride ion secondary batteries; and 
         FIG. 12  is a graph showing the relationship between the coulombic efficiency, the capacity, and the zirconium fluoride concentration of the negative electrode half cells of Examples 1 and 8 to 10 for fluoride ion secondary batteries. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
     Negative Electrode for Use in Fluoride Ion Secondary Battery 
     The negative electrode according to an embodiment of the present invention for use in a fluoride ion secondary battery includes zirconium fluoride as a negative electrode active material. A negative electrode including zirconium fluoride and being for use in a fluoride ion secondary battery has not been known so far. The negative electrode according to the embodiment for use in a fluoride ion secondary battery is characterized by including zirconium fluoride. 
     Zirconium fluoride functions as a negative electrode active material during charging and discharging. Specifically, zirconium fluoride releases fluoride ions F −  during charging, and stores fluoride ions F −  during discharging. Zirconium fluoride may be a commercially available product. 
     In an embodiment of the present invention, zirconium fluoride is preferably in the form of particles with an average particle size of 100 nm or less, and specifically, zirconium fluoride is preferably in the form of nanoparticles with an average particle size of 100 nm or less. The negative electrode active material including zirconium fluoride in the form of nanoparticles with an average particle size of 100 nm or less can form a battery with an increased capacity. More preferably, zirconium fluoride is in the form of particles with an average particle size of 65 nm or less. 
     In an embodiment of the present invention, the negative electrode for use in a fluoride ion secondary battery preferably has a zirconium fluoride content of less than 50% by mass. The charging capacity may increase with increasing zirconium fluoride concentration of the negative electrode for use in a fluoride ion secondary battery. However, if the zirconium fluoride concentration of the negative electrode for use in a fluoride ion secondary battery reaches 50% by mass, the charging capacity may sharply decrease due to a voltage drop caused by an increase in the internal resistance of the negative electrode, which may make it difficult to perform charging and discharging. More preferably, the zirconium fluoride content is 40% by mass or less. 
     In this regard,  FIG. 1  is a view showing the properties of aluminum fluoride and zirconium fluoride.  FIG. 1  shows the literature values (theoretical values) and the measured values of the densities of aluminum fluoride AlF 3 , which has been studied in the conventional art, and zirconium fluoride ZrF 4  according to the embodiment.  FIG. 1  also shows their ionic conductivities at 140° C. where fluoride ion secondary batteries are assumed to be operated. 
     The data in  FIG. 1  indicates that zirconium fluoride can be more densified than the conventional aluminum fluoride and can also have a higher ionic conductivity. Therefore, zirconium fluoride can be used at a higher concentration than aluminum fluoride and can form a battery with a larger capacity. Moreover, the increase in the volume of zirconium fluoride with increasing concentration can be kept relatively low, which will make it possible to increase the content of a solid electrolyte including a fluoride ion-conducting fluoride material as described later and to increase the content of a conductive aid, so that higher ionic conductivity can be achieved. 
     In an embodiment of the present invention, the negative electrode active material preferably further includes metallic zirconium. The use of the negative electrode active material further including metallic zirconium makes it possible to improve the coulombic efficiency, which is the ratio of discharging capacity to charging capacity, and to improve the reversibility of charging and discharging. Metallic zirconium may be a commercially available product. 
     Specifically, the negative electrode according to the embodiment for use in a fluoride ion secondary battery preferably includes a solid electrolyte including a fluoride ion-conducting fluoride material as described later. In this case, if the solid electrolyte undergoes a certain reaction, the discharging capacity may be at an insufficient level relative to the charging capacity level. To address this problem, metallic zirconium can be added according to the embodiment to prevent the solid electrolyte from undergoing such a reaction, which will result in a high charging capacity and thus result in an improvement in the reversibility of charging and discharging. Moreover, metallic zirconium can work for discharging with a small loss of charging capacity, which can be expected to result in an increase in the lifetime of the fluoride ion secondary battery according to an embodiment of the present invention. 
     The metallic zirconium is preferably in the form of particles with an average particle size of 75 μm or less. In other words, the metallic zirconium preferably forms particles with an average particle size of 75 μm or less. The addition of metallic zirconium in the form of particles with an average particle size of 75 μm or less makes it possible to provide a battery with increased capacity and improved reversibility of charging and discharging. 
     The negative electrode according to the embodiment for use in a fluoride ion secondary battery preferably has a metallic zirconium content of 8% by mass or less. As the metallic zirconium content of the negative electrode for use in a fluoride ion secondary battery is increased, the coulombic efficiency may increase, but the amount of the electrolyte may decrease so that the internal resistance of the negative electrode may increase and the voltage may decrease, which may result in a decrease in charging capacity. However, when the negative electrode for use in a fluoride ion secondary battery has a metallic zirconium content of 8% by mass or less, such a decrease in charging capacity can be prevented. More preferably, the metallic zirconium content is 5% by mass or less. 
     The negative electrode according to the embodiment for use in a fluoride ion secondary battery preferably further includes a fluoride ion-conducting fluoride solid electrolyte and a conductive aid in addition to the zirconium fluoride and the metallic zirconium as negative electrode active materials. 
     The fluoride ion-conducting fluoride may be any fluoride having fluoride ion conductivity. Examples of the fluoride ion-conducting fluoride include CeBaF x  and BaLaF y , such as Ce 0.95 Ba 0.05 F 2.95  and Ba 0.6 La 0.4 F 2.4 . When containing such a fluoride ion-conducting fluoride, the negative electrode according to the embodiment for use in a fluoride ion secondary battery can have improved fluoride ion conductivity. 
     The fluoride ion-conducting fluoride is preferably in the form of particles with an average particle size in the range of 0.1 μm to 100 μm. The fluoride ion-conducting fluoride in the form of particles with an average particle size in such a range can form an electrode thin layer having relatively high ionic conductivity. More preferably, the fluoride ion-conducting fluoride is in the form of particles with an average particle size in the range of 0.1 μm, to 10 μm. 
     The conductive aid may be any type having electron conductivity. For example, the conductive aid may be carbon black or the like. The carbon black may be furnace black, Ketjen black, or acetylene black. When containing such a conductive aid, the negative electrode according to the embodiment for use in a fluoride ion secondary battery can have improved electron conductivity. 
     The conductive aid is preferably in the form of particles with an average particle size in the range of 20 nm to 50 nm. The conductive aid in the form of particles with an average particle size in such a range can form a lightweight electrode having high electron conductivity. 
     The negative electrode according to the embodiment for use in a fluoride ion secondary battery may further include additional components, such as a binder, as long as such components do not impair the advantageous effects of the embodiment. 
     Next, methods for producing the negative electrode according to the embodiment for use in a fluoride ion secondary battery will be described in detail with reference to  FIGS. 2 and 3 .  FIG. 2  is a diagram showing an exemplary method for producing the negative electrode according to the embodiment for use in a fluoride ion secondary battery.  FIG. 3  is a diagram showing another exemplary method for producing the negative electrode according to the embodiment for use in a fluoride ion secondary battery. 
     In the exemplary production method shown in  FIG. 2 , first, a mixture is prepared of 700 mg of CeBaF x , (Ce 0.95 Ba 0.05 F 2.95 ), which is a fluoride ion-conducting fluoride solid electrolyte, and 50 mg of carbon black (acetylene black AB), which is a conductive aid. 
     Subsequently, 250 mg of zirconium fluoride ZrF 4  is added to the mixture, and then the resulting mixture is subjected to, for example, 40 cycles of ball milling at 400 rpm for 15 minutes. As a result, a material mixture ZrFCB is obtained which is for the negative electrode according to the embodiment for use in a fluoride ion secondary battery. The resulting material mixture ZrFCB and a negative electrode current collector, such as a gold foil, are then integrated by pressing at a predetermined pressure to form a negative electrode according to the embodiment for use in a fluoride ion secondary battery. 
     In this process, zirconium fluoride and the fluoride ion-conducting fluoride may be mixed in any selected ratio. As mentioned above, the negative electrode for use in a fluoride ion secondary battery preferably has a zirconium fluoride content of less than 50% by mass. For an increase in charging capacity, the fluoride ion-conducting fluoride as a source of fluorine is preferably mixed in a higher ratio. 
     The zirconium fluoride to be added is preferably in the form of particles having an average particle size selected such that the zirconium fluoride will be in the form of nanoparticles with an average particle size of 100 nm or less after being subjected to ball milling. For example, the zirconium fluoride may be added in the form of microparticles with an average particle size of 100 μm and then pulverized by ball milling. 
       FIG. 4  is an electron micrograph of zirconium fluoride in the form of microparticles to be subjected to ball milling.  FIG. 5  is an electron micrograph of zirconium fluoride in the form of microparticles resulting from ball milling.  FIGS. 4 and 5  show that even after the ball milling, the zirconium fluoride may remain in the form of microparticles although the ball milling can reduce the minimum particle size to about 300 nm. 
       FIG. 6  is an electron micrograph of zirconium fluoride in the form of nanoparticles resulting from ball milling.  FIG. 6  shows that the average particle size remains almost unchanged after the ball milling when zirconium fluoride is added in the form of particles with an average particle size of 65 nm or 20 nm as shown in  FIG. 2 . Namely, zirconium fluoride in the form of particles with an average particle size of 65 nm or 20 nm can remain unchanged in particle size. 
     Referring back to  FIG. 3 , which shows another production method, a mixture is first prepared of 700 mg of CeBaF x  (Ce 0.95 Ba 0.05 F 2.95 ), which is a fluoride ion-conducting fluoride solid electrolyte, and 50 mg of carbon black (acetylene black AB), which is a conductive aid, as in the example shown in  FIG. 2 . 
     Subsequently, 250 mg of zirconium fluoride ZrF 4  in the form of particles, for example, with an average particle size of 20 nm and 50 mg of metallic zirconium Zr in the form of particles, for example, with an average particle size of 2 μm are added to the mixture. The resulting mixture is then subjected to, for example, 40 cycles of ball milling at 400 rpm for 15 minutes. As a result, a material mixture ZrFCB is obtained which is for the negative electrode according to the embodiment for use in a fluoride ion secondary battery. The resulting material mixture ZrFCB and a negative electrode current collector, such as a gold foil, are then integrated by pressing at a predetermined pressure to form a negative electrode according to the embodiment for use in a fluoride ion secondary battery. 
     The negative electrode according to the embodiment described above for use in a fluoride ion secondary battery has advantageous effects as shown below. 
     The negative electrode according to the embodiment for use in a fluoride ion secondary battery includes zirconium fluoride as a negative electrode active material. As shown above, zirconium fluoride can be more densified than aluminum fluoride according to the conventional art and can have a higher ionic conductivity. Therefore, zirconium fluoride can be used at a higher concentration than aluminum fluoride according to the conventional art and can form a battery with a larger capacity. Moreover, the increase in the volume of zirconium fluoride with increasing concentration can be kept relatively low, which will make it possible to increase the content of a solid electrolyte including a fluoride ion-conducting fluoride and to increase the content of a conductive aid, so that higher ionic conductivity can be achieved. 
     As mentioned above, a fluoride ion secondary battery has been provided including, as a negative electrode active material, an aluminum fluoride material doped with lithium metal. At present, however, such a fluoride ion secondary battery is required to have further improved characteristics. Specifically, the lithium metal-doped aluminum fluoride active material has relatively low ionic conductivity, and the content of the negative electrode active material in the negative electrode cannot be increased sufficiently. Such a battery cannot have a high capacity density (capacity per mass of the battery) and has a coulombic efficiency as low as about 50% at the first charge/discharge cycle. To address this problem, the negative electrode according to the embodiment for use in a fluoride ion secondary battery may further include metallic zirconium as another negative electrode active material in addition to zirconium fluoride. The use of metallic zirconium as an additional negative electrode active material makes it possible to remarkably increase the coulombic efficiency, which is the ratio of discharging capacity to charging capacity, and to remarkably improve the reversibility of charging and discharging. 
     Fluoride Ion Secondary Battery 
     The fluoride ion secondary battery according to an embodiment of the present invention includes the negative electrode described above. The fluoride ion secondary battery according to the embodiment also includes a solid electrolyte layer including a fluoride ion-conducting solid electrolyte; and a positive electrode. 
     The solid electrolyte as a component of the solid electrolyte layer may be a conventionally known solid electrolyte. Specifically, the solid electrolyte may be a fluoride ion-conducting fluoride as described above. 
     The positive electrode may include a conventionally known positive electrode active material. The positive electrode preferably has a standard electrode potential sufficiently higher than that of the negative electrode according to the embodiment. A fluoride ion-free material may be selected as a positive electrode material to form a battery that can be charged at the start. In this case, the battery can be produced in a discharged state at a low energy level with improved stability of the active material in the electrode. 
     Examples of the positive electrode material include Pb, Cu, Sn, Bi, Ag, a conductive aid, and a binder. For example, a positive electrode material mixture including lead fluoride or tin fluoride and carbon black may be integrated with a positive electrode material for serving as a current collector, such as a lead foil, by pressing at a predetermined pressure to form a positive electrode. 
     Thus, the negative electrode according to the embodiment, the solid electrolyte layer, and the positive electrode may be stacked in order to form the fluoride ion secondary battery according to the embodiment. The fluoride ion secondary battery according to the embodiment can produce the same advantageous effects as shown for the negative electrode according to the embodiment described above. 
     The embodiments described above are not intended to limit the present invention and may be altered or modified within the scope of the invention where the objects of the present invention can be achieved. For example, while embodiments in which the present invention is applied to solid-state batteries have been described, such embodiments are not intended to limit the battery type. The present invention may also be applied to fluoride ion secondary batteries including an electrolytic solution in place of the solid electrolyte layer. 
     EXAMPLES 
     Next, examples of the present invention will be described, which are not intended to limit the scope of the present invention. 
     Examples 1 to 3 
     In each of Examples 1 to 3, a negative electrode material powder for use in a fluoride ion secondary battery was prepared according to the method shown in  FIG. 2  for producing the negative electrode according to the embodiment. Zirconium fluoride in the form of particles with average particles sizes of 65 nm, 20 nm, and 2 μm was used in Examples 1, 2, and 3, respectively. In all of Examples 1 to 3, the content of zirconium fluoride in the negative electrode for use in a fluoride ion secondary battery was 25% by mass. 
     Examples 4 to 7 
     In each of Examples 4 to 7, a negative electrode material powder for use in a fluoride ion secondary battery was prepared according to the method shown in  FIG. 2  for producing the negative electrode according to the embodiment. Zirconium fluoride in the form of particles with an average particles size of 65 nm was used in all of Examples 4 to 7. The content of zirconium fluoride in the negative electrode for use in a fluoride ion secondary battery was 12.5% by mass in Example 4, 30% by mass in Example 5, 40% by mass in Example 6, and 50% by mass in Example 7. 
     Examples 8 to 10 
     In each of Examples 8 to 10, a negative electrode for use in a fluoride ion secondary battery was prepared according to the method shown in  FIG. 3  for producing the negative electrode according to the embodiment. Metallic zirconium in the form of particles with an average particle size of 2 μm was used in all of Examples 8 to 10. The content of metallic zirconium in the negative electrode for use in a fluoride ion secondary battery was 8% by mass in Example 8, 1% by mass in Example 9, and 5% by mass in Example 10. 
     Comparative Examples 1 and 2 
     In each of Comparative Examples 1 and 2, a negative electrode for use in a fluoride ion secondary battery was prepared using a modified AlF 3  negative electrode active material, which is a lithium metal-doped aluminum fluoride material as disclosed in PCT/JP2019/039886, by the production method disclosed in PCT/JP2019/039886. Modified AlF 3  in the form of particles with an average particle size on the order of nanometers was used in all of Comparative Examples 1 and 2. The content of the modified AlF 3  in the negative electrode for use in a fluoride ion secondary battery was 12.5% by mass in Comparative Example 1 and 25% by mass in Comparative Example 2. 
     Charge and Discharge Test 
     The negative electrode prepared in each of the examples was used to form a half cell. The resulting half cells were subjected to a charge and discharge test at a constant current. Specifically, the charge and discharge test at a constant current was carried out in a vacuum environment at 140° C. at a charging current of 0.04 mA and a discharging current of 0.02 mA with a lower limit voltage of −2.2 V and an upper limit voltage of −0.1 V using a potentio-galvanostat system (SI 1287/1255B manufactured by Solartron). The test was started from the application of the charging current. 
     Each half cell was prepared in the form of a cylindrical columnar pellet cell by press-molding the materials at a pressure of 40 MPa in a tablet molding machine. Specifically, a gold foil (99.99%, 10 μm in thickness, manufactured by The Nilaco Corporation) as a negative electrode current collector, 10 mg of the negative electrode material mixture powder prepared in each of the examples, 200 mg of a solid electrolyte, 30 mg of a positive electrode material mixture powder, and a gold foil (99.99%, 20 μm in thickness, manufactured by the Nilaco Corporation) serving as a positive electrode material and a positive electrode current collector were placed in order in the tablet molding machine and then press-molded to form a half cell. 
     Results and Discussion 
       FIG. 7  is a graph showing the charging and discharging curves of the negative electrode half cells of Example 4 and Comparative Example 1 for fluoride ion secondary batteries.  FIG. 7  shows that the half cell of Example 4 having the negative electrode containing 12.5% by mass of zirconium fluoride ZrF 4  as a negative electrode active material has a high ratio of discharging capacity to charging capacity (a high coulombic efficiency) and improved reversibility of charging and discharging as compared to the half cell of Comparative Example 1 having the negative electrode containing 12.5% by mass of modified AlF 3 . 
       FIG. 8  is a graph showing the charging and discharging curves of the negative electrode half cells of Example 1 and Comparative Example 2 for fluoride ion secondary batteries.  FIG. 8  shows that the half cell of Example 1 having the negative electrode containing 25% by mass of zirconium fluoride ZrF 4  as a negative electrode active material has a large capacity and shows a utilization rate of 1001 as compared to the half cell of Comparative Example 2 having the negative electrode containing 25% by mass of modified AlF 3 , which showed almost no capacity. 
       FIG. 9  is a graph showing the charging and discharging curves of the negative electrode half cells of Examples 1 to 3 for fluoride ion secondary batteries. In this case, the actually available capacity is expressed by the utilization rate relative to the theoretical capacity. While zirconium fluoride at a concentration of 25% by mass provides a theoretical capacity of 1.6 mAh,  FIG. 9  shows that microparticles of zirconium fluoride with an average particle size of 100 μm (the particle size was smaller after the ball milling) in Example 3 provide a capacity of about 1.1 mAh. Nanoparticles of zirconium fluoride with an average particle size of 65 nm in Example 1 and nanoparticles of zirconium fluoride with an average particle size of 20 nm in Example 2 all provide a capacity of about 1.6 mAh. This result shows that the nanoparticles provide a higher utilization rate than the microparticles. 
       FIG. 10  is a graph showing the relationship between the capacity and the zirconium fluoride concentration of the negative electrode half cells of Examples 1 and 4 to 7 for fluoride ion secondary batteries.  FIG. 10  indicates that the charging capacity increases with increasing zirconium fluoride concentration of the negative electrode for a fluoride ion secondary battery and that when the zirconium fluoride concentration of the negative electrode reaches 50% by mass, the charging capacity sharply decreases due to a voltage drop caused by an increase in the internal resistance of the negative electrode, which makes it difficult to perform charging and discharging. This result suggests that the negative electrode for use in a fluoride ion secondary battery should preferably have a zirconium fluoride concentration of less than 50% by mass, more preferably 40% by mass or less. 
       FIG. 11  is a graph showing the charging and discharging curves of the negative electrode half cells of Examples 1 and 8 for fluoride ion secondary batteries.  FIG. 11  indicates that the half cell of Example 8 containing a negative electrode active material including zirconium fluoride and 8% by mass of metallic zirconium based on the mass of the negative electrode has a larger discharging capacity and improved reversibility of charging and discharging as compared to the half cell of Example 1 containing only zirconium fluoride as a negative electrode active material. 
       FIG. 12  is a graph showing the relationship between the coulombic efficiency, the capacity, and the zirconium fluoride concentration of the negative electrode half cells of Examples 1 and 8 to 10 for fluoride ion secondary batteries.  FIG. 12  indicates that as the metallic zirconium concentration of the negative electrode increases, the coulombic efficiency increases from about 80% or more to near 100% while the charging capacity decreases gradually. This is because as the metallic zirconium concentration increases, the amount of the electrolyte decreases so that the internal resistance of the electrode increases to reduce the voltage. The results suggest that the metallic zirconium concentration of the negative electrode for use in a fluoride ion secondary battery should be 8% by mass or less so that an improved coulombic efficiency can be provided while the reduction in charging capacity is kept low.