Negative electrode for fluoride ion secondary battery and fluoride ion secondary battery including the same

Abstract

The problem to be solved by the present invention is to provide a fluoride ion secondary battery having a larger battery capacity than before. To solve the above problems, there is provided a negative electrode for a fluoride ion secondary battery containing zirconium fluoride as a negative electrode active material and a fluoride ion secondary battery equipped with the negative electrode. The average particle size of zirconium fluoride can be 100 nm or less, and the content of zirconium fluoride in the negative electrode for a fluoride ion secondary battery can be less than 50% by mass. The negative electrode active material may further contain metallic zirconium, the average particle size of metallic zirconium can be 75 μm or less, and the content of metallic zirconium in the negative electrode for a fluoride ion secondary battery can be 8% by mass or less.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a fluoride ion secondary battery and a fluoride ion secondary battery including the same. Background Art

[0002] At present, a fluoride ion secondary battery using fluoride ions as carriers has been proposed (for example, refer to Patent Documents 1 to 6). In recent years, various studies have been conducted on fluoride ion secondary batteries, which are expected to have battery characteristics superior to those of lithium ion secondary batteries.

[0003] For example, aluminum-based materials are listed as candidates for negative electrode active materials of fluoride ion secondary batteries. Among them, although the use of aluminum fluoride has been studied, since aluminum fluoride has electrical insulation properties, there is a problem that it is difficult to cause an electrochemical reaction.

[0004] [Prior Art Documents]

[0005] (Patent Documents)

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2019-87403

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-50113

[0008] Patent Document 3: Japanese Patent Application Laid-Open No. 2019-29206

[0009] Patent Document 4: Japanese Patent Application Laid-Open No. 2018-206755

[0010] Patent Document 5: Japanese Patent Application Laid-Open No. 2018-198130

[0011] Patent Document 6: Japanese Patent Application Laid-Open No. 2018-92863 Summary of the Invention

[0012] [Problems to be Solved by the Invention]

[0013] Therefore, the applicant has realized a fluoride ion secondary battery using a negative electrode active material formed by doping lithium metal in aluminum fluoride. However, at present, it is necessary to further improve the battery characteristics. In particular, since the negative electrode active material doped with lithium metal in aluminum fluoride does not have good ion conductivity, it is impossible to increase the concentration of the negative electrode active material in the negative electrode, and it is difficult to increase the battery capacity.

[0014] The present invention has been made in view of the above circumstances, and an object thereof is to provide a fluoride ion secondary battery having a larger battery capacity than before.

[0015] [Means for Solving the Problems]

[0016] (1) The present invention provides a negative electrode for a fluoride ion secondary battery, which includes a negative electrode active material, and the aforementioned negative electrode active material includes zirconium fluoride.

[0017] (2) In the negative electrode for a fluoride ion secondary battery according to (1), the average particle size of the aforementioned zirconium fluoride is 100 nm or less.

[0018] (3) Optionally, in the negative electrode for a fluoride ion secondary battery according to (1) or (2), the content of the aforementioned zirconium fluoride in the negative electrode for a fluoride ion secondary battery is less than 50% by mass.

[0019] (4) Optionally, in the negative electrode for a fluoride ion secondary battery according to any one of (1) to (3), the aforementioned negative electrode active material further includes metallic zirconium.

[0020] (5) In the negative electrode for a fluoride ion secondary battery according to (4), the average particle size of the aforementioned metallic zirconium is 75 μm or less.

[0021] (6) In the negative electrode for a fluoride ion secondary battery according to (4) or (5), the content of the aforementioned metallic zirconium in the negative electrode for a fluoride ion secondary battery is 8% by mass or less.

[0022] (7) In addition, the present invention provides a fluoride ion secondary battery, which includes the negative electrode for a fluoride ion secondary battery according to any one of (1) to (6).

[0023] (Effects of the Invention)

[0024] According to the present invention, it is possible to provide a fluoride ion secondary battery having a larger battery capacity than conventional ones. Description of the Drawings

[0025] Figure 1 is a diagram showing the characteristics of aluminum fluoride and zirconium fluoride.

[0026] Figure 2 is a diagram showing an example of a method for manufacturing a negative electrode for a fluoride ion secondary battery according to an embodiment of the present invention.

[0027] Figure 3 is a diagram showing another example of the method for manufacturing a negative electrode for a fluoride ion secondary battery according to the above embodiment.

[0028] Figure 4 is an electron micrograph of zirconium fluoride composed of fine particles before ball milling.

[0029] Figure 5 is an electron micrograph of zirconium fluoride composed of fine particles after ball milling.

[0030] Figure 6 It is an electron microscope photograph of zirconium fluoride composed of nanoparticles after ball milling treatment.

[0031] Figure 7 It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Example 4 and Comparative Example 1.

[0032] Figure 8 It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Example 1 and Comparative Example 2.

[0033] Figure 9 It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Examples 1 to 3.

[0034] Figure 10 It is a graph showing the relationship between the zirconium fluoride concentration and the capacity of the negative electrode half-cells for fluoride ion secondary batteries of Examples 1, 4 to 6.

[0035] Figure 11 It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Examples 1 and 8.

[0036] Figure 12 It is a graph showing the relationship between the zirconium metal concentration and the capacity and coulombic efficiency of the negative electrode half-cells for fluoride ion secondary batteries of Examples 1, 8 to 10. Detailed implementation mode

[0037] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0038] [Negative electrode for fluoride ion secondary battery]

[0039] The negative electrode for a fluoride ion secondary battery of the present embodiment contains zirconium fluoride as a negative electrode active material. So far, no negative electrode for a fluoride ion secondary battery containing zirconium fluoride has been found, and the negative electrode for a fluoride ion secondary battery of the present embodiment is characterized by containing zirconium fluoride.

[0040] Zirconium fluoride acts as a negative electrode active material during charge and discharge. Specifically, zirconium fluoride releases fluoride ions F - during charging, and absorbs fluoride ions F - during discharging. As zirconium fluoride, commercially available products can be used.

[0041] The average particle size of zirconium fluoride in this embodiment is preferably 100 nm or less, that is, zirconium fluoride preferably consists of nanoparticles with an average particle size of 100 nm or less. Using zirconium fluoride composed of nanoparticles with an average particle size of 100 nm or less as the negative electrode active material can increase the battery capacity. A more preferable average particle size of zirconium fluoride is 65 nm or less.

[0042] The content of zirconium fluoride in the negative electrode for a fluoride ion secondary battery in this embodiment is preferably less than 50% by mass. The reason is that the higher the concentration of zirconium fluoride in the negative electrode for a fluoride ion secondary battery, the higher the charging capacity. On the other hand, if the concentration of zirconium fluoride in the negative electrode for a fluoride ion secondary battery reaches 50% by mass, due to the increase in the resistance inside the negative electrode, the voltage drops, and thus the charging capacity decreases sharply, making it difficult to perform charge and discharge. A more preferable content of zirconium fluoride is 40% by mass or less.

[0043] Here, Figure 1 is a graph showing the characteristics of aluminum fluoride and zirconium fluoride. In Figure 1 the literature values (theoretical values) and measured values of the densities of aluminum fluoride AlF3 investigated in the past and zirconium fluoride ZrF4 in this embodiment are shown, and the ionic conductivities of both at 140 °C when assuming the operation of a fluoride ion secondary battery are shown.

[0044] As Figure 1 shown, the density of zirconium fluoride can be higher than that of existing aluminum fluoride. In addition, its own ionic conductivity is also high. Therefore, compared with aluminum fluoride, the concentration of zirconium fluoride can be increased, and thus the battery capacity can be further improved. In addition, in zirconium fluoride, even if its concentration is increased, the increase in volume can be suppressed. Therefore, the content of the solid electrolyte including fluoride ion conductive fluoride and the content of the conductive aid described later can be increased, and as a result, a higher ionic conductivity can be obtained.

[0045] The negative electrode active material in this embodiment preferably further contains metallic zirconium. Using a negative electrode active material containing metallic zirconium can improve the ratio of discharge capacity to charging capacity, that is, the Coulomb efficiency, and can improve the reversibility of charge and discharge. As the metallic zirconium, commercially available products can be used.

[0046] In particular, the negative electrode for a fluoride ion secondary battery in this embodiment preferably contains the solid electrolyte including fluoride ion conductive fluoride described later. However, if a reaction of the solid electrolyte occurs, the discharge capacity cannot be sufficiently obtained relative to the charging capacity. In this regard, in this embodiment, metallic zirconium is contained, thereby being able to suppress the reaction of the solid electrolyte and obtain a higher discharge capacity. As a result, the reversibility of charge and discharge can be improved. In addition, since the charging capacity can be used for discharge with a small loss, an extension of the life of the fluoride ion secondary battery in this embodiment can be expected.

[0047] The average particle size of zirconium metal is preferably 75 μm or less. That is, zirconium metal preferably consists of particles with an average particle size of 75 μm or less. By containing zirconium metal composed of particles with an average particle size of 75 μm or less, the battery capacity can be increased and the reversibility of charge and discharge can be improved.

[0048] The content of zirconium metal in the negative electrode for a fluoride ion secondary battery of the present embodiment is preferably 8% by mass or less. The reason is that the higher the zirconium metal concentration in the negative electrode for a fluoride ion secondary battery, the higher the coulombic efficiency. On the other hand, the amount of electrolyte decreases, the resistance inside the negative electrode increases, and the voltage drops. As a result, the charge capacity decreases. However, if the content of zirconium metal in the negative electrode for a fluoride ion secondary battery is 8% by mass or less, the decrease in charge capacity can be suppressed. A more preferable content of zirconium metal is 5% by mass or less.

[0049] The negative electrode for a fluoride ion secondary battery of the present embodiment preferably further contains a solid electrolyte including a fluoride ion-conductive fluoride and a conductive aid in addition to zirconium fluoride and zirconium metal as the above-mentioned negative electrode active material.

[0050] As the fluoride ion-conductive fluoride, any fluoride having fluoride ion conductivity may be used, and there is no particular limitation. For example, CeBaF x and BaLaF y and other fluoride ion-conductive fluorides are listed. Specifically, Ce 0.95 Ba 0.05 F 2.95 or Ba 0.6 La 0.4 F 2.4 etc. can be used. By including these fluoride ion-conductive fluorides in the negative electrode for a fluoride ion secondary battery of the present embodiment, the fluoride ion conductivity can be improved.

[0051] The average particle size of the fluoride ion-conductive fluoride is preferably in the range of 0.1 μm to 100 μm. If the average particle size of the fluoride ion-conductive fluoride is within this range, it has high ion conductivity and can form a thin-film electrode. A more preferable range of the average particle size of the fluoride ion-conductive fluoride is 0.1 μm to 10 μm.

[0052] As the conductive aid, any material having electron conductivity may be used, and there is no particular limitation. For example, carbon black or the like is used as the conductive aid. As the carbon black, furnace black, Ketjen black, acetylene black, etc. can be used. By including these conductive aids in the negative electrode for a fluoride ion secondary battery of the present embodiment, the electron conductivity can be improved.

[0053] The average particle size of the conductive additive is preferably in the range of 20 nm to 50 nm. If the average particle size of the conductive additive is within this range, an electrode that is lighter in weight and has higher electron conductivity can be formed.

[0054] In addition, within the range that does not impair the effects of the present embodiment, the negative electrode for a fluoride ion secondary battery of the present embodiment may further contain other components such as a binder.

[0055] Next, the manufacturing method of the negative electrode for a fluoride ion secondary battery of the present embodiment will be described in detail with reference to Figure 2 and Figure 3 in detail.

[0056] Here, Figure 2 is a diagram showing an example of the manufacturing method of the negative electrode for a fluoride ion secondary battery of the present embodiment. In addition, Figure 3 is a diagram showing another example of the manufacturing method of the negative electrode for a fluoride ion secondary battery of the present embodiment.

[0057] In Figure 2 In an example of the manufacturing method shown, first, 700 mg of CeBaF x (Ce 0.95 Ba 0.05 F 2.95 ), which is a solid electrolyte including a fluoride ion conductive fluoride, and 50 mg of carbon black (acetylene black AB) as a conductive additive are mixed.

[0058] Next, after adding 250 mg of zirconium fluoride ZrF4 to the above mixture, a ball mill pulverization treatment such as 400 rpm, 15 minutes, and 40 cycles is performed. Thereby, a binder ZrFCB for the negative electrode of the fluoride ion secondary battery of the present embodiment can be obtained. Then, the obtained binder ZrFCB is pressed and integrated with a negative electrode current collector such as gold foil at a predetermined pressure, thereby manufacturing the negative electrode for a fluoride ion secondary battery of the present embodiment.

[0059] In addition, the mixing ratio of zirconium fluoride and the fluoride ion conductive fluoride can be arbitrarily selected. Among them, as described above, the content of zirconium fluoride in the negative electrode for a fluoride ion secondary battery is preferably less than 50% by mass, and from the viewpoint of increasing the charge capacity, the proportion of the fluoride ion conductive fluoride as a fluorine source is preferably high.

[0060] In addition, the average particle size of the added zirconium fluoride is preferably appropriately selected so that nanoparticles having an average particle size of 100 nm or less are formed after the ball mill pulverization treatment. For example, when zirconium fluoride composed of fine particles having an average particle size of 100 μm is added, zirconium fluoride can be obtained by ball mill pulverization treatment.

[0061] Figure 4It is an electron microscope photograph of zirconium fluoride composed of fine particles before ball milling. Additionally, Figure 5 is an electron microscope photograph of zirconium fluoride composed of fine particles after ball milling. As Figure 4 and Figure 5 show, zirconium fluoride composed of fine particles can be ground by the above ball mill to a minimum particle size of about 300 nm, but the average particle size remains that of fine particles.

[0062] In contrast, Figure 6 is an electron microscope photograph of ball-milled zirconium fluoride composed of nanoparticles. As Figure 6 shows, when adding zirconium fluoride with an average particle size of 65 nm or 20 nm as shown in Figure 2 , the average particle size hardly changes after ball milling. That is, for zirconium fluoride with an average particle size of 65 nm or 20 nm, the particle size remains unchanged.

[0063] Return Figure 3 , in Figure 3 another example of the manufacturing method shown, similar to the example shown in Figure 2 , first, 700 mg of CeBaF as a solid electrolyte including a fluoride ion-conductive fluoride x (Ce 0.95 Ba 0.05 F 2.95 ) and 50 mg of carbon black (acetylene black AB) as a conductive aid are mixed.

[0064] Next, after adding, for example, 250 mg of zirconium fluoride ZrF4 with an average particle size of 20 nm and, for example, 50 mg of metallic zirconium Zr with an average particle size of 2 μm to the above mixture, a ball milling treatment is performed, for example, at 400 rpm for 15 minutes for 40 cycles. Thus, a binder ZrFCB for the negative electrode of the fluoride ion secondary battery of the present embodiment is obtained. Then, the obtained binder ZrFCB is pressed and integrated with a negative electrode current collector such as a gold foil at a predetermined pressure, thereby manufacturing the negative electrode for the fluoride ion secondary battery of the present embodiment.

[0065] The negative electrode for the fluoride ion secondary battery according to the present embodiment described above has the following effects.

[0066] In the negative electrode for a fluoride ion secondary battery according to this embodiment, it is configured to include zirconium fluoride as a negative electrode active material. As described above, in addition to the density of zirconium fluoride being able to be higher than that of the existing aluminum fluoride, its own ionic conductivity is also high. Therefore, compared with the existing aluminum fluoride, the concentration of zirconium fluoride can be increased, and thus the battery capacity can be further increased. In addition, in zirconium fluoride, even if its concentration is increased, an increase in volume can be suppressed, so that the content of the solid electrolyte including the fluoride ion-conductive fluoride and the content of the conductive aid can be increased. As a result, a higher ionic conductivity can be obtained.

[0067] In addition, as described above, the present applicant has realized a fluoride ion secondary battery that uses a negative electrode active material in which lithium metal is doped in aluminum fluoride. However, at present, it is necessary to further improve the battery characteristics. In particular, since the negative electrode active material in which lithium metal is doped in aluminum fluoride does not have good ionic conductivity, the concentration of the negative electrode active material in the negative electrode cannot be increased. Therefore, the capacity density (battery capacity per unit mass) of the battery cannot be increased, and the Coulomb efficiency in the first charge-discharge cycle is as low as about 50%. Therefore, in the negative electrode for a fluoride ion secondary battery according to this embodiment, it is configured to include not only zirconium fluoride but also metal zirconium as a negative electrode active material. Thereby, by using the negative electrode active material containing metal zirconium, the ratio of the discharge capacity to the charge capacity, that is, the Coulomb efficiency, can be significantly increased, and the reversibility of charge and discharge can be significantly improved.

[0068] [Fluoride Ion Secondary Battery]

[0069] The fluoride ion secondary battery according to this embodiment includes the above-described negative electrode for a fluoride ion secondary battery. In addition, the fluoride ion secondary battery according to this embodiment includes a solid electrolyte layer and a positive electrode formed of a solid electrolyte having fluoride ion conductivity.

[0070] As the solid electrolyte constituting the solid electrolyte layer, a currently well-known solid electrolyte is used. Specifically, the same solid electrolyte as the above-described fluoride ion-conductive fluoride can be used.

[0071] As the positive electrode, a currently well-known positive electrode active material is used. Preferably, a positive electrode having a sufficiently high standard electrode potential is used with respect to the standard electrode potential of the negative electrode for a fluoride ion secondary battery according to this embodiment. In addition, a material that does not have fluoride ions is selected as the positive electrode, whereby a battery that can be charged can be realized. That is, the battery can be manufactured in a discharged state with a lower energy state, and the stability of the active material in the electrode can be further improved.

[0072] As specific positive electrode materials, conductive aids such as Pb, Cu, Sn, Bi, Ag, binders, etc. can be cited. For example, a positive electrode mixture containing lead fluoride or tin fluoride, carbon black, etc. is pressed and integrated with a positive electrode material and a lead foil as a current collector at a prescribed pressure, whereby a positive electrode can be manufactured.

[0073] Therefore, by sequentially laminating the negative electrode for a fluoride ion secondary battery, the solid electrolyte layer, and the positive electrode of the present embodiment described above, the fluoride ion secondary battery of the present embodiment can be manufactured. The fluoride ion secondary battery according to the present embodiment can achieve the same effects as the negative electrode for a fluoride ion secondary battery of the present embodiment described above.

[0074] The present invention is not limited to the above-described embodiments, and modifications and improvements made within the scope where the object of the present invention can be achieved are included in the present invention.

[0075] For example, in the above-described embodiment, an example of applying the present invention to a solid-state battery was described, but it is not limited thereto. An electrolytic solution can also be used instead of the solid electrolyte layer for a fluoride ion secondary battery.

[0076] [Examples]

[0077] Next, examples of the present invention will be described, but the present invention is not limited to these examples.

[0078] [Examples 1 to 3]

[0079] According to Figure 2 the manufacturing method of the negative electrode for a fluoride ion secondary battery of the present embodiment, the negative electrode mixture powders for fluoride ion secondary batteries of Examples 1 to 3 were produced. In Example 1, zirconium fluoride having an average particle diameter of 65 nm was used, in Example 2, zirconium fluoride having an average particle diameter of 20 nm was used, and in Example 3, zirconium fluoride having an average particle diameter of 2 μm was used. In addition, in Examples 1 to 3, the content of zirconium fluoride in the negative electrode for a fluoride ion secondary battery was set to 25% by mass.

[0080] [Examples 4 to 7]

[0081] According to Figure 2 the manufacturing method of the negative electrode for a fluoride ion secondary battery shown, the negative electrode mixture powders for fluoride ion secondary batteries of Examples 4 to 7 were produced. In Examples 4 to 7, zirconium fluoride having an average particle diameter of 65 nm was used. In addition, the content of zirconium fluoride in the negative electrode for a fluoride ion secondary battery was set to 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.

[0082] [Examples 8 to 10]

[0083] According to Figure 3 the method for manufacturing a negative electrode for a fluoride ion secondary battery according to the present embodiment shown, negative electrodes for fluoride ion secondary batteries of Examples 8 to 10 were fabricated. In Examples 8 to 10, zirconium metal with an average particle size of 2 μm was used. In addition, the content of zirconium metal in the negative electrode for a fluoride ion secondary battery was set to 8% by mass in Example 8, 1% by mass in Example 9, and 5% by mass in Example 10.

[0084] [Comparative Examples 1, 2]

[0085] As existing negative electrodes for fluoride ion secondary batteries, modified AlF3 negative electrode active materials were fabricated as negative electrodes for fluoride ion secondary batteries of Comparative Examples 1 and 2 according to the manufacturing method described in PCT / JP2019 / 039886. The modified AlF3 negative electrode active material was formed by doping lithium metal in aluminum fluoride as proposed by the present applicant in PCT / JP2019 / 039886. In Comparative Examples 1 and 2, modified AlF3 with a nanoscale average particle size was used, and the content of modified AlF3 in the negative electrode for a fluoride ion secondary battery was set to 12.5% by mass in Comparative Example 1 and 25% by mass in Comparative Example 2.

[0086] [Charge and Discharge Tests]

[0087] Half cells were fabricated respectively, and the half cells used the negative electrodes for fluoride ion secondary batteries fabricated in each example, and a constant current charge and discharge test was implemented. Specifically, a potentiostat (manufactured by Soltron Corporation, SI1287 / 1255B) was used, and in an environment of 140 °C under vacuum, with a charging current of 0.04 mA and a discharging current of 0.02 mA, and a lower limit voltage of -2.2 V and an upper limit voltage of -0.1 V were set, and the constant current charge and discharge test was implemented starting from the charging current.

[0088] In addition, as each half cell, it was pressed using a tablet press at a pressure of 40 MPa to fabricate a cylindrical granular battery formed by powder pressing. Specifically, a gold foil (99.99%, thickness 10 μm) manufactured by Nilaco Co., Ltd. as a negative electrode current collector, 10 mg of the negative electrode binder powder for a fluoride ion secondary battery fabricated in each example, 200 mg of a solid electrolyte, 30 mg of a positive electrode binder powder, a positive electrode material, and a gold foil (99.99%, thickness 20 μm) manufactured by Nilaco Co., Ltd. as a positive electrode current collector were successively put into the tablet press to fabricate each half cell.

[0089] [Results and Discussion]

[0090] Figure 7It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Example 4 and Comparative Example 1. As Figure 7 shown, if Example 4 using zirconium fluoride ZrF4 and Comparative Example 1 using modified AlF3 with the negative electrode active material concentrations both being 12.5% by mass are compared, it can be confirmed that the ratio of the discharge capacity to the charge capacity (coulombic efficiency) of Example 4 is larger and the reversibility of charge-discharge is improved.

[0091] Figure 8 It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Example 1 and Comparative Example 2. As Figure 8 shown, if Example 1 using zirconium fluoride ZrF4 and Comparative Example 2 using modified AlF3 with the negative electrode active material concentrations both being 25% by mass are compared, it can be confirmed that almost no capacity is obtained in Comparative Example 2, whereas a larger capacity is obtained in Example 1 and the utilization rate is 100%.

[0092] Figure 9 It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Examples 1 to 3. Here, the actually obtained capacity is represented by the utilization rate with respect to the theoretical capacity. The theoretical capacity of zirconium fluoride is 1.6 mAh when the concentration is 25% by mass. As Figure 9 shown, in the fine particles of Example 3 with an average particle size of 100 μm (the particle size after ball milling is smaller), the capacity is about 1.1 mAh. In contrast, in the nanoparticles of Example 1 with an average particle size of 65 nm and the nanoparticles of Example 2 with an average particle size of 20 nm, the capacity is about 1.6 mAh in both cases. From this result, it can be confirmed that the utilization rate of nanoparticles is higher than that of fine particles.

[0093] Figure 10 It is a graph showing the relationship between the zirconium fluoride concentration and the capacity in the negative electrode half-cells for fluoride ion secondary batteries of Examples 1, 4 to 7. As Figure 10 shown, it can be seen that the higher the zirconium fluoride concentration in the negative electrode for fluoride ion secondary batteries, the higher the charge capacity. If the zirconium fluoride concentration in the negative electrode for fluoride ion secondary batteries reaches 50% by mass, the voltage drops due to the increase in the internal resistance of the negative electrode, and thus the charge capacity drops sharply and it is difficult to perform charge-discharge. From this result, it can be confirmed that the preferred concentration of zirconium fluoride in the negative electrode for fluoride ion secondary batteries is less than 50% by mass, and more preferably 40% by mass or less.

[0094] Figure 11 It is a graph showing the charge-discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Examples 1 and 8. As Figure 11As shown, compared with Example 1 in which zirconium fluoride is used as the negative electrode active material, in Example 8, in addition to adding zirconium fluoride, 8% by mass of metallic zirconium is added as the negative electrode active material to the entire negative electrode for the fluoride ion secondary battery. According to Example 8, it was confirmed that the discharge capacity increased and the reversibility of charge and discharge was improved.

[0095] Figure 12 FIG. is a graph showing the relationship between the metallic zirconium concentration and the capacity and Coulomb efficiency in the negative electrode half-cell for the fluoride ion secondary battery of Examples 1 and 8 to 10. As Figure 12 shown, the higher the metallic zirconium concentration in the negative electrode for the fluoride ion secondary battery, the Coulomb efficiency increases from about 80% or more to nearly 100%. On the other hand, the charge capacity decreases slowly. The reason is that if the metallic zirconium concentration increases, the amount of electrolyte decreases, the resistance inside the negative electrode increases, and the voltage decreases. Therefore, it was confirmed from this result that if the metallic zirconium concentration in the negative electrode for the fluoride ion secondary battery is 8% by mass or less, the decrease in the charge capacity can be suppressed and the Coulomb efficiency can be improved.

Claims

1. A negative electrode for a fluoride ion secondary battery, which is a negative electrode for a fluoride ion secondary battery using fluoride ions as a carrier and containing a negative electrode active material. The negative electrode active material contains zirconium fluoride and metallic zirconium. Among them, The content of zirconium fluoride in the negative electrode for the fluoride ion secondary battery is less than 50% by mass, and the content of the metallic zirconium in the negative electrode for the fluoride ion secondary battery is 8% by mass or less.

2. The negative electrode for a fluoride ion secondary battery according to claim 1, wherein the average particle size of the zirconium fluoride is 100 nm or less.

3. The negative electrode for a fluoride ion secondary battery according to claim 1, wherein the average particle size of the metallic zirconium is 75 μm or less.

4. A fluoride ion secondary battery comprising the negative electrode for a fluoride ion secondary battery according to any one of claims 1 to 3.

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