Fluoride ion battery, negative electrode active material for fluoride ion battery, and method for producing negative electrode active material for fluoride ion battery

Abstract

To provide a fluoride ion battery that can achieve a high charge-discharge capacity.SOLUTION: A fluoride ion battery of the present disclosure has a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer. In the fluoride ion battery of the present disclosure, the negative electrode active material layer contains magnesium metal, magnesium fluoride, and calcium barium fluoride, and the mass ratio of the magnesium metal to the magnesium fluoride is 0.1-10.0.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a fluoride ion battery, a negative electrode active material for a fluoride ion battery, and a method for manufacturing a negative electrode active material for a fluoride ion battery.

Background Art

[0002] As a high-voltage and high-energy density battery, for example, a lithium-ion battery is known. A lithium-ion battery is a cation-based battery that uses lithium ions as carriers. On the other hand, as an anion-based battery, a fluoride ion battery that uses fluoride ions as carriers is known.

[0003] For example, Patent Document 1 discloses a fluoride ion battery including a negative electrode material containing a Mg material containing an Mg element and a fluoride ion conductive material containing at least one metal element (excluding the Mg element) and an F element.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a fluoride ion battery, it is required to achieve a high charge-discharge capacity.

[0006] An object of the present disclosure is to provide a fluoride ion battery capable of achieving a high charge-discharge capacity.

Means for Solving the Problems

[0007] The present inventors have found that the above problems can be solved by the following means. <Aspect 1> A fluoride ion battery having a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer, wherein the negative electrode active material layer contains metallic magnesium, magnesium fluoride, and calcium barium fluoride, and the mass ratio of the metallic magnesium to the magnesium fluoride is from 0.1 to 10.0. A fluoride ion battery. <Aspect 2> The fluoride ion battery according to Aspect 1, wherein the ratio of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of the magnesium fluoride measured using CuKα rays to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of NIST standard CeO2 is 1.5 or more. <Aspect 3> The fluoride ion battery according to Aspect 1 or 2, wherein the electrolyte layer has calcium barium fluoride. <Aspect 4> The fluoride ion battery according to any one of Aspects 1 to 3, having a charge-discharge capacity of 100 mAh / g or more. <Aspect 5> Containing magnesium fluoride, wherein the ratio of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of the magnesium fluoride measured using CuKα rays to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of NIST standard CeO2 is 1.5 or more. A negative electrode active material for a fluoride ion battery. <Aspect 6> A method for manufacturing a negative electrode active material according to Aspect 5, including changing the ratio of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of magnesium fluoride, measured using CuKα rays, after applying mechanical shock to magnesium fluoride, to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of NIST standard CeO2 to be 1.5 or more. <Aspect 7> The method according to Aspect 6, wherein the mechanical shock is applied by a ball mill. <Aspect 8> The method according to Aspect 7, wherein the rotation speed of the ball mill is 300 rpm or more. [Effect of the Invention]

[0008] According to the present disclosure, a fluoride ion battery capable of realizing a high charge-discharge capacity can be provided. [Brief Description of the Drawings]

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0010] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the disclosure.

[0011] 《Fluoride Ion Battery》 The fluoride ion battery of the present disclosure has a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer. In the fluoride ion battery of the present disclosure, the negative electrode active material layer contains metallic magnesium, magnesium fluoride, and calcium barium fluoride, and the mass ratio of metallic magnesium to magnesium fluoride is 0.1 to 10.0.

[0012] By having the negative electrode active material layer contain metallic magnesium and magnesium fluoride together with calcium barium fluoride and setting the mass ratio of metallic magnesium to magnesium fluoride within a predetermined range, the fluoride ion battery having this negative electrode active material layer can achieve a high charge-discharge capacity.

[0013] Although not intending to be bound by any theory, the reason is presumed as follows. That is, first, as the first-stage reaction during charging, magnesium fluoride reacts to release fluoride ions and metallic magnesium is generated. Next, as the second-stage reaction, the metallic magnesium in the negative electrode active material layer containing the metallic magnesium generated in the first stage forms an alloy with calcium or barium in calcium barium fluoride and releases fluoride ions. During discharging, the reverse reaction causes the occlusion of fluoride ions. That is, it is considered that the fluoride ion battery of the present disclosure can achieve a high charge-discharge capacity because two stages of fluoride ion release (charging) and occlusion (discharging) occur. Furthermore, since the blended metallic magnesium also functions as an electron conduction assistant and contributes to the efficient progress of charge and discharge, it is considered that the fluoride ion battery of the present disclosure can achieve a high charge-discharge capacity.

[0014] FIG. 1 is a schematic diagram showing an example of a fluoride ion battery in the present disclosure. The fluoride ion battery 1 shown in FIG. 1 includes a positive electrode active material layer 20, a negative electrode active material layer 40, an electrolyte layer 30 formed between the positive electrode active material layer 20 and the negative electrode active material layer 40, a positive electrode current collector 10 for collecting current from the positive electrode active material layer 20, and a negative electrode current collector 50 for collecting current from the negative electrode active material layer 40.

[0015] Note that the fluoride ion battery of the present disclosure may be a liquid battery or a solid battery, particularly an all-solid-state battery. In the present disclosure, a solid battery means a battery using a solid electrolyte. Also, the fluoride ion battery in the present disclosure may be a primary battery or a secondary battery. Examples of the shape of the fluoride ion battery in the present disclosure include a coin type, a laminate type, a cylindrical type, and a rectangular type.

[0016] 〈Negative Electrode Active Material Layer〉 In the present disclosure, the negative electrode active material layer contains metallic magnesium, magnesium fluoride, and calcium barium fluoride.

[0017] (Metallic Magnesium) In the present disclosure, the negative electrode active material layer contains metallic magnesium.

[0018] The blending ratio of metallic magnesium in the negative electrode active material layer is, for example, 1% by weight or more, and may be 3% by weight or more, or 5% by weight or more. The blending ratio of metallic magnesium is, for example, 55% by weight or less, and may be 45% by weight or less, or 35% by weight or less.

[0019] (Magnesium Fluoride) In the present disclosure, the negative electrode active material layer contains magnesium fluoride (MgF2). MgF2 functions as a negative electrode active material.

[0020] In the present disclosure, the ratio (FWHMsam. / FWHMref.) of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of MgF2 measured using CuKα radiation to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of CeO2 may be 1.5 or more.

[0021] Crystallinity (crystallite size) is known to be correlated with the peak half-width FWHM (full width at half maximum) in XRD measurement. The smaller the FWHM, the higher the crystallinity (the larger the crystallite size). However, since the absolute value of the calculated FWHM depends on the XRD measurement apparatus, conditions, and calculation method, in the present disclosure, it is evaluated by the ratio of FWHMsam. of the sample (MgF2 in the negative electrode active material layer) to FWHMref. of the standard substance CeO2 (a product distributed by the National Institute of Standards and Technology, NIST) measured and calculated in the same manner as the sample. The smaller the FWHMsam. / FWHMref., the higher the crystallinity.

[0022] Note that although the FWHM is calculated individually for each peak in the XRD pattern, in the present disclosure, among the peaks derived from MgF2 in the negative electrode active material layer as the sample, as a peak with relatively high intensity and easy separation from other peaks, FWHMsam. is calculated for the peak of the MgF2 (111) plane confirmed at around 2θ = 40.4 deg. in the measurement using CuKα radiation. For the standard substance, FWHMref. is calculated for the peak of the CeO2 (220) plane confirmed at around 2θ = 47.5 deg.

[0023] The value of FWHMsam. / FWHMref. is preferably 1.5 or more, and more preferably 8 or more. The value of FWHMsam. / FWHMref. is preferably 10.0 or less, and more preferably 9.0 or less. When FWHMsam. / FWHMref. is within the above range, the charge-discharge capacity of the fluoride ion battery is improved.

[0024] Although not intending to be bound by any theory, it is considered that when the crystallinity of the MgF2 crystal is decreased (the crystallite size is reduced), the diffusion distance of fluoride ions in the active material particles of MgF2 becomes shorter, enabling the charge and discharge reactions to proceed efficiently.

[0025] The blending ratio of magnesium fluoride in the negative electrode active material layer is, for example, 1% by weight or more, and may be 3% by weight or more, or may be 5% by weight or more. The blending ratio of magnesium fluoride is, for example, 55% by weight or less, and may be 45% by weight or less, or may be 35% by weight or less.

[0026] The mass ratio of metallic magnesium to magnesium fluoride (Mg / MgF2) is from 0.1 to 10.0, preferably from 0.1 to 5.0. When Mg / MgF2 is within the above range, the charge and discharge capacity of the fluoride ion battery can be increased.

[0027] (Method for manufacturing negative electrode active material) The method of the present disclosure for manufacturing a negative electrode active material for a fluoride ion battery includes applying mechanical impact to magnesium fluoride to change the ratio of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of magnesium fluoride, measured using CuKα radiation, to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of NIST standard CeO2 to 1.5 or more.

[0028] Examples of the method for applying mechanical impact include mechanical milling, and specifically, ball milling. In this case, the rotation speed and mixing time of the ball mill can be adjusted to control FWHMsam. / FWHMref.

[0029] The rotation speed of the ball mill is, for example, 300 rpm or more, preferably 400 rpm or more. The rotation speed of the ball mill is, for example, 600 rpm or less, preferably 500 rpm or less.

[0030] Mixing with a ball mill can be carried out for, for example, 1 hour or more, 2 hours or more, or 3 hours or more.

[0031] (Calcium Barium Fluoride) In the present disclosure, the negative electrode active material layer is made of calcium barium fluoride (Ca 1-x Ba x F2). 1-x Ba x F2 also functions as a negative electrode active material. 1-x Ba x The negative electrode active material is generated self-assembled from F2.

[0032] Ca 1-x Ba x F2 can be obtained, for example, by mechanical milling a mixture containing calcium fluoride (CaF2) and barium fluoride (BaF2). The mechanical milling method can include, but is not limited to, mixing using a ball mill.

[0033] Ca 1-x Ba x x in F2 is 0 <x<1を満たすものであれば特に限定されないが、xは、0.30以上であってもよく、0.35以上であってもよく、0.40以上であってもよい。xは、0.70以下であってもよく、0.65以下であってもよく、0.60以下であってもよい。特に、xが0.45≦x≦0.65を満たす場合、可逆容量が良好になる。

[0034] Ca in the negative electrode active material layer 1-x Ba x The blending ratio of F2 is, for example, 45% by weight or more, may be 50% by weight or more, or may be 55% by weight or more. 1-x Ba x The blending ratio of F2 is, for example, 75% by weight or less, may be 70% by weight or less, or may be 65% by weight or less.

[0035] (others) The negative electrode active material layer in the present disclosure may further contain at least one of an electron conductive assistant and a binder, if necessary. Examples of the electron conductive assistant include carbon materials. Examples of the carbon materials include carbon blacks such as acetylene black, ketjen black, furnace black, thermal black, graphene, fullerene, and carbon nanotubes. Examples of the binder include fluorine-based binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0036] The thickness of the negative electrode active material layer is not particularly limited and can be appropriately adjusted according to the configuration of the battery.

[0037] 〈Positive Electrode Active Material Layer〉 The positive electrode active material layer in the present disclosure is a layer containing at least a positive electrode active material. Further, the positive electrode active material layer may further contain at least one of a solid electrolyte, an electron conductive assistant, and a binder, if necessary.

[0038] The positive electrode active material is usually an active material that undergoes defluorination during discharge. Examples of the positive electrode active material include a simple metal, an alloy, a metal oxide, and fluorides thereof. Examples of the metal element contained in the positive electrode active material include Cu, Ag, Ni, Co, Pb, Ce, Mn, Au, Pt, Rh, V, Os, Ru, Fe, Cr, Bi, Nb, Sb, Ti, Sn, Zn, etc. Among them, the positive electrode active material is Cu, CuF z , Fe, FeF z , Ag, AgF zIt is preferably such. Note that the above z is a real number greater than 0. Further, as other examples of the positive electrode active material, carbon materials and their fluorides can be mentioned. Examples of the carbon material include graphite, coke, and carbon nanotubes. Further, as still other examples of the positive electrode active material, polymer materials can be mentioned. Examples of the polymer material include polyaniline, polypyrrole, polyacetylene, and polythiophene. Further, as still other examples of the positive electrode active material, metal sulfides can be mentioned. Examples of the metal sulfide include Cu2S, CuS, FeS, and FeS2.

[0039] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but it is preferably large from the viewpoint of capacity. The content of the positive electrode active material is, for example, 30% by weight or more, may be 50% by weight or more, and may be 70% by weight or more.

[0040] Regarding the solid electrolyte, reference can be made to the description of the solid electrolyte layer of the present disclosure.

[0041] Regarding the electron conduction assistant and the binder, reference can be made to the description of the negative electrode active material layer of the present disclosure.

[0042] The thickness of the positive electrode active material layer is not particularly limited and can be appropriately adjusted according to the configuration of the battery.

[0043] 〈Electrolyte layer〉 The electrolyte layer in the present disclosure is a layer formed between the positive electrode active material layer and the negative electrode active material layer.

[0044] When the fluoride ion battery of the present disclosure is a liquid battery, the electrolyte layer can be composed of, for example, an electrolytic solution and an optional separator.

[0045] The electrolyte can contain, for example, a fluoride salt and an organic solvent. Examples of the fluoride salt include inorganic fluoride salts, organic fluoride salts, and ionic liquids. As an example of the inorganic fluoride salt, XF (where X is Li, Na, K, Rb, or Cs) can be mentioned. As an example of the cation of the organic fluoride salt, an alkylammonium cation such as a tetramethylammonium cation can be mentioned.

[0046] The organic solvent of the electrolyte is usually a solvent that dissolves the fluoride salt. Examples of the organic solvent include glymes such as triethylene glycol dimethyl ether (G3) and tetraethylene glycol dimethyl ether (G4), cyclic carbonates such as ethylene carbonate (EC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), propylene carbonate (PC), and butylene carbonate (BC), and chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Also, an ionic liquid may be used as the organic solvent.

[0047] The separator is not particularly limited as long as it has a composition that can withstand the use range of the fluoride ion battery. Examples of the separator include polymer non-woven fabrics such as polypropylene non-woven fabric and polyphenylene sulfide non-woven fabric, and microporous films of olefin resins such as polyethylene and polypropylene.

[0048] When the fluoride ion battery of the present disclosure is a solid battery, the electrolyte layer can be, for example, a layer of a solid electrolyte. In this case, the electrolyte layer may further contain a binder as needed.

[0049] The solid electrolyte is not particularly limited as long as it is a material that can be used in a fluoride ion battery, and inorganic fluorides are exemplified. Examples of the inorganic fluoride include fluorides containing lanthanoid elements such as La and Ce, fluorides containing alkaline earth elements such as Ca, Sr, and Ba, fluorides containing Group 14 elements such as Pb and Sn, and fluorides containing alkali elements such as Li, Na, K, Rb, and Cs. The solid electrolyte is preferably Ca 1-x Ba x F2.

[0050] Regarding the binder, reference can be made to the description of the negative electrode active material layer of the present disclosure.

[0051] <Other configurations> The fluoride ion battery in the present disclosure preferably has a positive electrode current collector that collects current from the positive electrode active material layer, a negative electrode current collector that collects current from the negative electrode active material layer, and a battery case that houses the above-described members. Examples of the shape of the current collector include foil shape, mesh shape, porous shape, and the like. As the battery case, a conventionally known battery case can be used.

[0052] The charge-discharge capacity of the fluoride ion battery of the present disclosure can be 100 mAh / g or more as the specific capacity normalized by the weight of the negative electrode composite material.

Examples

[0053] <Example 1> (Preparation of negative electrode composite material) CaF2 and BaF2 were prepared as raw materials and weighed so that the molar ratio was CaF2:BaF2 = 50:50. The weighed CaF2 and BaF2 were mixed by mechanical milling using a ball mill apparatus (manufactured by Fritsch, planetary ball mill premium line PL-7) at 600 rpm for 20 hours in a dry argon atmosphere, and the mixture was reacted to obtain powdery Ca 0.5 Ba 0.5 F2.

[0054] Mg, MgF2, the above-mentioned Ca0.5 Ba 0.5 F2 and acetylene black (AB) as an electron conduction aid were weighed at the compounding ratios (by weight) shown in Table 1, and using a ball mill apparatus (manufactured by Fritsch, planetary ball mill premium line PL-7), they were mixed at 600 rpm for 3 hours in a dry argon atmosphere to obtain a powdery negative electrode composite material.

[0055] (Fabrication of solid electrolyte) CaF2 and BaF2 were prepared as raw materials and weighed so that the molar ratio was CaF2:BaF2 = 60:40. These were mechanically milled using a ball mill apparatus (manufactured by Fritsch, planetary ball mill premium line PL-7) and mixed at 600 rpm for 20 hours in a dry argon atmosphere, and the mixture was reacted to obtain a powdery solid electrolyte (Ca 0.6 Ba 0.4 F2).

[0056] (Fabrication of fluoride ion battery) The following were used as the respective constituent members of the fluoride ion battery. · Negative electrode current collector: Platinum foil · Negative electrode active material layer: A compact formed using 10 mg of the powder of the above negative electrode composite material · Electrolyte layer: A compact formed using 100 mg of the powder of the above solid electrolyte · Positive electrode active material layer: Lead plate (220 mg) · Positive electrode current collector: Aluminum foil

[0057] The negative electrode current collector, negative electrode active material layer, electrolyte layer, positive electrode active material layer, and positive electrode current collector were laminated in this order to fabricate an all-solid-state fluoride ion battery.

[0058] 〈Examples 2 to 5, Comparative Examples 1 to 4〉 All-solid-state fluoride ion batteries of Examples 2 to 5 and Comparative Examples 1 to 4 were fabricated in the same manner as in Example 1, except that the compounding ratio of the negative electrode composite material was changed as shown in Table 1.

[0059] 〈Charge and discharge test〉 For the fluoride ion batteries of Examples 1 to 5 and Comparative Examples 1 to 4, while evacuating in a sealed container, a charge and a discharge were performed at a test temperature of 200°C and a current density of 0.05 mA / cm 2 <000026>

[0060] Table 1, Figure 2 (Examples 1 to 5) and Figure 3 (Comparative Examples 1 to 4) show the results of the charge and discharge tests. The charge capacity and discharge capacity of each test are shown as specific capacities normalized by the weight of the negative electrode composite material.

[0061]

Table 1

[0062] As shown in Table 1, Figure 2 and Figure 3, the charge and discharge capacities of the fluoride ion batteries of the examples in which Mg / MgF2 was within the scope of the present disclosure were large.

[0063] Although not intending to be bound by any theory, the reason is presumed as follows. That is, first, as the first-stage reaction during charging, MgF2 reacts to release fluoride ions and Mg is generated. Next, as the second-stage reaction, Mg in the negative electrode composite material containing Mg generated in the first stage forms an alloy with Ca 1-x Ba x in CaBaF2 and releases fluoride ions. During discharging, occlusion of fluoride ions occurs due to this reverse reaction. That is, it is considered that high charge and discharge capacities are obtained by the occurrence of two-stage release (charging) and occlusion (discharging) of fluoride ions.

[0064] Incidentally, in the above mechanism, it is considered that the smaller the Mg / MgF2, the higher the theoretical capacity. However, the actual charge-discharge capacity was maximized with a composition containing a small amount of Mg (Example 5). On the other hand, even when Mg was included, charge-discharge became impossible when Mg / MgF2 was less than the lower limit value of the present disclosure (Comparative Example 1 and Comparative Example 2). This is presumably because the incorporated Mg also functions as an electron conduction aid, and the incorporation of a small amount of Mg contributes to the efficient progress of charge-discharge. This presumption is supported by the fact that in Comparative Example 3 where Mg / MgF2 was made smaller than the range of the present disclosure and the proportion of AB, which is an electron conduction aid, was increased, although the charge-discharge capacity value was smaller than that of the example, charge-discharge became possible.

[0065] <Examples 6 to 10, Comparative Example 5> All-solid-state fluoride ion batteries of Examples 6 to 10 and Comparative Example 5 were produced in the same manner as in Example 4, except that the rotation speed and time of ball mill mixing (BM) in the production of the negative electrode composite material were changed as shown in Table 2.

[0066] <Effect of MgF2 crystallinity> In order to investigate the effect of the crystallinity of MgF2 in the negative electrode composite material on the charge-discharge capacity, charge-discharge tests and XRD measurements were performed on the negative electrode composite materials obtained in Example 4, Examples 6 to 10, and Comparative Example 5. The method of the charge-discharge test is as described above, and the method of XRD measurement is as shown below.

[0067] (XRD measurement) Measurement was performed by the focusing method of irradiating CuKα rays under the conditions of a tube voltage of 45 kV and a tube current of 200 mA using a SmartLab device manufactured by Rigaku Corporation. Under the same conditions, the above standard substance CeO2 was measured. The obtained XRD patterns shown in FIGS. 4 and 5 were subjected to pattern fitting analysis processing using PDXL analysis software manufactured by Rigaku Corporation to calculate the full width at half maximum FWHM. The results are shown in Table 2 and FIG. 6.

[0068]

Table 2

[0069] As shown in Table 2 and Figure 6, the charge-discharge capacity of the fluoride ion battery in the example where the crystallinity of MgF2 in the negative electrode composite material was reduced to make FWHMsam. / FWHMref. greater than 1.5 was large.

[0070] Although not intending to be bound by any theory, this is presumably because reducing the crystallinity of the MgF2 crystal (decreasing the crystallite size) shortens the fluoride ion diffusion distance within the active material particles of MgF2, enabling the charge-discharge reaction to proceed efficiently.

Explanation of Reference Signs

[0071] 1 Fluoride ion battery 10 Positive electrode current collector layer 20 Positive electrode active material layer 30 Electrolyte layer 40 Negative electrode active material layer 50 Negative electrode current collector layer

Claims

1. A fluoride ion battery having a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer, wherein the negative electrode active material layer contains metallic magnesium, magnesium fluoride, and calcium barium fluoride, the mass ratio of the metallic magnesium to the magnesium fluoride is 0.1 to 10.0, and the ratio of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of the magnesium fluoride, measured using CuKα radiation, to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of NIST standard CeO₂ is 1.5 or more, a fluoride ion battery.

2. The fluoride ion battery according to claim 1, wherein the blending ratio of the metallic magnesium in the negative electrode active material layer is 5% by weight or more.

3. The fluoride ion battery according to claim 1, wherein the mass ratio of the metallic magnesium to the magnesium fluoride is 0.1 to 5.

0.

4. The fluoride ion battery according to any one of claims 1 to 3, wherein the electrolyte layer has calcium barium fluoride.

5. The fluoride ion battery according to any one of claims 1 to 3, having a charge-discharge capacity of 100 mAh / g or more.

6. A negative electrode active material for a fluoride ion battery, containing metallic magnesium, magnesium fluoride, and calcium barium fluoride, wherein the mass ratio of the metallic magnesium to the magnesium fluoride is 0.1 to 10.

0. The ratio of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of the magnesium fluoride measured using CuKα radiation to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of NIST standard CeO ２ is 1.5 or more,

7. The ratio of the first FWHM (XRD half-width) of the peak near 2θ = 40.4 deg. in the XRD spectrum of magnesium fluoride, which was measured using CuKα radiation after applying mechanical shock to the magnesium fluoride, to the second FWHM (XRD half-width) of the peak near 2θ = 47.5 deg. in the XRD spectrum of NIST standard CeO ２ is changed to 1.5 or more, the method for producing a negative electrode active material according to claim 6, comprising this.

8. The method according to claim 7, wherein the mechanical shock is applied by a ball mill.

9. The method according to claim 8, wherein the rotation speed of the ball mill is 300 rpm or more.

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