Negative electrode active material and fluoride ion secondary battery

Abstract

A negative electrode active material according to the present disclosure is a negative electrode active material for a fluoride ion secondary battery, and contains scandium fluoride that has a crystallite size of less than 63 nm. A fluoride ion secondary battery according to the present disclosure comprises: a positive electrode 2; a negative electrode 4; and an electrolyte layer 3 that is disposed between the positive electrode 2 and the negative electrode 4. The negative electrode 4 contains the negative electrode active material according to the present disclosure. The negative electrode active material may additionally contain at least one metal that is selected from the group consisting of alkali metals and alkaline earth metals.

Description

Negative electrode active material and fluoride ion secondary battery

[0001] The present disclosure relates to a negative electrode active material and a fluoride ion secondary battery.

[0002] Fluoride ion secondary batteries are considered promising as batteries having higher energy density than lithium ion secondary batteries, and research into them is ongoing.

[0003] Patent Document 1 describes a fluoride ion secondary battery using LaF as the negative electrode active material. Patent Document 2 describes a fluoride ion secondary battery using AlF as the negative electrode active material.

[0004] JP 2017-84506 A International Publication No. 2019 / 187943

[0005] Since Al is a lighter element than La, AlF3 can theoretically achieve a higher weight energy density than LaF3. However, AlF3 has poor electrical conductivity, which makes it difficult for electrochemical reactions to occur.

[0006] An object of the present disclosure is to provide a negative electrode active material suitable for a fluoride ion secondary battery.

[0007] The present disclosure provides a negative electrode active material for a fluoride ion secondary battery, the negative electrode active material comprising scandium fluoride, wherein the scandium fluoride has a crystallite size of less than 63 nm.

[0008] According to the present disclosure, it is possible to provide a negative electrode active material suitable for a fluoride ion secondary battery.

[0009] FIG. 1A is a schematic cross-sectional view of a fluoride ion secondary battery according to a second embodiment of the present disclosure. FIG. 1B is a schematic cross-sectional view of a fluoride ion secondary battery according to a modified example. FIG. 2A is a graph showing X-ray diffraction patterns of negative electrode active materials of Examples 1, 2, 3, and 5, and Comparative Example 2. FIG. 2B is a partially enlarged view of the X-ray diffraction pattern shown in FIG. 2A. FIG. 3A is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 1. FIG. 3B is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 2. FIG. 3C is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 3. FIG. 3D is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 4. FIG. 3E is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 5. FIG. 3F is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 6. Fig. 3G is a graph showing the charge / discharge curves for the first cycle of the fluoride ion secondary battery of Example 7. Fig. 3H is a graph showing the charge / discharge curves for the first cycle of the fluoride ion secondary battery of Example 8. Fig. 3I is a graph showing the charge / discharge curves for the first cycle of the fluoride ion secondary battery of Comparative Example 1. Fig. 3J is a graph showing the charge / discharge curves for the first cycle of the fluoride ion secondary battery of Comparative Example 2.

[0010] (Findings that Form the Basis of the Present Disclosure) The present inventors focused on scandium(III) fluoride (ScF) as a metal fluoride that can achieve a higher weight energy density than LaF. It was known that scandium fluoride could potentially be used as an active material for fluoride ion secondary batteries (Figure 1 of Patent Document 2). However, due to the low ionic conductivity of scandium fluoride, it was thought that fluoride ion secondary batteries using scandium fluoride could only achieve a discharge capacity significantly below the theoretical capacity (789 mAh / g).

[0011] The present inventors have conducted extensive research into fluoride ion secondary batteries that use a negative electrode active material containing scandium fluoride, and as a result have discovered that the crystallite size of scandium fluoride is involved in the initial discharge capacity, leading to the completion of the technology of the present disclosure.

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0013] First Embodiment The negative electrode active material of this embodiment is a negative electrode active material for a fluoride ion secondary battery, and contains scandium fluoride. Scandium fluoride (ScF) is a material that can absorb and release fluoride ions during charge and discharge of the fluoride ion secondary battery.

[0014] Scandium fluoride may be a major component of the negative electrode active material. "Major component" refers to the component that is contained in the largest amount by mass.

[0015] In the negative electrode active material of this embodiment, the crystallite size of scandium fluoride is less than 63 nm. When the crystallite size is within this range, a fluoride ion secondary battery using the negative electrode active material of this embodiment exhibits a large initial discharge capacity. The following reasons are thought to be the reasons why such an effect is obtained. When the crystallite size is too large, the distance from the interface between the crystallites to the depths of the crystallites is long, so fluoride ions cannot be sufficiently conducted inside the crystallites, and the initial discharge capacity is presumed to be reduced. When the crystallite size is small, fluoride ions can be smoothly conducted deep into the crystallites. Furthermore, when the crystallite size is small, the fluoride ion conductivity within the particles of the negative electrode active material is improved, and as a result, a large initial discharge capacity can be achieved.

[0016] The crystallite size of the scandium fluoride may be 60 nm or less. This configuration enhances the effect of improving the initial discharge capacity.

[0017] The lower limit of the crystallite size of scandium fluoride is not particularly limited. In one example, the crystallite size of scandium fluoride is greater than 9 nm. When the lower limit of the crystallite size is such a value, it is possible to avoid an excessive increase in the treatment time required to reduce the crystallite size.

[0018] The negative electrode active material may have a particulate shape, such as a spherical shape, an elliptical shape, a scale shape, or a fiber shape.

[0019] The crystallite size can be adjusted, for example, by milling the scandium fluoride powder. By adjusting the milling time and the number of revolutions of the milling machine, a negative electrode active material having a desired crystallite size can be obtained.

[0020] The crystallite size can be calculated based on the Scherrer equation from the half-width of a specific diffraction peak in the X-ray diffraction pattern of the negative electrode active material measured using Cu-Kα radiation. The specific diffraction peak can be the strongest diffraction peak among the diffraction peaks of the ScF3 crystal. In this embodiment, the specific diffraction peak is a diffraction peak attributable to the (100) plane of the cubic ScF3 crystal, and appears at a diffraction angle 2θ of approximately 22°.

[0021] The negative electrode active material of this embodiment may further contain at least one selected from the group consisting of alkali metals and alkaline earth metals. By containing an alkali metal and / or alkaline earth metal, the initial discharge efficiency of a fluoride ion secondary battery using scandium fluoride as the negative electrode active material is improved. Although the reason for this is not necessarily clear, it is presumed that one reason is that the alkali metal and / or alkaline earth metal improves the ionic conductivity of the negative electrode active material.

[0022] The alkaline earth metal may exist in the negative electrode active material in the form of a crystalline fluoride or an amorphous fluoride. When the negative electrode active material contains a crystalline fluoride, the X-ray diffraction pattern of the negative electrode active material exhibits a peak attributed to the alkaline earth metal fluoride. When the negative electrode active material does not contain a crystalline fluoride, the X-ray diffraction pattern of the negative electrode active material does not exhibit a peak attributed to the alkaline earth metal fluoride. In either case, the effect of improving initial discharge efficiency can be obtained.

[0023] As with alkaline earth metals, alkali metals exist in negative electrode active materials in the form of crystalline fluorides and amorphous fluorides. When crystalline fluorides are contained in the negative electrode active material, the X-ray diffraction pattern of the negative electrode active material exhibits peaks attributable to alkali metal fluorides. When crystalline fluorides are not contained in the negative electrode active material, the X-ray diffraction pattern of the negative electrode active material does not exhibit peaks attributable to alkali metal fluorides. In either case, it is presumed that the effect of improving initial discharge efficiency can be obtained.

[0024] The ratio of the amount of alkali metal atoms to the amount of scandium atoms is adjusted, expressed as a percentage, for example, to 1% to 50%, preferably 5% to 25%. The ratio of the amount of alkaline earth metal atoms to the amount of scandium atoms is adjusted, expressed as a percentage, for example, to 1% to 50%, preferably 5% to 25%. The ratio of the total amount of alkali metal atoms and alkaline earth metal atoms to the amount of scandium atoms is adjusted, expressed as a percentage, for example, to 1% to 50%, preferably 5% to 25%. By appropriately adjusting these ratios, the effect of improving the utilization rate of scandium fluoride can be sufficiently obtained, and the content ratio of scandium fluoride in the negative electrode material can also be sufficiently ensured. As a result, the capacity of the negative electrode can be improved.

[0025] The alkaline earth metal may include at least one selected from the group consisting of Ba, Ca, and Sr. Ba, Ca, and Sr are considered to be elements that improve the ionic conductivity of the negative electrode active material, and are therefore suitable as elements to be included in the negative electrode active material of the present disclosure.

[0026] The alkali metal may include at least one selected from the group consisting of K and Rb. K and Rb are considered to be elements that improve the ionic conductivity of the negative electrode active material, and are therefore suitable as elements to be contained in the negative electrode active material of the present disclosure.

[0027] The median diameter (D50) of the particles of the negative electrode active material is preferably 5.0 μm or less, more preferably 2.0 μm or less. The smaller the upper limit of the median diameter, the more the fluorination reaction of the metal active material during discharge progresses, and the decrease in initial discharge capacity is suppressed. The lower limit of the median diameter is, for example, 0.1 μm. The median diameter of the particles of the negative electrode active material may be 0.1 μm or more and 5.0 μm or less, or 0.1 μm or more and 2.0 μm or less. The median diameter is determined from a volume-based particle size distribution obtained by a laser diffraction scattering method.

[0028] The negative electrode active material containing scandium fluoride and an alkaline earth metal can be produced by the following method.

[0029] In one method, a mixed powder of scandium fluoride powder and alkaline earth metal fluoride powder is milled. The alkaline earth metal fluoride is, for example, at least one selected from the group consisting of barium fluoride (BaF), calcium fluoride (CaF), and strontium fluoride (SrF). In the mixed powder, the content of scandium fluoride is greater than the content of the alkaline earth metal fluoride, based on the amount of substance. The ratio of the amount of substance of the alkaline earth metal fluoride to the amount of substance of scandium fluoride is adjusted, expressed as a percentage, to a range of, for example, 1% to 50%, preferably 5% to 25%. The milling process can be carried out using a grinding device such as a ball mill. The milling process causes a solid-phase reaction between scandium fluoride and alkaline earth metal fluoride, resulting in the negative electrode active material of this embodiment. By adjusting the conditions of the milling treatment, such as the treatment time and the rotation speed of the milling device, a negative electrode active material having a desired crystallite size can be obtained.

[0030] In another method, a mixed powder of scandium fluoride powder and alkaline earth metal fluoride powder is heat-treated. The mixed powder may be pelletized in advance. The ambient temperature during the heat treatment is, for example, 550°C to 1100°C. The heat treatment time is, for example, 0.5 to 10 hours. The heat treatment may be performed in an inert atmosphere such as an argon atmosphere or a nitrogen atmosphere. The heat treatment causes a solid-phase reaction between scandium fluoride and alkaline earth metal fluoride. After the heat treatment, the product is pulverized to obtain a negative electrode active material having a desired crystallite size. The product can be pulverized using a pulverizer such as a ball mill. The crystallite size of scandium fluoride in the negative electrode active material varies depending on the pulverization conditions, such as the rotation speed of the pulverizer, the diameter of the pulverization media, and the material of the pulverization media. By adjusting the pulverization conditions, a negative electrode active material having a desired crystallite size can be obtained.

[0031] When an alkali metal fluoride is used as a raw material instead of an alkaline earth metal fluoride, a negative electrode active material containing scandium fluoride and an alkali metal can be obtained. When an alkali metal fluoride is used as a raw material together with an alkaline earth metal fluoride, a negative electrode active material containing scandium fluoride, an alkali metal, and an alkaline earth metal can be obtained. The alkali metal fluoride is, for example, at least one selected from the group consisting of potassium fluoride (KF) and rubidium fluoride (RbF).

[0032] Second Embodiment Fig. 1A is a cross-sectional view of a fluoride ion secondary battery 10 according to a second embodiment of the present disclosure. The fluoride ion secondary battery 10 includes a positive electrode 2, a negative electrode 4, and an electrolyte layer 3. The positive electrode 2 has a positive electrode current collector 5 and a positive electrode active material layer 6. The positive electrode active material layer 6 is disposed on the positive electrode current collector 5. The negative electrode 4 has a negative electrode current collector 7 and a negative electrode active material layer 8. The negative electrode active material layer 8 is disposed on the negative electrode current collector 7. An electrolyte layer 3 is disposed between the positive electrode 2 and the negative electrode 4. The positive electrode 2 and the negative electrode 4 face each other with the electrolyte layer 3 interposed therebetween.

[0033] When the fluoride ion secondary battery 10 is charged, fluoride ions (F -) migrates from the negative electrode 4 to the positive electrode 2. A defluorination reaction occurs at the negative electrode 4, and a fluorination reaction occurs at the positive electrode 2. When the fluoride ion secondary battery 10 is discharged, fluoride ions migrate from the positive electrode 2 to the negative electrode 4. A fluorination reaction occurs at the negative electrode 4, and a defluorination reaction occurs at the positive electrode 2.

[0034] The fluorination reaction and defluorination reaction include both a form involving a chemical reaction between the active material and fluoride ions and a form not involving a chemical reaction, such as intercalation. The chemical reaction includes a reaction that forms a compound and a reaction that forms a complex that is not a compound. Examples of complexes that are not compounds include alloys and solid solutions.

[0035] The positive electrode current collector 5 is made of a metal material such as copper, chromium, nickel, titanium, platinum, gold, aluminum, tungsten, iron, molybdenum, zirconium, or an alloy containing at least one of these metals. In one example, the positive electrode current collector 5 is an aluminum foil coated with a carbon material.

[0036] The positive electrode active material layer 6 includes a positive electrode active material. The positive electrode active material is a material capable of absorbing and releasing fluoride ions during charging and discharging of the fluoride ion secondary battery. The positive electrode active material is a material having a more noble potential than scandium. The positive electrode active material includes, for example, at least one element selected from the group consisting of Co, Cu, Bi, Sn, Pb, Fe, and C. The positive electrode active material may be a simple element selected from these groups, an alloy containing at least one element selected from these groups, a solid solution containing at least one element selected from these groups, or a compound containing at least one element selected from these groups. The compound is, for example, a fluoride.

[0037] The positive electrode active material may have a particle shape. Examples of the particle shape include spherical, elliptical, scaly, fibrous, etc. The particles of the positive electrode active material have a median diameter (D50) of, for example, 0.2 μm to 5.0 μm.

[0038] The positive electrode active material layer 6 may contain other materials such as a solid electrolyte, a conductive additive, and a binder. Examples of the solid electrolyte having fluoride ion conductivity include Ba. x Ca y F z (0.3<x<0.7, 0.3<y<0.7, 1.5<z<2.5) and Ce x Sr y F z At least one selected from the group consisting of (0.9<x<1, 0<y<0.1, 2<z<3) can be used. Examples of the conductive additive include carbon materials such as acetylene black and carbon nanotubes. Polymer materials known in the field of batteries can be used as the binder.

[0039] The negative electrode current collector 7 is made of a metal material such as copper, chromium, nickel, titanium, platinum, gold, aluminum, tungsten, iron, molybdenum, zirconium, or an alloy containing at least one of these metals. In one example, the negative electrode current collector 7 is an aluminum foil coated with a carbon material.

[0040] The negative electrode active material layer 8 contains the negative electrode active material described in the first embodiment. With this configuration, the fluoride ion secondary battery 10 exhibits a large initial discharge capacity.

[0041] The negative electrode active material layer 8 may contain other materials such as a solid electrolyte, a conductive additive, a binder, etc. As the other materials, the same materials as those used in the positive electrode active material layer 6 can be used.

[0042] In this embodiment, the ratio M2 / M1 of the mass M2 of scandium fluoride to the mass M1 of the negative electrode material is, for example, 40 mass% to 90 mass%, and preferably 44 mass% to 63 mass%, expressed as a percentage. By appropriately adjusting the ratio M2 / M1, the discharge capacity per unit mass of the negative electrode material can be increased.

[0043] The electrolyte layer 3 includes a solid electrolyte having fluoride ion conductivity. The solid electrolyte having fluoride ion conductivity is, for example, Ba. x Ca y F z(0.3<x<0.7, 0.3<y<0.7, 1.5<z<2.5) and Ce x Sr y F z At least one selected from the group consisting of (0.9<x<1, 0<y<0.1, 2<z<3) can be used.

[0044] 1A, the fluoride ion secondary battery 10 is a solid-state battery. However, the fluoride ion secondary battery 10 may be a battery that uses an electrolytic solution.

[0045] When the fluoride ion secondary battery 10 is a battery using an electrolytic solution, the electrolyte layer 3 can be formed of a resin porous membrane such as a porous membrane made of polyolefin. The positive electrode active material layer 6, the negative electrode active material layer 8, and the electrolyte layer 3 are impregnated with the electrolytic solution.

[0046] The electrolyte solution includes a solvent and an electrolyte salt.

[0047] The solvent is, for example, a non-aqueous solvent, such as a cyclic ether, a chain ether, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester, or a chain carboxylic acid ester.

[0048] Examples of cyclic ethers include 4-methyl-1,3-dioxolane, 2-methyltetrahydrofuran, and crown ether. Examples of chain ethers include 1,2-dimethoxyethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, and 4,5-difluoroethylene carbonate. Examples of chain carbonates include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone. Examples of chain carboxylic acid esters include ethyl acetate, propyl acetate, and butyl acetate. One or a mixture of two or more selected from these can be used as the non-aqueous solvent.

[0049] Examples of the electrolyte salt include inorganic fluoride salts, organic fluoride salts, and ionic liquids.

[0050] Examples of inorganic fluoride salts include lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, cesium fluoride, ammonium fluoride, etc. Examples of organic fluoride salts include tetramethylammonium fluoride, neopentyltrimethylammonium fluoride, trineopentylmethylammonium fluoride, tetraneopentylammonium fluoride, 1,3,3,6,6-hexamethylpiperidinium fluoride, 1-methyl-1-propylpiperidinium fluoride, tetramethylphosphonium fluoride, tetraphenylphosphonium fluoride, trimethylsulfonium fluoride, etc.

[0051] Examples of cations of ionic liquids include 1-ethyl-3-methylimidazolium cation, 1-ethylpyridinium cation, 1-methoxyethyl-1-methylpyrrolidinium cation, N-methyl-N-propylpiperidinium cation, trimethylbutylammonium cation, N,N-diethyl-N-methylmethoxyethylammonium cation, tetrabutylphosphonium cation, triethyl-(2-methoxyethyl)phosphonium cation, triethylsulfonium cation, and diethyl-(2-methoxyethyl)sulfonium cation.

[0052] Examples of the anion of the ionic liquid include a bis(fluorosulfonyl)amide anion, a bis(trifluoromethanesulfonyl)amide anion, a hexafluorophosphate anion, a tris(pentafluoroethyl)trifluorophosphate anion, a trifluoromethanesulfonate anion, and a tetrafluoroborate anion.

[0053] One or a mixture of two or more selected from the above materials can be used as the electrolyte salt.

[0054] 1B is a schematic cross-sectional view of a fluoride ion secondary battery according to a modified example. The fluoride ion secondary battery 11 further includes a positive electrode buffer layer 9 in addition to the components of the fluoride ion secondary battery 10 described with reference to FIG. 1A. The positive electrode buffer layer 9 is disposed between the electrolyte layer 3 and the positive electrode active material layer 6.

[0055] The positive electrode buffer layer 9 is a layer containing a positive electrode buffer material. The positive electrode buffer material is, for example, a compound containing the same metal element as the metal contained in the positive electrode active material. With this configuration, the cycle characteristics of the fluoride ion secondary battery 11 can be improved. For example, when the positive electrode active material is lead, the positive electrode buffer material is Pb 1-a Sn a F2, where a may satisfy, for example, 0≦a≦1 or 0<a<1.

[0056] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0057] (Technology 1) A negative electrode active material for a fluoride ion secondary battery, comprising scandium fluoride, the scandium fluoride having a crystallite size of less than 63 nm.

[0058] According to the present disclosure, it is possible to provide a negative electrode active material suitable for a fluoride ion secondary battery.

[0059] (Technology 2) The negative electrode active material according to Technology 1, wherein the crystallite size is greater than 9 nm. When the lower limit of the crystallite size is such a value, it is possible to avoid an excessive increase in the treatment time for reducing the crystallite size.

[0060] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the crystallite size is 60 nm or less. With this configuration, the effect of improving the initial discharge capacity is enhanced.

[0061] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, further comprising at least one selected from the group consisting of alkali metals and alkaline earth metals. By including an alkali metal and / or alkaline earth metal, the initial discharge efficiency of a fluoride ion secondary battery using scandium fluoride as the negative electrode active material is improved.

[0062] (Technology 5) The negative electrode active material according to Technology 4, wherein the alkaline earth metal includes at least one selected from the group consisting of Ba, Ca, and Sr. Ba, Ca, and Sr are considered to be elements that improve the ionic conductivity of the negative electrode active material, and are therefore suitable as elements to be contained in the negative electrode active material of the present disclosure.

[0063] (Technology 6) The negative electrode active material according to Technology 4 or 5, wherein an X-ray diffraction pattern of the negative electrode active material exhibits a peak attributed to the alkaline earth metal fluoride. With this configuration, the initial discharge efficiency of a fluoride ion secondary battery using scandium fluoride as the negative electrode active material is improved.

[0064] (Technology 7) The negative electrode active material according to Technology 4 or 5, wherein an X-ray diffraction pattern of the negative electrode active material does not show a peak attributed to the alkaline earth metal fluoride. With this configuration, the initial discharge efficiency of a fluoride ion secondary battery using scandium fluoride as the negative electrode active material is improved.

[0065] (Technology 8) The negative electrode active material according to any one of Technologies 4 to 7, wherein the alkali metal includes at least one selected from the group consisting of K and Rb. K and Rb are considered to be elements that improve the ionic conductivity of the negative electrode active material, and are therefore suitable as elements to be contained in the negative electrode active material of the present disclosure.

[0066] (Technology 9) A fluoride ion secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte layer disposed between the positive electrode and the negative electrode, wherein the negative electrode contains the negative electrode active material according to any one of Technologies 1 to 8.

[0067] In the following examples and comparative examples, a planetary ball mill "PL-7, manufactured by Fritsch GmbH" was used. The container and balls of the planetary ball mill were made of silicon nitride. A heat treatment device "KSL-1100X, manufactured by MTI GmbH" was used. The sample container was made of alumina.

[0068] Example 1: ScF powder (manufactured by Apollo Scientific) was milled using a planetary ball mill at 800 rpm. The milling process consisted of 40 cycles of 15 minutes of operation and 5 minutes of rest. This produced a negative electrode active material for Example 1.

[0069] Example 2 ScF powder and BaF powder (manufactured by Kojundo Chemical Research Institute) were mixed in a mass ratio of ScF:BaF = 75:25 to obtain a mixed powder. The mixed powder was milled using a planetary ball mill at 600 rpm for 1 hour. The obtained powder was placed in a mold and pressed to produce pellets. The pellets were placed in a heat treatment device in an argon atmosphere and heat treated at 1000°C for 1 hour. After the heat treatment, the pellets were placed in a mortar and roughly crushed to obtain a pulverized material. The pulverized material was milled using a planetary ball mill at 800 rpm. During the milling process after the heat treatment, a 15-minute run followed by a 5-minute break was repeated 40 times. This produced the negative electrode active material of Example 2.

[0070] Example 3: ScF powder and BaF powder were mixed in a mass ratio of ScF:BaF = 75:25 to obtain a mixed powder. The mixed powder was milled using a planetary ball mill at 400 rpm. The milling process consisted of 40 cycles of 15 minutes of operation and 5 minutes of rest. This produced the negative electrode active material of Example 3.

[0071] Example 4 ScF powder and BaF powder were mixed in a mass ratio of ScF:BaF = 95:5 to obtain a mixed powder. The mixed powder was milled using a planetary ball mill at 600 rpm for 1 hour. The obtained powder was placed in a mold and pressed to produce pellets. The pellets were placed in a heat treatment device in an argon atmosphere and heat treated at 1000°C for 1 hour. After the heat treatment, the pellets were placed in a mortar and roughly crushed to obtain a pulverized material. The pulverized material was milled using a planetary ball mill at 500 rpm. In the milling process after the heat treatment, a 15-minute operation followed by a 5-minute break was repeated 40 times. This produced the negative electrode active material of Example 4.

[0072] Example 5: ScF powder and CaF powder (manufactured by Kojundo Chemical Research Institute) were mixed in a mass ratio of ScF:CaF = 75:25 to obtain a mixed powder. The mixed powder was milled using a planetary ball mill at 800 rpm. The milling process consisted of 40 cycles of 15 minutes of operation and 5 minutes of rest. This produced the negative electrode active material of Example 5.

[0073] Example 6 A negative electrode active material of Example 6 was prepared in the same manner as in Example 2, except that SrF2 powder was used instead of BaF2 powder.

[0074] Example 7 A negative electrode active material of Example 7 was prepared in the same manner as in Example 2, except that KF powder was used instead of BaF2 powder.

[0075] Example 8 A negative electrode active material of Example 8 was prepared in the same manner as in Example 2, except that RbF powder was used instead of BaF2 powder.

[0076] Comparative Example 1 A negative electrode active material of Comparative Example 1 was produced in the same manner as in Example 1, except that the rotation speed of the planetary ball mill was changed to 400 rpm.

[0077] Comparative Example 2 A negative electrode active material of Comparative Example 2 was produced in the same manner as in Example 3, except that the rotation speed of the planetary ball mill was changed to 300 rpm.

[0078] [Powder X-ray Diffraction Measurement] The negative electrode active materials of the Examples and Comparative Examples were subjected to powder X-ray diffractometry under the following conditions.

[0079] Rigaku Corporation, SmartLab Radiation source: CuKα ray (45 kV, 200 mA) Detector: HyPix-3000 Scan step: 0.02 deg Scan speed: 4 deg / min 2θ: 20-80 deg

[0080] FIG. 2A is a graph showing the X-ray diffraction patterns of the negative electrode active materials of Examples 1, 2, 3, 5, and Comparative Example 2. As shown in FIG. 2A, the X-ray diffraction patterns of the Examples and Comparative Example all had a peak near 22 degrees. This peak is attributed to reflection from the crystal plane of ScF. The X-ray diffraction patterns of Example 3 and Comparative Example 2 had a peak near 25 degrees. This peak is attributed to reflection from the crystal plane of BaF. The X-ray diffraction pattern of Example 5 had a peak near 28 degrees. This peak is attributed to reflection from the crystal plane of CaF. The X-ray diffraction pattern of Example 2 showed only a peak attributed to reflection from the crystal plane of ScF, and no peak of BaF. The reason BaF was not detected in the raw materials is presumed to be due to the heat treatment. In Example 2, Ba is presumed to be contained in an amorphous phase. From this, it is presumed that the negative electrode active material preferably contains an amorphous alkaline earth metal fluoride and / or an amorphous alkali metal fluoride.

[0081] Figure 2B is a partially enlarged view of the X-ray diffraction pattern shown in Figure 2A. The peak appearing at a diffraction angle 2θ of approximately 22 degrees is a diffraction peak attributed to the (100) plane of cubic ScF3. The crystallite size was calculated from the half-width of this diffraction peak. The crystallite size was calculated using the software "PDXL2" manufactured by Rigaku Corporation. A Scherrer constant of 0.94 was used. The results are shown in Table 1.

[0082] [Preparation of Solid Electrolyte] CaF powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and BaF powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were mixed in a molar ratio of CaF:BaF = 1:1 to prepare a mixture. The mixture was milled using a planetary ball mill. As a result, a solid electrolyte having the composition formula Ca 0.5 Ba 0.5 A solid electrolyte represented by F (hereinafter referred to as BCF) was obtained. The ball mill rotation speed during the milling treatment was 800 rpm. The milling treatment was performed by repeating 60 minutes of operation and 5 minutes of rest 10 times.

[0083] [Preparation of Negative Electrode Material] A mixture was prepared by mixing BCF and carbon nanotubes (CNT) as a conductive additive in a mass ratio of BCF:CNT = 4:1. Single-wall carbon nanotubes (manufactured by Zeon Corporation) were used as the carbon nanotubes. The mixture was milled using a planetary ball mill. The ball mill rotation speed during the milling process was 800 rpm. The milling process consisted of 20 cycles of 30 minutes of operation and 5 minutes of rest. The negative electrode active material of Example 1 was added to the obtained material and dispersed. This resulted in the negative electrode material of Example 1. The ScF3 content in the negative electrode material was 50%.

[0084] The negative electrode materials of Examples 2 to 8 and Comparative Examples 1 and 2 were prepared using the same method. In Examples 2 to 8 and Comparative Example 2, the ScF content in the negative electrode materials was the value shown in Table 1. The ScF content was calculated using the mass of the ScF powder, which was the raw material powder. In other words, the ScF content shown in Table 1 was obtained by dividing the mass of the ScF powder, which was the raw material powder, by the mass of the negative electrode material.

[0085] [Preparation of Positive Electrode Buffer Material] PbF powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and SnF powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were mixed in a mass ratio of PbF:SnF = 15:7 to prepare a mixture. The mixture was milled using a planetary ball mill. As a result, a positive electrode buffer material having the composition formula Pb 0.58 Sn 0.42A compound represented by formula F (hereinafter referred to as PSF) was obtained. The ball mill rotation speed during the milling treatment was 600 rpm. During the milling treatment, a cycle of 15 minutes of operation and 5 minutes of rest was repeated 40 times.

[0086] A mixture was prepared by mixing PSF and acetylene black (manufactured by Denka Co., Ltd.) as a conductive additive at a mass ratio of PSF:acetylene black = 14:1. The mixture was milled using a planetary ball mill. This resulted in a positive electrode buffer material containing PSF and acetylene black. The ball mill rotation speed during the milling process was 600 rpm. The milling process consisted of 12 cycles of 15 minutes of operation followed by a 5-minute break.

[0087] [Fabrication of Fluoride Ion Secondary Battery] 4 mg of the negative electrode material of Example 1, 50 mg of BCF, 50 mg of the positive electrode buffer material, lead foil (manufactured by Nilaco Corporation, thickness 200 μm) as the positive electrode active material, and aluminum foil (manufactured by Nilaco Corporation, thickness 10 μm) as the positive electrode current collector were placed in this order in a cylindrical mold with a diameter of 10 mm and pressed. Platinum foil (manufactured by Nilaco Corporation, thickness 20 μm) as the negative electrode current collector was placed on top of the layer of negative electrode material. This resulted in the production of the fluoride ion secondary battery of Example 1. Using the negative electrode materials of Examples 2 to 8 and Comparative Examples 1 and 2, the fluoride ion secondary batteries of Examples 2 to 8 and Comparative Examples 1 and 2 were produced by the same method as Example 1.

[0088] [Measurement of Initial Discharge Capacity of Fluoride Ion Secondary Battery] The initial discharge capacity of the fluoride ion secondary battery of the example and comparative example was measured using a potentiogalvanostat (SP300, manufactured by Biologic). The fluoride ion secondary battery was placed in a vacuum thermostatic chamber at 150°C and subjected to constant current charging at an end voltage of -2.7 V and a current value of 40 μA. Subsequently, constant current discharging was performed at an end voltage of -0.05 V and a current value of 40 μA. The results are shown in Table 1 and Figures 3A to 3J. The charge curves of Example 1, Example 8, Comparative Example 1, and Comparative Example 2 showed a step or voltage increase that was thought to be due to a side reaction. In Example 1, Example 8, Comparative Example 1, and Comparative Example 2, the end point of charging could not be determined, so charging was manually stopped.

[0089]

[0090] 3A to 3J are graphs showing the charge / discharge curves for the first cycle of the fluoride ion secondary batteries of Examples 1 to 8 and Comparative Examples 1 and 2, respectively. The horizontal axis of the graph represents the capacity per unit mass of the negative electrode material, that is, the amount of electricity charged or discharged. The vertical axis of the graph represents the potential of the negative electrode based on the oxidation-reduction equilibrium potential between lead and lead fluoride. For example, since the content ratio of scandium fluoride in the negative electrode material of Example 1 is 50 mass%, doubling the initial discharge capacity in Table 1 gives the initial discharge capacity per unit mass of scandium fluoride.

[0091] As can be seen from the results of Example 1 and Comparative Example 1, adjusting the crystallite size of scandium fluoride to less than 63 nm significantly improved the initial discharge capacity of the battery, i.e., the proportion of scandium fluoride that contributed to the charge-discharge reaction significantly increased.

[0092] As can be seen from the results of Examples 2 to 6, the initial discharge capacity of the battery was further improved by including an alkaline earth metal in the negative electrode active material.

[0093] According to the X-ray diffraction patterns shown in FIG. 2A, the negative electrode active materials of Example 3 and Comparative Example 2 are thought to contain crystalline BaF2. However, as can be seen from the charge / discharge curves of FIGS. 3C and 3J, BaF2 does not contribute to the discharge capacity of the battery. The redox equilibrium potential of ScF3 is −2.17 V based on the redox equilibrium potential between lead and lead fluoride. The redox equilibrium potential of BaF2 is lower than this potential, at approximately −3.0 V. The initial discharge capacities of the batteries of Example 3 and Comparative Example 2 were higher than that of ScF3. 3- This is due to the fluorination reaction from ScF to ScF.

[0094] As can be seen from the results of Examples 5 and 6, even when the alkaline earth metal contained in the negative electrode active material was Ca or Sr, the effect of improving the initial discharge capacity was obtained in the same way as when Ba was used.

[0095] As can be seen from the results of Examples 2 and 4, the initial discharge capacity of the battery was significantly improved by appropriately adjusting the crystallite size of scandium fluoride and by including an alkaline earth metal in the negative electrode active material. As can be seen from the X-ray diffraction pattern of the negative electrode active material of Example 2 ( FIG. 2A ), no BaF crystalline phase remained in the negative electrode active material of Example 2.

[0096] The initial discharge capacities of the batteries of Examples 2, 4, and 6 tended to be higher than the initial discharge capacities of the batteries of Examples 1, 3, and 5. From this, it is presumed that the heat treatment and milling treatment are effective in improving the utilization rate of ScF.

[0097] As can be seen from the results of Examples 7 and 8, even when the negative electrode active material contained an alkali metal, the effect of improving the initial discharge capacity was obtained in the same way as when the negative electrode active material contained an alkaline earth metal.

[0098] The initial discharge capacities of the batteries of Examples 1 to 6, which used negative electrode active materials containing alkaline earth metals, tended to be higher than the initial discharge capacities of the batteries of Examples 7 and 8, which used negative electrode active materials containing alkali metals.

[0099] The technology of the present disclosure is useful for fluoride ion secondary batteries.

Claims

1. A negative electrode active material for a fluoride ion secondary battery, comprising scandium fluoride, wherein the scandium fluoride has a crystallite size of less than 63 nm.

2. The negative electrode active material according to claim 1, wherein the crystallite size is greater than 9 nm.

3. The negative electrode active material according to claim 1, wherein the crystallite size is 60 nm or less.

4. The negative electrode active material according to claim 1, further comprising at least one selected from the group consisting of alkali metals and alkaline earth metals.

5. The negative electrode active material according to claim 4, wherein the alkaline earth metal comprises at least one selected from the group consisting of Ba, Ca, and Sr.

6. The negative electrode active material according to claim 4, wherein an X-ray diffraction pattern of the negative electrode active material exhibits peaks attributable to the alkaline earth metal fluoride.

7. The negative electrode active material according to claim 4, wherein the X-ray diffraction pattern of the negative electrode active material does not show any peaks attributed to the alkaline earth metal fluoride.

8. The negative electrode active material according to claim 4, wherein the alkali metal comprises at least one selected from the group consisting of K and Rb.

9. A fluoride ion secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte layer disposed between the positive electrode and the negative electrode, wherein the negative electrode contains the negative electrode active material according to claim 1.

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