Negative electrode active material, negative electrode material, and fluoride ion secondary battery

Abstract

This disclosure provides a negative electrode active material which is for a fluoride ion secondary battery, which contains yttrium fluoride, and for which the crystallite size is 7-27 nm. A fluoride ion secondary battery 10 of this disclosure comprises: a positive electrode 2; a negative electrode 4; and an electrolyte layer 3 disposed between the positive electrode 2 and the negative electrode 4. The negative electrode 4 includes the negative electrode active material of this disclosure.

Description

Anode active material, anode material, and fluoride ion secondary battery

[0001] The present disclosure relates to a negative electrode active material, a negative electrode 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 yttrium fluoride, the negative electrode active material having a crystallite size of more than 7 nm and less than 27 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 third 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. 2 is a graph showing an X-ray diffraction pattern of a negative electrode active material of Example 1. 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 Comparative Example 1. FIG. 3E is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Comparative Example 2. FIG. 3F is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Comparative Example 3. FIG. 4 is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 4.

[0010] (Findings that Form the Basis of the Present Disclosure) The present inventors focused on yttrium (III) fluoride (YF) as a metal fluoride that can achieve a higher weight energy density than LaF and has better electrical conductivity than AlF. However, when the present inventors actually fabricated fluoride ion secondary batteries using yttrium fluoride, they found that in some cases the initial discharge capacity was significantly lower than expected.

[0011] The present inventors have conducted extensive research into fluoride ion secondary batteries using a negative electrode active material containing yttrium fluoride, and as a result, have discovered that the crystallite size 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 yttrium fluoride. Yttrium fluoride is a material that can absorb and release fluoride ions as the fluoride ion secondary battery is charged and discharged. Yttrium fluoride is the main component of the negative electrode active material. The term "main component" refers to the component that is contained in the largest amount by mass.

[0014] The crystallite size of the negative electrode active material is greater than 7 nm and less than 27 nm. When the crystallite size is adjusted to this range, a fluoride ion secondary battery using the negative electrode active material of this embodiment exhibits a large initial discharge capacity. Although the reason for this effect is not necessarily clear, it is presumed that if the crystallite size is too large, fluoride ions cannot reach the depths of the crystallite, resulting in a decrease in initial discharge capacity. On the other hand, if the crystallite size is too small, it is presumed that the ionic conductivity decreases due to an increase in grain boundaries and a decrease in bulk yttrium fluoride, resulting in a decrease in initial discharge capacity.

[0015] The crystallite size of the negative electrode active material may be 25 nm or less, preferably 20 nm or less. The crystallite size of the negative electrode active material may be 10 nm or more and 25 nm or less, or 10 nm or more and 20 nm or less. This configuration enhances the effect of improving the initial discharge capacity.

[0016] 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.

[0017] The crystallite size can be adjusted, for example, by milling the powder of the negative electrode active material. By adjusting the milling time and the rotation speed of the milling device, a negative electrode active material having a desired crystallite size can be obtained. Alternatively, the crystallite size can be adjusted by heat-treating the powder of the negative electrode active material. By adjusting the temperature and time of the heat treatment, a negative electrode active material having a desired crystallite size can be obtained.

[0018] 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. In this embodiment, the specific diffraction peak is a diffraction peak attributable to the (111) plane of the YF crystal.

[0019] The negative electrode active material of this embodiment may have an orthorhombic crystal structure. The crystal structure of YF is an orthorhombic crystal. The crystal space group of YF is Pnma.

[0020] The median diameter (D50) of the particles of the negative electrode active material is preferably 7.0 μm or less, more preferably 5.0 μm or less, and even more preferably 2.0 μm or less. The smaller the upper limit of the median diameter, the more the fluorination reaction of the metal that is the negative electrode active material progresses during discharge, and the more the decrease in initial discharge capacity is suppressed. The lower limit of the median diameter is, for example, 0.1 μm. The median diameter is determined from the volume-based particle size distribution obtained by laser diffraction scattering.

[0021] In the negative electrode active material of this embodiment, it is desirable that the content of metal elements other than yttrium is sufficiently reduced. By limiting the content of metal elements other than yttrium, it is expected that the resistance of the fluoride ion secondary battery will be reduced and the initial discharge capacity will be increased. In particular, it is desirable that the content of Na is sufficiently low. Although the theoretical capacity of NaF, a compound of Na and F, is large at 638 mAh / g, the reversibility of NaF is not necessarily good. Therefore, it is desirable that the content of Na in the negative electrode active material of this embodiment is sufficiently low. In one example, the content of Na is desirably less than 80 ppm by mass, and more desirably less than 50 ppm by mass. When the content of Na is reduced, the effect of Na on a fluoride ion secondary battery using the negative electrode active material of this embodiment is considered to be small. In this specification, "ppm" represents the content by mass.

[0022] Another metal element whose content should be limited is Bi. The Bi content in the negative electrode active material is preferably less than 10 ppm by mass. When the Bi content is less than 10 ppm, the initial discharge capacity of a fluoride ion secondary battery using the negative electrode active material of this embodiment is expected to increase.

[0023] The lower limits of the Na and Bi contents are not particularly limited, and the Na and Bi contents may be below the detection limit of an analytical instrument.

[0024] The content of metal elements in the negative electrode active material can be determined by an analytical method such as ICP (inductively coupled plasma) emission spectrometry, ICP mass spectrometry, or atomic absorption spectrometry.

[0025] Yttrium fluoride can be obtained by reacting yttrium (III) oxide or yttrium (III) hydroxide with hydrogen fluoride. Yttrium can be separated and purified from the yttrium-containing mineral so that the Na content in the negative electrode active material is less than 80 ppm. The separation and purification of yttrium can be performed, for example, by ion exchange. Similarly, yttrium can be separated and purified from the yttrium-containing mineral so that the Bi content in the negative electrode active material is less than 10 ppm.

[0026] Examples of other metal elements whose contents should be limited include Al, Mg, Fe, and Zr. The Al content in the negative electrode active material may be less than 250 ppm by mass. The Mg content in the negative electrode active material may be less than 100 ppm by mass. The Fe content in the negative electrode active material may be less than 70 ppm by mass. The Zr content in the negative electrode active material may be less than 50 ppm by mass. With this configuration, a fluoride ion secondary battery using the negative electrode active material of this embodiment is expected to exhibit a large initial discharge capacity.

[0027] The lower limits of the contents of Al, Mg, Fe, and Zr are not particularly limited, and the contents of Al, Mg, Fe, and Zr may be below the detection limits of analytical instruments.

[0028] The total content of metal elements other than yttrium is preferably less than 500 ppm, more preferably 300 ppm or less, and even more preferably 50 ppm or less. With this configuration, it is expected that a fluoride ion secondary battery using the negative electrode active material of this embodiment will exhibit a large initial discharge capacity.

[0029] Second Embodiment The negative electrode material of this embodiment contains the negative electrode active material described in the first embodiment and a fibrous carbon material. A mixture of the negative electrode active material and the fibrous carbon material is used as the negative electrode material.

[0030] According to the findings of the present inventors, when YF is used as a negative electrode active material, a side reaction occurs during charging of a fluoride ion secondary battery, and the side reaction may hinder charging of the fluoride ion secondary battery. According to this embodiment, the fibrous carbon material suppresses the side reaction during charging, thereby increasing the charge capacity of the fluoride ion secondary battery.

[0031] Examples of fibrous carbon materials include solid carbon fibers, carbon nanotubes, etc. Among these, carbon nanotubes are considered to be more effective in suppressing side reactions, and are therefore suitable for the negative electrode material of this embodiment.

[0032] The carbon nanotubes may be single-walled carbon nanotubes, multi-walled carbon nanotubes, or a mixture thereof. Single-walled carbon nanotubes are carbon nanotubes formed from a single layer of graphene. Multi-walled carbon nanotubes are carbon nanotubes having a structure in which multiple tubes formed from graphene are stacked. Single-walled carbon nanotubes are suitable for the negative electrode material of this embodiment because of their high effect of suppressing side reactions.

[0033] It is possible to distinguish between single-walled and multi-walled carbon nanotubes by observing the carbon nanotubes with a transmission electron microscope.

[0034] The average diameter of the single-walled carbon nanotubes is preferably 1 nm or more and 7 nm or less, more preferably 3 nm or more and 5 nm or less. The average diameter of the multi-walled carbon nanotubes is 300 nm or more and 500 nm or less. The average length of the fibrous carbon material is, for example, 100 μm or more and 600 μm or less. The average diameter and average length are the average values ​​of the diameters and lengths of a plurality of (for example, 10) fibrous carbon material fibers. The diameter and length of the fibrous carbon material may be values ​​measured in an electron microscope image of the fibrous carbon material.

[0035] In this embodiment, the ratio M2 / M1 of the mass M2 of the negative electrode active material to the mass M1 of the negative electrode material is, expressed as a percentage, preferably 40% by mass or more and 80% by mass or less, more preferably 40% by mass or more and 70% by mass or less, and even more preferably 50% by mass or more and 60% by mass or less. The ratio M3 / M1 of the mass M3 of the fibrous carbon material to the mass M1 of the negative electrode material is, expressed as a percentage, preferably 0.5% by mass or more and 15% by mass or less, more preferably 6% by mass or more and 12% by mass or less, and even more preferably 8% by mass or more and 10% by mass or less. By appropriately adjusting the ratios M2 / M1 and M3 / M1, the effect of suppressing side reactions is enhanced.

[0036] When the fibrous carbon material was analyzed by Raman spectroscopy, the intensity of the D band I D G-band intensity I G Ratio I G / I D The D band may be at 1360 cm in the Raman spectrum of the fibrous carbon material. -1 The G band is a peak that appears around 1580 cm in the Raman spectrum of the fibrous carbon material and is associated with disorders and defects in the graphite structure. -1 The peak that appears around the graphite peak is derived from graphite. G / I D is an indicator of the amount of defects in the fibrous carbon material. G / I D A larger ratio I indicates that the fibrous carbon material has fewer defects. G / I DA small ratio indicates that the fibrous carbon material has many defects. A fibrous carbon material with few defects is thought to exhibit high chemical stability. The higher the chemical stability of the fibrous carbon material, the greater the effect of suppressing side reactions is expected to be. G / I D The upper limit is not particularly limited, and is, for example, 100.

[0037] The negative electrode material of this embodiment may contain an electrolyte having fluoride ion conductivity. The electrolyte is typically a solid electrolyte. 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. x Ca y F z and Ce x Sr y F z has high fluoride ion conductivity and is therefore suitable as the negative electrode material of this embodiment.

[0038] In this embodiment, the ratio M4 / M1 of the mass M4 of the electrolyte to the mass M1 of the negative electrode material is, for example, 24 mass % or more and 48 mass % or less, expressed as a percentage.

[0039] The negative electrode material of this embodiment may contain other materials such as a conductive additive and a binder other than the fibrous carbon material. Examples of the conductive additive other than the fibrous carbon material include spherical conductive additives such as acetylene black. Polymer materials known in the field of batteries can be used as the binder.

[0040] 1A is a cross-sectional view of a fluoride ion secondary battery 10 according to a third 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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 yttrium. 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.

[0045] 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.

[0046] The positive electrode active material layer 6 may contain other materials such as a solid electrolyte, a conductive additive, and a binder. The solid electrolyte may be the material described in the second embodiment. The conductive additive may be a carbon material such as acetylene black or carbon nanotubes. The binder may be a polymer material known in the field of batteries.

[0047] 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.

[0048] 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.

[0049] The negative electrode active material layer 8 may contain the negative electrode material described in the second embodiment. With such a configuration, the fluoride ion secondary battery 10 exhibits a large initial discharge capacity.

[0050] The electrolyte layer 3 contains a material having fluoride ion conductivity. The electrolyte layer 3 may be made of, for example, the solid electrolyte described in the second embodiment.

[0051] 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.

[0052] 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.

[0053] The electrolyte solution includes a solvent and an electrolyte salt.

[0054] 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.

[0055] 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.

[0056] Examples of the electrolyte salt include inorganic fluoride salts, organic fluoride salts, and ionic liquids.

[0057] 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.

[0058] Examples of the cation of the ionic liquid include a 1-ethyl-3-methylimidazolium cation, a 1-ethylpyridinium cation, a 1-methoxyethyl-1-methylpyrrolidinium cation, an N-methyl-N-propylpiperidinium cation, a trimethylbutylammonium cation, an N,N-diethyl-N-methylmethoxyethylammonium cation, a tetrabutylphosphonium cation, a triethyl-(2-methoxyethyl)phosphonium cation, a triethylsulfonium cation, and a diethyl-(2-methoxyethyl)sulfonium cation.

[0059] 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.

[0060] One or a mixture of two or more selected from the above materials can be used as the electrolyte salt.

[0061] 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.

[0062] 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.

[0063] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0064] (Technology 1) A negative electrode active material for a fluoride ion secondary battery, comprising: yttrium fluoride; and a crystallite size of the negative electrode active material being greater than 7 nm and less than 27 nm.

[0065] According to the present disclosure, it is possible to provide a negative electrode active material suitable for a fluoride ion secondary battery.

[0066] (Technology 2) The negative electrode active material according to Technology 1, wherein the crystallite size is 10 nm or more and 25 nm or less. With this configuration, it is expected that a fluoride ion secondary battery using the negative electrode active material of this embodiment will exhibit a large initial discharge capacity.

[0067] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the content of Na in the negative electrode active material is less than 80 ppm by mass. With this configuration, it is expected that a fluoride ion secondary battery using the negative electrode active material of this embodiment will exhibit a large initial discharge capacity.

[0068] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, wherein the total content of metal elements other than yttrium in the negative electrode active material is less than 500 ppm. With this configuration, a fluoride ion secondary battery using the negative electrode active material of the present disclosure is expected to exhibit a large initial discharge capacity.

[0069] (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein the median diameter of the particles of the negative electrode active material is 0.1 μm or more and 5.0 μm or less. As the upper limit of the median diameter becomes smaller, a fluorination reaction of the metal of the negative electrode active material progresses during discharge, and a decrease in initial discharge capacity is suppressed.

[0070] (Technology 6) A negative electrode material comprising: the negative electrode active material according to any one of Technologies 1 to 5; and a fibrous carbon material.

[0071] According to the present disclosure, the carbon material fiber suppresses side reactions during charging, which can increase the charge capacity of the fluoride ion secondary battery.

[0072] (Technology 7) The negative electrode material according to Technology 6, wherein the fibrous carbon material includes carbon nanotubes. Carbon nanotubes are considered to be effective in suppressing side reactions, and therefore are suitable for the negative electrode material of the present disclosure.

[0073] (Technology 8) The negative electrode material according to Technology 7, wherein the carbon nanotubes include single-walled carbon nanotubes. Single-walled carbon nanotubes are suitable for the negative electrode material of the present disclosure because they are highly effective in suppressing side reactions.

[0074] (Technology 9) The negative electrode material according to any one of Technologies 6 to 8, wherein a ratio M3 / M1 of a mass M3 of the fibrous carbon material to a mass M1 of the negative electrode material is, expressed as a percentage, 6 mass% or more and 12 mass% or less. By appropriately adjusting the ratio M3 / M1, the effect of suppressing side reactions is enhanced.

[0075] (Technology 10) The negative electrode material according to any one of Technologies 6 to 9, wherein a ratio M2 / M1 of a mass M2 of the negative electrode active material to a mass M1 of the negative electrode material is, expressed as a percentage, 40 mass% or more and 80 mass% or less. By appropriately adjusting the ratio M2 / M1, the effect of suppressing side reactions is enhanced.

[0076] (Technology 11) The negative electrode material according to any one of Technologies 6 to 10, further comprising at least one selected from the group consisting of an electrolyte represented by the following composition formula (1) and an electrolyte represented by the following composition formula (2): In composition formula (1), 0.3<x<0.7, 0.3<y<0.7, and 1.5<z<2.5 are satisfied, and in composition formula (2), 0.9<x<1, 0<y<0.1, and 2<z<3 are satisfied. Ba x Ca y F z and Ce x Sr y F z has high fluoride ion conductivity and is therefore suitable as the negative electrode material of the present disclosure. x Ca y F z ... (1) Ce x Sr y F z ...(2)

[0077] (Technology 12) 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 5.

[0078] With this configuration, the fluoride ion secondary battery exhibits a large initial discharge capacity.

[0079] Five types of YF powders A (manufactured by Sigma-Aldrich), B (manufactured by Sigma-Aldrich), C (manufactured by Kanto Chemical), D (manufactured by Sigma-Aldrich), E (manufactured by ANGENE), and F (manufactured by ANGENE) were prepared for use in the examples and comparative examples. Quantitative analysis of the elements contained in each of the YF powders A to E was performed. An atomic absorption spectrometer (AA240FS, manufactured by VARIAN) was used to analyze Na. An ICP mass spectrometer (7700x, manufactured by Agilent Technologies) was used to analyze Bi. An ICP optical emission spectrometer (ICPS-8100, manufactured by Shimadzu Corporation) was used to analyze Al, Mg, and Fe. As a result, the purity of YF in each of the YF powders A to E was 99.99% by mass or higher. The Na content, Bi content, Al content, Mg content, and Fe content in YF3 powders A to D were all below the detection limit of the respective analytical instruments. The Na content in YF3 powder E was 80 ppm, the Bi content was below the detection limit of the analytical instrument, the Al content was 250 ppm, the Mg content was 100 ppm, and the Fe content was 70 ppm. In YF3 powder E, the total content of Na, Bi, Al, Mg, and Fe was 500 ppm.

[0080] 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.

[0081] Example 1: YF powder (manufactured by Sigma-Aldrich) was milled using a planetary ball mill. This resulted in a negative electrode active material (YF powder A) of Example 1. The ball mill rotation speed during the milling process was 800 rpm. The milling process consisted of 40 cycles of 15 minutes of operation and 5 minutes of rest. The median particle diameter (D50) of the negative electrode active material of Example 1 was 5.0 μm.

[0082] (Example 2) Using a planetary ball mill, YF powder (manufactured by Sigma-Aldrich) was milled under the same conditions as in Example 1. Then, the milled product was heat-treated in an argon atmosphere at 300°C for 1 hour. This yielded a negative electrode active material (YF powder B) for Example 2.

[0083] (Example 3) Using a planetary ball mill, YF powder (manufactured by ANGENE) was milled under the same conditions as in Example 1, and then heat-treated under the same conditions as in Example 2. This yielded a negative electrode active material of Example 3 (YF powder E).

[0084] Comparative Example 1 Using a planetary ball mill, YF3 powder (manufactured by Kanto Chemical Co., Inc.) was milled under the same conditions as in Example 1. This yielded a negative electrode active material of Comparative Example 1 (YF3 powder C).

[0085] (Comparative Example 2) Using a planetary ball mill, YF powder (manufactured by Sigma-Aldrich) was milled under the same conditions as in Example 1. Then, the milled product was heat-treated in an argon atmosphere at 400°C for 1 hour. This yielded a negative electrode active material (YF powder D) of Comparative Example 2.

[0086] Comparative Example 3 Using a planetary ball mill, YF powder (manufactured by ANGENE) was milled under the same conditions as in Example 1, and then heat-treated under the same conditions as in Comparative Example 2. This resulted in a negative electrode active material (YF powder F) of Comparative Example 3.

[0087] [Powder X-ray Diffraction Measurement] Powder X-ray diffraction measurements were carried out on the negative electrode active materials of the Examples and Comparative Examples. The measurement conditions were as follows: Figure 2 shows the X-ray diffraction pattern of the negative electrode active material of Example 1.

[0088] 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

[0089] The crystallite size was calculated from the half-width of the diffraction peak assigned to the (111) plane of YF. The crystallite size was calculated using the software "PDXL2" manufactured by Rigaku Corporation. The diffraction peak assigned to the (111) plane of YF appears at a diffraction angle 2θ of approximately 27.8 degrees. The results are shown in Table 1.

[0090]

[0091] [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.

[0092] [Preparation of Negative Electrode Material] BCF and carbon nanotubes (CNTs) serving as a conductive additive were mixed at a mass ratio of BCF:CNT = 4:1 to prepare a mixture. The mixture was milled using a planetary ball mill. The ball mill rotation speed during milling 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 resulting material and dispersed. This yielded the negative electrode material of Example 1. The mass ratio of the negative electrode active material in the negative electrode material was 55%. The negative electrode materials of Examples 2, 3, Comparative Examples 1, 2, and 3 were obtained using the same method. Single-wall carbon nanotubes (manufactured by Zeon Corporation) were used as the carbon nanotubes.

[0093] [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.

[0094] 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.

[0095] [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, 3, Comparative Examples 1, 2, and 3, the fluoride ion secondary batteries of Examples 2, 3, Comparative Examples 1, 2, and 3 were produced by the same method as Example 1.

[0096] [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 Inc.). 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. Thereafter, constant current discharging was performed at an end voltage of -0.05 V and a current value of 40 μA.

[0097] 3A to 3F are graphs showing charge / discharge curves for the first cycle of the fluoride ion secondary batteries of Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2, and Comparative Example 3, respectively. The horizontal axis of the graph represents the capacity per unit mass of the negative electrode active 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.

[0098] As shown in Figures 3A and 3B, the initial discharge capacities of the fluoride ion secondary batteries of Example 1 and Example 2 were large. The initial discharge capacity of the fluoride ion secondary battery of Example 1 was 467 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Example 2 was 456 mAh / g. The charge / discharge curve was smooth, and side reactions were suppressed. The theoretical capacity of the fluoride ion secondary battery of Example 1 was 551 mAh / g.

[0099] For the fluoride ion secondary battery of Example 1, a cross section of the negative electrode active material after discharge was analyzed by SEM-EDX (scanning electron microscope-energy dispersive X-ray spectroscopy). In active materials with large particle diameters, the fluorination reaction of the metal, which is the state during charging, did not progress sufficiently inside the particles compared to the surface side. Therefore, it is understood that the smaller the upper limit of the median diameter, the more the fluorination reaction of the metal, which is the negative electrode active material, progresses during discharge, and the decrease in initial discharge capacity is suppressed.

[0100] As shown in Figures 3D and 3E, the initial discharge capacities of the fluoride ion secondary batteries of Comparative Example 1 and Comparative Example 2 were small. The initial discharge capacity of the fluoride ion secondary battery of Comparative Example 1 was 194 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Comparative Example 2 was 237 mAh / g. A step (near 250 mAh / g) that is thought to be due to a side reaction was observed in the charge curve of Comparative Example 1. A step (near 300 mAh / g) that is thought to be due to a side reaction was observed in the charge curve of Comparative Example 2. In Comparative Examples 1 and 2, the end of charging could not be determined, so charging was stopped manually.

[0101] As shown in FIGS. 3C and 3F , the initial discharge capacity of the fluoride ion secondary battery of Example 3 was larger than that of the fluoride ion secondary battery of Comparative Example 3. The initial discharge capacity of the fluoride ion secondary battery of Example 3 was 285 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Comparative Example 3 was 270 mAh / g. A step (near 400 mAh / g) that is thought to be due to a side reaction was observed in the charge curve of Example 3. A step (near 350 mAh / g) that is thought to be due to a side reaction was observed in the charge curve of Comparative Example 3. In Example 3 and Comparative Example 3, the end point of charging could not be determined, so charging was stopped manually.

[0102] Comparing Example 2 (FIG. 3B) and Example 3 (FIG. 3C), which have the same crystallite size, the initial discharge capacity of the fluoride ion secondary battery of Example 2 using YF3 powder B was greater than the initial discharge capacity of the fluoride ion secondary battery of Example 3 using YF3 powder E. The Na content of YF3 powder B was less than 80 ppm by mass, and the Na content of YF3 powder E was 80 ppm by mass. Therefore, it is desirable that the Na content in the YF3 powder is sufficiently low.

[0103] Example 4 A fluoride ion secondary battery of Example 4 was produced in the same manner as in Example 2, except that acetylene black (manufactured by Denka Co., Ltd.) was used as a conductive additive instead of carbon nanotubes when producing the negative electrode material.

[0104] The initial discharge capacity of the fluoride ion secondary battery of Example 4 was measured by the method described above. FIG. 4 is a graph showing the charge / discharge curve of the first cycle of the fluoride ion secondary battery of Example 4. The initial discharge capacity of the fluoride ion secondary battery of Example 4 was 312 mAh / g. A step (near 400 mAh / g) that is thought to be due to a side reaction was observed in the charge curve of Example 4. In Example 4, the end point of charging could not be determined, so charging was stopped manually.

[0105] As shown in FIG. 4, the initial discharge capacity of the fluoride ion secondary battery of Example 4 was larger than the initial discharge capacities of the fluoride ion secondary batteries of Comparative Examples 1 to 3.

[0106] Comparing Example 2 ( FIG. 3B ) with Example 4 ( FIG. 4 ), the initial discharge capacity of Example 2, which used carbon nanotubes as the conductive additive, was greater than the initial discharge capacity of Example 4, which used acetylene black as the conductive additive. Therefore, it is considered that a fluoride ion secondary battery using carbon nanotubes as the conductive additive in the negative electrode material is more likely to exhibit the effect of suppressing side reactions than a fluoride ion secondary battery using acetylene black as the conductive additive in the negative electrode material.

[0107] 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 yttrium fluoride, and a crystallite size in the negative electrode active material is greater than 7 nm and less than 27 nm.

2. The negative electrode active material according to claim 1, wherein the crystallite size is 10 nm or more and 25 nm or less.

3. The negative electrode active material according to claim 1, wherein the Na content in the negative electrode active material is less than 80 ppm by mass.

4. The negative electrode active material according to claim 1, wherein the total content of metal elements other than yttrium in the negative electrode active material is less than 500 ppm.

5. The negative electrode active material according to claim 1, wherein the particles of the negative electrode active material have a median diameter of 7.0 μm or less.

6. A negative electrode material comprising the negative electrode active material according to claim 1 and a fibrous carbon material.

7. The negative electrode material according to claim 6, wherein the fibrous carbon material comprises carbon nanotubes.

8. The negative electrode material of claim 7, wherein the carbon nanotubes comprise single-walled carbon nanotubes.

9. The negative electrode material according to claim 6, wherein a ratio M3 / M1 of a mass M3 of the fibrous carbon material to a mass M1 of the negative electrode material is, expressed as a percentage, 6 mass % or more and 12 mass % or less.

10. The negative electrode material according to claim 9, wherein a ratio M2 / M1 of a mass M2 of the negative electrode active material to a mass M1 of the negative electrode material is, expressed as a percentage, 40 mass % or more and 80 mass % or less.

11. The negative electrode material according to any one of claims 6 to 10, further comprising at least one selected from the group consisting of an electrolyte represented by the following composition formula (1) and an electrolyte represented by the following composition formula (2). x Ca y F z ... (1) Ce x Sr y F z ...(2) In composition formula (1), 0.3<x<0.7, 0.3<y<0.7, and 1.5<z<2.5 are satisfied, and in composition formula (2), 0.9<x<1, 0<y<0.1, and 2<z<3 are satisfied.

12. 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 claims 1 to 5.

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