Fluoride ion secondary battery

Abstract

A fluoride ion secondary battery 10 according to the present disclosure comprises: a positive electrode 2; a negative electrode 4; and an electrolyte 3 disposed between the positive electrode 2 and the negative electrode 4. The negative electrode 4 contains a negative electrode active material and a fibrous carbon material. The potential of the negative electrode 4 when charging has ended is less than -2.5 V, with a standard hydrogen electrode serving as a reference. The fibrous carbon material may include carbon nanotubes.

Description

Fluoride-ion secondary battery

[0001] The present disclosure relates to fluoride ion secondary batteries.

[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] One of the problems with conventional fluoride ion secondary batteries is that they do not provide a sufficient initial discharge capacity.

[0006] The present disclosure provides a fluoride ion secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the negative electrode contains a negative electrode active material and a fibrous carbon material, and the potential of the negative electrode at the end of charging is lower than −2.5 V with respect to a standard hydrogen electrode.

[0007] According to the technology of the present disclosure, the initial discharge capacity of a fluoride ion secondary battery can be increased.

[0008] FIG. 1A is a schematic cross-sectional view of a fluoride ion secondary battery according to a first 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 a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 1. FIG. 2B is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 2. FIG. 2C is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Comparative Example 1. FIG. 3A is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 3. FIG. 3B is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 4. FIG. 3C is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Comparative Example 2. FIG. 4A is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 5. 4B is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Example 6. FIG. 4C is a graph showing a charge / discharge curve for the first cycle of the fluoride ion secondary battery of Comparative Example 3.

[0009] (Findings that Form the Basis of the Present Disclosure) A large potential difference between the positive electrode and the negative electrode is advantageous for achieving a high energy density. The present inventors fabricated fluoride ion secondary batteries using various negative electrode active materials and investigated their charge / discharge characteristics. As a result, they found that when the potential of the negative electrode at the end of charge is sufficiently base, the initial discharge capacity is sometimes significantly lower than the value predicted from the theoretical capacity. After extensive investigation, they found that the initial discharge capacity also depends on the type of conductive additive. When the potential of the negative electrode at the end of charge is a noble potential, the initial discharge capacity does not tend to depend on the type of conductive additive. Based on these findings, the present inventors have completed the technology of the present disclosure.

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0011] 1A is a cross-sectional view of a fluoride ion secondary battery 10 according to a first 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] The positive electrode active material layer 6 includes a positive electrode active material. A material capable of absorbing and releasing fluoride ions during charging and discharging of the fluoride ion secondary battery 10 is used as the positive electrode active material. The positive electrode active material includes, for example, at least one element selected from the group consisting of Co, Cu, Bi, Sn, Pb, Fe, Zn, Ga, and C. The positive electrode active material may be a simple element selected from this group, an alloy containing at least one element selected from this group, a solid solution containing at least one element selected from this group, or a compound containing at least one element selected from this group. The compound is, for example, a fluoride.

[0016] The positive electrode active material may have a particle shape. Examples of the particle shape include spherical, elliptical, scaly, and fibrous shapes. The particles of the positive electrode active material have a median diameter (D50) of, for example, 0.2 μm to 5.0 μm. The median diameter is determined from a volume-based particle size distribution obtained by a laser diffraction scattering method.

[0017] The positive electrode active material layer 6 may contain other materials such as a solid electrolyte, a conductive additive, and a binder.

[0018] The solid electrolyte may be an electrolyte having fluoride ion conductivity. 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), Ce x Sr y F z (0.9<x<1, 0<y<0.1, 2<z<3) x Ca y F z and Ce x Sr y F z has high fluoride ion conductivity and is therefore suitable for the fluoride ion secondary battery 10 of this embodiment.

[0019] Examples of the conductive additive include carbon materials such as acetylene black and carbon nanotubes.

[0020] As the binder, polymeric materials known in the field of batteries can be used.

[0021] 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.

[0022] The negative electrode active material layer 8 includes a negative electrode active material and a fibrous carbon material. The negative electrode active material is a material capable of absorbing and releasing fluoride ions during charging and discharging of the fluoride ion secondary battery 10. The potential of the negative electrode 4 at the end of charging is lower than −2.5 V relative to the standard hydrogen electrode. This configuration allows for a large potential difference between the positive electrode 2 and the negative electrode 4, thereby providing a fluoride ion secondary battery 10 with a high energy density. The fibrous carbon material suppresses side reactions that may occur at the negative electrode 4 during charging of the fluoride ion secondary battery 10. By suppressing side reactions, the initial discharge capacity of the fluoride ion secondary battery 10 can be increased. While the reason why the fibrous carbon material can suppress side reactions is not entirely clear, it is presumed that the chemical stability of the fibrous carbon material contributes to the suppression of side reactions. The “end of charging” is determined by a control circuit that controls the charging and discharging of the fluoride ion secondary battery 10 depending on the type of negative electrode active material and the type of positive electrode active material. The present disclosure can provide an electricity storage system including a fluoride ion secondary battery 10 and a control circuit.

[0023] The potential of the negative electrode 4 at the end of charging may be a potential less noble than −3.082 V with respect to the standard hydrogen electrode. It is believed that the more noble the potential of the negative electrode 4 at the end of charging, the more likely side reactions will occur. In such cases, the technology of the present disclosure is particularly effective.

[0024] The negative electrode active material may be a material whose oxidation-reduction equilibrium potential between the fluorination reaction and the defluorination reaction is less noble than −2.5 V or less noble than −3.082 V with respect to the standard hydrogen electrode.

[0025] The negative electrode active material may contain at least one element selected from the group consisting of lanthanum, cerium, magnesium, sodium, and yttrium. Specifically, the negative electrode active material may be a fluoride containing at least one element selected from the group consisting of these metal elements. With this configuration, a fluoride ion secondary battery 10 having a high energy density can be provided.

[0026] The negative electrode active material may have a particle shape. 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 of 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.

[0027] Examples of the fibrous carbon material 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 fluoride ion secondary battery 10 of this embodiment.

[0028] 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 fluoride ion secondary battery 10 of this embodiment because they are highly effective in suppressing side reactions.

[0029] It is possible to distinguish between single-walled and multi-walled carbon nanotubes by observing the carbon nanotubes with a transmission electron microscope.

[0030] 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.

[0031] 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 active material layer 8 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 active material layer 8 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.

[0032] 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.

[0033] The negative electrode active material layer 8 may contain other materials such as a solid electrolyte and a binder. As the solid electrolyte, the materials described above as materials usable for the positive electrode active material layer 6 may be used. As the binder, a polymer material known in the field of batteries may be used. The negative electrode active material layer 8 may contain a conductive additive other than the fibrous carbon material.

[0034] The negative electrode active material layer 8 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 for the fluoride ion secondary battery 10 of this embodiment.

[0035] In this embodiment, the ratio M4 / M1 of the mass M4 of the electrolyte to the mass M1 of the negative electrode active material layer 8 is, for example, 24 mass % or more and 48 mass % or less, expressed as a percentage.

[0036] 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 above as a material usable for the positive electrode active material layer 6.

[0037] 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.

[0038] 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.

[0039] The electrolyte solution includes a solvent and an electrolyte salt.

[0040] 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.

[0041] 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.

[0042] Examples of the electrolyte salt include inorganic fluoride salts, organic fluoride salts, and ionic liquids.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] One or a mixture of two or more selected from the above materials can be used as the electrolyte salt.

[0047] 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.

[0048] 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.

[0049] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0050] (Technology 1) A fluoride ion secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the negative electrode contains a negative electrode active material and a fibrous carbon material, and the potential of the negative electrode at the end of charging is lower than −2.5 V with respect to a standard hydrogen electrode.

[0051] According to the technology of the present disclosure, the initial discharge capacity of a fluoride ion secondary battery can be increased.

[0052] (Technology 2) The fluoride ion secondary battery according to Technology 1, 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 fluoride ion secondary battery of the present disclosure.

[0053] (Technology 3) The fluoride ion secondary battery according to Technology 2, wherein the carbon nanotubes include single-walled carbon nanotubes. Single-walled carbon nanotubes are suitable for the fluoride ion secondary battery of the present disclosure because they are highly effective in suppressing side reactions.

[0054] (Technology 4) The fluoride ion secondary battery according to any one of Technologies 1 to 3, wherein the potential of the negative electrode at the end of charging is more base-noble than −3.082 V with respect to a standard hydrogen electrode. It is considered that the more base the potential of the negative electrode at the end of charging is, the more likely a side reaction is to occur. In such cases, the technology of the present disclosure is particularly effective.

[0055] (Technology 5) When the fibrous carbon material is analyzed by Raman spectroscopy, the intensity of the D band I D G-band intensity I G The fluoride ion secondary battery according to any one of claims 1 to 4, wherein the ratio of I is 1.1 or more. G / I D A larger ratio I indicates that the fibrous carbon material has fewer defects. G / I D A small value indicates that the carbon fiber has many defects. A carbon fiber with few defects is thought to exhibit high chemical stability.

[0056] (Technology 6) The fluoride ion secondary battery according to any one of Technologies 1 to 5, wherein the negative electrode active material contains at least one selected from the group consisting of lanthanum, cerium, magnesium, sodium, and yttrium. With this configuration, a fluoride ion secondary battery having a high energy density can be provided.

[0057] (Technology 7) The fluoride ion secondary battery according to any one of Technologies 1 to 6, wherein the negative electrode has a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer contains the negative electrode active material and the fibrous carbon material, and a ratio M3 / M1 of a mass M3 of the fibrous carbon material to a mass M1 of the negative electrode active material layer 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.

[0058] (Technology 8) The fluoride ion secondary battery according to any one of Technologies 1 to 7, wherein the negative electrode has a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer contains the negative electrode active material and the fibrous carbon material, and a ratio M2 / M1 of a mass M2 of the negative electrode active material to a mass M1 of the negative electrode active material layer 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.

[0059] (Technology 9) The fluoride ion secondary battery according to any one of Technologies 1 to 8, wherein the electrolyte contains at least one selected from the group consisting of a material represented by the following composition formula (1) and a material 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 for the fluoride ion secondary battery of the present disclosure. x Ca y F z ... (1) Ce x Sr y F z ...(2)

[0060] 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.

[0061] Example 1: YF powder (manufactured by Sigma-Aldrich) was milled using a planetary ball mill. This resulted in the production of a negative electrode active material of Example 1. The ball mill rotation speed during the milling process was 800 rpm. During the milling process, a cycle of 15 minutes of operation and 5 minutes of rest was repeated 40 times.

[0062] [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.

[0063] [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 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 yielded the negative electrode material of Example 1. The mass ratio of the negative electrode active material in the negative electrode material of Example 1 was 55%. Single-wall carbon nanotubes (manufactured by Zeon Corporation) were used as the carbon nanotubes.

[0064] Example 2 A negative electrode material of Example 2 was obtained in the same manner as in Example 1, except that multi-walled carbon nanotubes (manufactured by Tokyo Chemical Industry Co., Ltd.) were used instead of single-walled carbon nanotubes.

[0065] Comparative Example 1 A negative electrode material for Comparative Example 1 was obtained in the same manner as in Example 1, except that acetylene black (manufactured by Denka Co., Ltd.) was used instead of the single-walled carbon nanotubes.

[0066] Example 3 A mixture was prepared by mixing NaF powder (manufactured by Sigma-Aldrich) and CeF powder (manufactured by Sigma-Aldrich) in a molar ratio of 1:1. The mixture was milled using a planetary ball mill. This resulted in the production of the negative electrode active material of Example 3. The ball mill rotation speed during the milling process was 300 rpm. The milling process consisted of 40 cycles of 15 minutes of operation followed by a 5-minute break.

[0067] [Preparation of Negative Electrode Material] The negative electrode material of Example 3 was obtained by using the negative electrode active material of Example 3 instead of the negative electrode active material of Example 1. The mass ratio of the negative electrode active material in the negative electrode material of Example 3 was 60%.

[0068] Example 4 A negative electrode material of Example 4 was obtained in the same manner as in Example 3, except that multi-walled carbon nanotubes (manufactured by Tokyo Chemical Industry Co., Ltd.) were used instead of single-walled carbon nanotubes.

[0069] Comparative Example 2 A negative electrode material for Comparative Example 2 was obtained in the same manner as in Example 3, except that acetylene black (manufactured by Denka Co., Ltd.) was used instead of the single-walled carbon nanotubes.

[0070] Example 5: LaF powder (manufactured by Sigma-Aldrich) was milled using a planetary ball mill. This resulted in the production of a negative electrode active material of Example 5. The ball mill was rotated at 300 rpm during the milling process. The milling process consisted of 40 cycles of 15 minutes of operation and 5 minutes of rest.

[0071] [Preparation of Negative Electrode Material] The negative electrode material of Example 5 was obtained by using the negative electrode active material of Example 5 instead of the negative electrode active material of Example 1. The mass ratio of the negative electrode active material in the negative electrode material of Example 5 was 65%.

[0072] Example 6 A negative electrode material of Example 6 was obtained in the same manner as in Example 5, except that multi-walled carbon nanotubes (manufactured by Tokyo Chemical Industry Co., Ltd.) were used instead of single-walled carbon nanotubes.

[0073] Comparative Example 3 A negative electrode material for Comparative Example 3 was obtained in the same manner as in Example 5, except that acetylene black (manufactured by Denka Co., Ltd.) was used instead of the single-walled carbon nanotubes.

[0074] [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 0.42 A 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.

[0075] 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.

[0076] [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 6 and Comparative Examples 1 to 3, the fluoride ion secondary batteries of Examples 2 to 6 and Comparative Examples 1 to 3 were produced by the same method as Example 1.

[0077] [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.

[0078] 2A to 2C are graphs showing first-cycle charge / discharge curves of the fluoride ion secondary batteries of Example 1, Example 2, and Comparative Example 1, respectively. FIGS. 3A to 3C are graphs showing first-cycle charge / discharge curves of the fluoride ion secondary batteries of Example 3, Example 4, and Comparative Example 2, respectively. FIGS. 4A to 4C are graphs showing first-cycle charge / discharge curves of the fluoride ion secondary batteries of Example 5, Example 6, 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 negative electrode potential based on the oxidation-reduction equilibrium potential between lead and lead fluoride. The relationship between the oxidation-reduction equilibrium potential between lead and lead fluoride and the standard hydrogen electrode potential is 0 V vs. Pb / PbF2 = -0.44 V vs. SHE.

[0079] 2A , 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 1 was sufficiently large. The theoretical capacity of the fluoride ion secondary battery of Example 1 is 551 mAh / g.

[0080] As shown in FIG. 2B , the initial discharge capacity of the fluoride ion secondary battery of Example 2 was 392 mAh / g. In Example 2, a potential step was observed near 480 mAh / g on the charge curve. This step indicates that a side reaction occurred at the negative electrode during charging of the fluoride ion secondary battery. The initial discharge capacity of the fluoride ion secondary battery of Example 2 was large, but was lower than the initial discharge capacity of Example 1.

[0081] 2C , the initial discharge capacity of the fluoride ion secondary battery of Comparative Example 1 was 312 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Comparative Example 1 was smaller than those of Examples 1 and 2. In Comparative Example 1, a rise in potential was observed near 400 mAh / g on the charge curve, so charging was manually stopped.

[0082] 3A, the initial discharge capacity of the fluoride ion secondary battery of Example 3 was 412 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Example 3 was sufficiently large. The theoretical capacity of the fluoride ion secondary battery of Example 3 was the sum of the theoretical capacity of NaF and the theoretical capacity of CeF, i.e., 112 + 337 = 449 mAh / g.

[0083] 3B, the initial discharge capacity of the fluoride ion secondary battery of Example 4 was 389 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Example 4 was large, but was lower than the initial discharge capacity of Example 3.

[0084] 3C , the initial discharge capacity of the fluoride ion secondary battery of Comparative Example 2 was 292 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Comparative Example 2 was smaller than those of Examples 3 and 4. In Comparative Example 2, a rise in potential was observed near 380 mAh / g on the charge curve, so charging was manually stopped.

[0085] 4A , the initial discharge capacity of the fluoride ion secondary battery of Example 5 was 311 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Example 5 was sufficiently large. The theoretical capacity of the fluoride ion secondary battery of Example 5 is 410 mAh / g.

[0086] 4B , the initial discharge capacity of the fluoride ion secondary battery of Example 6 was 296 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Example 6 was large, but was lower than the initial discharge capacity of Example 4.

[0087] 4C , the initial discharge capacity of the fluoride ion secondary battery of Comparative Example 3 was 275 mAh / g. The initial discharge capacity of the fluoride ion secondary battery of Comparative Example 3 was smaller than those of Examples 5 and 6. In Comparative Example 3, a rise in potential was observed near 350 mAh / g on the charge curve, so charging was manually stopped.

[0088] Table 1 shows the ratio of the initial discharge capacity to the theoretical capacity for Examples 1 to 6 and Comparative Examples 1 to 3. In all of Examples 1 to 6, the ratio of the initial discharge capacity to the theoretical capacity was improved compared to Comparative Examples 1 to 3, which used the same type of negative electrode active material.

[0089]

[0090] (Reference Examples 1 and 2) Using a potentiogalvanostat (SP300, manufactured by Biologic Corporation), the initial discharge capacities of the fluoride ion secondary batteries of Reference Examples 1 and 2 were measured. The fluoride ion secondary battery of Example 5 was used as the fluoride ion secondary battery of Reference Example 1. The fluoride ion secondary battery of Comparative Example 3 was used as the fluoride ion secondary battery of Reference Example 2. The fluoride ion secondary batteries of Reference Examples 1 and 2 were placed in a vacuum thermostatic chamber at 150°C and subjected to constant current charging at an end voltage of -2.108 V (=-2.49 V vs. SHE) 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. The results are shown in Table 2.

[0091]

[0092] As shown in Table 2, no difference was observed in the initial discharge capacity when charging was performed at a cut-off voltage of −2.108 V. In other words, when the potential of the negative electrode was a noble potential, no side reaction occurred regardless of the type of conductive additive.

[0093] [Raman Spectroscopic Analysis] Raman spectroscopic analysis was carried out on the negative electrode material of Example 1 and the negative electrode material of Comparative Example 1. From the obtained spectra, the intensity I of the D band was D G-band intensity I G Ratio I G / I D The results are shown in Table 3.

[0094]

[0095] Ratio I in the negative electrode material of Example 1 G / I D The ratio I in the negative electrode material of Comparative Example 1 was 1.1 or more. G / I Dwas smaller than 1.1. In other words, the amount of disorder and defects in the graphite structure in the negative electrode material of Example 1 was smaller than the amount of disorder and defects in the graphite structure in the negative electrode material of Comparative Example 1.

[0096] The technology of the present disclosure is useful for fluoride ion secondary batteries.

Claims

1. A fluoride ion secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the negative electrode contains a negative electrode active material and a fibrous carbon material, and the potential of the negative electrode at the end of charging is lower than −2.5 V with respect to a standard hydrogen electrode.

2. The fluoride ion secondary battery according to claim 1, wherein the fibrous carbon material includes carbon nanotubes.

3. The fluoride ion secondary battery according to claim 2, wherein the carbon nanotubes include single-walled carbon nanotubes.

4. The fluoride ion secondary battery according to claim 1, wherein the potential of the negative electrode at the end of charging is lower than −3.082 V with respect to a standard hydrogen electrode.

5. When the fibrous carbon material is analyzed by Raman spectroscopy, the intensity of the D band I D G-band intensity I G The fluoride ion secondary battery according to claim 1 , wherein the ratio is 1.1 or more.

6. The fluoride ion secondary battery according to claim 1, wherein the negative electrode active material contains at least one selected from the group consisting of lanthanum, cerium, magnesium, sodium and yttrium.

7. The fluoride ion secondary battery according to claim 1, wherein the negative electrode has a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer contains the negative electrode active material and the fibrous carbon material, and a ratio M3 / M1 of a mass M3 of the fibrous carbon material to a mass M1 of the negative electrode active material layer is, expressed as a percentage, 6 mass % or more and 12 mass % or less.

8. The fluoride ion secondary battery according to claim 7, wherein a ratio M2 / M1 of a mass M2 of the negative electrode active material to a mass M1 of the negative electrode active material layer is, expressed as a percentage, 40 mass % or more and 80 mass % or less.

9. The fluoride ion secondary battery according to claim 1, wherein the electrolyte contains at least one selected from the group consisting of a material represented by the following composition formula (1) and a material represented by the following composition formula (2): Ba 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.

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