Positive electrode active material, method for producing positive electrode active material, and fluoride ion secondary battery

Abstract

Provided is a positive electrode active material for use in a fluoride ion secondary battery, the positive electrode active material comprising Cu particles and Bi particles, the positive electrode active material having a first peak in the range of a diffraction angle 2 [theta] of 26.2 + / -0.15 DEG and a second peak in the range of a diffraction angle 2 [theta] of 30.4 + / -0.25 DEG in an XRD spectrum measured using a Cu-K [alpha] line, and a third peak exists in a range where the diffraction angle 2 [theta] is 27.0 + / -0.25 degrees.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a fluoride ion secondary battery, a method for manufacturing the positive electrode active material, and a fluoride ion secondary battery. Background Art

[0002] In recent years, research and development of secondary batteries that contribute to improving energy efficiency have been carried out to ensure that more people can obtain affordable, reliable, sustainable, and advanced energy.

[0003] Patent Document 1 describes a fluoride ion battery having at least a positive electrode active material layer and a solid electrolyte layer. The positive electrode active material layer includes positive electrode active material particles mainly composed of Cu and Sn, and the solid electrolyte layer includes a solid electrolyte containing Pb, Sn, and F.

[0004] [Prior Art Documents]

[0005] (Patent Document)

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-73753 Summary of the Invention

[0007] [Problems to be Solved by the Invention]

[0008] However, it is desired to increase the initial discharge capacity of the fluoride ion battery.

[0009] An object of the present invention is to provide a positive electrode active material that can increase the initial discharge capacity of a fluoride ion secondary battery.

[0010] [Means for Solving the Problems]

[0011] (1) A positive electrode active material for a fluoride ion secondary battery, the positive electrode active material including Cu particles and Bi particles. In an X-ray diffraction (XRD) spectrum measured using Cu-Kα radiation, a first peak exists in a range where the diffraction angle 2θ is 26.2 ± 0.15°, a second peak exists in a range where the diffraction angle 2θ is 30.4 ± 0.25°, and a third peak exists in a range where the diffraction angle 2θ is 27.0 ± 0.25°.

[0012] (2) The positive electrode active material according to (1), wherein the intensity ratio of the first peak to the third peak is 0.4 or more.

[0013] (3) The positive electrode active material according to (1) or (2), wherein the Cu particles are nanoparticles.

[0014] (4) A method for manufacturing a positive electrode active material is a method for manufacturing the positive electrode active material according to any one of (1) to (3). A raw material composition including Cu particles, Bi particles, and particles of a compound represented by the following general formula is ball-milled and mixed at a rotation speed of 200 rpm or more and 400 rpm or less for 10 minutes or more and 20 minutes or less. Thereafter, it is paused for 5 minutes or more and 20 minutes or less, and this cycle is carried out 40 times or more and 120 times or less.

[0015] K x Bi 1-x F 3-2x

[0016] In the formula, x is 0.02 or more and 0.12 or less.

[0017] (5) A fluoride ion secondary battery includes a positive electrode mixture layer including the positive electrode active material according to any one of (1) to (3).

[0018] (Effects of the Invention)

[0019] According to the present invention, a positive electrode active material can be provided that can improve the initial discharge capacity of a fluoride ion secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 are XRD spectra of the powder compositions for the positive electrode mixture layers of Examples 1 to 3 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0021] The embodiments of the present invention will be described below.

[0022] [Positive Electrode Active Material]

[0023] The positive electrode active material of the present embodiment is used for a fluoride ion secondary battery and includes Cu particles and Bi particles. In addition, in the XRD spectrum measured using Cu-Kα rays of the positive electrode active material of the present embodiment, a first peak exists in the range of diffraction angle 2θ of 26.2 ± 0.15°, a second peak exists in the range of diffraction angle 2θ of 30.4 ± 0.25°, and a third peak exists in the range of diffraction angle 2θ of 27.0 ± 0.25°. Therefore, the initial discharge capacity of the fluoride ion secondary battery is improved. It is presumed that this is because fluoride ion conductive compounds such as KBiO 2.35 F9 are generated during ball-milling and mixing when manufacturing the positive electrode active material as described below.

[0024] The intensity ratio of the first peak to the third peak is preferably 0.4 or more, and more preferably 0.6 or more. When the intensity ratio of the first peak to the third peak is 0.4 or more, the initial discharge capacity of the fluoride ion secondary battery increases.

[0025] The Cu particles are preferably nanoparticles. Thereby, the initial discharge capacity of the fluoride ion secondary battery increases. The particle size of the Cu particles is not particularly limited, for example, it is 10 nm or more and 100 nm or less.

[0026] The mass ratio of the Cu particles to the Bi particles in the positive electrode active material of the present embodiment is not particularly limited, for example, it is 1 / 2 or more and 3 or less. The particle size of the Bi particles is not particularly limited, for example, it is 20 nm or more and 1 μm or less.

[0027] [Manufacturing method of positive electrode active material]

[0028] The manufacturing method of the positive electrode active material of the present embodiment is to ball-mill and mix a raw material composition including Cu particles, Bi particles, and particles of a compound represented by the general formula

[0029] K x Bi 1-x F 3-2x

[0030] (wherein x is 0.02 or more and 0.12 or less)

[0031] at a rotation speed of 200 rpm or more and 400 rpm or less for 10 minutes or more and 20 minutes or less, and then pausing for 5 minutes or more and 20 minutes or less, and repeating this cycle 40 times or more and 120 times or less. Thereby, the initial discharge capacity of the fluoride ion secondary battery increases. It is speculated that this is because fluoride ion conductive compounds such as KBiO 2.35 F9 are generated.

[0032] When ball-milling and mixing the raw material composition, the rotation speed is 200 rpm or more and 400 rpm or less, preferably 300 rpm or more and 350 rpm or less. In addition, the time for ball-milling and mixing the raw material composition is 10 minutes or more and 20 minutes or less, preferably 15 minutes or more and 20 minutes or less. The time for pausing after ball-milling and mixing the raw material composition is 5 minutes or more and 20 minutes or less, preferably 5 minutes or more and 10 minutes or less. The number of times of repeating this cycle of pausing after ball-milling and mixing the raw material composition is 40 times or more and 120 times or less, preferably 80 times or more and 100 times or less.

[0033] The particles of the compound represented by the above general formula are preferably nanoparticles. The particle size of the particles of the compound represented by the above general formula is not particularly limited, for example, it is 10 nm or more and 100 nm or less.

[0034] In addition, in addition to the positive electrode active material, the raw material composition may further include a solid electrolyte, a conductive additive, etc. In this case, a powder composition for a positive electrode mixture layer including the positive electrode active material of the present embodiment can be manufactured.

[0035] [Fluoride ion secondary battery]

[0036] The fluoride ion secondary battery of the present embodiment includes a positive electrode mixture layer, and the positive electrode mixture layer includes the positive electrode active material of the present embodiment. The fluoride ion secondary battery of the present embodiment further includes, for example, a positive electrode current collector foil, a solid electrolyte layer, a negative electrode mixture layer, and a negative electrode current collector foil.

[0037] (Positive electrode mixture layer)

[0038] The positive electrode mixture layer includes the positive electrode active material of the present embodiment, and may further include a solid electrolyte, a conductive additive, etc. as needed. In addition, the positive electrode active material of the present embodiment may further include a positive electrode active material other than Cu particles and Bi particles.

[0039] As the positive electrode active material other than Cu particles and Bi particles, there is no particular limitation, and examples thereof include those represented by the general formula

[0040] K x Bi 1-x F 3-2x

[0041] (where x is 0.02 or more and 0.12 or less)

[0042] Particles of the compound represented.

[0043] The positive electrode active material other than Cu particles and Bi particles is preferably nanoparticles. The particle size of the positive electrode active material other than Cu particles and Bi particles is, for example, 10 nm or more and 100 nm or less.

[0044] As the solid electrolyte, as long as it has fluoride ion conductivity and does not defluorinate during discharge of the fluoride ion secondary battery, there is no particular limitation, and examples thereof include metal fluoride particles. As the metal fluoride particles, for example, Ce 0.92 Sr 0.08 F 2.92 Particles.

[0045] The solid electrolyte is preferably nanoparticles. The particle size of the solid electrolyte is, for example, 10 nm or more and 100 nm or less.

[0046] As the conductive additive, as long as it has electronic conductivity, there is no particular limitation, and examples thereof include acetylene black.

[0047] (Positive current collector foil)

[0048] As the positive current collector foil, any material having electron conductivity can be used without particular limitation. For example, metal foils such as gold foil and platinum foil can be mentioned.

[0049] (Solid electrolyte layer)

[0050] As the solid electrolyte constituting the solid electrolyte layer, any material having fluoride ion conductivity and not releasing fluorine during discharge of the fluoride ion secondary battery can be used without particular limitation. For example, metal fluorides can be mentioned. As the metal fluoride, for example, Ce 0.95 Sr 0.05 F 2.85 .

[0051] (Negative electrode mixture layer)

[0052] The negative electrode mixture layer includes a negative electrode active material, and if necessary, a conductive additive and the like can also be included. As the negative electrode active material, there is no particular limitation. For example, PbSnF4 particles can be mentioned. As the conductive additive, any material having electron conductivity can be used without particular limitation. For example, acetylene black can be mentioned.

[0053] (Negative current collector foil)

[0054] As the negative current collector foil, any material having electron conductivity can be used without particular limitation. For example, metal foils such as aluminum foil can be mentioned.

[0055] The fluoride ion secondary battery of the present embodiment is obtained, for example, by sequentially laminating a positive current collector foil, a powder composition for a positive electrode mixture layer, a solid electrolyte layer, a powder composition for a negative electrode mixture layer, and a negative current collector foil, and then performing press molding. Here, the powder composition for the positive electrode mixture layer includes, for example, the positive electrode active material, the solid electrolyte, and the conductive additive of the present embodiment. In addition, the powder composition for the negative electrode mixture layer includes, for example, the negative electrode active material and the conductive additive.

[0056] The embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments, and the above embodiments can be appropriately changed within the scope of the gist of the present invention.

[0057] [Examples]

[0058] Examples of the present invention will be described below, but the present invention is not limited to the examples.

[0059] (K 0.06 Bi 0.94 F 2.88 powder)

[0060] Weigh potassium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and bismuth fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.). After that, use an agate mortar and pestle to premix for about one hour to obtain a raw material mixed powder.

[0061] Use a stainless steel sieve with a mesh size of 500 μm to perform a classification process on the obtained raw material mixed powder. Then, use an agate mortar and pestle to mix the raw material mixed powder that did not pass through the sieve and perform the classification process until all the raw material mixed powder passes through the sieve.

[0062] In addition, to prevent the fluoride from absorbing moisture, the weighing, premixing, and classification of the raw materials are carried out inside a purge type (DBO type) glove box (manufactured by Miwa Mfg Co., Ltd.).

[0063] Take out the sealed powder hopper containing the classified raw material mixed powder from the glove box and connect it to the high-frequency inductively coupled plasma nanoparticle synthesis device TP-40020NPS (manufactured by JEOL Ltd.). Then, supply argon to the plasma torch, melt the raw material mixed powder using the thermal plasma to form a raw material melt, and spray the raw material melt into the chamber under reduced pressure. The raw material melt sprayed into the chamber undergoes a cooling process and is made into nanoparticles, becoming K 0.06 Bi 0.94 F 2.88 powder. Then, capture the K 0.06 Bi 0.94 F 2.88 powder using an exhaust filter. After that, use a valve to block the upstream and downstream of the exhaust filter and transport it into the glove box to recover the K 0.06 Bi 0.94 F 2.88 powder with a particle size of 10 nm or more and 100 nm or less. Here, the composition of the K 0.06 Bi 0.94 F 2.88 powder is analyzed using inductively coupled plasma (ICP) emission spectrometry.

[0064] (Cu powder)

[0065] Except for using copper (manufactured by Kojundo Chemical Laboratory Co., Ltd.) to replace the raw material mixed powder, obtain Cu powder with a particle size of 10 nm or more and 100 nm or less in the same way as the K 0.06 Bi 0.94 F 2.88 powder.

[0066] (Ce 0.92 Sr 0.08 F 2.92 powder)

[0067] Weigh cerium fluoride (manufactured by High Purity Chemical Research Institute) and strontium fluoride (manufactured by High Purity Chemical Research Institute), and then use an agate mortar and pestle to premix for about one hour to obtain a raw material mixed powder.

[0068] Except for using the obtained raw material mixed powder, in the same manner as for the K 0.06 Bi 0.94 F 2.88 powder, Ce 0.92 Sr 0.08 F 2.92 powder with a particle size of 10 nm or more and 100 nm or less is obtained. Here, the composition of the Ce 0.92 Sr 0.08 F 2.92 powder is analyzed by ICP emission spectrometry.

[0069] (Powder composition for negative electrode binder layer)

[0070] Using a silicon nitride jar mill with a capacity of 45 mL and 10 silicon nitride balls with a diameter of 10 mm, 6 g of lead fluoride (manufactured by High Purity Chemical Research Institute) and 2.8 g of stannous fluoride (manufactured by High Purity Chemical Research Institute) are ball-milled and mixed. At this time, ball-milling and mixing are carried out at 600 rpm for 3 hours, then paused for 5 minutes, and this cycle is implemented 8 times. Then, 0.619 g of acetylene black is added to 8.669 g of the mixture, and ball-milling and mixing are carried out in the same manner as above. Then, under an argon atmosphere, heat treatment is carried out at 400 °C for 1 hour to obtain a powder composition for the negative electrode binder layer.

[0071] (Ce 0.95 Sr 0.05 F 2.85 powder)

[0072] 19.3510 g of cerium fluoride (manufactured by High Purity Chemical Research Institute) and 0.6490 g of strontium fluoride (manufactured by High Purity Chemical Research Institute) are ball-milled and mixed, and then sintered at 1100 °C for 6 hours under an argon atmosphere to obtain Ce 0.95 Sr 0.05 F 2.85 powder. When carrying out ball-milling and mixing, after ball-milling and mixing at 600 rpm for 1 hour, pause for 5 minutes, and this cycle is implemented 40 times.

[0073] (Example 1)

[0074] (Powder Composition for Positive Electrode Mixing Layer)

[0075] The powder composition for the positive electrode mixing layer was prepared inside a purge type (DBO type) glove box (manufactured by Miwa Seisakusho) filled with Ar gas. Specifically, 0.524 g of Cu powder as the positive electrode active material, 0.175 g of Bi powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.), 0.267 g of K 0.06 Bi 0.94 F 2.88 powder, and 0.023 g of acetylene black (manufactured by Denka Company Limited) as the conductive additive were weighed. Next, using a silicon nitride pot mill with a capacity of 45 mL and 40 g of silicon nitride balls with a diameter of 2 mm, the weighed materials were ball-milled and mixed in 8 g of cyclohexane, and then dried on a hot plate at 65 °C to obtain the powder composition for the positive electrode mixing layer. When ball-milling and mixing the weighed materials, ball-milling was carried out at 300 rpm for 15 minutes and then paused for 5 minutes, and this cycle was repeated 40 times.

[0076] (Single Battery Cell)

[0077] Inside a purge type (DBO type) glove box (manufactured by Miwa Seisakusho) filled with Ar gas, a single battery cell was fabricated using an alumina tube with an inner diameter of 10 mm. Specifically, first, 150 mg of Ce 0.95 Sr 0.05 F 2.85 powder was uniaxially pressed at a surface pressure of 740 MPa to obtain a solid electrolyte layer. Next, a Pt foil as the positive electrode current collector foil, 10 mg of the powder composition for the positive electrode mixing layer, the solid electrolyte layer, 30 mg of the powder composition for the negative electrode mixing layer, and an Al foil as the negative electrode current collector foil were laminated in sequence, and then uniaxially pressed at 700 MPa to obtain a single battery cell. Then, the single battery cell was encapsulated in a sealed glass container under a constrained pressure of approximately 340 MPa.

[0078] (Example 2)

[0079] A single battery cell was obtained in the same manner as in Example 1, except that when ball-milling and mixing the weighed materials, ball-milling was carried out at 400 rpm for 15 minutes and then paused for 5 minutes, and this cycle was repeated 40 times.

[0080] (Example 3)

[0081] A single battery cell was obtained in the same manner as in Example 1, except that when ball-milling and mixing the weighed materials, ball-milling was carried out at 300 rpm for 15 minutes and then paused for 5 minutes, and this cycle was repeated 80 times.

[0082] (Comparative Example 1)

[0083] A battery cell was obtained in the same manner as in Example 1, except that when the weighed substances were ball-milled and mixed, the ball-milling and mixing was carried out at 100 rpm for 15 minutes and then paused for 5 minutes, and this cycle was carried out 40 times.

[0084] (Comparative Example 2)

[0085] A battery cell was obtained in the same manner as in Example 1, except that when the weighed substances were ball-milled and mixed, the ball-milling and mixing was carried out at 200 rpm for 15 minutes and then paused for 5 minutes, and this cycle was carried out 40 times.

[0086] (Comparative Example 3)

[0087] A battery cell was obtained in the same manner as in Example 1, except that when the weighed substances were ball-milled and mixed, the ball-milling and mixing was carried out at 200 rpm for 15 minutes and then paused for 5 minutes, and this cycle was carried out 80 times.

[0088] [XRD Spectrum]

[0089] Using a fully automatic multifunctional X-ray diffractometer SmartLab (manufactured by Rigaku), the XRD spectrum of the powder composition for the positive electrode mixture layer was measured. At this time, Cu-Kα line (λ = 1.5418 Å) was used as the X-ray.

[0090] In Figure 1 the XRD spectra of the powder compositions for the positive electrode mixture layers of Examples 1 to 3 and Comparative Examples 1 to 3 are shown.

[0091] [Discharge Capacity]

[0092] Using a potentiostat / galvanostat SI1287 / 1255B (manufactured by Solartron), the inside of the glass container was depressurized using a vacuum pump, and the glass container was placed in a thermostat with the temperature set to 140 °C to carry out a constant current charge and discharge test of the battery cell. Specifically, first, a current of 0.120 mA was passed, and then a current of 0.040 mA was passed to charge until the voltage reached 1.5 V (relative to Pb / PbF2). Then, a current of 0.120 mA was passed, and then a current of 0.040 mA was passed to discharge until the voltage reached -0.5 V (relative to Pb / PbF2), and the discharge capacity was determined.

[0093] The evaluation results of the initial discharge capacity of the battery cell are shown in Table 1. In addition, the initial discharge capacity is the capacity per gram of the positive electrode mixture layer.

[0094] [Table 1]

[0095]

[0096] As can be seen from Table 1, the initial discharge capacities of the battery cells of Examples 1 to 3 are relatively high. In contrast, for the battery cells of Comparative Examples 1 to 3, no peak exists in the XRD spectrum of the positive electrode active material within the range of 30.4 ± 0.25°, and thus the initial discharge capacities are low.

Claims

1. A positive electrode active material for a fluoride ion secondary battery, wherein the positive electrode active material includes Cu particles and Bi particles, in an XRD spectrum measured using Cu-Kα rays, a first peak exists in a range where the diffraction angle 2θ is 26.2 ± 0.15°, a second peak exists in a range where the diffraction angle 2θ is 30.4 ± 0.25°, and a third peak exists in a range where the diffraction angle 2θ is 27.0 ± 0.25°.

2. The positive electrode active material according to claim 1, wherein The intensity ratio of the aforementioned first peak to the aforementioned third peak is 0.4 or more.

3. The positive electrode active material according to claim 1 or 2, wherein The aforementioned Cu particles are nanoparticles.

4. A method for manufacturing a positive electrode active material, which is a method for manufacturing the positive electrode active material according to claim 1 or 2, a raw material composition including Cu particles, Bi particles, and particles of a compound represented by the following general formula is ball-milled and mixed at a rotational speed of 200 rpm or more and 400 rpm or less for 10 minutes or more and 20 minutes or less, then paused for 5 minutes or more and 20 minutes or less, and this cycle is carried out 40 times or more and 120 times or less, K x Bi 1-x F 3-2x wherein x is 0.02 or more and 0.12 or less.

5. A fluoride ion secondary battery, which includes a positive electrode mixture layer, and the positive electrode mixture layer includes the positive electrode active material according to claim 1 or 2.

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