Positive electrode active material, positive electrode and fluoride ion secondary battery

Abstract

To provide a positive electrode active material capable of improving the charge and discharge capacity of a fluoride ion secondary battery.SOLUTION: A positive electrode active material is a complex fluoride in which copper and a fluoride represented by a formula BaxCa1-xF2 (in the formula, x is 0.2 to 0.8) are made to be complex.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material, a positive electrode, and a fluoride ion secondary battery. [Background technology]

[0002] In recent years, research and development into secondary batteries that contribute to energy efficiency has been conducted to ensure that many people have access to affordable, reliable, sustainable and advanced energy.

[0003] Fluoride ion secondary batteries are known as solid-state batteries in which a solid electrolyte layer is disposed between a positive electrode and a negative electrode. As a positive electrode active material for fluoride ion secondary batteries, composite fluorides, which are composites of metals and fluorides having fluoride ion conductivity, are known (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 187942 Summary of the Invention [Problem to be solved by the invention]

[0005] However, it is desirable to improve the charge / discharge capacity of fluoride ion secondary batteries.

[0006] An object of the present invention is to provide a positive electrode active material that can improve the charge / discharge capacity of a fluoride ion secondary battery. [Means for solving the problem]

[0007] One embodiment of the present invention is a positive electrode active material comprising copper and a compound represented by the formula Ba x Ca 1-x F2 (wherein x is 0.2 or more and 0.8 or less). and complex fluorides in which is complexed.

[0008] The complex fluorides may be produced by an aerosol process.

[0009] Another embodiment of the present invention includes a positive electrode containing the above positive electrode active material.

[0010] Another embodiment of the present invention is a fluoride ion secondary battery having the above positive electrode. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a positive electrode active material that can improve the charge / discharge capacity of a fluoride ion secondary battery. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a graph showing the measurement results of the fluoride ion conductivity of the composite fluorides of Examples 1 to 3 and Comparative Examples 1 and 2. [Figure 2] 1 is an XRD spectrum of the complex fluoride of Example 1. [Figure 3] 1 shows a BF STEM image, an ADF STEM image, and an EELS mapping image of a thin section of the complex fluoride of Example 1. [Figure 4] 1 is a graph showing second cycle charge / discharge curves of cells of fluoride ion secondary batteries of Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described.

[0014] [Cathode active material] The positive electrode active material of this embodiment is a material containing copper and a compound represented by the formula Ba x Ca 1-x F2 (wherein x is 0.2 or more and 0.8 or less). and a composite fluoride in which is composited. Therefore, the fluoride ion conductivity of the positive electrode active material of this embodiment is improved. As a result, when the positive electrode active material of this embodiment is applied to a fluoride ion secondary battery, the charge / discharge capacity of the fluoride ion secondary battery is improved.

[0015] Here, x is 0.2 or more and 0.8 or less, and preferably 0.4 or more and 0.6 or less.

[0016] The copper content in the positive electrode active material of this embodiment is preferably 40 at % or more and 70 at % or less, and more preferably 50 at % or more and 60 at % or less. When the copper content in the positive electrode active material of this embodiment is 40 at % or more and 70 at % or less, when the positive electrode active material of this embodiment is applied to a fluoride ion secondary battery, the charge / discharge capacity of the fluoride ion secondary battery is improved.

[0017] The average particle size of the positive electrode active material of this embodiment is preferably 35 nm or less, and more preferably 25 nm or less. When the average particle size of the positive electrode active material of this embodiment is 35 nm or less, the effective area contributing to the electrode reaction of the positive electrode active material of this embodiment increases. As a result, when the positive electrode active material of this embodiment is applied to a fluoride ion secondary battery, the temperature characteristics of the charge / discharge capacity of the fluoride ion secondary battery are improved, and the battery can operate sufficiently even in a low-temperature environment. The average particle size of the positive electrode active material of this embodiment is not particularly limited, but is, for example, 20 nm or more.

[0018] Here, the average particle size means the particle size of primary particles calculated from the specific surface area by a constant volume gas adsorption method.

[0019] The positive electrode active material of this embodiment can be produced by an aerosol process. For example, a positive electrode active material containing copper and a compound represented by the formula Ba x Ca 1-x F2 (wherein x is 0.2 or more and 0.8 or less). and a fluoride represented by the formula (I) are melted and then sprayed under reduced pressure.

[0020] [Positive electrode] The positive electrode of the present embodiment includes the positive electrode active material of the present embodiment, and for example, a positive electrode composite layer is formed on a positive electrode current collector. In this case, the positive electrode composite layer includes the positive electrode active material of the present embodiment, and may further include a positive electrode active material other than the positive electrode active material of the present embodiment, a solid electrolyte, a conductive additive, etc., as necessary.

[0021] The positive electrode current collector is not particularly limited as long as it has electronic conductivity, and examples thereof include gold foil. The solid electrolyte is not particularly limited as long as it has fluoride ion conductivity, and examples thereof include PbSnF4. The conductive additive is not particularly limited as long as it has electronic conductivity, and examples thereof include acetylene black.

[0022] The positive electrode of this embodiment may have a porous structure, which improves the electrochemical reaction efficiency of the fluoride ion secondary battery.

[0023] The positive electrode of this embodiment can be obtained, for example, by molding a powder composition containing the positive electrode active material of this embodiment, a solid electrolyte, and a conductive additive.

[0024] [Fluoride-ion secondary battery] The fluoride ion secondary battery of this embodiment has the positive electrode of this embodiment, and for example, a solid electrolyte layer is sandwiched between the positive electrode of this embodiment and the negative electrode.

[0025] The negative electrode has, for example, a negative electrode mixture layer formed on a negative electrode current collector. In this case, the negative electrode mixture layer contains a negative electrode active material and may further contain a solid electrolyte, a conductive additive, etc., as necessary.

[0026] The negative electrode current collector is not particularly limited as long as it has electronic conductivity, and examples thereof include gold foil. The negative electrode active material is not particularly limited as long as it has fluoride ion conductivity, and examples thereof include lead. The solid electrolyte is not particularly limited as long as it has fluoride ion conductivity, and examples thereof include PbSnF4. The conductive additive is not particularly limited as long as it has electronic conductivity, and examples thereof include acetylene black.

[0027] As the negative electrode, a lead foil may be used which serves as both a negative electrode current collector and a negative electrode active material.

[0028] The solid electrolyte constituting the solid electrolyte layer is not particularly limited as long as it has fluoride ion conductivity, and examples thereof include PbSnF4.

[0029] The fluoride ion secondary battery of the present embodiment is obtained, for example, by sequentially stacking materials constituting the positive electrode (e.g., a positive electrode current collector and a powder composition for a positive electrode composite layer), materials constituting the solid electrolyte, and materials constituting the negative electrode (e.g., a negative electrode current collector and a powder composition for a negative electrode composite layer), and then integrally molding them.

[0030] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be modified as appropriate within the scope of the spirit of the present invention. [Example]

[0031] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0032] [Example 1] Copper (manufactured by Kojundo Chemical Laboratory) with an average particle size of 1 μm, barium fluoride (manufactured by Kojundo Chemical Laboratory), and calcium fluoride (manufactured by Kojundo Chemical Laboratory) were weighed out to a mass ratio of 90:7:3, and then premixed for approximately 1 hour using an agate mortar and pestle to obtain a raw material mixed powder.

[0033] The weighing and pre-mixing of the raw materials were carried out in a purged (DBO type) glove box (manufactured by Miwa Seisakusho) to prevent moisture absorption by the fluoride and oxidation of the copper.

[0034] The resulting raw material mixed powder was classified using a stainless steel mesh with 500 μm openings. Next, the raw material mixed powder that did not pass through the mesh was mixed using an agate mortar and pestle, and then classified, until all the raw material mixed powder passed through the mesh.

[0035] The sealed powder hopper containing the classified raw material mixed powder was removed from the glove box and connected to a high-frequency induction thermal plasma nanoparticle synthesis device TP-40020NPS (manufactured by JEOL Ltd.). Next, argon gas was supplied to the plasma torch, and the raw material mixed powder was melted by thermal plasma to form a raw material melt, which was then sprayed into a chamber in a reduced pressure environment. The raw material melt sprayed into the chamber was cooled and turned into nanoparticles, forming a composite fluoride (Cu-Ba 0.5 Ca 0.5 Next, the complex fluorides were collected using an exhaust filter, and then the upstream and downstream of the exhaust filter were blocked with valves, and the gas was transported into a glove box, where the complex fluorides were recovered.

[0036] [Example 2] Cu-Ba complex fluoride 0.8 Ca 0.2 A complex fluoride was obtained in the same manner as in Example 1, except that barium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and calcium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so as to obtain F2.

[0037] [Example 3] Cu-Ba complex fluoride 0.2 Ca 0.8 A complex fluoride was obtained in the same manner as in Example 1, except that barium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and calcium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so as to obtain F2.

[0038] [Comparative Example 1] A complex fluoride was obtained in the same manner as in Example 1, except that barium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and calcium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so that the complex fluoride would be Cu-BaF2.

[0039] Comparative Example 2 A composite fluoride was obtained in the same manner as in Example 1, except that barium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and calcium fluoride (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so that the composite fluoride would be Cu-CaF2.

[0040] [Fluoride ion conductivity] 4t / cm of composite fluoride powder 2 The powder was compressed and molded to prepare a compacted pellet. Next, gold foil (current collector) was placed on both sides of the compacted pellet, and the fluoride ion conductivity was measured by the AC impedance method.

[0041] FIG. 1 shows the measurement results of the fluoride ion conductivity of the composite fluorides of Examples 1 to 3 and Comparative Examples 1 and 2.

[0042] It can be seen from FIG. 1 that the complex fluorides of Examples 1 to 3 have higher fluoride ion conductivities than the complex fluorides of Comparative Examples 1 and 2.

[0043] [Crystal structure] The crystal structure of the complex fluoride of Example 1 was analyzed using a fully automatic multipurpose X-ray diffractometer SmartLaB (manufactured by Rigaku, Cu-Kα radiation source, λ=1.5418 Å).

[0044] Figure 2 shows the XRD spectrum of the composite fluoride of Example 1. Note that Figure 2 also shows the XRD spectrum of Cu, Ba 0.5 Ca 0.5 The XRD spectra of F2, BaF2 and CaF2 are also shown.

[0045] From FIG. 2, the complex fluoride of Example 1 has a peak at around 35°, which indicates that it contains Cu, Ba, 0.5 Ca 0.5It can be seen that the crystal structure is different from that of F2, BaF2 and CaF2.

[0046] [Domain Structure] Thin sections of the composite fluoride of Example 1 were prepared using a focused ion beam processing and observation system (FIB) FB-2100 (Hitachi High-Technologies) (not open to the atmosphere, cooled) and a precision ion polishing system Model 695 PIPS II (Gatan) (not open to the atmosphere, cooled).

[0047] The domain structure of the thin flake of the composite fluoride of Example 1 was observed using an atomic resolution analytical electron microscope JEM-ARM200F NEOARM (manufactured by JEOL Ltd.) (not open to the atmosphere, cooled) and a CCD camera GIF Quantum-ER (for EELS) (manufactured by Gatan Ltd.).

[0048] Figure 3 shows a BF STEM image (see Figure 3(a)), an ADF STEM image (see Figure 3(b)), and an EELS mapping image (see Figure 3(c)) of a thin piece of the complex fluoride of Example 1. In the EELS mapping image, Cu (blue) and Ba (yellow) are mapped.

[0049] It can be seen from FIG. 3 that the complex fluoride of Example 1 has a domain containing Cu and a domain containing Ba.

[0050] [Fabrication of fluoride ion secondary batteries] Using the composite fluorides of Example 1 and Comparative Example 1, fluoride ion secondary batteries were fabricated.

[0051] (solid electrolyte) PbSnF4 was used as the solid electrolyte.

[0052] (Positive electrode current collector) A gold foil was used as the positive electrode current collector.

[0053] (Powder composition for positive electrode composite layer) A composite fluoride as a positive electrode active material, PbSnF4 as a solid electrolyte, and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive additive were weighed out to a mass ratio of 30:65:5, and then thoroughly mixed to obtain a powder composition for a positive electrode composite layer.

[0054] (Negative electrode) A 200 μm thick lead foil (manufactured by Nilaco) that served as both a negative electrode current collector and a negative electrode active material was cut into a diameter of 10 mm to obtain a negative electrode.

[0055] (cell) A positive electrode current collector, a powder composition for a positive electrode composite layer (20 mg), a solid electrolyte (400 mg), and a negative electrode were layered in this order in a mold with a diameter of 10 mm, and then 4 t / cm 2 The resulting cell was a fluoride ion secondary battery cell. Gold wires were attached to the surfaces of the positive electrode current collector and the negative electrode of the cell with carbon paste to serve as terminals for charge / discharge measurements.

[0056] [Charge / discharge capacity] A constant-current charge-discharge test of a fluoride-ion secondary battery cell was conducted at 140°C. Specifically, a potentio / galvanostat SI1287 / 1255B (manufactured by Solartron) was used to conduct the constant-current charge-discharge test under the following conditions: current during charge and discharge: 40 μA, end-of-charge voltage: 1.3 V (vs. Pb / PbF2), end-of-discharge voltage: 0.3 V. To control the cell temperature during charge and discharge, the constant-current charge-discharge test was conducted by placing the cell in a small environmental test chamber SU261 (manufactured by Espec).

[0057] Fig. 4 shows the charge / discharge curves for the second cycle of the cells of the fluoride ion secondary batteries of Example 1 and Comparative Example 1. The capacity on the horizontal axis of Fig. 4 is the capacity per 1 g of composite fluoride.

[0058] 4 shows that the cell of the fluoride ion secondary battery of Example 1 has a higher charge / discharge capacity than the cell of the fluoride ion secondary battery of Comparative Example 1. This is presumably because the fluoride ion conductivity of the positive electrode active material of Example 1 is higher than the fluoride ion conductivity of the positive electrode active material of Comparative Example 1 (see FIG. 1).

Claims

1. Copper and the formula No ｘ Ca １－ｘ F ２ (In the formula, x is 0.2 or more and 0.8 or less.) and a complex fluoride in which A positive electrode active material having a copper content of 40 at % or more and 70 at % or less.

2. A method for producing the positive electrode active material according to claim 1, comprising: A method for producing a positive electrode active material, comprising the step of producing the complex fluoride by an aerosol process.

3. A positive electrode comprising the positive electrode active material of claim 1.

4. A fluoride ion secondary battery comprising the positive electrode according to claim 3 .

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