Fluoride ion secondary battery

Abstract

To provide a fluoride ion secondary battery capable of improving the initial charge and discharge capacity and the initial charge and discharge efficiency.SOLUTION: A fluoride ion secondary battery has a positive electrode including a positive electrode active material. The positive electrode active material is a complex fluoride in which copper and bismuth fluoride are made to be complex. A discharge final voltage of the fluoride ion secondary battery is equal to or less than 0.3 V (vs.Pb / PbF2).SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a fluoride ion secondary battery.

Background Art

[0002] In recent years, research and development on secondary batteries that contribute to energy efficiency have been carried out to enable more people to secure access to affordable, reliable, sustainable and advanced energy.

[0003] As a solid battery having a solid electrolyte layer disposed between a positive electrode and a negative electrode, a fluoride ion secondary battery is known. As a positive electrode active material for a fluoride ion secondary battery, a composite fluoride in which a metal and a fluoride having fluoride ion conductivity are composited is known (see, for example, Patent Document 1).

Prior Art Literature

Patent Literature

[0004]

Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0005] However, it has been desired to improve the initial charge-discharge capacity and initial charge-discharge efficiency of fluoride ion secondary batteries.

[0006] An object of the present invention is to provide a fluoride ion secondary battery capable of improving the initial charge-discharge capacity and the initial charge-discharge efficiency.

Means for Solving the Problem

[0007] One aspect of the present invention is a fluoride ion secondary battery having a positive electrode containing a positive electrode active material, wherein the positive electrode active material contains a composite fluoride in which copper and bismuth fluoride are combined, and the discharge termination voltage is 0.3V (vs.Pb / PbF2) or less.

[0008] The aforementioned composite fluoride may contain 20% by mass or more of bismuth.

[0009] The positive electrode may further contain a solid electrolyte containing fluorine.

[0010] The solid electrolyte may be a metal fluoride containing cerium and strontium.

[0011] The solid electrolyte may have a primary particle diameter of 10 nm or more and 200 nm or less.

[0012] The positive electrode further comprises a second positive electrode active material, the second positive electrode active material having a general formula K x Bi 1-x F 3-2x (In the equation, x is between 0.02 and 0.12.) It may be a compound represented by and having a hexagonal structure. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a fluoride-ion secondary battery that can improve the initial charge / discharge capacity and initial charge / discharge efficiency. [Brief explanation of the drawing]

[0014] [Figure 1] This is the XRD spectrum of the composite fluoride from Example 1. [Figure 2] This graph shows the charge-discharge curves for the first cycle of the fluoride-ion secondary battery cells in Example 1 and Comparative Example 1. [Modes for carrying out the invention]

[0015] Embodiments of the present invention will be described below.

[0016] [Fluoride ion secondary battery] The fluoride ion secondary battery of this embodiment has a positive electrode containing a positive electrode active material, but for example, a solid electrolyte layer is sandwiched between the positive electrode and the negative electrode of this embodiment. The positive electrode active material contains a composite fluoride in which copper and bismuth fluoride are compounded.

[0017] The discharge termination voltage of the fluoride-ion secondary battery in this embodiment is 0.3V (vs.Pb / PbF2) or less, and preferably 0.0V (vs.Pb / PbF2) or less. If the discharge termination voltage of the fluoride-ion secondary battery exceeds 0.3V (vs.Pb / PbF2), the initial charge / discharge capacity and initial charge / discharge efficiency will decrease. The discharge termination voltage of the fluoride-ion secondary battery in this embodiment is not particularly limited, but for example, it is -0.5V (vs.Pb / PbF2) or more.

[0018] The charging termination voltage of the fluoride-ion secondary battery in this embodiment is not particularly limited, but for example, it is between 1.3V (vs.Pb / PbF2) and 2.0V (vs.Pb / PbF2).

[0019] The bismuth content in the composite fluoride is preferably 20% by mass or more, and more preferably 30% by mass or more. When the bismuth content in the composite fluoride is 20% by mass or more, the capacity retention rate of the fluoride ion secondary battery of this embodiment is improved. The bismuth content in the composite fluoride is not particularly limited, but for example, it is 70% by mass or less.

[0020] The average particle diameter of the composite fluoride is preferably 35 nm or less, more preferably 25 nm or less. When the average particle diameter of the composite fluoride is 35 nm or less, the effective area of the composite fluoride contributing to the electrode reaction increases. As a result, the temperature characteristics of the charge / discharge capacity of the fluoride ion secondary battery of the present embodiment are improved, and the battery can operate sufficiently even in a low-temperature environment. The average particle diameter of the composite fluoride is not particularly limited, but is, for example, 20 nm or more.

[0021] Here, the average particle diameter means the particle diameter of primary particles calculated from the specific surface area measured by the constant-volume gas adsorption method.

[0022] The positive electrode active material of the present embodiment can be produced by an aerosol process. For example, after melting copper and bismuth fluoride, the molten mixture is sprayed under reduced pressure.

[0023] (Positive Electrode) As described above, the positive electrode contains the positive electrode active material, and for example, a positive electrode mixture layer is formed on a positive electrode current collector. In this case, the positive electrode mixture layer contains the composite fluoride, and may further contain a positive electrode active material other than the composite fluoride, a fluorine-containing solid electrolyte, a conductive aid, and the like as necessary. When the positive electrode further contains a fluorine-containing solid electrolyte, since the fluorine-containing solid electrolyte is present during defluorination of bismuth fluoride upon discharge of the fluoride ion secondary battery, bismuth fluoride can be effectively utilized. Accordingly, the initial discharge capacity of the fluoride ion secondary battery is improved.

[0024] 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 is a metal fluoride that does not undergo defluorination during discharge of the fluoride ion secondary battery, and examples thereof include PbSnF₄, Ce 0.975 Sr 0.025 F 2.975 , La 0.93 Ba 0.07 F 2.93 , Ca 0.5 Sr 0.5 F₂, Sr 0.7 Y0.3 F 2.3 Ba 0.7 S 0.3 F 2.3 La 0.9 Sr 0.1 F 2.9 Ba 0.5 Ca 0.5 Examples include F2. Among these, metal fluorides containing cerium and strontium are preferred. The conductive additive is not particularly limited as long as it has electronic conductivity, but examples include acetylene black.

[0025] The primary particle size of the solid electrolyte is preferably between 10 nm and 200 nm, and more preferably between 10 nm and 100 nm. When the primary particle size of the solid electrolyte is between 10 nm and 200 nm, the initial discharge capacity of the fluoride ion secondary battery is improved.

[0026] The positive electrode further contains a second positive electrode active material, and the second positive electrode active material has a general formula K x Bi 1-x F 3-2x (In the equation, x is between 0.02 and 0.12.) It is preferable that the compound is represented as and has a hexagonal structure. The second positive electrode active material has a higher fluoride ion conductivity than bismuth fluoride, thus improving the initial discharge capacity of the fluoride ion secondary battery.

[0027] The positive electrode may have a porous structure. This improves the electrochemical reaction efficiency of the fluoride ion secondary battery of this embodiment.

[0028] 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.

[0029] (Negative electrode) The negative electrode, for example, has a negative electrode composite layer formed on a negative electrode current collector. In this case, the negative electrode composite layer contains a negative electrode active material and may further contain a solid electrolyte, a conductive additive, etc., as needed.

[0030] The negative electrode current collector is not particularly limited as long as it has electronic conductivity, but examples include gold foil. The negative electrode active material is not particularly limited as long as it has electronic conductivity, but examples include lead. The solid electrolyte is not particularly limited as long as it has fluoride ion conductivity, but examples include PbSnF4. The conductive additive is not particularly limited as long as it has electronic conductivity, but examples include acetylene black.

[0031] Furthermore, lead foil that serves as both the negative electrode current collector and the negative electrode active material may be used as the negative electrode.

[0032] The solid electrolyte constituting the solid electrolyte layer is not particularly limited as long as it has fluoride ion conductivity, but examples include PbSnF4.

[0033] The fluoride ion secondary battery of this embodiment is obtained, for example, by sequentially stacking a material constituting the positive electrode (e.g., a powder composition for the positive electrode current collector and the positive electrode composite layer), a material constituting the solid electrolyte, and a material constituting the negative electrode (e.g., a powder composition for the negative electrode current collector and the negative electrode composite layer), and then integrally molding them.

[0034] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and the above embodiments may be modified as appropriate within the scope of the spirit of the present invention. [Examples]

[0035] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0036] [Example 1] Copper (manufactured by Kojun Chemical Laboratory) and bismuth fluoride (manufactured by Kojun Chemical Laboratory), with an average particle size of 1 μm, were weighed in a mass ratio of 61:39. These were then pre-mixed for approximately 1 hour using an agate mortar and pestle to obtain a raw material mixed powder.

[0037] Furthermore, the weighing and pre-mixing of the raw materials were carried out inside a purge-type (DBO type) glove box (manufactured by Miwa Seisakusho) to prevent moisture absorption of fluoride and oxidation of copper.

[0038] The obtained raw material mixture powder was subjected to classification using a stainless steel mesh with a mesh size of 500 μm. Next, the raw material mixture powder that did not pass through the mesh was mixed using an agate mortar and pestle, and then subjected to classification again. This process was repeated until all of the raw material mixture powder passed through the mesh.

[0039] A sealed powder hopper containing the classified raw material mixture powder was removed from the glove box and connected to a high-frequency induction thermal plasma nanoparticle synthesis apparatus TP-40020NPS (manufactured by JEOL). Next, argon gas was supplied to the plasma torch, and the raw material mixture powder was melted by thermal plasma to form a raw material molten material, which was then sprayed into a chamber under reduced pressure. The raw material molten material sprayed into the chamber underwent a cooling process, was converted into nanoparticles, and became a composite fluoride (Cu-BiF3). Subsequently, the composite fluoride was collected by an exhaust filter, and then the upstream and downstream of the exhaust filter were shut off with valves to transport it into the glove box for recovery of the composite fluoride.

[0040] [Example 2] A composite fluoride was obtained in the same manner as in Example 1, except that copper (manufactured by Kojun Chemical Laboratory) and bismuth fluoride (manufactured by Kojun Chemical Laboratory), with an average particle size of 1 μm, were weighed in a mass ratio of 71:29.

[0041] [Comparative Example 1] A composite fluoride (Cu-BaF2) was obtained in the same manner as in Example 1, except that copper (manufactured by Kojun Chemical Laboratory) and barium fluoride (manufactured by Kojun Chemical Laboratory), with an average particle size of 1 μm, were weighed in a mass ratio of 90:10.

[0042] [Crystal structure] The crystal structure of the composite fluoride of Example 1 was analyzed using a fully automated multi-purpose X-ray diffractometer, SmartLaB (manufactured by Rigaku, Cu-Kα source, λ=1.5418Å).

[0043] Figure 1 shows the XRD spectra of the composite fluoride from Example 1. Figure 1 also shows the XRD spectra of Cu and BiF3.

[0044] Figure 1 shows that the crystal structure of the composite fluoride in Example 1 is a composite of the crystal structure of Cu and the crystal structure of BiF3.

[0045] [Fabrication of Fluoride Ion Secondary Battery 1] Fluoride ion secondary batteries were fabricated using the composite fluorides of Examples 1 and 2 and Comparative Example 1.

[0046] (solid electrolyte) PbSnF4 was used as the solid electrolyte.

[0047] (Positive electrode current collector) Gold foil was used as the positive electrode current collector.

[0048] (Powder composition for positive electrode composite layer) A composite fluoride as the positive electrode active material, PbSnF4 as the solid electrolyte, and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as the conductive additive were weighed in a mass ratio of 30:65:5 and then thoroughly mixed to obtain a powder composition for the positive electrode composite layer.

[0049] (Negative electrode) A 200 μm thick lead foil (manufactured by Nirako), which serves as both the negative electrode current collector and the negative electrode active material, was processed to a diameter of 10 mm to obtain the negative electrode.

[0050] (cell) After sequentially layering a positive electrode current collector, a powder composition for the positive electrode composite layer (20 mg), a solid electrolyte (400 mg), and a negative electrode in a 10 mm diameter mold, the process is carried out at 4 t / cm². 2The cells for a fluoride ion secondary battery were obtained by integral molding under pressure. At this time, gold wires were bonded 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 and discharge measurement.

[0051] [Charge / discharge capacity] Constant current charge-discharge tests were conducted on fluoride-ion secondary battery cells at 140°C. Specifically, using a potentiometer / galvanostat SI1287 / 1255B (manufactured by Solartron), constant current charge-discharge tests were performed under the following conditions: charge current of 40 μA, charge termination voltage of 1.3 V (vs. Pb / PbF2), and discharge termination voltage of -0.5 V. To control the cell temperature during charge and discharge, the cells were placed in a small environmental test chamber SU261 (manufactured by ESPEC) and the constant current charge-discharge tests were performed.

[0052] Figure 2 shows the charge-discharge curves for the first cycle of the fluoride-ion secondary battery cells of Example 1 and Comparative Example 1. The capacity on the horizontal axis of Figure 2 represents the capacity per gram of composite fluoride.

[0053] Figure 2 shows that the fluoride-ion secondary battery cell of Example 1 has a higher initial charge / discharge capacity than the fluoride-ion secondary battery cell of Comparative Example 1. This is presumed to be because the fluoride-ion conductivity of the positive electrode active material of Example 1 is higher than that of the positive electrode active material of Comparative Example 1.

[0054] [Initial charge / discharge efficiency] The ratio of the initial discharge capacity to the initial charge capacity was calculated and defined as the initial charge / discharge efficiency.

[0055] [Capacity maintenance rate] The ratio of the charging capacity after the 5th cycle to the initial charging capacity was calculated and defined as the capacity retention rate.

[0056] Table 1 shows the evaluation results for the initial charge-discharge efficiency and capacity retention rate of fluoride-ion secondary battery cells.

[0057] [Table 1]

[0058] Table 1 shows that the fluoride-ion secondary batteries of Examples 1 and 2 have higher initial charge-discharge capacity and initial charge-discharge efficiency than the fluoride-ion secondary battery of Comparative Example 1.

[0059] [Fabrication of Fluoride Ion Secondary Battery 2] A fluoride ion secondary battery was fabricated using the composite fluoride from Example 1.

[0060] (Ce 0.92 Sr 0.08 F 2.92 (Production of) After weighing cerium fluoride (manufactured by Koshu Chemical Laboratory) and strontium fluoride (manufactured by Koshu Chemical Laboratory), the mixture was pre-mixed for approximately one hour using an agate mortar and pestle to obtain a raw material mixed powder.

[0061] The obtained raw material mixture powder was subjected to classification using a stainless steel mesh with a mesh size of 500 μm. Next, the raw material mixture powder that did not pass through the mesh was mixed using an agate mortar and pestle, and then subjected to classification again. This process was repeated until all of the raw material mixture powder passed through the mesh.

[0062] Furthermore, the weighing, pre-mixing, and classification of the raw materials were carried out inside a purge-type (DBO type) glove box (manufactured by Miwa Seisakusho) to prevent moisture absorption of the fluoride.

[0063] A sealed powder hopper containing the classified raw material mixture powder was removed from the glove box and connected to the high-frequency induction thermal plasma nanoparticle synthesis apparatus TP-40020NPS (manufactured by JEOL). Next, argon gas was supplied to the plasma torch, and the raw material mixture powder was melted by thermal plasma to form a raw material molten material, which was then sprayed into a chamber under reduced pressure. The raw material molten material sprayed into the chamber underwent a cooling process and was converted into nanoparticles, which then produced Ce 0.92 Sr 0.08 F 2.92 Next, Ce in the exhaust filter0.92 Sr 0.08 F 2.92 After collecting the material, the upstream and downstream of the exhaust filter are shut off with valves and transported into the glove box, Ce 0.92 Sr 0.08 F 2.92 We recovered it. Here, Ce 0.92 Sr 0.08 F 2.92 The composition was analyzed by ICP emission spectrometry.

[0064] (Powder composition for positive electrode composite layer) Composite fluoride as positive electrode active material, Ce as solid electrolyte 0.92 Sr 0.08 F 2.92 Furthermore, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.), used as a conductive additive, was weighed in a mass ratio of 30:65:5 and thoroughly mixed to obtain a powder composition for the positive electrode composite layer.

[0065] A fluoride-ion secondary battery 2 was prepared in the same manner as for fluoride-ion secondary battery 1, except that the obtained positive electrode composite layer powder composition was used.

[0066] Similar to fluoride-ion secondary battery 1, the initial charge-discharge efficiency and capacity retention rate of the cells of fluoride-ion secondary battery 2 were evaluated, and the initial charge capacity was 528 mAhg. -1 Initial discharge capacity is 532mAhg -1 The initial charge / discharge efficiency was 100.8%, and the capacity retention rate was 80.2%.

[0067] [Fabrication of Fluoride Ion Secondary Battery 3] A fluoride ion secondary battery was fabricated using the composite fluoride from Example 1.

[0068] [K 0.06 Bi 0.94 F 2.88 [Production of] After weighing potassium fluoride (manufactured by Koshu Chemical Laboratory) and bismuth fluoride (manufactured by Koshu Chemical Laboratory), the mixture was pre-mixed for approximately one hour using an agate mortar and pestle to obtain a raw material mixture powder.

[0069] The obtained raw material mixture powder was subjected to classification using a stainless steel mesh with a mesh size of 500 μm. Next, the raw material mixture powder that did not pass through the mesh was mixed using an agate mortar and pestle, and then subjected to classification again. This process was repeated until all of the raw material mixture powder passed through the mesh.

[0070] Furthermore, the weighing, pre-mixing, and classification of the raw materials were carried out inside a purge-type (DBO type) glove box (manufactured by Miwa Seisakusho) to prevent moisture absorption of the fluoride.

[0071] A sealed powder hopper containing the classified raw material mixture powder was removed from the glove box and connected to the high-frequency induction thermal plasma nanoparticle synthesis apparatus TP-40020NPS (manufactured by JEOL). Next, argon gas was supplied to the plasma torch, and the raw material mixture powder was melted by thermal plasma to form a raw material molten material, which was then sprayed into a chamber under reduced pressure. The raw material molten material sprayed into the chamber underwent a cooling process to be converted into nanoparticles, and K 0.06 Bi 0.94 F 2.88 Next, the exhaust filter K 0.06 Bi 0.94 F 2.88 After collecting the material, the upstream and downstream of the exhaust filter are shut off with valves and transported into the glove box, K 0.06 Bi 0.94 F 2.88 We recovered it. Here, K 0.06 Bi 0.94 F 2.88 The composition was analyzed by ICP emission spectrometry.

[0072] (Powder composition for positive electrode composite layer) Composite fluorides as positive electrode active materials, K as positive electrode active materials 0.06 Bi 0.94 F 2.88 Ce as a solid electrolyte 0.92 Sr 0.08 F 2.92Furthermore, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.), used as a conductive additive, was weighed in a mass ratio of 30:10:55:5, and then thoroughly mixed to obtain a powder composition for the positive electrode composite layer.

[0073] A fluoride-ion secondary battery 3 was prepared in the same manner as for fluoride-ion secondary battery 1, except that the obtained positive electrode composite layer powder composition was used.

[0074] Similar to fluoride-ion secondary battery 1, the initial charge-discharge efficiency and capacity retention rate of the cells of fluoride-ion secondary battery 3 were evaluated, and the initial charge capacity was 529 mAhg. -1 The initial discharge capacity is 544mAhg -1 The initial charge-discharge efficiency was 102.8%, and the capacity retention rate was 81%. Compared to fluoride-ion secondary batteries, K 0.06 Bi 0.94 F 2.88 It can be seen that this contributes to the discharge capacity.

Claims

1. Having a positive electrode containing a positive electrode active material, The positive electrode active material is a composite fluoride in which copper and bismuth fluoride are combined, and a general formula K x Bi 1-x F 3-2x (In the formula, x is between 0.02 and 0.12.) It is represented as and includes a compound having a hexagonal structure, Discharge termination voltage is 0.3V (vs. Pb / PbF) ２ ) Below, a fluoride ion secondary battery.

2. The fluoride ion secondary battery according to claim 1, wherein the composite fluoride has a bismuth content of 20% by mass or more.

3. The fluoride ion secondary battery according to claim 1 or 2, wherein the positive electrode further comprises a solid electrolyte containing fluorine.

4. The fluoride ion secondary battery according to claim 3, wherein the solid electrolyte is a metal fluoride containing cerium and strontium.

5. The fluoride ion secondary battery according to claim 3, wherein the solid electrolyte has a primary particle size of 10 nm or more and 200 nm or less.

Images