Positive electrode active material for fluoride-ion secondary batteries, positive electrode using the active material, fluoride-ion secondary battery and method for manufacturing the active material

Abstract

Positive electrode active material for fluoride-ion secondary batteries, wherein the positive electrode active material is a complex fluoride, comprising: a first component made of metal; and a second component consisting of a fluorine compound that exhibits fluoride ion conductivity, the complex fluoride contains perovskite fluoride.

Description

[0001] This application is based on the Japanese patent application JP 2018-059 702 A filed on March 27, 2018, and claims priority from that application, the contents of which are incorporated herein by reference. TECHNICAL FIELD

[0002] The present invention relates to a positive electrode active material for fluoride-ion secondary batteries, a positive electrode which uses the active material, a fluoride-ion secondary battery and a method for producing the active material. STATE OF THE ART

[0003] Conventionally, the lithium-ion secondary battery is widely used as a high-energy-density secondary battery. The lithium-ion secondary battery has a structure in which a separator is present, formed by placing a separator between the positive and negative electrodes, and a liquid electrolyte (electrolyte solution) is filled within it.

[0004] The electrolyte solution of the lithium-ion secondary battery is normally a flammable organic solvent, and therefore there have been cases of safety issues related to heat, which becomes a particular problem.

[0005] Therefore, instead of the electrolyte of liquid form of an organic system, a solid-state battery formed by the use of an electrolyte of solid form of an inorganic system has been proposed (see patent document 1).

[0006] In comparison to a battery that uses an electrolytic solution, a solid-state battery made from a solid electrolytic substance can solve the problem of heat and improve the voltage through stacking, and also meet the requirements of compactness.

[0007] A fluoride-ion secondary battery has been considered as a battery made from such a solid electrolyte.

[0008] The fluoride-ion secondary battery is a secondary battery with fluoride ions (F₂). - ) as a carrier and it is known to have a high theoretical energy.

[0009] It is therefore expected that their battery properties will surpass those of lithium-ion secondary batteries.

[0010] The following materials have been reported as positive electrode active materials for fluoride-ion secondary batteries: BiF3, CuF2, KBiF4, etc. (see non-patent documents 1 to 10).

[0011] However, in the fluoride-ion secondary battery manufactured with these currently reported positive electrode active materials, an example with an operating temperature of no more than 100°C has not been confirmed, and thus there has been a limitation in the usage environment. Patent document 1: Japanese unexamined patent application, publication number JP 2000-106 154 A Non-patent document 1: M. Anji Reddy, M. Fichtner, J. Mater. Chemie, 21 (2011) 17059-17062 Non-patent document 2: JH Kennedy and JC Hunter, J. Electrochem. Soc., 123, 10 (1976). Non-patent document 3: JM Reau and J. Portier, Solid Electrolytes, edited by P. Hagenmuller and W. van Gool (Academic, New York, 1978), p. 313. Non-patent document 4: RN Zakirov and AS Marinin, Proceedings of the IX All-Union Symposium on the Chemistry of Inorganic Fluorides, Cherepovets, 1990, p.136; ibid., p.137; ibid., p.138. Non-patent document 5: I. Kosacki, Appl. Phys., A49, 413 (1989). Non-patent document 6: IV Murin, OV Glumov, and II Kosacki, Appl. Phys., A49, 413 (1989). Leningr. Univ., No. 22, Issue 4, 87 (1980). Non-patent document 7: J. Schoonman and A. Wolfert, Solid State Ionic, 3-4, 373 (1981). Non-patent document 8: IV Murin, Doctoral Dissertation (Tech.) (LGU, Leningrad, 1984). Non-patent document 9: AA Potanin, Russ. Chem. J., 45 (2001) 61-66. Non-patent document 10: W. Baukal, R. Knodler, W. Kuhn, Chem. Ingenieur Technik, 50 (1978) 245-249.

[0012] From US patent 2014 / 0170493A1, a positive electrode is known consisting of an electrochemically active material. The electrochemically active material comprises (i) a metal component and (ii) a lithium compound component, which is mixed with the metal component at a distance of about 20 nm or less. Furthermore, when fully charged and forming a compound of the metal component and an anion of the lithium compound, the electrochemically active material exhibits a reversible specific capacity of about 350 mAh / g or more when discharged with lithium ions at a rate of at least about 200 mA / g.

[0013] DE 10 2009 017 262 A1 discloses a cathode material for fluoride-based conversion electrodes. The cathode material contains - Alkali metal ions, preferably lithium ions or sodium ions, - Fluoride ions as well as - Metal nanoparticles with a maximum size of 20 nm and - which are distributed in a matrix of graphitic nanocarbon, preferably multi-walled nanocarbon.

[0014] From WO 2018 / 112 400 A1, an electrochemically active structure is known which comprises: a core containing an active material; and a fluoride-containing shell that at least partially surrounds the active material. Furthermore, this document discloses electrochemically active materials and, in particular, fluoride-ion battery systems comprising electrode materials with tailored structures and compositions to improve battery performance. DISCLOSURE OF THE INVENTION The problems solved by the invention

[0015] The present invention was carried out taking into account the aforementioned prior art and provides a positive electrode active material for a fluoride-ion secondary battery according to the subject matter of main claim 1, as well as a method for producing the positive electrode active material for fluoride-ion secondary batteries according to claim 10. In particular, the present invention describes: providing a positive electrode active material for a fluoride-ion secondary battery, a positive electrode which uses the active material, a fluoride-ion secondary battery, and a method for producing the active material which can realize a fluoride-ion secondary battery which is also sufficiently operational in a low-temperature environment. Means to solve the problems

[0016] The inventors in the present case focus on the fact that the ionic conductivity of the solid electrolyte of fluoride ions is at an essentially equivalent level (10 -3 ~10 -5 S / cm at room temperature) such as the solid electrolyte of lithium ions (see Solid State Ionics 239 (2013), 41-49, title: “Fast Fluoride ion conducting materials in solid state ionics: An overview”, authors: LN Patro, K. Hariharan).

[0017] Because of this fact, the step that determines the rate of charging and discharging reactions, which is a factor for the operating temperature of fluoride-ion secondary batteries above 100°C, is not the ionic conductivity in the solid electrolyte, and it was considered that this depends strongly on the reaction activity of the fluorination / defluorination of the active material itself.

[0018] Here, the fluoride ion conductivity of the active material can be given as an example of a physical property that serves as an index for the reaction activity of fluorination / defluorination.

[0019] For example, the fluoride ion conductivity of the active material of the positive CuF2 electrode is not measurable because it decreases and leaves the measuring range of the measuring instrument (SI126096 from Solartron).

[0020] Therefore, if one extrapolates to 25°C and assumes that the temperature dependence of the ionic conductivity follows the Arrhenius law, it is only on the order of 10 -16 S / cm.

[0021] As described above, in the case where a substance exhibiting very low fluoride ion conductivity is used as the positive electrode active material, the dispersion of the fluoride ion in the positive electrode active material becomes the step that determines the rate, and it becomes impossible to carry out the charging or discharging continuously in the state of constant current.

[0022] In order to operate a fluoride-ion secondary battery in different temperature environments, it is therefore necessary to realize an active material that exhibits high fluorination / defluorination reaction activity at room temperature (approx. 25°C).

[0023] In contrast, according to the present invention, a positive electrode active material for fluoride-ion secondary batteries is provided, which is a complex fluoride comprising a first metal component and a second fluorine compound component which exhibits fluoride-ion conductivity.

[0024] The complex fluoride contains perovskite fluoride.

[0025] A domain formed by the first component and a domain formed by the second component in the complex fluoride can be complex.

[0026] The average particle size of the complex fluoride cannot exceed 35nm.

[0027] The content of the first component, in relation to the complex fluoride, can be anywhere in the range of 40 to 70 atomic percent.

[0028] The metal can be at least one type selected from the group consisting of Cu, Co, Ag and Bi.

[0029] The fluorine compound can be at least one type selected from the group consisting of lead fluoride, tin fluoride, bismuth fluoride, lanthanum fluoride, cerium fluoride, sodium fluoride, potassium fluoride and barium fluoride.

[0030] The fluorine compound can be at least one type selected from the group consisting of sodium fluoride, potassium fluoride and barium fluoride.

[0031] Furthermore, another aspect of the invention is a positive electrode for fluoride-ion secondary batteries, comprising the above-mentioned positive electrode active material for fluoride-ion secondary batteries.

[0032] Furthermore, another aspect of the invention is a fluoride-ion secondary battery comprising the above-mentioned positive electrode for fluoride-ion secondary batteries, a solid electrolyte and a negative electrode.

[0033] Furthermore, another aspect of the invention is a method for producing the above-mentioned positive electrode active material for fluoride-ion secondary batteries, wherein the method comprises an aerosol process for spraying a raw material melt containing the metal and the fluorine compound under reduced pressure. Effects of the invention

[0034] According to the positive electrode active material for fluoride-ion secondary batteries of the present invention, it is possible to increase the fluoride ion conductivity in the fluoride-ion secondary battery. Furthermore, it is possible to increase the effective area for the charge / discharge reaction.

[0035] As a result, the temperature characteristics of the charging / charging capacity are improved, making it possible to realize a fluoride-ion secondary battery that is sufficiently operational even in low-temperature environments. BRIEF DESCRIPTION OF THE FIGURES Fig. Figure 1 is a view showing the structure of a complex fluoride of the present invention; Fig. Figure 2 is a graph showing the ionic conductivity of various fluorine compounds and the solid electrolyte PbSnF4 used in the present invention; Fig. Figure 3 is a schematic drawing of a manufacturing apparatus for the complex fluoride of the present invention; Fig. Figure 4 is an XRD chart of the complex fluoride from Example 9; Fig. Figure 5 shows the charge / discharge curves at each temperature of the complex fluorides of comparison example 1, example 2 and example 3. Fig. Section 6 provides charge / discharge curves at 40°C for the complex fluorides from the examples and comparison examples; Fig.Figure 7 is a graph showing the relationship between the Cu content and the charge / discharge capacity of the complex fluorides in the examples and comparison examples; Fig. Figure 8 is a STEM-HAADF photograph of the complex fluoride from Example 10; Fig. 9 is an EELS mapping of copper in the complex fluoride of Example 10; Fig. Example 10 is an EELS mapping of barium in the complex fluoride of Example 10; and Fig. 11 is an EELS mapping of the complex fluoride from example 10. PREFERRED MODE FOR EXECUTING THE INVENTION

[0036] One embodiment of the invention is explained below. <Positives Elektroden-Aktivmaterial für Fluorid-Ionen-Sekundärbatterie>

[0037] The positive electrode of a fluoride-ion secondary battery must be able to store fluoride ions (F-) during the charging process and release fluoride ions (F-) during the discharging process.

[0038] The positive electrode active material for fluoride-ion secondary batteries of the present invention is a complex fluoride comprising a first component of metal and a second component of a fluorine compound having fluoride ion conductivity.

[0039] The positive electrode active material for conventional fluoride-ion secondary batteries has mainly been a metal with a single composition or metal fluoride.

[0040] On the other hand, the positive electrode active material for fluoride-ion secondary batteries of the present invention is not a single composition and is characterized in that it is a complex fluoride configured from two or more types of components (raw materials).

[0041] The complex fluoride, which is the positive electrode active material for fluoride-ion secondary batteries of the present invention, makes it possible to increase the fluoride ion conductivity of the fluoride-ion secondary battery.

[0042] Furthermore, it is possible to increase the effective area for the charging / discharging reaction.

[0043] As a result, the temperature characteristics of the charging / discharging capacity are improved, making it possible to realize a fluoride-ion secondary battery that is sufficiently operational even in a low temperature environment. (First component (metal))

[0044] The first component of the complex fluoride, which is the positive electrode active material for fluoride-ion secondary batteries of the present invention, is metal.

[0045] For example, it is possible to illustrate silver (Ag), copper (Cu), nickel (Ni), cobalt (Co), lead (Pb), cerium (Ce), manganese (Mn), gold (Au), platinum (Pt), rhodium (Rh), vanadium (V), osmium (Os), ruthenium (Ru), bismuth (Bi), iron (Fe), etc., and it is possible to use one or more of these types in the present invention.

[0046] In the present invention, it is preferably at least one type of these metals selected from the group consisting of copper (Cu), cobalt (Co), silver (Ag) and bismuth (Bi). As long as this is the case, a battery with 0.6 V to 4.0 V of electromotive force can be produced in a single battery. (Second component (fluorine compound))

[0047] The second component of the complex fluoride, which serves as the positive electrode active material for the fluoride ion secondary battery of the present invention, is a fluoride that exhibits fluoride ion conductivity.

[0048] As long as it is a fluorine compound with fluoride ion conductivity, it is not particularly limited.

[0049] In the present invention, due to its particularly high ionic conductivity, it is preferably a type selected from the group consisting of lead fluoride (PbF2), tin fluoride (SnF2), bismuth fluoride (BiF3), lanthanum fluoride (LaF3), cerium fluoride (CeF3), sodium fluoride (NaF), potassium fluoride (KF) and barium fluoride (BaF2).

[0050] In the present invention, it is furthermore preferably at least one type selected from the group consisting of sodium fluoride (NaF), potassium fluoride (KF) and barium fluoride (BaF2).

[0051] As long as this is the case, due to the formation of a compound with ionic conductivity as the product after the charging reaction (i.e., after the fluorination reaction), the subsequent fluorination and defluorination reactions proceed easily, and therefore a large capacity can be achieved. (Contents of the first component)

[0052] In the complex fluoride, which serves as the positive electrode active material for fluoride-ion secondary batteries of the present invention, the content of the above-mentioned first component, based on the total complex fluoride, is preferably 40 to 70 atomic percent.

[0053] As long as the content is in the range of 40 to 70 atomic percent, it is possible to increase, in particular, the charging / discharging capacity of the fluoride-ion secondary battery.

[0054] The content of the first component in relation to the total complex fluoride is particularly preferably 40 to 60 atomic percent and most preferably 50 to 60 atomic percent. (Perovskite fluoride)

[0055] The complex fluoride, which serves as the positive electrode active material for fluoride-ion secondary batteries of the present invention, contains perovskite fluoride.

[0056] Fig. Figure 2 shows the ionic conductivity of various fluorine compounds and the solid electrolyte PbSnF4.

[0057] As in Fig. As shown in Figure 1, perovskite fluoride has an ionic conductivity 1000 to 100,000 times higher than CuF2.

[0058] Since the complex fluoride, which serves as the positive electrode active material for fluoride-ion secondary batteries of the present invention, is a perovskite fluoride, the subsequent fluorination reaction proceeds easily during the charging process.

[0059] As a result, the temperature characteristics of the charging / discharging capacity are improved, and it is possible to obtain a larger capacity at low temperatures, such as not exceeding 100°C.

[0060] For example, in one case, a complex fluoride with copper (Cu) as the first component and potassium fluoride (KF) as the second component becomes a positive electrode active material consisting of Cu-KF complex fluoride; however, the Cu-KF complex fluoride forms a perovskite fluoride, such as KCuF3 or K2CuF4, through the charging reaction (fluorination reaction).

[0061] Similarly, in this case, a complex fluoride with copper (Cu) as the first component and barium fluoride (BaF2) as the second component becomes a positive electrode active material consisting of Cu-BaF2 complex fluoride; however, the active material forms a perovskite fluoride, such as BaCuF4 or Ba2CuF6, from Cu-BaF2 complex nanoparticles through the charging reaction (fluorination reaction). (Domain structure)

[0062] The complex fluoride, which serves as the positive electrode active material for fluoride-ion secondary batteries of the present invention, has a fine domain structure with one domain through the first component and one domain through the second component.

[0063] Fig. Figure 1 is a view showing the structure of the complex fluoride of the present invention.

[0064] As in Fig.As shown in Figure 1, the complex fluoride of the present invention has in a particle a domain 1 of the first component and a domain 2 of the second component.

[0065] By constructing such a fine domain structure, the effective area contributing to the electrode reaction is increased, the temperature characteristics of the charge / discharge capacity are consequently improved, and it is possible to realize a fluoride-ion secondary battery that is sufficiently functional even in a low-temperature environment. (Average particle size)

[0066] The average particle size of the complex fluoride, which serves as the positive electrode active material for fluoride-ion secondary batteries of the present invention, is preferably no more than 35 nm. It is particularly preferably no more than 25 nm and most preferably no more than 20 nm.

[0067] Since the particles are nanoscale, the effective area contributing to the electrode reaction is increased, the temperature characteristics of the charge / discharge capacity are consequently improved, and it is possible to realize a fluoride-ion secondary battery that is sufficiently functional even in low-temperature environments.

[0068] Herein, the average particle size indicates the size of the primary particles of the complex fluoride, which is calculated from the specific area by a constant volume gas adsorption process, and as mentioned above, the complex fluoride of the present invention has the domain of the first component and the domain of the second component in a single particle.

[0069] In other words, the complex fluoride of the present invention has a fine domain in primary particles of nanosize and, depending on the case, these primary particles aggregate to form secondary particles.

[0070] To increase the effective area of ​​the charge / discharge reaction, it is effective to use nano-sized particles as the active material of the battery.

[0071] However, nanoparticles have limitations in their production and in the work process, and generally about 20 nm is considered the limit.

[0072] In the complex fluoride of the present invention, the individual particle itself has a domain structure; therefore, it becomes possible to sufficiently increase the effective area without reducing the average particle size to the limit. <Positive Elektrode für Fluorid-Ionen-Sekundärbatterien>

[0073] The positive electrode for fluoride-ion secondary batteries of the present invention is characterized in that it contains the positive electrode active material for fluoride-ion secondary batteries of the present invention.

[0074] As long as the positive electrode active material for fluoride-ion secondary batteries of the present invention is included, other configurations are not particularly limited.

[0075] To increase the electrochemical reaction efficiency of the fluoride-ion secondary battery, an expansion of the surface of the material from which the positive electrode is made is effective.

[0076] Therefore, the positive electrode for fluoride-ion secondary batteries of the present invention preferably has a structure, e.g. a porous structure, which increases the contact area with the solid electrode as a structure with a large surface area.

[0077] Furthermore, the positive electrode for fluoride-ion secondary batteries of the present invention can contain other components in addition to the positive electrode active material for fluoride-ion secondary batteries of the present invention. Examples of such other components include conductive materials, binders, etc.

[0078] The positive electrode for fluoride-ion secondary batteries of the present invention can be obtained, for example, by applying a mixture of the positive electrode active material for fluoride-ion secondary batteries of the present invention, a conductive agent and a binder to a current collector and then drying it. <Fluorid-Ionen-Sekundärbatterie>

[0079] The fluoride-ion secondary battery of the present invention comprises a positive electrode for fluoride-ion secondary batteries, which contains the positive electrode active material for fluoride-ion secondary batteries of the present invention, a solid electrolyte and a negative electrode.

[0080] The fluoride-ion secondary battery of the present invention is not particularly limited to other configurations, as long as a positive electrode is used which contains the positive electrode active material for fluoride-ion secondary batteries of the present invention.

[0081] By selecting a negative electrode material that provides a sufficiently low standard electrode potential relative to the standard electrode potential of the positive electrode for fluoride-ion secondary batteries, which contains the positive electrode active material for fluoride-ion secondary batteries of the present invention, the characteristic as a fluoride-ion secondary battery is high and thus it is possible to realize the desired battery voltage. <Verfahren zum Herstellen von positivem Elektroden-Aktivmaterial für Fluorid-Ionen-Sekundärbatterien>

[0082] The method for producing the active material of the positive electrode for fluoride-ion secondary batteries of the present invention comprises an aerosol process in which a raw material melt containing a metal serving as the first component and a fluorine compound serving as the second component is sprayed under reduced pressure.

[0083] For example, one method for producing nanoparticles uses mechanical comminution, such as a ball mill, and another uses an aerosol method that condenses the raw material vapor, which has been evaporated by any heat source such as direct heating by an electric furnace, laser irradiation, plasma and flame, through physical cooling or chemical reaction.

[0084] In the present invention, an aerosol method is used from the perspective that the control of the particle size is simple and the mixing of impurities, which leads to a problem in mechanical grinding processes, can be prevented.

[0085] In the process for producing the positive electrode active material for fluoride-ion secondary batteries of the present invention, the required amounts of the first component, which consists of a metal, and the second component, which consists of a fluorine compound, which serve as raw materials for the complex fluoride, are first weighed and a premix is ​​carried out to obtain the mixed powder of the raw material.

[0086] Next, it is preferable to carry out the classification processing on the obtained raw material mixed powder.

[0087] Next, the raw material mixture powder, which undergoes classification processing as required, is melted by thermal plasma or similar to produce a raw material melt, and then the raw material melt is sprayed into a chamber with a negative pressure environment.

[0088] The raw material melt is then processed into nanoparticles by a cooling process, making it possible to obtain complex fluoride particles that serve as the positive electrode active material for fluoride-ion secondary batteries of the present invention. Examples

[0089] Next, examples of the present invention will be explained; however, the present invention is not intended to be limited to these examples. <Beispiele 1 bis 5>

[0090] In examples 1 to 5, Cu-KF complex fluorides were prepared using copper (Cu) as the first component and potassium fluoride (KF) as the second component. (Production of complex fluoride) (Weighing and premixing of raw materials)

[0091] Cu metal powder (average particle size: 1 µm, purity: 99.99%, manufactured by High Purity Chemical Laboratory Co., Ltd.) and potassium fluoride (anhydrous, purity: 99%, manufactured by High Purity Chemical Laboratory Co., Ltd.) were weighed to yield a total of 400 grams in the ratio shown in Table 1 and then premixed for about 1 hour using an agate mortar and pestle to obtain the raw material mixture powder.

[0092] It should be noted that the weighing and premixing of the raw materials was carried out in a glove box (model DBO-1.5BNK-SQ1, manufactured by Miwa Mfg. Co., Ltd.) to prevent moisture absorption of the fluoride and oxidation of the metal particles. (Classification processing)

[0093] The classification process was carried out using a stainless steel mesh (sieve opening: 200 µm to 500 µm) on the obtained raw material mixed powder.

[0094] The classification process was repeated on the raw material mixture powder that did not pass through the sieve after mixing with an agate mortar and pestle, and continued until all the raw material mixture powder passed through the sieve. (Aerosol process)

[0095] Fig.Figure 3 shows a schematic representation of a manufacturing apparatus for the complex fluoride, which was produced using the aerosol method.

[0096] In the examples, the complex fluoride was produced using a high-frequency thermal plasma.

[0097] A direct venting powder filling hopper, filled with raw material mixed powder after classification processing, was removed from the glove box and then connected to a high-frequency induction thermal plasma nanoparticle synthesizer (TP-40020NPS, manufactured by JEOL Ltd.).

[0098] Argon gas was supplied to the plasma torch, the raw material mixture powder was melted by the thermal plasma to produce a raw material molten metal, and the raw material molten metal was sprayed into a chamber with reduced pressure.

[0099] The molten raw material sprayed into the chamber underwent a cooling process to become the Cu-KF complex fluoride, which had become nanoparticle-shaped.

[0100] Next, the Cu-KF complex fluoride was collected with an outlet filter downstream of the device, the upstream and downstream sections of the filter collection were blocked by valves and transported into the glove box, followed by recovery of the complex fluoride nanoparticles, thus obtaining the final Cu-KF complex fluoride. <Beispiele 6 bis 11>

[0101] In examples 6 to 11, Cu-BaF2 complex fluoride was prepared using copper (Cu) as the first component and barium fluoride (BaF2) as the second component. (Weighing raw materials)

[0102] Cu-BaF2 complex fluoride was obtained similarly to that in Examples 1 to 5, except for the use of barium fluoride in the ratio shown in Table 2 (99.9% purity, manufactured by High Purity Chemical Laboratory Co., Ltd.) instead of potassium fluoride. <Vergleichsbeispiel 1>

[0103] Similar operations were performed as in the examples, using only copper (Cu) as the raw material, without using the fluorine compound as the second component to obtain the Cu nanoparticles. <Bewertung des Komplexfluorids > (Elemental composition)

[0104] Elemental analysis was performed using an FE-SEM: SE6600 manufactured by Hitachi High Technology Co., Ltd. and an energy-dispersive X-ray elemental analyzer (EMAX x-act detector, model 067-H) manufactured by HORIBA, Ltd.

[0105] The results are shown in Table 1 and Table 2. (Actual density of the powder)

[0106] The actual density of the powder was calculated using a gas-phase substitution method based on pycnometer theory, employing an automated dry densimeter (AccuPyc II1340) manufactured by Micromeritics Instrument Corp. (Average particle size)

[0107] After measuring the specific surface area with an automatic measuring device for measuring the specific surface area / pore size distribution (BELSORP-mini II, manufactured by Microtrac BEL Corp.), the average particle size was calculated using the formula described below, assuming that the particles are spherical.

[0108] The calculation results are shown in Table 1 and Table 2. d=6000 / (S·ρ)

[0109] (In the formula, 'd' represents the particle size (nm), 'S' the specific surface area (m²)2 / g) and ,ρ' for the actual density of the powder (g / cc), which is obtained as mentioned above). (Crystal structure)

[0110] The crystal structure of Example 9 was analyzed using XRD (“Smartlab”, manufactured by Rigaku Corp., Cu-Kα radiation source, λ= 1.5418 Å).

[0111] The results are in Fig. 4 shown. (Domain structure)

[0112] Under observation of the domain structure, a focused ion beam processing observation device: FIB (FB-2100, manufactured by Hitachi High-Tech Corp.) (closed to the atmosphere, cooled) and a precision ion polishing system (Model 695 (PIPSII), manufactured by “Gatan”) (closed to the atmosphere, cooled) were used as devices for the thin section preparation of samples.

[0113] An imaging image of the Cu-L end and the Ba-M end of the complex fluoride particles, as well as a STEM-HAADF image, were acquired using a transmission scanning electron microscope equipped with a spherical aberration correction function (Cs-(S)TEM (JEM-ARM200F (cold FEG), manufactured by JEOL Ltd.) (closed to the atmosphere, cooled)) and a CCD camera (GIF Quantum-ER (for EELS), manufactured by Gatan) while observing the sample.

[0114] A STEM-HAADF photographer of the complex fluoride of Example 10 is in Fig. 8. The EELS mapping of copper is in Fig. 9, the EELS mapping of barium is in Fig. 10 and the EELS mapping of the entire Cu-BaF2 complex fluoride is in Fig. 11 shown. [Table 1] Raw material ratio (at%) Classification mesh size Obtained composition (at%) Average particle size (nm) Cu KF Cu O K F Comparative example 1 100 0 Zero 77,7 22,3 0 0 27,4 Example 1 75 25 Zero 76,8 19,6 1,5 2,1 31,1 Example 2 66 34 Zero 71 21 2,9 5,1 28,2 Example 3 75 25 500µm 61,8 13,5 8,7 16 19,3 Example 4 50 50 500µm 51,6 10,5 15,6 22,3 15,5 Example 5 50 50 200µm 23,70 12,4 29, 1 34,8 11,2 [Table 2] Raw material ratio (at%) Classification mesh size Obtained composition (at%) Average particle size (nm) Cu BaF2 Cu O Ba F Example 6 50 50 Zero 16,9 6,5 19,1 57,5 18,6 Example 7 66 34 Zero 17,7 11 16,1 55,3 30, 1 Example 8 83 17 Zero 37,6 11,6 14,4 36,4 22,4 Example 9 87 13 Zero 41,9 14 12,3 31,9 28,9 Example 10 90 10 Zero 51,3 9, 1 10,1 29,5 26,4 Example 11 95 5 Zero 70,10 11,4 5,2 13,3 28,3

[0115] The elements comprising the second component were identified and their average particle size was confirmed to be no more than 35 nm in all examples from Table 1 and Table 2.

[0116] Furthermore, it emerged from the in Fig. The XRD chart shown in Example 9 shows that the crystal structure of the Cu metal, which is the first component, and the crystal structure of BaF2, which is the second component, coexist in the complex fluoride nanoparticle. <Herstellung einer Fluorid-Ionen-Sekundärbatterie>

[0117] A fluoride-ion secondary battery was manufactured using the following procedure and materials. (Solid electrolyte)

[0118] PbSnF4 (hereinafter referred to as PSF), which is a solid electrolyte of the fluorite type, was used.

[0119] PbSnF4 is a known compound (Documents 11 to 13) and was produced according to the method disclosed in Document 12. Document 11: Journal of Solid State Chemistry 253(2017)287-293 Document 12: J. Phys. Chem. C121(2017)2627-2634 Document 13: Solid State Ionics 86-88(1996)77-82 (Positive electrode mixture powder)

[0120] The complex fluoride produced in the examples or the Cu nanoparticles of comparison example 1, the solid electrolyte (PSF) to provide a pathway for ionic conductivity and acetylene black (produced by Denki Kagaku Kogyo KK) to provide a pathway for electronic conductivity were mixed sufficiently in a mass ratio of 30:65:5 to produce a positive electrode mixture powder. (Negative electrode)

[0121] Lead foil (99.99% purity, 200 µm thick, manufactured by Nilaco Corp.) was machined to a diameter of 10 mm and used as the negative electrode. (Fluoride-ion secondary battery)

[0122] The positive electrode mixture powder (20 mg), the solid electrolyte (400 mg), and the negative electrode, prepared in the manner described above, were pressed at a pressure of 4 t / cm². 2 integrally cast in a 10 mm diameter mold to obtain a cast product which serves as the fluoride-ion secondary battery.

[0123] Gold wire, used as terminals in charge / discharge measurement, was glued to the positive / negative electrode surfaces of the resulting cast product using carbon paste. <Bewertung der Fluorid-Ionen-Sekundärbatterie>

[0124] Constant current charging / discharging tests were performed in the temperature range of 25°C to 140°C. <Messung der anfänglichen Lade- / Entladekapazität>

[0125] Using a potentiostat / galvanostat device (SI1287 / 1255B, manufactured by Solartron), constant current charge / discharge tests were performed with currents of 0.02 mA charge and 0.01 mA discharge, an upper limit voltage of 1.25 V and a lower limit voltage of 0.40 V.

[0126] To control the ambient temperature during the charge / discharge measurement, the prepared fluoride-ion secondary battery was placed in a constant temperature warm air circulation bath (SU261, manufactured by ESPEC Corp.) and the measurement was performed.

[0127] The charging / discharging curves at each temperature from 25°C to 140°C are shown in Fig. 5 shown.

[0128] Fig.Figure 5(a) is a charge / discharge curve of the fluoride-ion secondary battery produced using Cu nanoparticles of comparison example 1; Fig. 5(b) is the charge / discharge curve of the fluoride-ion secondary battery produced using the complex fluoride from Example 2, and Fig. 5(c) is the battery produced using Example 3.

[0129] Out of Fig. 5 can be confirmed that the production of the positive electrode active material in Examples 2 and 3 with the Cu-KF complex fluoride, which is produced by adding KF as the second component, increased the discharge capacity at all temperatures more than in Example 1. (Charge / discharge curve at 40°C)

[0130] The charging / discharging curves at 40°C for examples 1 to 11 and comparison example 1 are shown in Fig. 6 shown.

[0131] Fig.6(a) provides the charging / discharging curves of comparison example 1 and examples 1 to 5 (Cu-KF complex fluoride), Fig. 6(b) presents the charging / discharging curves of comparison example 1 and examples 8 to 10 (Cu-BaF2 complex fluoride) and Fig. 6(c) provides the charging / discharging curves of examples 6 and 7 (Cu-BaF2 complex fluoride).

[0132] For all examples, it can be confirmed that the charging / discharging capacity has increased more than in comparison example 1. (Relationship between Cu ratio (At%) and charging / discharging capacity)

[0133] Fig. Figure 7 shows the relationship between the ratio (atomic percent: at%) of copper (Cu), which serves as the first component, and the charging / discharging capacity for examples 1 to 11.

[0134] Out of Fig.7. It can be confirmed that the charging / discharging capacity, regardless of the type of the second component, has increased particularly remarkably in the range of 40 to 70 atomic percent (at%) of the content of the first component relative to the total complex fluoride. EXPLANATION OF REFERENCE SYMBOLS 1 Domain of the first component 2 Domain of the second component 3 Raw material feed connection 4 Complex fluoride compound

Claims

[1] Positive electrode active material for fluoride-ion secondary batteries, wherein the positive electrode active material is a complex fluoride, comprising: a first component made of metal; and a second component consisting of a fluorine compound that exhibits fluoride ion conductivity, the complex fluoride contains perovskite fluoride. [2] Positive electrode active material for fluoride-ion secondary batteries according to claim 1, wherein the complex fluoride has a fine domain structure with a domain through the first component and a domain through the second component in a single particle. [3] Positive electrode active material for fluoride-ion secondary batteries according to one of claims 1 or 2, wherein an average particle size of the complex fluoride is not more than 35 nm. [4] Positive electrode active material for fluoride-ion secondary batteries according to any one of claims 1 to 3, wherein the content of the first component is in the range of 40 to 70 atomic percent with respect to the complex fluoride. [5] Positive electrode active material for fluoride-ion secondary batteries according to any one of claims 1 to 4, wherein the metal is at least one type selected from the group consisting of Cu, Co, Ag and Bi. [6] Positive electrode active material for fluoride-ion secondary batteries according to any one of claims 1 to 5, wherein the fluorine compound is at least one type selected from the group consisting of lead fluoride, tin fluoride, bismuth fluoride, lanthanum fluoride, cerium fluoride, sodium fluoride, potassium fluoride and barium fluoride. [7] Positive electrode active material for fluoride-ion secondary batteries according to any one of claims 1 to 5, wherein the fluorine compound is at least one type selected from the group consisting of sodium fluoride, potassium fluoride and barium fluoride. [8] Positive electrode for fluoride-ion secondary batteries, wherein the positive electrode comprises the positive electrode active material for fluoride-ion secondary batteries according to any one of claims 1 to 7. [9] Fluoride-ion secondary batteries comprising the positive electrode for fluoride-ion secondary batteries according to claim 8, a solid electrolyte and a negative electrode. [10] Method for producing the positive electrode active material for fluoride-ion secondary batteries, wherein the positive electrode active material is a complex fluoride, comprising: a first component made of metal; and a second component consisting of a fluorine compound exhibiting fluoride ion conductivity, wherein the method comprises an aerosol process for spraying a raw material melt containing the metal and the fluorine compound under reduced pressure.

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