Negative electrode composite complex for fluoride ion secondary battery, negative electrode for fluoride ion secondary battery using the complex, secondary battery, and method for manufacturing the complex

Abstract

The present invention provides a negative electrode composite complex for a fluoride ion secondary battery, a negative electrode for a fluoride ion secondary battery using the complex, a secondary battery, and a method for manufacturing the complex. In the fluoride ion secondary battery, the negative electrode composite complex for a fluoride ion secondary battery can achieve a fluoride ion secondary battery with high initial charge-discharge efficiency. Aluminum with a nanoparticle size is used as a negative electrode active material and forms a complex together with other components of the negative electrode composite, thereby suppressing the coverage caused by aluminum fluoride formed by the refluorination reaction after defluorination and suppressing the aggregation of particles of the negative electrode active material with each other.

Description

Technical Field

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

[0002] Conventionally, as a secondary battery having a high energy density, a lithium ion secondary battery has been widely popularized. The lithium ion secondary battery has a structure in which a separator is provided between a positive electrode and a negative electrode, and a liquid electrolyte (electrolyte solution) is filled.

[0003] Since the electrolyte solution of the lithium ion secondary battery is usually a flammable organic solvent, there is a problem in particular with respect to thermal safety. In response to this, a solid battery has been proposed, which uses an inorganic solid electrolyte instead of an organic liquid electrolyte (see Patent Document 1).

[0004] As a battery using such a solid electrolyte, a secondary battery using fluoride ions has also been studied (see Patent Document 2). It is known that a fluoride ion secondary battery is a secondary battery using fluoride ions (F - ) as a carrier and has a high theoretical energy. Moreover, it is expected that the battery characteristics of this fluoride ion secondary battery will exceed those of the lithium ion secondary battery.

[0005] Here, as a negative electrode active material for a fluoride ion secondary battery, for example, MgF2, CaF2, CeF3, etc. have been reported (see Non-Patent Documents 1 and 2). However, the charge-discharge efficiency of a fluoride ion secondary battery using these negative electrode active materials is 10 to 20%, and there is a problem of low energy efficiency as a secondary battery. In addition, regarding the charge-discharge capacity, at present, it is only about 10 to 20% of the theoretical capacity, and high capacity has not been achieved compared with existing lithium ion secondary batteries or Ni-MH batteries.

[0006] As a solid electrolyte used in a fluoride ion secondary battery, for example, La 1－x Ba x F 3－x 、x = 0.01 to 0.2 (hereinafter referred to as LBF) can be cited (see Non-Patent Documents 1 to 4). The reduction-side potential window of LBF is as Figure 1 shown, and is restricted by the potential of La / LaF3 calculated from the Gibbs energy, that is, -2.41 V vs. Pb / PbF2.

[0007] In contrast, the potential of the negative electrode active material of the fluoride ion secondary battery reported at present is as Figure 1As shown, the potential of MgF2 is -2.35 to -2.87 V vs. Pb / PbF2, that of CaF2 is -2.85 to -2.89 V vs. Pb / PbF2, and that of CeF3 is -2.18 to -2.37 V vs. Pb / PbF2. Therefore, under the constraint of -2.41 V which is the reduction potential window of LBF, the current situation is that considering this overvoltage, the defluorination / refluorination reaction of the negative electrode active material described above cannot be provided.

[0008] On the other hand, if we focus on the positive electrode reaction, for example, charge-discharge test results showing high utilization or reversible reaction of positive electrode active materials such as Cu / CuF2 and Bi / BiF3 have been reported (refer to Patent Documents 3 to 4 and Non-Patent Documents 1 to 3).

[0009] Therefore, in a fluoride ion secondary battery, in order to achieve a practical full cell reaction that combines the positive and negative electrode reactions, a negative electrode active material material that exhibits a reversible negative electrode reaction with high utilization is required.

[0010] In response to this requirement, in Patent Document 5, a negative electrode active material is proposed. It focuses on aluminum fluoride (AlF3: -1.78 V vs. Pb / PbF2) in which a charge-discharge reaction (defluorination / refluorination reaction) exists under the constraint of the potential window of -2.41 V of LBF which is a fluoride ion solid electrolyte. Furthermore, a part of the fluoride ions (F - ) is pre-detached from the structure of the complete crystal of the 6-coordinate octahedron of aluminum fluoride (AlF3), and aluminum fluoride (AlF3) is modified to create vacancies at the positions where fluorine atoms are present.

[0011] According to the negative electrode active material of Patent Document 5, the vacancies provided at the positions where fluorine atoms are present become the starting points of the defluorination / refluorination reaction, enabling the desired negative electrode reaction to be exhibited with high utilization and reversibility.

[0012] [Prior Art Documents]

[0013] [Patent Documents]

[0014] Patent Document 1: Japanese Patent Application Laid-Open No. 2000-106154

[0015] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-050113

[0016] Patent Document 3: Japanese Patent Application Laid-Open No. 2018-206755

[0017] Patent Document 4: Japanese Patent Application Laid-Open No. 2019-087403

[0018] Patent Document 5: Japanese Patent Application No. 2018-059703

[0019] [Non - Patent Literature]

[0020] Non - Patent Literature 1: J.Mater.Chem.A.2014.2.20861 - 20872

[0021] Non - Patent Literature 2: J.Solid State Electrochem(2017)21:1243 - 1251

[0022] Non - Patent Literature 3: J.Mater.Chem.,2011,21,17059

[0023] Non - Patent Literature 4: Dalton Trans.,2014,43,15771 - 15778 Summary of the Invention

[0024] [Problems to be Solved by the Invention]

[0025] However, regarding the fluoride ion secondary battery using the negative electrode active material proposed in Patent Document 5, the electrochemical efficiency in the first cycle is about 50%, and further improvement is required.

[0026] The present invention has been completed in view of the above - mentioned background art, and an object of the present invention is to provide a negative electrode composite material complex for a fluoride ion secondary battery, a negative electrode for a fluoride ion secondary battery using the complex, a secondary battery, and a manufacturing method of the complex, and the negative electrode composite material complex for a fluoride ion secondary battery can achieve a fluoride ion secondary battery with high initial charge - discharge efficiency in a fluoride ion secondary battery.

[0027] [Technical Means for Solving the Problems]

[0028] The present inventors have diligently studied the reason for the low electrochemical efficiency of the negative electrode active material proposed in Patent Document 5. And it is considered that perhaps an insulating layer is formed by aluminum fluoride formed through the re - fluorination reaction after defluorination covering the surface of the negative electrode active material, thus reducing the reactivity.

[0029] In addition, it is considered that since the negative electrode active material is nanoparticles, the particles aggregate during the initial charge - discharge, and as a result, the electron conduction path and the ion conduction path are not sufficiently formed.

[0030] Furthermore, the inventors have found that if aluminum with a nanoparticle size is used as the negative electrode active material and forms a composite with other components of the negative electrode composite material, it is possible to suppress the coverage caused by aluminum fluoride formed through the refluorination reaction after defluorination, and it is possible to suppress the aggregation of the particles of the negative electrode active material. As a result, a fluoride ion secondary battery with a high initial charge-discharge efficiency can be achieved, and thus the present invention has been completed.

[0031] That is to say, the present invention is a negative electrode composite material complex for a fluoride ion secondary battery, which contains a negative electrode active material and a fluoride ion conductive fluoride, and the negative electrode active material is aluminum.

[0032] The average particle size of the aluminum can be 10 to 200 nm.

[0033] The negative electrode composite material complex for a fluoride ion secondary battery may further contain carbon black.

[0034] The negative electrode composite material complex for a fluoride ion secondary battery may be in the shape of particles.

[0035] The average particle size of the negative electrode composite material complex for a fluoride ion secondary battery can be 0.5 to 10 μm.

[0036] Another aspect of the present invention is a negative electrode for a fluoride ion secondary battery, which contains the negative electrode composite material complex for a fluoride ion secondary battery.

[0037] Another aspect of the present invention is a fluoride ion secondary battery, which includes the negative electrode for a fluoride ion secondary battery, a solid electrolyte, and a positive electrode.

[0038] Another aspect of the present invention is a method for manufacturing a negative electrode composite material complex for a fluoride ion secondary battery, which includes: a mixing step of mixing a negative electrode active material, a fluoride ion conductive fluoride, and carbon black to obtain a negative electrode composite material mixture; and a composite particle forming step of obtaining composite particles by subjecting the negative electrode composite material mixture to a pulverization and mixing treatment to composite the negative electrode active material, the fluoride ion conductive fluoride, and the carbon black; the negative electrode active material is aluminum.

[0039] In the method for manufacturing the negative electrode composite material complex for a fluoride ion secondary battery, the average particle size of the aluminum can be 10 to 200 nm.

[0040] In the method for manufacturing the negative electrode composite material complex for a fluoride ion secondary battery, the pulverization and mixing treatment can be dry pulverization.

[0041] In the method for manufacturing the negative electrode composite material complex for a fluoride ion secondary battery, the pulverization and mixing treatment can be carried out using a ball mill.

[0042] [Effect of the Invention]

[0043] The negative electrode composite material for a fluoride ion secondary battery according to the present invention enables a fluoride ion secondary battery with high initial charge-discharge efficiency. As a result of the battery having high initial charge-discharge efficiency, it is possible to discharge without losing the capacity during charging, and thus the energy density of the battery can be increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a graph showing the potential calculated from the Gibbs energy.

[0045] Figure 2 It is a scanning electron microscope (SEM) photograph of the negative electrode composite material for a fluoride ion secondary battery fabricated in Example 1.

[0046] Figure 3 It is an SEM photograph of the negative electrode composite material for a fluoride ion secondary battery fabricated in Comparative Example 1.

[0047] Figure 4 It is an X-ray diffraction (XRD) pattern of various substances and the negative electrode composite material for a fluoride ion secondary battery of Example 1.

[0048] Figure 5 It is a cross-sectional scanning transmission electron microscope (STEM) photograph of the negative electrode composite material for a fluoride ion secondary battery of Example 1.

[0049] Figure 6A It is Figure 5 An enlarged photograph of the dashed part in

[0050] Figure 6B It is showing Figure 5 A graph of the distribution of the conductive additive obtained by energy dispersive X-ray spectroscope (EDX) analysis of the dashed part in

[0051] Figure 6C It is showing Figure 5 A graph of the distribution of aluminum obtained by EDX analysis of the dashed part in

[0052] Figure 6D It is showing Figure 5 A graph of the distribution of the solid electrolyte obtained by EDX analysis of the dashed part in

[0053] Figure 7 These are diagrams showing the manufacturing methods of fluoride ion secondary batteries in the examples and comparative examples.

[0054] Figure 8 These are cross-sectional views of the fluoride ion secondary batteries manufactured in the examples and comparative examples.

[0055] Figure 9 These are charge-discharge curves of the fluoride ion secondary batteries manufactured in Examples 1 to 3.

[0056] Figure 10 These are charge-discharge curves of the fluoride ion secondary batteries manufactured in Example 1 and Example 4.

[0057] Figure 11 These are charge-discharge curves of the fluoride ion secondary batteries manufactured in Example 1 and Comparative Example 1.

[0058] Figure 12 This is a graph showing the capacity change of the fluoride ion secondary battery manufactured in Example 1. Detailed Implementation Modes

[0059] Hereinafter, the implementation modes of the present invention will be described.

[0060] <Negative Electrode Composite Material Complex for Fluoride Ion Secondary Battery>

[0061] The negative electrode of a fluoride ion secondary battery must be able to accommodate fluoride ions (F - ) during discharge and release fluoride ions (F - ) during charging.

[0062] The negative electrode composite material complex for a fluoride ion secondary battery of the present invention is a complex that includes a negative electrode active material and a fluoride ion conductive fluoride, and the negative electrode active material is aluminum.

[0063] The negative electrode composite material complex for a fluoride ion secondary battery of the present invention only needs to include aluminum as the negative electrode active material and a fluoride ion conductive fluoride as constituent components, and can also be a complex that arbitrarily includes other components.

[0064] In addition, in the negative electrode composite material complex for a fluoride ion secondary battery of the present invention, aluminum as the negative electrode active material forms an alloy with other constituent components of the complex and does not exist in the form of elemental aluminum.

[0065] [Shape of the Complex]

[0066] The shape of the negative electrode composite complex for a fluoride ion secondary battery of the present invention is preferably spherical after granulation. And within each particle, there are aluminum as the negative electrode active material, a fluoride ion-conductive fluoride, and any other components.

[0067] By being spherical after granulation, when pressing the electrode, an electrode that can be filled more gaplessly can be produced, thereby improving the volumetric energy density of the battery.

[0068] By having the constituent components of the complex within each composite particle, an electron conduction path and an ion conduction path for the fluorination / defluorination reaction required for the electrochemical reaction can be formed in nanosize.

[0069] In addition, in order to improve the electrochemical reaction efficiency of the fluoride ion secondary battery, it is effective to increase the surface area of the material constituting the negative electrode. Therefore, when the shape of the negative electrode composite complex is spherical, the negative electrode for the fluoride ion secondary battery that is an aggregate of spherical objects has a large surface area structure. As a result, the contact area with the solid electrolyte contained in the adjacent solid electrolyte layer can be increased.

[0070] (Average particle size)

[0071] The average particle size of the negative electrode composite complex for a fluoride ion secondary battery of the present invention is preferably in the range of 0.5 to 10 μm. Particularly preferably, it is in the range of 1 to 5 μm.

[0072] If the average particle size of the negative electrode composite complex for a fluoride ion secondary battery is within the above range, then during the pulverization and mixing process for obtaining the composite particles, the particles will collide with each other and granulate, thereby firmly bonding to form an electron conduction path and an ion conduction path for the fluorination / defluorination reaction within the micro-sized particles. The particle structure having the electron conduction path and the ion conduction path can follow the volume change caused by the reaction of aluminum as the negative electrode active material, so the structure collapse of the negative electrode layer can be suppressed, and thus the reversibility of the electrochemical reaction can be improved.

[0073] [Negative electrode active material]

[0074] The negative electrode active material of the negative electrode composite complex for a fluoride ion secondary battery of the present invention is aluminum. The potential of aluminum fluoride AlF3 as the fluoride of aluminum is Figure 1 shown as -1.78V vs. Pb / PbF2, and there is a charge-discharge reaction (defluorination / refluorination reaction) under the constraint of the potential window of -2.41V of LBF as the fluoride ion solid electrolyte.

[0075] Therefore, under the constraint of the reduction potential window of LBF, i.e., -2.41 V, even considering this overvoltage, the defluorination / refluorination reaction of aluminum will proceed sufficiently. In addition, since aluminum is an inexpensive material, it is also economically advantageous.

[0076] In addition, an oxide film may also exist on the surface of aluminum.

[0077] (Shape)

[0078] The shape of aluminum as the negative electrode active material is preferably spherical. By being spherical, when pressing the electrode, an electrode that can be filled more gaplessly can be fabricated, thereby improving the volumetric energy density of the battery.

[0079] (Average particle size)

[0080] The average particle size of aluminum is preferably in the range of 10 to 200 nm, and particularly preferably in the range of 40 to 100 nm.

[0081] If the average particle size of aluminum as the negative electrode active material is in the range of 10 to 200 nm, then the negative electrode composite material for the fluoride ion secondary battery obtained becomes a granule close to a true sphere.

[0082] [Fluoride ion conductive fluoride]

[0083] The fluoride ion conductive fluoride, which is an essential constituent of the negative electrode composite material for the fluoride ion secondary battery of the present invention, is not particularly limited as long as it is a fluoride having fluoride ion conductivity. For example, Ce 0.95 Ba 0.05 F 2.95 , Ba 0.6 La 0.4 F 2.4 etc.

[0084] Among these fluorides, in terms of having high ion conductivity, it is preferable to use Ce 0.95 Ba 0.05 F 2.95 .

[0085] (Average particle size)

[0086] The average particle size of the fluoride ion conductive fluoride is preferably in the range of 0.1 to 100 μm, and particularly preferably in the range of 0.1 to 10 μm.

[0087] If the average particle size of the fluoride ion conductive fluoride is in the range of 0.1 to 100 μm, then an electrode having a relatively high ion conductivity and a thin layer can be formed.

[0088] [Other components]

[0089] The negative electrode composite material complex for a fluoride ion secondary battery of the present invention may optionally contain other components in addition to the essential components, namely aluminum as the negative electrode active material and a fluoride ion conductive fluoride. Examples of other components include a conductive aid or a binder.

[0090] (Conductive aid)

[0091] In the negative electrode composite material complex for a fluoride ion secondary battery of the present invention, it is particularly preferable to contain carbon black as a conductive aid. By allowing carbon black to exist within the composite particles, it is possible to easily form an electron conduction path and an ion conduction path for the fluorination / defluorination reaction required for the electrochemical reaction.

[0092] The type of carbon black is not particularly limited, and examples include furnace black, Ketjen black, acetylene black, etc.

[0093] Regarding the average particle diameter of the carbon black, there is no particular limitation, and it is preferably in the range of 20 to 50 nm.

[0094] If the average particle diameter of the carbon black is in the range of 20 to 50 nm, then an electrode having high electron conductivity can be formed with a small weight.

[0095] [Composition]

[0096] (Aluminum)

[0097] With respect to the entire negative electrode composite material complex for a fluoride ion secondary battery of the present invention, the ratio of aluminum in the negative electrode composite material complex for a fluoride ion secondary battery is preferably set to 5 to 25% by mass, and more preferably in the range of 5 to 13% by mass.

[0098] In the negative electrode composite material complex for a fluoride ion secondary battery of the present invention, if the ratio of aluminum is within the above range, then the capacity per unit weight of the obtained fluoride ion secondary battery increases.

[0099] (Fluoride ion conductive fluoride)

[0100] With respect to the entire negative electrode composite material complex for a fluoride ion secondary battery of the present invention, the ratio of the fluoride ion conductive fluoride in the negative electrode composite material complex for a fluoride ion secondary battery is preferably set to 70 to 90% by mass, and more preferably in the range of 80 to 90% by mass.

[0101] In the negative electrode composite material complex for a fluoride ion secondary battery of the present invention, if the ratio of the fluoride ion conductive fluoride is within the above range, then an electrode having high ion conductivity can be formed.

[0102] (Conductive aid)

[0103] When the negative electrode composite material for a fluoride ion secondary battery of the present invention contains a conductive additive, the ratio of the conductive additive is preferably set to 5 to 25% by mass, more preferably in the range of 5 to 10% by mass, based on the entire negative electrode composite material for a fluoride ion secondary battery.

[0104] In the negative electrode composite material for a fluoride ion secondary battery of the present invention, if the ratio of the conductive additive is within the above range, an electrode having high electron conductivity can be formed.

[0105] <Negative electrode for fluoride ion secondary battery>

[0106] The negative electrode for a fluoride ion secondary battery of the present invention is characterized by containing the negative electrode composite material for a fluoride ion secondary battery of the present invention. As long as it contains the negative electrode composite material for a fluoride ion secondary battery of the present invention, other configurations are not particularly limited.

[0107] <Fluoride ion secondary battery>

[0108] The fluoride ion secondary battery of the present invention includes a negative electrode for a fluoride ion secondary battery containing the negative electrode composite material for a fluoride ion secondary battery of the present invention, a solid electrolyte, and a positive electrode. As long as the negative electrode containing the negative electrode composite material for a fluoride ion secondary battery of the present invention is used in the fluoride ion secondary battery of the present invention, other configurations are not particularly limited.

[0109] In the present invention, by selecting the following positive electrode material, the characteristics of the fluoride ion secondary battery are high and the desired battery voltage can be achieved. The positive electrode material can provide a standard electrode potential that is sufficiently high relative to the standard electrode potential of the negative electrode for a fluoride ion secondary battery containing the negative electrode composite material for a fluoride ion secondary battery of the present invention.

[0110] <Method for manufacturing negative electrode composite material for fluoride ion secondary battery>

[0111] The method for manufacturing the negative electrode composite material for a fluoride ion secondary battery of the present invention includes a mixing step and a composite particle forming step.

[0112] [Mixing step]

[0113] The mixing step in the method for manufacturing the negative electrode composite material for a fluoride ion secondary battery of the present invention is a step of mixing a negative electrode active material, a fluoride ion conductive fluoride, and carbon black to obtain a negative electrode composite mixture. In the present invention, the negative electrode active material is aluminum.

[0114] Aluminum as the negative electrode active material, fluoride ion-conductive fluoride, and carbon black as the conductive assistant are the same as above. In addition, as long as aluminum, fluoride ion-conductive fluoride, and carbon black are included as essential components, other substances can be arbitrarily blended.

[0115] The mixing method is not particularly limited, as long as the desired masses of each component are measured and simultaneously or gradually introduced into the same space for mixing. In addition, in the case of gradual introduction, the introduction order is not particularly limited.

[0116] [Composite particle manufacturing process]

[0117] The composite particle manufacturing process is a process of obtaining composite particles by subjecting the negative electrode composite material mixture obtained in the mixing process to a pulverization and mixing treatment to composite the negative electrode active material, fluoride ion-conductive fluoride, and carbon black.

[0118] In the composite particle manufacturing process, the negative electrode active material, fluoride ion-conductive fluoride, and carbon black constituting the negative electrode composite material mixture form an alloy and are granulated.

[0119] Aluminum as the negative electrode active material is a relatively soft material, so during the pulverization and mixing treatment, it is loaded onto the fluoride ion-conductive fluoride, which is a hard material, due to the impact. And it is considered that by being nanoparticles, heat diffusion can occur inside the composite using the heat during the pulverization and mixing treatment, and as a result, the composite can form an alloy and be granulated.

[0120] The pulverization and mixing treatment for forming an alloy and granulating the negative electrode composite material mixture is not particularly limited as long as it is a method capable of pulverizing and mixing the negative electrode composite material mixture in an inert environment.

[0121] The pulverization and mixing treatment can be dry pulverization or wet pulverization, but in terms of the oxide film on the particle surface peeling off during the pulverization and mixing treatment, exposing the active surface, dry pulverization in an inert environment is preferred.

[0122] In the present invention, it is particularly preferred to perform the pulverization and mixing treatment using a ball mill. If it is a ball mill, it is a closed type, so the mixing ratio does not change during pulverization and dispersion, and a stable pulverization and mixing treatment can be performed. Among them, in terms of high pulverization power, fine pulverization can be achieved, and the pulverization time can be shortened, a planetary ball mill is preferred. Regarding the pulverization and mixing conditions when using a ball mill, there is no particular limitation either, for example, set to 400 rpm and 10 hours.

[0123] [Examples]

[0124] Next, examples of the present invention will be described, but the present invention is not limited to these examples.

[0125] <Examples 1 to 4>

[0126] In Examples 1 to 4, aluminum as a negative electrode active material, CeBaF as a fluoride ion-conductive fluoride, and acetylene black as a conductive additive were used to produce a negative electrode composite complex for a fluoride ion secondary battery. 2.95 And a negative electrode composite complex for a fluoride ion secondary battery was produced.

[0127] [Mixing process]

[0128] As shown in Table 1, weigh the nanoparticle-like aluminum, Ce 0.95 Ba 0.05 F 2.95 , and acetylene black shown in Table 1. After weighing, put the aluminum into a silicon nitride ball mill container (manufactured by Fritsh, Germany, PL-7 special container, internal volume: 45 cc), and then put Ce 0.95 Ba 0.05 F 2.95 , and acetylene black. Further, put 40 g of silicon nitride balls with a diameter of 2 mm and seal the ball mill container.

[0129] [Composite particle process]

[0130] The sealed ball mill container was rotated at a speed of 400 rpm for 10 hours to perform a pulverization and mixing treatment, and a negative electrode composite complex for a fluoride ion secondary battery was obtained. After the pulverization and mixing treatment, the treated powder was recovered. The recovery rate is shown in Table 1.

[0131] [Table 1]

[0132]

[0133]

[0134] <Comparative Example 1>

[0135] Modified aluminum fluoride described in Japanese Patent Application No. 2018-059703 was used as the negative electrode active material instead of aluminum, and the rest was the same as in Examples 1 to 4 to obtain a negative electrode composite for a fluoride ion secondary battery.

[0136] The following shows the operation for obtaining the modified aluminum fluoride. In addition, the recovery rate of the obtained negative electrode composite for a fluoride ion secondary battery is shown in Table 1.

[0137] [Modified aluminum fluoride]

[0138] Lithium (Li) metal was used to make aluminum fluoride (AlF3) into modified aluminum fluoride.

[0139] (Weighing and premixing of raw materials)

[0140] Weigh aluminum fluoride (AlF3) and lithium (Li) metal in such a way that the molar ratio of aluminum fluoride to lithium is 90:10 and the total amount becomes 6.0 g. Using an agate mortar and pestle, premix for about 1 hour to obtain a raw material mixed powder.

[0141] In addition, since both aluminum fluoride (AlF3) and lithium (Li) metal have extremely high reactivity with moisture, the weighing and premixing of the raw materials are carried out inside a glove box (manufactured by Miwa Seisakusho Co., Ltd., model DBO-1.5BNK-SQ1).

[0142] <Evaluation of the negative electrode composite complex for fluoride ion secondary batteries>

[0143] For the negative electrode composite complex for fluoride ion secondary batteries and the negative electrode composite for fluoride ion secondary batteries produced in the examples and comparative examples, various observations and evaluations are carried out.

[0144] [SEM Observation]

[0145] For the negative electrode composite complex for fluoride ion secondary batteries produced in Example 1 and the negative electrode composite for fluoride ion secondary batteries produced in Comparative Example 1, secondary electron images are obtained using SEM. Figure 1 The negative electrode composite complex for fluoride ion secondary batteries produced in Example 1 is shown in Figure 2 The SEM photograph of the negative electrode composite for fluoride ion secondary batteries produced in Comparative Example 1 is shown in

[0146] As Figure 1 shown, the negative electrode composite complex for fluoride ion secondary batteries of the present invention becomes spherical particle shape. On the other hand, it can be seen that the negative electrode composite for fluoride ion secondary batteries produced in Comparative Example 1 is not granulated.

[0147] [X-ray Diffraction Pattern]

[0148] Use XRD (manufactured by Rigaku Corporation, SmartLaB, Cu-Kα ray source, ) to analyze the crystal structures of the negative electrode composite complex for fluoride ion secondary batteries produced in Example 1, aluminum (denoted as nano Al) as the negative electrode active material, and Ce 0.95 Ba 0.05 F 2.95 (denoted as CBF) as the fluoride ion conductive fluoride. The XRD diagram is shown in Figure 4 .

[0149] As Figure 4As shown, in the negative electrode composite material for a fluoride ion secondary battery fabricated in Example 1, a single peak of aluminum (nano Al) was not confirmed. Therefore, it can be understood that in the negative electrode composite material for a fluoride ion secondary battery fabricated in Example 1, aluminum exists in an alloyed state.

[0150] [STEM Observation]

[0151] Using a scanning transmission electron microscope (STEM: Scanning Transmission Electron Microscopy, manufactured by JEOL, JEM-ARM200F), a cross-sectional observation was made of the negative electrode composite material for a fluoride ion secondary battery fabricated in Example 1. Figure 5 The cross-sectional STEM photograph of the negative electrode composite material for a fluoride ion secondary battery fabricated in Example 1 is shown in. Additionally, Figure 6A is shown in Figure 5 an enlarged photograph of the dashed-line portion in.

[0152] Figure 5 and Figure 6A in, the portion outside the particles that appears black is a carbon protective film for preventing damage during focused ion beam (FIB) processing. Figure 6A in, the central portion of the particles is white and the outside is black. From this, it can be understood that Ce 0.95 Ba 0.05 F 2.95 as a fluoride ion-conductive fluoride mainly exists in the central portion, and aluminum mainly exists on the outside.

[0153] [EDX Analysis]

[0154] Using an energy dispersive X-ray spectroscope (EDX: Energy Dispersive X-ray Spectroscope), an analysis was made of the dashed-line portion in Figure 5 . Figure 6B is a graph showing the distribution of the conductive aid obtained by EDX analysis of the dashed-line portion in Figure 5 , Figure 6C is a graph showing the distribution of aluminum obtained by EDX analysis of the dashed-line portion in Figure 5 , Figure 6D is a graph showing the distribution of the solid electrolyte obtained by EDX analysis of the dashed-line portion in Figure 5 .

[0155] According to Figure 6B it can be understood that acetylene black as a conductive aid is generally present in the structure, indicating that a good electron conduction path is formed. According to Figure 6C and Figure 6D ​​It can be seen that in some parts of the aluminum, there are portions that appear as blocks of about 100 nm. However, aluminum and fluoride ion-conductive fluorides are generally present in the tissue in a contacting manner, indicating that an ion conduction path has been well formed.

[0156] <Fabrication of a Fluoride Ion Secondary Battery>

[0157] The following materials are used and the following method is employed to fabricate a fluoride ion secondary battery.

[0158] [Negative electrode composite powder]

[0159] The negative electrode composite complex for a fluoride ion secondary battery fabricated in Examples 1 to 4, or the negative electrode composite for a fluoride ion secondary battery fabricated in Comparative Example 1 is used.

[0160] [Solid electrolyte]

[0161] La used as a tysonite-based solid electrolyte 0.95 Ba 0.05 F 2.95 (LBF). LBF is a well-known compound (refer to Non-Patent Documents 5 to 7) and is fabricated using the method described in Document 5.

[0162] Non-Patent Document 5: ACS Appl. Mater. Interfaces 2014, 6, 2103 - 2110

[0163] Non-Patent Document 6: J. Phys. Chem. C 2013, 117, 4943 - 4950

[0164] Non-Patent Document 7: J. Phys. Chem. C 2014, 118, 7117 - 7129

[0165] [Positive electrode composite powder]

[0166] 63.7 mass% of lead fluoride powder (manufactured by Kanto Chemical Co., Inc.), 29.6 mass% of tin fluoride (manufactured by Kanto Chemical Co., Inc.), and 6.7 mass% of acetylene black (manufactured by Denka Co., Ltd.) are mixed using a ball mill and then calcined at 400 °C for 1 hour in an argon atmosphere to obtain the positive electrode composite powder.

[0167] [Fabrication method of a fluoride ion secondary battery]

[0168] Figure 7 The fabrication method of a fluoride ion secondary battery is shown. As Figure 7As shown, using tablet formers (1a and 1b), battery materials 3 are sequentially introduced into the ceramic tube 2 and pressed from above and below at a pressure of 40 MPa, thereby fabricating a pelletized single cell formed by powder pressing. As the battery materials 3, a gold foil (manufactured by Nilaco Corporation, 99.9+%, thickness: 10 μm) as a negative electrode current collector, 10 mg of the negative electrode composite material powder, 200 mg of a solid electrolyte, 30 mg of a positive electrode composite material powder, and a lead foil (manufactured by Nilaco Corporation, purity: 99.99%, thickness: 200 μm) as a positive electrode current collector are sequentially introduced.

[0169] Figure 8 The cross-sectional view of the fabricated fluoride ion secondary battery is shown. As Figure 8 shown, in the fabricated pelletized fluoride ion secondary battery, a positive electrode composite layer 4, a solid electrolyte layer 5, and a negative electrode composite layer 6 are laminated in a state held by the tablet former.

[0170] <Evaluation of Fluoride Ion Secondary Battery>

[0171] [Constant Current Charge-Discharge Test]

[0172] The obtained pelletized fluoride ion secondary battery is heated to 140°C in a vacuum environment to perform an electrochemical reaction (charge-discharge reaction). Specifically, using a potentiostat-galvanostat (Solartron Corporation, SI1287 / 1255B), a constant current charge-discharge test is performed with a charging current of 0.02 mA and a discharging current of 0.01 mA, and a lower limit voltage of -2.35 V and an upper limit voltage of -0.1 V. The charge-discharge curve is shown in Figures 9 to 12 .

[0173] Figure 9 are the charge-discharge curves of the fluoride ion secondary batteries fabricated in Examples 1 to 3 in which the blending amount of aluminum as a negative electrode active material is changed. According to Figure 9 it can be seen that in the negative electrode composite complex for the fluoride ion secondary battery of the present invention, the lower the concentration of the negative electrode active material, the larger the capacity of the obtained fluoride ion secondary battery. It is considered that the reason is that the supply of fluoride ions to the negative electrode active material is the rate-determining factor.

[0174] Figure 10 are the charge-discharge curves of the fluoride ion secondary batteries fabricated in Examples 1 and 4 in which the average particle size of aluminum as a negative electrode active material is changed. According to Figure 10It can be seen that in the negative electrode composite material for a fluoride ion secondary battery of the present invention, the smaller the average particle size of the negative electrode active material, the larger the capacity of the obtained fluoride ion secondary battery. It is considered that the reason is that the movement of fluoride ions inside the negative electrode active material becomes a factor determining the reaction rate of the charge-discharge reaction (defluorination / refluorination reaction).

[0175] Figure 11 are the charge-discharge curves of the first cycle and the second cycle of the fluoride ion secondary batteries fabricated in Example 1 and Comparative Example 1. According to Figure 11 It can be seen that the reversibility of the electrochemical reaction of the fluoride ion secondary battery using the negative electrode composite material for a fluoride ion secondary battery of the present invention is improved.

[0176] Figure 12 is a graph showing the capacity change when the fluoride ion secondary battery fabricated in Example 1 is repeatedly charged and discharged 9 times. Figure 12 On the vertical axis of, the ratio of the discharge capacity change represents the ratio of the discharge capacity in each cycle to the discharge capacity in the first cycle. Figure 12 On the vertical axis of, the ratio of the Coulomb efficiency change represents the ratio of the discharge capacity to the charge capacity in each cycle. According to Figure 12 It can be seen that the fluoride ion secondary battery using the negative electrode composite material for a fluoride ion secondary battery of the present invention still shows a high Coulomb efficiency after 2 cycles.

[0177] Reference numeral

[0178] 1a, 1b: Tablet former

[0179] 2: Ceramic tube

[0180] 3: Battery material

[0181] 4: Positive electrode composite layer

[0182] 5: Solid electrolyte layer

[0183] 6: Negative electrode composite layer.

Claims

1. A negative electrode composite material for a fluoride ion secondary battery, which comprises a negative electrode active material and a fluoride ion conductive fluoride. The negative electrode active material is aluminum. By diffusing the aluminum in the negative electrode composite material for the fluoride ion secondary battery, an alloy is formed in the composite material.

2. The negative electrode composite material for a fluoride ion secondary battery according to claim 1, wherein the average particle size of the aluminum is 10 to 200 nm.

3. The negative electrode composite material for a fluoride ion secondary battery according to claim 1, which further comprises carbon black.

4. The negative electrode composite material for a fluoride ion secondary battery according to claim 1, which is in the shape of particles.

5. The negative electrode composite material for a fluoride ion secondary battery according to claim 4, wherein the average particle size is 0.5 to 10 μm.

6. A negative electrode for a fluoride ion secondary battery, which comprises the negative electrode composite material for a fluoride ion secondary battery according to claim 1.

7. A fluoride ion secondary battery, which comprises the negative electrode for a fluoride ion secondary battery according to claim 6, a solid electrolyte, and a positive electrode.

8. A method for manufacturing a negative electrode composite material for a fluoride ion secondary battery, which comprises: A mixing step of mixing a negative electrode active material, a fluoride ion conductive fluoride, and carbon black to obtain a negative electrode composite mixture; and A composite particle forming step of obtaining composite particles by subjecting the negative electrode composite mixture to a pulverization and mixing treatment to composite the negative electrode active material, the fluoride ion conductive fluoride, and the carbon black; The negative electrode active material is aluminum, and the average particle size of the aluminum is 10 to 200 nm.

9. The method for manufacturing a negative electrode composite material for a fluoride ion secondary battery according to claim 8, wherein the pulverization and mixing treatment is dry pulverization.

10. The method for manufacturing a negative electrode composite material for a fluoride ion secondary battery according to claim 8, wherein the pulverization and mixing treatment is performed using a ball mill.

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