Positive electrode active material for fluoride ion batteries, fluoride ion batteries, and methods for manufacturing the same.

Abstract

To provide a positive electrode active material for a fluoride ion battery, which is lightweight, has a high utilization rate of an active material, and can be easily produced.SOLUTION: A positive electrode active material for a fluoride ion battery is an alloy including an intermetallic compound of a rare earth element of La, Ce, or combinations thereof, and a transition metal of Co, Ni, Cu, or combinations thereof.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a cathode active material for a fluoride ion battery, a fluoride ion battery, and methods for manufacturing them.

Background Art

[0002] [[ID=eleven]] A fluoride ion battery can be expected to have an energy density of 1000 Wh / kg <00001​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Fluoride-ion batteries charge and discharge via fluorination and defluorination reactions of metal active material particles used in the positive and negative electrodes. Conventionally, pure metals such as transition metals have been used as the positive electrode active material in fluoride-ion batteries. However, because the fluoride ion conductivity of the metal fluorides formed on the surface of the active material particles during the charging (fluorination) process is low, the fluorination reaction does not proceed to the interior of the positive electrode active material particles. Therefore, the utilization rate of pure metal positive electrode active material (hereinafter also referred to as utilization rate or active material utilization rate) is low, at a few percent or less.

[0006] For example, in a fluoride-ion battery using Cu as the positive electrode active material and Ce as the negative electrode active material, the positive electrode active material becomes CuF2 when charged. As discharge occurs, the positive electrode active material becomes Cu, and the negative electrode becomes CeF3. In the active materials of fluoride-ion batteries, intercalation like that in lithium-ion batteries does not occur, and the crystal structure of the active material changes with the inflow and outflow of fluorine.

[0007] Figure 1 shows schematic cross-sectional diagrams of a pure metal positive electrode active material used in a conventional fluoride-ion battery, both before and after charging. As schematically shown in Figure 1, the positive electrode active material fluorinates during charging of a fluoride-ion battery, but the fluorination stops near the surface of the positive electrode active material, at a thickness of approximately 10 nm, and does not penetrate to the interior of the positive electrode active material. For example, when Cu is used as the positive electrode active material, the Cu fluorinates during charging, forming a layer of CuF2 with a thickness of approximately 4.9 nm on the surface of the Cu, and the fluorination does not proceed further. This is because the fluoride ion conductivity and electronic conductivity of the fluoride layer such as CuF2 are low, which inhibits the exchange of fluoride ions and electrons in the positive electrode active material.

[0008] During charging and discharging, the exchange of fluoride ions and electrons in the active material is essential. However, if a passivation layer forms on the surface of the active material, the fluorination reaction does not proceed into the active material, resulting in a low utilization rate of the active material.

[0009] To improve the utilization rate of active materials, core-shell structured active materials with particle sizes of 50 nm or less have been proposed (Patent Document 1, Non-Patent Document 1). However, these methods have the drawbacks of complex synthesis methods for the nanoparticle-based core-shell structure and the significant influence of surface oxidation due to the small size of the active material particles.

[0010] To improve the utilization rate of the active material, it has been proposed to use thin-film electrodes with a thickness of about 2.3 nm (Non-Patent Literature 2). However, this method has the challenge of requiring highly precise control of sputtering conditions in order to fabricate extremely thin films.

[0011] Furthermore, it has been proposed to use BiF3 as the active material in order to improve the utilization rate of the active material (Non-Patent Literature 3). Although Bi has relatively good fluoride ion conductivity, it has the problem that its atomic weight is large, about 209u, which results in a large mass for the fluoride ion battery.

[0012] Therefore, there is a need for a material that is lightweight, has a high utilization rate, and can be easily manufactured as the positive electrode active material for fluoride-ion batteries. [Means for solving the problem]

[0013] The gist of this invention is as follows: (1) A positive electrode active material for a fluoride ion battery, which is an alloy containing an intermetallic compound of a rare earth element La, Ce, or a combination thereof with a transition metal Co, Ni, Cu, or a combination thereof. (2) The positive electrode active material described in (1) above, wherein the intermetallic compound has a composition represented by AB5 (wherein A is a rare earth element of La, Ce, or a combination thereof, and B is a transition metal of Co, Ni, Cu, or a combination thereof). (3) The positive electrode active material according to (1) or (2) above, wherein the alloy comprises a network of fluorides of the rare earth element, a network of the transition metal, a network of fluorides of the transition metal, or a network of a combination thereof. (4) A positive electrode active material for a fluoride ion battery, which is an alloy comprising a network of fluorides of rare earth elements La, Ce, or combinations thereof, a network of transition metals Co, Ni, Cu, or combinations thereof, a network of fluorides of the transition metals, or a network of combinations thereof. (5) A fluoride ion battery containing the positive electrode active material described in any of (1) to (4) above. (6) Dissolve a rare earth element such as La, Ce, or a combination thereof, and a transition metal such as Co, Ni, Cu, or a combination thereof to obtain an alloy containing an intermetallic compound between the rare earth element and the transition metal. A method for producing a positive electrode active material for a fluoride-ion battery, including the above. (7) The manufacturing method according to (6) above, comprising solution treatment of the alloy. (8) The method of production according to (6) or (7) above, wherein the intermetallic compound has a composition represented by AB5 (wherein A is a rare earth element of La, Ce, or a combination thereof, and B is a transition metal of Co, Ni, Cu, or a combination thereof). (9) The alloy is subjected to a fluoride treatment to decompose at least a portion of the intermetallic compound to generate a network of fluorides of the rare earth element and a network of the transition metal, a network of fluorides of the transition metal, or a network of a combination thereof. The manufacturing method described in any of (6) to (8) above. (10) A method for producing a fluoride ion battery, comprising obtaining a laminate comprising a negative electrode current collector, a negative electrode layer, a fluorine-based electrolyte layer, a positive electrode layer containing a positive electrode active material obtained by the manufacturing method described in any of (6) to (9) above, and a positive electrode current collector. [Effects of the Invention]

[0014] According to this disclosure, it is possible to provide a positive electrode active material for fluoride-ion batteries that is lightweight, has a high utilization rate of the active material, and can be easily manufactured. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a schematic cross-sectional view of a conventional positive electrode active material used in fluoride-ion batteries, both before and after charging. [Figure 2] Figure 2 is a schematic cross-sectional view of the vicinity of the surface of the positive electrode active material particles before and during the initial stages of charging. [Figure 3] Figure 3 is a schematic diagram of an all-solid-state fluoride-ion battery containing the positive electrode active material of this disclosure as the positive electrode. [Figure 4] Figure 4 shows the charge-discharge curve of the fabricated all-solid-state fluoride-ion battery. [Figure 5] Figure 5 shows an annular dark-field scanning transmission electron microscope (ADF-STEM) image of the cathode active material in the very early stages of fluorination. [Figure 6] Figure 6 shows an ADF-STEM image of the positive electrode active material particles during the initial stages of charging. [Figure 7] Figure 7 shows an ADF-STEM image of the positive electrode active material particles during the later stages of charging. [Figure 8] Figure 8 shows the results of electron energy loss spectroscopy (EELS) analysis of the La content of the positive electrode active material before charging, during the initial charging phase, during the later stages of charging, and during the later stages of discharge. [Figure 9] Figure 9 shows the EELS analysis results for the Ni content of the positive electrode active material before charging, during the initial charging phase, during the later stages of charging, and during the later stages of discharge. [Figure 10] Figure 10 shows the EELS maps (two-dimensional maps) of LaF3, NiF2, and Ni obtained in combination with ADF-STEM images. [Figure 11] Figure 11 shows the results of a simulation regarding the formation of the internal network of positive electrode active material particles during the initial stages of charging. [Figure 12] Figure 12 shows a comparison between the two-dimensional maps of LaF3 and Ni obtained from simulations (calculated) and the EELS maps of LaF3 and Ni obtained in combination with ADF-STEM (experimental). [Figure 13] Figure 13 shows the measurement results for the first to fifth charge-discharge cycles of the fluoride-ion battery fabricated in Example 1. [Figure 14]Figure 14 shows the measurement results for the first to fifth charge-discharge cycles of the fluoride-ion battery fabricated in Example 2. [Modes for carrying out the invention]

[0016] This disclosure relates to positive electrode active materials for fluoride-ion batteries, which are alloys comprising intermetallic compounds of rare earth elements La, Ce, or combinations thereof with transition metals Co, Ni, Cu, or combinations thereof.

[0017] The positive electrode active material of this disclosure is an alloy containing an intermetallic compound in which a transition metal (transition element) that is not easily fluorinated is added to a rare earth element (rare earth metal) that is easily fluorinated. The positive electrode active material of this disclosure is lighter than BiF3 because it is composed of elements lighter than Bi, can be easily manufactured by melting alone, and has excellent active material utilization rate when used as a positive electrode active material for fluoride ion batteries.

[0018] The utilization rate of the active material can be calculated as the ratio of the actual charge capacity to the theoretical charge capacity, or the ratio of the actual discharge capacity to the theoretical discharge capacity. The actual charge capacity and actual discharge capacity can be determined by measuring the charge-discharge curve of the battery containing the positive electrode active material. The utilization rate of the positive electrode active material of this disclosure during the first charge-discharge is preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. The utilization rate of the positive electrode active material of this disclosure during the fifth charge-discharge is preferably 10% or more, more preferably 20% or more, and even more preferably 30% or more.

[0019] Co, Ni, and Cu are transition metals that form a passive film on their surface and are not easily fluorinated, while La and Ce are typical negative electrode materials in fluoride batteries and are easily fluorinated. Transition metals that are not easily fluorinated have low fluorine storage capacity, while rare earth elements that are easily fluorinated have high fluorine storage capacity. Intermetallic compounds can be obtained from these transition metals and rare earth metals by dissolution treatment.

[0020] As a result of diligent research, the inventors have discovered that alloys containing intermetallic compounds, which combine two or more elements—a transition metal that is difficult to fluoride with a rare earth element that is easily fluorinated—exhibit a different structure and properties from pure metals conventionally used as positive electrode active materials in fluoride-ion batteries, and can effectively absorb and release fluoride ions. By utilizing two or more elements with different responses to fluorine, the utilization rate of the positive electrode active material can be improved compared to conventional methods.

[0021] The intermetallic compound in this positive electrode active material is an alloy in which rare earth elements with high fluorine storage capacity and transition metals with low fluorine storage capacity are regularly arranged at the atomic level. By making the positive electrode active material an alloy containing the above intermetallic compound, two or more elements with different responses to fluorine can be utilized more effectively, and the utilization rate of the positive electrode active material can be improved compared to conventional methods. The intermetallic compound naturally includes those with slightly different composition ratios of two or more elements, or those with slightly disordered atomic arrangements. This is because the effects of the present invention are obtained when rare earth elements with high fluorine storage capacity and transition metals with low fluorine storage capacity are present in the alloy in a certain degree of proximity.

[0022] The intermetallic compound preferably has a composition represented by AB5 (wherein A is a rare earth element of La, Ce, or a combination thereof, and B is a transition metal of Co, Ni, Cu, or a combination thereof).

[0023] Alloys with the AB5 composition are used as hydrogen storage alloys, but it has also been found to be useful as positive electrode active materials for fluoride-ion batteries. The atomic arrangement structure of the AB5 composition allows for more effective utilization of rare earth elements and transition metals with different responses to fluorine, thereby improving the utilization rate of the positive electrode active material.

[0024] The atomic weight of AB5, an alloying active material, is 68.9 to 77.6, which is significantly lighter than Bi, which has an atomic weight of approximately 209 u. For example, the atomic weight of LaNi5 is approximately 72 u.

[0025] Preferably, the alloy includes a network of fluorides of rare earth elements, a network of transition metals, a network of fluorides of transition metals, or a network of combinations thereof.

[0026] In the initial stages of the first charge (first charge) of a fluoride-ion battery containing the positive electrode active material of this disclosure, the intermetallic compounds of the positive electrode active material decompose into nanocrystals of rare earth element fluorides and nanocrystals of transition metals. As a result, in the initial stages of the first charge, the rare earth elements in the positive electrode active material are fluorinated, forming nanoscale networks of rare earth element fluorides and nanoscale networks of transition metals. The formation of the above nanoscale microstructure in the positive electrode active material greatly improves the utilization rate of the active material.

[0027] A nanoscale network refers to a network in which the width of the network perpendicular to the longitudinal direction of the network, consisting of fluorides of rare earth elements, transition metals, or fluorides of transition metals, is preferably 20 nm or less, more preferably 10 nm or less, and even more preferably 5 nm or less. The lower limit of the width is not particularly limited as long as the network is connected, but for example, it may be 1 nm or more, 2 nm or more, or 3 nm or more.

[0028] Figure 2 shows schematic cross-sectional diagrams of the vicinity of the surface of the positive electrode active material particles before charging and at the initial stage of the first charge. The positive electrode active material 10 is in contact with the fluorine-based electrolyte 20. At the initial stage of the first charge, the fluorination reaction of rare earth elements proceeds from the surface side of the positive electrode active material particles. Along with this fluorination reaction, a network of rare earth element fluorides 12 and a nanoscale network of transition metals 14 are formed from the surface side of the positive electrode active material particles, changing the atomic arrangement from before charging.

[0029] In the initial stage of the first charge, a nanoscale network 12 of rare earth element fluorides and a nanoscale network 14 of transition metals exist on the surface side of the positive electrode active material particles, while a regular atomic arrangement of rare earth elements and transition metals from before charge may exist on the interior side of the positive electrode active material particles. That is, the nanoscale microstructure obtained in this process is characterized by a network of rare earth fluorides and refinement of transition metals. The spontaneous formation of this microstructure during the first charge (fluorination) process is the origin of the high utilization rate of the positive electrode active material of this disclosure.

[0030] Further charging of a fluoride-ion battery containing the positive electrode active material of this disclosure in the positive electrode allows the transition metals forming the nanoscale network to fluoride, thereby forming a nanoscale network of transition metal fluorides.

[0031] As charging progresses from the early to the later stages, the networks of rare earth element fluorides and the nanoscale networks of transition metals further extend into the interior of the positive electrode active material particles, and the transition metals constituting the nanoscale networks already formed on the surface side become fluorinated, forming a nanoscale network of transition metal fluorides. Although not bound by theory, the transition metals contained in the alloy constituting the positive electrode active material of this disclosure are not easily fluorinated on their own, but it is thought that the transition metals constituting the nanoscale network formed in combination with the nanoscale network of rare earth element fluorides become more easily fluorinated because fluorine is supplied from the adjacent rare earth element fluorides which have fluoride ion conductivity. The network of rare earth element fluorides exhibits fluoride ion conductivity, and the network of transition metals exhibits electron conductivity.

[0032] As the charge is followed by discharge, the fluorides of the transition metals constituting the nanoscale network are defluorinated, and a new nanoscale network of transition metals is formed. Between the initial, late, and late stages of charging, rare earth elements form fluorides, and transition metals undergo both fluorination and defluorination. Therefore, depending on the charging and discharging state, fluorinated and defluorinated transition metals can coexist. Consequently, the positive electrode active material may include a network of transition metals, a network of transition metal fluorides, or a combination thereof, in addition to a network of rare earth element fluorides.

[0033] As described above, the positive electrode active material of this disclosure spontaneously forms a microstructure of rare earth element fluorides, transition metals, transition metal fluorides, or combinations thereof within the positive electrode active material particles during charging and discharging. The network of rare earth fluorides and the refinement of the transition metals formed within the positive electrode active material particles allow fluoride ions to diffuse into the interior of the positive electrode active material particles, improving the utilization rate of the active material. The transition metals constituting the nanoscale network that has developed within the active material particles are fluorinated during charging and defluorinated during discharging, resulting in excellent utilization rate of the active material.

[0034] Therefore, the positive electrode active material of this disclosure does not require the formation of a microstructure such as a core-shell structure or a thin film with a thickness of several nanometers when fabricating the positive electrode of a fluoride-ion battery, and even when incorporated into the positive electrode of a fluoride-ion battery with a particle size of several hundred nanometers or more, it enables charge and discharge with excellent utilization efficiency.

[0035] To illustrate with an example where the positive electrode active material is an alloy containing the composition of AB5, in the initial stages of charging, rare earth element A and transition metal B are as follows: AB5+5F - →AF3+5B+3e - The reaction occurs, and AB5 splits into the rare earth element fluoride AF3 and the transition metal B.

[0036] Next, by the late stages of charging, transition metal B is as follows: B+2F- → BF2 + 2e - The reaction of occurs, and the transition metal B is fluorinated.

[0037] That is, from the initial stage to the later stage of charging, the rare earth element A and the transition metal B are as follows: AB5 + 13F - → AF3 + 5BF2 + 13e - The reaction of proceeds, and the rare earth element A and the transition metal B are phase-separated and their respective fluorination reactions occur.

[0038] Thus, at the initial stage of charging, the rare earth element A is first fluorinated to form a nanoscale network of the fluoride of the rare earth element A and a nanoscale network of the transition metal B. When charging is further continued, the transition metal B is also fluorinated to form a nanoscale network of the fluoride of the transition metal B.

[0039] Next, until the later stage of discharging, the transition metal B is as follows: BF2 + 2e - → B + 2F - The reaction of occurs, and BF2 returns to B. The fluoride of the transition metal formed in the later stage of charging is defluorinated to form a nanoscale network of the transition metal again. Furthermore, when charging and discharging are repeated, the nanoscale network of the fluoride of the rare earth element and the nanoscale network of the transition metal, the fluoride of the transition metal, or a combination thereof further progresses to the interior of the active material particles respectively.

[0040] When the positive electrode active material of the present disclosure is repeatedly charged and discharged, finally, the atomic arrangement before charging decreases or substantially disappears, and the atomic arrangement before charging that has decreased or substantially disappeared changes to a network of the fluoride of the rare earth element and a network of the transition metal, a network of the fluoride of the transition metal, or a combination thereof.

[0041] Accordingly, the disclosure also relates to a positive electrode active material for a fluoride-ion battery, which is an alloy comprising a network of fluorides of rare earth elements La, Ce, or combinations thereof, and a network of transition metals Co, Ni, Cu, or combinations thereof, a network of fluorides of the transition metals, or a network of combinations thereof.

[0042] The positive electrode active material is preferably an alloy consisting of a network of fluorides of rare earth elements such as La, Ce, or a combination thereof, a network of transition metals such as Co, Ni, Cu, or a combination thereof, a network of fluorides of the transition metals, or a network of combinations thereof.

[0043] The above-mentioned principles apply to the configuration of alloys, networks of fluorides of rare earth elements, networks of transition metals, networks of fluorides of transition metals, or networks of combinations thereof.

[0044] The disclosure also relates to positive electrode active materials for fluoride-ion batteries, which are at least one of the following: (A) alloys comprising an intermetallic compound of a rare earth element La, Ce, or a combination thereof with a transition metal Co, Ni, Cu, or a combination thereof; and (B) alloys comprising a network of fluorides of a rare earth element La, Ce, or a combination thereof with a network of transition metals Co, Ni, Cu, or a combination thereof, a network of fluorides of the transition metal, or a network of combinations thereof.

[0045] The positive electrode active material of this disclosure before the preparation of the positive electrode composite material may be relatively coarse-grained, and its volume-average particle size (hereinafter also referred to as D50) can be preferably 1 to 50 μm, more preferably 2 to 30 μm, and even more preferably 3 to 10 μm. When preparing a positive electrode composite material using the positive electrode active material of this disclosure, the positive electrode active material, electrolyte, conductive additive, etc. can be mixed and crushed and / or pulverized, so the volume-average particle size (hereinafter also referred to as D50) of the positive electrode active material of this disclosure in the positive electrode composite material is preferably 10 to 0.05 μm, more preferably 5 to 0.1 μm, and even more preferably 1 to 0.5 μm. The positive electrode active material of this disclosure exhibits good charge-discharge characteristics even when incorporated into the positive electrode with the above preferred particle size, but it also exhibits good charge-discharge characteristics even when further refined and incorporated into the positive electrode.

[0046] A fluoride-ion battery can be obtained using the positive electrode active material of this disclosure. Since the positive electrode active material of this disclosure has good ionic conductivity and electronic conductivity, a fluoride-ion battery using the positive electrode active material of this disclosure has good properties. The fluoride-ion battery is preferably a fluoride-ion secondary battery. The fluoride-ion battery comprises a negative electrode current collector, a negative electrode layer, a fluorine-based electrolyte layer, a positive electrode layer, and a positive electrode current collector.

[0047] The positive electrode layer is a layer containing the positive electrode active material of this disclosure. The positive electrode layer may contain other positive electrode active materials in addition to the positive electrode active material of this disclosure. The positive electrode active material layer may contain other components in addition to the positive electrode active material. The positive electrode active material layer may further contain at least one of a conductive additive, an electrolyte, and a binder in addition to the positive electrode active material.

[0048] The conductive additive is not particularly limited as long as it has the desired electronic conductivity, but examples include carbon materials. Examples of carbon materials include carbon black such as acetylene black, Ketjen black, furnace black, and thermal black, as well as graphene, fullerene, carbon nanofibers, or carbon nanotubes. The lower limit of the proportion of the conductive additive in the positive electrode active material layer may be, for example, 1% by weight or more, or 5% by weight or more, and a better electronic conduction path can be formed within this range. The upper limit of the proportion of the conductive additive in the positive electrode active material layer may be, for example, 20% by weight or less, or 15% by weight or less, and by setting it within this range, the proportion of active material in the positive electrode active material can be ensured, and a better energy density can be obtained.

[0049] The binder is not particularly limited as long as it is chemically and electrically stable, but examples of fluorine-based binders include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0050] The electrolyte can be one that has been conventionally used in fluoride-ion batteries, and preferably, an electrolyte used in a fluorine-based electrolyte layer can be used.

[0051] The content of the positive electrode active material in the positive electrode layer is, for example, 30% by weight or more, 50% by weight or more, or 70% by weight or more. The thickness of the positive electrode layer can be, for example, 0.1 μm to 1000 μm, 1 to 100 μm, or 20 to 40 μm.

[0052] The negative electrode layer is a layer containing a negative electrode active material. The negative electrode active material can be an active material conventionally used in fluoride ion batteries, such as PbF2, MgF2, or CeF3. The negative electrode active material layer may also contain other components in addition to the negative electrode active material. In addition to the negative electrode active material, the negative electrode active material layer may further contain at least one of the following: a conductive additive, an electrolyte, and a binder.

[0053] The content of the negative electrode active material in the negative electrode layer is, for example, 30% by weight or more, 50% by weight or more, or 70% by weight or more. The thickness of the negative electrode layer can be, for example, 0.1 μm to 1000 μm, 1 to 100 μm, or 20 to 50 μm.

[0054] The fluorine-based electrolyte layer is a layer containing a fluorine-based electrolyte formed between the positive electrode layer and the negative electrode layer. The fluorine-based electrolyte can be an electrolyte conventionally used in fluoride ion batteries, such as Ce 0.9 La 0.05 Sr 0.05 F 2.95 , La 0.9 Sr 0.1 F 2.9、 Ba 0.6 La 0.4 F 2.4 The electrolyte constituting the fluorine-based electrolyte layer may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a polymer electrolyte. The thickness of the fluorine-based electrolyte layer can be, for example, 10 to 1000 μm, 50 to 500 μm, or 100 to 200 μm.

[0055] The positive electrode current collector and the negative electrode current collector can be current collectors conventionally used in fluoride-ion batteries, and can be foil-shaped, mesh-shaped, or porous.

[0056] The disclosure also relates to a method for producing a positive electrode active material for a fluoride-ion battery, comprising dissolving a rare earth element, La, Ce, or a combination thereof, and a transition metal, Co, Ni, Cu, or a combination thereof, to obtain an alloy containing an intermetallic compound between the rare earth element and the transition metal.

[0057] The positive electrode active material of this disclosure can be easily prepared by dissolving a transition metal that is difficult to fluoride and a rare earth element that is easily fluorinated.

[0058] Rare earth elements such as La, Ce, or combinations thereof can be used in any form, including granular or lump forms. While higher purity is preferable for La, Ce, or combinations thereof, mischmetal is also acceptable.

[0059] Co, Ni, Cu, or combinations thereof of transition metals can be used in any shape, such as granular or lump form. While high purity is preferable for Co, Ni, Cu, or combinations thereof of transition metals, they may contain impurities.

[0060] The melting temperature is preferably 1500 to 2000°C. The melting time is preferably 1 to 30 minutes, more preferably 3 to 15 minutes, and even more preferably 5 to 10 minutes. The melting atmosphere can be a vacuum or an inert gas atmosphere. The melting method is preferably vacuum melting, arc melting, or plasma melting, and more preferably arc melting. An alloy can be produced by arc melting two or more metals together.

[0061] Preferably, the alloy is subjected to solution treatment. By performing solution treatment on the alloy obtained by the melting process, microsegregation in the alloy can be removed. The solution treatment temperature can be, for example, 600 to 1200°C, depending on the alloy composition. The solution treatment time can be 100 to 500 hours. To prevent oxidation of the alloy, the heat treatment atmosphere can be a vacuum or an inert gas atmosphere. For example, the alloy can be sealed in quartz and subjected to solution treatment in a vacuum atmosphere.

[0062] The intermetallic compound preferably has a composition represented by AB5 (wherein A is a rare earth element such as La, Ce, or a combination thereof, and B is a transition metal such as Co, Ni, Cu, or a combination thereof). The atomic arrangement structure having the AB5 composition allows for more effective utilization of rare earth elements and transition metals with different responses to fluorine, thereby improving the utilization rate of the positive electrode active material.

[0063] Preferably, the alloy is subjected to fluoride treatment to decompose at least a portion of the intermetallic compounds, thereby generating a network of rare earth element fluorides, a network of transition metals, a network of transition metal fluorides, or a network of combinations thereof. By decomposing a portion of the intermetallic compounds by fluoride treatment of the alloy, the alloy may contain, in addition to the remaining intermetallic compounds, a network of rare earth element fluorides, a network of transition metals, a network of transition metal fluorides, or a network of combinations thereof. By decomposing all of the intermetallic compounds by fluoride treatment of the alloy, the intermetallic compounds disappear, and the alloy may contain a network of rare earth element fluorides, a network of transition metals, a network of transition metal fluorides, or a network of combinations thereof.

[0064] Fluorination treatment includes charging, discharging, or repeatedly doing so, a battery constructed by arranging a positive electrode active material adjacent to a fluorine-based electrolyte, or chemically fluorinating the positive electrode active material.

[0065] By charging a battery constructed with a positive electrode active material positioned adjacent to a fluorine-based electrolyte, fluoride ions are supplied from the fluorine-based electrolyte, allowing fluoridation to be performed from the surface of the positive electrode active material in contact with the fluorine-based electrolyte toward the interior. During charging, intermetallic compounds separate into rare earth element fluorides and transition metals (transition elements), and as charging continues, the transition metals become fluorinated. Upon discharge, the fluorinated transition metals are defluorinated. Upon recharging, the separation of rare earth element fluorides and transition metals further progresses within the particles of the positive electrode active material, and as charging continues, the transition metals become fluorinated. As described above, the positive electrode active material can be fluorinated by charging, discharging, or repeating these processes.

[0066] The positive electrode active material may be fluorinated by chemical fluorination treatment without forming a battery. Chemical fluorination treatment can be carried out by immersing the positive electrode active material in a fluorinating agent. The fluorinating agent can be, for example, bis(2-methoxyethyl)aminosulfur trifluoride, diethylaminosulfur=trifluoride, or bis(2-methoxyethyl)aminosulfur trifluoride diluted with acetonitrile solution.

[0067] The positive electrode active material disclosed herein is for use in fluoride-ion batteries, but can be manufactured and sold in any form, such as as a standalone positive electrode active material, as a positive electrode composite material, as a positive electrode layer, or incorporated into a fluoride-ion battery. The positive electrode active material disclosed herein can be sold either before or after fluorination treatment. The positive electrode active material disclosed herein can be incorporated into a battery, charged, discharged, or repeatedly subjected to these processes, and then removed from the battery and sold in any form, such as positive electrode active material, positive electrode composite material, or positive electrode layer. The positive electrode active material disclosed herein can also be sold after being chemically fluorinated.

[0068] This disclosure also relates to a method for manufacturing a fluoride-ion battery, comprising obtaining a laminate comprising a negative electrode current collector, a negative electrode layer, a fluorine-based electrolyte layer, a positive electrode layer containing a positive electrode active material obtained by the above manufacturing method, and a positive electrode current collector.

[0069] The configuration of the negative electrode current collector, negative electrode layer, fluorine-based electrolyte layer, and positive electrode current collector can be adapted to the above-described method. The laminate including the negative electrode current collector, negative electrode layer, fluorine-based electrolyte layer, positive electrode layer, and positive electrode current collector can be manufactured by any method, such as pressure molding. For example, the laminate of the negative electrode layer, fluorine-based electrolyte layer, and positive electrode layer can be manufactured by any method, such as pressure molding, and then the current collector can be placed. When the laminate is pressure-molded, the pressure of the pressure molding can be 10 to 600 MPa. The positive electrode current collector and negative electrode current collector can be formed by any method, such as lamination, coating, or vapor deposition. [Examples]

[0070] (Example 1) (Fabrication of positive electrode active material) A LaNi5 alloy was prepared by mixing 4.98 g of granular La (99.99% purity, D50=8 mm) and 10.52 g of granular Ni (99.99% purity, D50=4 mm) and arc melting them in an Ar atmosphere at 1500°C for 10 minutes using an arc melting apparatus.

[0071] The prepared alloy was vacuum-sealed in quartz and held at 1100°C for 336 hours to perform a solution treatment, thereby removing microsegregation in the alloy. The solution-treated LaNi5 alloy was lightly crushed in an alumina mortar to prepare LaNi5 powder with a D50 of 5 μm.

[0072] (Preparation of solid electrolytes) CeF3 powder, LaF3 powder, and SrF2 powder (manufactured by Koshu Chemical) were mixed in a molar ratio of 0.9:0.05:0.05. Mechanical milling was performed using a planetary ball mill at a rotation speed of 600 rpm for 12 hours. The powder after mechanical milling was calcined at 900°C to obtain a fluorine-based solid electrolyte. 0.9 La 0.05 Sr 0.05 F 2.95 We prepared it.

[0073] (Preparation of positive electrode composite material) Prepared LaNi5 powder and Ce 0.9 La 0.05 Sr 0.05 F 2.95 These were used as the positive electrode active material and the fluorine-based solid electrolyte, respectively. 0.003 g of LaNi5 was used as the positive electrode active material, and 0.03 g of Ce was used as the fluorine-based solid electrolyte. 0.9 La 0.05 Sr 0.05 F 2.95 A cathode composite material was prepared by mixing 0.03 g of carbon nanofiber (VGCF®, fiber diameter 150 nm) as a conductive additive and mechanically milling it at 100 rpm for 10 hours using a ball mill. The D50 of the cathode active material in the cathode composite material was 0.5 μm. The D50 of the cathode active material was measured using a STEM at a magnification of 30,000 to 40,000 times, and the size was 26 to 47 μm. 2This is the average value obtained by acquiring five images of the field of view and measuring the particle size of 17 positive electrode active materials.

[0074] (Preparation of negative electrode composite material) A negative electrode composite was prepared by mixing 0.95 g of PbF2 (high-purity chemical product, 99.9% purity) as the negative electrode active material and 0.05 g of acetylene black (D50=48 nm) as a conductive additive, and mechanically milling the mixture using a ball mill at 600 rpm for 3 hours.

[0075] (Battery construction) The electrolyte used in the electrolyte layer is a prepared fluorine-based solid electrolyte called Ce 0.9 La 0.05 Sr 0.05 F 2.95 Using this method, the prepared negative electrode mixture, fluorine-based solid electrolyte, and positive electrode mixture were pressure-molded at 392 MPa to produce a pellet with a diameter of 11 mm, comprising a 30 μm thick positive electrode layer, a 150 μm thick fluorine-based solid electrolyte layer, and a 35 μm thick negative electrode layer.

[0076] A solid-state fluoride ion battery was fabricated by placing a 20 μm thick Pt foil on the positive electrode layer as the positive electrode current collector, and a 200 μm thick Pb foil and a 15 μm thick Al foil on the negative electrode layer as the negative electrode current collector. Figure 3 shows a schematic diagram of the solid-state fluoride ion battery comprising the fabricated negative electrode current collector 70, negative electrode layer 50, fluorine-based solid electrolyte layer 40, positive electrode layer 30, and positive electrode current collector 60.

[0077] (Measurement of charge / discharge characteristics) The charge-discharge characteristics of the fabricated battery were evaluated using a VMP-300 multipotentiostat (BioLogic) at 140°C in a vacuum, under galvanostat mode. The current density was 17 mAg. -1 The specific capacity was calculated from the mass of LaNi5. Figure 4 shows the charge-discharge curve for the first cycle. During charging, see below: LaNi5+13F - →LaF3+5NiF2+13e - The reaction proceeded, and the fluorination reaction of the rare earth element La and the transition metal Ni occurred. The theoretical charging capacity is 805.8 mAhg. -1 In contrast, the actual charging capacity is 701mAhg -1 The active material utilization rate was 87%.

[0078] Regarding discharge, see below: NiF2+2e - →Ni+2F - The reaction proceeded, and the defluorination reaction of the transition metal Ni occurred. The theoretical discharge capacity was 619.9 mAhg. -1 In contrast, the actual discharge capacity is 396mAhg -1 The active material utilization rate was 64%. The theoretical capacity was 274 mAhg. -1 It showed superior characteristics compared to lithium-ion batteries.

[0079] (Tissue observation) The positive electrode active material in different charge states—Stage I (initial charging), Stage II (late charging), and Stage III (late discharging)—as shown in Figure 4 was subjected to structural analysis using an annular dark-field scanning transmission electron microscope (ADF-STEM, ARM300CF, manufactured by JEOL Ltd.).

[0080] In addition to observing stages I, II, and III, and in order to perform structural analysis of the very first stage of fluorination while avoiding surface damage by ball milling, the cathode active material was fluorinated using the following chemical method, and structural analysis was performed by ADF-STEM. 0.1 grams of LaNi5 powder, lightly crushed in an agate mortar, was immersed for 1 hour under an Ar atmosphere in 50 ml of acetonitrile solution of bis(2-methoxyethyl)aminosulfur trifluoride, a fluoridating agent, at a concentration of 1.4 mol / L. Then, it was washed three times with acetonitrile to obtain LaNi5 powder for evaluating the very first stage of fluorination.

[0081] Figure 5 shows an ADF-STEM image of LaNi5 powder in the initial stages of fluorination using a chemical method. A regular atomic arrangement was observed on the interior side of the positive electrode active material particles, but on the surface side of the positive electrode active material particles, no regular atomic arrangement was observed, the crystal structure was disordered, and nanoscale fluctuations in contrast, indicated by the dashed circles, were observed.

[0082] The lower left of Figure 5 shows a magnified ADF-STEM image of the interior of the positive electrode active material particle. Inside the positive electrode active material particle, La and Ni were regularly arranged at the atomic level. The lower right of Figure 5 shows a magnified ADF-STEM image of the area indicated by the arrow on the surface of the positive electrode active material particle. On the surface of the positive electrode active material particle, there was a region where Ni nanocrystals were present.

[0083] This indicates that the fluorination reaction proceeds from the surface side of the positive electrode active material particles, disrupting the crystal structure before charging.

[0084] Figure 6 shows the ADF-STEM image and Fourier power spectrum of the positive electrode active material particles in Stage I (initial charging). As shown in Figure 6(a), in the initial charging stage, no long-range periodic lattice contrast was observed, and medium-range ordered (MRO) lattice fringes of several nanometers were seen in both bright and dark contrast regions. These MRO lattice fringes were also confirmed by the Fourier power spectrum, as shown in Figure 6(b). Detailed observation of the MRO lattice fringes revealed the presence of two types of nanocrystals: tisonite-structured LaF3 ([1-103] projection) and face-centered cubic (fcc)-structured Ni (

[0110] projection), as shown in Figures 6(c) and (d). The "-1" is a notation where a horizontal line is normally placed above the number.

[0085] Therefore, in Stage I, La is fluorinated individually, and LaNi5 is decomposed into nanocrystals of LaF3 and Ni. In other words, the positive electrode active material particles have clear crystalline properties before charging, and Ni and La are mixed at the atomic level. When charging (fluorination) proceeds from this state, the easily fluorinated La is preferentially fluorinated, and along with microcrystallization, it is found to separate into Ni and lanthanum fluoride.

[0086] To explain using a reaction equation, in the initial stage of charging, the following occurs: LaNi5+5F - →LaF3+5Ni+3e - It was found that the following reaction occurred, causing LaNi5 to split into LaF3 and Ni.

[0087] Figure 7 shows the ADF-STEM image and Fourier power spectrum of the positive electrode active material particles in Stage II (late charging stage). As shown in Figures 7(c) and (d), in addition to LaF3 nanocrystals, NiF2 (projected by

[0001] ) nanocrystals were also observed, indicating that in Stage II, the Ni nanocrystals were fluorinated into NiF2 nanocrystals. In other words, the crystal structures of LaF3 and NiF2 were observed in the positive electrode active material particles in the late charging stage.

[0088] To explain using a reaction equation, in the later stages of charging, No+2F - →NiF2+2e - It was found that the reaction occurs, Ni is fluorinated, and phases LaF3 and NiF2 are formed.

[0089] During discharge, NiF2+2e - →Ni+2F - It was found that the reaction occurred, and NiF2 was converted back to Ni.

[0090] Ni is inherently difficult to fluoride, but it is thought that its fluorination becomes easier when it forms microcrystals. When LaNi5 separates into LaF3 and Ni during charging, Ni is dispersed and formed as microcrystals of the size shown by the dashed line, which is thought to make it easier to fluoride.

[0091] (Electron Energy Loss Spectroscopy (EELS) Analysis) Figure 8 shows the EELS analysis results for the La-containing components of the positive electrode active material before charging, at stage I (initial charging), stage II (late charging), and stage III (late discharge). Figure 8 shows the FK and La-M obtained at fluoridation stages I, II, and III of LaNi5. 4,5 , and Ni-L 2,3 This is the edge EEL spectrum. Before charging, a peak based on LaNi5 was observed, and in the early stages of charging, a peak based on LaF3 (where LaNi5 decomposes and La becomes fluorinated) was observed. In the later stages of charging and discharging, similar peaks based on LaF3 were observed.

[0092] Figure 9 shows the EELS analysis results for the Ni content of the positive electrode active material before charging, at stage I (initial charging), stage II (late charging), and stage III (late discharge). Figure 9 shows the FK and La-M obtained at fluoridation stages I, II, and III of LaNi5. 4,5 , and Ni-L 2,3 This is the edge EEL spectrum. Before charging, a peak based on LaNi5 is observed, and in Stage I (initial charging), a peak based on Ni generated from the decomposition of LaNi5 is observed.

[0093] In Stage I (initial charging), the Ni-L2 edge has a broad profile without FK edges, indicating that the Ni nanocrystal state is metallic. In Stage II (later charging), Ni-L 2,3 The edges become very sharp, indicating that the Ni has been oxidized (fluorinated). In Stage II, a strong FK edge was also observed, confirming that the Ni nanocrystals had been fluorinated to become NiF2.

[0094] In Stage III (late discharge), the FK edge completely disappears, and Ni-L 2,3 The edge profile broadened again, indicating the recovery of the metallic properties of Ni. Therefore, it was found that the discharge was caused by the defluorination of NiF2 to Ni nanocrystals. The EEL spectra in Figures 8 and 9 were obtained using a Quantum spectrometer (Gatan) mounted on an ARM300CF microscope.

[0095] Figure 10 shows the EELS maps (two-dimensional maps) of LaF3, NiF2, and Ni obtained from the same region as the ADF-STEM image. As shown by the dashed circles, the La-rich and Ni-rich regions with diameters of approximately 1-5 nm are well spatially separated at the nanoscale. It was found that LaF3 and Ni in the early stages of charging, and NiF2 appearing in the later stages of charging, are not isolated but are distributed in a network-like manner. Since LaF3 exhibits fluoride ion conduction and Ni exhibits electron conduction, it was found that these form a nanoscale network from the surface to the interior of the positive electrode active material particles, thereby creating pathways for fluoride ion conduction and electron conduction from the surface to the interior of the positive electrode active material particles. In other words, it was shown that fluoride ions can be absorbed from the surface to the interior of the positive electrode active material.

[0096] (Simulation of network formation) Monte Carlo simulations using a voxel evolution system were performed to investigate the formation of a network within the positive electrode active material particles during the initial stages of charging, specifically for the case where the positive electrode active material is an alloy with a LaNi5 composition. Based on ADF-STEM experimental results, the following (i) to (iii) were used as simulation conditions: (i) Fluoridation of LaNi5 begins at the surface (interface between the fluorinated electrolyte and LaNi5); (ii) LaNi5 is decomposed into LaF3 nanocrystals and Ni nanocrystals by fluoridation; (iii) Since LaF3 can supply fluoride ions through its network, fluoridation occurs at the fluorinated electrolyte or at the interface between LaF3 and LaNi5. That is, since LaF3 is fluoride ion conductive, LaNi5 in contact with LaF3 is fluorinated and decomposes into LaF3 and Ni.

[0097] Figure 11 shows the results of a simulation of the formation of the internal network of positive electrode active material particles during the initial stages of charging. The initial state was set by assigning the voxel with x=0 to the fluorine-based electrolyte and the other voxels to LaNi5. Figure 11(a) shows the initial state of the simulation, where the gray voxels represent the fluorine-based electrolyte and the transparent voxels represent LaNi5.

[0098] In the first step of charging, one LaNi5 voxel adjacent to the fluorinated electrolyte is randomly selected, and another LaNi5 voxel adjacent to the selected voxel is also randomly selected. As shown in Figure 11(b), these two LaNi5 voxels are transformed into LaF3 nanocrystals and Ni nanocrystals. In the next step, a LaNi5 voxel adjacent to either the fluorinated electrolyte or a LaF3 voxel is randomly selected and converted into LaF3 and Ni voxels. By repeating this fluorination process, fluorination of LaNi5 progresses from the surface to the interior, as shown in Figures 11(c) to (e). The simulation was carried out until there were no more deformable LaNi5 voxels left.

[0099] As LaNi5 fluorinated from the surface into the interior, LaF3 forms a three-dimensional labyrinthine network. New LaF3 voxels are generated by connecting to previously generated LaF3 voxels.

[0100] Two-dimensional maps of LaF3 and Ni were calculated from the three-dimensional map in Figure 11(e) obtained from the simulation. Figure 12 shows a comparison between the two-dimensional maps of LaF3 and Ni obtained from the simulation (calculated) and the EELS maps of LaF3 and Ni obtained in combination with ADF-STEM (experimental).

[0101] The two-dimensional map obtained from the simulation and the EELS map obtained in combination with ADF-STEM showed very similar distributions. Therefore, the simulation model was validated in which, as fluorination progresses from the surface of the active material particle, LaNi5 decomposes into LaF3 and Ni, and the areas in contact with LaF3, which has fluoride ion conductivity, are fluorinated and decompose into LaF3 and Ni, and the formation of a LaF3 and Ni network progresses from the surface to the interior of the active material particle.

[0102] The charge-discharge cycles of the fabricated all-solid-state fluoride-ion battery were evaluated. Figure 13 shows the measurement results for the first to fifth charge-discharge cycles. The first charge capacity was 701 mAhg. -1 (Utilization rate 87%), and the first discharge capacity is 396mAhg -1 (Utilization rate was 64%). The fifth charging capacity was 139mAhg. -1 (Utilization rate 17%), and the fifth discharge capacity is 127mAhg -1 (The utilization rate was 20%).

[0103] (Example 2) The positive electrode active material for LaCo5 was prepared in the same manner as in Example 1, except that Co was used instead of Ni. Using the prepared LaCo5, an all-solid-state fluoride ion battery was fabricated in the same manner as in Example 1, and the charge-discharge characteristics of the first to fifth charge-discharge cycles were evaluated. Figure 14 shows the measurement results for the first to fifth charge-discharge cycles. Theoretical capacity of the first charge: 803.6 mAhg -1And the theoretical capacity after the first discharge is 618.2 mAhg -1 In contrast, the first charging capacity is 454mAhg -1 (Utilization rate 56%), and the first discharge capacity is 301mAhg -1 (Utilization rate was 49%). The fifth charging capacity was 180mAhg -1 (Utilization rate 29%), and the fifth discharge capacity is 174mAhg -1 (Utilization rate was 28%). LaCo5 showed a lower first charge / discharge capacity compared to LaNi5, but demonstrated a good fifth discharge capacity. [Explanation of Symbols]

[0104] 10 Cathode active material 12. Network of fluorides of rare earth elements 14. Network of transition metals 20 Fluorine-based electrolytes 30 positive electrode 40 Fluorine-based electrolyte layer 50 Negative electrode layer 60 Positive electrode current collector 70 Negative electrode current collector

Claims

1. A positive electrode active material for a fluoride ion battery, which is an alloy containing an intermetallic compound of a rare earth element La, Ce, or a combination thereof, and a transition metal Co, Ni, Cu, or a combination thereof, The alloy is capable of forming a network of fluorides of the rare earth element, a network of the transition metal, a network of fluorides of the transition metal, or a network of combinations thereof by fluorination treatment. The fluorination treatment includes charging, discharging, or repeatedly charging a battery configured by arranging the positive electrode active material adjacent to a fluorine-based electrolyte. Positive electrode active material for fluoride-ion batteries.

2. The intermetallic compound is AB ５ The positive electrode active material according to claim 1, having a composition represented by (wherein A is a rare earth element of La, Ce, or a combination thereof, and B is a transition metal of Co, Ni, Cu, or a combination thereof).

3. The positive electrode active material according to claim 1 or 2, wherein the alloy comprises a network of fluorides of the rare earth element, a network of transition metals, a network of fluorides of the transition metal, or a network of combinations thereof.

4. A positive electrode active material for a fluoride-ion battery, comprising an alloy containing a network of fluorides of rare earth elements La, Ce, or combinations thereof, and a network of transition metals Co, Ni, Cu, or combinations thereof, a network of fluorides of the transition metals, or a network of combinations thereof.

5. A fluoride ion battery comprising the positive electrode active material according to any one of claims 1 to 4.

6. A method for producing a positive electrode active material for a fluoride ion battery, comprising dissolving a rare earth element La, Ce, or a combination thereof with a transition metal Co, Ni, Cu, or a combination thereof to obtain an alloy containing an intermetallic compound between the rare earth element and the transition metal, The alloy is capable of forming a network of fluorides of the rare earth element, a network of the transition metal, a network of fluorides of the transition metal, or a network of combinations thereof by fluorination treatment. The fluorination treatment includes charging, discharging, or repeatedly charging a battery configured by arranging the positive electrode active material adjacent to a fluorine-based electrolyte. A method for producing positive electrode active material for fluoride-ion batteries.

7. The manufacturing method according to claim 6, comprising solution treatment of the alloy.

8. The intermetallic compound is AB ５ The manufacturing method according to claim 6 or 7, having a composition represented by (wherein A is a rare earth element of La, Ce, or a combination thereof, and B is a transition metal of Co, Ni, Cu, or a combination thereof).

9. The process includes subjecting the alloy to the fluoride treatment to decompose at least a portion of the intermetallic compound, thereby generating a network of fluorides of the rare earth element, a network of the transition metal, a network of fluorides of the transition metal, or a network of combinations thereof. The manufacturing method according to any one of claims 6 to 8.

10. A method for manufacturing a fluoride ion battery, comprising obtaining a laminate comprising a negative electrode current collector, a negative electrode layer, a fluorine-based electrolyte layer, a positive electrode layer containing a positive electrode active material obtained by the manufacturing method described in any one of claims 6 to 9, and a positive electrode current collector.

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