Negative electrode for fluoride ion secondary battery and fluoride ion secondary battery having the same

Abstract

The problem to be solved by the present invention is to provide a fluoride ion secondary battery having a larger battery capacity than before. To solve the above problem, a negative electrode for a fluoride ion secondary battery is provided, wherein the negative electrode for a fluoride ion secondary battery contains a complex of Li3AlF6 and AlF3 as the negative electrode active material. The molar ratio of AlF3 to Li3AlF6 in the complex of Li3AlF6 and AlF3 is preferably 0.1 to 2, the content of the complex of Li3AlF6 and AlF3 in the negative electrode for a fluoride ion secondary battery is preferably less than 25% by mass, and the complex of Li3AlF6 and AlF3 is preferably amorphous.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a fluoride ion secondary battery and a fluoride ion secondary battery including the negative electrode. Background Art

[0002] Currently, fluoride ion secondary batteries using fluoride ions as carriers have been proposed (for example, see Patent Documents 1 to 6). In recent years, fluoride ion secondary batteries are expected to have battery properties superior to those of lithium ion secondary batteries, and various studies have been conducted.

[0003] For example, aluminum-based materials are listed as candidates for negative electrode active materials for fluoride ion secondary batteries. Among them, the use of aluminum fluoride has been studied, but aluminum fluoride has electrical insulating properties, making it difficult to undergo electrochemical reactions.

[0004] [Prior Art Literature]

[0005] (Patent Document)

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2019-87403

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-50113

[0008] Patent Document 3: Japanese Patent Application Laid-Open No. 2019-29206

[0009] Patent Document 4: Japanese Patent Application Laid-Open No. 2018-206755

[0010] Patent Document 5: Japanese Patent Application Laid-Open No. 2018-198130

[0011] Patent Document 6: Japanese Patent Application Laid-Open No. 2018-92863 Summary of the Invention

[0012] [Problems to be solved by the invention]

[0013] Therefore, the applicant has achieved a fluoride ion secondary battery using modified AlF3, formed by doping aluminum fluoride with lithium metal, as the negative electrode active material. However, there is a need to further improve battery performance. In particular, because modified AlF3, formed by doping aluminum fluoride with lithium metal, does not have good ion conductivity, it is impossible to increase the concentration of the negative electrode active material in the negative electrode, making it difficult to increase the battery capacity.

[0014] The present invention has been made in view of the above, and an object of the present invention is to provide a fluoride ion secondary battery having a larger battery capacity than conventional batteries.

[0015] [Technical means to solve the problem]

[0016] (1) The present invention provides a negative electrode for a fluoride ion secondary battery, which includes a negative electrode active material, wherein the negative electrode active material includes a complex of Li3AlF6 and AlF3.

[0017] (2) Alternatively, in the negative electrode for a fluoride ion secondary battery of (1), the molar ratio of AlF3 to Li3AlF6 in the composite of Li3AlF6 and AlF3 is 0.1 to 2.

[0018] (3) In the negative electrode for a fluoride ion secondary battery of (1) or (2), the content of the composite of Li3AlF6 and AlF3 in the negative electrode for a fluoride ion secondary battery is 25% by mass or less.

[0019] (4) Optionally, in the negative electrode for a fluoride ion secondary battery according to any one of (1) to (3), the composite of Li3AlF6 and AlF3 is amorphous.

[0020] (5) The present invention also provides a fluoride ion secondary battery comprising the negative electrode for a fluoride ion secondary battery according to any one of (1) to (4).

[0021] (Effects of the Invention)

[0022] According to the present invention, it is possible to provide a fluoride ion secondary battery having a larger battery capacity than conventional batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a diagram illustrating a method for synthesizing a composite of Li3AlF6 and AlF3 as a negative electrode active material according to one embodiment of the present invention.

[0024] Figure 2 This is an X-ray diffraction spectrum of a complex of Li3AlF6 and AlF3 as the negative electrode active material in the above embodiment.

[0025] Figure 3 This figure shows the characteristics of the composite of Li3AlF6 and AlF3 and the existing modified AlF3.

[0026] Figure 4 This is a diagram illustrating an example of a method for producing a negative electrode for a fluoride ion secondary battery according to one embodiment of the present invention.

[0027] Figure 5 This is a diagram illustrating an example of a conventional method for producing a negative electrode for a fluoride ion secondary battery.

[0028] Figure 6FIG. 1 is a graph showing an NMR spectrum of a complex of Li 3 AlF 6 and AlF 3 as the negative electrode active material in the above embodiment.

[0029] Figure 7 Graphs showing charge and discharge curves of negative electrode half-cells for fluoride ion secondary batteries of Examples 1-2, Reference Example 1, and Comparative Example 1. DETAILED DESCRIPTION

[0030] Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0031] [Negative electrode for fluoride ion secondary battery]

[0032] The negative electrode for a fluoride ion secondary battery of this embodiment contains a complex of Li₃AlF₆ and AlF₃ as the negative electrode active material. To date, no negative electrode for a fluoride ion secondary battery containing a complex of Li₃AlF₆ and AlF₃ has been discovered. However, the negative electrode for a fluoride ion secondary battery of this embodiment is characterized by containing a complex of Li₃AlF₆ and AlF₃.

[0033] The complex of Li3AlF6 and AlF3 acts as a negative electrode active material during charge and discharge. Specifically, the complex of Li3AlF6 and AlF3 releases fluoride ions F during charge. - , absorbs fluoride ions F during discharge - .

[0034] The Li3AlF6 and AlF3 complex of this embodiment is a complex of Li3AlF6 and AlF3 within a single particle. In this complex, the ion-conductive Li3AlF6 acts as a fluorine source and a catalyst that promotes the defluorination of AlF3, which is generally difficult to defluorinate.

[0035] The molar ratio of AlF3 to Li3AlF6 in the Li3AlF6 / AlF3 complex is preferably 0.1 to 2. That is, the complex is preferably constructed so that Li3AlF6:AlF3 coexists in a molar ratio of 1 mol:0.1 mol to 1 mol:2 mol. If the molar ratio of AlF3 to 1 mol of Li3AlF6 is less than 0.1 mol, the defluorination of AlF3 described above cannot be effectively promoted. On the other hand, if the molar ratio of AlF3 to 1 mol of Li3AlF6 exceeds 2 mol, the presence of a large amount of insulating AlF3 reduces ion conductivity, making it impossible to function as a battery.

[0036] Next, for the synthesis method of the composite of Li3AlF6 and AlF3, refer to Figure 1 Provide explanation.

[0037] Figure 1 : is a diagram illustrating a method for synthesizing a composite of Li3AlF6 and AlF3 as a negative electrode active material according to this embodiment. Figure 1 As shown, a mixture of LiF and AlF₃ at a predetermined ratio is sintered to synthesize the Li₃AlF₆ and AlF₃ composite of this embodiment. Next, the mixture is subjected to a ball milling process, for example, at 400 rpm for 15 minutes for 40 cycles, followed by a sintering process, for example, at 900°C for 3 hours. After sintering, the mixture is pulverized to obtain the Li₃AlF₆ and AlF₃ composite serving as the negative electrode active material of this embodiment.

[0038] The sintering temperature is preferably between 850°C and 900°C. This is because the melting point of the raw material, LiF, is 850°C. Therefore, if the sintering temperature is within this range, the molten LiF and AlF3 are uniformly mixed. However, if the sintering temperature exceeds 900°C, the weight after sintering begins to decrease significantly, and the raw materials evaporate, which is not preferred.

[0039] Furthermore, when the sintering temperature is 850°C to 900°C, the sintering time is preferably within the range of 2 to 3 hours. If the sintering time is less than 2 hours, the reaction between LiF and AlF3 is insufficient, which is not preferred. If the sintering time exceeds 3 hours, the raw materials evaporate, resulting in a decrease in yield, which is also not preferred.

[0040] The pulverization after the sintering process can be performed in, for example, an agate mortar, and the pulverized particles are fine particles. These fine particles are further pulverized in a ball mill during the preparation of the negative electrode mixture powder described below.

[0041] Here, the mixing ratio of LiF and AlF₃ is preferably within the range of LiF:AlF₃ = 1:1 to 3:1.1 on a molar basis. By mixing and sintering the two within this range, a composite having a molar ratio of AlF₃ to Li₃AlF₆ of 0.1 to 2 is obtained, as described above. If the ratio of LiF is less than 1 mol per 1 mol of AlF₃, a large amount of insulating AlF₃ remains, reducing ion conductivity. On the other hand, if the ratio of LiF exceeds 3 mol per 1.1 mol of AlF₃, a large amount of insulating LiF remains, reducing ion conductivity.

[0042] Furthermore, when LiF:AlF3=3:1 is mixed and sintered, Li3AlF6 can be synthesized, which can function as a negative electrode active material. However, in this case, the use of lithium may increase costs. However, according to this embodiment, the amount of LiF used can be reduced, so it can be said to be preferable from a cost perspective.

[0043] Figure 2 This is an X-ray diffraction spectrum of the composite of Li3AlF6 and AlF3 as the negative electrode active material of this embodiment. Figure 2 From top to bottom, the following are shown: Figure 1 X-ray diffraction spectra of synthesized products obtained by the synthesis method, AlF3 (theoretical calculated value), LiF (theoretical calculated value), and Li3AlF6 (theoretical calculated value). The synthesized products are shown, from top to bottom, in the following order: a product synthesized with a molar ratio of LiF to AlF3 of LiF:AlF3 = 3:1, a product synthesized with a molar ratio of LiF to AlF3 of LiF:AlF3 = 2:1, and a product synthesized with a molar ratio of LiF to AlF3 of LiF:AlF3 = 1:1.

[0044] like Figure 2 As shown, according to Figure 1 In the X-ray diffraction spectra of the synthetic products synthesized by the synthetic method, the peak derived from LiF as the raw material disappears in the synthetic products synthesized by setting the molar ratio of LiF to AlF3 to LiF:AlF3=2:1 and the synthetic products synthesized by setting the molar ratio of LiF to AlF3 to LiF:AlF3=1:1, while the peak derived from Li3AlF6 and the peak derived from AlF3 can be observed. In contrast, in the synthetic product synthesized by setting the molar ratio of LiF to AlF3 to LiF:AlF3=3:1, the peak derived from LiF as the raw material and the peak derived from AlF3 both disappear, and only the peak derived from Li3AlF6 can be observed. That is, from this Figure 2 It can be observed in the X-ray diffraction spectrum that Figure 1 In the synthesis method, the mixing molar ratio of LiF and AlF3 is set to be in the range of LiF:AlF3=1:1 to 3:1.1, thereby obtaining the Li3AlF6 and AlF3 complex of this embodiment.

[0045] The negative electrode active material of this embodiment is preferably amorphous. Figure 2The X-ray diffraction spectrum shows that the composite of Li3AlF6 and AlF3 as the negative electrode active material synthesized in the above manner is crystalline, but amorphization occurs during the manufacturing process of the negative electrode for the fluoride ion secondary battery of this embodiment, which will be described later. It is believed that the composite of Li3AlF6 and AlF3 as the negative electrode active material synthesized in the above manner has an unstable crystal structure, and it is believed that this crystal structure will be destroyed by the ball mill crushing process in the manufacturing process, which will be described later, and amorphization will occur. In this way, because the negative electrode active material of this embodiment is amorphous, it is possible to tightly combine Li3AlF6 with the solid electrolyte and the conductive additive, and a high-quality interface can be formed.

[0046] The content of the composite of Li3AlF6 and AlF3 in the negative electrode for a fluoride ion secondary battery of this embodiment is preferably less than 25% by mass. Here, as described above, in the modified AlF3 formed by doping lithium metal in aluminum fluoride, which has been discovered by the present applicant, the upper limit of the content in the negative electrode for a fluoride ion secondary battery is 12.5% ​​by mass. In contrast, in the composite of Li3AlF6 and AlF3 of this embodiment, the upper limit of the content in the negative electrode for a fluoride ion secondary battery can be increased to 25% by mass. Thus, according to this embodiment, the battery capacity can be significantly increased compared to the past.

[0047] The average particle size of the composite of Li3AlF6 and AlF3 in this embodiment is preferably in the micron order. Existing modified AlF3 formed by doping aluminum fluoride with lithium metal consists of nanoparticles with an average particle size of the nanometer order. In contrast, in this embodiment, the composite of Li3AlF6 and AlF3, which serves as the negative electrode active material, is composed of microparticles with an average particle size of the micrometer order, thereby further increasing the density. Therefore, higher ionic conductivity can be achieved, and the battery capacity can be increased. Furthermore, in order to obtain a composite of Li3AlF6 and AlF3 composed of microparticles with an average particle size of the micrometer order, AlF3 and LiF, each composed of microparticles with an average particle size of the micrometer order, can be used as raw materials. Furthermore, unlike existing modified AlF3, the composite of Li3AlF6 and AlF3 in this embodiment undergoes a sintering process, and therefore, this sintering process also increases the particle size.

[0048] Here, Figure 3This graph shows the properties of a Li3AlF6 / AlF3 complex and a conventional modified AlF3 formed by doping aluminum fluoride with lithium metal. Specifically, measured density values ​​are shown for Li3AlF6 / AlF3 complexes synthesized with a LiF:AlF3 molar ratio of 2:1 and a 1:1 molar ratio, as well as for a Li3AlF6 / AlF3 complex synthesized with a LiF:AlF3 molar ratio of 3:1 (i.e., Li3AlF6) and conventional modified AlF3. The graph also shows the ionic conductivity of both at 140°C, assuming the operation of a fluoride ion secondary battery.

[0049] like Figure 3 As shown, the composite of Li3AlF6 and AlF3 is similar to Li3AlF6 in that it can have a higher density than conventional modified AlF3 and also has higher ionic conductivity. Therefore, compared to modified AlF3, the concentration of the composite of Li3AlF6 and AlF3 can be increased, further increasing the battery capacity as described above. Furthermore, even when the concentration of the composite of Li3AlF6 and AlF3 is increased, the volume increase can be suppressed. Therefore, the content of the solid electrolyte composed of a fluoride ion-conductive fluoride and the content of the conductive additive (described later) can be increased, resulting in higher ionic conductivity.

[0050] The negative electrode for the fluoride ion secondary battery of this embodiment preferably contains a solid electrolyte composed of a fluoride ion conductive fluoride and a conductive auxiliary agent in addition to the composite of Li3AlF6 and AlF3 as the above-mentioned negative electrode active material.

[0051] The fluoride ion conductive fluoride is not particularly limited as long as it has fluoride ion conductivity. For example, CeBaF X and BaLaF y Fluoride ion conductive fluoride, specifically, Ce 0.95 Ba 0.05 F 2.95 Or Ba 0.6 La 0.4 F 2.4 By including these fluoride ion conductive fluorides in the negative electrode for a fluoride ion secondary battery of this embodiment, fluoride ion conductivity is improved.

[0052] The average particle size of the fluoride ion conductive fluoride is preferably in the range of 0.1 μm to 100 μm. When the average particle size of the fluoride ion conductive fluoride is within this range, it exhibits high ion conductivity and can form a thin-layer electrode. The more preferred range of the average particle size of the fluoride ion conductive fluoride is 0.1 μm to 10 μm.

[0053] The conductive additive is not particularly limited as long as it has electron conductivity. For example, carbon black can be used as a conductive additive. Examples of carbon black include furnace black, Ketjen black, and acetylene black. Inclusion of these conductive additives in the negative electrode for the fluoride ion secondary battery of this embodiment can improve electron conductivity.

[0054] The average particle size of the conductive additive is preferably in the range of 20 nm to 50 nm. If the average particle size of the conductive additive is in this range, a lightweight electrode with high electron conductivity can be formed.

[0055] Furthermore, the negative electrode for a fluoride ion secondary battery of this embodiment may further contain other components such as a binder within a range not impairing the effects of this embodiment.

[0056] Next, the method for manufacturing the negative electrode for the fluoride ion secondary battery of this embodiment is described with reference to Figure 4 and Figure 5 Explain in detail.

[0057] Here, Figure 4 This is a diagram illustrating an example of a method for manufacturing a negative electrode for a fluoride ion secondary battery according to this embodiment. Figure 5 This is a diagram showing an example of a conventional method for manufacturing a negative electrode for a fluoride ion secondary battery. Figure 5 The manufacturing method shown represents a conventional method proposed by the present applicant for manufacturing modified AlF3 formed by doping aluminum fluoride with lithium metal.

[0058] exist Figure 4 In an example of the manufacturing method of the present embodiment shown in the figure, first, 700 mg of CeBaF as a solid electrolyte composed of a fluoride ion conductive fluoride is mixed. x (Ce 0.95 Ba 0.05 F 2.95 ) and 50 mg of carbon black (acetylene black AB) as a conductive aid.

[0059] Then, 250 mg of Figure 1 After synthesizing the Li3AlF6 and AlF3 complex using the synthesis method shown, a ball mill pulverization process is performed, for example, at 300 rpm for 15 minutes for 40 cycles. This produces the LiAlFCB mixture for the negative electrode of the fluoride ion secondary battery of this embodiment. The obtained LiAlFCB mixture is then pressed and integrated with a negative electrode current collector such as gold foil at a predetermined pressure to produce the negative electrode of the fluoride ion secondary battery of this embodiment.

[0060] The mixing ratio of the complex of Li3AlF6 and AlF3 to the fluoride ion conductive fluoride can be arbitrarily selected. However, as described above, the content of the complex of Li3AlF6 and AlF3 in the negative electrode for a fluoride ion secondary battery is preferably 25% by mass or less. From the perspective of increasing the charge capacity, a higher proportion of the fluoride ion conductive fluoride as a fluorine source is preferred.

[0061] In addition, if you compare Figure 4 The method for manufacturing the negative electrode for the fluoride ion secondary battery of the present embodiment shown in FIG. Figure 5 As shown in the conventional method for manufacturing a negative electrode for a fluoride ion secondary battery, the difference between the two manufacturing methods lies in the different negative electrode active materials added to the mixture of fluoride ion conductive fluoride and conductive auxiliary agent. In the manufacturing method of the negative electrode for a fluoride ion secondary battery of this embodiment, a complex of Li3AlF6 and AlF3 synthesized by the above-mentioned synthesis method is added as the negative electrode active material, thereby obtaining a negative electrode mixture for a fluoride ion secondary battery consisting of a mixture of fluoride ion conductive fluoride, conductive auxiliary agent and Li3AlF6 and AlF3. In addition, the details of the synthesis method of the modified AlF3 formed by doping lithium metal in aluminum fluoride added to the conventional method for manufacturing a negative electrode for a fluoride ion secondary battery are described in PCT / JP2019 / 039886.

[0062] By the way, when using Figure 4 In the negative electrode for the fluoride ion secondary battery of the present embodiment manufactured by the manufacturing method shown, as described above, since the complex of Li3AlF6 and AlF3 as the negative electrode active material is unstable, the crystal structure is destroyed by the ball mill treatment, thereby becoming amorphous. That is, even if the complex of Li3AlF6 and AlF3 as the negative electrode active material of the present embodiment is measured by X-ray diffraction, no peak can be confirmed. Therefore, as a measurement method instead of X-ray diffraction measurement, NMR measurement can be cited. According to this NMR measurement, the amorphized complex of Li3AlF6 and AlF3 as the negative electrode active material of the present embodiment can be detected.

[0063] Figure 6 This is an NMR spectrum of the composite of Li3AlF6 and AlF3 as the negative electrode active material of this embodiment. More specifically, Figure 6 According to the above Figure 1 Solid-state NMR spectrum of a composite of Li3AlF6 and AlF3 synthesized by the synthesis method shown. The measurement conditions for the NMR measurement are as follows.

[0064] (NMR measurement conditions)

[0065] NMR equipment: JEOL "JNM-ECA600"

[0066] Probe: Agilent 1.6mm triple resonance MAS probe

[0067] Temperature: Room temperature

[0068] Rotation condition: 35kHz

[0069] Reference materials: 7 Li is LiCl, 19 F is CFCl3, 27 Al is Al(NO3)3

[0070] like Figure 6 As shown, in the NMR spectrum of the complex of Li3AlF6 and AlF3, a large peak is observed at a chemical shift of 180 ppm. This large peak is attributed to 19 The peak of F origin is characteristic of Li3AlF6. In addition, a larger peak is also observed at the chemical shift of 170ppm. This larger peak is attributed to 19 The peak originating from F is characteristic of AlF 3 . Therefore, it can be seen that the presence of a composite of Li 3 AlF 6 and Al F 3 amorphized by the above-mentioned production method can be confirmed by solid-state NMR measurement.

[0071] According to the negative electrode for a fluoride ion secondary battery of the present embodiment described above, the following effects are achieved.

[0072] In the negative electrode for the fluoride ion secondary battery of the present embodiment, a complex comprising Li3AlF6 and AlF3 is constructed as a negative electrode active material. The complex of Li3AlF6 and AlF3 is a complex formed by the composite of Li3AlF6 and AlF3 in one particle. Li3AlF6 with ion conductivity acts as a fluorine source and acts as a catalyst to promote the defluorination of AlF3, which is usually difficult to defluorinate. In addition, as mentioned above, the complex of Li3AlF6 and AlF3 has a higher density than the modified AlF3 formed by doping lithium metal in the existing aluminum fluoride, and its own ionic conductivity is also high. Therefore, compared with the existing modified AlF3, the concentration of the complex of Li3AlF6 and AlF3 can be increased, and therefore, the battery capacity can be further increased. In addition, in the complex of Li3AlF6 and AlF3, the volume increase can be suppressed even if the concentration is increased. Therefore, the content of the solid electrolyte composed of fluoride ion conductive fluoride and the content of the conductive additive can be increased. As a result, higher ion conductivity can be obtained and the battery capacity can be further increased.

[0073] Furthermore, the negative electrode for a fluoride ion secondary battery of this embodiment achieves high active material utilization and high coulombic efficiency during the first charge-discharge cycle. Specifically, while existing modified AlF3 exhibits low active material utilization of approximately 40% and coulombic efficiency of 50%, the composite of Li3AlF6 and AlF3 of this embodiment achieves a high active material utilization of approximately 70% and a high coulombic efficiency of approximately 80%.

[0074] [Fluoride ion secondary battery]

[0075] The fluoride ion secondary battery of this embodiment includes the above-described negative electrode for a fluoride ion secondary battery. Furthermore, the fluoride ion secondary battery of this embodiment includes a solid electrolyte layer composed of a solid electrolyte having fluoride ion conductivity and a positive electrode.

[0076] As the solid electrolyte constituting the solid electrolyte layer, a conventionally known solid electrolyte can be used. Specifically, the same solid electrolyte as the above-mentioned fluoride ion-conductive fluoride can be used.

[0077] As the positive electrode, a currently known positive electrode active material is used, and a positive electrode having a sufficiently high standard electrode potential is preferably used relative to the standard electrode potential of the fluoride ion secondary battery negative electrode of the present embodiment. In addition, a material without fluoride ions is selected as the positive electrode, whereby a battery with charging turned on can be achieved. That is, a battery can be manufactured in a discharge state in which the energy state is relatively low, and the stability of the active material in the electrode can be further improved.

[0078] Specific examples of positive electrode materials include conductive additives such as Pb, Cu, Sn, Bi, and Ag, and binders. For example, a positive electrode mixture containing lead fluoride or tin fluoride, carbon black, and the like is pressed together with the positive electrode material and a lead foil as a current collector at a predetermined pressure to form a whole, thereby producing a positive electrode.

[0079] Therefore, the fluoride ion secondary battery of this embodiment can be manufactured by sequentially stacking the negative electrode for the fluoride ion secondary battery of this embodiment, the solid electrolyte layer, and the positive electrode. According to the fluoride ion secondary battery of this embodiment, the same effects as those of the negative electrode for the fluoride ion secondary battery of this embodiment can be achieved.

[0080] The present invention is not limited to the above-described embodiment, and modifications and improvements made within the scope that can achieve the object of the present invention are included in the present invention.

[0081] For example, in the above embodiment, the present invention is described as being applied to a solid-state battery, but the present invention is not limited thereto. An electrolyte solution may be used in place of a solid electrolyte layer in a fluoride ion secondary battery.

[0082] [Example]

[0083] Next, examples of the present invention will be described, but the present invention is not limited to these examples.

[0084] [Examples 1 and 2]

[0085] according to Figure 4 The method for manufacturing a negative electrode for a fluoride ion secondary battery according to the present embodiment is shown, and negative electrodes for fluoride ion secondary batteries of Examples 1 and 2 are produced. Both Examples 1 and 2 use a composite of Li₃AlF₆ and AlF₃ with an average particle size of micrometers (10 μm to 100 μm), a fluoride ion conductive fluoride with an average particle size of 0.1 to 100 μm, and a conductive additive with an average particle size of 20 to 50 nm. The content of the composite of Li₃AlF₆ and AlF₃ in the negative electrode for a fluoride ion secondary battery is set to 25% by mass. In Example 1, the composite of Li₃AlF₆ and AlF₃ is synthesized at a molar ratio of LiF to AlF₃ of 1:1, while in Example 2, the composite of Li₃AlF₆ and AlF₃ is synthesized at a molar ratio of LiF to AlF₃ of 2:1.

[0086] [Comparative Example 1]

[0087] according to Figure 5 The negative electrode for a fluoride ion secondary battery of Comparative Example 1 was prepared using the conventional method for manufacturing a negative electrode for a fluoride ion secondary battery and the synthesis method described in PCT / JP2019 / 039886. In Comparative Example 1, modified AlF3 with an average particle size of nanometers was used, and the content of the modified AlF3 in the negative electrode for a fluoride ion secondary battery was set to 25% by mass.

[0088] [Reference Example 1]

[0089] according to Figure 4The method for manufacturing a negative electrode for a fluoride ion secondary battery according to the present embodiment is described, and a negative electrode for a fluoride ion secondary battery of Reference Example 1 is produced. Specifically, a negative electrode synthesized with a molar ratio of LiF to AlF3 of LiF:AlF3 = 3:1 is used as Reference Example 1. Reference Example 1 uses Li3AlF6 with an average particle size of micrometers (10 μm to 100 μm), a fluoride ion conductive fluoride with an average particle size of 0.1 to 100 μm, and a conductive additive with an average particle size of 20 to 50 nm. Furthermore, the Li3AlF6 content in the negative electrode for a fluoride ion secondary battery is set to 25% by mass.

[0090] [Charge and discharge test]

[0091] Half-cells were fabricated using the negative electrodes for fluoride ion secondary batteries prepared in each example, and constant current charge-discharge tests were performed. Specifically, a potentiostat (Soltron, SI1287 / 1255B) was used in a vacuum environment at 140°C. Constant current charge-discharge tests were performed starting with the charging current, with a charge current of 0.04 mA and a discharge current of 0.02 mA, and a lower voltage limit of -2.44 V and an upper voltage limit of -0.1 V.

[0092] Each half-cell was then pressed using a tablet press at a pressure of 40 MPa to produce a cylindrical pellet-type battery formed from powder. Specifically, the tablet press was loaded with gold foil (99.99%, 10 μm thick) manufactured by Nilaco Co., Ltd. as the negative electrode current collector, 10 mg of the fluoride ion secondary battery negative electrode mixture powder prepared in each example, 200 mg of the solid electrolyte, 30 mg of the positive electrode mixture powder, the positive electrode material, and lead foil (99.99%, 200 μm thick) manufactured by Nilaco Co., Ltd. as the positive electrode current collector. Each half-cell was produced.

[0093] [Results / Investigation]

[0094] Figure 7 Graph showing the charge and discharge curves of the negative electrode half-cells for fluoride ion secondary batteries of Examples 1-2, Reference Example 1, and Comparative Example 1. More specifically, Figure 7 The charge-discharge curves in the first charge-discharge cycle of Examples 1-2, Reference Example 1 and Comparative Example 1 are shown. Figure 7As shown, in Comparative Example 1, where the modified AlF3 content in the fluoride ion secondary battery negative electrode was 25 mass%, almost no charge / discharge capacity was achieved. In contrast, Examples 1 and 2, where the composite of Li3AlF6 and AlF3 in the fluoride ion secondary battery negative electrode was 25 mass%, were able to achieve charge / discharge capacities comparable to those of Reference Example 1, which contained 25 mass% Li3AlF6 as the negative electrode active material. This result confirms that, according to this example, increasing the composite of Li3AlF6 and AlF3 in the fluoride ion secondary battery negative electrode to 25 mass% can achieve a higher battery capacity than before.

[0095] In addition, the actual capacity obtained is expressed as the utilization rate of active material relative to the theoretical capacity. In this regard, the theoretical capacity of the complex of Li3AlF6 and AlF3 is 2.48 mAh, but Figure 7 The results show that the charge capacity in Examples 1 and 2 is about 1.7 mAh, and the active material utilization rate of about 68% can be obtained according to this embodiment. Figure 7 The results also confirmed that, according to Example 1, a discharge capacity of about 1.3 mAh can be obtained relative to a charge capacity of about 1.7 mAh, and a coulombic efficiency as high as about 80% can be obtained.

Claims

1. A negative electrode for a fluoride ion secondary battery, comprising a negative electrode active material, The negative electrode active material comprises a complex of Li3AlF6 and AlF3. The molar ratio of AlF3 to Li3AlF6 in the above-mentioned complex of Li3AlF6 and AlF3 is 0.1 to 2.

2. The negative electrode for a fluoride ion secondary battery according to claim 1, wherein The content of the composite of Li 3 AlF 6 and AlF 3 in the negative electrode for the fluoride ion secondary battery is 25% by mass or less.

3. The negative electrode for a fluoride ion secondary battery according to claim 1, wherein The aforementioned complex of Li3AlF6 and AlF3 is amorphous. 4 . A fluoride ion secondary battery comprising the negative electrode for a fluoride ion secondary battery according to claim 1 .

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