Anode for fluoride ion secondary battery and fluoride ion secondary battery including the same

Abstract

To provide a fluoride ion secondary battery larger, in battery capacity, than conventional ones.SOLUTION: A negative electrode for a fluoride ion secondary battery contains Li3AlF6 as a negative electrode active material, and a fluoride ion secondary battery has the negative electrode. In the negative electrode for a fluoride ion secondary battery, the content of the negative electrode active material may be 25 mass% or less, Li3AlF6 as the negative electrode active material may be amorphous, and the average particle size of Li3AlF6 may be in a micrometer order.SELECTED DRAWING: Figure 8

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 same. [Background technology]

[0002] Conventionally, fluoride ion secondary batteries using fluoride ions as a carrier have been proposed (for example, see Patent Documents 1 to 6). Fluoride ion secondary batteries are expected to have battery characteristics that exceed those of lithium ion secondary batteries, and various studies have been conducted in recent years.

[0003] For example, aluminum-based materials are candidates for the negative electrode active material of fluoride ion secondary batteries. Among them, the use of aluminum fluoride has been studied, but aluminum fluoride has an issue in that it is difficult for electrochemical reactions to occur because it has electrical insulation properties. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-87403 A [Patent Document 2] JP 2017-50113 A [Patent Document 3] JP 2019-29206 A [Patent Document 4] JP 2018-206755 A [Patent Document 5] JP 2018-198130 A [Patent Document 6] JP 2018-92863 A Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, the applicant has developed a modified AlF3 As a negative electrode active material, fluoride ion secondary batteries using aluminum fluoride have been developed, but there is currently a demand for further improvements in battery characteristics. In particular, modified AlF 3 However, since the ionic conductivity of such a negative electrode active material is not good, it is not possible to increase the concentration of the negative electrode active material in the negative electrode, and it is not easy to increase the battery capacity.

[0006] The present invention has been made in view of the above, and has an object to provide a fluoride ion secondary battery having a larger battery capacity than conventional batteries. [Means for solving the problem]

[0007] (1) The present invention relates to a negative electrode for a fluoride ion secondary battery, comprising a negative electrode active material, the negative electrode active material being Li 3 AlF 6 The present invention provides a negative electrode for a fluoride ion secondary battery, comprising:

[0008] (2) In the negative electrode for a fluoride ion secondary battery of (1), the content of the negative electrode active material in the negative electrode for a fluoride ion secondary battery may be 25 mass % or less.

[0009] (3) In the negative electrode for a fluoride ion secondary battery according to (1) or (2), the Li 3 AlF 6 may be amorphous.

[0010] (4) In the negative electrode for a fluoride ion secondary battery according to any one of (1) to (3), the Li 3 AlF 6 The average particle size may be on the order of micrometers.

[0011] (5) The present invention also provides a fluoride ion secondary battery comprising a negative electrode for a fluoride ion secondary battery according to any one of (1) to (4). Effect of the Invention

[0012] 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]

[0013] [Figure 1] FIG. 2 is a diagram showing a method for synthesizing Li3AlF6 as a negative electrode active material according to one embodiment of the present invention. [Diagram 2] FIG. 2 is an X-ray diffraction spectrum of Li3AlF6 as the negative electrode active material according to the embodiment. [Diagram 3] FIG. 1 is a diagram showing the characteristics of Li3AlF6 and conventional modified AlF3. [Figure 4] 1A to 1C are diagrams illustrating an example of a method for producing a negative electrode for a fluoride ion secondary battery according to an embodiment of the present invention. [Diagram 5] FIG. 1 is a diagram showing an example of a conventional method for producing a negative electrode for a fluoride ion secondary battery. [Figure 6] FIG. 2 is an NMR spectrum diagram of Li3AlF6 as the negative electrode active material according to the embodiment. [Figure 7] FIG. 2 is a diagram showing charge / discharge curves of negative electrode half-cells for fluoride ion secondary batteries according to Example 1 and Comparative Example 1. [Figure 8] FIG. 13 is a diagram showing charge / discharge curves of negative electrode half-cells for fluoride ion secondary batteries according to Example 2 and Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0015] [Negative electrodes for fluoride ion secondary batteries] The negative electrode for a fluoride ion secondary battery according to this embodiment contains Li as a negative electrode active material. 3 AlF 6 Contains: 3 AlF 6 A negative electrode for a fluoride ion secondary battery containing Li has not been found so far, and the negative electrode for a fluoride ion secondary battery according to this embodiment contains Li 3 AlF6 The present invention is characterized in that it includes the following:

[0016] Li 3 AlF 6 Li functions as a negative electrode active material during charging and discharging. 3 AlF 6 During charging, fluoride ions F - During discharge, fluoride ions F - Li 3 AlF 6 can be synthesized, for example, as follows:

[0017] FIG. 1 is a graph showing a Li 3 AlF 6 As shown in FIG. 1, first, LiF and AlF 3 LiF:AlF 3 For example, in the example shown in Figure 1, 2.4 g of LiF and 2.6 g of AlF 3 Then, the mixture is subjected to a ball mill pulverization process at 400 rpm for 15 minutes for 40 cycles, and then sintered at 900° C. for 3 hours. After sintering, the mixture is pulverized to obtain Li as the negative electrode active material according to the present embodiment. 3 AlF 6 is obtained.

[0018] Here, the temperature of the sintering process is preferably within a range of 850° C. to 900° C. Since the melting point of the raw material LiF is 850° C., if the temperature of the sintering process is within this range, the molten LiF and AlF 3 If the sintering temperature exceeds 900°C, the weight loss after sintering starts to increase and the raw materials evaporate, which is not preferable.

[0019] The sintering time is preferably within a range of 2 to 3 hours when the sintering temperature is 850° C. to 900° C. If the sintering time is less than 2 hours, LiF and AlF 3If the sintering time exceeds 3 hours, the raw materials are evaporated, resulting in a poor yield.

[0020] The pulverization after the sintering process may be performed by, for example, crushing in an agate mortar, and the particles after crushing are microparticles. These microparticles are further crushed by a ball mill crushing process during the production of the negative electrode composite powder, which will be described later.

[0021] FIG. 2 is a graph showing the structure of a negative electrode active material according to the present embodiment. 3 AlF 6 In FIG. 2, from the top to bottom, there are shown the synthesized product obtained according to the synthesis method in FIG. 1, AlF 3 (theoretical calculation value), LiF (theoretical calculation value), Li 3 AlF 6 As shown in FIG. 2, the X-ray diffraction spectrum of the synthetic product obtained by the synthesis method of FIG. 1 shows that the raw material AlF 3 The peaks due to LiF and LiF both disappeared, while the peaks due to Li 3 AlF 6 In other words, from the X-ray diffraction spectrum in Figure 2, it is clear that the Li 3 AlF 6 It is confirmed that it is possible to synthesize

[0022] The negative electrode active material according to the present embodiment is preferably amorphous. 3 AlF 6 As is clear from the X-ray diffraction spectrum of FIG. 2, Li is crystalline, but becomes amorphous during the manufacturing process of the negative electrode for a fluoride ion secondary battery according to the present embodiment, which will be described later. 3 AlF 6It is believed that Li has an unstable crystal structure, and that the crystal structure is destroyed and made amorphous by the ball mill crushing process in the manufacturing process described later. 3 AlF 6 It is believed that this allows the material to bond closely with the solid electrolyte and conductive additive, making it possible to form a high-quality interface.

[0023] Li in the negative electrode for the fluoride ion secondary battery according to this embodiment 3 AlF 6 The content of is preferably 25 mass % or less. Here, as described above, the present applicant has previously discovered modified AlF 2 by doping aluminum fluoride with lithium metal. 3 In the case of Li 2 O 3 , the upper limit of the content in the negative electrode for a fluoride ion secondary battery is 12.5 mass %. 3 AlF 6 In this embodiment, the upper limit of the content in the negative electrode for a fluoride ion secondary battery can be increased to 25 mass %, thereby making it possible to greatly increase the battery capacity compared to the conventional case.

[0024] Li as the negative electrode active material according to this embodiment 3 AlF 6 The average particle size of the modified AlF is preferably on the order of micrometers. 3 In contrast, in this embodiment, the negative electrode active material is Li 3 AlF 6 By using microparticles with an average particle size on the order of micrometers, the density can be increased. This results in higher ionic conductivity and increases the battery capacity. 3 AlF 6 In order to obtain this, the raw material must be AlF, which is composed of microparticles with an average particle size on the order of micrometers. 3and LiF. 3 Unlike the Li 3 AlF 6 Since the powder undergoes a sintering process, the grain size also increases due to this sintering process.

[0025] Here, Figure 3 shows Li 3 AlF 6 and modified AlF, which is made by doping conventional aluminum fluoride with lithium metal. 3 FIG. 3 is a graph showing the characteristics of the Li 3 AlF 6 and modified AlF 3 The figures show the actual measured densities of the two materials, as well as the ionic conductivities of the two materials at 140°C, which is the temperature at which a fluoride ion secondary battery is operated.

[0026] As shown in Figure 3, Li 3 AlF 6 Conventional modified AlF 3 It can be seen that the density can be increased more than that of Li, and the ionic conductivity of the Li is also high. 3 AlF 6 Modified AlF 3 Since the concentration can be increased compared to Li, the battery capacity can be increased as described above. 3 AlF 6 In the case of the fluoride-ion conductive solid electrolyte, the content of the fluoride-ion conductive solid electrolyte and the content of the conductive additive can be increased, so that a higher ion conductivity can be obtained.

[0027] The negative electrode for a fluoride ion secondary battery according to this embodiment contains Li as the negative electrode active material. 3 AlF 6 In addition, it is preferable that the battery further contains a solid electrolyte made of a fluoride ion-conductive fluoride and a conductive assistant.

[0028] The fluoride ion conductive fluoride is not particularly limited as long as it has fluoride ion conductivity. For example, CeBaF x and BaLaF y Examples of fluoride ion conductive fluorides include Ce. 0.95 Ba 0.05 F 2.95 YaBa 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 according to this embodiment, the fluoride ion conductivity can be improved.

[0029] The average particle size of the fluoride ion conductive fluoride is preferably in the range of 0.1 μm to 100 μm. If the average particle size of the fluoride ion conductive fluoride is within this range, a thin-layer electrode can be formed while having relatively high ion conductivity. The average particle size of the fluoride ion conductive fluoride is more preferably in the range of 0.1 μm to 10 μm.

[0030] The conductive assistant may be any one having electronic conductivity and is not particularly limited. For example, carbon black or the like is used as the conductive assistant. As the carbon black, furnace black, ketjen black, acetylene black, or the like can be used. By including these conductive assistants in the negative electrode for a fluoride ion secondary battery according to this embodiment, the electronic conductivity can be improved.

[0031] The average particle size of the conductive assistant is preferably in the range of 20 nm to 50 nm. If the average particle size of the conductive assistant is within this range, an electrode having high electronic conductivity can be formed with a small weight.

[0032] Moreover, the negative electrode of the fluoride ion secondary battery according to this embodiment may contain other components such as a binder, within a range that does not impair the effects of this embodiment.

[0033] Next, a method for producing the negative electrode for a fluoride ion secondary battery according to this embodiment will be described in detail with reference to FIGS. Here, Fig. 4 is a diagram showing an example of a method for producing a negative electrode for a fluoride ion secondary battery according to the present embodiment. Fig. 5 is a diagram showing an example of a conventional method for producing a negative electrode for a fluoride ion secondary battery. The production method shown in Fig. 5 is a method for producing a modified AlF 2 O 4 obtained by doping lithium metal into conventional aluminum fluoride, which has already been proposed by the present applicant. 3 A negative electrode using This shows a manufacturing method of the above.

[0034] In one example of the manufacturing method of this embodiment shown in FIG. 4, first, CeBaF as a solid electrolyte made of a fluoride ion conductive fluoride is prepared. x (Ce 0.95 Ba 0.05 F 2.95 ) and 50 mg of carbon black (acetylene black AB) as a conductive additive are mixed.

[0035] Next, the mixture was treated with Li 3 AlF 6 After adding 250 mg of, for example, ball mill pulverization is performed at 300 rpm for 15 minutes for 40 cycles. This results in a composite LiAlFCB for the negative electrode for a fluoride ion secondary battery according to the present embodiment. The resulting composite LiAlFCB is then pressed together with a negative electrode current collector such as gold foil at a predetermined pressure to form an integrated structure, thereby producing the negative electrode for a fluoride ion secondary battery according to the present embodiment.

[0036] In addition, Li 3 AlF 6 The mixing ratio of Li and fluoride ion conductive fluoride can be selected arbitrarily. However, as described above, in the negative electrode for a fluoride ion secondary battery, 3 AlF 6 The content is preferably 25 mass % or less, and from the viewpoint of increasing the charge capacity, the higher the ratio of the fluoride ion-conductive fluoride, which is the fluorine source, the more preferable.

[0037] In addition, as is clear from a comparison between the method for producing a negative electrode for a fluoride ion secondary battery according to the present embodiment shown in FIG. 4 and the method for producing a negative electrode for a fluoride ion secondary battery according to the related art shown in FIG. 5, the difference between the two production methods is that the negative electrode active material added to the mixture of the fluoride ion conductive fluoride and the conductive assistant is different. In the method for producing a negative electrode for a fluoride ion secondary battery according to the present embodiment, Li synthesized by the above-mentioned synthesis method is used as the negative electrode active material. 3 AlF 6 By adding fluoride ion conductive fluoride, conductive additive and Li 3 AlF 6 In addition, the negative electrode mixture for a fluoride ion secondary battery is obtained by doping lithium metal into aluminum fluoride, which is added in the conventional method for producing a negative electrode for a fluoride ion secondary battery. 3 Details of the synthesis method etc. are as described in PCT / JP2019 / 039886.

[0038] Incidentally, in the negative electrode for a fluoride ion secondary battery according to the present embodiment produced by the production method shown in FIG. 4, Li 3 AlF 6 Since Li is unstable, the crystal structure is destroyed and made amorphous by the ball mill treatment. 3 AlF 6 However, even if X-ray diffraction measurement is performed, no peak can be confirmed. Therefore, an alternative measurement method to X-ray diffraction measurement is NMR measurement. According to this NMR measurement, the amorphous Li 3 AlF 6 It is possible to detect.

[0039] FIG. 6 shows a Li negative electrode active material according to this embodiment. 3 AlF 6 More specifically, FIG. 6 shows the NMR spectrum of Li synthesized according to the synthesis method shown in FIG. 3 AlF 6The solid-state NMR spectrum of the synthesized product is shown below. The NMR measurement conditions are as follows:

[0040] (NMR measurement conditions) NMR device: “JNM-ECA600” manufactured by JEOL Probe: Agilent 1.6 mm triple resonance MAS probe Temperature: room temperature Spin condition: 35kHz Reference material: 7 Li is LiCl, 19 F is CFCl 3 , 27 Al is Al(NO 3 ) 3

[0041] As shown in Figure 6, Li 3 AlF 6 In the NMR spectrum of the synthesized product, a large peak is observed at a chemical shift of 180 ppm. This large peak is due to Li 3 AlF 6 Characteristic of 19 The peaks are assigned to F. Therefore, the Li 3 AlF 6 The presence or absence of Li can be confirmed by solid-state NMR measurement. 3 AlF 6 Although a LiF peak is observed in the NMR spectrum of the synthesized product, this is LiF remaining unreacted.

[0042] According to the negative electrode for a fluoride ion secondary battery according to the present embodiment described above, the following effects are achieved.

[0043] In the negative electrode for a fluoride ion secondary battery according to this embodiment, Li is used as the negative electrode active material. 3 AlF 6 As mentioned above, the Li 3 AlF 6 is a modified AlF 3It can be made denser than Li and has high ionic conductivity. 3 AlF 6 Conventional modified AlF 3 The concentration can be increased compared to Li, which can increase the battery capacity. 3 AlF 6 In the case of the present invention, since the increase in the volume can be suppressed even if the concentration is increased, the content of the solid electrolyte made of the fluoride ion-conductive fluoride and the content of the conductive additive can be increased, and as a result, higher ion conductivity can be obtained, and the battery capacity can be further increased.

[0044] In addition, the negative electrode for a fluoride ion secondary battery according to this embodiment can obtain a high active material utilization rate in the first charge / discharge cycle, and can also obtain a high coulombic efficiency. 3 In the conventional battery, the active material utilization rate is low at about 40% and the Coulombic efficiency is also low at about 50%, whereas in this embodiment, a high active material utilization rate of about 70% and a high Coulombic efficiency of about 80% can be obtained.

[0045] [Fluoride ion secondary battery] The fluoride ion secondary battery according to this embodiment includes the above-mentioned negative electrode for fluoride ion secondary batteries. The fluoride ion secondary battery according to this embodiment also includes a solid electrolyte layer made of a solid electrolyte having fluoride ion conductivity, and a positive electrode.

[0046] As the solid electrolyte constituting the solid electrolyte layer, a conventionally known solid electrolyte can be used. Specifically, the same fluoride ion-conducting fluoride as described above can be used.

[0047] As the positive electrode, a conventionally known positive electrode active material is used, and it is preferable to use a positive electrode that can obtain a sufficiently high standard electrode potential compared to the standard electrode potential of the negative electrode for the fluoride ion secondary battery according to this embodiment. In addition, by selecting a material that does not have fluoride ions as the positive electrode, a battery that can be started by charging can be realized. In other words, it becomes possible to manufacture a battery in a discharge state where the energy state is low, and the stability of the active material in the electrode can be improved.

[0048] Specific examples of the positive electrode material include Pb, Cu, Sn, Bi, Ag, etc., conductive additives, binders, etc. For example, a positive electrode can be manufactured by pressing a positive electrode mixture containing lead fluoride, tin fluoride, carbon black, etc. together with a positive electrode material and a lead foil or the like as a current collector at a predetermined pressure to integrate them.

[0049] Therefore, the fluoride ion secondary battery according to the present embodiment can be manufactured by sequentially stacking the negative electrode for the fluoride ion secondary battery according to the present embodiment, the solid electrolyte layer, and the positive electrode. The fluoride ion secondary battery according to the present embodiment exhibits the same effects as the negative electrode for the fluoride ion secondary battery according to the present embodiment.

[0050] The present invention is not limited to the above-described embodiment, and the present invention includes modifications and improvements within the scope that can achieve the object of the present invention. For example, in the above embodiment, the present invention is described as being applied to a solid-state battery, but is not limited thereto, and may be applied to a fluoride ion secondary battery that uses an electrolytic solution instead of a solid electrolyte layer. EXAMPLES

[0051] Next, examples of the present invention will be described, but the present invention is not limited to these examples.

[0052] [Examples 1 and 2] According to the method for producing the negative electrode for a fluoride ion secondary battery according to the present embodiment shown in FIG. 4, the negative electrodes for a fluoride ion secondary battery according to Examples 1 and 2 were produced. In both Examples 1 and 2, Li 3 AlF 6 The negative electrode for a fluoride ion secondary battery is made of a fluoride ion conductive fluoride having an average particle size of 0.1 to 100 μm and a conductive assistant having an average particle size of 20 to 50 nm. 3 AlF 6 The content was 12.5 mass % in Example 1 and 25 mass % in Example 2.

[0053] [Comparative Examples 1 and 2] According to the conventional method for producing a negative electrode for a fluoride ion secondary battery shown in FIG. 5 and the synthesis method described in PCT / JP2019 / 039886, negative electrodes for a fluoride ion secondary battery according to Comparative Examples 1 and 2 were produced. In both Comparative Examples 1 and 2, modified AlF 3 Modified AlF in the negative electrode for fluoride ion secondary batteries 3 The content was 12.5 mass % in Comparative Example 1 and 25 mass % in Comparative Example 2.

[0054] [Charge / discharge test] A half cell was prepared using the negative electrode for fluoride ion secondary battery prepared in each Example, and a constant current charge / discharge test was carried out. Specifically, using a potentiogalvanostat device (Sotron, SI1287 / 1255B), a constant current charge / discharge test was carried out by applying a charge current of 0.02 mA for charge and 0.01 mA for discharge under a vacuum at 140° C. for the one having an active material concentration of 12.5% ​​by mass, with a lower limit voltage of −2.35 V and an upper limit voltage of −0.1 V. For the one having an active material concentration of 25% by mass, a constant current charge / discharge test was carried out by applying a charge current of 0.04 mA for charge and 0.02 mA for discharge, with a lower limit voltage of −2.44 V and an upper limit voltage of −0.1 V.

[0055] Each half cell was prepared as a cylindrical pellet-type cell by pressing at a pressure of 40 MPa using a tablet press. Specifically, each half cell was prepared by putting gold foil (99.99%, thickness 10 μm) manufactured by Nilaco Corporation as a negative electrode current collector, 10 mg of the negative electrode composite powder for fluoride ion secondary batteries prepared in each Example, 200 mg of solid electrolyte, 30 mg of positive electrode composite powder, and lead foil (99.99%, thickness 200 μm) manufactured by Nilaco Corporation as a positive electrode material and positive electrode current collector in this order into a tablet press.

[0056] [Results / Discussion] Fig. 7 is a diagram showing charge / discharge curves of the negative electrode half cells for fluoride ion secondary batteries according to Example 1 and Comparative Example 1. More specifically, Fig. 7 shows charge / discharge curves in the first charge / discharge cycle in Example 1 and Comparative Example 1. As shown in Fig. 7, Li in the negative electrode for fluoride ion secondary batteries 3 AlF 6 and Example 1, in which the content of AlF is 12.5 mass %, and 3 It was found that the charge capacity was equivalent to that of Comparative Example 1, in which the content was 12.5 mass %. On the other hand, it was found that the discharge capacity was smaller in Comparative Example 1 than in Example 1, and it was confirmed that the Coulomb efficiency, which is the ratio of the discharge capacity to the charge capacity, was higher in this example. From this result, it was confirmed that the reversibility of charge and discharge can be improved compared to the conventional example.

[0057] Fig. 8 is a diagram showing charge / discharge curves of the negative electrode half cells for fluoride ion secondary batteries according to Example 2 and Comparative Example 2. More specifically, Fig. 8 shows charge / discharge curves in the first charge / discharge cycle in Example 2 and Comparative Example 2. As shown in Fig. 8, the modified AlF 3 In Comparative Example 2, in which the content of Li was 25 mass %, almost no charge / discharge capacity was obtained. 3 AlF 6It was confirmed that a large charge / discharge capacity was obtained in Example 2, in which the content of Li was 25 mass %. From this result, it was confirmed that the Li content in the negative electrode for a fluoride ion secondary battery was 25 mass %. 3 AlF 6 It was confirmed that the content of can be increased to 25 mass %, resulting in a battery capacity larger than that of conventional batteries.

[0058] In addition, the capacity that can actually be obtained is expressed by the utilization rate of the active material compared to the theoretical capacity. 3 AlF 6 The theoretical capacity of the battery is 2.48mAh, but the capacity in Example 2 is about 1.7mAh from the results in Fig. 8, and it was confirmed that a high active material utilization rate of about 68% was obtained according to this example. Furthermore, it was confirmed from the results in Fig. 8 that a discharge capacity of about 1.3mAh was obtained with Example 2 for a charge capacity of about 1.7mAh, and a high Coulombic efficiency of about 80% was obtained.

Claims

1. A negative electrode for a fluoride ion secondary battery comprising a negative electrode active material, The negative electrode active material is Li ３ AlF ６ Including, The negative electrode for a fluoride ion secondary battery, wherein the Li 3 AlF 6 is amorphous.

2. The Li in the negative electrode for the fluoride ion secondary battery ３ AlF ６ The negative electrode for a fluoride ion secondary battery according to claim 1 , wherein the content of

3. The Li ３ AlF ６ The negative electrode for a fluoride ion secondary battery according to claim 1 or 2, wherein the average particle diameter of the negative electrode is 10 μm to 100 μm.

4. A fluoride ion secondary battery comprising the negative electrode for a fluoride ion secondary battery according to claim 1 .

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