Fluoride-ion battery

Abstract

To provide means enabling charge / discharge performance to be improved in a fluoride ion battery.SOLUTION: A fluoride ion battery is provided, including: a positive electrode having a positive electrode active material layer containing a positive electrode active material and a positive electrode current collector collecting a current of the positive electrode active material layer; a negative electrode having a negative electrode active material layer containing a negative electrode active material and a negative electrode current collector collecting a current of the negative electrode active material layer; and an electrolyte layer containing an electrolyte having fluoride ion conductivity and disposed between the positive electrode and the negative electrode. Here, in the fluoride ion battery, at least a part of the positive electrode active material layer and the negative electrode active material layer contains a boroxine ring-containing polymer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fluoride ion battery. [Background technology]

[0002] Fluoride ion (F) is an innovative battery that significantly improves the energy density of conventional Li-ion batteries. - There is an anion-based fluoride-ion battery that utilizes the reaction of fluoride with metals to generate polyvalent metal fluorides and the reverse reaction (defluorination of metal fluorides). In this battery, the lightest halide ion, F, moves between the two electrodes. - ion (M=19), and sometimes 2-3 or more F atoms per metal atom. - Ions (and an equal number of electrons) participate in the electrode reactions, which are the primary factor in achieving the high energy density mentioned above.

[0003] The fluoride-ion battery does not require a host lattice, which is an essential component of LIBs. - This type of battery, in which ions play a major role in both the electrode reaction and the charge transfer in the electrolyte, is called FIB (Fluoride Ion Battery) or F - It is known by the abbreviation FSB (Fluoride Shuttle Battery), which emphasizes the role of bidirectional ion movement.

[0004] The reduction (defluorination) reaction of metal fluorides, one of the fundamental elements of fluoride-ion batteries, has been reported in considerable numbers since the 1970s, when solid electrolytes were in the spotlight. However, it was only relatively recently that significant reversibility of charge and discharge, which is essentially essential for secondary batteries, was confirmed, albeit incompletely, within the framework of all-solid-state batteries.

[0005] Furthermore, due to limitations generally associated with the ionic conductivity of solid electrolytes and the electrochemical reactivity at the solid / solid interface, there have only been a few reports of room-temperature operation of solid-state FIB batteries published very recently.

[0006] In principle, the above limitations are eliminated in wet batteries that use a non-aqueous electrolyte to transport fluoride ions. There is a wealth of prior patents and academic papers on electrolytes that can be used for this purpose. One example is a method using an electrolyte containing a fluoride salt, a boron compound, and a non-aqueous solvent (Patent Document 1). In this method, the boron compound has the function of promoting the dissociation of the fluoride salt and the function of coordinating with and stabilizing the fluoride ions. It has been reported that the use of a boroxine-based compound in particular facilitates the smooth movement of fluoride ions in the electrolyte. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-10865 Summary of the Invention [Problem to be solved by the invention]

[0008] The fluoride ion battery described in Patent Document 1 utilizes a fluorination reaction of a metal and a defluorination reaction of a metal fluoride. However, according to the investigations of the present inventors, it has been found that such a fluoride ion battery does not provide sufficient charge / discharge performance.

[0009] Therefore, an object of the present invention is to provide a means for improving the charge / discharge performance of a fluoride ion battery. [Means for solving the problem]

[0010] In order to achieve the above object, the present inventors have conducted extensive research and have found that charge-discharge performance can be significantly improved by carrying out charge-discharge reactions in a state in which a boroxine ring-containing polymer is present in the active material layer of a fluoride ion battery, thereby completing the present invention.

[0011] That is, one aspect of the present invention provides a fluoride ion battery comprising: a positive electrode having a positive electrode active material layer containing a positive electrode active material and a positive electrode current collector that collects current from the positive electrode active material layer, a negative electrode having a negative electrode active material layer containing a negative electrode active material and a negative electrode current collector that collects current from the negative electrode active material layer, and an electrolyte layer disposed between the positive electrode and the negative electrode and containing an electrolyte having fluoride ion conductivity. The fluoride ion battery is characterized in that at least a portion of the positive electrode active material layer and the negative electrode active material layer contains a boroxine ring-containing polymer. [Effects of the Invention]

[0012] According to the present invention, the boroxine ring-containing polymer is contained in at least a portion of the positive electrode active material layer and the negative electrode active material layer, thereby suppressing the elution of metal ions from the positive electrode active material, deformation of the electrode, and / or reductive decomposition of the electrolyte at the negative electrode, thereby improving the charge / discharge performance of the fluoride ion battery. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a conceptual diagram showing the principle of a fluoride ion battery according to one embodiment of the present invention. [Figure 2] (A) to (C) in FIG. 2 are graphs showing the charge / discharge curves of Comparative Example 1-1, Comparative Example 1-2, and Example 1-1, respectively. [Figure 3] FIG. 3 shows photographs of Comparative Example 1-1, Comparative Example 1-2, and Example 1-1, showing the coloration of the electrolyte before and after the charge-discharge test. [Figure 4] (A) to (C) in FIG. 4 are graphs showing the charge / discharge curves of Comparative Example 2-1, Comparative Example 2-2, and Example 2-1, respectively. [Figure 5] FIG. 5 shows photographs of Comparative Example 2-1, Comparative Example 2-2, and Example 2-1, showing the deformation of the electrodes before and after the charge-discharge test. [Figure 6] FIGS. 6A and 6B are graphs showing the charge-discharge curves of Comparative Example 3-1 and Example 3-1, respectively. [Figure 7] (A) to (D) in FIG. 7 are graphs showing the charge / discharge curves of Comparative Example 1-1, Example 1-2, Example 1-1, and Example 1-3, respectively. [Figure 8] FIG. 8 shows XPS spectra of the electrodes of Examples 1-1, 2-1, and 3-1 before and after the charge-discharge test. DETAILED DESCRIPTION OF THE INVENTION

[0014] According to one aspect of the present invention, there is provided a fluoride ion battery comprising: a positive electrode having a positive electrode active material layer containing a positive electrode active material and a positive electrode current collector that collects current from the positive electrode active material layer; a negative electrode having a negative electrode active material layer containing a negative electrode active material and a negative electrode current collector that collects current from the negative electrode active material layer; and an electrolyte layer disposed between the positive electrode and the negative electrode and containing an electrolyte having fluoride ion conductivity. The fluoride ion battery is characterized in that at least a portion of the positive electrode active material layer and the negative electrode active material layer contains a boroxine ring-containing polymer.

[0015] Hereinafter, embodiments of a fluoride ion battery according to the present invention will be described in detail with reference to the drawings.

[0016] FIG. 1 is a conceptual diagram illustrating the principle of a fluoride ion battery according to one embodiment of the present invention. As shown in FIG. 1, the fluoride ion battery 1 includes a positive electrode 10, a negative electrode 20, and an electrolyte layer 30 disposed between the positive electrode 10 and the negative electrode 20. In the embodiment shown in FIG. 1, the positive electrode 10 has a configuration in which a positive electrode active material layer 12 is disposed on one surface of a positive electrode current collector made of aluminum (Al) foil 11. The positive electrode active material layer 12 is a composite layer containing a positive electrode active material 13, a binder (not shown), and a conductive additive (not shown). In contrast, the negative electrode 20 has a configuration in which a negative electrode active material 21 is supported within pores of foamed aluminum 22, which is a conductive porous structure (conductive porous body). Here, the foamed aluminum 22 on the negative electrode 20 side functions as a negative electrode current collector. The electrolyte layer 30 of this embodiment contains fluoride ions (F - ) is used. In the fluoride ion battery 1 of this embodiment, a boroxine ring-containing polymer (not shown) is dissolved in the electrolyte solution (liquid electrolyte) and injected into the electrolyte layer 30. The boroxine ring-containing polymer penetrates into the voids in the positive electrode active material layer and the negative electrode active material layer together with the electrolyte solution (liquid electrolyte), so that the boroxine ring-containing polymer is present in the positive electrode active material layer and the negative electrode active material layer. The positive electrode current collector made of aluminum (Al) foil 11 and the negative electrode current collector made of foamed aluminum 22 are connected to an external load 40.

[0017] With this configuration, when the fluoride ion battery 1 is discharged, a voltage is applied to the external load 40, causing a current to flow. At this time, the fluoride ions (F - ) mediates the battery reaction by moving through the electrolyte layer 30 from the positive electrode 10 side to the negative electrode 20 side. On the other hand, when charging the fluoride ion battery 1, an external power supply is connected to the positive electrode 10 and the negative electrode 20 instead of the external load 40, and fluoride ions (F - Charging is achieved by forcing the

[0018] The shape of the fluoride ion battery is not particularly limited, but examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, and square type.

[0019] The main components of the fluoride ion battery according to this embodiment will be described below.

[0020] [Positive electrode current collector] The positive electrode current collector is a member that collects current from the positive electrode active material layer. Therefore, the positive electrode current collector must be made of a conductive material. In the positive electrode according to this embodiment, the material of the positive electrode current collector is not particularly limited, but may be an aluminum (Al) foil as shown in FIG. 1, other metal current collectors used in conventionally known secondary batteries, or a resin current collector having a conductive resin layer.

[0021] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material. Positive electrode active materials that can be used in the positive electrode according to this embodiment include pure metals, alloys, metal oxides, and metal fluorides. Examples of elements contained in the positive electrode active material include one or more of Ag, Pt, Au, Cr, Mo, W, V, Fe, Co, Ni, Cu, Zn, S, Sb, Bi, Sn, Pb, and C. Among these, the positive electrode active material preferably contains one of Fe, Co, Ni, Cu, Pb, Bi, and Zn. The positive electrode active material more preferably contains at least one of Cu and Bi, and even more preferably contains Cu. Specifically, the positive electrode active material preferably contains at least one selected from the group consisting of Cu metal, Cu alloy, Cu fluoride, Bi metal, Bi alloy, and Bi fluoride, more preferably contains at least one selected from the group consisting of Cu fluoride and Bi fluoride, further preferably contains one or both of CuF2 and BiF3, and particularly preferably contains CuF2. As shown in the examples and comparative examples described below, these positive electrode active materials have a significant problem of reduced charge / discharge performance due to metal ions eluted from the positive electrode active material, making the effects achieved by the present invention more pronounced.

[0022] The positive electrode active material layer according to this embodiment may further contain a binder, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or polyimide (PI).

[0023] The positive electrode active material layer according to this embodiment may further contain a conductive additive, such as carbon black (e.g., acetylene black), graphite, carbon materials (e.g., carbon nanotubes), or metal particles.

[0024] [Negative electrode current collector] The negative electrode current collector is a member that collects current from the negative electrode active material layer. Therefore, the negative electrode current collector must be made of a conductive material. In the negative electrode according to this embodiment, the material of the negative electrode current collector is not particularly limited, but is preferably aluminum (Al), magnesium (Mg), or an alloy thereof. By adopting such a configuration, the progression of side reactions due to reductive decomposition of the electrolyte can be suppressed.

[0025] The shape of the negative electrode current collector is not particularly limited as long as it can hold the negative electrode active material (or a composite containing the negative electrode active material and an additive such as a binder). Examples of the shape of the negative electrode current collector include a foil shape and a conductive porous structure (conductive porous body). The conductive porous body is a member having a porous structure (numerous voids) inside and exhibiting electrical conductivity. Examples of conductive porous bodies include conductive porous bodies made of aluminum (Al), magnesium (Mg), or an alloy thereof. The conductive porous body constituting the negative electrode current collector is preferably made of foamed metal, and more preferably made of foamed aluminum. The use of a conductive porous body enables the formation of a good conductive path. Therefore, when a conductive porous body is used as the negative electrode current collector, no conductive additive is required in the negative electrode active material layer.

[0026] [Negative electrode active material layer] The negative electrode active material layer contains a negative electrode active material. Examples of the negative electrode active material that can be used for the negative electrode according to this embodiment include pure metals, alloys, metal oxides, metal fluorides, and the like. Among them, it is preferable that the negative electrode active material contains a metal fluoride. There is no particular limitation on the specific type of the metal fluoride. As an example, the metal fluoride is Al 1-x (Ti) x F3 (where 0 < x ≦ 0.5), Ti 1-y (Al) y F3 (where 0 ≦ y ≦ 0.5) and at least one selected from the group consisting of CeF3. Here, x is not particularly limited as long as it is a real number greater than 0 and less than or equal to 0.5, preferably 0.01 or more and 0.1 or less, and more preferably 0.02 or more and 0.5 or less. Also, y is not particularly limited as long as it is a real number greater than or equal to 0 and less than or equal to 0.5, preferably 0.01 or more and 0.1 or less, and more preferably 0.02 or more and 0.08 or less. Among them, from the viewpoint of being able to particularly construct a high-capacity fluoride ion battery, the metal fluoride more preferably contains Al 1-x (Ti) x F3 (where 0 < x ≦ 0.5). Note that for Al 1-x (Ti) x F3 and Ti 1-y (Al) y F3 (y > 0), under an inert atmosphere such as argon or nitrogen, powders of aluminum trifluoride (AlF3) and titanium trifluoride (TiF3) are weighed in a desired mass ratio, and a mixing treatment using a mixing device such as a ball mill can be performed for production.

[0027] The negative electrode active material layer constituting the negative electrode according to this embodiment may further contain a binder and a conductive assistant described in the column of the positive electrode active material layer.

[0028] As described above, when a conductive porous body is used as a negative electrode current collector, a conductive additive is not required in the negative electrode active material layer. In such cases, the negative electrode active material layer preferably does not substantially contain a conductive additive. In this specification, "substantially does not contain a conductive additive" refers to a conductive additive content of 3% by mass or less relative to the total solid content of the negative electrode active material layer. The content is preferably 1% by mass or less, and more preferably 0% by mass (i.e., no conductive additive is contained).

[0029] The fluoride ion battery according to this embodiment is characterized in that the positive electrode active material layer and the negative electrode active material layer contain a boroxine ring-containing polymer in at least a part thereof, i.e., the boroxine ring-containing polymer is contained in either the positive electrode active material or the negative electrode active material, or in both the positive electrode active material layer and the negative electrode active material layer.

[0030] In this specification, a boroxine ring-containing polymer refers to a polymer containing two or more repeating units having a boroxine ring (a six-membered ring structure in which boron atoms and oxygen atoms are arranged alternately). Therefore, compounds having one boroxine ring (monocyclic boroxine compounds), such as 2,4,6-trimethoxyboroxine (TMBx), used in the comparative examples described below, do not fall under the category of boroxine ring-containing polymers in this specification.

[0031] As the boroxine ring-containing polymer, for example, a trialkoxyboroxine polymer represented by the following formula is preferably used.

[0032] [ka]

[0033] In the above formula, R a and R b each independently represents an alkylene group; R ceach independently represents an alkyl group, m and n represent the number of repetitions of the oxyalkylene group and are each independently an integer of 1 to 10, and X and Y represent the number of repetitions of the boroxine ring-containing unit and are each independently an integer of 1 or more.

[0034] The alkylene group is preferably an alkylene group having 1 to 10 carbon atoms, more preferably an alkylene group having 2 to 8 carbon atoms, even more preferably an alkylene group having 2 to 6 carbon atoms, and particularly preferably an alkylene group having 2 to 4 carbon atoms. Specific examples of such alkylene groups include a methylene group, an ethylene group, a trimethylene group, a propylene group, an isopropylene group, a butylene group, an isobutylene group, a sec-butylene group, a tert-butylene group, a pentamethylene group, and a hexamethylene group.

[0035] The alkyl group is preferably an alkyl group having 1 to 6 carbon atoms, and more preferably an alkyl group having 1 to 3 carbon atoms. Specific examples of such alkyl groups include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an iso-pentyl group, a tert-pentyl group, a neopentyl group, and an n-hexyl group.

[0036] The above m and n represent the number of repetitions of the oxyalkylene group, and are each independently an integer of preferably 2 to 8, and more preferably an integer of 4 to 8.

[0037] The trialkoxyboroxine polymer represented by the above formula is a compound of boron oxide (B2O3) and polyalkylene glycol monoalkyl ether (HO(R b O) n R c ) and polyalkylene glycol (HO(R a O) mH) can be synthesized. More detailed methods for synthesizing trialkoxyboroxine polymers are described in Chemistry Letters, The Chemical Society of Japan, 1997, p. 915; JP-A No. 11-54151; JP-A No. 2005-093376, etc.

[0038] The inclusion of a boroxine ring-containing polymer in the positive electrode active material layer of this embodiment provides improved charge-discharge performance, such as an increased reversible capacity and improved cycle characteristics. The present inventors speculate that the mechanism by which such effects are achieved is as follows.

[0039] It is believed that a dissolution equilibrium defluorination reaction and an anode dissolution side reaction occur on the positive electrode side of a fluoride ion battery. In either reaction, according to conventional techniques, a large amount of metal ions dissolved from the positive electrode active material diffuses into the electrolyte, resulting in a decrease in reversible capacity and electrode deformation due to dendrite growth. On the other hand, according to the present invention, it is believed that the boroxine ring-containing polymer contained in the positive electrode active material layer forms a coating on the positive electrode surface during charge and discharge. Because the electron pairs of fluoride ions can coordinate to the vacant orbital of boron (B) contained in the coating, the coating has fluoride ion conductivity. Furthermore, the presence of the coating suppresses the diffusion of metal ions into the electrolyte. As a result, it is possible to suppress a decrease in reversible capacity due to a large amount of metal ions diffusing into the electrolyte and electrode deformation due to dendrite growth without inhibiting the fluorination / defluorination reaction at the positive electrode, thereby improving charge / discharge characteristics.

[0040] Furthermore, by including a boroxine ring-containing polymer in the negative electrode active material layer of this embodiment, reductive decomposition of the electrolyte solution (liquid electrolyte) at the negative electrode can be suppressed, thereby improving charge-discharge performance. The present inventors speculate that the mechanism by which such effects are achieved is as follows.

[0041] In the past, a problem with the negative electrode of a fluoride ion battery has been a deterioration in charge-discharge characteristics due to a reductive decomposition side reaction of the electrolyte (liquid electrolyte) on the surface of the negative electrode current collector and / or negative electrode active material. According to the present invention, similar to the mechanism at the positive electrode described above, it is believed that the boroxine ring-containing polymer forms a fluoride ion-conductive coating upon charge and discharge. By coating the surface of the negative electrode current collector and / or negative electrode active material with this coating, the reductive decomposition of the electrolyte (liquid electrolyte) can be suppressed without inhibiting the fluorination-defluorination reaction at the negative electrode, thereby improving the charge-discharge characteristics.

[0042] Although the specific structure of the coating is unknown, the B1s peak is observed in the XPS spectra of Examples 1-1, 2-1, and 3-1 described below, confirming the presence of a coating containing boron on the electrode surface.

[0043] Therefore, the fluoride ion battery according to one embodiment of the present invention has a boron-containing coating on at least a part of the surface of the positive electrode active material layer and the negative electrode active material layer.

[0044] In the fluoride ion battery according to this embodiment, the content of the boroxine ring-containing polymer is not particularly limited, but is preferably more than 0% by mass and not more than 10% by mass, more preferably 1% by mass or more and not more than 10% by mass, even more preferably 2.5% by mass or more and not more than 7.5% by mass, and particularly preferably 2.5% by mass or more and not more than 5% by mass, relative to the total mass of the boroxine ring-containing polymer and the electrolyte. When the content of the boroxine ring-containing polymer is within the above range, charge / discharge performance can be further improved.

[0045] [Electrolyte layer] The electrolyte layer is a component placed between the positive electrode (positive electrode active material layer) and the negative electrode (negative electrode active material layer), and contains fluoride ions (F -) can pass through the electrolyte layer. In the fluoride ion battery according to this embodiment, the electrolyte contained in the electrolyte layer can be a liquid electrolyte (electrolytic solution) or a solid electrolyte, without any particular limitation, and a combination of these may also be used. When the electrolyte layer is a liquid electrolyte (electrolytic solution), a separator may be used in the electrolyte layer. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the fluoride ion battery, and examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabric, polyphenylene sulfide nonwoven fabric, and aramid nonwoven fabric, and microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0046] As the liquid electrolyte (electrolyte), a conventionally known electrolyte for fluoride ion batteries may be used. However, it is particularly preferable to use an organic electrolyte containing a single or mixed organic solvent of an ester or lactone having an α-position hydrogen (e.g., γ-butyrolactone or ε-caprolactone) and an alkali metal fluoride having an alkali metal cation and a fluoride ion (e.g., cesium fluoride). For the organic electrolyte, the technology described in JP 2021-36512 A can be appropriately adopted. By using such an electrolyte as the electrolyte, excellent capacity characteristics can be exhibited. Examples of solid electrolytes include fluorides containing at least one element selected from the group consisting of lanthanoid elements (e.g., La, Ce), alkali metal elements (e.g., Li, Na, K, Rb, Cs), and alkaline earth elements (e.g., Ca, Sr, Ba).

[0047] [Alkali metal fluorides] Ionically conductive fluoride ions that directly participate in the electrode reaction can also be introduced into the electrolyte by dissolving organic fluorides, but this is not suitable for high-capacity fluoride-ion batteries due to issues such as the electrochemical stability of the coexisting organic cations. Considering other conditions, including the effect on the gravimetric energy density of secondary batteries, the most desirable source of fluoride ions is an alkali metal fluoride such as lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), rubidium fluoride (RbF), or cesium fluoride (CsF).

[0048] [Mixed lithium salts] This fluoride ion-conductive electrolyte, in which only alkali metal fluorides are dissociated and dissolved at a concentration of 1 mM or more, is prone to undergo a side reduction reaction involving the solvent derived from fluoride ions in the high negative potential region, which competes with the fluorination reaction of metals less noble than zinc and interferes with the intended battery operation. This problem can be easily solved by adding an excess of any lithium salt that can be co-dissolved in the electrolyte of this embodiment, such as LiFSA (FSA: bis(fluorosulfonyl)amide), to the electrolyte, making it possible to operate a battery utilizing the fluorination reaction of the most noble metal species, such as aluminum or lanthanum.

[0049] In preparing the mixed electrolyte solution, the concentration of the excess lithium salt added is desirably adjusted to at least 3 times, preferably 5 times, and more preferably about 10 times the concentration of the dissociated alkali metal fluoride. If the concentration (amount added) of this lithium salt is 3 times or more, the fluoride ions and lithium ions do not form an insoluble solid, and therefore the fluoride ion concentration is not reduced, which is desirable.

[0050] The anion species of the lithium salt is FSA - Besides TFSA - , BETA - , BF4 - , PF6 - , ClO4 -That is, the lithium salt added in excess is preferably any one of LiFSA, LiTFSA, LiBETA, LiBF4, LiPF6, and LiClO4, or a mixture of two or more thereof.

[0051] [Mixed barium salts] Although the effect of the lithium salt cannot be expected, the irreversible reduction current in the high negative potential region can also be suppressed by mixing an excess amount of a barium salt, such as Ba(FSA)2, into the electrolyte. In preparing the above mixed electrolyte, it is desirable to prepare the concentration of the excess barium salt to be at least 3 times, preferably 5 times, and more preferably about 10 times the concentration of the dissociated alkali metal fluoride. If the concentration (amount added) of this barium salt is 3 times or more, fluoride ions and barium ions do not form an insoluble solid, and therefore the fluoride ion concentration is not reduced, which is desirable. As the anion species of the barium salt, FSA - Besides TFSA - , BETA - , BF4 - That is, the barium salt added in excess is preferably any one of Ba(FSA)2, Ba(TFSA)2, Ba(BETA)2, and Ba(BF4)2, or a mixture of two or more thereof. [Example]

[0052] The present invention will be explained in more detail below with reference to experimental examples.

[0053] [Manufacturing Example 1] (Preparation of CuF2 electrodes) Commercially available CuF powder (Sigma-Aldrich) was used as the active material. The conductive additive, acetylene black (AB), was added to the powder in a mass ratio of 90:10. The mixture was then mixed in a ball mill under an Ar atmosphere to prepare a CuF / AB composite. The conductive additive, AB, and the binder, polyvinylidene fluoride (PVdF), were then added to the resulting composite to a mass ratio of 75:12.5:12.5. N-methyl-2-pyrrolidone (NMP) was then added and mixed to prepare a slurry with an appropriate viscosity. The resulting slurry was applied to one surface of an aluminum (Al) foil current collector and vacuum-dried at 120°C. The resulting mixture was then pressed to a porosity of 40-50% to prepare an electrode sheet. The obtained positive electrode sheet was cut into a size of 5 mm x 10 mm, and a part of the composite surface was further peeled off so that the composite surface had a size of 5 mm x 5 mm, thereby producing an electrode of this production example.

[0054] [Manufacturing Example 2] (Preparation of BiF3 electrodes) The electrode of this production example was produced in the same manner as in Production Example 1, except that commercially available BiF3 powder (manufactured by Sigma-Aldrich) was used as the active material.

[0055] [Manufacturing Example 3] (Ti 0.97 Al 0.03 Synthesis of F3) Commercially available TiF powder (Sigma-Aldrich) was mixed with commercially available AlF powder (Fujifilm Wako Pure Chemical Industries, Ltd.) as an additive at a predetermined ratio, and the mixture was mixed in a ball mill under an argon atmosphere to form Ti, which is the active material. 0.97 Al 0.03 F3 was synthesized.

[0056] (Preparation of Ti(Al)F3 electrode) The active material prepared above and polyvinylidene fluoride (PVdF) binder were mixed in a mass ratio of 95:5, and N-methyl-2-pyrrolidone (NMP) was added to form a slurry with an appropriate viscosity. The resulting slurry was filled into a conductive porous aluminum foam (CELMET®, manufactured by Sumitomo Electric Industries, Ltd., aluminum, porosity 96%), dried, and pressed to produce an electrode for this manufacturing example.

[0057] [Example 1-1] (Synthesis of Boroxine Ring-Containing Polymers) The following procedures were performed using a glove box, gas bag, Schlenk apparatus, etc. to avoid working in the atmosphere. Dry toluene (300 mL), polyethylene glycol monomethyl ether (PEGMME350; 60.2 g), and tetraethylene glycol (TEG; 25.1 g) were charged into a reaction vessel equipped with a Dean-Stark water separator and stirred at room temperature (25 °C). Boron oxide (15 g) was gradually added to the solution. After the addition was complete, the mixture was heated and stirred at an internal temperature of 108 °C. Heating and stirring continued while removing the generated water. After 12 hours, heating was stopped. Approximately 9.0 g of water was discharged from the Dean-Stark water separator. After allowing the reaction solution to cool to room temperature, insoluble matter was removed by filtration through a membrane filter (1.0 μm). The filtrate was concentrated under reduced pressure. A liquid nitrogen trap, an oil pump (vacuum: 0.05 mmHg), and an oil bath (temperature: 90 °C) were then attached, and the mixture was evaporated under reduced pressure while stirring. After confirming that the weight loss had ceased, a light brown, transparent, viscous liquid (80 g) was obtained. 1 From the results of H-NMR analysis and IR analysis, it was confirmed that the polymer was a boroxine ring-containing polymer (BxP) represented by the following formula.

[0058] [ka]

[0059] (Preparation of Electrolyte) An electrolyte solution was prepared by dissolving LiFSA (0.14 M) and CsF (0.012 M) in gamma-butyrolactone (GBL). Specifically, a CsF / RBL electrolyte solution was prepared by dissolving a predetermined amount of CsF in gamma-butyrolactone through a multi-stage dissolution process, following the method described in Example 1 of JP 2021-36512 A. Then, a predetermined amount of LiFSA was dissolved in the CsF / RBL electrolyte solution to prepare an electrolyte. Note that to indicate the possibility that significant modification of the solvent itself occurs through the multi-stage dissolution process, the name RBL (Reformed Butyrolactone) is used to distinguish the solvent from pure GBL solvent.

[0060] (Cell preparation) A beaker cell of this example was fabricated using the CuF2 electrode fabricated in Production Example 1 as the working electrode, an electrode sheet (activated carbon:PTFE=70:30 (mass ratio)) made of activated carbon and a binder polytetrafluoroethylene (PTFE) as the counter electrode, and an Ag wire as the reference electrode. The beaker cell was then filled with the electrolyte solution prepared above and the boroxine ring-containing polymer (BxP) synthesized above. The electrolyte solution and BxP were poured into the beaker cell so that the BxP content was 5 mass% relative to the total mass of the BxP and electrolyte solution. The beaker cell of this example was fabricated using the above method.

[0061] [Example 1-2] A beaker cell of this example was produced in the same manner as in Example 1-1, except that the content of BxP was set to 2.5 mass % with respect to the total mass of BxP and the electrolyte solution.

[0062] [Examples 1-3] A beaker cell of this example was produced in the same manner as in Example 1-1, except that the content of BxP was 10 mass % with respect to the total mass of BxP and the electrolyte solution.

[0063] [Comparative Example 1-1] The beaker cell of this comparative example was produced in the same manner as in Example 1-1, except that BxP was not added to the beaker cell.

[0064] [Comparative Example 1-2] Instead of BxP, 2,4,6-trimethoxyboroxine (TMBx) represented by the following formula was poured into the beaker cell so that the amount was 5 mass % relative to the total mass of TMBx and the electrolyte. A beaker cell of this comparative example was fabricated in the same manner as in Example 1-1.

[0065] [ka]

[0066] [Example 2-1] A beaker cell of this example was produced in the same manner as in Example 1-1, except that the BiF3 electrode produced in Production Example 2 was used as the working electrode instead of the CuF2 electrode.

[0067] [Comparative Example 2-1] The beaker cell of this comparative example was produced in the same manner as in Example 2-1, except that BxP was not added to the beaker cell.

[0068] [Comparative Example 2-2] Instead of BxP, 2,4,6-trimethoxyboroxine (TMBx) was poured into the beaker cell so that the amount was 5 mass % relative to the total mass of TMBx and the electrolyte solution. Except for this, the beaker cell of this comparative example was fabricated in the same manner as in Example 2-1.

[0069] [Example 3-1] A beaker cell of this example was produced in the same manner as in Example 1-1, except that the Ti(Al)F3 electrode produced in Production Example 3 was used as the working electrode instead of the CuF2 electrode.

[0070] [Comparative Example 3-1] The beaker cell of this comparative example was produced in the same manner as in Example 3-1, except that BxP was not added to the beaker cell.

[0071] <Charge / discharge test> A charge-discharge test was conducted on the beaker cells fabricated in each of the above-mentioned examples and comparative examples. Specifically, a charge-discharge test was conducted on each beaker cell using a constant current test, and the reversible capacity of each cycle was measured. The measurements were conducted in an argon atmosphere at room temperature (25°C). The beaker cells using CuF2 and BiF3 electrodes were charged and discharged at a charge-discharge rate equivalent to 0.04C. The beaker cell using Ti(Al)F3 electrodes was charged and discharged at a charge-discharge rate equivalent to 0.02C.

[0072] The results are shown in Tables 1 to 4 below and Figures 2 to 8.

[0073] (1) Effect of additives on the behavior of CuF2 electrodes in a beaker cell

[0074] [Table 1]

[0075] Table 1 summarizes the effects of additives on beaker cells using CuF2 electrodes. Figure 2 (A) to (C) are graphs showing charge-discharge curves for each example after 1 to 3 charge-discharge cycles. Figure 3 shows photographs of the coloration of the electrolyte before and after a 3-cycle charge-discharge test.

[0076] As shown in FIG. 2(A), Comparative Example 1-1 has a small reversible capacity and poor charge-discharge performance (shown by × in Table 1). As shown in FIG. 2(B), Comparative Example 1-2 has a large reversible capacity but poor cycle characteristics (large capacity loss due to cycling), resulting in partially improved charge-discharge performance (shown by △ in Table 1). As shown in FIG. 2(C), Example 1-1 has a large reversible capacity and excellent cycle characteristics (small capacity loss due to cycling), resulting in excellent charge-discharge performance (shown by ○ in Table 1).

[0077] 3, the electrolyte of Comparative Example 1-1 after three charge-discharge cycles (second from the left in FIG. 3) is colored blue. This blue color appears when Cu ions form aqua complex ions, and indicates that Cu ions have dissolved into the electrolyte.

[0078] From the above results, it can be seen that Example 1-1 according to the present invention has a large reversible capacity and excellent cycle characteristics, and therefore has improved charge / discharge characteristics. In Example 1-1, no Cu ion elution into the electrolyte solution was observed, suggesting that the formation of a coating derived from the boroxine ring-containing polymer on the electrode surface during charge / discharge inhibits the diffusion of Cu ions into the electrolyte solution.

[0079] (2) Effect of additives on the performance of a beaker cell using a BiF3 electrode

[0080] [Table 2]

[0081] Table 2 summarizes the effects of additives on beaker cells using BiF3 electrodes. Figure 4 (A) to (C) are graphs showing charge-discharge curves for each example after 1 to 3 charge-discharge cycles. Figure 5 shows photographs of the electrode deformation before and after a 5-cycle charge-discharge test.

[0082] As shown in FIG. 4(A), Comparative Example 2-1 has a large reversible capacity but poor cycle characteristics (large capacity loss due to cycling), resulting in some improvement in charge-discharge performance (shown as △ in Table 2). As shown in FIG. 4(B), Comparative Example 2-2 has a large resistance and a small reversible capacity, resulting in poor charge-discharge performance (shown as × in Table 2). As shown in FIG. 4(C), Example 2-1 has a large reversible capacity and excellent cycle characteristics (small capacity loss due to cycling), resulting in excellent charge-discharge performance (shown as ○ in Table 2).

[0083] Furthermore, as shown in Fig. 5, the electrode of Comparative Example 2-1 (second from the left in Fig. 5) after the charge-discharge test after 5 cycles was significantly deformed. This deformation was thought to be due to the formation of dendrites by Bi ions that dissolved and diffused in the electrolyte.

[0084] From the above results, it can be seen that Example 2-1 according to the present invention has a large reversible capacity and excellent cycle characteristics, and therefore has improved charge-discharge characteristics. In Example 1-1, no deformation of the electrode was observed, suggesting that the formation of a coating derived from the boroxine ring-containing polymer on the electrode surface during charge-discharge cycles suppresses the diffusion of Bi ions into the electrolyte.

[0085] (3) Effect of additives on the Ti(Al)F3 electrode beaker cell

[0086] [Table 3]

[0087] Table 3 summarizes the effects of additives on beaker cells using Ti(Al)F3 electrodes. Figures 6(A) and (B) are graphs showing the charge-discharge curves for each example after 1 to 6 charge-discharge cycles.

[0088] As shown in Figure 6(A), Comparative Example 3-1 has a small reversible capacity, indicating poor charge-discharge performance (shown by x in Table 3). As shown in Figure 6(B), Example 3-1 has a large reversible capacity, indicating excellent charge-discharge performance (shown by ◯ in Table 3).

[0089] When the electrodes of each example were observed after the charge-discharge test after 6 cycles, no deformation was observed in any of the electrodes.

[0090] From the above results, it can be seen that Example 3-1 according to the present invention has a large reversible capacity and therefore improved charge / discharge characteristics. In Example 3-1, it is presumed that the formation of a coating derived from the boroxine ring-containing polymer on the electrode surface by charge / discharge suppresses the reductive decomposition of the electrolyte on the electrode (negative electrode) surface, resulting in a large reversible capacity.

[0091] (4) Effect of boroxine ring-containing polymer (BxP) content in a beaker cell using a CuF2 electrode

[0092] [Table 4]

[0093] Table 4 summarizes the effect of the boroxine ring-containing polymer content in a beaker cell using a CuF2 electrode. Figures 7(A) to 7(D) are graphs showing the charge-discharge curves for each example after 1 to 3 charge-discharge cycles.

[0094] The charge-discharge performance of Comparative Example 1-1 and Example 1-1 is as described above. As shown in FIG. 7(B), Example 1-2 shows an improved reversible capacity and improved charge-discharge performance (shown by ○ in Table 1). As shown in FIG. 7(D), Example 1-3 shows a large reversible capacity but poor cycle characteristics (large capacity loss due to cycling), resulting in a partial improvement in charge-discharge performance (shown by △ in Table 1).

[0095] In addition, when the electrolyte solution of each example was observed after the charge-discharge test for three cycles, no coloration of the electrolyte solution was observed in Examples 1-2 and 1-3.

[0096] (5) XPS measurement of electrode surface For Examples 1-1, 2-1, and 3-1, each containing 5% by mass of the boroxine ring-containing polymer, X-ray photoelectron spectroscopy (XPS) was performed on the electrode surface before and after three charge-discharge cycles. Specifically, the electrodes were removed from the beaker cells, washed with GBL, and vacuum-dried to prepare measurement samples. XPS measurements were performed under the following conditions:

[0097] (XPS measurement conditions) Measurement equipment: ULVAC-PHI, Inc. Quantera SXM X-ray source: AlKα ray (1486.6eV) X-ray output: 25W 15kV Detection area: 100 μmφ Detection location: Center of electrode sample Detection depth: several nm (take-off angle 45°) Measured spectra: Cu2p, Bi4f, Ti2p, B1s.

[0098] The results are shown in Figure 8. Figure 8 shows XPS spectra of the electrodes of Examples 1-1, 2-1, and 3-1 before the charge-discharge test and after three cycles of the charge-discharge test. As shown in Figure 8, no peaks derived from boron (B) were observed in the electrodes before the charge-discharge test, but peaks derived from boron (B) were observed in the electrodes after the charge-discharge test. This suggests that a coating derived from the boroxine ring-containing polymer is formed on the electrode surface by charge-discharge. [Explanation of symbols]

[0099] 1 fluoride-ion battery, 10 positive electrode, 11 Aluminum (Al) foil (positive electrode current collector), 12 positive electrode active material layer, 13 Positive electrode active material, 20 negative electrode, 21 negative electrode active material, 22 Foamed aluminum (negative electrode current collector), 30 electrolyte layer, 40 external load, F- Fluoride ions.

Claims

1. a positive electrode having a positive electrode active material layer containing a positive electrode active material and a positive electrode current collector that collects current from the positive electrode active material layer; a negative electrode having a negative electrode active material layer containing a negative electrode active material and a negative electrode current collector that collects current from the negative electrode active material layer; and an electrolyte layer disposed between the positive electrode and the negative electrode, the electrolyte layer containing an electrolyte having fluoride ion conductivity; and a boroxine ring-containing polymer is contained in at least a portion of the positive electrode active material layer and the negative electrode active material layer; A fluoride ion battery, wherein the content of the boroxine ring-containing polymer is more than 0 mass % and 10 mass % or less with respect to the total mass of the boroxine ring-containing polymer and the electrolyte.

2. 2. The fluoride ion battery according to claim 1, wherein a coating containing boron is provided on at least a portion of the surface of the positive electrode active material layer and the negative electrode active material layer.

3. 3. The fluoride ion battery according to claim 1, wherein the positive electrode active material contains at least one element of Cu and Bi.

4. The fluoride ion battery according to any one of claims 1 to 3, wherein the positive electrode active material contains Cu element.

5. 5. The fluoride ion battery according to claim 1, wherein the electrolyte is an organic electrolytic solution containing an ester-based or lactone-based organic solvent alone or in combination having an α-position hydrogen, and an alkali metal fluoride having an alkali metal cation and a fluoride ion.

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