JP7724079B2 - fluoride-ion battery - Google Patents

Description

The present invention relates to a fluoride ion battery.

Anion-based fluoride-ion batteries are an innovative type of battery that significantly improves the energy density of conventional Li-ion batteries. These batteries utilize the reaction of fluoride ions (F ^- ) with metals to produce polyvalent metal fluorides, and the reverse reaction (defluorination of metal fluorides). In these batteries, the lightest halide ions, F ^- ions (M = 19), move between the electrodes, and in some cases, two to three or more F ^- ions (and the same number of electrons) per metal atom are involved in the electrode reaction. These are the main factors that result in the high energy density mentioned above.

The fluoride ion battery does not require a host lattice, which is an essential component of LIBs. This type of battery, in which F ^ions play a key role in both the electrode reaction and charge transfer in the electrolyte, is known as an FIB (Fluoride Ion Battery) or an FSB (Fluoride Shuttle Battery), which emphasizes the role of the bidirectional transfer of F ^ions .

The reduction (defluorination) reaction of metal fluorides, one of the basic elements of fluoride-ion batteries, was already widely reported in research conducted in the 1970s, when solid electrolytes were attracting attention. However, it was only relatively recently that significant reversibility in charge and discharge, which is essentially essential for secondary batteries, was confirmed, albeit incompletely, within the framework of all-solid-state batteries.

In addition, due to limitations generally associated with the ionic conductivity of solid electrolytes and the electrochemical reactivity at solid-state interfaces, there have only been a few recent reports of room-temperature operation of solid-state FIB batteries.

In principle, the above restrictions 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 coordinating with and stabilizing the fluoride ions. It has been reported that the use of boroxine-based compounds in particular facilitates the smooth movement of fluoride ions in the electrolyte.

JP 2017-10865 A

The fluoride ion battery described in Patent Document 1 utilizes a metal fluorination reaction and a metal fluoride defluorination reaction. However, the inventors' investigations have revealed that such a fluoride ion battery does not provide sufficient charge/discharge performance.

The present invention therefore aims to provide a means for improving the charge/discharge performance of fluoride ion batteries.

To achieve the above objectives, the inventors conducted extensive research. As a result, they discovered that by conducting charge-discharge reactions in the presence of a boroxine ring-containing polymer in the active material layer of a fluoride ion battery, charge-discharge performance can be significantly improved, leading to the completion of the present invention.

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. Here, 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.

According to the present invention, the inclusion of a boroxine ring-containing polymer in at least a portion of the positive electrode active material layer and the negative electrode active material layer suppresses 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. As a result, it is possible to improve the charge/discharge performance of fluoride ion batteries.

FIG. 1 is a conceptual diagram showing the principle of a fluoride ion battery according to one embodiment of the present invention. 2A to 2C are graphs showing the charge/discharge curves of Comparative Example 1-1, Comparative Example 1-2, and Example 1-1, respectively. FIG. 3 is a photograph showing the coloration of the electrolyte solution before and after the charge-discharge test for Comparative Example 1-1, Comparative Example 1-2, and Example 1-1. 4A to 4C are graphs showing the charge/discharge curves of Comparative Example 2-1, Comparative Example 2-2, and Example 2-1, respectively. 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. FIGS. 6A and 6B are graphs showing the charge/discharge curves of Comparative Example 3-1 and Example 3-1, respectively. 7A to 7D are graphs showing the charge/discharge curves of Comparative Example 1-1, Example 1-2, Example 1-1, and Example 1-3, respectively. FIG. 8 shows XPS spectra of the electrodes of Examples 1-1, 2-1, and 3-1 before and after the charge-discharge test.

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. This 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.

Embodiments of a fluoride ion battery according to the present invention will be described in detail below with reference to the drawings.

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 includes a positive electrode active material layer 12 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 includes a negative electrode active material 21 supported within pores of aluminum foam 22, which is a conductive porous structure (conductive porous body). Here, the aluminum foam 22 on the negative electrode 20 side functions as a negative electrode current collector. The electrolyte layer 30 of this embodiment uses an electrolytic solution (liquid electrolyte) containing fluoride ions (F ⁻ ). In the fluoride ion battery 1 of this embodiment, a boroxine ring-containing polymer (not shown) is dissolved in the electrolytic 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 electrolytic 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.

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, fluoride ions (F ⁻ ) contained in the electrolyte layer 30 mediate 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 the fluoride ion battery 1 is charged, an external power supply is connected to the positive electrode 10 and the negative electrode 20 instead of the external load 40, and charging is achieved by forcibly moving fluoride ions (F ⁻ ) from the negative electrode 20 to the positive electrode 10 through the electrolyte layer 30.

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

The main components of the fluoride ion battery according to this embodiment are described below.

[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.

[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 CuF ₂ and BiF ₃ , and particularly preferably contains CuF _2. As shown in the examples and comparative examples described below, these positive electrode active materials have a significant problem of reduced charge and discharge performance due to metal ions eluted from the positive electrode active material, and therefore the effects achieved by the present invention become more pronounced.

The positive electrode active material layer according to this embodiment may further contain a binder. Examples of binders include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and polyimide (PI).

The positive electrode active material layer according to this embodiment may further contain a conductive additive. Examples of conductive additives include carbon materials such as carbon black (e.g., acetylene black), graphite, and carbon nanotubes, as well as metal particles.

[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.

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 foil and a conductive porous structure (conductive porous body). A conductive porous body is a conductive material having a porous structure (numerous voids) inside, and examples include conductive porous bodies made of aluminum (Al), magnesium (Mg), or an alloy thereof. Preferably, the conductive porous body constituting the negative electrode current collector is made of foamed metal, and more preferably, 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, a conductive additive is not required in the negative electrode active material layer.

[Negative electrode active material layer]
The negative electrode active material layer includes a negative electrode active material. Examples of negative electrode active materials that can be used in the negative electrode according to this embodiment include pure metals, alloys, metal oxides, and metal fluorides. Among these, it is preferable that the negative electrode active material includes a metal fluoride. While there are no particular limitations on the specific type of metal fluoride, as an example, the metal fluoride preferably includes at least one selected from the group consisting of Al _1-x (Ti) _x F ₃ (where 0<x≦0.5), Ti _1-y (Al) _y F ₃ (where 0≦y≦0.5), and CeF _3. Here, x is not particularly limited as long as it is a real number greater than 0 and less than 0.5, but is preferably 0.01 or greater and 0.1 or less, and more preferably 0.02 or greater and 0.5 or less. Furthermore, y is not particularly limited as long as it is a real number greater than 0 and less than 0.5, but is preferably 0.01 or greater and 0.1 or less, and more preferably 0.02 or greater and 0.08 or less. Among these, from the viewpoint of being able to construct a particularly high-capacity fluoride ion battery, it is more preferable that the metal fluoride contains Al _1-x (Ti) _x F ₃ (where 0<x≦0.5). Note that Al _1-x (Ti) _x F ₃ or Ti _1-y (Al) _y F ₃ (y>0) as the negative electrode active material can be produced by weighing aluminum trifluoride (AlF ₃ ) powder and titanium trifluoride (TiF ₃ ) powder in a desired mass ratio in an inert atmosphere such as argon or nitrogen, and mixing the powder using a mixing device such as a ball mill.

The negative electrode active material layer constituting the negative electrode according to this embodiment may further contain a binder or conductive additive as described in the section on the positive electrode active material layer.

As mentioned 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, it is preferable that the negative electrode active material layer is substantially free of a conductive additive. In this specification, "substantially free of a conductive additive" means that the content of the conductive additive is 3% by mass or less relative to the total solid content of the negative electrode active material layer. This content is preferably 1% by mass or less, and more preferably 0% by mass (i.e., no conductive additive is included).

The fluoride ion battery according to this embodiment 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. That is, 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.

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.

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

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

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 methylene, ethylene, trimethylene, propylene, isopropylene, butylene, isobutylene, sec-butylene, tert-butylene, pentamethylene, and hexamethylene.

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 methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, iso-pentyl, tert-pentyl, neopentyl, and n-hexyl.

The above m and n represent the number of repeating oxyalkylene groups, and are each independently an integer of preferably 2 to 8, and more preferably an integer of 4 to 8.

The trialkoxyboroxine polymer represented by the above formula can be synthesized by reacting boron oxide (B ₂ O ₃ ), polyalkylene glycol monoalkyl ether (HO(R ^b O) _n R ^c ), and polyalkylene glycol (HO(R ^a O) _m H). More detailed methods for synthesizing trialkoxyboroxine polymers are described in Chemistry Letters, The Chemical Society of Japan, 1997, p. 915; JP-A-11-54151; JP-A-2005-093376, etc.

By including a boroxine ring-containing polymer in the positive electrode active material layer of this embodiment, improved charge/discharge performance, such as increased reversible capacity and improved cycle characteristics, is achieved. The inventors speculate that the mechanism by which these effects are achieved is as follows.

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, conventional technology has shown that metal ions dissolved from the positive electrode active material diffuse into the electrolyte in large quantities, resulting in a decrease in reversible capacity and electrode deformation due to dendrite growth. In contrast, according to the present invention, the boroxine ring-containing polymer contained in the positive electrode active material layer is believed to form a coating on the positive electrode surface during charge and discharge. Because the electron pairs of fluoride ions can coordinate to the vacant orbitals of boron (B) contained in the coating, the coating possesses 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 the decrease in reversible capacity and electrode deformation due to dendrite growth, which would otherwise occur due to the diffusion of large amounts of metal ions into the electrolyte, without inhibiting the fluorination/defluorination reaction at the positive electrode, thereby improving charge/discharge characteristics.

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 is suppressed, thereby improving charge/discharge performance. The inventors speculate that the mechanism behind this effect is as follows.

Conventionally, a problem with the negative electrode of a fluoride ion battery is 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 for the positive electrode described above, it is believed that the boroxine ring-containing polymer forms a fluoride ion-conductive coating during charge/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 charge/discharge characteristics.

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.

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

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

[Electrolyte layer]
The electrolyte layer is a component disposed between the positive electrode (positive electrode active material layer) and the negative electrode (negative electrode active material layer) and is a layer that is conductive to fluoride ions (F ⁻ ) so that fluoride ions (F − ) can travel back and forth during charge and discharge. 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, 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 a fluoride ion battery. Examples of the separator include polymer nonwoven fabrics such as polypropylene nonwoven fabrics, polyphenylene sulfide nonwoven fabrics, and aramid nonwoven fabrics, and microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

The liquid electrolyte (electrolyte solution) may be any conventionally known electrolyte solution for fluoride ion batteries. However, it is particularly preferable to use an organic electrolyte solution containing a single or mixed organic solvent of an ester or lactone with an α-position hydrogen (e.g., gamma-butyrolactone or epsilon-caprolactone) and an alkali metal fluoride with an alkali metal cation and a fluoride ion (e.g., cesium fluoride). For this organic electrolyte solution, the technology described in JP 2021-36512 A can be appropriately adopted. By using such an electrolyte solution as the electrolyte, excellent capacity characteristics can be achieved. 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).

[Alkali metal fluorides]
Ion-conductive fluoride ions that directly participate in the electrode reaction can also be introduced into the electrolyte by dissolving an organic fluoride, 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 the influence on the weight energy density of secondary batteries as another condition, 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).

[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 more base than zinc and interferes with the original battery operation. This problem can be easily solved by mixing an excess of any lithium salt that can be co-dissolved in the electrolyte solution of this embodiment, such as LiFSA (FSA: bis(fluorosulfonyl)amide), into the electrolyte solution, making it possible to operate a battery utilizing the fluorination reaction of the most base metal species, such as aluminum or lanthanum.

When preparing the above mixed electrolyte solution, it is desirable to prepare the concentration of the excess lithium salt to be at least three times, preferably five times, and more preferably about ten times the concentration of the dissociated alkali metal fluoride. If the concentration (amount added) of this lithium salt is three times or more, the fluoride ions and lithium ions do not form an insoluble solid, and this is desirable as it does not reduce the fluoride ion concentration.

The anion species of the lithium salt may _be ^TFSA , ^BETA , _BF , ^{PF ,} or _ClO other than ^FSA . That is, the lithium salt added in excess is preferably any one of LiFSA, _LiTFSA , _LiBETA , LiBF, LiPF, or _LiClO , or ^a mixture of two or more thereof.

[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 of a barium salt, such as Ba(FSA) ₂ , into the electrolyte. In preparing the above-mentioned mixed electrolyte, the concentration of the barium salt added in excess is preferably at least three times, preferably five times, and more preferably about ten times the concentration of the dissociated alkali metal fluoride. If the concentration (amount added) of this barium salt is three times or more, the fluoride ions and barium ions do not form an insoluble solid, and therefore the fluoride ion concentration is not reduced, which is desirable. In addition to FSA ^- , TFSA ^- , BETA ^- , and BF ₄ ^- can also be used as the anion species of the barium salt. That is, the barium salt added in excess is preferably any one of Ba(FSA) ₂ , Ba(TFSA) ₂ , Ba(BETA) ₂ and Ba(BF ₄ ) ₂ or a mixture of two or more thereof.

The following experimental examples will further illustrate the present invention.

[Production Example 1]
(Preparation of _CuF2 electrode)
Commercially available CuF ₂ powder (Sigma-Aldrich), which is the active material, was added with acetylene black (AB), a conductive additive, in a mass ratio of CuF ₂ :AB = 90:10, and the mixture was mixed in a ball mill under an Ar atmosphere to produce a CuF ₂ /AB composite. To the resulting composite, the conductive additive AB and the binder polyvinylidene fluoride (PVdF) were added, resulting in a mass ratio of CuF ₂ :AB:PVdF = 75:12.5:12.5, followed by kneading with N-methyl-2-pyrrolidone (NMP) to produce a slurry with an appropriate viscosity. The resulting slurry was applied to one surface of a current collector made of aluminum (Al) foil and vacuum dried at 120 ° C. Next, an electrode sheet was produced by pressing to achieve a porosity of 40 to 50%. 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.

[Production Example 2]
(Preparation of _BiF3 electrode)
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.

[Production Example 3]
₍ Synthesis of _{Ti0.97Al0.03F3 )}
Commercially available _TiF3 powder (manufactured by Sigma-Aldrich) was mixed with commercially available _AlF3 powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as an additive in a predetermined ratio _, and the mixture was mixed in a ball mill under an argon atmosphere to synthesize the _active material _{Ti0.97Al0.03F3} .

(Preparation of Ti(Al) _F3 electrode)
The active material prepared above and polyvinylidene fluoride (PVdF) as a binder were mixed in a mass ratio of 95:5, and N-methyl-2-pyrrolidone (NMP) was added to prepare a slurry with an appropriate viscosity. The resulting slurry was filled into a conductive porous aluminum foam (CELMET (registered trademark), manufactured by Sumitomo Electric Industries, Ltd., made of aluminum, with a porosity of 96%), dried, and pressed to prepare an electrode for this manufacturing example.

[Example 1-1]
(Synthesis of Boroxine Ring-Containing Polymer)
The following operations were performed using a glove box, gas bag, Schlenk apparatus, etc., and operations under atmospheric pressure were avoided. 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 this solution. After the addition was completed, the mixture was heated and stirred at an internal temperature of 108°C. Heating and stirring were continued while removing the generated water. After 12 hours, heating was stopped. The amount of water flowing out of the Dean-Stark water separator was approximately 9.0 g. After allowing the reaction solution to cool to room temperature, insoluble matter was removed by filtration through a membrane filter (1.0 μm), and the filtrate was concentrated under reduced pressure. Furthermore, a liquid nitrogen trap, an oil pump (vacuum pressure: 0.05 mmHg), and an oil bath (temperature: 90°C) were attached, and distillation under reduced pressure was performed while stirring. After confirming that the weight loss had ceased, a pale brown, transparent, viscous liquid (80 g) was obtained. From the results of ¹ H-NMR analysis and IR analysis, it was confirmed that the polymer was a boroxine ring-containing polymer (BxP) represented by the following formula:

(Preparation of Electrolyte)
An electrolyte solution was prepared by dissolving LiFSA (0.14 M) and CsF (0.012 M) in γ-butyrolactone (GBL). Specifically, a CsF/RBL electrolyte solution was prepared by dissolving a predetermined amount of CsF in γ-butyrolactone through a multi-stage dissolution process according to 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 solution. Note that, to indicate the possibility that significant modification of the solvent itself occurs as a result of the multi-stage dissolution process, the name RBL (reformed butylactone) is used to distinguish it from pure GBL solvent as the name of the solvent that constitutes the electrolyte solution.

(Cell Preparation)
A beaker cell of this example was produced using the _CuF2 electrode produced in Production Example 1 as the working electrode, an electrode sheet made of activated carbon and a binder polytetrafluoroethylene (PTFE) (activated carbon:PTFE = 70:30 (mass ratio)) 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 the electrolyte solution. The beaker cell of this example was produced using the above method.

[Example 1-2]
The 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.

[Examples 1-3]
The 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 10 mass % with respect to the total mass of BxP and the electrolyte solution.

[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.

[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 produced in the same manner as in Example 1-1.

[Example 2-1]
A beaker cell of this example was produced in the same manner as in Example 1-1, except that the BiF ₃ electrode produced in Production Example 2 was used as the working electrode instead of the CuF ₂ electrode.

[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.

[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. Except for this, the beaker cell of this comparative example was produced in the same manner as in Example 2-1.

[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) _F electrode produced in Production Example 3 was used as the working electrode instead of the _CuF electrode.

[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.

<Charge/discharge test>
A charge/discharge test was conducted on the beaker cells prepared 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 measurement was conducted in an argon atmosphere at room temperature (25°C). The beaker cells using _CuF2 electrodes and _BiF3 electrodes were charged/discharged at a charge/discharge rate equivalent to 0.04C. The beaker cells using Ti(Al) _F3 electrodes were charged/discharged at a charge/discharge rate equivalent to 0.02C.

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

(1) Effect of additives on the _CuF2 electrode in a beaker cell

Table 1 summarizes the effects of additives on beaker cells using _CuF2 electrodes. Figures 2A to 2C 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.

As shown in Figure 2(A), Comparative Example 1-1 has a small reversible capacity and poor charge/discharge performance (indicated by an x in Table 1). As shown in Figure 2(B), Comparative Example 1-2 has a large reversible capacity but poor cycle characteristics (large capacity loss due to cycling), resulting in some improvement in charge/discharge performance (indicated by a △ in Table 1). As shown in Figure 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 (indicated by a ○ in Table 1).

Furthermore, as shown in Figure 3, the electrolyte of Comparative Example 1-1 after three charge-discharge cycles (second from the left in Figure 3) is colored blue. This blue color appears when Cu ions form aqua complex ions, indicating that Cu ions have dissolved into the electrolyte.

These results demonstrate that Example 1-1 of the present invention has a large reversible capacity and excellent cycle characteristics, resulting in improved charge/discharge characteristics. In Example 1-1, no Cu ion elution into the electrolyte was observed, suggesting that a coating derived from the boroxine ring-containing polymer is formed on the electrode surface during charge/discharge, thereby suppressing the diffusion of Cu ions into the electrolyte.

(2) Effect of additives on the beaker cell using _BiF3 electrodes

Table 2 summarizes the effects of additives on beaker cells using _BiF3 electrodes. Figures 4A to 4C 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.

As shown in Figure 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 a △ in Table 2). As shown in Figure 4(B), Comparative Example 2-2 has a large resistance and small reversible capacity, resulting in poor charge/discharge performance (shown as an x in Table 2). As shown in Figure 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 a ○ in Table 2).

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

These results demonstrate that Example 2-1 of the present invention has a large reversible capacity and excellent cycle characteristics, resulting in improved charge/discharge characteristics. In Example 1-1, no electrode deformation was observed, suggesting that a coating derived from the boroxine ring-containing polymer is formed on the electrode surface during charge/discharge, thereby suppressing the diffusion of Bi ions into the electrolyte.

(3) Effect of additives on Ti(Al) _F3 electrode in beaker cell

Table 3 summarizes the effects of additives on the beaker cell using a Ti(Al) _F3 electrode. Figures 6(A) and 6(B) are graphs showing the charge-discharge curves for each example after 1 to 6 charge-discharge cycles.

As shown in Figure 6(A), Comparative Example 3-1 has a small reversible capacity, indicating poor charge/discharge performance (indicated by an x in Table 3). As shown in Figure 6(B), Example 3-1 has a large reversible capacity, indicating excellent charge/discharge performance (indicated by a circle in Table 3).

In addition, when the electrodes of each example were observed after six charge-discharge cycles, no deformation was observed in any of the electrodes.

These results demonstrate that Example 3-1 of the present invention has improved charge/discharge characteristics due to its large reversible capacity. In Example 3-1, a coating derived from the boroxine ring-containing polymer is formed on the electrode surface during charge/discharge, suppressing the reductive decomposition of the electrolyte on the electrode (negative electrode) surface. This is thought to result in a large reversible capacity.

(4) Effect of boroxine ring-containing polymer (BxP) content in a beaker cell using a _CuF2 electrode

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.

The charge-discharge performance of Comparative Example 1-1 and Example 1-1 is as described above. As shown in Figure 7(B), Example 1-2 shows improved reversible capacity and improved charge-discharge performance (shown by a circle in Table 1). As shown in Figure 7(D), Example 1-3 has 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 a triangle in Table 1).

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

(5) XPS Measurement of Electrode Surface For Examples 1-1, 2-1, and 3-1, in which the content of the boroxine ring-containing polymer was 5% by mass, X-ray photoelectron spectroscopy (XPS) measurement was performed on the electrode surface before and after three charge-discharge cycles. Specifically, the electrode was removed from the beaker cell, washed with GBL, and vacuum-dried to prepare a measurement sample. XPS measurement was performed under the following measurement conditions.

(XPS measurement conditions)
Measuring device: Quantera SXM manufactured by ULVAC-PHI, Inc.
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.

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 and after three charge-discharge cycles. 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 as a result of charge and discharge.

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 ion.

Claims (5)

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 including 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;
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 . The fluoride ion battery according to claim 1, wherein at least a portion of the surface of the positive electrode active material layer and the negative electrode active material layer has a coating containing boron. The fluoride ion battery according to claim 1 or 2, wherein the positive electrode active material contains at least one of Cu and Bi. The fluoride ion battery according to any one of claims 1 to 3, wherein the positive electrode active material contains Cu. 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.