JP2017010865A - Secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery which achieves high capacity exceeding capacity of existing battery systems that include lithium ion secondary batteries.SOLUTION: A secondary battery of the present invention is characterized to include: an electrode including a metal fluoride represented by composition formula: MF(where, M represents Bi, Pb or Sn, and X represents the valence of metal M); and an electrolyte including a boron compound and an alkali metal fluoride salt. In the secondary battery, a fluoride ion (F) moves between a positive electrode and a negative electrode via the electrolyte during a charge/discharge reaction.SELECTED DRAWING: None

Description

  The present invention relates to a secondary battery.

  Among battery systems currently in practical use, lithium ion secondary batteries are known as those having a higher energy density than lead acid batteries, nickel metal hydride batteries, and the like (see, for example, Patent Document 1). Lithium ion secondary batteries convert electrical energy into chemical energy and store it when lithium ions move from the positive electrode to the negative electrode through the electrolyte during charging. Conversely, during discharge, lithium ions return from the negative electrode to the positive electrode through the electrolyte, thereby generating electric energy. Research and development are also being conducted on advanced lithium ion secondary batteries using materials with higher energy density than current lithium ion secondary batteries.

JP 2011-165392 A

However, an energy density that is a performance limit of the lithium ion secondary battery system is also pointed out. Further, even in advanced lithium ion secondary battery systems, there is an upper limit for improving performance such as capacity.
Considering the recent high performance of electronic devices and expectations for electric vehicles, an innovative battery that achieves a higher capacity than existing battery systems including lithium ion secondary batteries is desired.

  Then, the subject of this invention is providing the secondary battery which implement | achieves the high capacity | capacitance exceeding the existing battery system containing a lithium ion secondary battery.

The present inventors have found that the above problem can be achieved by a secondary battery in which fluoride ions (F ⁻ ) move between a positive electrode and a negative electrode through an electrolytic solution in a charge / discharge reaction, and the present invention has been achieved. did.
The secondary battery of the present invention includes an electrode including a metal fluoride represented by a composition formula MF _X (wherein M is a metal and X is a valence of the metal M), a boron compound, and a fluoride. And an electrolyte solution containing a salt.

  According to the secondary battery of the present invention, it is possible to achieve a higher capacity than existing battery systems including conventional lithium ion secondary batteries.

It is a longitudinal cross-sectional view which represents typically the internal structure of the secondary battery which concerns on embodiment of this invention.

Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.
The secondary battery of the present invention is a novel fluoride ion secondary battery that can achieve a higher capacity than a conventional lithium ion secondary battery (see, for example, Patent Document 1).

The secondary battery according to the present invention is mainly characterized by including an electrode containing a metal fluoride described later and an electrolytic solution containing a boron compound described later and a fluoride salt described later.
Below, after explaining the whole structure of a secondary battery, it demonstrates in more detail about an electrode and electrolyte solution. The metal fluoride can be included in either the positive electrode or the negative electrode depending on the selection of the counter electrode. However, in the secondary battery of the present embodiment described below, at least in the state before discharge, the metal fluoride is contained in the positive electrode. The thing including is described.

<Overall configuration of secondary battery>
FIG. 1 is a longitudinal sectional view schematically showing the internal structure of a secondary battery 1 according to an embodiment of the present invention.
As shown in FIG. 1, the secondary battery 1 according to the present embodiment includes a battery container 13, a positive electrode 10, a separator 11, a negative electrode 12, a battery lid 20, and an electrolytic solution 22.

  The battery container 13 in the present embodiment has a bottomed substantially cylindrical shape. In the battery container 13, an electrode group having the positive electrode 10, the separator 11, and the negative electrode 12, and the electrolytic solution 22 are accommodated. There is no particular limitation on the outer shape of the battery container 13.

  The material of the battery container 13 is selected from materials that are corrosion resistant to the electrolytic solution 22. Specific examples of the material of the battery container 13 include aluminum, stainless steel, nickel-plated steel, and the like.

  The positive electrode 10, the separator 11, and the negative electrode 12 are overlapped with each other and wound around the axis 21 to form a substantially cylindrical electrode group. The separator 11 is inserted between the positive electrode 10 and the negative electrode 12. The electrode group will be described in detail later together with the electrode material.

  The shaft center 21 is not particularly limited as long as it can support the positive electrode 10, the separator 11, and the negative electrode 12 wound around the shaft center 21, and a known shaft center can be used.

  The battery lid 20 is formed by integrating an inner lid 16, an internal pressure release valve 17, a gasket 18, and a positive temperature coefficient resistance element 19. Incidentally, the gasket 18 hermetically and liquid-tightly seals the inside and outside of the battery container 13 and electrically insulates the battery container 13 and the battery lid 20. The battery lid 20 is disposed on the opening side of the battery container 13 and is attached to the battery container 13 by welding, caulking, or the like.

The electrolytic solution 22 is injected into the battery container 13 from the opening side before the battery cover 20 is attached to the battery container 13. As a result, the electrode group is immersed in the electrolytic solution 22.
Incidentally, the electrolytic solution 22 can be injected through an inlet (not shown) provided in the battery lid 20 after the battery lid 20 is attached to the battery container 13. This inlet is closed after the electrolyte 22 is injected. The electrolytic solution 22 will be described in detail later.

In FIG. 1, reference numeral 14 denotes a positive electrode current collecting tab, and reference numeral 15 denotes a negative electrode current collecting tab.
The positive electrode current collecting tab 14 electrically connects the positive electrode 10 of the electrode group accommodated in the battery container 13 and the battery cover 20 (inner cover 16) of the battery container 13.
The negative electrode current collecting tab 15 electrically connects the negative electrode 12 of the electrode group accommodated in the battery container 13 and the inner wall of the battery container 13.

<Electrode group>
Next, the electrode group will be described.
The electrode group includes a positive electrode 10, a negative electrode 12, and a separator 11 (see FIG. 1). Below, it demonstrates in order of the positive electrode 10, the negative electrode 12, and the separator 11. FIG. In addition, about the positive electrode 10 and the negative electrode 12, it demonstrates on the basis of the positive electrode 10 and the negative electrode 12 in the initial stage of a discharge process.

(Positive electrode)
The positive electrode 10 in the present embodiment includes a metal fluoride (active material), a conductive agent, a binder (binder), and a current collector.
Metal fluoride is represented by a composition formula MF _X. However, in the said compositional formula, M is a metal and X is the valence of the metal M.
Examples of the metal M include Bi, Pb, Sn, Co, Mn, Ni, Cu, Fe, Al, Mg, and Ce. Among them, the metal M is desirably Bi, Pb, and Sn, and Bi is most desirable.

Specific examples of the metal fluoride include, but are not limited to, BiF ₃ , PbF ₂ , SnF _{2 and the} like. Among these, BiF ₃ is preferably used from the viewpoint of capacity and reaction potential.
The metal fluoride is also acceptable that a part of the fluorine atoms of formula MF _X is replaced by an oxygen atom, a nitrogen atom as an impurity.
In addition, as the metal fluoride, those in which the M is composed of a plurality of elements are allowed.

The conductive agent is a powder that supplements the electrical conductivity of the electrode, and can be included in the positive electrode 10 as necessary. The positive electrode 10 containing a conductive agent is desirable.
Examples of the conductive agent include graphite, graphitizable carbon, non-graphitizable carbon, graphene, carbon black, graphitized carbon black, ketjen black, vapor grown carbon fiber, pitch-based carbon fiber, mesocarbon microbead, multilayer / Single-walled carbon nanotubes, Vulcan, acetylene black, oxygen-containing functional group-introduced hydrophilic carbon and the like. However, the conductive agent is not limited to these.

The binder bonds the metal fluoride particles. When the positive electrode 10 contains a conductive agent, the metal fluoride particles and the conductive agent particles are also bonded to each other.
Examples of the binder include polyvinylidene fluoride (PVDF), an acrylic polymer, and a polymer having an imide group or an amide group. However, the binder is not limited to these.

  Examples of the current collector include metal foil, metal mesh (expanded metal), foam metal, and punching metal. Examples of the material for the current collector include aluminum, copper, brass, nickel, zinc, and stainless steel. Further, as the current collector, a non-woven fabric imparted with conductivity can also be used. However, the current collector is not limited to the above.

The positive electrode 10 can be formed by adhering a composition containing the metal fluoride, conductive agent, and binder to the current collector.
As a specific method of attaching the composition, for example, a slurry is formed by adding a predetermined liquid medium to the metal fluoride, conductive agent, and binder, and the slurry is applied to the surface of the current collector. Is mentioned. Examples of the liquid medium include 1-methyl-2-pyrrolidone (N-methylpyrrolidone), tetrahydrofuran, hexanol, butanol, propanol, isopropanol, and water. However, the liquid medium is not limited to these.

Examples of the application method include a doctor blade method, a dipping method, and a spray method. However, the coating method is not limited to these.
The positive electrode 10 can be obtained by drying the slurry applied to the current collector and, if necessary, pressing with a roll press or the like. Moreover, at the time of this pressure molding, it can also heat as needed.

  The positive electrode 10 can also be obtained by adhering the composition containing the metal fluoride, conductive agent, and binder to the surface of the current collector by pressure bonding, welding, or the like.

  In addition, a plurality of electrode layers formed on the surface of the current collector by the composition containing the metal fluoride, the conductive agent, and the binder may be stacked on the surface of the current collector.

Although content of the binder in an electrode layer can be 5 mass%-20 mass% with respect to the total amount of a metal fluoride, a electrically conductive agent, and a binder, it is not limited to this, for example.
When the electrode layer contains a conductive agent, the content of the conductive agent is preferably about 5% by mass to 30% by mass.

(Negative electrode)
Examples of the negative electrode 12, fluoride ions during discharge (F ^-) occludes a fluorine atom, fluoride ions during charging ^- not particularly limited as long as it emits (F).
As the negative electrode 12 in the present embodiment, for example, a material containing an active material showing a lower potential than a metal fluoride (MF _X ) can be used.

Examples of the active material of the negative electrode 12 include a metal M simple substance constituting a metal fluoride (MF _X ).
Examples of the metal M alone include Bi, Pb, Sn, Co, Mn, Ni, Cu, Fe, Al, Mg, and Ce as described above. Among them, the metal M is preferably Mg, Al, and Ce.
In addition, the negative electrode 12 can contain a binder, a conductive agent, a current collector, and the like as necessary together with the active material. The same binder, conductive agent, and current collector as those of the positive electrode 10 can be used.

(Separator)
The separator 11 is inserted between the positive electrode 10 and the negative electrode 12 as described above, and prevents a short circuit due to direct contact between the positive electrode 10 and the negative electrode 12.
Examples of the separator 11 include a microporous polymer film such as polyethylene, polypropylene, and aramid resin, and a film in which a heat resistant material such as alumina particles is coated on the surface of the polymer film.

<Electrolyte>
The electrolytic solution 22 contains a boron compound, a fluoride salt, and a nonaqueous solvent.
(Boron compound)
Boron compounds promote the dissociation of fluoride salts with relatively low solubility in non-aqueous solvents. The boron compound also has a role of coordinating and stabilizing fluoride ions.
It is desirable to select a boron compound that is used alone or in combination with a fluoride salt and that dissolves in a non-aqueous solvent. Further, it is desirable that the boron compound is stable in the range of 1V to -2V with respect to the standard hydrogen electrode.
Examples of the boron compound include organic boron compounds and fluorine-containing organic boron compounds.

As the organoboron compound, the following formula: B≡ (R ¹ ) ₃
(Wherein R ¹ are monovalent organic groups which may be the same or different from each other)
The thing shown by is mentioned.

Examples of the organic group for R ¹ include an alkyl group, an alkoxy group, a cyclohexyl group, an aryl group, and an aryloxy group.

As the fluorine-containing organic boron compound, the following formula: FB = (R ¹ ) ₂
(Wherein R ¹ has the same meaning as described above, and may be the same as or different from each other).
Indicated by
The following formula: B≡ (R ² ) ₃
(In the formula, R ² is a monovalent alkyl group or alkoxy group in which all or part of the hydrogen atoms are substituted with fluorine atoms, and R ² may be the same or different from each other.)
And the like shown in FIG.

Further, as the organoboron compound and the fluorine-containing organoboron compound, a boroxine compound having a boroxine ring (a heterocycle represented by ≡ (BO) ₃ obtained by removing three hydrogen atoms from boroxine) can also be used.
Examples of boroxine compounds include the following formula: R ³ ₃ (BO) ₃
(Wherein R ³ are monovalent organic groups which may be the same or different from each other)
The thing shown by is mentioned.
Examples of the organic group for R ³ include an alkyl group, an alkoxy group, a cyclohexyl group, a cyclohexyloxy group, an aryl group, an aryloxy group, and all or part of hydrogen atoms of these groups substituted with fluorine atoms. Etc.

Among these, preferred boron compounds are B≡ (OPh) ₃ [where Ph is a phenyl group], B≡ (Ph) ₃ [where Ph is a phenyl group], FB = (C ₉ H ₁₁ ) ₂ [Dimesi Tylfluoroborane], B≡ (OCH ₂ CF ₂ CHF ₂ ) ₃ , (Ph) ₃ (BO) ₃ [where Ph is a phenyl group], (C ₆ H ₄ F) ₃ (BO) ₃ [where C ₆ H ₄ F includes a group in which one hydrogen atom of a phenyl group is substituted with a fluorine atom (for example, 2,4,6-tris (4-fluorophenyl) boroxine group)] and the like.
The above boron compounds can be used alone or in combination of two or more.

(Fluoride salt)
The fluoride salt is preferably an alkali metal salt.
Examples of the fluoride salt include lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride.
Among these, as the fluoride salt, cesium fluoride is particularly desirable from the viewpoint of solubility in a non-aqueous solvent.
Fluoride salts can be used alone or in combination of two or more.

(Non-aqueous solvent)
Examples of the non-aqueous solvent include tetraethylene glycol dimethyl ether (tetraglyme), triethylene glycol dimethyl ether (triglyme), diethylene glycol dimethyl ether (diglyme), ethylene glycol dimethyl ether (monoglyme), ethylene carbonate, propylene carbonate, γ-butyrolactone, dimethyl carbonate. , Ethyl methyl carbonate, diethyl carbonate and the like.
Among these, tetraethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and propylene carbonate are desirable as the non-aqueous solvent from the viewpoint of ease of dissolution of the fluoride salt.
A non-aqueous solvent can also be used individually or in combination of 2 or more types.

When the concentration of the fluoride salt in the electrolytic solution 22 is high, the ionic conductivity of the electrolytic solution tends to be improved. Therefore, the concentration of the fluoride salt is desirably 0.1 mol / L or more, and more desirably 0.5 mol / L or more.
Moreover, in order to dissolve more fluoride salts, it is necessary to add more boron compounds. The concentration of the boron compound in the electrolytic solution 22 is desirably 0.1 mol / L or more, and more desirably 0.5 mol / L or more.

  In addition, an ion conductivity improver, an overcharge inhibitor, a flame retardant imparting agent, a wettability improver, and the like can be added to the electrolytic solution 22 as necessary.

Next, the effects of the secondary battery 1 will be described while explaining the operation of the secondary battery 1 according to the present embodiment. Here, in the initial state of the discharge process, the positive electrode 10 includes the metal fluoride (MF _X ), and the negative electrode 12 includes a metal fluoride (M′F _Y ) different from the metal fluoride (MF _X ). The secondary battery 1 including a single metal M ′ will be described as an example. It is assumed that the metal M ′ included in the negative electrode 12 has a lower potential than the metal fluoride (MF _X ) and has a valence of Y as described later. First, the discharge process of the secondary battery 1 will be described.

  The following equation (1) is a reaction equation showing the reaction mechanism of the positive electrode 10 in the discharge process. Formula (2) is a reaction formula showing the reaction mechanism of the negative electrode 12 in the discharge process.

Positive electrode 10:
MF _X + Xe ⁻ → M + XF ⁻ Formula (1)
Negative electrode 12:
(X / Y) M ′ + XF ⁻ → (X / Y) M′F _Y + Xe ⁻ (2)
(In the formulas (1) and (2), M, M ′, X, and F ⁻ are as defined above, Y is the valence of the metal M ′, and e ⁻ is an electron)

As shown in Formula (1), in the positive electrode 10 in the discharge process, the defluorination reaction proceeds by the reduction reaction of the metal fluoride (MF _X ) as the active material. In the positive electrode 10, metal (M) and fluoride ions (F ⁻ ) are generated.
Fluoride ions (F ⁻ ) generated at the positive electrode 10 move to the negative electrode 12 through the electrolytic solution 22.

As shown in the formula (2), in the negative electrode 12 in the discharge process, a metal fluoride (M′F _Y ) is obtained by a reaction between the metal M ′ (simple substance) as an active material of the negative electrode 12 and fluoride ions (F ⁻ ). ) Is generated. In the negative electrode 12, electrons (e ⁻ ) are generated by this oxidation reaction. That is, in the discharge process in the secondary battery 1, electrons (e ⁻ ) flow from the negative electrode 12 connected to each other toward the positive electrode 10 by the oxidation-reduction reaction in each of the positive electrode 10 and the negative electrode 12. The current flows from the positive electrode 10 toward the negative electrode 12.

In contrast, in the charging process, the reverse reaction in Formula (1) proceeds at the positive electrode 10, and the reverse reaction in Formula (2) proceeds at the negative electrode 12. That is, in the positive electrode 10 of the secondary battery 1 that has undergone this charging process, a metal fluoride (MF _X ) as an active material is generated again. Thereby, the secondary battery 1 can be restored and prepared for the next discharge process.
In the example shown in the above formula (1) and (2), as the metal M'used for the negative electrode, it is assumed that different from the metal M of the positive electrode of the MF _X, metals in the positive electrode to the negative electrode The same metal M as the seed can also be used. The negative electrode reaction in this case is represented by the following formula (2) ′.
M + XF ⁻ → MF _X + Xe ⁻ Expression (2) ′

Further, the electrolytic solution 22 in the secondary battery 1 of the present embodiment contains a boron compound and a fluoride salt. Thereby, the movement of fluoride ions (F ⁻ ) flowing between both electrodes of the positive electrode 10 and the negative electrode 12 becomes smooth. In particular, the boron compound promotes dissolution of fluoride ions (F ⁻ ) in the electrolytic solution 22.

Thus, when the fluoride ion (F ⁻ ) moves between both the positive electrode 10 and the negative electrode 12 through the electrolytic solution 22, for example, compared with the case where the fluoride ion (F ⁻ ) moves in the solid electrolyte. Accordingly, the diffusion of fluoride ions (F ⁻ ) is remarkably accelerated. Thereby, the secondary battery 1 of the present embodiment is further improved in output and rate characteristics.

Moreover, the electrolyte solution 22 containing an alkali metal fluoride salt and an organoboron compound makes the movement of fluoride ions (F ⁻ ) smoother. Among them, the boroxine-based compound can further improve the performance of the electrolytic solution 22.

In addition, the secondary battery 1 having the electrolytic solution 22 that dissolves until the fluoride salt is saturated can achieve good cycle characteristics. This is because the fluoride salt in a saturated state, it is possible to suppress the electrode (positive electrode 10, negative electrode 12) metal fluoride (MF X, _{M'F Y)} used for the elution into the electrolytic solution 22 Because.

As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment, It can implement with a various other form.
In the above-described embodiment, the secondary battery 1 including the metal fluoride in the positive electrode 10 has been described. However, the present invention may be a secondary battery 1 in which the negative electrode 12 includes a metal fluoride instead of a single metal. That is, the secondary battery 1 of the present invention has a configuration including at least a metal fluoride (MF _X ) in the working electrode.

  In the embodiment, the electrode group has a cylindrical shape, but in addition, various shapes such as a laminate of strip-shaped electrodes or a positive electrode 10 and a negative electrode 12 wound in an arbitrary flat shape are used. be able to. Further, the shape of the battery case 13 may be selected from shapes such as a flat oval shape, a flat oval shape, and a square shape. Furthermore, the axis 21 can be omitted depending on the battery shape or for the purpose of improving the volume occupancy of the electrode inside the battery.

  Moreover, since the secondary battery 1 of the said embodiment has the above advantages, it can be used for various applications. In particular, since the secondary battery 1 has good battery characteristics in addition to the advantage of being excellent in safety, it is not only used as a secondary battery for driving power sources of mobile information devices such as mobile phones and laptop computers, It can be widely used as a power source for various devices such as electric vehicles and hybrid electric vehicles.

  Hereinafter, Examples 1 to 15 and Comparative Examples 1 to 3 for verifying the effects of the present invention will be described. In addition, this invention is not limited to a following example, In the range which does not deviate from the summary of this invention, it can change arbitrarily and can implement.

<Production of electrode>
In Examples and Comparative Examples, as shown in Table 1, BiF ₃ , PbF ₂ , and SnF ₂ were used as electrode active materials, respectively.

First, 60 parts by mass of an active material, 25 parts by mass of acetylene black (conductive agent), and 15 parts by mass of polyvinylidene fluoride (binder) were mixed, and then 1-methyl-2-pyrrolidone 350- 400 parts by mass was added to prepare a slurry composition part.
This slurry composition was applied to one side of an aluminum foil as a current collector. Next, the aluminum foil coated with the slurry composition was heat-dried at 110 ° C. and then press-molded. From this press-molded product, a section having an area with a reaction area of 5 mm × 5 mm was cut out to form an electrode.

<Preparation of electrolyte>
An electrolyte solution was prepared using the fluoride salt, boron compound and non-aqueous solvent shown in Table 1. Table 1 shows the concentration of the fluoride salt and the concentration of the boron compound in the electrolytic solution.
In Table 1, the boron compounds represented by the composition formula are as follows.
FB = (C ₉ H ₁₁ ) ₂ [Dimesitylfluoroborane]
B≡ (OPh) ₃ [Ph is a phenyl group]
B≡ (Ph) ₃ [Ph is a phenyl group]
(Ph) ₃ (BO) ₃ [Ph is a phenyl group, (BO) ₃ is a boroxine ring]
(C ₆ H ₄ F) ₃ (BO) ₃ [2,4,6-Tris (4-fluorophenyl) boroxine]

<Production of prototype battery>
Place the prepared electrode and a platinum mesh as a counter electrode in a predetermined container, and then immerse the electrode and the counter electrode in the electrolyte by introducing the prepared electrolyte in the container. It was. For the reference electrode, a solution containing 0.1 mol / L AgNO ₃ and 0.1 mol / L C ₈ H ₂₀ ClNO ₄ in acetonitrile was used.

<Charge / discharge test of prototype battery>
The charge / discharge test was performed in the order of discharge and charge. Moreover, charging / discharging was made to perform 2 cycles with the electric current equivalent to 1 / 40C. Discharging was performed up to the theoretical capacity, and charging was performed up to 0 V with respect to the reference electrode. However, when a plateau region where the potential becomes constant was observed and then the potential suddenly decreased or increased, the end of charge / discharge was controlled by the potential.
Table 2 shows the charge capacity and charge / discharge termination conditions in the first and second cycles in Examples 1 to 13 and Comparative Examples 1 to 3.

(Performance evaluation of secondary battery)
In Comparative Examples 1 to 3, the charge capacity was not obtained. This is because the electrolyte solution used in Comparative Examples 1 to 3 does not contain fluoride salt, and therefore the electrolyte solution hardly exhibits ionic conductivity.
On the other hand, in Examples 1-12, the charge capacity was obtained. This is because a fluoride salt is dissolved in the electrolyte and an electrolyte that exhibits fluoride ion conduction is used.

  In Example 1, the charge capacity becomes half or less from the first cycle to the second cycle, but in other examples, high cycle characteristics are shown.

In Examples 2-5, since the fluoride salt is in a saturated state, it has high cycle characteristics. In Examples 6 to 12, the use of boroxin compounds [(Ph) ₃ (BO) _3, (C ₆ H ₄ F) ₃ (BO) ₃ ] as a boron compound is related to high cycle characteristics. doing.

Further, Examples 13 to 15 show that reversible charging / discharging is possible even when PbF ₂ or SnF ₂ is used as the electrode material.

DESCRIPTION OF SYMBOLS 1 Secondary battery 10 Positive electrode 11 Separator 12 Negative electrode 13 Battery container 14 Positive electrode current collection tab 15 Negative electrode current collection tab 16 Inner cover 17 Internal pressure release valve 18 Gasket 19 Positive temperature coefficient resistance element 20 Battery cover 21 Shaft center 22 Electrolyte

Claims (7)

An electrode containing a metal fluoride represented by a composition formula MF _X (wherein M is a metal and X is a valence of the metal M);
A secondary battery comprising: an electrolytic solution containing a boron compound and a fluoride salt. The secondary battery according to claim 1,
The secondary battery, wherein the metal M is Bi, Pb, or Sn. The secondary battery according to claim 1,
The secondary battery according to claim ₁ , wherein the metal fluoride is BiF ₃ , PbF _2, or SnF ₂ . The secondary battery according to claim 1,
The secondary battery is a boroxine compound. The secondary battery according to claim 1,
The secondary battery is characterized in that the fluoride salt is an alkali metal fluoride salt. The secondary battery according to claim 1,
The secondary battery is characterized in that the fluoride salt is cesium fluoride. The secondary battery according to claim 1,
The secondary battery is characterized in that the fluoride salt is dissolved in the electrolytic solution until saturated.

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