JP5473969B2 - Secondary battery positive electrode and secondary battery equipped with the same - Google Patents

Description

 The present invention relates to a positive electrode for a secondary battery and a secondary battery including the same.

 In recent years, secondary batteries with large capacity and high efficiency are expected. A lithium ion secondary battery (hereinafter abbreviated as “LIB”) is known as one of secondary batteries that can meet such requirements. LIB has features such as higher voltage, higher energy density, and higher coulomb efficiency than other secondary batteries such as lead-acid batteries.

For example, the LIB has a structure in which a positive electrode and a negative electrode are separated and accommodated in a battery container that stores an electrolytic solution via a separator. The positive electrode and the negative electrode are obtained by applying an electrode active material to a current collector. As an electrode active material, one made of a metal fluoride coated with carbon is known (for example, see Patent Document 1).
According to this electrode active material, it is considered that a high-performance secondary battery can be realized, but there is an improvement in reducing the manufacturing cost of the secondary battery.

Therefore, in order to enable the production of a secondary battery at a low cost, the present invention and the like first include a mixture containing lithium fluoride (LiF) and elemental metal iron (Fe) on the surface of the current collector. A method of producing LiFe ^(III) F ₃ , which is an electrode active material, on the surface of the current collector by reacting lithium fluoride with iron by controlling the charging of the current collector I put it out.

JP 2008-130265 A

However, in the method using the mixture containing lithium fluoride and iron described above, it is difficult to control the particle size of iron or lithium fluoride, and the formation of a conductive path by contact of these particles is not sufficient. Therefore, an electrode using this electrode active material has a high internal resistance and is not suitable for a secondary battery application that requires high output.
Further, in the method described in Patent Document 1, the amount of lithium doped with respect to iron fluoride (FeF ₃ ) is not sufficient, and only an electrode active material having a lower capacity than the theoretical charge / discharge capacity of iron fluoride can be obtained. There was a problem that not.

 The present invention has been made in view of the above circumstances, and an object thereof is to provide a positive electrode for a secondary battery having a low internal resistance and a high capacity, and a secondary battery including the same.

In the present invention, the following means are adopted in order to achieve the object.
The positive electrode for a secondary battery of the present invention is prepared by mixing lithium oxalate and iron fluoride, heating the mixture at 200 ° C. or higher and 400 ° C. or lower, and using the obtained product as an electrode active material. The battery is manufactured by charging and discharging between the electrode and the counter electrode (negative electrode).

In the present invention, in the positive electrode for a secondary battery, iron and lithium fluoride are uniformly dispersed at the atomic level, so that a conductive path by contact between iron and lithium fluoride is sufficiently formed, and the internal resistance is lowered. Further, since a sufficient amount of lithium is doped in iron fluoride (FeF ₃ ), the charge / discharge capacity can be increased.

 In the positive electrode for a secondary battery of the present invention, the iron fluoride is preferably produced by reacting low cost iron oxide and hydrofluoric acid as a raw material. Also, iron fluoride can be produced by reacting iron chloride or nitrate that is a low-cost raw material with hydrofluoric acid.

 In this way, in the reaction between lithium oxalate and iron fluoride, iron and lithium fluoride can be uniformly dispersed at the atomic level. Moreover, the material cost of the positive electrode for secondary batteries can be reduced.

 The secondary battery of the present invention comprises the positive electrode for a secondary battery of the present invention, a negative electrode, and a nonaqueous electrolyte.

 Since the secondary battery positive electrode of the present invention is provided, the output can be increased.

According to the positive electrode for a secondary battery of the present invention, since iron and lithium fluoride are uniformly dispersed at the atomic level, a conductive path is sufficiently formed by contact between iron and lithium fluoride, and the internal resistance is reduced. . Further, since a sufficient amount of lithium is doped in iron fluoride (FeF ₃ ), the charge / discharge capacity can be increased.

It is an exploded perspective view which shows the structural example of a secondary battery. It is a flowchart which shows roughly one Embodiment of the manufacturing method of the positive electrode for secondary batteries, and the manufacturing method of a secondary battery. It is a graph which shows the thermal-analysis result at the time of heating the mixture of lithium oxalate and iron fluoride trihydrate at 300 degreeC for 8 hours. It is a graph which shows the result of the XRD (X-ray diffraction) measurement of the obtained product after heating the mixture of lithium oxalate and iron fluoride trihydrate at 300 degreeC for 8 hours.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In the drawings used for explanation, in order to show characteristic parts in an easy-to-understand manner, dimensions and scales of structures in the drawings may be different from actual structures.
Before describing the secondary battery positive electrode according to the present invention, a configuration example of the secondary battery according to the present invention will be described.

"Secondary battery"
FIG. 1 is an exploded perspective view showing a configuration example of a secondary battery.
As shown in FIG. 1, the secondary battery 1 includes a battery container 10 that stores an electrolytic solution therein.
The secondary battery 1 is, for example, a lithium ion secondary battery.
The battery container 10 is, for example, an aluminum hollow container. The battery container 10 of this example has a substantially prismatic shape (substantially rectangular parallelepiped shape). The battery container 10 includes a cylindrical body 10a having an opening and a lid 10b that closes the opening of the cylindrical body 10a.

The lid 10b is provided with electrode terminals 11 and 12 and an electrolyte inlet 13. For example, the electrode terminal 11 is a positive terminal and the electrode terminal 12 is a negative terminal.
The electrodes 14 and 15 and the separator 16 are accommodated in the battery container 10.

The electrodes 14 and 15 have a sheet-like current collector (conductor) such as a conductor foil or a conductor thin plate as a base material, and a surface of the base material is coated with an electrode active material according to the type of the electrolyte. .
The electrode 14 is, for example, a positive electrode plate, and a positive electrode active material layer containing an electrode active material (positive electrode active material) made of lithium-containing iron fluoride is formed on the surface of an aluminum base material.
The electrode 15 is, for example, a negative electrode plate, and a portion in contact with the electrolytic solution is made of graphite.

The electrode 14 is disposed to face the electrode 15. The electrodes 14 and 15 are repeatedly arranged in directions facing each other. A separator 16 is provided between the electrodes 14 and 15 so that the electrodes 14 and 15 do not contact each other.
Separator 16 consists of insulating materials, such as a porous resin film, for example.

An electrode tab 14 a is formed at the end of the electrode 14 on the electrode terminal 11 side. The electrode tabs 14 a of the plurality of electrodes 14 that are repeatedly arranged are collectively connected to the electrode terminal 11.
An electrode tab 15a is formed at the end of the electrode 15 on the electrode terminal 12 side. The electrode tabs 15 a of the plurality of electrodes 15 that are repeatedly arranged are collectively connected to the electrode terminal 12.

The electrolyte solution is stored in the battery container 10 so as to be in contact with the electrodes 14 and 15.
Examples of the electrolyte solution for the lithium ion secondary battery include a solution in which a lithium salt such as lithium hexafluorophosphate or lithium tetrafluoroborate is dissolved in an organic solvent such as ethylene carbonate, diethyl carbonate, or propyl carbonate. Can be mentioned.
Further, when the secondary battery 1 is a sodium secondary battery, examples of the electrolyte solution for the sodium secondary battery include a solution in which a sodium salt such as sodium perchlorate is dissolved in an organic solvent.

"Positive electrode for secondary battery"
The positive electrode for a secondary battery according to the present invention is used, for example, as the electrode 14 described above, and is manufactured by a method for manufacturing a positive electrode for a secondary battery, which will be described later, and lithium (LiF) is added to iron fluoride (FeF ₃ ). ) Is doped.

“Method for producing positive electrode for secondary battery and method for producing secondary battery”
FIG. 2 is a flowchart schematically showing one embodiment of a method for producing a positive electrode for a secondary battery and a method for producing a secondary battery.
To manufacture the electrode 14, first, lithium iron fluoride is generated (step S1).

In this step S1, lithium oxalate (LiCOO) ₂ and iron fluoride trihydrate (FeF ₃ .3H ₂ O) are mixed, and the mixture is 200 ° C. or higher and 400 ° C. or lower in an inert gas atmosphere. Heat. Thereby, the chemical reaction shown in the following formula (1) or formula (2) mainly occurs, and divalent iron fluoride and lithium fluoride (Fe ^(II) F ₂ , LiF) are generated.
The chemical reaction shown in Formula (1) and Formula (2) is a reduction reaction of iron fluoride and iron fluoride trihydrate with lithium oxalate.

Examples of the iron fluoride trihydrate include iron nitrate (Fe (NO ₃ ) ₃ ) nonahydrate or iron oxide (Fe ₂ O ₃ ) and an aqueous solution of hydrogen fluoride (HF) (hydrofluoric acid, hydrofluoric acid ) And a product produced by reaction. Iron nitrate (Fe (NO ₃ ) ₃ ) nonahydrate and hydrogen fluoride cause a chemical reaction represented by the following formula (3). Moreover, iron oxide (Fe ₂ O ₃ ) and hydrogen fluoride cause a chemical reaction represented by the following formula (4).

The compounding ratio of lithium oxalate and iron fluoride trihydrate (lithium oxalate / iron fluoride trihydrate) is preferably 1.0 / 2.0 to 1.5 / 2.0 in molar ratio. More preferably, it is 1.0 / 2.0 to 1.2 / 2.0.
When the mixing ratio of lithium oxalate and iron fluoride trihydrate is less than 1.0 / 2.0 in terms of molar ratio, the chemical reaction represented by formula (1) or formula (2) does not proceed sufficiently, and two Valent iron fluoride and lithium fluoride (Fe ^(II) F ₂ , LiF) may not be generated. On the other hand, when the compounding ratio of lithium oxalate and iron fluoride trihydrate exceeds 1.5 / 2.0 in terms of molar ratio, a chemical reaction other than the chemical reaction represented by formula (1) or formula (2) occurs. As a result, a large amount of substances (impurities) other than divalent iron fluoride and lithium fluoride (LiF, Fe ^(II) F ₂ ) may be generated.

As the inert gas, nitrogen (N ₂ ), helium (He), neon (Ne), argon (Ar), or the like is used.

The temperature at which the mixture of lithium oxalate and iron fluoride trihydrate is heated is 200 ° C. or higher, and preferably 250 to 300 ° C.
When the temperature for heating the mixture of lithium oxalate and iron fluoride trihydrate is less than 200 ° C., the chemical reaction represented by formula (1) or formula (2) does not proceed sufficiently, and divalent iron fluoride and Lithium fluoride (Fe ^(II) F ₂ , LiF) is not sufficiently generated, and dehydrated iron fluoride (FeF ₃ ) and the raw material lithium oxalate remain, or when impurities increase, Water and carbon dioxide (CO ₂ ) generated by the reaction may not be sufficiently removed.

The time for heating the mixture of lithium oxalate and iron fluoride trihydrate is preferably 8 hours or more, more preferably 8 to 16 hours.
When the time for heating the mixture of lithium oxalate and iron fluoride trihydrate is less than 8 hours, the chemical reaction represented by formula (1) or formula (2) does not proceed sufficiently, and divalent iron fluoride and Lithium fluoride (Fe ^(II) F ₂ , LiF) may not be sufficiently generated, the amount of impurities may increase, and moisture and carbon dioxide (CO ₂ ) generated by the reaction may not be sufficiently removed. In addition, when the time for heating the mixture of lithium oxalate and iron fluoride trihydrate exceeds 16 hours, a chemical reaction other than the chemical reaction represented by formula (1) or formula (2) occurs, Substances other than iron fluoride and lithium fluoride (Fe ^(II) F ₂ , LiF) (such as iron oxide (Fe ₂ O ₃ ) which is an impurity) may be produced in large amounts.

In step S1, since the chemical reaction by heating is completed, carbon dioxide (CO ₂ ) and moisture generated by this chemical reaction are removed, and carbon dioxide and moisture do not cause problems in the subsequent process.

The product obtained by the chemical reaction between lithium oxalate and iron fluoride trihydrate in step S1 is mainly composed of divalent iron fluoride and lithium fluoride (Fe ^(II) F ₂ , LiF). And other impurities (Li _0.5 FeF ₃ , Li ₃ FeF ₆ , Fe ₂ O _3, etc.) are also included.

Here, the thermal analysis result at the time of heating the mixture of lithium oxalate and iron fluoride trihydrate at 300 degreeC for 8 hours is shown in FIG.
From the results shown in FIG. 3, the weight loss reached 24.8% at about 200 ° C., indicating that the weight reduction corresponding to the completion of dehydration of iron fluoride trihydrate was completed, and further, 300 ° C., 8 hours. The weight after heating is reduced by 42%, which is close to the weight loss of 45% when completely decarboxylated. Therefore, iron fluoride trihydrate and lithium oxalate react until decarboxylation. It was confirmed that it had progressed.

Here, a mixture of lithium oxalate and iron fluoride trihydrate was heated in an electric furnace at 300 ° C. for 8 hours, and the result of XRD (X-ray diffraction) measurement of the obtained product is shown in FIG. Shown in
From the result of FIG. 4, it was confirmed that the product obtained by heating for 8 hours contains divalent iron fluoride and lithium fluoride (Fe ^(II) F ₂ , LiF).
From the X-ray analysis results of FIG. 4, the main components are divalent iron fluoride (Fe ^(II) F ₂ ) around 31.2 degrees and lithium fluoride (LiF) around 45.2 degrees and 52.7 degrees. It was confirmed that.

 Next, using the product containing lithium iron fluoride obtained in step S1 as an electrode active material, a mixture containing this electrode active material is prepared, and this mixture is formed into a sheet shape as a base material of the electrode 14 which is a positive electrode. The surface of the current collector is coated (step S2).

 In step S2, the product containing lithium iron fluoride was uniformly dispersed by dissolving the product containing lithium iron fluoride, the conductive additive and the binder in a solvent, or kneading with the solvent. A slurry is prepared and this slurry is used as a mixture.

As the conductive assistant, a conductive material is appropriately selected and used. Specific examples of the conductive assistant include acetylene black, carbon black, activated carbon and the like.
The binder is appropriately selected and used depending on the thickness of the coating film formed on the surface of the current collector by the above mixture. Specific examples of the binder include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF).
As the solvent, water or various organic solvents are used.

Next, the coated film of the mixture is pressure-bonded to the current collector, for example, by pressing the current collector coated with the mixture (step S3).
In step S3, the pressure for pressing (pressing) the coating film formed on the surface of the current collector is not particularly limited, and is appropriately adjusted within a range in which the coating film can have a predetermined shape and thickness. .

Next, the coating film formed on the surface of the current collector is dried (step S4).
In step S4, the temperature at which the coating film is dried is not particularly limited, and is appropriately adjusted according to the thickness of the coating film, the type of solvent, and the like.

Next, the current collector having the coating film is processed into a predetermined electrode shape by punching (die cutting) or the like, and an intermediate body (hereinafter referred to as “positive electrode intermediate body”) that later becomes the electrode 14 is formed. (Step S5).
In step S2 to step S5, the sheet-like current collector is processed in-line, for example.

Next, a positive electrode intermediate, a separator 16, and an intermediate (hereinafter referred to as “negative electrode intermediate”) that will later become the electrode 15 are stacked and fixed to each other, and the positive electrode intermediate, separator 16, and negative electrode A laminate composed of an intermediate is formed (step S6).
In step S6, the electrode tab 14a is formed in the edge part arrange | positioned at the electrode terminal 11 side in the intermediate body of a positive electrode. Similarly, the electrode tab 15a is formed in the edge part distribute | arranged to the electrode terminal 12 side in the intermediate body of a negative electrode.

In addition, the electrode 15 which is a negative electrode is formed by a forming material and a forming method selected as appropriate. As a specific example of the intermediate of the negative electrode, a current collector having at least a surface made of graphite (C ₆ ) is used.

 Next, the cylindrical body 10a and the lid 10b constituting the battery container 10 are prepared, and the above-described stacked body is accommodated in the cylindrical body 10a (step S7).

The lid 10b is provided with electrode terminals 11 and 12 and an inlet 13 in advance.
In step S <b> 7, the plurality of electrode tabs 14 a formed on the positive electrode intermediate body constituting the laminated body are collectively connected to the electrode terminal 11. Further, the plurality of electrode tabs 15 a formed on the negative electrode intermediate body constituting the laminate are collectively connected to the electrode terminal 12.
And after closing the opening part of the cylindrical body 10a with the lid | cover 10b, the lid | cover 10b is welded to the cylindrical body 10a.

 Next, after injecting an electrolytic solution into the battery container 10 from the inlet 13, the inlet 13 is sealed (step S8).

Next, the electrode terminals 11 and 12 are electrically connected to a charger or the like, and a voltage is applied between the positive electrode intermediate body and the negative electrode intermediate body (charge control is performed) (step S9). obtain.
Then, the chemical reaction shown in the following formula (5) occurs in the intermediate of the positive electrode.
The chemical reaction represented by the formula (5) is a one-electron reaction in which one electron is emitted from the positive electrode intermediate (LiF + Fe ^(II) F ₂ ). By this one-electron reaction, Fe ^(III) F ₃ is formed in the intermediate of the positive electrode.

When the discharge is further controlled through the one-electron reaction shown in the formula (5), a chemical reaction shown in the following formula (6) occurs.
The chemical reaction shown in Formula (6) is a one-electron reaction in which the positive electrode intermediate (Fe ^(III) F ₃ ) receives lithium ions and one electron. By this one-electron reaction, LiFe ^(II) F ₃ is formed on the surface of the electrode 14.

When the reaction shown in Formula (6) is completed and LiFe ^(II) F ₃ is formed on the surface of the electrode 14, the electrode 14 becomes a completed electrode, and the secondary battery 1 functions as a secondary battery. be able to.

Moreover, when charge control is performed through the one-electron reaction shown in Formula (6), the chemical reaction shown in Formula (7) below occurs.
The chemical reaction shown in Formula (7) is the reverse reaction of the one-electron reaction shown in Formula (6).

In the second and subsequent charge / discharge control, the LiFeF ₃ intercalation reaction, that is, the chemical reaction shown in the equations (6) and (7) is repeated.

In the method for producing a positive electrode for a secondary battery and the method for producing a secondary battery as described above, divalent iron fluoride obtained by a chemical reaction between lithium oxalate and iron fluoride trihydrate, LiFe ^(II) F _3, which is an electrode active material, is applied to the surface of the current collector by applying a mixture of lithium fluoride uniformly dispersed at the atomic level to the current collector and controlling the charge of the current collector. As a result, a conductive path by contact between iron fluoride and lithium fluoride is sufficiently formed, and as a result, a positive electrode for a secondary battery having low internal resistance is obtained. Therefore, a secondary battery provided with this positive electrode for secondary electrodes can be increased in output.

That is, compared with the conventional method using a mixture containing lithium fluoride and iron, iron and lithium fluoride can be uniformly dispersed, and a conductive path is sufficiently formed by contact of these particles. Can do. Further, since a sufficient amount of lithium is doped in iron fluoride (FeF ₃ ), the charge / discharge capacity can be increased.

 In the present embodiment, the electrode terminals 11 and 12 are formed outside the battery container 10, and the intermediate body of the electrodes 14 and 15 is accommodated inside the battery container 10, and then an electrolyte is injected into the battery container 10, Next, since charge control is performed using the electrode terminals 11 and 12, the electrodes 14 and 15 can be formed using the components of the secondary battery 1. For example, as compared with a method in which the electrodes 14 and 15 are completed before the electrodes 14 and 15 are accommodated in the battery container 10, the electrolytic solution tank and the electrodes 14 and 15 for bringing the electrodes 14 and 15 into contact with the electrolytic solution are held. A jig, a jig for supplying power to the electrodes 14 and 15, and the like can be omitted. Therefore, the cost of the apparatus used for manufacturing the secondary battery can be reduced, and the manufacturing cost can be reduced.

 The technical scope of the present invention is not limited to the above-described embodiment. Various modifications are possible without departing from the gist of the present invention. For example, the mixture may be reacted to form an electrode active material and an electrode may be manufactured before being accommodated in the battery container, and the manufactured electrode may be accommodated in the battery container to produce a secondary battery.

 In this case, for example, the current collector is immersed in the electrolytic solution stored in the electrolytic solution tank before the sheet-shaped current collector coated with the mixture is processed by punching or the like. To supply power. An electrode active material can be formed also by such a method, and an electrode having an electrode active material can be manufactured.

DESCRIPTION OF SYMBOLS 1 ... Secondary battery, 10 ... Battery container, 10a ... Cylindrical body, 10b ... Cover, 11, 12 ... Electrode terminal, 13 ... Injection port, 14 ... Electrode (Positive electrode), 14a ... electrode tab, 15 ... electrode (negative electrode), 15a ... electrode tab, 16 ... separator, S1-S9 ... step.

Claims (3)

 Lithium oxalate and iron fluoride are mixed, the mixture is heated at 200 ° C. or higher and 400 ° C. or lower, and an electrode is prepared using the obtained product as an electrode active material. The electrode and the counter electrode (negative electrode) A positive electrode for a secondary battery manufactured by charging and discharging between the electrodes.  The positive electrode for a secondary battery according to claim 1, wherein the iron fluoride is produced by reacting iron nitrate, iron oxide, or iron chloride with hydrofluoric acid. A secondary battery comprising the positive electrode for a secondary battery according to claim 1, a negative electrode, and a nonaqueous electrolyte.

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