JP2003077538A - Solid secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid secondary battery for permitting the shut-off of internal ion conductivity to prevent the thermal runaway of the battery due to internal short-circuit during the malfunction such as overcharge. SOLUTION: The solid secondary battery having a positive electrode, a negative electrode and a solid electrolyte comprises a composite electrolyte containing γ-butyrolactone, polyvinylidene fluoride and electrically inactive inorganic particles and existing between the electrode and the solid electrolyte or constituting the solid electrolyte itself.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery structure including a lithium ion secondary battery and the like, and more particularly to an electrolyte structure.

[0002]

2. Description of the Related Art In recent years, there have been remarkable developments in mobile devices, and high energy batteries such as lithium ion secondary batteries have been a major driving force for such development. Currently, the market for lithium secondary ion batteries is 300 annually.
It is expected to continue to grow in various portable devices over ¥ 100 million, and there is a demand for advances in battery manufacturing technology.

Such a lithium ion secondary battery is usually composed of a positive electrode, a liquid or solid electrolyte layer, and a negative electrode. This positive / negative electrode material is formed by mixing a positive electrode active material / a negative electrode active material with a conductive auxiliary agent and a binder, and coating the mixture on a current collector. As a development trend of such a lithium ion secondary battery, higher energy density of the battery is required, and development of a thin battery is progressing as a measure for that.

As one of the methods for producing a thin and lightweight battery, there is a polymer battery in which the electrolyte portion, which was a solution, is made solid so as to be thin. This technology itself is already known, but in recent years, the characteristics have improved
The battery characteristics are improved to the extent that they cannot be compared with the initial disclosure.

There are various forms of such solid polymer secondary batteries, but they are roughly classified into the following three types.

(1) A type that uses lithium ion conduction in a polymer as an electrolyte. (2) A type that uses lithium ion conduction in a plasticized polymer as an electrolyte. (3) A type in which lithium ion conduction in a polymer polymer plasticized with an organic solvent or a plasticizer is used as an electrolyte.

Among these, the battery component obtained by mixing the solvent component belonging to (3), the organic polymer component, and the electrolyte salt and gelling (solidifying) shows the characteristics comparable to the solution type battery, so that it is put into practical use. There is.

As a typical example of a method for producing such a battery, there is a battery described in US Pat. No. 545600. This is to make a solid polyvinylidene fluoride solid electrolyte medium, join it with the positive electrode / negative electrode, extract the plasticizer from the whole battery body, and inject the electrolyte solution to gel the whole. is there. By gelling the entire battery body in this manner, the liberated electrolytic solution does not exist inside the battery. Therefore, it can be said that the configuration is completely different from that of the conventional solution type battery. further,
According to the contents disclosed in this document, the battery characteristics are also excellent.

However, when this solid gelled electrolyte is used, there are no problems during normal use, but there are inherent problems to be solved in the event of some abnormality.

That is, from the viewpoint of battery characteristics, the above-mentioned conventional gelled electrolytes were superior to the conventional electrolytes utilizing the ionic conductivity inside the polymer. However, there is no problem in normal use, but when overcharging, etc., the current is not cut off due to the melting of the resin, or when it is in a molten state, it has conductivity and current concentration occurs, causing an internal short circuit. Was sometimes reached.

When an internal short circuit occurs, rapid heat generation occurs inside the battery, causing the battery to be in a thermal runaway state and leading to ignition. Therefore, even when a solid electrolyte is used, it is necessary to have a function of reaching a safe state during overcharge.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a solid secondary battery capable of blocking internal ionic conductivity and preventing thermal runaway of the battery due to an internal short circuit when the battery is abnormal such as overcharge. Is to provide.

[0013]

The solid state secondary battery of the present invention is a solid state secondary battery having a positive electrode, a negative electrode and a solid state electrolyte, and comprises γ-butyrolactone, polyvinylidene fluoride, and an electrically non-conductive material. It has a composite electrolyte containing active inorganic particles between the electrode and the solid electrolyte, or constitutes the solid electrolyte itself.

The solid state secondary battery of the present invention comprises a collector material,
A positive electrode active material material coated on a current collector material, a positive electrode material comprising a binder, more specifically, a metal aluminum foil as a current collector material, a material for inserting and releasing lithium such as lithium cobalt oxide as a positive electrode active material, It has a positive electrode made of polyvinylidene fluoride as a binder. Similarly, a current collector material, a negative electrode active material applied to the current collector material, a negative electrode material comprising a binder,
More specifically, it has a copper foil as a current collector material, a material such as graphite as a negative electrode active material for inserting and releasing lithium, and a negative electrode made of polyvinylidene fluoride as a binder.

In the present invention, a function of blocking ionic conduction between the electrode and the solid electrolyte layer is added to the positive or negative electrode side of the solid electrolyte, or to the solid electrolyte itself at a certain temperature or higher potential. That is, since it does not affect the ionic conductivity under normal conditions, it does not affect the battery characteristics, but has a composite electrolyte that blocks ionic conductivity when the electrode falls into an abnormal state due to overcharge or the like for some reason. This makes it possible to prevent an internal short circuit, which is a drawback of the conventional solid electrolyte.

The composite electrolyte of the present invention is for preventing a short circuit, which is usually observed during overcharge. At this time, the short circuit is not completely prevented, and the circuit is interrupted in the presence of a certain resistance layer. That is, at the time of overcharging, temperature rise occurs due to heat generation inside the battery. At this time, if a certain resistance layer exists and a short circuit phenomenon occurs, the current continues to flow. That is, although a short circuit occurs, the potential drops to the extent that thermal runaway does not occur, so heat generation in the battery as a whole is reduced. As a result, the function of the battery safely stops even when overcharged.

Therefore, due to the solid electrolyte and electrode structure of the present invention, there is no ignition and heat generation even during overcharge, and the potential is lowered, so that the battery system is in a safe state.

This phenomenon is manifested by the solid electrolyte component used in the present invention, although the clear reason and mechanism are not known. As the solid electrolyte, there are the following two types.

Solid electrolyte 1 A solid electrolyte prepared by previously forming a film of a non-aqueous solvent such as lithium salt, ethylene carbonate or propylene carbonate, and a gelling polymer material such as polyvinylidene fluoride.

Solid electrolyte 2 Using a microporous membrane made of a polymer material to be gelled, it is integrated with an electrode to prepare a battery body precursor, and an electrolyte solution is injected in a later step, and the whole is heated and cooled. A gelled solid electrolyte.

The present invention is effective for both of these solid electrolytes. However, in the present invention, an inactive substance is further added.

Gelation component In forming the solid electrolyte, γ is used as a solvent component.
-Must contain butyrolactone. γ
-Butyrolactone may be present alone, but in the present invention, in order to improve the characteristics as a secondary battery, it may be mixed with one or more other solvents.

The solvent is not particularly limited, but polar organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), which do not decompose even at a high operating voltage in a lithium battery or the like,
Butylene carbonate, dimethyl carbonate (DM
C), carbonates such as diethyl carbonate and ethyl methyl carbonate, tetrahydrofuran (TH
F), cyclic ethers such as 2-methyltetrahydrofuran, 1,3-dioxolane, cyclic ethers such as 4-methyldioxolane, and sulfolane are preferably used. Further, 3-methylsulfolane, dimethoxyethane, diethoxyethane, ethoxymethoxyethane, ethyl diglyme and the like may be used. Among these, ethylene carbonate (EC) is particularly preferable.

The mixing ratio of γ-butyrolactone and other solvent is γ-butyrolactone: other solvent (preferably EC).
Is preferably 3: 7 to 9: 1, more preferably 7: 3 to 8: 2.

As the electrolyte salt to be dissolved in the solvent, for example,
For example, LiBF_Four, LiPF₆, LiAsF₆, LiSO
₃CF₃, LiClO_Four, LiN (SO₂ CF₃ )₂ ,
LiC (CF₃SO₂ ]₃ Lithium salts such as
However, in the present invention, especially LiBF_Four Is preferred
Yes. In addition, LiBF_Four If it always contains
It may be a mixture with the above-mentioned salt.

As the gelling material, a polyvinylidene fluoride-based material is also used. Polyvinylidene fluoride-based material, as a gel component of the solid electrolyte,
Functions as a binder for inorganic particles. Examples of such polyvinylidene fluoride-based materials include PVDF homopolymers, but copolymers may also be used. Such a PVDF homopolymer is commercially available, and can be obtained, for example, as kynar 741,301F manufactured by Elf Atfiner.

Inorganic particles In the present invention, the solid electrolyte is used as a porous body to conduct ions.
In order to carry the
Inorganic particles are included in the solid electrolyte to form a composite electrolyte. Including
The inorganic particles that have are electrically inactive and
Reaction with electrolyte without affecting conductivity
There is no particular limitation as long as it has low property. Ingredient
Physically, oxides such as silica (SiO 2₂ ),Aluminum
Na (Al ₂O₃ ) And other ceramic powders
Wear. Among these, especially silica (SiO₂ ) Is preferred
Good The particle size of such inorganic particles is preferably
It is 1 to 20 μm, especially 2 to 15 μm. Grain of inorganic particles
The diameter is usually the average primary particle size, but by forming secondary particles
If so, it is the average secondary particle size.

The content of the inorganic particles in the solid electrolyte is preferably 10:90 to 30:70, more preferably about 20:80 to 25:75 in terms of mass ratio of PVDF: inorganic particles (preferably SiO ₂ ). Is. It is desirable that PVDF and inorganic particles are uniformly mixed and dispersed.

The thickness of the composite electrolyte is preferably 1
It is from 0 to 50 μm, especially from 15 to 30 μm. If the film thickness is too thin, the current suppressing function will be insufficient, and if it is too thick, the ionic conductivity will deteriorate and the battery characteristics will deteriorate.

Further, as an embodiment of the invention, as described above, either one of the positive electrode side and the negative electrode side may be interposed, or the solid electrolyte may be interposed between the positive electrode side and the negative electrode side. As another form, a particle layer that does not swell with respect to an organic solvent-based material and melts at a certain temperature may be provided in the solid electrolyte layer or on the surface of the layer.

The electrode to be combined with the solid polymer electrolyte may be appropriately selected and used from those known as electrodes for secondary batteries, particularly lithium secondary batteries, and preferably an electrode active material and a gel electrolyte are required. Therefore, a composition with a conductive additive is used.

For the negative electrode, a carbon material, (lithium) metal,
A negative electrode active material such as a (lithium) alloy or an oxide material is used, and a positive electrode active material such as an oxide or a carbon material capable of intercalating / deintercalating metal (lithium) ions is used for the positive electrode. It is preferable. By using such an electrode, a lithium secondary battery having good characteristics can be obtained.

The carbon material used as the electrode active material may be appropriately selected from, for example, mesocarbon microbeads (MCMB), natural or artificial graphite, resin-fired carbon material, carbon black, carbon fiber and the like. These are used as powder. Of these, graphite is preferable, and its average particle diameter is preferably 1 to 30 μm, and particularly preferably 5 to 25 μm. If the average particle size is too small, the charge / discharge cycle life tends to be short, and the capacity variation (individual difference) tends to increase. If the average particle size is too large, the variation in capacity becomes extremely large and the average capacity becomes small. When the average particle diameter is large, the variation in capacity is considered to be due to the variation in the contact between graphite and the current collector and the contact between graphite.

As the oxide capable of intercalating / deintercalating metal (lithium) ions, a complex oxide containing lithium is preferable, for example, LiCo.
O _2, such as _{LiMn 2 O 4, LiNiO 2,} LiV 2 O 4 and the like. The average particle size of these oxide powders is 1 to 4
It is preferably about 0 μm.

A conductive auxiliary agent is added to the electrode if necessary. Examples of the conductive aid include graphite, carbon black, carbon fibers, metals such as nickel, aluminum, copper, silver and the like, and graphite and carbon black are particularly preferable.

The electrode composition of the positive electrode is preferably in the range of active material: conduction aid: binder = 80 to 94: 2 to 8: 2 to 18 by weight ratio, and in the negative electrode, the active material: conduction aid is in weight ratio. Agent: Binder = The range of 70 to 97:00 to 25: 3 to 10 is preferable.

As the binder, a thermoplastic elastomer resin such as a fluororesin, a polyolefin resin, a styrene resin, an acrylic resin, or a rubber resin such as fluororubber can be used. Specific examples thereof include polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polyacrylonitrile, nitrile rubber, polybutadiene, butylene rubber, polystyrene, styrene-butadiene rubber, polysulfide rubber, nitrocellulose, cyanoethyl cellulose, carboxymethyl cellulose and the like. Particularly, it is preferable to use the same PVDF homopolymer or copolymer as the above solid electrolyte.

To manufacture the electrode, first, an active material and, if necessary, a conductive auxiliary agent are dispersed in a binder solution to prepare a coating solution.

Then, the electrode coating solution is applied to the current collector. The means for applying is not particularly limited and may be appropriately determined depending on the material and shape of the current collector. Generally, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, a screen printing method and the like are used.
Then, if necessary, rolling treatment is performed by a flat plate press, a calendar roll, or the like.

The current collector may be appropriately selected from ordinary current collectors depending on the shape of the device used by the battery, the method of arranging the current collector in the case, and the like. Generally, aluminum or the like is used for the positive electrode and copper, nickel or the like is used for the negative electrode.
A metal foil, a metal mesh, or the like is usually used as the current collector. Although the metal mesh has a smaller contact resistance with the electrode than the metal foil, the metal foil can also obtain a sufficiently small contact resistance.

Then, the solvent is evaporated to produce an electrode. The coating thickness is preferably about 50 to 400 μm.

Such a positive electrode, a solid electrolyte and a negative electrode are laminated in this order and pressure-bonded to obtain a battery body.

The structure of the battery can be applied to either a wound type structure or a laminated type structure. In the case of the laminated type structure, the positive electrode, the negative electrode, the solid electrolyte layer, and the ion conduction blocking layer are sequentially formed. Since the laminated structure is adopted, film strength required for a ticket die is not required, and material and mechanical restrictions are reduced.

The present invention will be described below with reference to examples.

EXAMPLES <Example 1> The positive electrode and the negative electrode were made of the following materials. Positive electrode Current collector Aluminum foil Positive electrode active material Lithium cobaltate binder PVDF polymer Conductive agent acetylene black Negative electrode negative electrode active material Graphite binder PVDF homopolymer Conductive agent Acetylene black

Solid electrolyte As a solid electrolyte, a microporous membrane containing PVDF as a main component was formed. At this time, silica was used as the electrically inactive inorganic particles to form a composite, which was used as a microporous film. At this time, PVDF: SiO ₂ has a mass ratio of 2
It was set to 5:75.

When the thus obtained microporous membrane was gelled, ethylene carbonate (EC) and γ-butyrolactone (γBL) were used as solvents as gelling constituents.

Electrolyte salt Boron tetrafluoride (LiBF ₄ )
Was used. Further, here, the ratio of γ-butyrolactone is changed and EC: γBL = 3: 7 (mass ratio) is changed to A1, EC:
γBL = 1: 3 (mass ratio) is A2, EC: γBL = 2:
A solid electrolyte was produced by setting 8 (mass ratio) to A3.

A1, A2 and A3 produced in this way
Got the battery.

Example 2 A battery was obtained in the same manner as in A1 of Example 1 except that a material composed of lithium manganese composite oxide and lithium nickel composite oxide was used as the positive electrode active material. These were designated as A4 and A5.

Comparative Example In place of γ-butyrolactone as the gelling constituent in A1 of Example 1, DEC,
A battery was prepared in the same manner as in Example 1 except that DMC and EMC were used. Replace these batteries with B1, B2
It was set to B3.

An overcharge test was conducted on the batteries obtained in these Examples and Comparative Examples. Table 1 shows the results of the overcharge test. In the overcharge test, a current equivalent to 2 C was applied to bring the battery into an overcharged state, and the presence or absence of abnormality of the battery was observed.

[0052]

[Table 1]

As shown in Table 1, Examples A1 to A5
The samples of the above example do not ignite. The battery potential after completion was 1-2 volts.

As is clear from the above results, the battery can be produced by exhibiting the function of rapidly interrupting the ionic conduction between the electrode and the solid electrolyte layer at a temperature above the temperature of the solid electrolyte or its positive or negative electrode side. When abnormalities such as overcharging occur, the ionic conductivity is blocked internally, and thermal runaway of the battery can be prevented due to an internal short circuit. That is, in a normal state, this layer does not contribute to ionic conductivity, but when the electrode falls into an abnormal state such as overcharging, by blocking the ionic conductivity, an internal short circuit, which is a defect in the conventional solid electrolyte, is prevented. Can be prevented.

[0055]

As described above, according to the present invention, when the battery is abnormal such as overcharged, the internal ionic conductivity is shut off and the thermal runaway of the battery due to an internal short circuit can be prevented. Can be provided.

Claims (3)

[Claims] 1. A solid secondary battery having a positive electrode, a negative electrode, and a solid electrolyte, wherein a composite electrolyte containing γ-butyrolactone, polyvinylidene fluoride, and electrically inactive inorganic particles is mixed with the electrode. Solid-state secondary battery having between solid electrolytes or constituting the solid electrolyte itself. 2. The solid secondary battery according to claim 1, wherein the content of the inorganic particles is 10:90 to 30:70 by mass ratio of PVDF: inorganic particles. 3. The solid secondary battery according to claim 1, further comprising ethylene carbonate as a solvent.