JP2003257496A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having superior self-safety function even in trouble without sacrificing battery characteristics. <P>SOLUTION: The lithium secondary battery comprises positive electrodes 3a, 3b, negative electrodes 2a, 2b, an electrolytic solution and a separator. A back-coat layer 11 is formed on the outermost-layer electrode of an electrode structure with the positive electrodes 3a, 3b and the negative electrodes 2a, 2b arranged in plural. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a safety mechanism for a lithium secondary battery such as a lithium ion secondary battery.

[0002]

2. Description of the Related Art In recent years, the development of portable devices has been remarkable, and one of the driving forces thereof is that high energy batteries such as lithium ion secondary batteries make a great contribution. At present, the market for lithium-ion secondary batteries exceeds 300 billion a year, and it is expected that various mobile devices will continue to develop in the future, and there is a demand for advances in battery manufacturing technology accordingly.

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 and negative electrode material is obtained by mixing a positive electrode active material and a negative electrode active material with a conductive auxiliary agent and a binder and applying the mixture on a current collector. In such a lithium-ion secondary battery, there is a demand for higher energy density of the battery as a development trend, and development of a thin battery is progressing as a measure for that.

As a method of obtaining such a thin and lightweight battery, there is a polymer battery in which the electrolyte portion, which was a solution, is made into a solid state so as to make the battery thinner. Although this technology is already known, for example, in US Pat. No. 5,418,091, the characteristics of the battery have been improved in recent years, and the battery characteristics have been improved to such an extent that they cannot be compared with the beginning when the technology was first disclosed.

There are various types of batteries using such solid electrolytes, which are roughly classified into the following three types.

(1) Type using lithium ion conduction in polymer polymer as electrolyte (2) Type using lithium ion conduction in plasticized polymer polymer as electrolyte (3) Plasticizing with organic solvent and plasticizer as electrolyte Type using lithium ion conduction in polymerized polymer, solvent component belonging to (3) among these, organic polymer component,
Batteries that have been gelled (solidified) by mixing an electrolyte salt have characteristics comparable to solution batteries and are being put to practical use.

As a typical example of a method for producing a gelled solid-state battery of the type (3), there is a battery manufacturing method described in, for example, US Pat. No. 5,296,318 and US Pat. No. 5,418,091. . This is to prepare a solid polyvinylidene fluoride solid electrolyte medium, join this with the positive and negative electrodes, and extract the plasticizer from the entire battery body,
Further, an electrolytic solution is injected to gelate the whole.

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 US Pat. No. 5,296,318 and US Pat. No. 5,418,091, the battery characteristics are also excellent.

However, when the above solid gelled electrolyte is used, no problem occurs during normal use, but there is a problem to be solved in the case of an abnormality. That is, during the battery safety test, there are an overcharge test, a nailing test simulating an internal short circuit (so-called hard short circuit), a heating test, and the like.
Among these, the internal short-circuit test is one in which an internal short circuit is forcibly caused by nailing in a fully charged state, and especially as the capacity of the battery increases, the short-circuit current also increases, resulting in a rapid temperature rise of the battery. It happens and the battery itself goes into thermal runaway. As a countermeasure, for example, there is a method of using a separator having a shirt down function. However, this method depends on the shirt down response of the separator film, and therefore only this method is used for the nailing test that causes a sudden short circuit. Then there is a limit.

A measure to increase the internal impedance of the battery is also conceivable, but in this case, the charge / discharge characteristics of the battery are sacrificed, and although it can be implemented depending on the application, there is a practical limit. I can say.

By the way, a lithium secondary battery having a laminated type electrode structure has a higher degree of freedom in shape than a wound type lithium secondary battery, and is characterized in that a thin battery having a large area can be constructed.

The outermost layer of the electrode laminate is a negative electrode or a positive electrode, but when an electrode having an electrode active material applied to one surface of a current collector is used as the outermost layer, the outermost electrode is warped. In particular, when the metal foil serving as a current collector is thinner than 30 μm and the electrode active material layer is thicker than 50 μm, the warpage of the electrode becomes remarkable, which is a serious problem in manufacturing the electrode structure. In addition, when the outermost electrode is warped, the adhesion between the electrodes is deteriorated, which causes the battery characteristics such as cycle characteristics to be deteriorated.

[0013]

[0014]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery which is excellent in self-safety function even in an abnormal state without sacrificing battery characteristics.

Further, in a lithium secondary battery provided with an electrode structure of a laminated structure, the outermost electrode of the lithium secondary battery is prevented from warping, and the lithium secondary battery is excellent in handling in the manufacturing process and excellent in cycle characteristics. That is.

[0016]

That is, the above object is achieved by the following constitution of the present invention. (1) A lithium secondary battery having a positive electrode, a negative electrode, an electrolytic solution, and a separator, wherein a back coat layer is formed on the outermost electrode of the electrode structure in which the positive electrode and the negative electrode are arranged in plural. (2) The lithium secondary battery according to (1), wherein the back coat layer has a function of preventing short circuit between electrodes. (3) The lithium secondary battery according to (1) or (2), wherein the back coat layer has a function of preventing the electrode from warping. (4) The lithium secondary battery according to any one of (1) to (3) above, wherein the backcoat layer contains at least an inorganic material and / or a resin as a filler. (5) The lithium secondary battery according to any of (1) to (4) above, wherein the filler is the same material as the electrode active material. (6) The lithium secondary battery according to any one of (1) to (5), wherein the separator has a puncture strength of 50 gf or more. (7) The back coat layer has a thickness of 50 to 500 μm.
The lithium secondary battery according to any one of (1) to (6) above, wherein m is m. (8) The above (1) to (1) in which the electrode structure has a laminated structure.
The lithium secondary battery according to any one of (7).

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium secondary battery of the present invention has a positive electrode, a negative electrode, and an electrolyte solution or a solid electrolyte, and a back electrode is provided on the outermost electrode of the electrode structure in which the positive electrode and the negative electrode are arranged in plural. The coat layer is formed.

Preferably, the back coat layer is
It contains an electrode active material and / or an inorganic insulating material.

As described above, by providing the back coat layer on the outermost layer electrode of the electrode structure, a hard internal short circuit does not occur even in an abnormal state assumed in a nailing test or the like. , Discharge inside safely,
An extremely safe lithium secondary battery can be obtained. Moreover, since the back coat layer is provided only on the outermost layer of the electrode structure, the battery characteristics are not sacrificed.
That is, the back coat layer does not function as a battery constituent element.

That is, in the normal state, the outermost layer of the electrode structure does not contribute to the battery characteristics at all, but when the internal electrode structure falls into a hard internal short-circuiting state such as nailing, each internal Prevent short-circuiting of components (especially between electrodes). In particular, when the battery structure is a stack type battery and the outermost layer is made of only a metal current collector, if a hard internal short circuit is forcibly attempted with a nail clipper, the metal current collector foil will be They get caught together and cause an internal short circuit. On the other hand, in the structure of the present invention, since the outer layer surface is covered with the material layer that prevents a hard internal short circuit, even if nailing is performed, the current collector foil is less entangled and hard. Does not cause an internal short circuit.

The basic structure of the lithium secondary battery of the present invention is shown in FIG. The battery of the illustrated example has a negative electrode collector 2a and a negative electrode active material-containing layer 2b that form a negative electrode, a positive electrode collector 3a and a positive electrode active material-containing layer 3b that form a positive electrode, and a separator 4a between these electrodes. An electrolyte having a solid electrolyte 4b is arranged in between. Then, these are negative electrode / electrolyte / positive electrode / electrolyte / negative electrode ... negative electrode / electrolyte / positive electrode / electrolyte /
It is sequentially laminated with the negative electrode. The back coat layer 1 is formed on the outermost layers (the uppermost end and the lowermost end in the figure) of this electrode laminate.
1 is formed and arranged. In addition, in the battery of FIG. 1, the exterior body in which the stacked body is housed is omitted.

The back coat layer is used in the nailing test.
It has the function of preventing the formation of burrs when holes are opened in the collector metal foil. This back coat layer is preferably formed directly on the current collector of the electrode. Also, the back coat layer is
If the outermost layer of the electrode structure, that is, the electrode structure is a laminated type, it may be formed on the surface of the current collector at the upper and lower ends, and the back surface side may be an electrode of any pole. Good.
However, in order to improve safety and production efficiency, it is preferable that the negative electrode current collector is formed on the outermost surface.

The material for the back coat layer may be any material which is electrochemically inactive, particularly solvent resistance to the battery electrolyte, and is specifically used as a solid electrolyte material for the battery or as an electrode binder. Resin, eg PVD
Preferred examples include F and the like.

However, when the back coat layer is formed only of the resin material, the film thickness may be insufficient or the short-circuit preventing function may be deteriorated. Therefore, the back coat layer is
It is preferable to contain a filler made of a predetermined inorganic material.

As the filler, the same material as the electrode active material can be used. Specific examples thereof include carbon materials such as carbon black and graphite.

The back coat layer may conveniently have the same composition as the electrode, especially the negative electrode material, that is, the same as the electrode active material-containing layer. Specifically, it may be a mixture of a carbonaceous material and a resin. In particular, a composite of graphite and resin, which can reduce friction and increase slidability, is preferable. Further, from the viewpoint of productivity, it is more preferable to use a double-sided coated negative electrode, which has the effect, for the outermost layer without producing a special electrode for the outermost layer.

On the other hand, when the electrode active material is used for the back coat layer which does not contribute to the battery, various adverse effects may occur. In particular, when a carbonaceous material is used as the filler of the back coat layer, depending on the type of electrolyte used, the carbon material, particularly graphite, and further the agglomerated graphite may be decomposed to deteriorate the cycle characteristics. Such a phenomenon is remarkable especially when propylene carbonate (PC) is used as a main solvent or a mixed solvent containing a large amount of PC.

As described above, when it is difficult to use the electrode active material as a filler or when battery characteristics such as cycle characteristics are important, it is preferable to use an inorganic insulating material as the filler.

As the inorganic insulating material, wallacenite,
Silicates such as sericite, kaolin, mica, clay, bentonite, asbestos, talc, alumina silicate, metal compounds such as alumina, silicon chloride, magnesium oxide, zirconium oxide, titanium oxide, calcium carbonate, magnesium carbonate, lithium carbonate, dolomite Examples thereof include carbonates, calcium sulfate, sulfates such as barium sulfate, boron nitride, aluminum nitride, nitrides such as Si ₃ N ₄ , glass beads, silicon carbide and silica, which may be hollow. .

These fillers may have a spherical shape, an irregular shape such as crushed powder, or any shape, but are preferably spherical or lumpy. The filler may be primary particles or secondary particles.

The particle size of the filler varies depending on the shape and the material, but is an average particle size when the inorganic insulating material is converted into a spherical shape, preferably 0.1 to 10 μm, and particularly 0.5 to 6 μm. The range is good. The BET specific surface area is preferably 0.1 to 60 m ² / g, more preferably 0.5 to 40 m ² / g. When a carbon material such as acetylene black is used, the average particle size is preferably 10 nm to 1 μm, particularly 10 to 200 nm.
The BET specific surface area is preferably 10 to 20.
0 m ² / g, more preferably from 20 to 100 m ² / g.

These fillers can be used in combination of two or more kinds, and the addition amount is preferably 20 to 99% by mass, particularly preferably 50 to 98% by mass, when the resin component is 100 by mass ratio. Further, if necessary, it can be used after being subjected to a vinyl monomer graft treatment or a surface treatment with various coupling agents such as silane type, chromium type and titanium type. If the amount of addition is too small, the film thickness of the back coat layer becomes thin, and it becomes difficult to obtain the effect of suppressing warpage and preventing internal short-circuiting during nailing. If there is too much filler, it becomes difficult to carry it on the current collector.

The present invention works effectively against a hard internal short circuit, especially when a laminated battery is constructed.

The thickness of the back coat layer is not particularly limited, but if it is too thin, it becomes difficult to obtain the short-circuit preventing effect, and if it is too thick, there is an adverse effect such as an increase in the exclusive deposition of the portion that does not contribute to the battery characteristics. Will occur. Specifically, it is preferably about 50 to 500 μm, more preferably 80 to 200 μm, like the electrode.

As the method for producing the back coat layer, first,
An inorganic substance is dispersed in a binder solution to prepare a slurry.
The obtained slurry is applied to the outermost layer of the electrode. The means for applying the slurry may be the same as in the electrode manufacturing method described later. After applying the back coat layer, the electrode active material layer may be applied to the surface opposite to the back coat layer, or the back coat layer may be applied after applying the electrode active material layer.

The structure of the lithium secondary battery of the present invention is not particularly limited, but is usually composed of a positive electrode, a negative electrode and a solid electrolyte / separator, and is applied to a laminated battery, a wound battery or the like.

The electrode to be combined with the solid polymer electrolyte may be appropriately selected and used from those known as electrodes for lithium secondary batteries, preferably an electrode active material, a gel electrolyte and, if necessary, a conductive auxiliary agent. Is used.

For the negative electrode, a negative electrode active material such as carbon material, lithium metal, lithium alloy or oxide material is used, and for the positive electrode, oxide or carbon material capable of intercalating / deintercalating lithium ions. It is preferable to use a positive electrode active material such as 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. Among them, graphite is preferable, and bulk graphite is preferable in order to realize a high capacity, and its average particle size is 1
It is preferably from 30 to 30 μm, particularly preferably from 5 to 25 μm.
Also, artificial graphite is preferable. 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.

Lithium ion is an intercalating day
Intercalatable oxides include lithium
Complex oxides are preferred, for example LiCoO 2.₂, LiM
n₂O _Four, LiNiO₂, LiV₂O_FourAnd so on.
The average particle size of these oxide powders is about 1-40 μm
Is preferred.

A conductive aid 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: conductive auxiliary agent: binder = 80 to 94: 2 to 8: 2 to 18 by mass ratio, and in the negative electrode, the active material: conductive auxiliary agent is in mass 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.

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.

The matrix resin constituting the solid electrolyte includes (1) polyalkylene oxide such as polyethylene oxide and polypropylene oxide, (2) copolymer of ethylene oxide and acrylate, (3) copolymer of ethylene oxide and glycyl ether. Coalescing, (4)
Copolymer of ethylene oxide, glycyl ether and allyl glycyl ether, (5) polyacrylate (6) polyacrylonitrile (7) polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-3 fluoride Examples thereof include fluoropolymers such as ethylene oxide copolymers, vinylidene fluoride-hexafluoropropylene fluororubber, and vinylidene fluoride "tetrafluoroethylene-hexafluoropropylene fluororubber."

Among these resins, polyvinylidene fluoride (PVDF), polyethylene oxide, polyacrylonitrile and the like are preferable, and polyvinylidene fluoride homopolymer is particularly preferable. The PVDF homopolymer has a wide redox window, is electrochemically stable, and has excellent long-term stability.

The separator sheet forming the separator has a constituent material of one or two or more kinds of polyolefins such as polyethylene and polypropylene (in the case of two or more, there is a laminate of two or more layers of film),
Examples thereof include polyesters such as polyethylene terephthalate, thermoplastic fluororesins such as ethylene-tetrafluoroethylene copolymer, and celluloses. The form of the sheet is such that the air permeability measured by the method specified in JIS-P8117 is about 5 to 2000 seconds / 100 cc, and the thickness is 5 to 100.
Examples include microporous film of about μm, woven fabric, and non-woven fabric.

In the present invention, it is desirable to use a so-called shutdown separator as the separator. By using the shutdown separator, as the temperature inside the electrochemical device rises, the fine pores of the separator are closed, conduction of ions is suppressed, current is suppressed, and thermal runaway can be prevented. Examples of such a shutdown separator include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDP) described in Japanese Patent No. 2642206.
E) A separator made of a synthetic resin film having at least one kind of fine pores, containing 1% by weight or more of ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 5 or more described in JP-A-2520316, Made of a microporous membrane composed of polyethylene composition with an average molecular weight / number average molecular weight of 10 to 300, thickness of 0.1 to 25 μm, porosity of 40
A method for producing a lithium battery separator having an average through-hole diameter of 0.001 to 0.1 μm and a breaking strength of 10 mm width of 0.5 kg or more, wherein the polyethylene composition is an aliphatic hydrocarbon or a cyclic hydrocarbon. Alternatively, the solution is heated and dissolved in a non-volatile solvent composed of a mineral oil fraction to form a uniform solution, and the solution is extruded through a die to form a gel-like sheet. After removing the non-volatile solvent, the solution is stretched at least uniaxially by a factor of 2 or more. Examples thereof include a lithium battery separator and the like.

By using a solid electrolyte for such a separator, it is possible to obtain a high-performance electrochemical device having both the characteristics of the separator and the characteristics of the solid electrolyte. That is, the adhesiveness with the electrode is improved, and the film strength can be maintained, and an electrochemical device excellent in environmental change and mechanical strength can be obtained. In particular, by using a solid electrolyte formed by a wet phase separation method and a shutdown separator in the manufacturing process, a device with high safety and good electric characteristics can be obtained.

As another form, a particle layer which 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 coating method of the matrix resin is not particularly limited, and a known coating method can be used. Specifically, 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. At this time, for the purpose of improving the adhesion between the separator and the matrix resin, an additive such as a surfactant which improves surface wettability may be used.

Thereafter, if necessary, rolling treatment may be carried out by a flat plate press, a calender roll or the like.

After the matrix resin is formed, it may be heated and dried at an optimum temperature.

After the drying step, the matrix resin may be heat-bonded to the separator sheet by heat treatment. The heating temperature at this time varies depending on the matrix resin used, but is specifically about 100 to 120 ° C.

The obtained gel electrolyte sheet precursor is sandwiched between a positive electrode and a negative electrode and laminated to form a laminated body. After putting this laminated body in an exterior body such as an aluminum laminated film, an electrolytic solution is injected to impregnate the matrix resin. In such gelling treatment in the subsequent step, it is necessary to provide the matrix resin with sufficient openings as described above.

Finally, the outer package is sealed and hot pressed to obtain a solid electrolyte electrochemical device.

The structure of the lithium secondary battery of the present invention can be applied to both a wound type structure and a laminated type structure. In the case of the laminated type structure, the positive electrode, the negative electrode, the solid electrolyte layer and the separator layer are used. Since it has a structure in which the back coat layer is sequentially laminated, it is easy to arrange the back coat layer as the outermost layer. Further, the film strength required in the winding type is not required, and the mechanical restriction of the material for the separator is reduced.

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

When the present invention is applied to a winding type, the negative electrode portion may be formed longer than the positive electrode, and the outermost layer may be wound only once with the negative electrode at least once.

The electrolytic solution with which the solid electrolyte / separator is impregnated generally comprises an electrolyte salt and a solvent. Examples of the electrolyte salt include LiBF ₄ , LiPF ₆ , LiAsF ₆ ,
LiSO ₃ CF ₃ , LiClO ₄ , LiN (SO ₂ CF
₃ ) Lithium salts such as ₂ can be applied.

The solvent of the electrolytic solution is not particularly limited as long as it has good compatibility with the above-mentioned solid polymer electrolyte and electrolyte salt, but it is a polar organic solvent that does not decompose even at a high operating voltage in a lithium battery or the like. Solvents such as ethylene carbonate (EC), propylene carbonate (P
C), butylene carbonate, dimethyl carbonate (DMC), carbonates such as diethyl carbonate and ethyl methyl carbonate, tetrahydrofuran (T
HF), cyclic ethers such as 2-methyltetrahydrofuran, cyclic ethers such as 1,3-dioxolane and 4-methyldioxolane, lactones such as γ-butyrolactone, and sulfolane are preferably used. 3-Methylsulfolane, dimethoxyethane, diethoxyethane, ethoxymethoxyethane, ethyl diglyme and the like may be used.

The concentration of the electrolyte salt when considering that the solvent and the electrolyte salt constitute the electrolytic solution is preferably 0.3 to 5 mol.
l / l. Usually, the highest ionic conductivity is shown around 0.8 to 1.5 mol / l.

The outer package is composed of a laminate film in which a polyolefin resin layer such as polypropylene or polyethylene as a heat-adhesive resin layer or a heat-resistant polyester resin layer is laminated on both surfaces of a metal layer such as aluminum. . The exterior body is formed in a bag shape in which one side is opened by previously heat-bonding two laminated films to each other by thermally adhering the thermoadhesive resin layers on the end faces of the three sides. Alternatively, a single laminated film may be folded back and the end faces of both sides may be heat-bonded to form a seal portion to form a bag shape.

The present invention is particularly effective in the case of forming a laminated battery, or in the case of forming a laminated type and high capacity battery.

Further, if the present invention is combined with a shirt down separator or with a high impedance measure, the safety is further enhanced.

[0069]

EXAMPLES Next, the present invention will be described in more detail with reference to specific examples.

[Example 1] LiCoO 2 as a positive electrode active material
₂ 90 parts by weight and carbon black 6 as a conduction aid
Parts by weight and 4 parts by weight of PVDF: Kynar761A as a binder were mixed to form a positive electrode mixture, and N-methyl-2-pyrrolidone (NMP) was dispersed as a solvent to form a slurry. Coating the resulting slurry on an Al foil which is a current collector,
It was dried and used as a positive electrode.

90 parts by weight of artificial graphite powder as a negative electrode active material and 10 parts by weight of PVDF: Kynar761A as a binder were dispersed in N-methyl-2-pyrrolidone to form a slurry. This slurry was applied onto a Cu foil, which is a negative electrode current collector, and dried to obtain a negative electrode.

The following were used as solid electrolyte components. Matrix polymer: Kynar761A Polyolefin film: Asahi Kasei Polyethylene (PE)
H6022 25μm Stock solution: 2wt% Kynar761A / NMP + 1wt% L-77
(Made by Nippon Unicar Co., Ltd.)

The above-mentioned polyolefin film was dipped in a film-forming stock solution, and then the dipped product was squeezed with a roll to remove excess film-forming stock solution. The polymer in the stock solution for film formation was gelated in a porous state on the polyolefin film by dropping the sheet in water.

The positive electrode, the negative electrode, and the solid electrolyte separator described above are used as the lowermost layer of the double-sided coated negative electrode, and then the separator, followed by the double-sided coated positive electrode, the separator, the double-sided coated negative electrode, ...
Was repeatedly laminated, and the laminated structure was such that the double-sided coated negative electrode came to the uppermost layer. Ethylene-
A methacrylic acid copolymer was used.

An Al lead was bonded to the positive electrode of the obtained laminate and a Ni lead was bonded to the negative electrode, and the laminate was wrapped in an aluminum laminate film with only one open.

As a non-aqueous electrolytic solution, a mixed solvent of 70 parts by volume of ethylene carbonate and 30 parts by volume of diethyl carbonate was used as an electrolytic solution, and LiPF ₆ was used as a solute at a ratio of 2 mol dm ⁻³ , and the aluminum laminate film was opened. After injecting a predetermined amount from the part and impregnating, vacuum sealing was performed.

Thereafter, a hot pressing at 80 ° C. was applied to produce a laminated solid electrolyte lithium battery.

Example 2 LiNi as a positive electrode active material
_0.33 Mn _0.33 Co _0.33 O ₂ was used. Hereinafter, Example 1
A laminated solid electrolyte lithium battery was produced in the same manner as in.

Example 3 The negative electrode slurry prepared in the same manner as in Example 1 was applied to the back surface of the positive electrode prepared in the same manner as in Example 1. The obtained single-sided positive electrode / single-sided negative electrode was laminated on the lowermost layer and the uppermost layer, and an electrode and a separator were laminated in such a structure that the negative electrode faces outward. Hereinafter, a laminated solid electrolyte battery was produced in the same manner as in Example A-1.

Example 4 Same as Example 2 except that 30 parts by weight of natural graphite and 70 parts by weight of fibrous artificial graphite (MCF) were used as the filler for the negative electrode active material and the back coat layer. Then, a battery was manufactured.

Example 5 Except that a bulk artificial graphite was used as the filler of the negative electrode active material and the back coat layer, and a polyethylene separator having a puncture strength of 600 gf was used as the base material of the solid electrolyte separator.
A battery was prepared in the same manner as in.

Example 6 The case where a filler other than graphite is used as the back coat layer will be described. 90 parts by weight of Al ₂ O ₃ as filler, PVdF: K as binder
10 parts by weight of ynar761A was dispersed in N-methyl-2-pyrrolidone and mixed to form a slurry. Cu which is a collector
The resulting slurry was applied onto the foil and dried to form a back coat layer. The backcoat layer was a dry coating film having a thickness of 55 μm.

A negative electrode active material layer was applied to the opposite surface of the current collector coated with the back coat layer. 90 parts by weight of artificial graphite powder (MCF) as a negative electrode active material layer and PVdF as a binder:
10 parts by weight of Kynar761A was dispersed in N-methyl-2-pyrrolidone to form a slurry. This slurry was applied to the surface opposite to the side on which the back coat layer was applied and dried.

The electrode coated with the negative electrode active material layer and the back coat layer was rolled into an electrode used as the outermost layer of the electrode laminate.

A battery was prepared in the same manner as in Example 2, except that the negative electrode other than the outermost layer was an electrode in which negative electrode active material layers were coated on both sides of a current collector.

Example 7 A battery was prepared in the same manner as in Example 6 except that SiO ₂ was used as the filler for the back coat layer.

Example 8 A battery was produced in the same manner as in Example 6 except that acetylene black was used as the filler for the back coat layer.

Example 9 A mixture of propylene carbonate (PC), ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 2: 1: 7 was used as a solvent, and LiPF ₆ was used as a solvent. 1.5 mol dm ^-3
A battery was produced in the same manner as in Example 6 except that the nonaqueous electrolytic solution dissolved as the solute was used in the ratio of.

Example 10 A battery was produced in the same manner as in Example 2 except that a PVdF porous porous membrane was used as the solid electrolyte separator.

[Comparative Example 1] A negative electrode prepared in the same manner as in Example 1 was prepared by coating only one side, and the one-side coated negative electrode was placed in the outermost layer, and the uncoated portion faced to the outside. The electrodes and the separator were laminated in the structure. Hereinafter, a laminated solid electrolyte battery was produced in the same manner as in Example 1.

Comparative Example 2 A battery was produced in the same manner as in Example 2 except that a cellulose separator was used as the separator.

Comparative Example 3 A battery was prepared in the same manner as in Example 2 except that PVdF containing SiO ₂ as a filler was used as the separator.

Comparative Example 4 A battery was prepared in the same manner as in Example 9 except that a single-sided coated guest electrode was used as the outermost layer of the battery laminate and no back coat layer was provided.

After the initial evaluation of the batteries obtained in Examples 1 to 10 and Comparative Examples 1 to 4, constant current and constant voltage charging up to 4.2 V was performed, and then a nail having a diameter of 1.5 mm was used. A nailing test was performed. In addition, as a separator puncture strength test, a nail with a 3 mm diameter and a 2.5 mm tip with a conical tip,
A puncture strength test was carried out by penetrating the separator at a puncture speed of 200 mm / min. At the time of the test, the separator was fixed to a jig having a hole with a diameter of 12 mm, and the center portion thereof was pierced and measured. The capacity of the battery was 2.5 Ah (1 C discharge) in both the example and the comparative example. The results are shown in Table 1.

[0095]

[Table 1]

The batteries of Examples 1 to 10 did not cause a hard internal short circuit even if they were nailed during the nailing test, and the batteries did not burst or ignite. On the other hand, in the battery of the comparative example, a hard internal short circuit occurs at the moment of piercing the nail,
Shortly after, the battery burst and ignited.

As can be seen from the above results, Comparative Examples 1 to 4
It is considered that the battery produced in (1) was engulfed by the foil as the current collector during nailing, causing a hard internal short circuit in the battery, causing the battery to burst and ignite. On the other hand, in the batteries produced in the examples, it is considered that the foil, which is the current collector, was prevented from being rolled up during nailing, the hard internal short circuit did not occur, and the test could be completed safely.

Furthermore, among the produced batteries, Example 9,
11 and Comparative Example 4 were subjected to constant-voltage constant-current charging at 2.5 A (1 C) and a cutoff voltage of 4.2 V, and a charge-discharge cycle of discharging to 2.5 V at a constant current of 2.5 A was repeated. The ratio of the discharge capacity after 200 cycles to the initial discharge capacity was defined as the capacity retention rate. The results are shown in Table 2.

[0099]

[Table 2]

In each of Examples 1 to 4, since the outermost layer electrode of the electrode structure is less warped, it is excellent in handling in the manufacturing process. However, since the battery of Comparative Example 1 was not provided with the back coat layer, the warp of the outermost layer electrode was large and it was difficult to handle. Further, Examples 1 to 4 are all excellent in cycle characteristics, but Comparative Example 1 is inferior in cycle characteristics. It is considered that this is because the outermost layer electrode has a warp, and thus the adhesion of the outermost layer electrode deteriorates with the cycle, and the distance between the positive and negative electrodes becomes uneven.

In Example 5, bulk graphite, which is advantageous in terms of battery characteristics but is likely to cause a problem in the nailing test, was used. However, since a separator having high piercing strength was used, no problem occurred in the nailing test. . From these results, it can be seen that using a separator with high puncture strength is effective for the nail insertion test. However, a separator with high puncture strength has a low shutdown function and a low overcharge protection function.Therefore, using an inorganic insulating material for the back coat layer and using it in combination with a separator with an excellent shutdown function will improve the overall battery safety. It is desirable to improve the sex. The cycle characteristics of Example 9 are clearly improved as compared with Comparative Example 4. It is considered that this is because the back coat layer prevents the outermost layer electrode from warping and makes the distance between the electrodes uniform, resulting in uniform current density and improved cycle characteristics.

Furthermore, in Example 9, Al ₂ O ₃ :
When PVdF was set to 80:20 (weight ratio), the film thickness before and after rolling was halved, and the cycle characteristics deteriorated. On the other hand, A
When l ₂ O ₃ : PVdF was set to 95: 5 (weight ratio), the film thickness before and after rolling was nearly doubled and the cycle characteristics were also improved. From this, it is understood that when the content of the filler is too small, the film thickness is reduced, the effect of suppressing warpage and the cycle characteristics are reduced.

[0103]

As described above, according to the present invention, it is possible to provide a lithium secondary battery having an excellent self-safety function even in an abnormal condition without sacrificing battery characteristics.

Further, in a lithium secondary battery provided with an electrode structure having a laminated structure, the outermost layer of the electrode is prevented from being warped, and the lithium secondary battery is excellent in handling in the manufacturing process and has excellent cycle characteristics. be able to.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a basic configuration of a battery of the present invention.

[Explanation of symbols]

2a Negative electrode current collector 2b Layer containing negative electrode active material 3a Positive electrode current collector 3b Positive electrode active material-containing layer 4a separator 4b Solid electrolyte 11 Back coat layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Satoshi Maruyama             1-13-1, Nihonbashi, Chuo-ku, Tokyo             -In DC Inc. F term (reference) 5H029 AJ05 AJ12 AK03 AK06 AL02                       AL06 AL07 AL08 AL12 AM02                       AM03 AM04 AM05 AM07 AM16                       EJ03 EJ04 EJ12 HJ00 HJ04

Claims (8)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode, an electrolytic solution, and a separator, wherein a back coat layer is formed on an outermost electrode of an electrode structure in which a plurality of the positive electrodes and the negative electrodes are arranged. 2. The lithium secondary battery according to claim 1, wherein the back coat layer has a short circuit preventing function of preventing a short circuit between electrodes. 3. The lithium secondary battery according to claim 1, wherein the back coat layer has a function of preventing the electrode from warping. 4. The lithium secondary battery according to claim 1, wherein the back coat layer contains at least an inorganic material and / or a resin as a filler. 5. The lithium secondary battery according to claim 1, wherein the filler is the same material as the electrode active material. 6. The piercing strength of the separator is 50 g.
It is f or more, The lithium secondary battery in any one of Claims 1-5. 7. The back coat layer has a thickness of 50 to 5
The lithium secondary battery according to any one of claims 1 to 6, which has a size of 00 µm. 8. The electrode structure according to claim 1, which has a laminated structure.
7. The lithium secondary battery according to any one of 7.