JP3229740B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high energy density lithium secondary battery.

[0002]

2. Description of the Related Art A secondary battery is used as a power source for driving a portable electronic device or the like for the purpose of economy and resource saving, and its use has been rapidly expanding in recent years.

Also, with the miniaturization and higher performance of electronic devices, batteries used are required to be smaller, lighter and have higher capacities.

On the other hand, lead batteries and nickel cadmium batteries have been used as secondary batteries. In recent years, non-aqueous lithium secondary batteries with high energy density have been proposed or put into practical use.

However, since a lithium secondary battery using a non-aqueous electrolyte uses a non-aqueous solvent as an ion-conducting medium, the ionic conduction speed of the electrolyte is lower than that of a conventional secondary battery using an aqueous solution. There is a problem that the density is small.

In order to solve such problems, there have been proposed methods of increasing the surface area of the electrode and increasing the contact area with the electrolyte. That is, a thin electrode layer is formed by applying an electrode paint containing an active material and a polymer compound binder on a current collector such as a thin metal foil, and laminating or spirally winding them through a separator or the like. The method has been done.

For example, Japanese Patent Application Laid-Open No. 63-121260 describes a lithium secondary battery using a non-aqueous electrolyte, LiCoO 2 and / or LiNiO 2 as a positive electrode, and carbon as a negative electrode.

However, when an electrode layer is formed on a current collector such as a metal foil, if the charge and discharge are repeated many times, the adhesion at the interface between the current collector and the electrode layer deteriorates, and the discharge capacity of the electrode decreases. Therefore, there is a problem that the cycle life is not sufficient, and the fine powder of the electrode layer which has fallen off from the current collector causes a short circuit.

[0009] This is because the active material and the electrode layer repeatedly expand and contract due to doping and undoping of Li ions during charge and discharge, so that local shear stress is generated at the interface between the electrode layer and the current collector, and the It is considered that the adhesion is deteriorated. Therefore, there arises a problem that the internal resistance of the battery increases, and the life of the battery is shortened due to a decrease in capacity or an increase in internal temperature.

A method of preventing the deterioration of the adhesion between the electrode layer and the current collector as described above and increasing the current collection efficiency has been conventionally proposed.

For example, JP-B-62-186465, JP-A-2-148654 and JP-A-3-238770 disclose a conductive underlayer containing carbon black or nickel fine powder using SiO ₂ as a binder. A non-aqueous electrolyte battery provided between a layer and a current collector is disclosed, and these underlayers are effective in a primary battery.

However, in a secondary battery in which charge and discharge are repeated, the underlayer using SiO ₂ as a binder has poor flexibility, and thus the electrode layer expands and contracts due to charge and discharge, so that the interface between the electrode layer and the current collector interface is formed. The generated shear stress cannot be alleviated, and it is not possible to prevent the adhesion from being deteriorated when the battery is repeatedly charged and discharged.

Japanese Unexamined Patent Publication No. 63-266774 discloses a flat current collector of an ethylene resin film containing carbon black, in which a layer containing a polymer softer than the current collector and carbon black is provided. -Type organic electrolyte batteries are disclosed. However, in this battery, when repeatedly charged and discharged, there is a problem that the polymer of the binder is deteriorated by the charging and discharging, and is further eluted into the organic electrolyte, thereby deteriorating the adhesion.

Further, Japanese Patent Application Laid-Open No. 50-91719 discloses a current collector containing a chlorosulfonated polyethylene, tribasic lead maleate and carbon black, and provided with a heat-cured underlayer. ing. That is,
Japanese Patent Application Laid-Open No. 50-91719 discloses a technique in which a polymer compound is used as a binder for an underlayer, and the polymer compound is subjected to a thermosetting treatment. However, since the chlorosulfonated polyethylene, which is a polymer compound used for the underlayer disclosed in Japanese Patent Application Laid-Open No. 50-91719, contains chlorine, even if it is subjected to a thermosetting treatment, it has sufficient properties. Therefore, when the battery is repeatedly charged and discharged, the binder is deteriorated due to the charging and discharging, which causes a problem that the discharge capacity is extremely reduced.

[0015]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and in a lithium secondary battery that is repeatedly charged and discharged, deterioration of adhesion between a current collector and an electrode layer is prevented. It is another object of the present invention to provide an electrode having a large capacity and a small capacity reduction when charge and discharge are repeated even when the current density is increased.

[0016]

This and other objects are achieved by the present invention which is defined below as (1) to (6). (1) In a secondary battery provided with a non-aqueous electrolyte containing a lithium-containing electrolyte, an electrode layer constituting a negative electrode and / or a positive electrode is formed on a current collector through an underlayer, and the underlayer is formed of carbon black. A lithium secondary battery comprising a polymer compound containing a fluorine-based polymer compound and a thermosetting crosslinking agent, and having been subjected to a thermosetting treatment. (2) DB of carbon black contained in the underlayer
The lithium secondary battery according to the above (1), wherein the P oil absorption is 100 to 500 cc / 100 g. (3) The lithium secondary battery according to the above (1) or (2), wherein the underlayer has a carbon black content of 10 to 200 parts by weight based on 100 parts by weight of the polymer compound. (4) The lithium secondary battery according to any one of (1) to (3) above, wherein the content of the thermosetting crosslinking agent in the underlayer is 1 to 10 parts by weight with respect to 100 parts by weight of the polymer compound. . (5) The lithium secondary battery according to any one of the above (1) to (4), wherein the thickness of the underlayer is 0.1 to 20 μm. (6) The lithium secondary battery according to any one of (1) to (5), wherein the underlayer contains 1 to 15 parts by weight of the acid acceptor based on 100 parts by weight of the polymer compound.

[0017]

According to the present invention, an underlayer containing carbon black, a fluorine-based polymer compound and a thermosetting crosslinking agent and subjected to a thermosetting treatment is interposed between the electrode layer and the current collector.

By interposing the above-described underlayer, the adhesion between the electrode layer and the current collector is improved. Further, the underlayer having the above-described structure has a large flexibility, and thus the expansion and contraction of the electrode during charge and discharge can be improved. The shear stress generated between the electrode layer and the current collector caused by the shrinkage can be reduced, and the adhesion can be prevented from deteriorating. In addition, since the underlayer is formed by curing a polymer compound containing a fluorine-based polymer compound, it has high chemical resistance, and the resin layer at the current collector interface is decomposed by repeated charge and discharge. Or elution into the organic electrolyte does not occur. Therefore, the secondary battery having the configuration of the present invention prevents deterioration in adhesion between the current collector and the electrode layer due to charge and discharge, has a large capacity and has a small capacity decrease when repeated charge and discharge, and has excellent reliability. It can be a lithium secondary battery.

[0019]

[Specific Configuration] Hereinafter, a specific configuration of the present invention will be described in detail.

The lithium secondary battery of the present invention comprises an electrolyte comprising a non-aqueous solvent containing a lithium-containing electrolyte, and further comprises an electrode layer constituting the negative electrode and / or the positive electrode as shown in FIG. 9 is formed on the current collector 8 via an underlayer 10 which contains carbon black and a polymer compound and is crosslinked by a thermosetting treatment with a thermosetting crosslinking agent.

The polymer compound used for the underlayer is preferably a fluorine-based polymer compound containing a fluorine atom in the molecule, for example, polychlorotrifluoroethylene (PCTFE),
Polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber, vinylidene fluoride-tetrafluoroethylene fluororubber, tetrafluoroethylene-par Fluoroalkyl vinyl ether fluororubber, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene fluororubber, vinylidene fluoride-tetrafluoroethylene-perfluoroalkylvinylether fluororubber, tetrafluoroethylene-perfluoroalkylvinylether fluororubber, propylene-tetrafluoro At least one of ethylene fluorine rubber, fluorosilicone rubber, fluorine-containing phosphazene rubber, fluorine-based thermoplastic rubber, etc. Those thermosetting treatment by curing crosslinking agent.

As the thermosetting cross-linking agent, conventionally known ones may be used. For example, hexamethylene diamine carbamate, N, N'-dicinnamylidene-1,6-hexanediamine, 4,4'-methylene-bis- (cyclohexylamine ) Polyamines such as carbamates, polyols such as bisphenol AF [2,2-bis (4-hydroxyphenyl) hexafluoropropane], bisphenol A [2,2-bis (4-hydroxyphenyl) propane], and 2,5 Peroxides such as -dimethyl-2,5-di-t-butylperoxyhexane, 1,3-bis- (t-butylperoxy-isopropyl) benzene, and triallyl isocyanurate as a peroxide crosslinking aid; 6-dibutylamyl-1,3,5-triazine-2,4-dithio- Triazine dithiol etc. are preferred.

The content of the thermosetting crosslinking agent is preferably 1 to 10 parts by weight based on 100 parts by weight of the polymer compound. When the amount is less than 1 part by weight, a high crosslinking density of the polymer compound contained in the underlayer cannot be obtained, and the adhesion and the chemical stability of the underlayer become insufficient. If the amount is more than 10 parts by weight, the storage stability of the coating composition deteriorates, and it becomes difficult to form a thin underlayer film.

The underlayer may contain other polymer compounds such as polymethyl methacrylate (PMMA) and polycarbonate (PC), but these may be contained in 100 parts by weight of the fluorine-based polymer compound. The content is preferably about 50 parts by weight or less.

An additive such as an acid acceptor may be added to the underlayer. The acid acceptor may be a conventionally known one, and examples thereof include magnesium oxide and calcium hydroxide. The content of the acid acceptor is preferably in the range of about 1 to 15 parts by weight based on 100 parts by weight of the polymer compound.

The carbon black used in the underlayer of the present invention is preferably a conductive carbon black having a large structure in which primary particles have a chain. The size of the structure is generally represented by the amount of oil absorption by dibutyl phthalate (DBP), and the amount of DBP oil absorption (JIS K6221-
(Method A) is preferably in the range of 100 to 500 cc / 100 g.

When the DBP oil absorption is less than 100 cc / 100 g, the conductivity of the underlayer is not sufficient, the internal resistance of the electrode increases, and the discharge capacity decreases. DBP oil absorption 500
If it exceeds cc / 100 g, the flexibility and adhesion of the underlayer deteriorate, and the electrode layer expands when charging and discharging are repeated,
The shear stress due to shrinkage is not relaxed, and the discharge capacity of the battery decreases.

The type of the carbon black is not limited. For example, furnace black, acetylene black, Ketjen black and the like are preferable.

The content of carbon black used in the underlayer is not particularly limited, but is preferably about 10 to 200 parts by weight based on 100 parts by weight of the polymer compound. If the amount is less than 10 parts by weight, the conductivity of the underlayer deteriorates, and the capacity of the battery decreases.
If the amount exceeds 200 parts by weight, the strength and adhesion of the underlayer are not sufficient, and the electrode layer expands when charging and discharging are repeated,
The shear stress due to shrinkage is not sufficiently relaxed, and the discharge capacity of the battery decreases.

In the secondary battery of the present invention, the negative electrode active material is not particularly limited, but may be a carbon material capable of doping or intercalating lithium ions, a conductive polymer material, a lithium metal, or a lithium alloy. The carbon-based material may be appropriately selected from graphite, carbon black, mesophase carbon black, resin fired carbon material, gas-grown carbon fiber, carbon fiber, and the like. For example, Japanese Patent Publication No. 62-23433, Japanese Patent Application Laid-Open No. 3-137010 And those described in Japanese Patent Application Publication No.

The conductive polymer material may be selected from, for example, polyacetylene, polyphenylene, polyacene, and the like, and examples thereof include those described in JP-A-61-77275.

The positive electrode active material is not particularly limited, but may be a metal compound, metal oxide, metal sulfide, carbon material or conductive polymer material capable of doping or intercalating lithium ions.
O ₂ , LiNiO ₂ , LiMnO ₂ , Li ₂ MnO ₄ ,
V ₂ O ₅ , TiS ₂ , MoS ₂ , FeS ₂ , polyacetylene, polyaniline, polypyrrole, polythiophene,
Polyacene and the like, and JP-B-61-53828.
And Japanese Patent Publication No. 63-59507.

When a metal oxide, a metal sulfide, or the like is used as the positive electrode active material, it is preferable to include a carbon material such as graphite, acetylene black, and Ketjen black as a conductive agent.

To produce the secondary battery electrode of the present invention,
First, a base coating composition for forming a base layer on the current collector surface is applied.

The material and shape of the current collector are not particularly limited, and a band-shaped metal or alloy such as aluminum, copper, nickel, titanium, and stainless steel may be used in the form of a foil, a perforated foil, or a mesh. You can use it.

The polymer compound, thermosetting crosslinking agent, carbon black, and various additives are mixed and dispersed together with a solvent, if necessary, using a dispersing device such as a stirrer, a ball mill, a super sand mill, a pressure kneader, or the like. A coating composition is prepared.

The solvent used in the base coating composition is not particularly limited, and various solvents such as water, methyl ethyl ketone, cyclohexanone, isophorone, N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, and toluene are used. Should be selected according to

Such a base coating composition is applied to a belt-like current collector. The coating method is not particularly limited, and a known method such as an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, or a screen printing method may be used. Thereafter, a curing treatment is performed by a thermosetting device. Note that the curing treatment may be performed after forming the electrode layer.

The thermosetting treatment may be performed according to a known method.
A far-infrared furnace, an electric furnace, an oven, a vacuum oven, an inert gas replacement oven, or the like may be used.

The thickness of the underlayer is not limited, but may be 0.1 to 2
0 μm, more preferably 0.5 to 10 μm. 0.
If it is less than 1 μm, it is difficult to avoid occurrence of film defects such as pinholes. If the thickness exceeds 20 μm, the conductivity of the interface between the electrode and the current collector deteriorates, and the capacity of the battery decreases.

An electrode layer is formed on the undercoated current collector prepared as described above.

The electrode coating composition is prepared by mixing and dispersing the active material, binder resin, various additives, and the like, if necessary, with a solvent and the like using a stirrer, a ball mill, a super sand mill, a pressure kneader or the like.

The binder resin contained in the electrode layer is not limited, but may be selected from a thermoplastic resin, a thermosetting resin, a radiation-curable resin, and the like. For example, polyolefin resin, polyester resin, polyamide resin, epoxy resin Resins, acrylic resins, etc., among which fluorine-based resins containing a fluorine atom in the molecule, such as polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), and tetrafluoroethylene-hexafluoro Propylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVD)
F), vinylidene fluoride-hexafluoropropylene copolymer, polyvinyl fluoride (PVF), vinylidene fluoride fluororubber, propylene-tetrafluoroethylene fluororubber, tetrafluoroethylene-perfluoroalkylvinylether fluororubber, Fluorine acrylic resins, fluorinated polycarbonate resins and the like are preferred. These resins may be used alone or as a mixture, but are not necessarily limited thereto.

Such an electrode coating composition is applied to the above-mentioned base-treated current collector. The coating method is not particularly limited, and a known method such as an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, or a screen printing method may be used. Thereafter, if necessary, a rolling process using a flat plate press, a calender roll, or the like is performed. The electrode layer may be subjected to a curing treatment. The underlayer may be subjected to a curing treatment after the formation of the electrode layer. When the curing treatment method of the electrode layer is a heat curing treatment, it is preferable to carry out the curing treatment at the same time.

The electrolytic solution used in the secondary battery of the present invention comprises:
A solution in which a lithium-containing electrolyte is dissolved in a non-aqueous solvent is used. The lithium-containing electrolyte is not particularly limited,
For example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiA
sF _6, may be appropriately selected from LiCF ₃ SO ₃ and the like.

The non-aqueous solvent is not particularly limited.
For example, dimethyl sulfoxide, sulfolane, ethylene carbonate, propylene carbonate, γ-butyrolactone, γ-valerolactone, γ-octanoic lactone, 1,2-diethoxyethane, 1,2-dimethoxyethane, 1,2-dibutoxyethane , 1,3-dioxolan, tetrahydrofuran, 2-methyltetrahydrofuran or the like, or a mixture thereof.

The amount of the electrolyte dissolved in the non-aqueous solvent is not particularly limited, but is preferably about 0.1 to 2 Mol / l.

Although the structure of the lithium secondary battery of the present invention is not particularly limited, it is usually composed of a positive electrode, a negative electrode, and a separator provided as necessary, and can be a paper type battery, a button type battery, a stacked type battery, Examples include a cylindrical battery.

[0049]

The present invention will be described in detail below with reference to specific examples of the present invention.

<Examples 1, 2, 3, and 4> First, an underlayer paint was prepared as follows. The composition of the underlayer coating is as follows.

Underlayer coating composition carbon black 100 parts by weight (types are listed in Table 1) Polymer compound 100 parts by weight (Viton A: manufactured by Showa Denko / Dupont) 3 parts by weight of thermosetting crosslinking agent (type Table 1) MgO 15 parts by weight (Kyowa Mag # 30: manufactured by Kyowa Chemical Co., Ltd.) N-methylpyrrolidone 2000 parts by weight

The above composition was mixed and dispersed in a ball mill for 10 hours to prepare a paint for an underlayer. The underlayer paint thus obtained is applied to both sides of a 12 μm-thick strip-shaped copper foil by a doctor blade method, and then dried by hot air, so that the thickness of one side of the underlayer becomes 3 μm. It was applied as follows.

An electrode paint was prepared as follows. The composition of the electrode paint is as follows.

Electrode coating composition Active material Graphite 100 parts by weight (KS44: manufactured by Lonza) Binder resin 8 parts by weight (HYLAR720: manufactured by Montecatini) N-methylpyrrolidone 180 parts by weight

Next, the above composition was mixed and dispersed in a ball mill for 10 hours to prepare a coating material for an electrode. The electrode paint thus obtained was applied on both sides of the previously prepared copper foil subjected to the base treatment by a doctor blade method, followed by hot-air drying, and the thickness of one side of the electrode layer was 100 μm.
It was applied so that Next, a heat curing treatment was performed at 200 ° C. for 24 hours in an N ₂ -substituted oven.

The sample prepared as described above was
The electrode layer was cut at a width of 5 mm at the upper end to leave an electrode layer of 20 mm square. Electrodes A, B, C, and D (Table 1) were produced by spot welding a titanium wire as a lead to the upper end from which the electrode layer was removed.

Using these electrodes as working electrodes, the charge / discharge measurement cells of Examples 1 to 4 were prepared and charged / discharged as follows.

A lithium plate connected to a titanium wire was used for the counter electrode and the reference electrode, and ethylene carbonate,
1M in a mixed solvent of diethyl carbonate at a volume ratio of 1: 1
Used was dissolved lithium perchlorate. And
0 to 1 volt vs. Li / Li at a constant current of 4 mA
Charge / discharge was performed within the range of ⁺ . The results are shown in Table 1.

FIG. 1 is a sectional view of a cell for measuring charge / discharge characteristics. 1 is a 100 cm @ 3 glass beaker, 2 is a silicon stopper, 3 is a working electrode which is an electrode, 4 is a counter electrode, 5 is a reference electrode,
Reference numeral 6 denotes a lugine tube, and reference numeral 7 denotes an electrolytic solution.

[0060]

[Table 1]

<Comparative Example 1> An electrode E was prepared in the same manner as in Example 1 except that the base treatment of the current collector was not performed, and a cell of Comparative Example 1 was prepared using the electrode E in the same manner as described above. .

<Comparative Example 2> As the polymer compound in the base coating composition, HYLAR28 which is a thermoplastic resin was used.
An electrode F (Table 1) was produced in the same manner as in Example 1 except that 00 (manufactured by Montecatini) was used, and a cell of Comparative Example 2 was produced in the same manner as above using this.

The charge and discharge characteristics of the cells of these comparative examples were measured in the same manner as described above. The results are shown in Table 1 above.

Table 1 shows that the electrode samples A, B, C, and
It can be seen that the cells of Examples 1 to 4 using D have a small decrease in discharge capacity when charging and discharging are repeated. For example, comparing the discharge capacities at 20 cycles in Table 1, the electrodes A, B,
In the cells of Examples 1 to 4 using C and D, the discharge capacity was significantly improved as compared with the cell of the comparative example, and the cycle life was improved.

[0065]

According to the present invention, a highly reliable lithium secondary battery having a large capacity and a long battery life when repeatedly charged and discharged can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an electrode of a lithium secondary battery according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a cell for measuring charge and discharge characteristics. DESCRIPTION OF SYMBOLS 1 Beaker 2 Silicon stopper 3 Electrode (working electrode) 4 Counter electrode 5 Reference electrode 6 Luggin tube 7 Electrolyte 8 Current collector 9 Electrode layer 10 Underlayer

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yosuke Miyagi 1-1-13 Nihonbashi, Chuo-ku, Tokyo Inside TDK Corporation (56) References JP-A-3-238771 (JP, A) JP-A-3 -165458 (JP, A) JP-A-5-135759 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/62 H01M 10/40

Claims (6)

(57) [Claims] In a secondary battery provided with a nonaqueous electrolyte containing a lithium-containing electrolyte, an electrode layer constituting a negative electrode and / or a positive electrode is formed on a current collector via an underlayer,
A lithium secondary battery, wherein the underlayer contains carbon black, a polymer compound containing a fluorine-based polymer compound, and a thermosetting crosslinking agent, and is subjected to a thermosetting treatment. 2. The lithium secondary battery according to claim 1, wherein the carbon black contained in the underlayer has a DBP oil absorption of 100 to 500 cc / 100 g. 3. The method according to claim 1, wherein the content of carbon black in the underlayer is 10 to 100 parts by weight of the polymer compound.
3. The lithium secondary battery according to claim 1, wherein the amount is 200 parts by weight. 4. In the underlayer, the content of the thermosetting crosslinking agent is 1 to 10 parts by weight based on 100 parts by weight of the polymer compound.
4. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is in parts by weight. 5. A method according to claim 1, wherein said underlayer has a thickness of 0.1 to 20 μm.
The lithium secondary battery according to any one of claims 1 to 4, wherein 6. The lithium secondary battery according to claim 1, wherein the underlayer contains 1 to 15 parts by weight of an acid acceptor based on 100 parts by weight of the polymer compound.