JP2001357854A - Nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous secondary battery having an excellent safety function in overcharge and excellent in adhesion to a collector and an active material without causing the degradation of volume energy density and the increase of cost. SOLUTION: This battery is composed by using the collector having a positive temperature coefficient resistor function that increases its resistance value when exceeding a predetermined temperature and by covering the collector with a conductive layer comprising a crystalline thermoplastic resin, a conductive agent and a binder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous secondary battery, and more particularly to a non-aqueous secondary battery provided with a current collector having a safety function at the time of overcharge and a binding function with an active material layer. It is about.

[0002]

2. Description of the Related Art Information terminals such as portable telephones and notebook personal computers are becoming smaller, lighter and thinner year by year, and there is an increasing demand for smaller, lighter and thinner batteries as power sources. . Under these circumstances, lithium-ion secondary batteries have attracted attention because they can obtain a higher energy density than aqueous secondary batteries such as conventional lead-acid batteries, nickel-cadmium batteries, and nickel-metal hydride batteries.

[0003] However, in a lithium-ion secondary battery, an active material layer is applied to a current collector, dried, rolled, cut into a predetermined size, and then wound or laminated in a spiral shape. The problem is that the current collecting performance is reduced due to the separation and dropping of the substance from the current collector. To cope with this problem, for example, JP-A-63-112265 discloses that a conductive coating film composed of a conductive filler and a binder is formed on a current collector. Kaihei 11-329
No. 448 discloses that the surface of the current collector is roughened.

On the other hand, as a method for reducing the thickness, a lithium polymer secondary battery using a polymer as a separator material for holding an electrolyte is disclosed in Japanese Patent Application Laid-Open No. Hei 8-507407. VD
Using a copolymer P (VDF-HFP) of F) and hexafluoropropylene (HFP), the polymer, the positive electrode, and the negative electrode are integrated by heat fusion.

However, this lithium polymer secondary battery contains a large amount of polymer that does not contribute to the battery capacity in the positive electrode and the negative electrode, and thus has a lower battery capacity than the lithium ion secondary battery. For this reason, in order to increase the capacity of the lithium polymer secondary battery, it is necessary to reduce the amount of polymer in the electrode plate as far as integration is possible, but if the amount of polymer is reduced, the connection between the active material layer and the current collector is reduced. There is a problem that the adhesiveness is reduced and the contact resistance is increased. For this challenge,
For example, US Pat. No. 5,554,459 discloses a method using a metal current collector in which an insulating material on the current collector surface is removed and bonded with an adhesive conductive polymer composition, or a polymer P (V
A method is disclosed in which a material obtained by adding a conductive substance to DF-HFP is coated after removing the insulating material on the current collector surface.

[0006] As the performance of the battery is improved and the energy density is increased in this way, it is important to ensure the safety of the battery. For example, a positive temperature coefficient resistor (hereinafter referred to as P
By attaching a safety component such as a TC element) to the battery, the resistance of the PTC element rapidly rises due to abnormal heat generation or overcurrent, and the circuit is substantially cut off, thereby ensuring the safety of the battery. .

[0007]

However, the mounting of such a safety component to a battery causes a poor response due to a delay in heat, or lowers the energy density of the battery pack by the weight and space of the safety component. Not only inviting but also increasing the material cost.

Therefore, Japanese Patent Application Laid-Open No. H11-329503 discloses that any one of a positive electrode, a negative electrode, and a non-aqueous electrolyte has the characteristics of a positive temperature coefficient resistor (hereinafter, PTC). However, in order to impart PTC characteristics to these, it is necessary to add a large amount of additives that do not contribute to the battery capacity, and the energy density is undesirably reduced.

On the other hand, Japanese Patent Application Laid-Open No. Hei 10-241665 also discloses a method of bonding an electron conductive material having PTC characteristics to a current collector, but since the thickness of the electron conductive material is as large as 50 μm. However, the energy density of the entire battery is undesirably reduced.

Therefore, the present invention provides a high-performance non-aqueous secondary battery having a high safety function and excellent binding properties between the current collector and the active material layer without lowering the energy density or increasing the cost. A main object is to provide a non-aqueous secondary battery.

[0011]

According to the present invention, there is provided a non-aqueous secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the current collector of the positive electrode and / or the negative electrode is provided. Is coated with a conductive layer containing a conductive agent and a binder in addition to a crystalline thermoplastic resin having a function as a positive temperature coefficient resistor (PTC element) whose resistance value increases with an increase in temperature. The thickness of this conductive layer is from 0.1 μm to 5.
It is intended to provide a non-aqueous secondary battery set to 0 μm. More preferably, a non-aqueous secondary battery having a positive temperature coefficient resistor in which the resistance of a current collector coated with a conductive layer composed of a crystalline thermoplastic resin, a conductive agent and a binder increases to 100 Ωcm or more. .

When the temperature in the battery exceeds the melting point of the crystalline thermoplastic resin, the crystalline thermoplastic resin expands in volume, and the conductive agent dispersed in the conductive layer is separated from the conductive agent. Is reduced. As a result, when the battery internal temperature reaches the melting point of the crystalline thermoplastic resin due to the heat generated when the battery is overcharged, the resistance of the conductive coating film rapidly increases, and the current between the current collector and the active material layer increases. Is shut off.

In addition, since it has a conductive agent and a binder,
It also has a function of preventing the active material layer from peeling off or falling off from the current collector, and is particularly effective in the case of a lithium polymer battery using a polymer that absorbs and retains an electrolytic solution for the positive electrode and the negative electrode.

As described above, according to the non-aqueous secondary battery of the present invention, since the current collectors of the positive electrode and / or the negative electrode have a positive temperature coefficient resistor function, the current collector portion is generated when the battery generates heat. By increasing the resistance of the battery and interrupting the current, the battery can be prevented from being ruptured or ignited, and has an excellent function of binding to the active material layer.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

In the following, a lithium polymer battery will be described as an example of an embodiment of the nonaqueous secondary battery of the present invention, but the present invention is also applicable to nonaqueous secondary batteries other than the lithium polymer battery.

1, 2 and 3 are a sectional view of a positive electrode and a sectional view of a negative electrode, respectively, used in the non-aqueous secondary battery of the present invention.
FIG. The positive electrode 1 includes a positive electrode active material layer 4 and a positive electrode current collector 5 embedded in the positive electrode active material layer. Negative electrode 2
Similarly, a negative electrode active material layer 7 and a negative electrode current collector 8 embedded in the negative electrode active material layer are also provided. The polymer electrolyte 3 is provided between the positive electrode plate and the negative electrode plate, and is integrated with the positive electrode and the negative electrode by a heat welding method, to obtain a configuration sectional view as shown in FIG.

The positive electrode current collector 5 is made of aluminum foil, punched metal and expanded metal, and has on its surface a carbon-based conductive agent such as carbon black or graphite represented by acetylene black and polyvinylidene fluoride as a binder. A conductive layer 6 made of (PVDF) and a crystalline thermoplastic polyolefin resin having a positive temperature coefficient resistor function such as a polyethylene resin or a polypropylene resin is applied.

The negative electrode current collector 8 is made of copper or nickel foil, punched metal and expanded metal, and has a carbon conductive agent such as carbon black or graphite represented by acetylene black on the surface and a polyolefin as a binder. A conductive layer 9 made of vinylidene fluoride (PVDF) and a crystalline thermoplastic polyolefin resin having a positive temperature coefficient resistor function such as a polyethylene resin or a polypropylene resin is applied.

As a method of forming the conductive layer on the current collector, for example, acetylene black and polyethylene
A dispersion of DF in an N-methyl-2-pyrrolidone (NMP) solution was applied on a current collector, and then NMP was used as a solvent.
Is removed by drying.

In the conductive layer, the crystalline thermoplastic resin and the conductive agent are uniformly dispersed, the binder forms the skeleton of the coating film, and the conductive agent forms a conductive path in a mesh. However, the above-mentioned crystalline thermoplastic resin has a property of rapidly expanding when its melting point is exceeded, and when the temperature of the conductive layer exceeds the melting point of the crystalline thermoplastic resin, it is uniformly distributed in the conductive coating film. The dispersed crystalline thermoplastic resin rapidly expands in volume and separates the electrical contact between the conductive carbon powders in the conductive coating, and the conductive path is cut off, exhibiting the property of a sudden rise in resistance. I do. Due to this property, when the battery temperature exceeds the melting point of the crystalline thermoplastic resin due to heat generated by overcharging of the battery, the resistance between the active material layer and the current collector rises rapidly, and the current is interrupted.

The positive electrode active material layer and the negative electrode active material layer are
For example, a paste obtained by mixing an active material, a conductive agent, a polymer, and a plasticizer in an organic solvent such as acetone is directly applied to the current collector on which the conductive layer is formed, and then the solvent is removed by drying. . The polymer electrolyte is prepared by applying a paste obtained by mixing a polymer and a plasticizer in an organic solvent such as acetone on a polyester film, for example, and then removing the solvent by drying.

Here, examples of the active material used for the positive electrode mixture layer include lithium-containing metal oxides such as LiMO ₂ (M = Co, Ni, Mn) and metal sulfides. As the polymer, PVDF or a copolymer P of polyvinylidene fluoride and hexafluoropropylene (VDF-HF
P) is preferably used. Examples of the conductive agent include carbon-based conductive agents such as acetylene black, graphite, and carbon fiber.

On the other hand, examples of the active material used for the negative electrode material mixture layer include carbon materials such as graphite, carbon black, and activated carbon capable of inserting and extracting lithium, lithium metal, and lithium alloy. As the polymer, PVD
F or P (VDF-HFP) is preferably used. Examples of the conductive agent include carbon-based conductive agents such as acetylene black, graphite, and carbon fiber.

The above-described battery in which the positive electrode, the negative electrode, and the separator are laminated and integrated is immersed in xylene, and a plasticizer D
After extracting and removing BP and drying xylene, a non-aqueous electrolyte is injected to prepare a battery.

Here, the non-aqueous electrolyte used in the non-aqueous secondary battery of the present invention is a solution in which an electrolyte is dissolved in a non-aqueous solvent. Examples of the electrolyte include LiClO ₄ , Li
Lithium salts such as BF ₄ and LiPF ₆ are used. on the other hand,
As the non-aqueous solvent, for example, one or a mixture of several non-aqueous solvents such as propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and vinylene carbonate are used.

The non-aqueous secondary battery of the present invention is constituted by appropriately combining the above-described positive electrode, negative electrode, separator and non-aqueous electrolyte. The shape of the battery itself is sheet-like, cylindrical, rectangular, It can be applied to any shape such as a mold, a coin, and a button.

[0028]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

(Example 1) Polyvinylidene fluoride (PV
To 270 g of a solution of DF) in N-methyl-2-pyrrolidone (NMP) (solid content: 13%), 35 g of a crystalline polyethylene resin having a melting point of 110 ° C. and 30 g of a carbon-based conductive agent of acetylene black were added and kneaded with a planetary mixer. ,
Further, 440 g of NMP is added and diluted to prepare a conductive paste. This paste was applied on a glass plate and dried to form a conductive layer, and the volume resistivity was 0.15 Ωcm.

Example 2 The conductive paste prepared in Example 1 was uniformly applied to both surfaces of an aluminum or copper expanded metal using a die coater or a gravure coater and dried to form a conductive layer having a thickness of 0.5 μm. A coated current collector was produced. On the other hand, a copolymer P of vinylidene fluoride and hexafluoropropylene (VDF-HF
P) A solution obtained by dissolving 710 g in acetone (11300 g), lithium cobaltate (10000 g), acetylene black (530 g), dibutyl phthalate (DBP) (1100 g)
Is mixed to prepare a positive electrode paste, which is applied to both surfaces of an aluminum current collector coated with the conductive layer by a die coater, dried, and then rolled by a roll press to produce a positive electrode plate. Similarly, a solution prepared by dissolving 350 g of P (VDF-HFP) in 3210 g of acetone and 2450 g of spherical graphite
g, 200 g of carbon fiber, and 540 g of DBP to prepare a paste for a negative electrode. The paste was coated on both surfaces of a copper current collector coated with the conductive layer with a die coater, dried, and then rolled by a roll press to form a negative electrode plate. Is prepared. Also, P (VD
280 g of F-HFP) was dissolved in 1440 g of acetone,
A mixed solution to which 280 g of DBP was added was prepared, and this was coated on a polyester film by a die coater to prepare a polymer electrolyte sheet. The positive electrode, the negative electrode, and the electrolyte sheet are laminated as shown in FIG. 3 and passed through a heating roller to be integrated to produce a constituent battery.

The above integrated battery was immersed in xylene to extract and remove DBP, dried in vacuum, impregnated with an electrolyte, and inserted into an aluminum laminated film bag.
The battery was sealed and a battery of the present invention having a nominal capacity of 500 mAh was obtained.
Here, the electrolyte used was one in which LiPF ₆ was dissolved in a mixed solution of ethylene carbonate and ethyl methyl carbonate.

(Example 3) In the same manner as in Example 2, a battery of Example 3 having a current collector with a conductive coating having a thickness of 5.0 µm was produced.

Comparative Example 1 Polyvinylidene fluoride (PV
To 540 g of a solution of DF) in N-methyl-2-pyrrolidone (NMP) (solid content: 13%), 30 g of a carbon-based conductive agent such as acetylene black or graphite is added, kneaded with a planetary mixer, and further diluted with 220 g of NMP. And adjust the conductive paste. This paste was applied on a glass plate with a doctor blade and dried to form a conductive layer, and the volume resistivity was 0.13 Ωcm.

(Comparative Example 2) In the same manner as in Example 2 except that the conductive paste prepared in Comparative Example 1 was used, Comparative Example 2 having a conductive coating current collector having a thickness of 0.5 μm was used. A battery was manufactured.

(Comparative Example 3) A safety component having a positive temperature coefficient resistor function was connected in series to the battery of Comparative Example 2, and this was fixed to the outside of an aluminum laminate bag with tape.
The battery of Comparative Example 3 was obtained.

Comparative Example 4 In the same manner as in Example 2, a battery of Comparative Example 4 having a current-carrying current collector having a thickness of 25 μm was produced.

(Evaluation of Positive Temperature Coefficient Resistor Function) Embodiment 1
FIG. 4 shows the results obtained by measuring the temperature dependency of the volume resistivity of the conductive layer coating film produced in Comparative Example 1 using a four-point probe method. As is clear from FIG. 4, the conductive layer of Comparative Example 1 shows a tendency that the volume resistivity slightly decreases with an increase in temperature, while the conductive layer of the present invention shows a sharp increase in resistance at 100 ° C. or higher. It can be seen that it rises to 100 Ωcm or more.

(Binding property between current collector and active material layer) The weight of the active material per unit area of the positive electrode plates of Examples 2, 3, Comparative Example 2, Comparative Example 3, and Comparative Example 4 was applied. Measured immediately after drying, processed by roll pressing, cutting, punching, etc., and further measured immediately before assembling into a battery, and the active material shedding rate was determined by the following equation. Active material detachment rate (%) = (weight of active material after coating / drying−weight of active material before assembly) / weight of active material after coating / drying × 100

The active material of the positive electrode plate in Examples 2, 3 and Comparative Example 4 hardly fell off.
In the case of Comparative Example 2 and Comparative Example 3, falling off at the end face was observed at the time of cutting and punching.

(Evaluation of Energy Density of Battery) Example 2
The volume energy densities (Wh / l) of the batteries of Example 3, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were determined. Table 1 shows a relative comparison when the volume energy density of Example 2 was set to 100.
Shown in

(Evaluation of Battery Safety) Ten batteries each of Example 2, Example 3, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were prepared, and the batteries were operated at a constant current of 750 mA (1.5 CmA). The charge test was performed, and the number of smoked and ignited batteries is shown in Table 1.

[0042]

[Table 1]

From Table 1, it can be seen that the batteries of Examples 2 and 3 each having a conductive layer having a positive temperature coefficient resistor function having an optimum thickness on the current collector had the positive temperature coefficient resistor function. Compared to the battery of Comparative Example 2, no smoke or ignition occurred during overcharge. This is because the resistance of the conductive layer rapidly rises as shown in the result of FIG. 4 when the battery temperature suddenly rises due to Joule heat generated by the internal resistance of the battery due to the overcharge current and the heat generated by the active material itself. This is probably because the current flowing between the active material and the current collector was interrupted, and charging was stopped. On the other hand, in the battery of Comparative Example 2, the current continued to flow even when the battery temperature was increased due to overcharging, and thus the heat generation of the battery did not stop. As a result, the electrolyte sheet melted due to the heat generation, and the positive electrode and the negative electrode were short-circuited. It is thought that it was done.

The battery of Comparative Example 4 has a conductive layer having a function of a positive temperature coefficient resistor on the current collector. However, even if the thickness is increased, the responsiveness to rupture and ignition of the battery does not change. On the other hand, if the thickness of the conductive layer is large, the energy density decreases, which is not preferable.

Further, the result of Example 2 shows that the safety is excellent as in Comparative Example 3. By providing the positive temperature coefficient resistor function on the current collector, the positive temperature coefficient It has been found that the same effect as when a safety component having a resistor function is connected is obtained.

[0046]

As described above, the present invention relates to a non-aqueous secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the resistance of the current collector of the positive electrode and / or the negative electrode exceeds a predetermined temperature. 0.1 μm consisting of a crystalline thermoplastic resin having a positive temperature coefficient resistor function, a conductive agent and a binder
By coating with a conductive layer having a thickness of m to 5.0 μm, the resistance of the current collector increases due to heat generated during overcharging of the battery, and the current can be cut off to prevent the battery from bursting or firing. This makes it possible to provide a high-performance non-aqueous secondary battery which is excellent not only in safety but also in binding properties without lowering energy density or increasing costs.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a positive electrode plate for a non-aqueous secondary battery of the present invention.

FIG. 2 is a cross-sectional view of a negative electrode plate for a non-aqueous secondary battery of the present invention.

FIG. 3 is a cross-sectional view of the battery after configuration.

FIG. 4 is a characteristic diagram showing the temperature dependence of the volume resistivity of the conductive layer.

[Explanation of symbols]

 Reference Signs List 1 positive electrode 2 negative electrode 3 separator 4 positive electrode active material layer 5 positive electrode current collector 6 conductive layer 7 negative electrode active material layer 8 negative electrode current collector 9 conductive layer

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) HA17

Claims (3)

[Claims] 1. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the current collector of the positive electrode and / or the negative electrode has a positive temperature coefficient whose resistance increases with an increase in temperature. It is covered with a conductive layer containing a crystalline thermoplastic resin having a function of a resistor, a conductive agent and a binder, and the conductive layer has a thickness of 0.1 μm to 5.0 μm. Non-aqueous secondary battery. 2. The crystalline thermoplastic resin has a melting point of 100 ° C.
2. The non-aqueous secondary battery according to claim 1, wherein the current collector coated with the conductive layer has a volume resistivity of 100 Ωcm or more when the current collector is an olefin resin at −120 ° C. and exceeds a predetermined temperature. 3. The non-aqueous secondary battery according to claim 1, wherein the binder is a polyvinylidene fluoride resin.