JP2003331823A - Nonaqueous electrolyte secondary battery and method of manufacturing the battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of suppressing the shortage of liquid in an active material layer by charging and discharging and the deposition of metallic lithium, and providing excellent cycle characteristics and large current discharging characteristics. <P>SOLUTION: In this nonaqueous electrolyte secondary battery, the active material layer contains active material particles, binder, and conductive agent. The surfaces of the active material particles are covered with the binder including the conductive agent. The active material layer can be formed by using a mixture processed by dispersing the conductive agent and the active material particles in the binder by ultrasonic treatment. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the same.

[0002]

2. Description of the Related Art Generally, a non-aqueous electrolyte secondary battery has a structure in which a power generating element manufactured by alternately laminating a pair of positive and negative electrode plates and winding the electrode plate is enclosed inside a battery can together with an electrolytic solution. Has become.

[0003] Here, the electrode plate is one in which an active material layer is provided on the surface of a current collector made of a metal foil. This electrode plate is prepared by mixing a positive electrode active material or a negative electrode active material with a binder made of a polymer material, and adding a conductive agent if necessary to prepare a paste-like mixture, and then mixing this mixture. It is prepared by coating the surface of the current collector to form an active material layer.
At this time, the binder plays a role of binding the active material particles to each other and gelling by containing the electrolytic solution to retain the electrolytic solution in the active material layer.

[0004]

However, in the non-aqueous electrolyte secondary battery as described above, the electrolytic solution is decomposed by the reaction with the active material during charging and discharging. Alternatively, the SEI (So
Lid Electrolyte Interface: A protective film called a solid electrolyte interface is formed. If the electrolyte solution is consumed for these reasons, a part where the electrolyte solution does not exist locally occurs in the active material layer (liquid withdrawal phenomenon), and the charge / discharge cycle characteristics and high rate discharge characteristics deteriorate. There was a problem of doing.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a non-aqueous electrolyte secondary battery having excellent cycle characteristics and high rate discharge characteristics.

[0006]

Means for Solving the Problems The present inventor has earnestly studied to provide a positive electrode active material capable of producing a non-aqueous electrolyte secondary battery having excellent cycle characteristics and high rate discharge characteristics.
The following findings have been found.

In the non-aqueous electrolyte secondary battery as described above, the active material layer is formed of a paste obtained by mixing the active material, the binder and the conductive agent.

Since the binder used in the active material layer is non-conductive, when the contact area between the active material particles becomes small due to the interposition of the binder between the active material particles, the active material layer has a small contact area. Hinders electron conduction. Therefore, it is considered preferable that the binder does not form a film on the surface of the active material particles and the particles are kept in electrical contact with each other to some extent.

However, the binder also plays a role of holding the electrolytic solution in the active material layer. Here, when the binder is unevenly distributed in the active material layer, the amount of the electrolyte held is small in the portion where the binder is small, so that the liquid easily depletes by repeating charging and discharging. It is supposed to happen. Further, when the electrolyte solution is locally insufficient, ionic conduction is disturbed at that portion, which is considered to affect the high rate discharge characteristics.

Therefore, in order to secure both ionic conductivity and electronic conductivity and to improve the cycle characteristics and the high rate discharge characteristics, it is necessary to coat the entire surface of the active material particles with a binder. It is considered effective to secure the electrical connection between the active material particles by holding the electrolytic solution around the active material particles and including the conductive agent in the binder film. The present invention has been made based on such novel findings.

That is, the present invention is a non-aqueous electrolyte secondary battery comprising a pair of positive and negative electrodes formed by forming an active material layer on a current collector, wherein the active material layer comprises active material particles, It is characterized in that it contains a binder and a conductive agent, and the surface of the active material particles is covered with the binder containing the conductive agent.

[0012] Of the active materials of the present invention, the positive electrode active material used on the positive electrode side is not particularly limited as long as it is one normally used as the positive electrode active material of non-aqueous electrolyte secondary batteries.
For example, lithium cobalt oxide, lithium nickel oxide, spinel-based lithium manganate, which is a lithium-containing oxide of a transition metal, or a composite oxide thereof can be used. Further, the negative electrode active material used on the negative electrode side is not particularly limited as long as it is usually used as a negative electrode active material of a non-aqueous electrolyte secondary battery, and examples thereof include graphite, carbon black, activated carbon, carbon fiber, and coke. Carbon materials such as The average particle size of the active material is 5 μm to 30 μm
It is preferably m. If it is less than 5 μm, the contact area of the active material with the electrolytic solution becomes too large, so that the electrolytic solution is prone to be decomposed, and the liquid is liable to be exhausted, which is not preferable. Also, 30μ
If it is larger than m, it becomes difficult to control the thickness when forming the active material layer and it becomes difficult to smooth the surface, which is not preferable.

The binder of the present invention is not particularly limited as long as it is usually used in the active material layer of a non-aqueous electrolyte secondary battery. For example, polyvinylidene fluoride, vinylidene fluoride-tetrafluoropropylene copolymer Polymer materials such as polymer, vinylidene fluoride-hexafluoroethylene copolymer, vinylidene fluoride-tetrafluoropropylene-hexafluoroethylene copolymer, styrene-butadiene rubber, polyacrylonitrile, polyethylene oxide, low melting point polyethylene, acrylic resin Can be used. These polymeric materials may be used alone or in combination of two or more.

Particularly, a vinylidene fluoride / hexafluoropropylene copolymer excellent in both the binding property and the electrolytic solution retention property can be preferably used. The composition ratio of the vinylidene fluoride unit and the hexafluoropropylene unit is
The molar ratio is preferably 98: 2 to 94: 6. Hexafluoropropylene unit ratio is 6 mol%
If it is larger than this, the active material layer is easily peeled off from the current collector, which is not preferable. On the other hand, if the ratio of hexafluoropropylene units is less than 2 mol%, there is no problem in applying the active material layer to the current collector, but the retention of the electrolytic solution is reduced, which is not preferable.

The conductive agent of the present invention is not particularly limited as long as it is one normally used in the active material layer of a non-aqueous electrolyte secondary battery, and for example, fine particles of acetylene black, graphite, metal or the like can be used. Since the conductive agent needs to be able to exist in the binder coating formed on the surface of the active material particles, the average particle diameter thereof is preferably 1 μm or less. Further, the content of the conductive agent is preferably 10% by weight to 70% by weight with respect to the binder. If the content of the conductive agent is less than 10% by weight, electrical connection between the active material particles cannot be ensured and the current collecting property is reduced, which is not preferable. On the other hand, if the content of the conductive agent is more than 70%, the binding force is lowered and the active material layer is easily peeled off from the current collector, which is not preferable.

In the present invention, as a method for preparing active material particles whose surface is coated with a binder, for example, an ultrasonic treatment device is used to disperse the conductive agent and the active material particles in the binder. There is a method of making it. In addition, the jet mill method can be used to achieve the same purpose.

[0017]

According to the present invention, the surface of the active material particles is coated with a binder containing a conductive agent. With such a configuration, the electrolytic solution can be uniformly held on the entire surface of the active material particles. Therefore, it is possible to ensure the ionic conductivity of the active material layer and prevent local liquid depletion. Further, the binder contains a conductive agent. Therefore, even if the surface of the active material particles is covered with the binder, the active material particles can be electrically connected to each other through the conductive agent, and the electron conductivity can be secured. With these, a non-aqueous electrolyte secondary battery having excellent cycle characteristics and high rate discharge characteristics can be provided.

[0018]

EXAMPLES The present invention will be described in more detail with reference to examples.

[Charge / Discharge Cycle Test] <Example 1-1> 1. Preparation of Mixture 1) As the raw material binder, a vinylidene fluoride-hexafluoropropylene copolymer having a hexafluoropropylene unit content of 3 mol% (hereinafter sometimes simply referred to as “copolymer”) used. As the conductive agent, acetylene black having a primary particle diameter of 35 nm to 50 nm was used.
As the positive electrode active material, LiC having a particle size of 5 μm to 25 μm
oO ₂ was used. As the negative electrode active material, graphite having a particle size of 5 μm to 25 μm was used.

2) Preparation of positive electrode mixture A vinylidene fluoride-hexafluoropropylene copolymer was dissolved in N-methylpyrrolidone to prepare a binder solution having a concentration of 2.5% by weight. Acetylene black was added to this binder solution so that the weight ratio of the copolymer to the acetylene black was 3: 2 to prepare a slurry. By sonicating this slurry, acetylene black was dispersed in the copolymer. LiCoO ₂ was added to this slurry so that the total weight ratio of the copolymer and acetylene black to LiCoO ₂ was 10:90. This mixture was put into an ultrasonic stirrer (Ultrasonic Disperser USDS-1 manufactured by Nippon Seiki Seisakusho Ltd.), and ultrasonically treated while stirring to disperse LiCoO ₂ in the slurry. In this way, a paste-like positive electrode mixture was prepared.

3) Preparation of Negative Electrode Mixture A vinylidene fluoride-hexafluoropropylene copolymer was dissolved in N-methylpyrrolidone to prepare a binder solution having a concentration of 2.5% by weight. Acetylene black was added to this binder solution so that the weight ratio of the copolymer and acetylene black was 8: 1 to prepare a slurry. Graphite was added to this slurry so that the weight ratio of graphite to the total of the copolymer and acetylene black was 1.
It was added so that it might become 5:85. This mixture was added to the above 2)
In the same manner as above, the mixture was put into an ultrasonic stirrer and subjected to ultrasonic treatment while stirring to disperse graphite in the slurry. In this way, a paste-like negative electrode mixture was prepared.

2. Preparation of Lithium Ion Secondary Battery 1) Preparation of Positive Electrode A predetermined amount of the mixture for positive electrode prepared in 1.2) above is uniformly applied on both sides of a current collector made of an aluminum foil having a thickness of 20 μm and dried. After that, pressing was performed, and further vacuum drying was performed at 140 ° C. In this way, a strip-shaped positive electrode sheet having the positive electrode active material layer was produced. A positive electrode lead made of a 100 μm thick aluminum piece was welded to one end of this positive electrode sheet. The cross section of the formed positive electrode active material layer was observed with a scanning electron microscope (Hitachi scanning electron microscope S-2150).

2) Preparation of Negative Electrode A predetermined amount of the negative electrode mixture prepared in the above 1.3) was uniformly applied to both surfaces of a current collector made of a copper foil having a thickness of 20 μm,
After drying, pressing was performed and further vacuum drying was performed at 140 ° C. In this way, a strip-shaped negative electrode sheet provided with the negative electrode active material layer was produced. One end of this negative electrode sheet has a thickness of 1
A negative electrode lead made of a nickel piece of 00 μm was welded.
Further, as in the above, the cross section of the negative electrode active material layer was observed with a scanning electron microscope.

3) Preparation of Electrolyte Solution Ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1 to prepare a non-aqueous solvent. L, which is a lithium salt as an electrolyte, is added to this non-aqueous solvent.
iPF ₆ was added to a concentration of 1.2 mol / l to prepare a non-aqueous electrolytic solution.

4) Preparation of battery A positive electrode sheet, a separator, a negative electrode sheet and a separator are
The positive electrode lead and the negative electrode lead were laminated in this order with the end portions on the welded side on the same side, to obtain a laminated body. As the separator, a polypropylene / polyethylene / polypropylene three-layer film having a thickness of 25 μm was used. This laminated body was wound in an elliptical spiral shape around a polyethylene rectangular core as a center to manufacture a power generation element.
This power generating element was housed in a bag-shaped battery case made of a laminate film, and the positive electrode lead and the negative electrode lead were fixed to the battery case. Then, the electrolytic solution prepared in the above 3) was injected into the battery case in an amount such that the positive electrode sheet, the negative electrode sheet and the separator were sufficiently wet and there was no electrolytic solution which was not retained by the power generating element in the battery case. Then, the opening of the battery case was sealed by heating and pressure bonding. In this way, a laminated non-aqueous electrolyte secondary battery having a nominal capacity of 650 mAh was produced.

3. Cycle test Regarding the battery created by the above method, in a room temperature atmosphere,
After charging to 4.2 V with a constant current of 0.5 CA, charging was performed at a constant voltage of 4.2 V for 3 hours after the start of charging. Then, this battery was discharged to 2.7 V with a constant current of 0.5 CA, and the initial discharge capacity was measured. This was set as one cycle, and a predetermined cycle was repeated to measure the discharge capacity.
Further, the battery after the completion of 500 cycles was disassembled, and the cross sections of the positive electrode active material layer and the negative electrode active material layer were observed with a scanning electron microscope.

<Example 1-2> Except that the mixture for the negative electrode was prepared only by stirring using an ordinary stirring device (Poppard mixer LVT-A type manufactured by Kodaira Seisakusho) without using an ultrasonic stirring device. A battery was prepared in the same manner as in Example 1-1, and a cycle test was performed.

<Example 1-3> The same as Example 1-1, except that the positive electrode mixture was prepared only by stirring with the stirring device of Example 1-2 without using an ultrasonic stirring device. Then, a battery was manufactured and a cycle test was performed.

<Comparative Example 1> Without using an ultrasonic stirrer,
A battery was prepared and a cycle test was performed in the same manner as in Example 1-1, except that the positive electrode mixture and the negative electrode mixture were prepared only by stirring with the stirring device of Example 1-2.

[High Rate Discharge Test] <Example 2-1> A battery prepared by the same method as in Example 1-1 was charged to 4.2 V at a constant current of 0.5 CA in a room temperature atmosphere, Charging was performed at a constant voltage of 4.2 V for up to 3 hours after the start of charging. Then add 0.5
A constant current of CA was discharged to 2.7 V, and the initial discharge capacity was measured. Next, regarding this battery, in a room temperature atmosphere,
After charging to 4.2 V with a constant current of 0.5 CA, charging was performed at a constant voltage of 4.2 V for 3 hours after the start of charging. Then, this battery was discharged to 2.7 V at a constant current of 5 CA, and the high rate discharge capacity was measured.

<Example 2-2> A discharge test was conducted on a battery prepared by the same method as in Example 1-2, in the same manner as in Example 2-1.

<Example 2-3> A discharge test was conducted on a battery prepared by the same method as in Example 1-3, in the same manner as in Example 2-1.

<Comparative Example 2> A discharge test was conducted on a battery prepared by the same method as in Comparative Example 1 in the same manner as in Example 2-1.

[Results and Discussion] 1. Observation with Scanning Electron Microscope A photograph of the positive electrode active material layer (Example 1-1) formed by using the mixture prepared by performing ultrasonic agitation treatment by a scanning electron microscope is shown in FIG. A photograph of the positive electrode active material layer (Example 1-3) formed by using the prepared mixture is shown in FIG. 2 by a scanning electron microscope. In addition, FIG. 3 shows a photograph of a negative electrode active material layer (Example 1-1) formed by using the mixture prepared by performing the ultrasonic stirring treatment by a scanning electron microscope in FIG. A scanning electron microscope photograph of the negative electrode active material layer (Example 1-2) formed by using is shown in FIG.

As shown in FIGS. 1 and 3, when the active material layer is formed by using the mixture prepared by performing the ultrasonic stirring treatment, the entire surface of the active material particles is coated with the binder. It was observed that the conductive agent was dispersed in this binder film. On the other hand, as shown in FIGS. 2 and 4, in the active material layer formed by using the mixture prepared only by the stirring treatment, the binder and the conductive agent are localized in the active material layer. The situation was observed.

Although not shown in detail, when the battery was disassembled after 500 cycles and the active material layer was observed, the active material layer formed by using the mixture prepared by ultrasonic agitation treatment was observed. In the case of (the positive electrode of Example 1-1, the negative electrode, the positive electrode of Example 1-2, and the negative electrode of Example 1-3), 50
Even after the completion of 0 cycle, the liquid withdrawal phenomenon was not observed. On the other hand, the active material layer formed by using the mixture prepared by only stirring (the negative electrode of Example 1-2, and Example 1
-3, the positive electrode of Comparative Example 1, and the positive electrode and the negative electrode of Comparative Example 1) were observed to run out of liquid after 500 cycles. Further, deposition of metallic lithium was observed on the negative electrode active material layer.

2. Charge / Discharge Cycle Test FIG. 5 shows a graph showing the relationship between the number of cycles and the discharge capacity in Examples 1-1 to 1-3 and Comparative Example 1.

When an active material layer was formed using a mixture prepared by subjecting both the positive electrode and the negative electrode to ultrasonic agitation (Example 1-1), the discharge capacity after 900 cycles was
It was 75.2% of the initial discharge capacity, and the decrease in capacity was not so much seen. Further, when the active material layer was formed using the mixture prepared by subjecting only the positive electrode to ultrasonic agitation treatment (Example 1-2), the discharge capacity after 900 cycles was 70% of the initial discharge capacity. In the case where an active material layer was formed using a mixture prepared by subjecting only the negative electrode to ultrasonic agitation treatment (Example 1-2), the discharge capacity after 700 cycles was the initial value. The discharge capacity was 78.0%. In these batteries, although the discharge capacity was slightly reduced as compared with Example 1-1, good cycle characteristics were shown. On the other hand, in the case where the active material layer was formed using the mixture prepared only by the stirring treatment for both the positive electrode and the negative electrode (Comparative Example 1), the discharge capacity after 700 cycles was 65.0% of the initial discharge capacity. And the cycle characteristics were inferior.

3. High Rate Discharge Test When the active material layer was formed using the mixture prepared by subjecting the positive electrode and the negative electrode to ultrasonic agitation treatment (Example 2-1), the high rate discharge capacity was the initial discharge capacity. It was 82%, showing a good high rate discharge characteristic. When the active material layer was formed using the mixture prepared by subjecting only the positive electrode to ultrasonic agitation treatment (Example 2-2), the high rate discharge capacity was 78% of the initial discharge capacity. In the case where the active material layer was formed using the mixture prepared by ultrasonically stirring only the negative electrode (Example 2-3), the high rate discharge capacity was 79% of the initial discharge capacity. . These batteries exhibited good high rate discharge characteristics, although the discharge capacity was slightly reduced as compared with Example 2-1. On the other hand, in the case where the active material layer was formed using the mixture prepared only by the stirring treatment for both the positive electrode and the negative electrode (Comparative Example 2), the high rate discharge capacity was reduced to 70% of the initial discharge capacity. .

4. Summary As described above, when the active material layer is formed using the mixture prepared by performing the ultrasonic stirring treatment, the entire surface of the active material particles is covered with the binder, The conductive agent was dispersed in the agent film. Further, it has been found that the battery provided with such an active material layer suppresses liquid depletion of the active material layer due to charging and discharging and deposition of metallic lithium, and exhibits favorable cycle characteristics and high rate discharge characteristics.

[Brief description of drawings]

FIG. 1 is a photograph obtained by observing with a scanning electron microscope the appearance of a positive electrode active material layer formed by using a mixture prepared by ultrasonic agitation treatment.

FIG. 2 is a photograph of a state of a positive electrode active material layer formed by using a mixture prepared only by stirring treatment, observed by a scanning electron microscope.

FIG. 3 is a photograph obtained by observing a state of a negative electrode active material layer formed by using a mixture prepared by performing ultrasonic agitation treatment with a scanning electron microscope.

FIG. 4 is a photograph of the state of a negative electrode active material layer formed by using a mixture prepared only by stirring treatment, observed by a scanning electron microscope.

FIG. 5 is a graph showing the relationship between the number of charge / discharge cycles and the discharge capacity of a battery with and without ultrasonic agitation.

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Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a pair of positive and negative electrodes formed by forming an active material layer on a current collector, wherein the active material layer comprises active material particles, a binder and A non-aqueous electrolyte secondary battery containing a conductive agent, wherein the surface of the active material particles is coated with the binder containing the conductive agent. 2. A method for manufacturing a non-aqueous electrolyte secondary battery comprising a pair of positive and negative electrodes in which an active material layer is laminated on a current collector, wherein a conductive agent and active material particles are ultrasonically treated. A non-aqueous electrolyte two, characterized by undergoing a step of preparing a mixture by dispersing it in a binder, and a step of applying the mixture on the surface of the current collector to form the active material layer. Next battery manufacturing method.