JP5218965B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and relates to a non-aqueous electrolyte secondary battery in which a carbon material such as amorphous carbon is formed on a negative electrode current collector.

  Various non-aqueous electrolyte secondary batteries have been put into practical use, and one of them is a lithium ion secondary battery. Lithium ion secondary batteries are small, light, and have a high capacity, so that they are used in portable electronic devices, communication devices, electric assist bicycles, electric tools, and the like.

  The non-aqueous electrolyte secondary battery includes a positive electrode in which a positive electrode layer including a positive electrode active material, a positive electrode binder, and a conductive additive is formed on a positive electrode current collector, and a negative electrode layer including a negative electrode active material and a negative electrode binder. A laminate in which a negative electrode formed on a current collector is laminated or wound via a separator is housed in a metal case or a laminate film outer package.

  The negative electrode of the non-aqueous electrolyte secondary battery is composed of graphite, which is a negative electrode active material, and in some cases, a mixture of conductive aids, for example, mixed with a binder such as polyvinylidene fluoride (hereinafter referred to as PVDF), and from copper. Formed into a negative electrode current collector. However, the adhesion between the negative electrode active material and the negative electrode current collector (graphite and copper), which are different materials from each other as compared with the negative electrode active materials (graphite and graphite), is inferior.

  Therefore, in the charge / discharge cycle in which the negative electrode active material repeatedly expands and contracts, the adhesion between the negative electrode layer and the negative electrode current collector is gradually weakened. Such a decrease in adhesion causes an increase in impedance, which can contribute to cycle capacity deterioration. Therefore, a method is used in which the copper foil as the negative electrode current collector is surface-treated to improve the adhesion with the negative electrode layer. For example, Patent Documents 1 to 4 include a description of providing a copper oxide or hydroxide layer on a copper foil surface in order to improve adhesion. Further, Patent Documents 5 to 6 describe that a layer made of carbon particles and a binder is provided on a current collector metal foil.

Japanese Patent Application Laid-Open No. 07-109558 JP-A-11-310864 JP 11-293444 A JP 2000-45059 A JP 09-97625 A Japanese Patent Laid-Open No. 06-163030

  In the techniques of Patent Documents 1 to 4, the adhesion between the negative electrode layer and the current collector surface can be improved, but a resistor such as an oxide or hydroxide is provided between the negative electrode layer and the current collector. In the techniques of Patent Documents 5 to 6, since the carbon particles are bonded to the surface of the current collector metal foil with a binder, the surfaces of the carbon particles and the current collector metal foil are in point contact.

  The technical problem of the present invention is that good electrical contact is provided between the negative electrode layer and the negative electrode current collector surface, and the electrical characteristics are stable even when the charge / discharge cycle is repeated, and there is little capacity deterioration due to increase in internal resistance. The object is to provide a non-aqueous electrolyte secondary battery.

  The non-aqueous electrolyte secondary battery of the present invention includes a negative electrode and a positive electrode in which a negative electrode layer containing a negative electrode active material and a negative electrode binder is formed on a negative electrode current collector, and is stacked or wound via a separator and stored in an outer package. In the non-aqueous electrolyte secondary battery, the negative electrode current collector is made of a copper foil whose surface is coated with carbon.

  In the non-aqueous electrolyte secondary battery of the present invention, the carbon coating of the negative electrode current collector is preferably formed on a copper foil by thermal decomposition of a carbon precursor resin.

  In the non-aqueous electrolyte secondary battery of the present invention, it is preferable that the carbon precursor resin is coated on the surface of the copper foil by spray coating of the resin solution before thermal decomposition.

  The non-aqueous electrolyte secondary battery of the present invention is coated such that the coating of the carbon precursor resin is uniformly distributed in the form of polka dots on the copper foil, and the coverage is 30% or more and 60% or less. It is preferable.

  According to the present invention, in a non-aqueous electrolyte secondary battery using graphite as a negative electrode active material, the negative electrode current collector and the surface of the negative electrode current collector can be electrically isolated by using a copper foil coated with carbon on the negative electrode current collector. It is possible to provide a non-aqueous electrolyte secondary battery that reduces cycle capacity deterioration by improving adhesion while maintaining contact and suppressing an increase in resistance between the negative electrode layer and the negative electrode current collector surface due to charge / discharge cycles. .

  The non-aqueous electrolyte secondary battery of the present invention includes a negative electrode active material, a negative electrode binder, and a negative electrode in which a negative electrode layer containing a conductive auxiliary agent is formed on the negative electrode current collector, a positive electrode active material, and a positive electrode binder. After a positive electrode layer including a conductive auxiliary agent formed on a positive electrode current collector is stacked or wound with a separator interposed between the laminate and the non-aqueous electrolyte or polymer electrolyte is injected Sealed and manufactured.

  As the carbon coating, a phenol resin or the like can be used. For example, 5% by weight of a resol type phenolic resin solution is spray-coated on the negative electrode current collector and then carbonized. Artificial graphite and natural graphite can be used as the negative electrode active material. A negative electrode active material, a binder mixture, and optionally a conductive aid such as carbon black are dispersed in a solvent such as N-methyl-2-pyrrolidone (hereinafter referred to as NMP), a slurry is prepared, and carbon is coated. The negative electrode is formed by directly coating, drying and compressing on the negative electrode current collector.

As a positive electrode active material that can be used in the non-aqueous electrolyte secondary battery of the present invention, LiMO ₂ (wherein M represents at least one transition metal) can be used alone or a mixture of a plurality of types. For example, by dispersing a conductive additive such as a positive electrode active material, a positive electrode binder, and carbon black in a solvent such as NMP, preparing a slurry, coating directly on the positive electrode current collector, drying and compressing A positive electrode is formed.

  As the separator, a polyolefin resin such as polypropylene or polyethylene, a porous film such as a fluororesin, or the like can be used.

  Examples of the electrolyte include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, and vinylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, methyl formate, and methyl acetate. , Fatty carboxylic acid esters such as propionate ether, γ-lactones such as γ-butyrolactone, chain ethers such as 1,2-diethoxyethane and ethoxymethoxyethane, cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran , Dimethyl sulfoxide, 1,3-dioxirane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl Noglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether, etc. One or a mixture of two or more aprotic solvents is used, and the lithium salt dissolved in these organic solvents is dissolved.

Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , Li (CF ₃ SO ₂ ) ₂ , LiBr 3, LiCl 3, low fatty acid carboxylic acid lithium, and imides. Further, a polymer electrolyte may be used instead of the electrolytic solution.

  FIG. 1 is a schematic cross-sectional view showing a negative electrode of the present invention. The surface of the copper foil 11 is coated with carbon 12, and the negative electrode layer 13 is coated thereon.

  FIG. 2 is a schematic diagram showing a carbon-coated copper foil used in the present invention. Carbon 22 is coated on the surface of copper foil 21 at a coverage of 30% or more and 60% or less.

  FIG. 3 is a schematic view showing a carbon-coated copper foil used in the present invention, in which carbon 32 is coated on the surface of the copper foil 31 with a covering rate exceeding 60%.

  By using a copper foil coated with carbon on the negative electrode current collector, good electrical contact and adhesion between the negative electrode layer and the negative electrode current collector can be imparted, and cycle capacity deterioration due to an increase in impedance can be reduced. Specifically, since the resol type phenolic resin solution is directly coated on the surface of the copper foil and carbonized, the conductive carbon can be coated. Therefore, the surface of the carbon 12 and the copper foil 11 as shown in FIG. Since this is a surface contact, electrical contact is good.

  Also, the surface of the copper foil 11 coated with carbon 12 as shown in FIG. 1 is obtained by curing and carbonizing the carbon precursor resin coated on the copper foil surface while maintaining the shape of the droplets during spray coating. Is roughened to provide an anchor effect. Furthermore, the carbon coating on the surface of the copper foil has a polka dot shape and a uniform distribution reflecting the shape of the droplets during the spray coating of the carbon precursor resin solution as shown in FIG. It is possible to add the following property of carbon to expansion and contraction of the current collector and bending during winding.

  As a result, it is possible to suppress a decrease in adhesion between the negative electrode layer and the surface of the negative electrode current collector due to the charge / discharge cycle, and to reduce capacity deterioration due to internal resistance.

  In order to satisfy the above requirements, carbon is preferably applied to the copper foil surface at a coverage of 30% or more and 60% or less. That is, when the carbon coverage is less than 30%, the resulting anchor effect due to the unevenness of the carbon and the effect due to the affinity with the negative electrode layer are insufficient, so the adhesion between the negative electrode layer and the negative electrode current collector surface is sufficient. This is because a special effect cannot be obtained.

  Further, when the carbon coverage exceeds 60%, the sprayed droplets of the precursor resin solution aggregate to result in continuous carbon 32 as shown in FIG. This is because there is a possibility that followability with respect to may be impaired.

  Examples of the present invention are described in detail below.

Example 1
A 5% by weight solution of a resol type phenolic resin (hereinafter referred to as a phenolic resin) was spray-coated on both sides of a copper foil having a thickness of 10 μm so that the coverage was 30%. The copper foil coated with the phenol resin was heat-treated at 130 ° C. for 30 minutes in the air to cure the phenol resin. Further, the phenol resin coated on the surface of the copper foil by heat treatment at 600 ° C. for 60 minutes in a nitrogen atmosphere was carbonized to prepare a copper foil coated with carbon.

  Mix in an NMP solution in which the negative electrode binder is dissolved so that 93 parts by weight of artificial graphite as the negative electrode active material, 2 parts by weight of carbon black as the conductive auxiliary agent, and 5 parts by weight of PVDF as the negative electrode binder. A slurry was prepared, and this slurry was applied to both sides of a copper foil coated with carbon by a doctor blade method to form a negative electrode.

LiCoO ₂ was used as the positive electrode active material. A slurry was prepared by mixing the NMP solution in which the positive electrode binder was dissolved so that the positive electrode active material was 95 parts by weight, the carbon black as the conductive auxiliary agent was 2 parts by weight, and the PVDF as the positive electrode binder was 3 parts by weight. Then, the slurry was applied to both surfaces of a 15 μm thick aluminum foil by a doctor blade method to form a positive electrode.

As the electrolyte, a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in which LiPF ₆ was dissolved at a concentration of 1 mol / liter (mixing volume ratio: EC / DEC = 30/70) was used. The separator used was a porous polyethylene film having a thickness of 20 μm.

  The above-described negative electrode and positive electrode were laminated via a separator, housed in a laminate film outer package, injected with an electrolytic solution, and sealed to assemble a non-aqueous electrolyte secondary battery.

(Example 2)
A 5% by weight solution of phenol resin was spray-coated on both sides of a 10 μm thick copper foil so that the coverage was 40%. The copper foil coated with the phenol resin was heat-treated at 130 ° C. for 30 minutes in the air to cure the phenol resin. Further, the phenol resin coated on the surface of the copper foil by heat treatment at 600 ° C. for 60 minutes in a nitrogen atmosphere was carbonized to prepare a copper foil coated with carbon. A nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that this carbon-coated copper foil was used as the negative electrode current collector.

(Example 3)
A 5% by weight solution of phenol resin was spray-coated on both sides of a 10 μm thick copper foil so that the coverage was 50%. The copper foil coated with the phenol resin was heat-treated at 130 ° C. for 30 minutes in the air to cure the phenol resin. Further, the phenol resin coated on the surface of the copper foil by heat treatment at 600 ° C. for 60 minutes in a nitrogen atmosphere was carbonized to prepare a copper foil coated with carbon. A nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that this carbon-coated copper foil was used as the negative electrode current collector.

Example 4
A 5% by weight solution of phenol resin was spray-coated on both sides of a 10 μm thick copper foil so that the coverage was 60%. The copper foil coated with the phenol resin was heat-treated at 130 ° C. for 30 minutes in the air to cure the phenol resin. Further, the phenol resin coated on the surface of the copper foil by heat treatment at 600 ° C. for 60 minutes in a nitrogen atmosphere was carbonized to prepare a copper foil coated with carbon. A nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that this carbon-coated copper foil was used as the negative electrode current collector.

( Reference Example 1 )
A 5% by weight solution of phenol resin was spray-coated on both sides of a 10 μm thick copper foil so that the coverage was 70%. The copper foil coated with the phenol resin was heat-treated at 130 ° C. for 30 minutes in the air to cure the phenol resin. Further, the phenol resin coated on the surface of the copper foil by heat treatment at 600 ° C. for 60 minutes in a nitrogen atmosphere was carbonized to prepare a copper foil coated with carbon. A nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that this carbon-coated copper foil was used as the negative electrode current collector.

(Comparative Example 1)
A nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that a copper foil having a thickness of 10 μm and not coated with carbon was used as the negative electrode current collector.

The discharge capacities of 0.2C and 2.0C were measured for the nonaqueous electrolyte secondary batteries assembled in Examples 1 to 4, Reference Example 1 and Comparative Example 1, and the respective rate characteristics were compared from the ratios. The capacity measurement was performed at a charge voltage of 4.2 V (charge condition: current 1.0 C, 2.5 hours, 20 ° C.) and a discharge voltage of 3.0 V (discharge condition: current 0.2 C, 20 ° C.).

In addition, the non-aqueous electrolyte secondary battery assembled in Examples 1 to 4, Reference Example 1 and Comparative Example 1 was subjected to 500 cycles, and each discharge capacity was determined from the ratio of the initial discharge capacity and the discharge capacity after 500 cycles. The maintenance rates were compared. The charge / discharge cycle test was performed at a charge voltage of 4.2 V (charge condition: current 0.2 C, 2.5 hours, 20 ° C.) and a discharge voltage of 3.0 V (discharge condition: current 0.2 C, 20 ° C.).

Table 1 shows rate characteristics which are ratios of the 2.0 C discharge capacity to the 0.2 C discharge capacity of the nonaqueous electrolyte secondary batteries assembled in Examples 1 to 4, Reference Example 1 and Comparative Example 1, and the first discharge. The discharge capacity maintenance rate after 500 cycles with respect to the capacity is shown.

  FIG. 4 is a graph with the horizontal axis representing the carbon coverage with respect to the area of the copper foil and the vertical axis representing the rate characteristic.

  FIG. 5 is a graph in which the carbon coverage with respect to the area of the copper foil is plotted on the horizontal axis, and the discharge capacity retention ratio (C500 / C1) is plotted on the vertical axis.

4 that the rate characteristics of Examples 1 to 4 and Reference Example 1 are greater than those of Comparative Example 1. By using a copper foil coated with carbon on the negative electrode current collector, the adhesion is improved while maintaining good electrical contact between the negative electrode layer and the negative electrode current collector surface, and the negative electrode layer and the negative electrode current collector by charge / discharge cycle are improved. It can be seen that it is possible to provide a non-aqueous electrolyte secondary battery that reduces deterioration of cycle capacity by suppressing an increase in surface resistance, and the intended effect can be obtained.

  From FIG. 5, it was found that the discharge capacity retention rates of Examples 1 to 4 were equal to or higher than those of Comparative Example 1. By using a copper foil coated with carbon on the negative electrode current collector, the adhesion is improved while maintaining good electrical contact between the negative electrode layer and the negative electrode current collector surface, and the negative electrode layer and the negative electrode current collector by charge / discharge cycle are improved. It can be seen that it is possible to provide a non-aqueous electrolyte secondary battery that reduces deterioration of cycle capacity by suppressing an increase in surface resistance, and the intended effect can be obtained.

In consideration of the data of FIGS. 4 to 5 which are the results of Examples 1 to 4, Reference Example 1 and Comparative Example 1, the carbon coverage with respect to the range of Examples 1 to 4, that is, the area of the copper foil is If it is 30% or more and 60% or less, it turns out that the provision of the nonaqueous electrolyte secondary battery which reduces cycle capacity deterioration is possible.

  The embodiments of the present invention have been described above using the embodiments. However, the present invention is not limited to these embodiments, and the present invention is not limited to the scope of the present invention. Included in the invention. That is, various changes and modifications that can be naturally made by those skilled in the art are also included in the present invention.

The typical sectional view showing the negative electrode of the present invention. The schematic diagram showing the copper foil which coated the carbon of 30% or more of coverage and 60% or less used for this invention. The schematic diagram showing the copper foil which coated carbon exceeding the coverage 60%. The figure which shows the relationship between the coverage of carbon precursor resin with respect to the area of copper foil, and a rate characteristic. The figure which shows the relationship between the coverage of the carbon precursor resin with respect to the area of copper foil, and the discharge capacity maintenance factor after 500 cycles.

Explanation of symbols

11, 21, 31 Copper foil 12, 22, 32 Carbon 13 Negative electrode layer

Claims (2)

In the non-aqueous electrolyte secondary battery in which a negative electrode and a positive electrode in which a negative electrode layer including a negative electrode active material and a negative electrode binder is formed on a negative electrode current collector are stacked or wound via a separator and housed in an outer package, the negative electrode current collector, Ri Do a copper foil coated with carbon on the surface by pyrolysis of the carbon precursor resin, the carbon precursor resin, the thermal decomposition before uniformly with polka dots by spraying a coating resin solution distributed coverage of 30% or more, a non-aqueous electrolyte secondary battery, characterized Rukoto coated on the copper foil surface so that 60% or less. In the non-aqueous electrolyte secondary battery in which a negative electrode and a positive electrode in which a negative electrode layer including a negative electrode active material and a negative electrode binder is formed on a negative electrode current collector are stacked or wound via a separator and housed in an outer package, the negative electrode Non-aqueous electrolysis characterized in that a current collector is made of copper foil coated with carbon on the surface, the carbon coating is uniformly distributed in a polka dot shape, and the coverage is 30% or more and 60% or less Liquid secondary battery.

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