JP2015053283A - Negative electrode, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery having high peeling off strength even if the amount of a binder is reduced as much as possible and having high capacity and a long life.SOLUTION: A negative electrode having a negative electrode mix layer containing a negative electrode active substance, a binder, and a conductive agent, containing a polymer containing vinylidene fluoride as a repeating unit and a polymer containing acrylonitrile as a repeating unit, as the binder, and having the negative electrode mix layer containing vapor growth carbon fiber as the conductive agent is provided. A nonaqueous electrolyte secondary battery including the negative electrode is provided.

Description

  The present invention relates to a nonaqueous electrolyte secondary battery using a negative electrode containing a binder.

  In recent years, many portable electronic devices such as a camera-integrated VTR (video tape recorder), a digital camera, a mobile phone, a portable information terminal, and a notebook computer have appeared, and their size and weight have been reduced. As a portable power source for these electronic devices, research and development for improving the energy density of batteries, particularly secondary batteries, are being actively promoted.

  Among secondary batteries, lithium ion secondary batteries have a structure in which a positive electrode and a negative electrode are stacked and wound with a resin thin film separator made of polypropylene, polyethylene, or the like sandwiched between them. A non-aqueous electrolyte containing a solvent.

For example, an aluminum foil is used as a positive electrode current collector, LiCoO ₂ is used as a positive electrode active material, graphite is used as a conductive agent, and a positive electrode using polyvinylidene fluoride, Teflon (registered trademark) or the like is used as a binder. There is a lithium ion secondary battery equipped with a negative electrode using copper foil as an electrical conductor, amorphous carbon, coke, graphite, etc. as a negative electrode active material, and polyvinylidene fluoride, polyacrylonitrile, styrene butadiene rubber as a binder. Known (for example, Patent Documents 1 to 6).

As an electrolyte for a lithium ion secondary battery, a polymer lithium ion secondary battery containing a polymer such as polyvinylidene fluoride, polyacrylonitrile, or polyethylene oxide has also been proposed. In particular, a gel electrolyte battery using a gel electrolyte solidified by containing an electrolyte in a polymer has been widely put into practical use because it does not leak the electrolyte and has high reliability. In the gel electrolyte battery, the electrolyte existing inside the electrode and between the electrode and the separator is fixed, and the position of the battery member is fixed.
The polymer lithium ion secondary battery is lightweight because an aluminum laminate film can be used for the exterior, and a thin large-area battery that is difficult to process with a metal can exterior can be made (for example, Patent Document 7).

JP-A-8-315817 Japanese Examined Patent Publication No. 7-70319 JP 2003-282061 A JP 2003-317722 A Japanese Patent No. 2548460 JP 2003-132893 A JP 2001-167797 A

  The most important characteristics of a secondary battery are the “capacity” that is directly related to the duration and the “cycle characteristics” that have a long usable life. Since the active material of the lithium battery electrode is a powder, it is applied and fixed to the current collector with a polymer binder (binder), but this binder does not contribute to the battery capacity. The smaller the better. Further, since the binder coats the reaction part of the active material, the large current charge / discharge characteristics are deteriorated. If the charge / discharge characteristics decrease, the cycle life of the secondary battery also decreases.

However, when the amount of the binder is too small, the active material is peeled off from the current collector, which hinders the manufacturing process and deteriorates the cycle characteristics of the secondary battery. Since the battery active material undergoes a volume change due to lithium insertion / extraction associated with charge / discharge, if the amount of the binder is too small, the electrical contact of the active material particles becomes poor, and the capacity decreases with cycle performance.
In other words, the binder should have a minimum amount sufficient to ensure sufficient mechanical strength, but it should be as small as possible in terms of electrical and chemical properties. In reality, it is desirable that the number is small in terms of characteristics, but the fact is that it has to be increased because of lack of process suitability.

Furthermore, the electrode has a problem of electron conductivity in addition to binding properties. The electrode is required to have high electron conductivity. A carbon material used for a negative electrode of a lithium ion battery has electron conductivity, but graphite having high crystallinity has high electron conductivity, and amorphous carbon has low conductivity. That is, even if it is a graphite-type carbon material, when it has an amorphous layer on the surface, it does not necessarily have sufficient electronic conductivity.
In that case, it is well known to add carbon fiber to the electrode. Specifically, vapor grown carbon fiber (VGCF) is a well-known material. When VGCF is added, the electron conductivity of the electrode is improved, and the battery capacity, large current discharge, cycle life and the like are improved. Since VGCF is a bulky material, the absorbability of the electrolyte is also improved.
However, if it is too bulky due to its bulkiness, the filling property of the electrode is lowered, the volume efficiency is lowered, and the volume energy density of the battery is lowered. In addition, since VGCF has a large surface area, the amount of the binder increases, and the volume energy density may further decrease.

As the amount of bulky VGCF and binder increases, so does the solvent. If the solvent increases, sufficient drying cannot be achieved unless the coating speed is reduced, and productivity is lowered.
In other words, the amount of the binder and the conductive agent is essential for producing a battery that exhibits excellent characteristics.

  If the amount of the binder is too small, the battery active material is peeled off from the current collector. If the amount is too large, the solvent increases and the peel strength cannot be kept high unless the coating speed is slowed in the production process. If the amount of the conductive agent is too small, the electron conductivity of the electrode is low and the battery characteristics are deteriorated. If the amount is too large, the volume energy density is lowered even if the large current discharge and cycle characteristics are excellent. Since the binder increases, volume energy density and electrode productivity are further reduced.

  The present invention has been made in view of such problems, and has been devised so that a high peel strength can be obtained even if the binder is reduced as much as possible, and a battery having a high capacity and a long life can be provided. In particular, the present invention is effective for a non-aqueous electrolyte battery including VGCF in the electrode and an electrolyte containing a fluorine-based polymer.

That is, the present invention relates to the following negative electrode and nonaqueous electrolyte secondary battery.
[1] On a negative electrode current collector, a negative electrode active material, a binder composed of a polymer containing vinylidene fluoride as a repeating unit and a polymer containing acrylonitrile as a repeating unit, and vapor-grown carbon fiber are included. A negative electrode having a negative electrode mixture layer, wherein the total content of the binder is 8% by mass or more, and the peel strength between the negative electrode current collector and the negative electrode mixture layer is 77 N / m or more.
[2] A secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a negative electrode active material, a polymer containing vinylidene fluoride as a repeating unit, and acrylonitrile as a repeating unit on the negative electrode current collector. A negative electrode mixture layer containing a binder composed of a polymer and vapor-grown carbon fiber, the total content of the binder is 8% by mass or more, and the negative electrode current collector and the negative electrode mixture layer A secondary battery having a peel strength of 77 N / m or more.

In the present invention, a negative electrode coating film containing a negative electrode active material, a binder, and a conductive agent is applied to a current collector to produce a negative electrode. In this negative electrode, polyvinylidene fluoride (PVdF) is mainly used as a binder, and a small amount of polyacrylonitrile (PAN) is added thereto. With this PAN, high peel strength can be obtained even when the total amount of the binder is small.
The present invention is particularly effective in an electrode and a secondary battery including vapor grown carbon fiber (VGCF) that enhances the electronic conductivity of the negative electrode.

  Furthermore, this technique is particularly effective for an electrolyte battery in which the electrolytic solution contains a polymer compound such as polyvinylidene fluoride. Unless good adhesion is maintained between the electrolyte and the electrode active material particles, the chemical species in the solution (including the liquid retained in the polymer compound) cannot react with the active material particles that are solid. At this time, good adhesiveness is exhibited when the gel electrolyte and the electrode binder are polymers containing the same repeating unit.

It is one Embodiment of the nonaqueous electrolyte secondary battery of this invention, Comprising: It is a disassembled perspective view which shows an example of a laminate type battery. It is typical sectional drawing along the II-II line of the battery element shown in FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is an exploded perspective view showing an example of a laminated secondary battery as an embodiment of the nonaqueous electrolyte secondary battery of the present invention. As shown in the figure, this secondary battery is configured by enclosing a battery element 20 to which a negative electrode terminal 11 and a positive electrode terminal 12 are attached inside a film-shaped exterior member 30. The negative electrode terminal 11 and the positive electrode terminal 12 are led out from the inside of the exterior member 30 to the outside, for example, in the same direction. The negative electrode terminal 11 is made of a metal material such as copper (Cu) or nickel (Ni). The positive terminal 12 is made of a metal material such as aluminum (Al).

  The exterior member 30 is configured by a rectangular laminate film in which, for example, a nylon film, an aluminum foil, and a polyethylene film are bonded together in this order. The exterior member 30 is arrange | positioned so that the polyethylene film side and the battery element 20 may oppose, for example, and each outer edge part is mutually joined by melt | fusion or the adhesive agent. An adhesion film 31 is inserted between the exterior member 30 and the negative electrode terminal 11 and the positive electrode terminal 12 to prevent the outside air from entering. The adhesion film 31 is made of a material having adhesion to the negative electrode terminal 11 and the positive electrode terminal 12. For example, when the negative electrode terminal 11 and the positive electrode terminal 12 are made of the metal materials described above, polyethylene, polypropylene, modified It is preferably composed of a polyolefin resin such as polyethylene and modified polypropylene.

  In addition, the exterior member 30 may be configured by another structure, for example, a laminate film having no metal material, a polymer film such as polypropylene, or a metal film, instead of the above-described laminate film. Here, the general structure of an exterior member can be represented by the laminated structure of an exterior layer / metal foil / sealant layer (however, the exterior layer and the sealant layer may be composed of a plurality of layers), and the above. In this example, the nylon film corresponds to the exterior layer, the aluminum foil corresponds to the metal foil, and the polyethylene film corresponds to the sealant layer. In addition, as metal foil, it is sufficient if it functions as a moisture-permeable barrier film, and not only aluminum foil but also stainless steel foil, nickel foil and plated iron foil can be used, but it is thin and lightweight. Thus, an aluminum foil excellent in workability can be suitably used.

  As the exterior member, usable configurations are listed in the form of (exterior layer / metal foil / sealant layer): Ny (nylon) / Al (aluminum) / CPP (unstretched polypropylene), PET (polyethylene terephthalate) / Al / CPP, PET / Al / PET / CPP, PET / Ny / Al / CPP, PET / Ny / Al / Ny / CPP, PET / Ny / Al / Ny / PE (polyethylene), Ny / PE / Al / LLDPE (direct) Chain low density polyethylene), PET / PE / Al / PET / LDPE (low density polyethylene), and PET / Ny / Al / LDPE / CPP.

  FIG. 2 is a schematic cross-sectional view taken along the line II-II of the battery element 20 shown in FIG. In the figure, a battery element 20 is formed by winding a negative electrode 21 and a positive electrode 22 facing each other with a gel-like non-aqueous electrolyte layer 23 made of a gel-like non-aqueous electrolyte and a separator 24 therebetween. Yes, the outermost periphery is protected by a protective tape 25. It should be noted that the electrodes may be opposed one by one, or may be folded, or may have a wound structure. In the case of winding, volume efficiency can be improved by forming a mixture layer on both sides of the current collector.

[Negative electrode]
Here, the negative electrode 21 has a structure in which, for example, a negative electrode mixture layer 21B is provided on both surfaces or one surface of a negative electrode current collector 21A having a pair of opposed surfaces. The negative electrode current collector 21A has an exposed portion where the negative electrode mixture layer 21B is not provided at one end in the longitudinal direction, and the negative electrode terminal 11 is attached to the exposed portion. The negative electrode current collector 21A is made of a metal foil such as a copper foil, a nickel foil, or a stainless steel foil.

The negative electrode mixture layer 21B includes, as a negative electrode active material, one or more of negative electrode materials capable of occluding and releasing lithium ions and metallic lithium, and polyvinylidene fluoride as a binder. And polyacrylonitrile, and may contain a conductive agent as necessary. The area density of the negative electrode mixture layer 21B is preferably 15 mg / cm ² or more, more preferably 20 mg / cm ² or more on both sides.
In the present invention, the amount of solvent is small, which is advantageous in terms of production efficiency. Therefore, it is possible to suppress a decrease in manufacturing efficiency that occurs when the area density is increased. And even if it is a negative electrode of high area density as mentioned above, it manufactures, preventing the fall of manufacturing efficiency, and can obtain the battery of high energy density.
In the present invention, even a battery having a thick negative electrode mixture layer can be manufactured with high productivity, so that the energy density can be increased.
When the area density is low, the increase in the energy density of the entire battery is reduced, and the effects of the present invention cannot be fully utilized. Therefore, in order to sufficiently obtain the effect of increasing the energy density according to the present invention, a high area density as described above is preferable.

Examples of the negative electrode material capable of inserting and extracting lithium and the like include carbon materials, metals, metal oxides, silicon, and polymer materials. The carbon material is any one of carbon materials such as non-graphitizable carbon, graphitizable carbon, graphites, pyrolytic carbons, cokes, glassy carbons, organic polymer compound fired bodies, carbon fibers and activated carbon. Species or two or more can be used.
In particular, graphites such as natural graphite and artificial graphite are widely used in lithium ion batteries because they have high chemical stability and can repeatedly and stably undergo lithium ion deinsertion reactions and are easily available industrially. It has been broken.

As a material other than carbon, as a metal element or a metalloid element, for example, magnesium (Mg), boron (B), aluminum (Al), gallium (Ga), indium (In), which can form an alloy with lithium, Silicon (Si), germanium (Ge), tin (Sn), etc. are mentioned. These may be crystalline or amorphous.
In addition, 4B group silicon is known as a material from which Li can be inserted and removed. For tin and silicon, oxides and carbides containing oxygen and carbon can also be used. Examples of the polymer material include polyacetylene and polypyrrole.
For example, a combination of graphite material and tin oxide can be realized by mixing the above materials.

  The negative electrode mixture layer further includes a polymer containing vinylidene fluoride (VdF) as a repeating unit and a polymer containing acrylonitrile (AN) as a repeating unit. These polymers function as a binder. These polymers are preferably used as a mixture. This is because high peel strength can be maintained even when applied at high speed. In addition, the use of PVdF polymers as electrode binders is extremely effective when PVdF polymers are used for gel electrolytes. As a result, the electrode and the gel electrolyte layer are integrated to form a good ion conductive interface.

  The total content of the polymer containing vinylidene fluoride as a repeating unit and the polymer binder containing acrylonitrile as a repeating unit in the negative electrode mixture layer is preferably 2.5 to 8% by mass, more preferably 3 to 5% by mass. preferable. By setting it within the above range, both sufficient adhesiveness and high battery performance can be achieved.

The content of the polymer containing acrylonitrile (AN) as a repeating unit is 1 to 20% by mass (negative electrode mixture) based on the total content of the polymer containing vinylidene fluoride as a repeating unit and the polymer containing acrylonitrile as a repeating unit. In the layer is preferably 0.025 to 1.6% by mass), and more preferably 2 to 10% by mass (corresponding to 0.05 to 0.8% by mass in the negative electrode mixture layer). By setting the content within the above range, it is possible to achieve both the effect of the PVdF polymer as the main binder and the effect of the PAN polymer.
The polymer used for the binder may be a homopolymer composed of a single repeating unit or a copolymer.

Examples of the polymer containing vinylidene fluoride (VdF) as a repeating unit include polyvinylidene fluoride (PVdF) and a copolymer containing vinylidene fluoride as a component. Specific examples of the copolymer include vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-carboxylic acid copolymer, or vinylidene fluoride-hexafluoropropylene- Examples thereof include carboxylic acid copolymers. Examples of the vinylidene fluoride-hexafluoropropylene-carboxylic acid copolymer include a vinylidene fluoride-hexafluoropropylene-monomethylmaleic acid ester copolymer.
As a polymer containing acrylonitrile (AN) as a repeating unit, polyacrylonitrile (PAN) and other copolymerized PAN having an acrylate group or an acrylamide group can be used.
As the binder, one of the above polymers may be used alone, or two or more thereof may be mixed and used.

  The negative electrode mixture layer further includes vapor grown carbon fiber (VGCF). Thereby, the electronic conductivity of the electrode is improved, and the large current charge / discharge characteristics and the repeated charge / discharge life (cycle characteristics) of the battery are improved. As the vapor growth carbon fiber, one having an average fiber diameter of 150 nm and an average fiber length of 10 to 20 μm can be used. Moreover, 1-5 mass% is preferable and, as for content of the vapor growth carbon fiber in a negative mix layer, 2-3.5 mass% is more preferable. By setting it within the above range, high performance can be realized without causing a decrease in capacity due to bulkiness.

The negative electrode can be manufactured, for example, as follows.
First, for example, a negative electrode active material and a conductive material, and a binder in which PVdF and PAN are mixed at a ratio of 95: 5 are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) to obtain a negative electrode mixture slurry. Make it. Next, this negative electrode mixture slurry is applied to a negative electrode current collector, dried to remove the solvent, and then compression molded by a roll press or the like, and cut into a predetermined size. If necessary for this electrode, a lead wire for taking out the current is welded.
The peel strength between the negative electrode current collector and the negative electrode mixture layer is preferably 4 mN / mm or more, particularly 5 mN / mm or more. Thereby, the adhesion between the negative electrode current collector and the negative electrode mixture layer is improved, and the cycle characteristics are improved even under high temperature conditions.
The peel strength was determined by immersing the negative electrode in NMP at 80 ° C. for 1 hour, taking it out, drying it, placing the negative electrode with an adhesive tape on the support, and moving the adhesive tape in the 180 ° direction at a speed of 10 cm / min. When the negative electrode current collector is peeled off by pulling, it is the average value of the force per 1 mm required for peeling.

[Positive electrode]
On the other hand, like the negative electrode 21, the positive electrode 22 has a structure in which, for example, both surfaces or one surface of a positive electrode current collector 22A having a pair of opposed surfaces are coated with a positive electrode mixture layer 22B. The positive electrode current collector 22A has a portion that is exposed without being covered with the positive electrode mixture layer 22B at one end portion in the longitudinal direction, and the positive electrode terminal 12 is attached to the exposed portion. The positive electrode current collector 22A is made of a metal foil such as an aluminum foil.

  The positive electrode mixture layer 22B includes, for example, a positive electrode material capable of inserting and extracting lithium ions as a positive electrode active material, and may include a conductive agent and a binder as necessary. Here, it is preferable that the positive electrode active material, the conductive agent, and the binder are uniformly dispersed, and the mixing ratio is not limited.

As the positive electrode active material, a material capable of reversibly removing and inserting lithium ions and having a reaction potential of 3 to 4.5 V with respect to lithium is used. LiCoO ₂ is most often used, and LiMn ₂ O ₄ , LiNiO ₂ , and LiFePO ₄ are often used. Often used as a mixture of two or more species. Further, among these materials, materials in which other metal species are dissolved, such as LiCo _1-xy Al _x Mg _y O ₂ , are often used.

  Since the positive electrode active material is a semiconductor and has low electron conductivity, it is preferable to mix carbon powder as a conductive additive. Since the positive electrode is an oxidizing atmosphere, metal is often not used, but aluminum powder can be used. Like the negative electrode, the active material and the conductive agent are mixed with a binder and dissolved in a solvent such as NMP to form a slurry, which is applied onto a current collector, dried, pressed, slitted, and lead welded. Since the positive electrode has a harsh oxidizing atmosphere, the metal of the foil and lead is limited. Usually, aluminum foil is used. Aluminum is a suitable material that is passivated and does not melt, has good electrical conductivity, is light and inexpensive, is soft and has good workability.

[Nonaqueous electrolyte layer]
The nonaqueous electrolyte layer 23 includes, for example, an electrolytic solution and a matrix polymer compound that serves as a holding body that holds the electrolytic solution, and has a so-called gel shape. By such swelling, gelation or non-fluidization of the matrix polymer compound, it is possible to effectively suppress leakage of the nonaqueous electrolyte in the obtained battery.

  As the non-aqueous electrolyte, those generally used for lithium ion secondary batteries can be used. As such a nonaqueous electrolytic solution, a solution obtained by dissolving an electrolyte salt in a nonaqueous solvent can be used.

  Carbonates can be used as the non-aqueous solvent. Specific examples include ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. In addition, lactones such as gamma butyrolactone and gamma valerolactone, and carboxylic acid esters such as ethyl acetate are also used.

As the electrolyte salt, LiPF ₆ , LiBF ₄ , imide salt, or the like can be used. Although one kind or a plurality may be mixed, LiPF ₆ is most frequently used. The concentration of the electrolyte salt in the electrolytic solution is preferably 0.5 to 1.5 mol / kg.

  The matrix polymer compound is preferably a fluorine polymer compound such as polyvinylidene fluoride or a copolymer of vinylidene fluoride and hexafluoropropylene. Among these, from the viewpoint of redox stability, a vinylidene fluoride-hexafluoropropylene copolymer containing 3 to 7.5% by mass of hexafluoropropylene is preferable.

[Separator]
The separator 24 has a high ion permeability and a predetermined mechanical strength, such as a porous film made of a polyolefin-based organic resin such as polypropylene and polyethylene, or a porous film made of an inorganic material such as a ceramic nonwoven fabric. It is good also as a structure which laminated | stacked these 2 or more types of porous films. In particular, those containing a polyolefin-based porous membrane are preferable because they have excellent separability between the negative electrode 21 and the positive electrode 22 and can further reduce internal short circuit and open circuit voltage drop.

[Production method]
Next, an example of the manufacturing method of the nonaqueous electrolyte secondary battery mentioned above is demonstrated.
The laminate type secondary battery can be manufactured as follows. First, the negative electrode 21 is produced. For example, when a particulate negative electrode active material is used, a negative electrode mixture is prepared by mixing the negative electrode active material, the binder and the conductive agent described above, and dispersed in a dispersion medium such as N-methyl-2-pyrrolidone. Thus, a negative electrode mixture slurry is prepared. Next, the negative electrode mixture slurry is applied to the negative electrode current collector 21A, dried, and compression molded to form the negative electrode mixture layer 21B.

  Moreover, the positive electrode 22 is produced. For example, when a particulate positive electrode active material is used, a positive electrode mixture is prepared by mixing the positive electrode active material and, if necessary, a conductive agent and a binder, and a dispersion medium such as N-methyl-2-pyrrolidone. To produce a positive electrode mixture slurry. Thereafter, the positive electrode mixture slurry is applied to the positive electrode current collector 22A, dried, and compression molded to form the positive electrode mixture layer 22B.

  Next, the negative electrode terminal 11 is attached to the negative electrode 21, and the positive electrode terminal 12 is attached to the positive electrode 22. At this time, you may stick the protective tape 25 on the collector of the welding part of the negative electrode terminal 11 or the positive electrode terminal 12, and its back surface, or the boundary part of a mixture application part and an electrical power collector exposure part.

  Next, the nonaqueous electrolyte layer 23 is formed on one side or both sides of the obtained negative electrode 21. For example, an electrolyte salt such as lithium hexafluorophosphate, a nonaqueous solvent such as ethylene carbonate and propylene carbonate, and a matrix polymer such as polyvinylidene fluoride are mixed and dissolved in a diluent solvent such as dimethyl carbonate and (DMC). A sol-like nonaqueous electrolyte is prepared. The sol-like non-aqueous electrolyte is applied to the negative electrode 21 and the diluting solvent is volatilized to form a non-aqueous electrolyte layer 23 made of a gel-like non-aqueous electrolyte.

  Furthermore, the nonaqueous electrolyte layer 23 is formed on one side or both sides of the obtained positive electrode 22. For example, the electrolyte salt such as lithium hexafluorophosphate described above, a nonaqueous solvent such as ethylene carbonate and propylene carbonate, and a matrix polymer such as polyvinylidene fluoride are mixed with a diluting solvent such as dimethyl carbonate (DMC). Dissolve to produce a sol-like non-aqueous electrolyte. This sol-like non-aqueous electrolyte is applied to the positive electrode 22 and the diluting solvent is volatilized to form a non-aqueous electrolyte layer 23 made of a gel-like non-aqueous electrolyte.

  Thereafter, the separator 24, the positive electrode 22 with the non-aqueous electrolyte layer 23 formed thereon, the separator 24 and the negative electrode 21 with the non-aqueous electrolyte layer 23 formed thereon are sequentially stacked and wound, and the protective tape 25 is adhered to the outermost peripheral portion. Element 20 is formed. Further, the battery element 20 is packaged with an exterior member 30 to complete the laminate type secondary battery shown in FIGS.

  The nonaqueous electrolyte secondary battery may be manufactured as follows. For example, instead of wrapping the completed battery element with an exterior member, a matrix polymer monomer or polymer is applied on the negative electrode 21 and the positive electrode 22 or on the separator 24 and wound to produce a wound electrode body. Alternatively, the non-aqueous electrolyte layer 23 may be formed by injecting the non-aqueous electrolyte described above after being housed in the exterior member 30. However, it is preferable to polymerize the monomer inside the exterior member 30 because the bondability between the nonaqueous electrolyte layer 23 and the separator 24 is improved and the internal resistance can be lowered. In addition, it is preferable to inject a non-aqueous electrolyte into the exterior member 30 to form a gel-like non-aqueous electrolyte because it can be easily manufactured with fewer steps.

  In the secondary battery described above, when charged, lithium ions are released from the positive electrode mixture layer 22 </ b> B and inserted into the negative electrode mixture layer 21 </ b> B through the nonaqueous electrolyte layer 23. When discharge is performed, lithium ions are released from the negative electrode mixture layer 21 </ b> B and inserted into the positive electrode mixture layer 22 </ b> B through the nonaqueous electrolyte layer 23.

Further, specific embodiments of the present invention will be described in detail.
<Negative electrode production>
First, a binder and a solvent were mixed to prepare a polymer solution of the binder. A negative electrode active material and an additive were mixed there to prepare a paint, which was applied onto a copper foil current collector and the solvent was dried. After this was roll-pressed, it was cut into a predetermined width and a nickel terminal was attached to the end to create a negative electrode.
Artificial graphite and mesophase carbon microbeads having an average particle size of 20 μm were used as the negative electrode active material.
As the conductive agent, vapor grown carbon fiber (VGCF) having an average fiber length of 150 nm and an average fiber length of 10 to 20 μm was used.
The binders of Examples 1 to 19 and Comparative Examples 1 to 3 include a vinylidene fluoride (VdF): monomethylmaleic acid ester (MMM) (weight ratio 99: 1) copolymer having a number average molecular weight of 800,000. Polyacrylonitrile (PAN) was used.
The binders of Examples 20 to 22 include a vinylidene fluoride (VdF): monomethylmaleic acid ester (MMM) (weight ratio 98: 2) copolymer having a number average molecular weight of 700,000, and polyacrylonitrile (PAN). Were used at different ratios of PVdF and PAN.
The weight ratio of graphite to binder was 95: 5.
A copper foil having a thickness of 15 μm was used for the current collector.
The negative electrode layer was formed on both sides with a thickness of 50 μm on one side of the current collector. Application was performed at 30 m / min.
The experiment was conducted by changing the blending ratio of the binder. The ratio is shown in the attached table.

<Positive electrode fabrication>
LiCoO ₂ , graphite as a conductive agent, polyvinylidene fluoride as a binder, and N-methyl-2-pyrrolidone as a solvent are dispersed into a positive electrode mixture slurry, and then the positive electrode mixture slurry is thickened. A positive electrode current collector made of 20 μm aluminum foil was uniformly applied and dried, and compression molded with a roll press to form a positive electrode mixture layer, which was cut into a required size to produce a positive electrode. At that time, LiCoO ₂ , graphite, and polyvinylidene fluoride were mixed at a mass ratio of LiCoO ₂ : graphite: polyvinylidene fluoride = 91: 6: 3. After that, an aluminum positive electrode terminal was attached to one end of the positive electrode current collector.

<Gel electrolyte>
The gel electrolyte was prepared as follows.
As the matrix polymer, a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP) having a number average molecular weight of 700,000 and VdF: HFP = 93: 7 was used. In the electrolytic solution, LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) in EC: PC = 4: 6 (mass ratio) so as to have a molar concentration of 1 mol / kg. Dimethyl carbonate (DMC) was used as a dilution solvent. These were mixed and stirred at 90 ° C. in a ratio of polymer: electrolytic solution: diluting solvent = 1: 10: 10 to obtain a polymer solution to obtain a sol electrolyte. This sol was applied on the electrode, and the diluent DMC was volatilized to form a gel electrolyte film having a thickness of 20 μm on the electrode.

<Assembly>
The positive electrode on which the gel electrolyte was formed and the negative electrode were flatly wound through a 10 μm porous polyethylene separator in the meantime to produce a battery element. This element was inserted and sealed in an exterior using an aluminum laminate film. The aluminum laminate film is made by using nylon for the outer layer and crystalline polypropylene for the interior, and bonding 30 μm thick films. The battery element was inserted into this, and the three sides of the rectangle were bonded together by heat sealing under vacuum and sealed in a vacuum. The aluminum laminate film has an exterior that is not sagging at atmospheric pressure. A resin piece that adheres well to a metal such as an ionomer was adhered to the portion from which the battery terminal was taken out, and was made highly airtight.

<Characteristic evaluation>
-Peel strength test of electrode The peel strength test of the coating film of the electrode after application completion was conducted. When the adhesive tape is applied and peeled off from the electrode, it can be peeled off from the interface between the copper foil and the coating film. Half of the tape length was peeled off and inverted 180 ° there. The exposed portion of the copper foil and the adhesive tape were pulled with a tensile tester (Shimadzu Autograph AGS-50B, manufactured by Shimadzu Corporation), and the peel strength was measured. 40 N / m or more was regarded as a good product.
・ Volume energy density of the battery After charging the battery with 1 ItA, 4.2 V, constant current and constant voltage charging for 2.5 hours (this is assumed to be standard charging), 0.2 ItA constant current discharging is performed to 3 V, The battery capacity was measured. The discharge energy was calculated by applying the discharge average voltage, and the volume energy density was calculated by dividing this by the volume of the battery. The larger this value is, the better, and 480 Wh / L or more was regarded as a good product.
The conductivity of the electrode increases as the VGCF increases, but the volume energy density decreases as the VGCF increases. In the batteries of the example and the comparative example, the electrodes are formed by pressing at the same pressure, so that the thicker the VGCF is, the thicker the electrodes are. Since a film is used for the exterior of the battery, it is somewhat deformed. Therefore, when the electrode is thick, the volume of the battery increases and the energy density decreases. When the VGCF is small, the discharge performance is poor, the battery capacity is lowered, and the energy density is lowered.
-Cycle test The created secondary battery was subjected to a charge / discharge test, and the cycle characteristics were examined as follows. First, constant current / constant voltage charge of 1 ItA at 23 ° C. was performed to 4.2 V for 2.5 hours, followed by constant current discharge of 1 ItA to a final voltage of 3.0 V. This charge / discharge was repeated to determine the discharge capacity maintenance ratio at the 400th cycle relative to the discharge capacity at the first cycle. At this time, the maintenance rate of 70% or more of the first time was regarded as a non-defective product. Note that 1 ItA is a current value at which the theoretical capacity can be released in one hour. In this study, the capacity of the battery is 500 mAh, so 500 mA is 1 ItA. Since 0.2 ItA is 1/5 of the current value, it is 100 mA.
-Load characteristic test The battery was charged with 1 ItA-4.2 V, constant voltage and constant current charging for 2 and a half hours, discharged with constant current up to 3 V at 0.2 ItA, and the discharge capacity was measured. Similarly, the measurement result at 3 ItA Compared with.
A ratio of (3 ItA capacity) / (0.2 ItA capacity) of 80% or more was regarded as a non-defective product.

As can be seen from the results in Tables 1 and 2, the peel strength could be increased with a small amount of binder by adding PAN. In the case where PAN was not included, the peel strength was low unless the total amount of the binder was large, and it was peeled off during the process of producing the electrode, making it impossible to make a battery. Even if the battery could be made, the cycle characteristics deteriorated because it could not cope with the volume change accompanying charging and discharging. When the total amount of the binder is large, the peel strength can be increased. However, since there are many binders covering the electrode active material, the battery characteristics are deteriorated as compared with the binder containing PAN. On the other hand, it was found that if the ratio of PAN is increased too much, the interaction with the gel electrolyte is reduced, so that the characteristics are deteriorated. From this, the preferable content of the polymer containing acrylonitrile as a repeating unit is 1 to 20% by mass relative to the total content of the polymer containing vinylidene fluoride as a repeating unit and the polymer containing acrylonitrile as a repeating unit. I understood.
Moreover, it turned out that this effect is especially effective with respect to the negative electrode containing VGCF. By including VGCF, cycle characteristics, capacity, and current characteristics are improved. And from the said result, it turned out that the preferable content of the vapor growth carbon fiber in a negative mix layer is 1-5 mass%.
Thus, the battery characteristics can be improved by the binder containing PAN.

  Although the present invention has been described with reference to the embodiments and examples, the present invention is not limited to the embodiments and examples, and various modifications can be made. For example, in the above embodiments and examples, the secondary battery having a winding structure has been specifically described, but the present invention uses an exterior member such as a coin type, a sheet type, a button type, or a square type. The present invention can be similarly applied to secondary batteries having other shapes, or secondary batteries having a stacked structure in which a plurality of positive and negative electrodes are stacked.

DESCRIPTION OF SYMBOLS 11 ... Negative electrode terminal, 12 ... Positive electrode terminal, 20 ... Battery element, 21 ... Negative electrode, 21A ... Negative electrode collector, 21B ... Negative electrode mixture layer, 22 ... Positive electrode, 22A ... Positive electrode collector, 22B ... Positive electrode mixture layer , 23 ... Gel-like non-aqueous electrolyte layer, 24 ... Separator, 25 ... Protective tape, 30 ... Exterior member, 31 ... Adhesive film

Claims (11)

A negative electrode mixture comprising, on a negative electrode current collector, a negative electrode active material, a binder comprising a polymer containing vinylidene fluoride as a repeating unit and a polymer containing acrylonitrile as a repeating unit, and vapor grown carbon fiber Has a layer,
The total content of the binder is 8% by mass or more,
A negative electrode having a peel strength between the negative electrode current collector and the negative electrode mixture layer of 77 N / m or more.   2. The negative electrode according to claim 1, wherein the content of the polymer containing the acrylonitrile as a repeating unit with respect to the total content of the binder is 1 to 20% by mass.   The negative electrode according to claim 1 or 2, wherein the vapor grown carbon fiber has an average fiber length of 10 to 20 µm.   The negative electrode according to any one of claims 1 to 3, wherein a content of the vapor-grown carbon fiber in the negative electrode mixture layer is 1 to 5% by mass. A secondary battery comprising a positive electrode, a negative electrode and an electrolyte,
The negative electrode comprises, on a negative electrode current collector, a negative electrode active material, a binder composed of a polymer containing vinylidene fluoride as a repeating unit and a polymer containing acrylonitrile as a repeating unit, and vapor-grown carbon fiber. Having a negative electrode mixture layer containing,
The total content of the binder is 8% by mass or more,
A secondary battery, wherein a peel strength between the negative electrode current collector and the negative electrode mixture layer is 77 N / m or more.   The secondary battery according to claim 5, wherein the content of the polymer containing the acrylonitrile as a repeating unit with respect to the total content of the binder is 1 to 20% by mass.   The secondary battery according to claim 5 or 6, wherein an average fiber length of the vapor grown carbon fiber is 10 to 20 µm.   The secondary battery as described in any one of Claims 5 thru | or 7 whose content of the said vapor growth carbon fiber in the said negative mix layer is 1-5 mass%.   The secondary battery according to claim 5, wherein the electrolyte contains a fluorine-based polymer compound.   The secondary battery according to claim 9, wherein the fluorine-based polymer compound is a vinylidene fluoride-hexafluoropropylene copolymer containing hexafluoropropylene.   The secondary battery according to claim 10, comprising 3 to 7.5% by mass of the hexafluoropropylene.

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