JP5557619B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion battery, and a power source and an equipment system using the same.

  Lithium ion batteries are attracting attention as batteries for electric vehicles and power storage from the viewpoint of environmental problems. It is lighter than lead batteries and nickel cadmium batteries, and has characteristics such as high output and high energy density, and the demand for lithium ion batteries is expected to increase.

  However, conventional lithium ion batteries are required to further improve battery performance such as output, energy density, and cycle life. For example, as an improvement of the electrode material, it is proposed that two or more kinds of carbon bodies having different properties are used as an active material and mixed with an organic binder (binder) to produce a negative electrode for a secondary battery. (Patent Documents 1 to 6).

  Patent Document 1 is a negative electrode in which two types of carbons having different crystal structures are mixed, Patent Document 2 is a negative electrode in which carbon fiber such as a sphere or flake is mixed with carbon fiber, and Patent Document 3 is a negative electrode active material and a carbon-based conductive additive. Patent Document 4 discloses an invention relating to a negative electrode in which graphite and conductive carbon fine powder are mixed. Further, Patent Document 5 discloses an invention relating to a negative electrode in which first carbon particles containing a Li storage material and second carbon particles not containing a Li storage material are mixed, and discloses a method for improving battery characteristics. Yes. Patent Document 6 discloses an invention relating to a negative electrode using two types of graphite particles having different average particle diameters, and aims to improve cycle characteristics.

  The negative electrode which is the object of the present invention includes an active material capable of inserting and releasing lithium ions, a conductive auxiliary agent and a polyvinylidene fluoride (PVDF) system, a styrene-butadiene rubber (SBR; Styrene Butadiene Rubber), A negative electrode slurry prepared, mixed and stirred with a binder such as carboxymethyl cellulose (CMC) and an organic solvent or water is attached to a current collector sheet such as copper by the doctor blade method, and then heated. It can be produced by drying the organic solvent and press-molding it with a roll press.

  However, the physical properties of the active material and the conductive auxiliary agent may be different from each other, such as the particle size and specific surface area of the carbon particles. Sometimes. For this reason, the applied mixture layer does not necessarily have a uniform shape.

  When the obtained mixture layer of the negative electrode is observed with a scanning electron microscope (SEM), the states of the active material particles, the conductive auxiliary particles, and the binder can be confirmed. On the surface of the mixture layer, there are phases in which aggregates of conductive assistants adhere on a plurality of particles of the active material, phases in which aggregates of conductive assistants are mainly localized in the gaps between the plurality of particles of the active material, etc. is there. In addition, since the binder is generally a high-resistance material, when many binders are present at the interface between the battery current collector and the active material particles, a plurality of active materials on the surface of the mixture layer are used. When many binders are present on the particles, or when many particles are present between the active material particles, conduction inhibition occurs between the active materials, and the internal resistance of the mixture layer increases. There arises a problem that the charge / discharge capacity decreases.

  When problems such as aggregation of the conductive additive and unevenness of the binder occur in this way, not only the charge / discharge capacity decreases, but also the active material and conductive additive particles fall off from the current collector, the current becomes non-uniform, etc. And the reliability of battery quality is reduced.

  Under such circumstances, there is a strong demand for a negative electrode having a high capacity battery and a sound and strong conductive network.

Japanese Patent No. 3529802 Japanese Patent No. 3556270 Japanese Patent No. 3409756 JP-A-2002-134115 Japanese Patent Laid-Open No. 2004-349164 Japanese Patent Laid-Open No. 2008-140707

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery that solves the above-described problems, prevents the mixture layer from being peeled off by a charge / discharge cycle, and provides a high capacity. In particular, it aims to increase the capacity of lithium ion batteries.

  The problem to be solved by the present invention is solved by the following means. Here, the non-aqueous electrolyte secondary battery is a non-aqueous electrolyte secondary battery composed of a positive and negative electrodes capable of inserting and removing lithium ions and a porous film separating them, and uses other alkali metal ions. It can also be applied to secondary batteries.

  The first means is that the negative electrode is capable of electrochemically occluding and releasing lithium ions, and is capable of electrochemically occluding and releasing lithium ions, or does not substantially occlude lithium ions. An aggregate composed of particles of the second carbon and consisting of particles of the second carbon is mainly localized in the gaps between the plurality of particles of the first carbon, and the average particle size of the second carbon is the first carbon This is the first means of setting the average particle size to 15% or less.

  The second means is a method of applying a binarized image processing so that the first carbon and the second carbon can be distinguished from each other in the surface photograph of the negative electrode according to claim 2 obtained by a scanning electron microscope. The average area of the aggregate made of carbon is calculated, and the average area is set to be equal to or less than the square of the average particle diameter of the first carbon.

  A third means is a first means in which the aggregate composed of the second carbon particles has an aspect ratio of 1.25 or more and 50% or more of the total number of the aggregates.

  A fourth means is a third means in which the average particle diameter of the primary particles of the second carbon is 8% or less of the particle diameter of the first carbon, and the primary particles are in an aggregated state.

  The fifth means is the first means that the second carbon aggregate contains a binder and has conductivity.

  The sixth means is a first means in which a conductive fiber is added to the negative electrode, and a part of the fiber comes into contact with the second carbon aggregate.

  ADVANTAGE OF THE INVENTION According to this invention, peeling of the mixture layer by a charging / discharging cycle can be prevented, and the nonaqueous lithium secondary battery which has a high capacity | capacitance can be obtained.

It is a conceptual diagram which shows the relationship between several particle | grains of the 1st carbon of a mixture layer surface, and a 2nd carbon aggregate. It is a conceptual diagram which shows the relationship between several particle | grains of the 1st carbon of a mixture layer surface, and a 2nd carbon aggregate. It is sectional drawing of the coin-type lithium ion battery of this invention. 1 is a structural diagram of a cylindrical lithium ion battery of the present invention. It is a battery module comprising the cylindrical lithium ion battery of the present invention. It is the electrical storage system using the battery module of this invention. It is a data table of an Example and a comparative example.

In order to increase the charge / discharge capacity of the nonaqueous electrolyte secondary battery and prevent the mixture layer from being peeled off by the charge / discharge cycle, the present invention limits the area of the second carbon particle aggregate on the surface of the mixture layer and It was achieved by miniaturization. The nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode and a negative electrode capable of inserting and removing lithium ions, a separator separating the positive electrode and the negative electrode, and an electrolytic solution.
Hereinafter, these elements will be described.

  First, the positive electrode of a nonaqueous lithium secondary battery will be described. The positive electrode includes a positive electrode mixture layer made of a positive electrode active material, a conductive additive and a binder (binder), and a positive electrode current collector.

The positive electrode active material usable in the lithium ion battery according to the present invention comprises an oxide containing lithium. Examples of the oxide containing lithium include an oxide having a layered structure such as LiCoO ₂ , LiNiO ₂ , LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _0.4 Ni _0.4 Co _0.2 O ₂ , Lithium-manganese composite oxides having a spinel structure such as LiMn ₂ O ₄ and Li _{1 + x} Mn _2-x O ₄ , or in these oxides, part of Mn is replaced with other elements such as Al and Mg Can be used.

  Since the positive electrode active material generally has high resistance, the electrical conductivity of the positive electrode active material is supplemented by mixing carbon powder as a conductive additive. Since the positive electrode active material and the conductive additive are both powders, a binder is mixed to bond the powders together, and at the same time, this powder layer is bonded to the positive electrode current collector as a mixture layer.

  As the conductive additive, natural graphite, artificial graphite, coke, carbon black, amorphous carbon, or the like can be used. When the average particle size of the conductive auxiliary agent is smaller than the average particle size of the positive electrode active material powder, the conductive auxiliary agent tends to adhere to the surface of the positive electrode active material particles, and the electric resistance of the positive electrode is reduced by a small amount of conductive auxiliary agent. There are many. Therefore, what is necessary is just to select the material of a conductive support agent according to the average particle diameter of a positive electrode active material.

  The positive electrode current collector may be any material that is difficult to dissolve in the electrolyte, and aluminum foil is frequently used.

  The positive electrode can be produced by a method in which a positive electrode slurry in which a positive electrode active material, a conductive additive, a binder, and an organic solvent are mixed is applied to a current collector using a blade, that is, a doctor blade method. The positive electrode slurry applied to the current collector is heated to dry the organic solvent, and pressure-molded by a roll press. The positive electrode mixture layer is produced on the current collector by drying the organic solvent of the positive electrode slurry. In this way, a positive electrode in which the positive electrode mixture layer and the current collector are in close contact with each other can be produced.

  The negative electrode includes a negative electrode mixture layer composed of a negative electrode active material, a conductive additive and a binder (binder), and a negative electrode current collector. In some cases, a conductive additive is not used in the negative electrode mixture layer.

  As the negative electrode active material of the non-aqueous lithium ion battery according to the present invention, graphite or amorphous carbon capable of electrochemically occluding and releasing lithium ions can be used, but lithium ion can be occluded and released. There are no restrictions on the types and materials. Since the negative electrode active material to be used is generally used in a powder state, the binder is mixed to bond the powders together, and at the same time, the layer made of the negative electrode active material is bonded to the negative electrode current collector as a mixture layer I am letting.

  The first carbon is a carbon material capable of occluding and releasing lithium ions. For example, natural graphite, artificial graphite, amorphous carbon, and the like can be used.

  The second carbon can occlude and release lithium ions or does not substantially occlude lithium ions, but the first carbon material may be used as long as it is 15% or less of the average particle diameter of the first carbon. It is also possible to use carbon materials such as coke, carbon black, acetylene black, carbon fiber, ketjen black, carbon nanotube, mesocarbon microbead, and vapor grown carbon fiber.

  Although carbon black was used in the examples described below, the present invention is not limited to this. For example, carbon black may be replaced with the other second carbon described above, or several different types of carbon may be mixed and used.

  As the binder, in addition to polyvinylidene fluoride (PVDF), a fluorine-based polymer such as polytetrafluoroethylene, styrene butadiene rubber (SBR), acrylonitrile rubber, or the like may be used. Other binders not listed above may be used as long as they do not decompose at the reduction potential of the negative electrode and do not react with the nonaqueous electrolyte or the solvent in which it is dissolved. As the solvent used for preparing the negative electrode slurry, a known solvent suitable for the binder may be used. For example, a known solvent such as water in the case of SBR and acetone, toluene and the like in the case of PVDF can be used. A thickener can also be used to adjust the viscosity of the slurry. For example, carboxymethylcellulose (CMC) can be used for SBR.

  The negative electrode current collector is required to be a material that is difficult to be alloyed with lithium, and there is a metal foil made of copper, nickel, titanium, or an alloy thereof. In particular, copper foil is frequently used.

  The negative electrode is prepared by adhering a negative electrode slurry mixed with a negative electrode active material, a conductive additive, a binder, and an organic solvent to a current collector by a doctor blade method or the like, then heating to dry the organic solvent, and applying it by a roll press. It can be produced by pressure forming. The negative electrode mixture layer is produced on the current collector by drying the organic solvent of the negative electrode slurry.

  The separator is made of a polymer material such as polyethylene, polypropylene, and ethylene tetrafluoride, and is inserted between the positive electrode and the negative electrode manufactured as described above. The separator and the electrode sufficiently hold the electrolytic solution to ensure electrical insulation between the positive electrode and the negative electrode, and to allow the transfer of lithium ions between the positive electrode and the negative electrode.

  In the case of a coin-type battery, a positive electrode, a separator, and a negative electrode cut out in a circular shape are stacked in this order, the stack is stored in a coin-shaped container, the lid is placed on top, and then the entire battery is crimped. The

  In the case of a cylindrical battery, the electrode group is manufactured by winding with a separator inserted between the positive electrode and the negative electrode. In place of the separator, polymers such as polyethylene oxide (PEO), polymethacrylate (PMA), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), It is also possible to use a sheet-like solid electrolyte or gel electrolyte holding a lithium salt or a non-aqueous electrolyte. Further, when the electrodes are wound around two axes, an oval electrode group is also obtained.

  In the case of a prismatic battery, the positive electrode and the negative electrode are cut into strips, the positive electrode and the negative electrode are alternately laminated, and a polymer separator such as polyethylene, polypropylene, and tetrafluoroethylene is inserted between each electrode, and the electrode group Is made.

  In order to improve safety, a sandwich-type ceramic separator in which a polymer separator is sandwiched between electrically insulating ceramic particle layers such as alumina, silica, titania, and zirconia may be used as the separator. The ceramic particle layer may be formed only on one side of the polymer separator.

  The present invention does not depend on the structure of the electrode group described above, and any structure can be applied to the lithium ion battery according to the present invention.

Moreover, as a solvent of electrolyte solution, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl formate, ethyl formate, propyl formate, γ-butyrolactone, α-acetyl A mixture of at least one selected from -γ-butyrolactone, α-methoxy-γ-butyrolactone, dioxolane, sulfolane, and ethylene sulfite can be used. Preferred electrolytes include LiPF ₆ , LiBF ₄ , LiSO ₂ CF ₃ , LiN [SO ₂ CF ₃ ] ₂ , LiN [SO ₂ CF ₂ CF ₃ ] ₂ , LiB [OCOCF ₃ ] ₄ , LiB. A lithium salt electrolyte such as [OCOCF ₂ CF ₃ ] ₄ containing about 0.5 to 2 M in volume concentration can be used.

  The lithium ion battery can be produced by inserting the produced electrode group into a battery container made of aluminum, stainless steel, or nickel-plated steel, and then infiltrating the electrolytic solution into the electrode group. The shape of the battery can includes a cylindrical shape, a flat oval shape, a rectangular shape, and the like, and any shape can be selected as long as the electrode group can be accommodated.

  The negative electrode of the present invention includes a first carbon capable of electrochemically occluding and releasing lithium ions, and a second carbon capable of electrochemically occluding and releasing lithium ions or substantially not occluding lithium ions. , An electrode made of a binder. Aggregates composed of the second carbon particles are mainly localized in the gaps between the plurality of first carbon particles, and the average particle diameter of the second carbon is 15 which is the average particle diameter of the first carbon. % Or less.

  The concept of the present invention will be described in detail using models (FIGS. 1 and 2). The basis for the maximum size of the second carbon aggregate will be described with reference to FIG. 1, and the basis for the minimum size will be described with reference to FIG.

  FIG. 1 is a view of the state in which the aggregates of the second carbon are filled in the gaps between the four negative electrode active materials 11 when viewed from above the negative electrode surface. In this model, the negative electrode active material was assumed to be spherical and the particle size was the same. An actual negative electrode active material is not a perfect sphere, and its particle size is distributed in a certain range. However, it is applicable to such an actual negative electrode when considered statistically. A method applied to an actual negative electrode will be described later.

  Now, in FIG. 1, the aggregate which consists of 2nd carbon contains a binder, and is in the state which 2nd carbon and the binder compounded. This aggregate exhibits a binding force and at the same time has electronic conductivity. In the present invention, the second carbon aggregates are filled between the negative electrode active material particles, that is, in the gaps, and the aggregates connect the negative electrode active materials to each other to form an electronic network.

  The regions where the aggregates may exist are a space surrounded by a solid curve representing the outer shape of the negative electrode active material 11 and a place where the aggregates rise to the upper side of the negative electrode active material and partially cover the negative electrode active material ( This is a square portion 12) surrounded by a broken line in FIG. When the aggregate further spreads and covers the upper surface of the negative electrode active material, the occlusion / desorption of lithium ions is inhibited, resulting in a decrease in capacity. Therefore, the maximum size of the aggregate is the region 12 surrounded by the square in FIG.

  In the model described above, the negative electrode active material is assumed to be spherical and have the same particle size. Actually, this model is different from the actual shape because it is not a perfect sphere and has a wide particle size. However, in the examples described later, it is possible to apply this model by regarding the negative electrode active material as a sphere and replacing the average particle size obtained by measuring the particle size distribution with the particle size of FIG. I have confirmed.

  When the average particle diameter (average diameter) of the negative electrode active material is R, the length (gap length) of the side of the square 12 in FIG. 1 is (√2) R / 2. That is, its length is about 71% of R.

  Further, as a desirable mode, it is preferable that only the void portion in FIG. 1 is filled with the aggregate made of the second carbon and the surface of the negative electrode active material 11 is not substantially covered. If the size of the aggregate corresponds to the gap length, this condition is satisfied. That is, the length of the aggregate is 2R with respect to the average particle diameter R of the negative electrode active material.

  Next, the method for determining the minimum particle size and the basis for the determination will be described. FIG. 2 is a view of the state in which the second carbon aggregates are filled in the gaps between the three negative electrode active materials 21 when viewed from above the negative electrode surface. The reason for using the three particle model is that the gap formed from four particles as shown in FIG. 1 is large, the gap shown in FIG. 2 is smaller, and represents the lower limit size. It is. In the model of FIG. 2, the negative electrode active material was regarded as a sphere and the particle size was the same. The actual negative electrode active material is not completely spherical, and the particle size is distributed in a certain range, but it will be described later that it can be applied to such an actual negative electrode.

  Now, in FIG. 2, the aggregate made of the second carbon contains a binder, and the second carbon and the binder are in a composite state. This aggregate exhibits a binding force and at the same time has electronic conductivity.

  In order for the aggregate to connect the three negative electrode active materials to form an electron conduction path, the size is determined by the size of the void surrounded by the three negative electrode active materials. The size was a circle 23 having the maximum diameter in a space surrounded by a solid curve representing the outer shape of the negative electrode active material 21. If it is a circle smaller than this diameter, it will peel from the surface of any negative electrode active material, a binding force will also weaken, and an electronic network will also be interrupted. Conversely, if it is larger than this, three negative electrode active materials can be connected. The maximum value of the size has been described with reference to FIG.

  In the model described with reference to FIG. 2, the negative electrode active material is assumed to be spherical and have the same particle size. Actually, this model is different from the actual shape because it is not a perfect sphere and has a wide particle size. However, in the examples described later, it is possible to apply this model by regarding the negative electrode active material as a sphere and replacing the average particle size obtained by measuring the particle size distribution with the particle size of FIG. I have confirmed.

  When the average particle diameter of the negative electrode active material is R, the diameter of the circle in FIG. 2 (minimum diameter of the gap) is (2 / √3-1) · R. That is, the diameter is about 15% of R.

Example 1
FIG. 3 shows a cross section of a coin-type lithium secondary battery 301 of the present invention. The coin-type lithium ion battery 301 has a structure sealed with a positive electrode can 334, a negative electrode can 335, and a gasket 336. A positive electrode 307, a negative electrode 308, a separator 309, and an electrolytic solution are accommodated therein. The electrolytic solution is held in the gap 337 between the separator 309 and the battery. The positive electrode 307 includes a positive electrode mixture layer 330 and a positive electrode current collector 331. The negative electrode 308 includes a negative electrode mixture layer 332 and a negative electrode current collector 333.

  Below, the assembly method of the positive electrode 307, the negative electrode 308, and a coin-type battery is demonstrated in order.

The positive electrode active material used in this example is Li _1.05 Mn _1.95 O ₄ having an average particle diameter of 20 μm.
The conductive additive, the average particle diameter of 3 [mu] m, a specific surface area of 13m ² / g of natural graphite as an average particle size 0.04 .mu.m, and a carbon black having a specific surface area of 40 m ² / g, a weight ratio of 4: mixed so that 1 What was done was used. As the binder, a solution in which 8 wt% of polyvinylidene fluoride (PVDF) was previously dissolved in N-methyl-2-pyrrolidone was used.

  These positive electrode active material, conductive additive, and PVDF were mixed so as to have a weight ratio of 90: 4: 6 and sufficiently kneaded to obtain a positive electrode slurry. The positive electrode mixture layer 330 could be formed on the positive electrode current collector 331 by applying this positive electrode slurry to one surface of a positive electrode current collector 331 made of an aluminum foil having a thickness of 20 μm and drying. The positive electrode 307 was pressed using a roll press, and the positive electrode mixture layer 330 was compressed. Thereby, the internal resistance of the positive electrode mixture layer 330 was reduced, and the interface contact resistance between the positive electrode mixture layer 330 and the positive electrode current collector 331 was also reduced. This electrode was punched into a disk shape having a diameter of 15 mm to form a positive electrode 307.

The negative electrode 308 was produced by the following method. The first carbon of the negative electrode is lump graphite having an average particle diameter of 9 μm, and the second carbon is carbon black having an average particle diameter of 0.04 μm and a specific surface area of 40 m ² / g at a weight ratio of 95: 5. A mixture was used. Carbon black also serves as a conductive aid. As the binder, a solution in which 8% by weight of polyvinylidene fluoride was previously dissolved in N-methyl-2-pyrrolidone was used.

  A negative electrode slurry was prepared by mixing the carbon material composed of graphite and carbon black previously mixed with polyvinylidene fluoride so that the weight ratio was 90:10 and kneading sufficiently.

  Details of the distributed processing are as follows. First, N-methyl-2-pyrrolidone was added so that the ratio of the solid content composed of the active material, carbon black, and binder was in the range of 35 to 40%. This mixture is premixed with a planetary mixer. The rotation speed of the wings was a low speed of 10 to 20 rpm. This process is intended to disperse the materials because they are unevenly solidified immediately after the materials are added to the tank. In addition, the dispersion | distribution here is enough if the composition at the macro level of the grade which the composition when squeezing with a spoon etc. becomes a compounding composition is sufficient.

  Next, the slurry was placed in a disper device, and the stirrer (feather rotor) was rotated at a high speed of 2000 rpm for 15 to 30 minutes to treat the slurry. During the treatment, a jacket of circulating water was provided outside the container filled with the slurry so that the temperature of the slurry was kept constant at 20 to 25 ° C., and circulating water at a constant temperature was circulated through the jacket. In this treatment, the solid particles of the active material and carbon black are dispersed at a micro level, and further, the aggregated state of the binder is solved to aim at a state where the solid particles are appropriately entangled. Therefore, the process conditions such as the shape of the stirrer, the rotation speed, the time, and the slurry temperature during the dispersion treatment dominate the dispersion state of the material at the micro level. In this embodiment, an example of processing using the disperser is shown, but other known distributed processing devices can be used. Homogenizer, planetary mixer with disperser, high-speed stirrer (for example, film mix manufactured by Primix Co., Ltd.) that moves in a thin film, and a device that applies an impact force to slurry by injection (wet machine manufactured by Yoshida Machinery Co., Ltd.) Atomizer nanomizer). Whatever apparatus is used, the optimum process conditions for each apparatus may be set.

  This negative electrode slurry was applied to one surface of a negative electrode current collector 333 made of a copper foil having a thickness of 10 μm and temporarily dried to obtain a negative electrode mixture layer 332 on the negative electrode current collector 333. The negative electrode current collector 333 on which the negative electrode mixture layer 332 was formed was pressed with a roll press and then dried to produce an electrode. This electrode was punched into a disk shape having a diameter of 16 mm to form a negative electrode 308.

Here, in order to make a difference in the area occupied by the second carbon aggregates with respect to the entire area of the negative electrode mixture layer 332, in the following examples, negative electrode slurries with different dispersion conditions were prepared, and negative electrodes were produced. The surface state of these negative electrodes was observed using a scanning electron microscope (SEM). The left photograph in FIG. 4 shows an example of the negative electrode surface. The horizontal width is 125 μm, the vertical width is 90 μm, and the total area is about 11200 μm ² . The magnification is 1000 times.

  The occupied area of the second carbon aggregate with respect to the entire area of the SEM image is obtained by analyzing the particle shape using a known image processing software (for example, A image-kun (registered trademark) manufactured by Asahi Kasei Engineering). I was able to. The black part of the photograph on the right in FIG. 4 shows a region where the second carbon is agglomerated.

  The image processing software applied to the present invention preferably has an application capable of automatically separating and recognizing particles on the image one by one and measuring the area of the particles. It would be more desirable to have a function that can measure the area ratio, the maximum and minimum area length, and the number of particles.

  The procedure for obtaining the area is as follows. In the image, a plurality of particles of the first carbon and a plurality of particles of the second carbon with different sizes are seen. There is a mixture layer in which the second carbon particles form aggregates, and there is a mixture layer in which the size of the second carbon aggregates is different. Moreover, the mixture layer which the particle | grains of 2nd carbon make independently is seen. The image of the surface of the negative electrode mixture layer obtained by SEM was colored so that the plurality of particles of the first carbon and the plurality of particles of the second carbon could be distinguished. It is preferable to color the image scanned with any image processing software at as high a magnification as possible. This is because when a color is applied at the same magnification as the image obtained by SEM, an artificial error becomes large.

  Furthermore, it is difficult to perform binarized image processing on an image obtained by SEM as it is because a binarization threshold value cannot be determined. By coloring, binarized image processing is easy, and high data reliability is obtained. In order to facilitate image processing, a difference in contrast between the negative electrode active material (first carbon) and the second carbon in the SEM image may be given under the SEM observation conditions with a low acceleration voltage.

  In addition, in order to omit the above-described color painting work, commercially available image correction software can be used, and binarized image processing can be performed using the corrected image. By using such software together, an image of the negative electrode mixture layer obtained by SEM can be widely processed.

  Here, the reason for using commercially available image correction software is to eliminate gradation of contrast of the entire image and to eliminate an error in binarization processing due to a difference in the position of the negative electrode surface. That is, in order to avoid a problem that the tone of the light and shade is shifted while being the same material on the surface of the negative electrode mixture layer, and it is processed as if it were a different material. When such a gradation shift occurs, the gradation is corrected using image correction software (for example, software such as Premiere (registered trademark) Elements (registered trademark) 4 manufactured by ADOBE (registered trademark)). .

  When the same material portion can be binarized in the same manner, image processing is performed on a wide area of the negative electrode surface as much as possible. By doing so, it is possible to average the disturbance of the distribution state of the first carbon, the second carbon, or the binder, and to grasp a more correct distribution state.

  The threshold value for binarization is set so that parts of different materials are distinguished. Different materials can be judged from differences in the shape and size of particles observed in SEM images.

  FIG. 4 illustrates the result of the binarized image processing according to the procedure described above. The left side of FIG. 4 is a substantial image of the low acceleration voltage SEM. The right side of FIG. 4 is a distribution map in which the region where the second carbon exists is colored black by image processing. Using this image, A image-kun (registered trademark), the aspect ratio (= maximum area length / minimum area) in addition to the diameter (circular approximation), area, and number of aggregates composed of the second carbon Width). When the area is plotted on the horizontal axis and the number of aggregates is plotted on the vertical axis, the smaller the area, the larger the number, and the larger the area, the smaller the number.If the dispersion condition is inappropriate, the large area is coarse. Aggregated aggregates come out frequently. In the following examples, the number of aggregates is accumulated from the small area side toward the large area side, and the area when the value reaches 95% of the total number of aggregates is defined as the allowable maximum area. It was considered to correspond to an area of 12.

In this image analysis, the average area of the aggregates was calculated, and the average area was set to the square of the average particle diameter of the first carbon. In the case of FIG. 4, since the average particle diameter of the first carbon particles was 9 μm, the length of one side of the square in FIG. 1 is 6.4 μm, and the maximum area is 40 μm ² . This value could be confirmed because the maximum allowable area was in the range of 40 ± 5 μm when the dispersion treatment time was set to 15 to 20 minutes with the above composition. Thus, although it was the calculation by a model in FIG. 1, it turned out that the said model is applicable also to an actual negative electrode. A large number of negative electrode active materials exist in the average particle diameter, and as a result, the ratio of the number of gaps in the arrangement of FIG. 1 increases, and the area of the aggregate also increases with the ratio of the size shown in the broken line part of FIG. This is considered to be the reason. On the other hand, when the dispersion treatment time was 5 to 10 minutes, the ratio of aggregates having an area of 40 μm ² or more increased by 10 to 20%, and the allowable maximum area varied within a wide range of 80 to 160 μm ² .

In addition, in the case of an average particle diameter of 5 μm, a negative electrode was manufactured for each case of 15 μm, and the allowable maximum area of the aggregate obtained by the calculation of FIG. 1 was 15 ± 5 μm when the dispersion treatment time was 15 to 20 minutes. ² , 115 ± 10 μm ² . When the dispersion treatment time was 5 to 10 minutes, the coarsened aggregates increased, and the allowable maximum areas varied in the range of 30 μm ^{2 to} 90 μm ^{2 and in} the range of 130 μm ^{2 to} 200 μm ² , respectively.

Further, the minimum size when the average particle diameter is 5 μm is 0.7 μm in diameter and 0.4 μm ² in area from FIG. When the average particle size is 15 μm, the diameter is 2.3 μm and the area is 4 μm ² . When the dispersion time was set to 15 to 20 minutes, it was confirmed that almost 100% of the aggregates had an area of 0.4 μm ² or more. Thus, since the minimum diameter of the aggregate of the second carbon is about 1/10 of the average particle diameter of the first carbon, the primary particle of the second carbon is smaller than the minimum diameter of the aggregate. Is essential. If this condition is not satisfied, the primary diameter of the second carbon will exceed the minimum diameter. In such a case, the distance between the first carbon particles becomes too large, and the density of the negative electrode mixture is lowered, and as a result, the capacity density of the battery is reduced. Therefore, it is desirable that the average particle diameter of the primary particles of the second carbon is 1/10 or less of the particle diameter of the first carbon, and the aggregate composed of the primary particles is in the state shown in FIG.

  The average particle size of the second carbon is desirably 8% or less (for example, 0.1 μm or less), more desirably 1/100 or less (for example, 10 nm or less) with respect to the average particle size of the first stage. The reason for thinking in this way is that the second carbon is inserted into a fine gap where the two first carbons are in contact with each other, and has an effect of reducing the electronic resistance between the first carbons. The above 8% means that the maximum diameter of the second carbon in the gap of FIG. 1 is about 16% of the average particle diameter of the first carbon, so that the aggregate composed of at least two second carbon particles. The collection is given as the maximum diameter for being accommodated in the first carbon gap.

  Here, the average particle diameter of the second carbon is an average value of the diameters of the primary particles, and the primary particles may be aggregated in the state of the raw material powder when the slurry is prepared. This is because when the slurry is prepared, the agglomerated state is loosened and can be uniformly dispersed.

  Similarly, in the case of an average particle diameter of 5 μm, negative electrodes were manufactured using different graphites (first carbon) in the case of 15 μm. As a result, it was confirmed that 100% of the aggregates determined by the calculation of FIG.

Next, the coin-type lithium ion battery 301 shown in FIG. 3 was assembled using the negative electrode obtained by compressing the mixture layer using a roll press machine and the negative electrode having a dispersion time of 15 to 20 minutes. A positive electrode 307, a separator 309, and a negative electrode 308 were stacked, and the stacked body was housed in a positive electrode can 334 and a negative electrode can 335. The positive electrode 307 has a structure in which a positive electrode mixture layer 330 is bonded to one surface of a current collector 331. The negative electrode 308 has a structure in which a positive electrode mixture layer 332 is bonded to one surface of a current collector 333. The separator 309 is a polyethylene porous polymer sheet having a thickness of 40 μm. As the electrolytic solution, a mixed solution of 1.0 mol / dm ³ by dissolving LiPF ₆ in a mixed solution of ethylene carbonate and ethyl methyl carbonate (volume ratio of 1: 2) was used. The electrolyte exists in the separator 309 and the space 337 inside the battery. The battery 301 was completed by compressing the battery from the outside with a caulking machine.

The coin-type lithium ion battery 301 shown in Example 1 was subjected to a charge / discharge test under the following conditions in a temperature of 45 ° C. environment. First, the battery was charged at a constant current of 1 mA / cm ^{2 with} a current density of 1 mA / cm ² up to a voltage of 4.1 V, and then subjected to constant current and constant voltage charging at a constant voltage of 4.1 V for 3 hours. After charging was completed, a 1 hour rest period was provided, and the battery was discharged at a constant current of 1 mA / cm ² to a discharge end voltage of 3V. A two hour rest period was provided after the discharge was completed. A cycle test was repeated in which charging, resting, discharging, and resting were repeated. The discharge capacity of the lithium ion battery in the 100th cycle of this cycle test was compared.

The second carbon agglomerate area was observed with a SEM at a magnification of 1000 times, and the image of the surface of the negative electrode mixture layer surface obtained by photographing (colored in FIG. 3) was painted with image processing software, and the result of particle analysis (Image on the right in FIG. 3). Each slurry sample was prepared by subjecting it to a kneading condition at a low speed of 10 rpm for 10 minutes, followed by a different dispersion treatment. Example 1 is a result when the particle sizes of the first carbon are different, and is performed under high dispersion conditions (dispersion time 20 minutes). It was confirmed that the allowable maximum area was in a range including the value calculated in FIG. Comparative Examples 1 and 2 are the results when the dispersion time was 5 minutes for the average particle diameters of 5 μm and 15 μm, respectively. As a result of the particle analysis, the allowable maximum area in which the cumulative number of particles reaches 95% of the total range of Comparative Example 1 varies in the range of 80 to 110 μm ² , and Comparative Example 2 varies in the range of 110 to 280 μm ² . The average particle size was 15 ± 5 μm ² when the average particle size was 5 μm, 50 ± 5 μm ² when the average particle size was 10 μm, and 115 ± 10 μm ² when the average particle size was 15 μm.

  The ratio of the number of the second carbon aggregates having an aspect ratio of 1.25 or more is determined by obtaining the maximum length and the minimum width of the area of each second carbon aggregate at the time of the particle analysis described above, and the maximum length / minimum width = The aspect ratio was determined. The aspect ratio was divided into 1.24 or less and 1.25 or more, and the ratio was examined. The number of second aggregates at this time was 50 to 250. As a result of particle analysis, Comparative Example 1 was 45%, Comparative Example 2 was 48%, Example 1 was 49%, and Example 2 was 53%.

The density of the mixture layer was determined from the weight of the mixture layer / the area of the negative electrode / the thickness of the mixture layer after pressurizing the negative electrode with a target of 1.0 g / cm ³ . As a result of the charge / discharge cycle test, the discharge capacity was 330 mAh / g in Comparative Example 1, 348 mAh / g in Comparative Example 2, 362 mAh / g in Example 1, and 375 mAh / g in Example 2.

  As the area of the second carbon aggregate was smaller, the discharge capacity was improved. Further, comparing Example 1 and Example 2, when the ratio of the second carbon aggregate having an aspect ratio of 1.25 or more is 50% or more, the discharge capacity is clearly improved.

  The reason why the discharge capacity is improved is considered as follows. In Comparative Examples 1 and 2, on the surface of the mixture layer, there are many forms in which aggregates of conductive assistants are placed on a plurality of particles of the active material, and the particle area of the active material capable of inserting and releasing lithium ions is small. Thus, it is considered that the charge / discharge capacity has decreased. In Examples 1 and 2, aggregates of the conductive assistant were mainly localized in the gaps between the plurality of particles of the active material.

  The reason why the ratio of the aspect ratio of the second carbon aggregate to 1.25 or more is preferably 50% or more is that the elongated second carbon aggregate is organic through the gap between the active material particles. This is probably because a healthy and strong conductive network was formed.

  Thus, the charge / discharge capacity reduction can be prevented by limiting the size of the second carbon aggregate. The present invention is particularly effective in a lithium secondary battery having a large discharge capacity.

(Example 2)
In the negative electrode 308 manufactured in Example 1, a carbon fiber having a diameter of 100 nm and a length of 20 to 30 μm was added in a 2 weight ratio, and a negative electrode in which the amount of first carbon added was reduced to 93 weight ratio was manufactured. That is, the composition of the negative electrode is first carbon: second carbon: carbon fiber = 93: 5: 2. Other conditions such as the binder composition are the same as in Example 1. Further, the negative electrode manufacturing process was not changed, and a negative electrode slurry was prepared in the same procedure as in Example 1.

  This negative electrode slurry was applied to one surface of a negative electrode current collector 433 made of a copper foil having a thickness of 10 μm and temporarily dried, whereby a negative electrode mixture layer 432 was obtained on the negative electrode current collector 433. The negative electrode current collector 433 on which the negative electrode mixture layer 332 was formed was pressed with a roll press and then dried to produce an electrode. This electrode was punched into a disk shape having a diameter of 16 mm to form a negative electrode 408.

The coin type lithium ion battery 301 of FIG. 3 was manufactured using the same positive electrode 307, separator 309, electrolyte, positive electrode can 334, negative electrode can 335, and gasket 336 as in Example 1. After charging at a constant current of 1 mA / cm ^{2 with} a current density of up to 4.1 V, constant current and constant voltage charging, in which constant voltage charging was performed at 4.1 V, was performed for 3 hours. After charging was completed, a 1 hour rest period was provided, and the battery was discharged at a constant current of 1 mA / cm ² to a discharge end voltage of 3V. A two hour rest period was provided after the discharge was finished. Next, without changing the charging conditions, the discharge current was increased to 1.5 mA / cm ² and the discharge capacity was measured. As a result, the discharge capacity increased by 10% compared to the case of Example 1. Thus, it was found that by adding the conductive fiber to the negative electrode, the electronic resistance of the negative electrode was lowered and further excellent discharge characteristics were obtained.

  In the embodiment described above, a coin-type lithium ion battery is exemplified. The shape of these batteries, electrode specifications, and the like can be arbitrarily changed within the scope of the gist of the present invention, and the present invention is not limited to these examples.

(Example 3)
FIG. 5 schematically shows the internal structure of the nonaqueous electrolyte secondary battery 501. The non-aqueous electrolyte secondary battery 501 is a generic name for electrochemical devices that can store and use electrical energy by storing and releasing ions to and from electrodes in the non-aqueous electrolyte. In this embodiment, a lithium ion battery will be described as a representative example.

  In the lithium ion battery 501 of FIG. 5, an electrode group including a positive electrode 507, a negative electrode 508, and a separator 509 inserted between both electrodes is housed in a battery container 502 in a sealed state. A lid 503 is provided at the top of the battery container 502, and the lid 503 has a positive electrode external terminal 504, a negative electrode external terminal 505, and a liquid injection port 506. The positive external terminal 504 and the negative external terminal 505 are attached to the lid 504 via an insulating sealing material 512. After the electrode group was stored in the battery container 502, the lid 503 was placed on the battery container 502, and the outer periphery of the lid 503 was welded to be integrated with the battery container 502. For attachment of the lid 503 to the battery case 502, other methods such as caulking and adhesion can be employed in addition to welding. The positive electrode 507 is connected at the upper portion by a positive electrode lead wire 510 and connected to the positive electrode external terminal 504. Similarly, the negative electrode 508 is connected to the negative electrode lead wire 511 at the upper portion and connected to the negative electrode external terminal 505.

  As the positive electrode, the negative electrode, and the separator, those used in Example 1 can be used, but other materials may be used without departing from the scope of the present invention. A plurality of prismatic batteries shown in FIG. 5 were manufactured.

(Example 4)
FIG. 6 shows a battery system of the present invention in which two lithium ion batteries 601a and 601b manufactured in Example 3 are connected in series. The number of batteries can be arbitrarily changed in series and in parallel according to the voltage and capacity required by the system. Each of the lithium ion batteries 601a and 601b has an electrode group having the same specifications including a positive electrode 607, a negative electrode 608, and a separator 609, and a positive electrode external terminal 604 and a negative electrode external terminal 605 are provided on the upper part. An insulating seal member 612 is inserted between each external terminal and the battery container so that the external terminals are not short-circuited.

  A negative external terminal 605 of the lithium ion battery 601 a is connected to a negative input terminal of the charge / discharge controller 616 by a power cable 613. The positive external terminal 604 of the lithium ion battery 601a is connected to the negative external terminal 605 of the lithium ion battery 601b via the power cable 614. The positive external terminal 604 of the lithium ion battery 601 b is connected to the positive input terminal of the charge / discharge controller 616 by the power cable 615. With such a wiring configuration, the two lithium ion batteries 601a and 601b can be charged or discharged.

  The charge / discharge controller 616 exchanges power with an externally installed device (hereinafter referred to as an external device) 619 via power cables 617 and 618. The external device 619 includes various electric devices such as an external power source and a regenerative motor for supplying power to the charge / discharge controller 616, and an inverter, a converter, and a load that supply power from this system. An inverter or the like may be provided depending on the type of AC and DC that the external device supports. As these devices, known devices can be arbitrarily applied.

  In addition, a power generation device 622 that simulates the operating conditions of a wind power generator was installed as a device that generates renewable energy, and was connected to the charge / discharge controller 616 via power cables 620 and 621. When the power generation device 622 generates power, the charge / discharge controller 616 shifts to the charge mode, supplies power to the external device 619, and charges surplus power to the lithium ion batteries 601a and 612b. Further, when the power generation amount simulating the wind power generator is smaller than the required power of the external device 619, the charge / discharge controller 616 operates so as to discharge the lithium ion batteries 601a and 612b. Note that the power generation device 622 can be replaced with another power generation device, that is, any device such as a solar cell, a geothermal power generation device, a fuel cell, or a gas turbine generator. The charge / discharge controller 616 stores a program capable of automatic operation so as to perform the above-described operation.

  The lithium ion batteries 601a and 601b are normally charged so that a rated capacity can be obtained. For example, constant voltage charging of 4.1V or 4.2V can be performed for 0.5 hour at a charging current of 1 hour rate. The charging conditions are determined by the design of the type and amount of materials used for the lithium ion battery, so the optimum conditions are set for each battery specification.

  After charging the lithium ion batteries 601a and 601b, the charge / discharge controller 616 is switched to the discharge mode to discharge each battery. Normally, the discharge is stopped when a certain lower limit voltage is reached.

  The system described above is S1, and the external device 619 supplies power during charging and consumes power during discharging. In this example, up to 5 hour rate discharge was carried out, and a high capacity of 90% was obtained with respect to the capacity during 1 hour rate discharge. The capacity drop when 100 charge / discharge cycles were performed was not substantially observed, and the capacity under the above conditions was maintained at 90%. In addition, the power generation device 622 simulating a wind power generator could be charged at a rate of 3 hours during power generation.

  Based on the content described above, specific examples will be shown to clarify the effects of the present invention. Note that specific constituent materials, parts, and the like may be changed without departing from the scope of the present invention. In addition, if the constituent elements of the present invention are included, a known technique can be added or replaced by a known technique, and the power generator can be arbitrarily regenerated such as sunlight, geothermal heat, wave energy, etc. It can be replaced with an energy generation system.

(Comparative Example 1)
A negative electrode with a dispersion treatment time of 5 to 10 minutes with the composition of the negative electrode of Example 1 was manufactured, and a plurality of lithium ion batteries shown in FIG. 5 were manufactured. According to this comparative example, the second dispersion state is outside the scope of the present invention. Only the negative electrode dispersion treatment conditions were changed, and the other conditions were the same as in Example 4, and the system of FIG. 6 was manufactured. Let this system be S2.

  Using this system, the external device 619 supplies power during charging and consumes power during discharging. In this example, the discharge was carried out up to a 5-hour rate discharge, and a high capacity of 90% was obtained at the initial 10 cycles with respect to the capacity during the 1-hour rate discharge. However, as the number of cycles was increased, the capacity decreased greatly and decreased to 81% at 100 cycles.

The application of the nonaqueous electrolyte secondary battery according to the present invention is not particularly limited. For example, it can be used as a power source for portable information communication devices such as personal computers, word processors, cordless telephone cordless handsets, electronic book players, mobile phones, car phones, handy terminals, transceivers, and portable radios. Also, as a power source for various portable devices such as portable copiers, electronic notebooks, calculators, LCD TVs, radios, tape recorders, headphone stereos, portable CD players, video movies, electric shavers, electronic translators, voice input devices, memory cards, etc. Can be used. In addition, it can be used as household electric appliances such as refrigerators, air conditioners, televisions, stereos, water heaters, microwave ovens, dishwashers, dryers, washing machines, lighting fixtures, toys.
It can also be used as a battery for electric tools and nursing equipment (electric wheelchairs, electric beds, electric bathing facilities, etc.) regardless of whether they are for home use or business use. In addition, for industrial applications, as power sources for medical equipment, construction machinery, power storage systems, elevators, unmanned mobile vehicles, and mobiles such as electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, golf carts, turret vehicles, etc. The present invention can be applied as a power source. Furthermore, it is also possible to charge the battery module of the present invention with electric power generated from a solar cell or a fuel cell and use it as a power storage system that can be used outside the ground, such as a space station, spacecraft, or space base.

11 First carbon (negative electrode active material)
12 Diameter of the first carbon (average particle size)
21 1st carbon (negative electrode active material)
23 Minimum diameter of gap surrounded by first carbon 301 Coin type lithium ion batteries 307, 507, 607 Positive electrode 308, 508, 608 Negative electrode 309, 509, 609 Separator 330 Positive electrode mixture layer 331 Positive electrode current collector 332 Negative electrode combination Agent layer 333 Negative electrode current collector 334 Positive electrode can 335 Negative electrode can 336 Gasket 337 Space (region where electrolyte is added)
501, 601a, 601b Nonaqueous electrolyte secondary battery 502, 602 Battery container 503 Lid 504, 604 Positive electrode external terminal 505, 605 Negative electrode external terminal 506, 606 Injection port 510 Positive electrode lead wire 511 Negative electrode lead wire 512, 612 Insulating seal Material 613, 614, 615, 617, 618, 620, 621 Power cable 616 Charge / discharge controller 619 External device 622 Renewable energy generator

Claims (4)

In a non-aqueous electrolyte secondary battery comprising a positive and negative electrodes capable of inserting and removing lithium ions and a porous film separating them,
A first carbon capable of electrochemically occluding and releasing lithium ions;
Including a second carbon capable of electrochemically occluding and releasing lithium ions or substantially not occluding lithium ions;
Aggregates composed of the second carbon particle aggregates are mainly localized in the gaps between the plurality of first carbon particles;
An average particle size of primary particles of the second carbon is 8% or less of an average particle size of primary particles of the first carbon;
The primary particles of the second carbon are in an aggregated state,
The average particle size of average particle size and the primary particle of the first carbon of the primary particles of the second carbon is determined by measuring the particle size distribution,
The average particle size of the primary particles of the second carbon and the average particle size of the primary particles of the first carbon approximate the primary particles of the second carbon and the first carbon to spheres. In the surface photograph of the negative electrode obtained by a microscope, it is a value determined by performing binarized image processing so that the first carbon and the second carbon can be distinguished,
The non-aqueous electrolyte secondary battery characterized in that the aggregate composed of the second carbon particles has an aspect ratio of 1.25 or more with respect to the total number of the aggregates of 50% or more.   In the surface photograph of the negative electrode obtained by a scanning electron microscope, an average of aggregates composed of the second carbon after performing binarized image processing so as to distinguish the first carbon and the second carbon The non-aqueous electrolyte secondary battery according to claim 1, wherein an area is calculated and the average area is less than or equal to a square of an average particle diameter of the first carbon.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the second carbon aggregate contains a binder and has conductivity. Conductive fibers are added to the negative electrode,
The nonaqueous electrolyte secondary battery according to claim 1, wherein a part of the fiber is in contact with the second carbon aggregate.

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