JP4529511B2 - Lithium ion battery - Google Patents

Description

  The present invention relates to a lithium ion battery excellent in safety such as internal short circuit safety and heat resistance.

A non-aqueous electrolyte secondary battery such as a lithium ion battery includes a layer that electrically insulates a positive electrode plate and a negative electrode plate, and this insulating layer also has a role of holding an electrolyte solution. Examples of these insulating layers include microporous film separators made of polyethylene, polypropylene, etc. used in lithium ion batteries, or gel electrolyte sheets in which a polymer matrix used in polymer batteries is impregnated with a gel electrolyte. Etc.
Conventionally, examples in which the above insulating layer includes a fibrous material have been proposed for various reasons. For example, Patent Document 1 describes a gel electrolyte sheet using a porous sheet containing a fibrous inorganic filler as a core material.
Patent Document 2 describes a technique for forming an insulating layer by applying a fibrous polyethylene solution onto an electrode plate. However, separators and insulating layers, most of which are made of a resin material, generally tend to change shape at high temperatures. Therefore, when an internal short circuit or a sharply shaped protrusion such as a nail penetrates the battery, the insulation layer changes shape due to instantaneous short-circuit reaction heat, and the insulation between the positive and negative electrodes cannot be maintained. was there.

Patent Document 3 proposes a separator in which a fibrous organic compound having no melting point at 200 ° C. or less and a thermoplastic polymer are mixed as a technique for solving the heat resistance problem.
Japanese Patent Laid-Open No. 11-219727 JP 2002-083590 A Japanese Patent Laid-Open No. 11-040131

  All of the insulating layers described above are porous and function as a battery by ions moving through the pores. The fibrous material contained in the insulating layer also plays a role of securing these holes.

  However, in the technique using a gel electrolyte sheet using a porous sheet containing a fibrous inorganic filler as a core material, since the electrolyte is a polymer gel, its ionic conductivity is very low and it is difficult to discharge a large current. Met. In addition, since the main component of the porous sheet is a resin, even if some fibrous inorganic filler is included, it has not been possible to obtain heat resistance that can withstand heat generation during an internal short circuit.

  Even in the technique of forming an insulating layer by applying a solution of fibrous polyethylene on the electrode plate, the insulating layer is composed of polyethylene fibers, so that when the internal short circuit occurs, the shape deforms and melts, and between the positive and negative electrodes Insulation could not be maintained.

In a technique using a separator in which a fibrous organic compound having no melting point at 200 ° C. or less and a thermoplastic polymer are mixed, since a heat-resistant resin such as aramid is included, the heat resistance is considerably improved. . However, in an internal short circuit of a lithium ion battery, the aluminum positive electrode core material at the short circuit point may melt, and the maximum temperature at that time is considered to instantaneously reach 700 ° C. or higher. Even a resin that does not have a stable melting point, such as aramid, could not retain insulation between the positive and negative electrodes because it deformed at such a high temperature.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium ion battery that has a high capacity and high characteristics and is excellent in internal short circuit and nail penetration safety.

The method for producing a lithium ion battery of the present invention includes:
(A) cutting the fibrous ceramic filler to a desired fiber length using a pulverizing means;
(B) mixing at least the pulverized fibrous ceramic filler, a resin binder, and a solvent, and dispersing and slurrying;
(C) applying the slurry onto the active material layer of the positive electrode or the negative electrode and drying to form a porous insulating layer;
Is included.

Another manufacturing method of the present invention is as follows.
(A) Mixing at least a fibrous ceramic filler, a resin binder, and a solvent, and pulverizing the fibrous ceramic filler to a desired average fiber length using a dispersing means, and at the same time, the fibrous ceramic filler and the resin binder A step of dispersing the solvent to form a slurry;
(B) applying the slurry on the active material layer of the positive electrode or the negative electrode and drying to form a porous insulating layer.

  With the above configuration, the lithium ion battery of the present invention can be manufactured by a method that is rational and has good controllability.

According to the present invention, it is possible to provide a lithium ion secondary battery that is highly safe in internal short-circuiting and nail penetration tests, has a high capacity, and is excellent in charge / discharge characteristics.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  FIG. 1 is a cross-sectional view schematically showing the electrode plate configuration of a lithium ion battery according to an embodiment of the present invention. In FIG. 1, a separator 3 is interposed between a positive electrode 1 and a negative electrode 2, and a porous insulating layer 4 containing a ceramic filler and a resin binder is formed on the surface of either the positive electrode active material layer 1a or the negative electrode active material layer 2a. Is formed.

  First, the porous insulating layer 4 will be described.

  FIG. 2 is an enlarged schematic view of the porous insulating layer 4 in one embodiment of the present invention. The porous insulating layer contains a small amount of a resin binder (not shown) and a fibrous ceramic filler 5, and there are many pores 6 between the fibrous ceramic filler 5.

  Examples of the ceramic material constituting the fibrous ceramic filler 5 include oxide ceramics such as alumina, silica, titania, zirconia, zinc oxide, magnesia, mullite, nitride ceramics such as silicon nitride and titanium nitride, and others such as silicon. Ceramics such as carbide, calcium carbonate, talc, kaolin, and complex compounds thereof can be used. These materials may be used alone or in combination as necessary.

  The fiber diameter of the fibrous ceramic filler 5 is preferably 2 μm or less because holes for ion conduction can be efficiently provided. If the fiber diameter is larger than 2 μm, the vacancies become too large, which causes problems such as easy deposition of metallic lithium in the vacancies and an increase in internal short circuit of the battery.

  More preferably, it is the range of 0.1 micrometer-1 micrometer. Within this range, it is possible to obtain a porous insulating layer with particularly excellent battery performance, and it is easy to handle in the coating and coating process and has high productivity.

  The aspect ratio between the fiber length and the fiber diameter of the fibrous ceramic filler is desirably 3 or more. A fiber length of 10 μm or less is appropriate. When the aspect ratio is small and 3 or less, the effect of increasing the pores between the fillers is not sufficient. On the other hand, if the fiber length is 10 μm or more, the viscosity of the slurry becomes extremely high, handling such as transportation is difficult, or the pipe is clogged. In addition, as will be described later, when a thin insulating porous layer having a thickness of 10 μm or less is provided in order to ensure battery capacity, various coating processes such as being unable to be applied thinly or being clogged with a coating apparatus. A malfunction will occur.

A fibrous filler having a desired fiber length may be prepared and introduced into a slurrying process, or a filler cut to a desired fiber length before coating may be introduced. For the cutting, a pulverizing means such as a jet pulverizer, a ball mill, or a bead mill may be used. Further, there is a method of preparing a fiber having a relatively long fiber length and simultaneously cutting the filler while dispersing the filler, the binder and the solvent in the subsequent slurrying step. In that case, it is possible to determine the desired filler fiber length by controlling the degree of dispersion in the slurrying step, which is a rational method with excellent productivity.

  The inclusion of the fibrous ceramic filler, which is an important component of the present invention, is intended to ensure sufficient holes for ionic conduction. If the filler has a spherical shape or a shape close to that as in the conventional case, the filler becomes a densely packed film, and pores between the fillers cannot be increased. In the present invention, by using a fibrous ceramic filler, it is possible to obtain a porous insulating layer in which the filler is hardly clogged, that is, there are many voids between the fillers.

  In addition, a non-fibrous filler may be mixed and used together with the fibrous ceramic filler as described above as the ceramic filler. As the non-fibrous filler, for example, any other shape such as a spherical shape or a lump shape can be used. As the material, the same material as the fibrous ceramic filler described above can be used.

  In the lithium ion battery of the present invention, the presence of the porous insulating layer 4 on the active material layer 2a improves the safety in internal short circuit and nail penetration tests. In the case where the porous insulating layer 4 is not provided, when a hole is opened in the separator 3 due to a foreign substance or the like and the positive and negative electrodes are short-circuited, an excessive current may flow to the short-circuit point to generate Joule heat. In that case, the separator around the short-circuit point melts or contracts due to the generated heat, the hole expands, the short-circuit area further expands, and Joule heat generation continues.By repeating this, the temperature of the battery continues to rise, causing abnormal heat generation and It may cause external deformation. In the lithium ion battery of the present invention, when a hole is opened in the separator 3 and the positive and negative electrodes are short-circuited, the porous insulating layer 4 containing a ceramic filler is present even if the separator 3 melts or contracts to enlarge the hole. Therefore, the short-circuit area between the positive and negative electrodes does not increase. Therefore, the generation of Joule heat does not expand and does not lead to abnormal heat generation.

  The porous insulating layer 4 is obtained by applying a slurry of a filler and a resin binder dispersed together with a solvent and applying the slurry on the active material layer 2a and drying it.

  Here, when the filler is cut simultaneously with the dispersion, the particle size distribution of the slurry is measured to determine the filler fiber length in the resulting porous film.

  For the application onto the active material layer 2a, for example, a continuous application method such as gravure coating or die coating, a drawing method using an inkjet nozzle, a spray coating method, or the like is used.

  As the resin binder, a resin binder having heat resistance and electrolytic solution resistance is used. In addition to PVDF (polyvinylidene fluoride), for example, a rubbery polymer including a polyacrylonitrile unit having high heat resistance and rubber elasticity is preferable. A porous insulating layer containing such a material as a binder has an advantage that it can be produced while maintaining a high yield because cracking and peeling do not occur when the positive and negative electrodes are wound through a separator.

  Although the thickness of the porous insulating layer 4 is not particularly limited, from the viewpoint of maintaining the design capacity while demonstrating the effect of the porous insulating layer 4 described above, the sum of the thicknesses of the combined separators 3 is the current separator specification (15- 30 μm), that is, about 1 to 30 μm.

For the positive electrode 1, lithium cobaltate and its modified products (such as those obtained by eutectic aluminum and magnesium), lithium nickelate and its modified products (such as those obtained by partially replacing cobalt with nickel), and manganic acid Examples thereof include particles of composite oxides such as lithium and modified products thereof. As binder, polytetrafluoroethylene (PTFE) / modified acrylonitrile rubber particle binder combined with carboxymethyl cellulose (CMC) / polyethylene oxide (PEO) / soluble modified acrylonitrile rubber with thickening effect, PVDF and its modification The body is used. Moreover, acetylene black, various graphites, etc. are added as a electrically conductive agent. These positive electrode materials are applied onto a current collector as a slurry mixed with a solvent such as N-methylpyrrolidone, and a positive electrode 1 having an active material layer 1a is completed through drying and rolling processes.

  As for the negative electrode 2, particles of various natural graphites, silicon-based composite materials such as artificial graphite and silicide, and various alloy composition materials are used as active materials. Various binders such as PVDF and its modifications can be used as the binder. These negative electrode materials also become the negative electrode 2 through the same process as the positive electrode.

For the electrolytic solution, it is possible to use various lithium compounds such as LiPF ₆ and LiBF ₄ as a salt. Further, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) can be used alone or in combination as a solvent. In addition, vinylene carbonate (VC), cyclohexylbenzene (CHB), or the like can be added in order to form a good film on the positive and negative electrodes or to ensure stability during overcharge.

  The separator 3 is not particularly limited as long as it has a composition that can withstand the range of use of the lithium ion battery, but a microporous film of an olefin-based resin such as polyethylene or polypropylene is generally used singly or in combination. Also preferred as an embodiment. Although the thickness of the separator is not particularly limited, from the viewpoint of maintaining the design capacity while demonstrating the effect of the porous layer described above, the sum of the combined porous film thicknesses is about the same as the current separator specification (15 to 30 μm), that is, 5 More preferably, it is ˜25 μm.

  Note that an inexpensive separator such as a nonwoven fabric can be used as necessary. For example, it is preferable to use a separator having excellent heat resistance including, for example, an aramid resin or alumina fine particles because the safety is further improved.

  In the present embodiment, the example in which the insulating porous layer 4 is provided on the negative electrode active material layer 2a is shown in FIG. 1, but the insulating porous layer may be provided on either the positive electrode or the negative electrode. good.

  FIG. 2 conceptually shows the embodiment, and does not specifically limit the properties of the constituent elements of the present invention such as the filler and the insulating porous layer.

Hereinafter, the present invention will be described more specifically with reference to examples.
Example 1
(A) Preparation of positive electrode 3 kg of lithium cobaltate together with 1 kg of PVDF # 1320 (N-methylpyrrolidone (NMP) solution with a solid content of 12% by weight), 90 g of acetylene black and an appropriate amount of NMP manufactured by Kureha Chemical Co., Ltd. The mixture was stirred with a kneader to prepare a positive electrode paste. This paste was applied and dried on a 15 μm thick aluminum foil, rolled to a total thickness of 160 μm, and then slit to a width that could be inserted into a cylindrical type 18650 case to obtain a positive electrode plate.
(B) Preparation of negative electrode 3 kg of artificial graphite was mixed with Nippon Zeon Co., Ltd. styrene-butadiene copolymer rubber particle binder BM-400B (solid content 40% by weight) 75 g, CMC 30 g and an appropriate amount of water. The mixture was stirred in a combined machine to prepare a negative electrode paste. This paste was applied and dried on a 10 μm thick copper foil, rolled to a total thickness of 180 μm, and then slit into a width that could be inserted into a cylindrical 18650 case to obtain a negative electrode plate.
(C) Formation of porous insulating layer 1900 g of alumina short fibers having a fiber diameter of 0.05 μm to 4 μm and a fiber length of 10 to 100 μm are prepared, and 1250 g of a polyacrylonitrile-modified rubber binder BM-720H (solid content 8% by weight) An appropriate amount of solvent NMP was mixed and slurried while grinding and cutting the alumina short fibers with a bead disperser. While measuring the fiber length in the slurry with a particle size distribution meter, dispersion treatment was performed until the desired fiber length was achieved, and the fiber length was controlled. This slurry was applied onto the negative electrode active material layer by gravure printing and dried to form a porous insulating layer having a thickness of 8 μm.
(E) Battery assembly These positive and negative electrodes were wound using a polyethylene microporous film having a thickness of 13 μm as a separator, cut into predetermined dimensions, and inserted into a cylindrical case. An electrolyte solution in which 1M LiPF ₆ was dissolved in EC / DMC / EMC mixed solvent was injected into the case and sealed to prepare a cylindrical 18650 lithium ion battery having a design capacity of 2000 mAh.
(Example 2)
In Example 1, the fiber diameter of the fibrous ceramic filler after dispersion was changed as follows. An alumina short fiber having a fiber diameter of 0.2 μm and an average fiber length of 100 μm was prepared as a fibrous ceramic filler, and the fiber length was controlled from 0.2 μm to 15 μm by changing the dispersion time. In other steps, a battery was produced in the same manner as in Example 1, and Example 2 was obtained.
(Comparative Example 1)
A battery produced without including a fibrous ceramic filler in the porous insulating layer is referred to as Comparative Example 1. That is, 1900 g of alumina fine particles having an average particle size of 0.3 μm as a filler, 1250 g of a polyacrylonitrile-modified rubber binder BM-720H (solid content 8 wt%) and an appropriate amount of solvent NMP were slurried with a bead disperser, A slurry for forming a porous insulating layer containing a fine particle filler was prepared. This slurry was applied onto the negative electrode active material layer by gravure printing and dried to form a porous insulating layer. The thickness of the obtained porous insulating layer was 8 μm. All other steps were the same as in Example 1.

  Examples and comparative examples were evaluated as follows. The results are shown in (Table 1) together with the configuration conditions.

1. Evaluation of Porosity The slurry was applied onto a copper foil with an applicator to form a 30 μm thick porous insulating layer. The volume of the solid part was determined from the density and addition ratio of the ceramic filler and binder, and the volume was determined by dividing by the volume of the entire porous insulating layer.
2. Battery charge / discharge characteristics Finished charge / discharge of the finished battery is performed twice and stored for 7 days in a 45 ° C. environment. Then, the following charge / discharge tests are performed in the (1) and (2) conditions in a 20 ° C. environment. The 2C discharge of 3) was carried out at 20 ° C. and 0 ° C. environment.

(1) Constant current charging: 1400 mA (end voltage 4.2 V)
(2) Constant voltage charging: 4.2V (end current 100mA)
(3) Constant current discharge (2C discharge): 4000 mA (end voltage 3 V)
The charge / discharge capacity at this time is shown in Table 1.
3. Nail penetration safety The battery after the battery charge / discharge characteristics evaluation was charged as follows.

(1) Constant current charge: 1400 mA (end voltage 4.25 V)
(2) Constant voltage charging: 4.25 V (end current 100 mA)
Regarding the battery after charging, a heat generation state was observed when a 2.7 mm diameter iron round nail was penetrated at a speed of 5 mm / second in a 20 ° C. environment. Table 1 shows the temperature reached after 90 seconds in the vicinity of the penetration portion of the battery.

  Comparing the porosity of Examples 1 and 2 and the comparative example, it can be seen that the present invention using the fibrous filler has a clearly higher porosity. When the 2C discharge capacities of the example and the comparative example were compared in the battery characteristics, there was not much difference in the 20 ° C. environment, but in each of the examples, the capacity was larger than the comparative example in the 0 ° C. environment. It is considered that the superiority of the present invention is remarkably exhibited under severe test conditions of low temperature and high rate because the porosity is increased and the ion conductivity is improved by the effect of the present invention.

  In the nail penetration test, as in the comparative example, all of the batteries of the examples suppressed heat generation after nail penetration, and no defects such as abnormal heat generation and appearance deformation were observed.

  When the batteries of the examples were disassembled and examined after the test, the porous insulating layer was present on the active material layer in the same manner as before the test, and the melting of the separator remained in a slight range. It was. From this, it is considered that the porous insulating layer did not contract even in the heat generation due to the nail penetration short circuit and the expansion of the short circuit part could be suppressed, so that it was possible to prevent a significant overheating.

  In Example 1, those having a fiber diameter of 2 μm or less could be applied without any problem. However, in the case of a fiber diameter of 4 μm, uneven coating may occur when applying by gravure coating, and at that time, porosity is applied. Some parts were not seen. Moreover, the unevenness | corrugation on the surface of a porous layer may become large also in the location applied. Therefore, the fiber diameter of the fibrous filler is preferably 2 μm or less from the viewpoint of the coating process.

  In Example 2, a fiber having a fiber length of 10 μm or less could be applied without problems, but a fiber having a fiber length of 18 μm had a slightly high frequency of occurrence of uneven stripes during application. Therefore, it can be said that the fiber length of the fibrous filler is desirably 10 μm or less.

The results of the porosity measurement and battery characteristics shown in Table 1 were evaluated by selecting a portion having no coating unevenness or stripe unevenness as described above.
(Example 3)
In Example 1, the pulverization / cutting process of the fibrous ceramic filler was changed as follows. Further, a medialess disperser was used instead of the bead mill so that the fibrous ceramic filler was not further crushed and cut in the slurrying step. In other processes, a battery was produced in the same manner as Example 3.

  Alumina short fibers having a fiber diameter of 0.2 μm and 0.7 μm and a fiber length of 100 μm were prepared, and pulverized by a jet mill to cut the fiber lengths to 2 μm and 4 μm, respectively. 1900 g of these fibrous fillers, 1250 g of polyacrylonitrile-modified rubber binder BM-720H (solid content 8% by weight) and an appropriate amount of solvent NMP are mixed, and a medialess disperser CLEARMIX (product name of M Technique Co., Ltd.) Then, a dispersion treatment was performed under the condition of 12000 rpm-5 minutes to obtain a slurry for forming a porous layer. In this dispersion step, since there was no collision between the media such as beads and the filler, the fibrous ceramic filler was not crushed or cut in the dispersion step, and the fiber length did not change.

This slurry was applied onto the negative electrode active material layer by gravure printing and dried to form a porous insulating layer having a thickness of 8 μm.
Example 4
In Example 1, the type of the fibrous ceramic filler was changed as follows, and in the other steps, a battery was produced in the same manner, and Example 4 was obtained.

1. 1. Alumina / silica composite short fiber having a fiber diameter of 0.5 μm and a fiber length of 100 μm. 2. Zirconia fiber having a fiber diameter of 0.2 μm and a fiber length of 100 μm. 3. Calcium carbonate fiber having a fiber diameter of 0.1 μm and a fiber length of 50 μm. Silicon carbide fibers having a fiber diameter of 0.2 μm and a fiber length of 50 μm Each fiber was cut in the dispersion step and had a fiber length shown in Table 2 in the porous layer.

For Examples 3 and 4, the same evaluation as in Examples 1 and 2 was performed. The results are shown in Table 2.

Comparing the porosity of Examples 3 and 4 and the comparative example, it can be seen that the present invention using the fibrous filler clearly has a higher porosity. Also in the battery characteristics, as in Examples 1 and 2, the capacity was large in the 0 ° C. environment. It is considered that the superiority of the present invention is remarkably exhibited under severe test conditions of low temperature and high rate because the porosity is increased and the ion conductivity is improved by the effect of the present invention.

  In the nail penetration test, as in the comparative example, all of the batteries of the examples suppressed heat generation after nail penetration, and no defects such as abnormal heat generation and appearance deformation were observed. From the above, it was confirmed that the battery of the present invention includes a porous layer having a high porosity, is excellent in charge / discharge characteristics, and has high safety.

  The lithium ion battery of the present invention is useful as a portable power source having excellent safety.

Sectional drawing which showed the structure of the lithium ion battery of this invention typically The enlarged view which showed typically the porous insulating layer in one embodiment of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 1a Positive electrode active material layer 2 Negative electrode 2a Negative electrode active material layer 3 Separator 4 Porous insulating layer 5 Fibrous ceramic filler 6 Hole

Claims (2)

A positive electrode provided with a positive electrode active material layer, a negative electrode provided with a negative electrode active material layer, and a non-aqueous electrolyte solution, and fibrous ceramics on the surface of at least one of the positive electrode active material layer and the negative electrode active material layer A method for producing a lithium ion battery in which a porous insulating layer containing a filler and a resin binder is formed,
Cutting the fibrous ceramic filler to a desired fiber length using a grinding means;
Mixing at least the pulverized fibrous ceramic filler, a resin binder, and a solvent, and dispersing and slurrying; and
Applying the slurry onto the active material layer of the positive electrode or the negative electrode and drying to form a porous insulating layer;
A method for producing a lithium ion battery, comprising: A positive electrode including a positive electrode active material layer, a negative electrode including a negative electrode active material layer, and a non-aqueous electrolyte solution, and a ceramic filler on at least one surface of the positive electrode active material layer or the negative electrode active material layer A method for producing a lithium ion battery in which a porous insulating layer containing a resin binder is formed,
At least the fibrous ceramic filler, the resin binder, and the solvent are mixed, and the fibrous ceramic filler, the resin binder, and the solvent are simultaneously mixed while pulverizing the fibrous ceramic filler to a desired average fiber length by using a dispersion unit. A step of dispersing the slurry to form a slurry;
Applying the slurry onto the active material layer of the positive electrode or the negative electrode and drying to form a porous insulating layer;
A method for producing a lithium ion battery, comprising:

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