JP4992203B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery. More specifically, the present invention relates to a lithium ion secondary battery that has a separator having a specific structure and is excellent in safety and conductivity.

  A lithium ion secondary battery is usually manufactured by stacking two electrodes (positive electrode and negative electrode) through a separator, winding and folding them into a container, injecting the electrolyte into the container and sealing it. Has been.

  Conventionally, as the separator, a porous film or a nonwoven fabric made of polyolefin such as polyethylene or polypropylene is used. The separator made of a porous film has a function of preventing the electrical contact between the two electrodes and allowing ions in the electrolytic solution to pass therethrough. In addition, when the temperature of the electrochemical element is abnormally increased due to an external short circuit or the like, fine pores formed in the porous film are closed near the melting point of polyolefin as a separator material. Thereby, the passage of ions between the electrodes is blocked, so that it also has a function of interrupting the current and suppressing further temperature rise (hereinafter, this function is abbreviated as “shutdown function”).

  However, in these polyolefin porous membrane separators, when the temperature rises, the pores close and at the same time the entire membrane contracts. For this reason, the separator cannot cover the entire electrode, and a short circuit occurs inside the battery. As a result, the short circuit area may increase, and safety is not sufficient. Moreover, since the strength is insufficient, it is difficult to reduce the film thickness, and the internal resistance of the obtained battery may increase.

It has been proposed to form a porous film by binding organic or inorganic solid particles with a binder to form a separator (Patent Documents 1 and 2). However, even when these separators are used, there is a case where a short circuit cannot be completely prevented or a current cannot be interrupted.
JP 2000-149906 A Japanese Patent Laid-Open No. 7-220759

  In view of such circumstances, an object of the present invention is to provide a lithium secondary battery that is excellent in safety and conductivity and has a shutdown function.

A first aspect of the present invention is a lithium ion secondary battery having a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the separator is formed by binding organic solid particles having a melting point with a binder (A layer) ) and inorganic solid particles is composed of three layers which is formed from the bound formed by the layer (B layer) with a binding agent, the a layer and the B layer are alternately laminated, the a layer, one layer of the layer B This is a lithium ion secondary battery having a per-thickness of 1 to 5 μm .

  In the above lithium ion secondary battery, the organic solid particles preferably have a melting point of 100 to 250 ° C.

  The lithium ion battery of the present invention is excellent in high temperature safety and conductivity, and has a shutdown function.

The lithium ion secondary battery of this invention is comprised from a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte.
FIG. 1 schematically shows an embodiment of the separator and electrode in the present invention. In FIG. 1, the separator 30 is sandwiched between the positive electrode 10 and the negative electrode 20. The separator 30 in the present invention includes a layer 32 formed by bonding organic solid particles with a binder (hereinafter, may be abbreviated as “A layer 32”) and a layer 34 formed by bonding inorganic solid particles with a binder. (Hereinafter, it may be abbreviated as “B layer 34”.) In the separator composed of at least three layers, the A layer 32 and the B layer 34 are alternately laminated. .

<A layer 32>
The A layer 32 is a layer made of organic solid particles and a binder. The organic solid particles are organic particles having a melting point and existing as particles in the A layer. In the A layer 32, the organic solid particles are dispersed and held in the binder. As the organic solid particles, a powder made of an organic material having a melting point of 100 to 250 ° C., preferably 100 to 200 ° C., more preferably 100 to 150 ° C. can be used. For example, polystyrene, poly (meth) acrylic acid Particles made of various polymers such as esters, melamine resins, and phenol resins can be used. These polymers may be a modified body, a derivative, a mixture, a copolymer (any form of copolymer such as random, block, graft, or alternating), a crosslinked body, and the like.

  The particle diameter of the organic solid particles is preferably 0.1 to 5 μm from the viewpoint of uniformly dispersing in the separator material and the A layer 32 and improving the smoothness of the surface when forming the A layer. More preferably, it is 0.2-3 micrometers, and it is still more preferable that it is 0.5-2 micrometers. Here, the particle diameter is a median diameter measured using a laser diffraction particle size distribution measuring apparatus.

  As the binder for dispersing and holding the organic solid particles in the A layer 32, a binder that can hold the organic solid particles and stably exists in the battery is used. As the binder, various organic materials and inorganic materials can be used. For example, organic polymers, water glass, and the like can be used. As the form in which the binder holds the organic solid particles, the binder is an amorphous organic material or inorganic material, or organic polymer fine particles having no melting point, and the organic solid particles are bound to each other. Is mentioned. The organic polymer fine particles having no melting point used as a binder preferably have a particle size of 0.02 to 0.2 μm, and more preferably 0.05 to 0.1 μm. In addition, a fluororesin described later is used as a binder, and a form in which the binder forms a network structure and holds organic solid particles by applying a shearing force by kneading or pressing at the time of forming the A layer is also possible. it can.

  Examples of the organic polymer forming the binder include polyolefins obtained by polymerizing olefins such as ethylene, propylene, and styrene, and diolefins such as butadiene and isoprene; methyl poly (meth) acrylate, poly (meth) ethyl acrylate Poly (meth) acrylic acid esters such as polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene, and the like; polyvinyl acetate, polyvinyl alcohol, polyacrylonitrile, and the like can be used. These organic polymers may be a modified body, a derivative, a mixture, a copolymer (any form of copolymer such as random, block, graft, alternating, etc.), a crosslinked body, and the like. .

  The ratio of the organic solid particles in the A layer 32 is preferably 90% by mass or more, more preferably 95 to 99% by mass, based on the mass of the entire A layer 32 (100% by mass).

  The layer thickness of the A layer 32 is preferably 1 to 5 μm, and more preferably 2 to 4 μm. If the A layer 32 is too thick, the separator 30 on which it is laminated becomes too thick, and the internal resistance of the battery increases. On the other hand, if the A layer 32 is too thin, a short circuit may occur.

<B layer 34>
The B layer 34 is a layer made of inorganic solid particles and a binder. The inorganic solid particles are inorganic particles present as particles in the B layer. In the B layer 34, inorganic solid particles are dispersed and held in the binder. As inorganic solid particles, particles such as oxides such as iron oxide, silicon oxide, aluminum oxide, and titanium oxide; nitrides such as aluminum nitride and boron nitride can be used.

  The particle diameter of the inorganic solid particles is preferably 0.05 to 3 μm from the viewpoint of uniformly dispersing in the separator material and the B layer 34 and improving the smoothness of the surface when forming the B layer. More preferably, it is 0.1-1 micrometer. Here, the particle diameter is a median diameter measured using a laser diffraction particle size distribution measuring apparatus.

  As the binder for dispersing and holding the inorganic solid particles in the B layer 34, a binder that can hold the inorganic solid particles and stably exists in the battery is used. As such a binder, the binder described in the A layer 32 can be used.

  The proportion of the inorganic solid particles in the B layer 34 is preferably 90% by mass or more, more preferably 95 to 99% by mass, based on the mass of the entire B layer 34 (100% by mass).

  The layer thickness of the B layer 34 is preferably 1 to 5 μm, and more preferably 2 to 4 μm. If the B layer 34 is too thick, the separator 30 on which it is laminated becomes too thick, and the internal resistance of the battery increases. On the other hand, if the B layer 34 is too thin, a short circuit may occur.

<Layer structure of separator 30>
The layer structure of the separator 30 in the present invention is not particularly limited as long as it is composed of three or more layers formed from the A layer 32 and the B layer 34 and the A layers 32 and the B layers 34 are alternately laminated. Although the layer structure may be four or more layers, the three-layer structure is preferable from the viewpoint that it is easy to reduce the thickness of the separator, the production process is simple, and the productivity is excellent. Examples of the three-layer structure include those having the structure of A layer 32 / B layer 34 / A layer 32 as shown in FIG. 1A, and B layer 34 / A as shown in FIG. What has the structure of the layer 32 / B layer 34 can be mentioned. Since the lithium ion secondary battery of the present invention is excellent in safety and shutdown performance by using such a separator 30, it is not necessary to use a conventional separator such as a porous film or a nonwoven fabric.

  As thickness of separator 30 which laminated A layer 32 and B layer 34, it is preferred that it is 3-20 micrometers, and it is more preferred that it is 5-15 micrometers. If the thickness of the separator 30 is too thick, the internal resistance of the battery becomes large. Conversely, if the separator 30 is too thin, a short circuit may occur.

<Lamination method of separator 30>
The manufacturing method of the separator 30 in this invention is demonstrated below. The separator 30 in the lithium ion secondary battery of the present invention can be manufactured by applying a separator raw material to the positive electrode 10 or the negative electrode 20 and drying it. There are no particular limitations on the separator raw material coating method as long as the separator raw material can be uniformly applied to the positive electrode 10 or the negative electrode 20, for example, a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method. , Methods such as an extrusion method and a brush coating method.

  As a separator raw material for forming the A layer 32, a dispersion liquid in which organic solid particles and a binder are uniformly dispersed in a solvent can be used. Moreover, as a separator raw material for forming the B layer 34, a dispersion liquid in which inorganic solid particles and a binder are uniformly dispersed in a solvent can be used.

  As the solvent used in the separator raw material, either water or an organic solvent can be used. Among these, it is preferable to use a solvent capable of dissolving the binder, and examples of such a solvent include N-methyl-2-pyrrolidone and cyclohexane. Moreover, as a solvent used in the separator raw material for forming the A layer 32, it is necessary to select and use a solvent that dissolves only the binder without dissolving the organic solid particles.

  The separator raw material can be produced by mixing organic solid particles or inorganic solid particles, a binder, and various additives using a mixer. As the mixer, a ball mill, sand mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, Hobart mixer, or the like can be used.

  The separator 30 in the present invention is formed by alternately applying and drying the separator raw material for forming the A layer 32 and the B layer 34 on the positive electrode 10 or the negative electrode 20. For example, first, a separator raw material for forming the B layer 34 on the surface of the negative electrode 20 is applied and dried in such an application amount that the layer thickness of the B layer 34 after drying becomes the above predetermined thickness. And the separator raw material for forming A layer 32 is similarly apply | coated and dried on the formed B layer 34. FIG. Further, on the formed A layer 32, a separator raw material for forming the B layer 34 is applied and dried. Thereby, the separator 30 having a three-layer structure of B layer 34 / A layer 32 / B layer 34 can be formed on the negative electrode 20 surface. The separator in the present invention can also be formed by applying and drying a separator raw material on both the positive electrode 10 and the negative electrode 20. For example, the separator raw material for forming the A layer 32 and the B layer 34 is sequentially applied and dried on the surface of the positive electrode 10, and the separator raw material for forming the A layer 32 on the surface of the negative electrode 20 is applied and dried. By stacking, the separator 30 having a three-layer structure of A layer 32 / B layer 34 / A layer 32 can be formed.

  As a drying method of the applied separator raw material, for example, drying with warm air or low-humidity air, vacuum drying, or drying by irradiation with (far) infrared rays or electron beams can be mentioned.

  When applying the separator raw material in layers, the first applied layer may be completely dried before the next raw material may be applied, or the surface of the previous layer may be dried to apply the next raw material. When ready to be applied, the next raw material is applied to form the separator 30, and then the layers may be dried together.

<Lithium ion secondary battery>
In the lithium ion secondary battery of the present invention, for example, the separator 30 is formed on the negative electrode 20, the positive electrode 10 is overlaid on the separator 30, and this is wound into a predetermined shape, folded, etc. It can be manufactured by injecting an electrolytic solution into a container and sealing it.

  In the present invention, the positive electrode 10 and the negative electrode 20 (hereinafter sometimes collectively referred to as “electrode”) have a structure in which an active material layer composed of an electrode active material and a binder is bound on a current collector. Have. The current collector is not particularly limited as long as the material has conductivity and is electrochemically durable, but from the viewpoint of having heat resistance, for example, iron, copper, aluminum, nickel, stainless steel, Metal materials such as titanium, tantalum, gold and platinum are preferred. Among these, aluminum is particularly preferable for the positive electrode 10 of the nonaqueous electrolyte secondary battery, and copper is particularly preferable for the negative electrode 20. The shape of the current collector is not particularly limited, but a sheet shape having a thickness of about 0.001 to 0.5 mm is preferable.

  Said active material layer can be formed by apply | coating the slurry-form electrode composition which has an electrode active material and a binder on a collector, and drying it. In the electrode composition, the electrode active material is dispersed in water or an organic solvent capable of dissolving the binder. The electrode composition preferably contains a conductivity-imparting agent such as acetylene black, ketjen black, or carbon black. By including a conductivity imparting agent, electrical contact between the electrode active materials can be improved. As a method for applying and drying the electrode composition, a method similar to the method for applying and drying the separator raw material described above can be employed.

Examples of the electrode active material for the positive electrode 10 include lithium-containing composite metal oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiFePO ₄ , LiFeVO ₄ ; TiS ₂ , TiS ₃ , amorphous MoS _{3, and the} like. Transition metal sulfides; transition metal oxides such as Cu ₂ V ₂ O ₃ , amorphous V ₂ O—P ₂ O ₅ , MoO ₃ , V ₂ O ₅ , V ₆ O ₁₃ ; and in these compounds Examples are compounds in which a part of the transition metal is substituted with another metal. Further, a conductive polymer such as polyacetylene or poly-p-phenylene can be used. Iron-based oxides with poor electrical conductivity may be used in place of an electrode active material covered with a carbon material by the presence of a carbon source material during reduction firing.

  Examples of the electrode active material for the negative electrode 20 include carbonaceous materials such as amorphous carbon, graphite, natural graphite, mesocarbon microbeads (MCMB) and pitch-based carbon fibers, and conductive polymers such as polyacene. It is done. In addition, metals such as Si, Sn, Sb, Al, Zn, and W that can be alloyed with lithium are also included. As the electrode active material, a material obtained by attaching a conductivity imparting material to the surface by a mechanical modification method can also be used.

  As the binder, the same binder as described in the separator of the present invention can be used.

  As the non-aqueous electrolyte in the lithium ion secondary battery of the present invention, a liquid or gel non-aqueous electrolyte obtained by dissolving an electrolyte in an organic solvent can be used.

As the electrolyte, also known lithium salt is any conventionally _{available, LiClO 4, LiBF 4, LiPF} 6, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiB 10 Cl 10, LiAlCl 4, LiCl, LiBr, LiB ( C ₂ H ₅ ) ₄ , LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , LiC ₄ F ₉ S ₃ , Li (CF ₃ SO ₂ ) ₂ N, lower fatty acid lithium carboxylate and the like.

  The organic solvent (electrolytic solution solvent) for dissolving these electrolytes is not particularly limited. Specific examples include carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone; trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2 -Ethers such as ethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran; sulfoxides such as dimethyl sulfoxide, and mixed solvents thereof. Among them, carbonates are excellent in chemical, electrochemical and thermal stability. preferable.

  Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited thereto. In addition, the part and% in a present Example are a mass reference | standard unless there is particular notice.

<Preparation of positive electrode>
N-methyl-2-pyrrolidone (hereinafter sometimes referred to as “NMP”) is added to 90 parts of LiCoO _{2 as a} positive electrode active material, 5 parts of acetylene black as a conductivity imparting agent, and 5 parts of polyvinylidene fluoride as a binder. 30 parts were added and kneaded and dispersed for 1 hour with a planetary mixer to obtain a positive electrode composition. This was applied onto an aluminum foil having a thickness of 20 μm using an applicator and dried at 120 ° C. to form an active material layer on the aluminum foil. Then, a part of the active material layer was peeled off, and an aluminum lead was attached to the peeled portion to produce a positive electrode.

<Production of negative electrode>
67 parts of NMP was added to 90 parts of graphite as a negative electrode active material and 10 parts of polyvinylidene fluoride as a binder, and kneaded and dispersed for 1 hour with a planetary mixer to obtain a negative electrode composition. This was applied onto a copper foil having a thickness of 20 μm using an applicator and dried at 120 ° C. to form an active material layer on the copper foil. Then, a part of the active material layer was peeled off, and an aluminum lead was attached to the peeled portion to produce a negative electrode.

<Preparation of separator raw material A>
96 parts of polystyrene / 2-ethylhexyl acrylate copolymer particles (particle size: 1 μm, melting point: 120 ° C.) obtained by dispersion polymerization of styrene and 2-ethylhexyl acrylate and spray drying, 85 parts of NMP in 4 parts of polyvinylidene fluoride Was added, and kneading and dispersion treatment was performed for 1 hour with a planetary mixer to prepare separator raw material A.

<Preparation of separator raw material B>
96 parts of aluminum oxide (AKP-50, manufactured by Sumitomo Chemical Co., Ltd., particle size: 0.3 μm), 85 parts of NMP are added to 4 parts of polyvinylidene fluoride, and kneaded and dispersed for 1 hour with a planetary mixer. Prepared.

<Preparation of separator raw material C>
48 parts of the same polystyrene / 2-ethylhexyl acrylate copolymer particles used for the preparation of the separator raw material A, 48 parts of aluminum oxide (manufactured by Sumitomo Chemical Co., Ltd., AKP-50, particle size: 0.3 μm), polyvinylidene fluoride 4 85 parts of NMP was added to the part, and kneading and dispersion treatment was carried out for 1 hour with a planetary mixer to produce separator raw material C.

  In addition, the particle diameter of the particle | grains in said separator raw material is a median diameter measured using a laser diffraction type particle size distribution measuring apparatus (SALD-2000A; Shimadzu Corp. make).

Example 1
The separator raw material A prepared above was applied onto the negative electrode active material layer prepared above using an applicator with a coating width of 6 cm and a clearance of 250 mm, and dried at 120 ° C. to form an A layer. Thereafter, the separator raw material B was similarly applied onto the A layer using an applicator and dried to form the B layer. Further, the separator raw material A is applied onto the B layer in the same manner using an applicator, dried to form the A layer, and the electrode on which the separator having the three-layer structure of A layer / B layer / A layer is formed. Obtained. The separator raw material A and the separator raw material B are applied so that the dried A layer and B layer are 3 μm.

Then, the positive electrode was overlapped through a separator and wound to prepare a spiral electrode group. Insulating plates were attached to the upper and lower surfaces of the electrode plate group having a substantially cylindrical shape, and inserted into the battery case. Next, an electrolytic solution was poured into the battery case and sealed with a sealing plate to prepare a battery. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / liter in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 2 was used.

(Example 2)
In Example 1, except that the separator material B, the separator material A, and the separator material B were applied in this order on the negative electrode active material layer to form a B-layer / A-layer / B-layer separator. Produced a battery in the same manner as in Example 1.

(Comparative Example 1)
A battery was fabricated in the same manner as in Example 1 except that only the separator raw material A was applied to form a single-layer separator having an A layer.

(Comparative Example 2)
A battery was fabricated in the same manner as in Example 1, except that only the separator raw material B was applied to form a single-layer separator having a B layer.

(Comparative Example 3)
A battery was fabricated in the same manner as in Example 1 except that only the separator raw material C was applied to form a C-layer single-layer separator.

(Comparative Example 4)
Separator raw material B was applied onto the positive electrode active material layer and the negative electrode active material layer using an applicator and dried at 120 ° C. to form a B layer. Thereafter, the B layer side of each of the positive electrode and the negative electrode is superimposed on the non-woven fabric through a non-woven fabric made of Manila hemp (thickness is 80 μm and the weight per 1 m ² is 11 g). A group of plates was formed. Thereafter, a battery was produced in the same manner as in Example 1.

(Evaluation method)
Hereafter, the evaluation method of the battery produced by said Example and comparative example is demonstrated. In addition, "real part impedance" measured 100 each of batteries in the temperature range of 20-300 degreeC in the frequency of 10 KHz using the impedance analyzer. The real part impedance corresponds to the solution resistance through the separator and also corresponds to the internal resistance caused by the separator. Table 1 shows the evaluation results of each battery.

<Shutdown temperature>
The temperature at which the real part impedance value was 10 times or more compared with the value at 20 ° C. was taken as the shutdown temperature.

<High temperature safety>
The high temperature stability of each battery was evaluated according to the following criteria.
◯: The percentage of batteries in which the real part impedance suddenly decreased or became unmeasurable at the time of melting by visually checking the separator is 0 to 5%.
(Triangle | delta): The percentage of the battery whose real part impedance fell rapidly or became unmeasurable at the time of melting | dissolving by visually checking a separator is 6 to 20%.
X: The percentage of the battery in which the real part impedance suddenly decreased or became unmeasurable at the time of melting when the separator was visually confirmed was 21 to 100%.

<Conductivity>
For each 100 parts of separator raw material A, separator raw material B, and separator raw material C, ethylene carbonate (EC) / diethyl carbonate (DEC) (1/1 mass ratio) in which 1M concentration of LiPF ₆ was dissolved as a non-aqueous electrolyte was used. After adding 300 parts and then adding tetrahydrofuran (THF) as a coating solvent, the mixture was heated to 50 ° C. for dissolution to prepare a coating dope having a binder concentration of 12%. The obtained dope was applied on a PET film surface-treated with a release agent in the same manner as in the battery preparation in both the Examples and Comparative Examples, and then dried to prepare a separator film impregnated with an electrolytic solution.

The obtained electrolyte-carrying separator membrane was cut into 20 mmΦ, sandwiched between two SUS electrodes, and the conductivity was calculated from the AC impedance at 10 kHz. The evaluation criteria were as follows.
○: Ionic conductivity is 0.3 S / m or more.
Δ: Ionic conductivity is 0.15 S / m or more and less than 0.3 S / m.
X: The ionic conductivity is less than 0.15 S / m.

  As shown in Table 1, the batteries of the present invention (Example 1 and Example 2) showed good results in all evaluation items. On the other hand, the batteries of Comparative Examples 1 and 3 were inferior in high-temperature safety because the positive electrode and the negative electrode were short-circuited at the portion where the organic solid particles were dissolved at high temperatures. The battery of Comparative Example 2 was unable to exhibit the shutdown function at 100 to 200 ° C., which is a function required for the separator, because the inorganic solid particles forming the separator had a high melting point. The battery of Comparative Example 4 was poor in conductivity because of the large resistance between the electrodes.

  While the present invention has been described in connection with embodiments that are presently the most practical and preferred, the present invention is not limited to the embodiments disclosed herein. However, the present invention can be modified as appropriate without departing from the scope or spirit of the invention that can be read from the claims and the entire specification. Lithium ion secondary batteries with such modifications are also included in the technical scope of the present invention. Must be understood.

It is the schematic diagram which showed one Embodiment of the separator and electrode of this invention. It is the schematic diagram which showed one Embodiment of the separator and electrode of this invention.

Explanation of symbols

10 Positive electrode 20 Negative electrode 30 Separator 32 A layer 34 B layer

Claims (2)

A lithium ion secondary battery having a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte,
The separator is composed of three layers formed of a layer formed by binding organic solid particles having a melting point with a binder (layer A) and a layer formed by binding inorganic solid particles with a binder (layer B), The lithium ion secondary battery in which the A layer and the B layer are alternately stacked, and the thickness of each of the A layer and the B layer is 1 to 5 μm .   The lithium ion secondary battery according to claim 1, wherein the melting point of the organic solid particles is 100 to 250 ° C.

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