JP2014060122A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery that, even when a separator capable of achieving increased battery capacity and including a nonwoven fabric is used, sufficiently suppresses a short circuit, exhibits high heat resistance and is also excellent in output characteristics.SOLUTION: A lithium ion secondary battery comprises: an electrolyte containing a nonaqueous solvent, and a lithium salt; a positive electrode containing, as a positive electrode active material, one or more materials selected from the group consisting of materials capable of absorbing and desorbing lithium ions; a negative electrode containing, as a negative electrode active material, one or more materials selected from the group consisting of metal lithium and materials capable of absorbing and desorbing lithium ions; and a separator. The separator includes at least one layer of a first nonwoven fabric layer containing fibers having a fiber diameter of 4 μm or less, and the electrolyte further contains at least one compound selected from the group consisting of a carbonic acid ester having a carbon-carbon double bond, a cyclic carbonate having a fluorine atom, and sulfone.

Description

  The present invention relates to a lithium ion secondary battery.

  With the recent development of electronic technology and increasing interest in environmental technology, various electrochemical devices are used. In particular, there are many requests for energy saving, and expectations for what can contribute to it are increasing. For example, a solar cell is mentioned as a power generation device, and a secondary battery, a capacitor, a capacitor, etc. are mentioned as an electrical storage device. Lithium ion secondary batteries, which are representative examples of power storage devices, have been conventionally used mainly as rechargeable batteries for portable devices, but in recent years, they are also expected to be used as batteries for hybrid vehicles and electric vehicles.

A lithium ion secondary battery generally has a configuration in which a positive electrode and a negative electrode mainly composed of an active material capable of inserting and extracting lithium are arranged via a separator. The positive electrode of the lithium ion secondary battery includes LiCoO ₂ , LiNiO ₂ or LiMn ₂ O ₄ as a positive electrode active material, carbon black or graphite as a conductive agent, and polyvinylidene fluoride, latex or rubber as a binder. Is formed by covering a positive electrode current collector made of aluminum or the like. The negative electrode is formed by coating a negative electrode mixture made of copper or the like with a negative electrode mixture in which coke or graphite as a negative electrode active material and polyvinylidene fluoride, latex or rubber as a binder are mixed. The The separator is formed by a microporous membrane made of a synthetic resin such as porous polyolefin, and its thickness is very thin, from several μm to several hundred μm. The positive electrode, the negative electrode, and the separator are impregnated with an electrolytic solution in the battery. Examples of the electrolytic solution include an electrolytic solution in which a lithium salt such as LiPF ₆ or LiBF ₄ is dissolved in an aprotic solvent such as propylene carbonate or ethylene carbonate, or a polymer such as polyethylene oxide.

  Currently, lithium ion secondary batteries are mainly used as rechargeable batteries for portable devices (see, for example, Patent Document 1). In recent years, it is expected to be widely used as a battery for automobiles such as hybrid cars and electric cars. In order to expand the use of lithium ion secondary batteries, it is necessary to reduce the size and performance of the battery. One approach is to improve the separator. As a separator of a lithium ion secondary battery for portable devices, what is mainly used at present is a synthetic resin microporous membrane. Although the microporous membrane made of synthetic resin is a highly reliable membrane, it can be used as a separator for a lithium ion secondary battery for in-vehicle use, for example, in terms of capacity, current density, heat resistance and cost. There is a need for improvement.

  As an attempt to improve these performances, there is an example using a separator made of nonwoven fabric or paper (for example, see Patent Documents 2 and 3). Nonwoven fabric and paper are promising because they can produce a porous film at a low process cost, that is, a film that can increase the capacity of a battery, and can be formed from a material having high heat resistance (for example, Patent Document 4). reference). Patent Document 4 describes a polyvinylidene fluoride (PVdF) polymer porous membrane based on a polyethylene terephthalate (PET) nonwoven fabric as a preferred form of separator. This type of separator has high safety and heat resistance during overcharge and low cost.

In an ordinary lithium ion secondary battery, from the viewpoint of ion conductivity, a battery in which electrolyte LiPF ₆ is dissolved in a carbonate-based solvent is used as an electrolytic solution. Here, the carbonate-based solvent is generally a mixed solvent of a cyclic carbonate such as ethylene carbonate (EC) and a chain carbonate such as diethyl carbonate (DEC).

  When the separator containing the PET nonwoven fabric as described above and such an electrolytic solution are combined, there is a problem that storage characteristics at high temperatures are not good. In view of this problem, Patent Document 5 proposes a technique for adding vinylene carbonate (VC) to the electrolytic solution.

JP 2009-087648 A JP 2005-159283 A JP 2005-293891 A International Publication No. 01/67536 JP 2003-187867 A

However, although the technique described in Patent Document 5 has an effect of suppressing the decomposition reaction of the separator including the PET nonwoven fabric by adding VC to the electrolytic solution, it has been found that the effect of suppressing the short circuit is insufficient. . In addition, when a separator as described in Patent Documents 2 to 4 is used, a short circuit occurs, dendrite growth is observed, and various unstable charge / discharge behaviors are observed. It became clear that there was a problem in safety and safety. Furthermore, Patent Document 3 also shows a problem that the shape of the PET nonwoven fabric is not retained after the high-temperature storage test due to the reaction between LiPF ₆ and the PET nonwoven fabric at the negative electrode portion not facing the positive electrode.

  Therefore, the present invention has been made in view of the above circumstances, and even when a separator including a nonwoven fabric capable of increasing the capacity of a battery is used, a lithium ion secondary that sufficiently suppresses a short circuit and has excellent output characteristics. An object is to provide a battery.

  In order to achieve the above object, the present inventors have studied various combinations of separators and electrolytes in lithium ion secondary batteries. As a result, it was found that a lithium ion secondary battery using a separator having a specific structure and containing a specific solvent in the electrolyte sufficiently suppresses a short circuit and has excellent output characteristics, thereby completing the present invention. It came.

That is, the present invention is as follows.
[1] An electrolytic solution containing a nonaqueous solvent and a lithium salt, a positive electrode containing at least one material selected from the group consisting of materials capable of inserting and extracting lithium ions as a positive electrode active material, and a negative electrode A lithium ion secondary battery comprising a negative electrode containing a material capable of inserting and extracting lithium ions as an active material and one or more materials selected from the group consisting of metallic lithium, and a separator, The separator includes at least one first nonwoven fabric layer containing fibers having a fiber diameter of 4 μm or less, and the electrolytic solution is composed of a carbonic acid ester having a carbon-carbon double bond, a cyclic carbonate having a fluorine atom, and a sulfone. A lithium ion secondary battery further containing at least one compound selected from the group.
[2] The lithium ion secondary battery, wherein the separator includes a laminated nonwoven fabric composed of two or three or more nonwoven fabric layers having the first nonwoven fabric layer as an outermost layer.
[3] The lithium ion secondary battery, wherein the nonwoven fabric layers constituting the laminated nonwoven fabric are integrated with each other by chemical bonding and / or physical bonding.
[4] The separator has a second nonwoven fabric layer containing thermoplastic resin fibers having a fiber diameter of more than 4 μm and not more than 30 μm, and the second nonwoven fabric layer is formed of two or more first nonwoven fabric layers. The lithium ion secondary battery is sandwiched.
[5] The lithium ion secondary battery, wherein the thermoplastic resin fiber includes a thermoplastic synthetic long fiber.
[6] The lithium ion secondary battery, wherein the separator has a thickness of 10 to 60 μm.
[7] The lithium ion secondary battery, wherein the first nonwoven fabric layer is formed by a melt blown method.
[8] The lithium ion secondary battery, wherein the nonwoven fabric layer of the separator is calendered.
[9] The lithium ion secondary battery, wherein the carbonic acid ester having a carbon-carbon double bond includes vinylene carbonate.
[10] The lithium ion secondary battery, wherein the cyclic carbonate having a fluorine atom includes monofluoroethylene carbonate.
[11] The lithium ion secondary battery, wherein the separator includes a nonwoven fabric layer containing a nonwoven fabric containing a polyester resin.
[12] The lithium ion secondary battery, wherein the polyester resin includes polyethylene terephthalate.
[13] The lithium ion secondary battery, wherein the separator has a total basis weight of 30 g / m ² or less.
[14] The lithium ion secondary battery described above, wherein the basis weight of the first nonwoven fabric layer is 15 g / m ² or less.
[15] The lithium ion secondary battery, wherein the fibers contained in the first nonwoven fabric layer have a fiber diameter of 0.1 to 1 μm.

  According to the present invention, it is possible to provide a lithium ion secondary battery that sufficiently suppresses a short circuit and has excellent output characteristics even when a separator including a nonwoven fabric capable of increasing the capacity of the battery is used.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail. The lithium ion secondary battery of the present embodiment is one type selected from the group consisting of a specific separator, an electrolytic solution containing a specific compound, and a material capable of inserting and extracting lithium ions as a positive electrode active material. A positive electrode containing the above materials, and a negative electrode containing one or more materials selected from the group consisting of a material capable of inserting and extracting lithium ions as a negative electrode active material and metallic lithium.

<Separator>
The lithium ion secondary battery of the present embodiment includes a separator between the positive electrode and the negative electrode from the viewpoint of providing safety such as prevention of short circuit between the positive and negative electrodes and shutdown. As the separator, an insulating thin film having high ion permeability and excellent mechanical strength is preferable.

  The separator includes at least one first non-woven fabric layer (hereinafter also referred to as “non-woven fabric layer (I)”) containing fibers having a fiber diameter of 4 μm or less. That is, in the separator, the nonwoven fabric layer (I) may be used as a single layer, or two or more layers may be laminated directly or indirectly. The separator may have a configuration in which the nonwoven fabric layer (I) is laminated with other fiber layers. However, in order to suppress a short circuit more effectively and reliably and to obtain a high-power lithium ion secondary battery, from the viewpoint of preventing the diffusion of lithium ions by fibers having a large fiber diameter as much as possible, the separator is a non-woven fabric layer ( It is preferable that it is a laminate comprising two or three or more fiber layers having I) as the outermost layer, and a laminated nonwoven fabric comprising two or three or more nonwoven layers having the nonwoven fabric layer (I) as the outermost layer. More preferably, it is a laminated nonwoven fabric composed of two or three or more nonwoven fabric layers having the nonwoven fabric layer (I) as both outermost layers. In this specification, the “outermost layer” means a layer located at the outermost end in the stacking direction. When the nonwoven fabric layer (I) is provided as the outermost layer, better lithium ion diffusibility can be obtained in the vicinity of the electrode.

  If the fiber diameter is 4 μm or less, the size of the gap between the fibers in the nonwoven fabric layer can be prevented from becoming non-uniform or too large, so a denser and more uniform nonwoven fabric layer can be formed. In the lithium ion secondary battery, it is possible to sufficiently prevent a short circuit derived from the large maximum pore diameter of the nonwoven fabric layer. On the other hand, if the fiber diameter is 0.1 μm or more, the fiber can be easily formed, which is preferable. Hereinafter, fibers having a fiber diameter of 4 μm or less are also referred to as “ultrafine fibers”. Note that the uniformity of the nonwoven fabric layer and the separator means that the size of the gap between the fibers constituting them is uniform, and in addition, the thickness, fiber diameter, basis weight, and gap distribution are uniform. It means that.

  The non-woven fabric layer (I) may contain fibers other than the ultrafine fibers as long as the achievement of the object of the present invention is not impaired, but the ultrafine fibers are preferably 50% or more, more preferably 80% on a mass basis. More preferably, it contains 90% or more, particularly preferably only the above-mentioned ultrafine fibers. The fiber diameter of the ultrafine fibers contained in the nonwoven fabric layer (I) is preferably 0.3 to 4 μm, more preferably 0.3 to 3.5 μm, still more preferably 0.5 to 3 μm, Most preferably, it is 0.1-1 micrometer. In particular, when the fiber diameter is 0.1 to 1 μm, the output of the lithium ion secondary battery provided with such fibers in the nonwoven fabric layer (I) can be further increased. In addition, the “fiber diameter” in the present specification is a fiber diameter measured by a microscope, and more specifically, is measured according to the following examples.

  In the present embodiment, the material constituting the non-woven fabric layer (I) may be a thermoplastic resin, for example, a material used for a conventional non-woven fabric such as cellulose fibril, which is not a thermoplastic resin. Also good. Preferably, it is a thermoplastic resin like the below-mentioned nonwoven fabric layer (II). Specific examples of such thermoplastic resins include polyester resins and derivatives thereof, polyamide resins and derivatives thereof, polyoxymethylene ether resins, polyphenylene sulfide (PPS) resins, and polyphenylene oxide (PPO) resins. , Polyketone resins, polyketone resins such as polyetheretherketone (PEEK), and thermoplastic polyimide resins. Examples of the polyester resin include polyethylene terephthalate (PET) resin, polybutylene terephthalate (PBT) resin, and polyethylene naphthalate (PEN) resin. Further, a copolymer mainly composed of these resins (that is, a copolymer containing the largest amount of monomers of these resins as a monomer unit, preferably containing 50% or more) or a mixture (that is, most of these resins on a mass basis). Also preferred are mixtures containing many, preferably 50% by weight or more.

  These materials are used singly or in combination of two or more. In the present specification, the “PET resin” is a concept that always has a condensed structure of terephthalic acid, which is the basic skeleton of PET, and ethylene glycol in all the structural repeating units in addition to the PET resin. Examples of constituent repeating unit constituents other than terephthalic acid and ethylene glycol include dicarboxylic acids or derivatives thereof, oxyacids or derivatives thereof, and diols. Examples of dicarboxylic acids or derivatives thereof include phthalic acid, isophthalic acid, naphthalenedicarboxylic acid, 4,4′-diphenylsulfone dicarboxylic acid, 4,4′-biphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3- Examples thereof include phenylenedioxydiacetic acid and structural isomers thereof, aliphatic dicarboxylic acids such as malonic acid, succinic acid, and adipic acid, and esters of these dicarboxylic acids. As oxyacids or derivatives thereof, p-hydroxybenzoic acid Acids, p-hydroxybenzoic acid esters, and glycolic acid. Examples of the diol include diethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, pentamethylene glycol, hexamethylene glycol, neopentyl glycol and other aliphatic glycols, cyclohexane di Examples include alicyclic glycols such as methanol, aromatic dihydroxy compound derivatives such as bisphenol A and bisphenol S. Similarly, the other “... Resin” is a concept having a structure as a basic skeleton in all the repeating units.

  The material constituting the fibers of the nonwoven fabric layer (I) is preferably a material having a low water absorption rate. Examples of such materials include thermoplastic resins such as PET resins, PPS resins, PPO resins, polyketone resins, and PEEK resins. Moreover, the raw material which comprises the fiber of a nonwoven fabric layer (I) from a viewpoint of the ease of manufacture at the time of manufacturing a fiber and a nonwoven fabric, versatility, and cost is PET-type resin, PPS-type resin, and PEEK-type resin. And preferred.

In the present embodiment, the basis weight of the nonwoven fabric layer (I) is preferably 15 g / m ² or less. If the basis weight of the nonwoven fabric layer is 15 g / m ² or less, not only is it more advantageous to suppress short-circuiting, but even higher output characteristics can be obtained. The basis weight of the nonwoven fabric layer is more preferably 0.5 to 13 g / m ² , and further preferably 1 to 11 g / m ² . In addition, in this specification, a fabric weight is measured based on the method as described in an Example.

  The manufacturing method of the nonwoven fabric layer which concerns on this embodiment is not specifically limited including nonwoven fabric layer (I) and the below-mentioned nonwoven fabric layer (II). However, the method for producing the nonwoven fabric layer (I) is preferably a dry method or a wet method using ultrafine fibers, or an electrospinning and melt-blown method. From the viewpoint that the nonwoven fabric layer (I) can be easily and densely formed, the melt blown method is more preferable.

  The separator according to the present embodiment may include only the nonwoven fabric layer (I). However, from the viewpoint of further increasing the strength, it is preferable that the separator includes two or more fiber layers having the nonwoven fabric layer (I). It is more preferable to include two or three or more nonwoven fabric layers having the layer (I). Such a separator may contain only one layer of the nonwoven fabric layer (I) or two or more layers. Moreover, the separator according to the present embodiment may include a nonwoven fabric layer other than the nonwoven fabric layer (I) as necessary, and as such a nonwoven fabric layer, from the viewpoint of strength, denseness, and uniformity, the fiber diameter Is preferably a second non-woven fabric layer (hereinafter, also referred to as “non-woven fabric layer (II)”) containing thermoplastic resin fibers of more than 4 μm and 30 μm or less. In particular, when the separator includes three or more nonwoven fabric layers having two or more nonwoven fabric layers (I) and other nonwoven fabric layers, a layer sandwiched between two or more nonwoven fabric layers (I) (hereinafter referred to as “intermediate”) It is preferable to have a non-woven fabric layer (II) as “layer”.

  Nonwoven fabric layer (II) contains thermoplastic resin fibers having a fiber diameter of more than 4 μm and not more than 30 μm. If the fiber diameter is 30 μm or less, the fiber diameter is not too large and a more uniform inter-fiber distance can be obtained, so that a denser and more uniform nonwoven fabric layer can be formed. Moreover, if the fiber diameter of the fiber in the nonwoven fabric layer (II) is 30 μm or less, when the nonwoven fabric layer (I) and the nonwoven fabric layer (II) are laminated so as to be in direct contact with each other, The ultrafine fibers in the nonwoven fabric layer (I) are more uniformly disposed between the fibers in the nonwoven fabric layer (II). Thereby, in the laminated nonwoven fabric obtained by laminating those layers, it becomes possible to distribute the ultrafine fibers more uniformly. As a result, a denser and more uniform laminated nonwoven fabric can be formed through a layer of extra fine fibers distributed more uniformly. On the other hand, when the fiber diameter of the fibers in the nonwoven fabric layer (II) exceeds 4 μm, the laminated nonwoven fabric has more sufficient mechanical strength, and the separator can be wound more stably. Further, when the battery element is subsequently formed, the laminated nonwoven fabric in the separator is prevented from being deformed, so that the separator can be formed more stably. As a result, a separator with better performance can be formed. From such a viewpoint, the fiber diameter of the fibers in the nonwoven fabric layer (II) is more than 4 μm and not more than 30 μm, preferably 6 to 25 μm, more preferably 8 to 20 μm.

  The nonwoven fabric layer (II) may contain fibers other than thermoplastic resin fibers having a fiber diameter of more than 4 μm and not more than 30 μm, as long as the achievement of the object of the present invention is not impaired, but the fiber diameter is more than 4 μm and not more than 30 μm. The thermoplastic resin fiber is preferably 50% or more, more preferably 80% or more, still more preferably 90% or more, and particularly preferably only the thermoplastic resin fiber having a fiber diameter of more than 4 μm and 30 μm or less. .

  The thermoplastic resin constituting the fibers of the nonwoven fabric layer (II) can be appropriately selected from the thermoplastic resins described above according to the intended use of the lithium ion secondary battery. As the thermoplastic resin constituting the fibers of the nonwoven fabric layer (II), a crystalline resin having a melting point of 180 ° C. or higher is desirable. If the melting point of the thermoplastic resin is 180 ° C. or higher, a more stable separator can be formed even after a thermal history in each step of manufacturing the battery. The melting point of the thermoplastic resin is preferably 220 ° C. or higher, more preferably 240 ° C. or higher, and further preferably 350 ° C. or higher.

  The “crystalline resin” described in the present specification means a resin having a crystallinity of 10% or more measured with a differential scanning calorimeter (DSC) in a nonwoven fabric state.

  In the nonwoven fabric layer (II) according to this embodiment, specific examples of the crystalline resin having a melting point of 180 ° C. or higher are the same as those in the nonwoven fabric layer (I), polyester-based resins and derivatives thereof, polyamide-based resins, and Examples thereof include polyoxymethylene ether resins, PPS resins, PPO resins, polyketone resins, polyketone resins such as PEEK, and thermoplastic polyimide resins. Examples of the polyester resin include PET resin, PBT resin, and PEN resin. Further, a copolymer mainly composed of these resins (that is, a copolymer containing the largest amount of monomers of these resins as a monomer unit, preferably containing 50% or more) or a mixture (that is, most of these resins on a mass basis). Also preferred are mixtures containing many, preferably 50% by weight or more.

  These thermoplastic resins are used singly or in combination of two or more.

  More preferably, the thermoplastic resin constituting the fibers of the nonwoven fabric layer (II) is a thermoplastic resin having a low water absorption rate as in the nonwoven fabric layer (I). In addition, from the viewpoints of ease of manufacture, versatility, and cost when manufacturing fibers and nonwoven fabrics, the thermoplastic resin constituting the fibers of the nonwoven fabric layer (II) is PET resin, PPS resin, and PEEK resin. Is preferred. The thermoplastic resin constituting the fibers of the nonwoven fabric layer (II) can be appropriately selected according to the purpose of use of the lithium ion secondary battery of the present embodiment.

  In the laminated nonwoven fabric provided with the nonwoven fabric layer (I) and the nonwoven fabric layer (II), the materials constituting the fibers of the nonwoven fabric layer (I) and the nonwoven fabric layer (II) may be the same or different materials. However, in order to form a laminated nonwoven fabric more uniformly, it is preferable that they are the same substance. When the nonwoven fabric layer (I) and the nonwoven fabric layer (II) are formed from fibers of the same material, it is easy to form a nonwoven fabric having a more uniform gap between the fibers. It becomes a more uniform and dense separator.

  In this embodiment, it is desirable that the thermoplastic resin fiber in the nonwoven fabric layer (II) is a thermoplastic synthetic long fiber made of a long fiber of a thermoplastic synthetic resin. Even if the nonwoven fabric containing the thermoplastic synthetic continuous fiber is a micro slit product, it can have more sufficient mechanical strength. In addition, the nonwoven fabric containing the thermoplastic synthetic long fiber is more resistant to abrasion and more resistant to abrasion when slitting and when subjected to external friction or the like. As a result, the battery can be manufactured more stably, and a battery with higher performance can be obtained. As a thermoplastic synthetic long fiber, the long fiber comprised by the below-mentioned crystalline resin is mentioned, for example.

  On the other hand, when short fibers are used as the thermoplastic resin fibers, for example, a crystalline resin and a thermoplastic resin having a melting point lower than the melting point of the crystalline resin can be used in combination. Specifically, a fiber composed of a resin having a certain melting point and one or more fibers composed of a resin having a melting point different from that of the resin may be mixed. Two or more kinds of resins having different melting points may be contained in the fiber.

  The manufacturing method of the nonwoven fabric layer (II) is preferably a spunbond method from the viewpoint of mechanical strength.

  The separator according to this embodiment may further include other fiber layers, preferably a nonwoven fabric layer, in addition to the nonwoven fabric layer (I) and the nonwoven fabric layer (II) described above. However, from the viewpoint of ease of production, versatility, and cost when producing fibers and nonwoven fabrics, the separator of this embodiment is preferably a nonwoven fabric layer in which at least a part of the layer includes a polyester-based resin. More preferably, it is a non-woven fabric layer containing 50% by mass or more of a resin, desirably 80% by mass or more, more desirably 90% by mass, still more desirably 95% by mass or more, and particularly desirably 98% by mass or more. It is more preferable that it is a nonwoven fabric layer. The polyester-based resin preferably contains PET, and contains 50% by mass or more, desirably 80% by mass or more, more desirably 90% by mass, still more desirably 95% by mass or more, and particularly desirably 98% by mass or more. More preferably, PET is even more preferable.

When a separator contains a lamination nonwoven fabric, as the lamination aspect, the following aspect is mentioned, for example.
-Nonwoven fabric layer (I)-Nonwoven fabric layer (II)-Nonwoven fabric layer (I)
Nonwoven fabric layer (I)-Nonwoven fabric layer (II)-Nonwoven fabric layer (I)-Nonwoven fabric layer (II)-Nonwoven fabric layer (I)
Nonwoven fabric layer (I)-Nonwoven fabric layer (II)-Nonwoven fabric layer (II)-Nonwoven fabric layer (I)

In this embodiment, the total basis weight of the laminated nonwoven fabric is preferably 30 g / m ² or less. If the total basis weight of the laminated nonwoven fabric is 30 g / m ² or less, higher output characteristics can be obtained. Further, from the viewpoint of mechanical strength, the total basis weight of the laminated nonwoven fabric is preferably 4 g / m ² or more, more preferably 4 to 25 g / m ² , and still more preferably 5 to 20 g / m ² . .

  The thickness of the separator according to this embodiment is preferably 10 to 60 μm, more preferably 10 to 50 μm, still more preferably 15 to 40 μm, and particularly preferably 20 to 30 μm. The thickness of the separator is preferably 10 μm or more from the viewpoint of mechanical strength and from the viewpoint of separating the positive and negative electrodes and suppressing short circuits. Further, from the viewpoint of increasing the output density as a battery and suppressing the decrease in energy density, the thickness of the separator is preferably 60 μm or less.

  The separator according to the present embodiment is preferably one in which the porosity is controlled to some extent from the viewpoint of sufficiently ensuring ion permeability when used as a separator for a lithium ion secondary battery. For example, the porosity of the nonwoven fabric layer (I) when the separator has only the nonwoven fabric layer (I) as the nonwoven fabric layer, or the porosity of the laminated nonwoven fabric is preferably 55 to 90%, more preferably 60 to 80. %. When this porosity is 55% or more, higher output characteristics can be obtained, and when it is 90% or less, short circuit can be further suppressed. The porosity of the separator can be calculated by measuring the mass of the separator and the apparent volume, and using these measured values and the density of the material constituting the separator.

In the present embodiment, the total basis weight of the separator is preferably 30 g / m ² or less. If the total basis weight of the separator is 30 g / m ² or less, higher output characteristics can be obtained. From the viewpoint of mechanical strength, the total basis weight of the separator is preferably 4 g / m ² or more, more preferably 4 to 25 g / m ² , and further preferably 5 to 20 g / m ² .

  The separator according to the present embodiment has a nonwoven fabric layer (I) containing ultrafine fibers, whereby the distance between the fibers is reduced, that is, the pore diameter is reduced, and the gap between the fibers is more uniform. Easy to form. From such a viewpoint, the average pore diameter of the separator according to this embodiment is preferably 0.3 to 30 μm. The average pore diameter is more preferably 1 to 20 μm.

  As a method of forming a laminated nonwoven fabric by laminating nonwoven fabric layers (I) containing ultrafine fibers, or a nonwoven fabric layer (I) containing ultrafine fibers and another nonwoven fabric layer (for example, nonwoven fabric layer (II)). Are not particularly limited, but a method of integrating them by chemical and / or physical bonding is preferable. Examples of integration by chemical bonding include, for example, a method by chemical cross-linking, and examples of integration by physical bonding include, for example, a method by thermal bonding, a method of injecting a high-speed water flow and three-dimensional entanglement, and particles And a method of integrating with a fibrous or fibrous adhesive. Among these, examples of the integration method by thermal coupling include integration by hot embossing (thermal embossing roll method) and integration by high-temperature hot air (air-through method). Integration by thermal bonding is preferable from the viewpoint that the tensile strength and bending flexibility of the nonwoven fabric can be more effectively maintained, and the heat resistance stability can be more effectively maintained.

  Integration by thermal bonding is also preferable in that a laminated nonwoven fabric having a plurality of nonwoven fabric layers can be formed without using a binder. When a nonwoven fabric layer is integrated to form a laminated nonwoven fabric, if a binder is used, the binder remains in the separator. There is no particular problem as long as the binder does not deteriorate the battery performance. However, when the deterioration of the battery performance is promoted by the binder, a step for removing the binder is newly required. For the above reasons, when laminating a nonwoven fabric layer, a laminated nonwoven fabric that does not use a binder integrated only by heat is preferable.

  In the present embodiment, the nonwoven fabric layer and / or the laminated nonwoven fabric is preferably calendered. Thereby, it is possible to give the nonwoven fabric layer a structure having a more uniform gap between fibers, and the thickness and porosity of the separator can be easily controlled. Specifically, after joining the fibers by normal thermal bonding, for example, calendering is performed at a temperature higher by 10 ° C. or more than the thermal bonding temperature and at a linear pressure of 100 to 1000 N / cm. When the linear pressure in calendering is 100 N / cm or more, more sufficient adhesion can be obtained, and more sufficient strength tends to be expressed. Moreover, it is preferable from the viewpoint that the linear pressure in the calendering is 1000 N / cm or less, the deformation of the fibers is reduced, the fibers are more sufficiently bonded, and the effects of the present invention can be obtained more effectively and effectively. However, the calendering conditions are not limited to the above.

  In the present embodiment, it is also a preferable aspect that the nonwoven fabric is hydrophilized. When the nonwoven fabric is hydrophilized, the electrolyte solution is easily impregnated, so that a higher performance battery can be manufactured. As the hydrophilization processing, for example, physical processing, that is, hydrophilization by corona treatment or plasma treatment, and chemical processing method, that is, introduction of surface functional groups (for example, oxidation treatment, etc., sulfonic acid groups) , Introduction of carboxylic acid groups, etc.), water-soluble polymers (eg, PVA, polystyrene sulfonic acid, and polyglutamic acid) and surfactants (eg, nonionic species, anionic, cationic, and zwitterionic interfaces) And processing with a treating agent such as an activator). Since the hydrophilic processed nonwoven fabric is likely to contain moisture in the future and may cause deterioration of battery characteristics, the amount of the treatment agent and the functional group to be introduced with respect to the processing amount, that is, the mass of the nonwoven fabric. The mass is preferably 3% by mass or less.

<Electrolyte>
The electrolytic solution used in the present embodiment contains a nonaqueous solvent and a lithium salt, and further contains a carbonic acid ester having a carbon-carbon double bond (hereinafter referred to as “C═C bond”) and a fluorine atom. It contains at least one compound selected from the group consisting of cyclic carbonates (hereinafter referred to as “fluorinated cyclic carbonates”) and sulfones.

  Although various things can be used as a non-aqueous solvent, for example, an aprotic solvent is mentioned. When used as an electrolyte solution for a lithium ion secondary battery, an aprotic polar solvent is preferable in order to increase the ionization degree of a lithium salt that is an electrolyte that contributes to the charge and discharge. Specific examples thereof include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, trans-2,3-butylene carbonate, cis-2,3-butylene carbonate, 1,2-pentylene carbonate, and trans-2,3. -Cyclic carbonates typified by pentylene carbonate, cis-2,3-pentylene carbonate, trifluoromethylethylene carbonate, fluoroethylene carbonate and 4,5-difluoroethylene carbonate; typified by γ-butyrolactone and γ-valerolactone Lactones; cyclic ethers typified by tetrahydrofuran and dioxane; methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate Carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, ethyl propyl carbonate, and chain carbonate represented by methyl trifluoroethyl carbonate; nitrile represented by acetonitrile; chain ether represented by dimethyl ether; methyl propionate And a chain ether carbonate compound typified by dimethoxyethane. These are used singly or in combination of two or more.

  The non-aqueous solvent preferably contains one or more cyclic aprotic polar solvents in order to increase the ionization degree of the lithium salt. From the same viewpoint, the non-aqueous solvent more preferably contains one or more cyclic carbonates typified by ethylene carbonate and propylene carbonate.

  An ionic liquid can also be used as the non-aqueous solvent. An ionic liquid is a liquid composed of ions obtained by combining an organic cation and an anion.

  Examples of the organic cation include imidazolium ions such as a dialkylimidazolium cation and a trialkylimidazolium cation, a tetraalkylammonium ion, an alkylpyridinium ion, a dialkylpyrrolidinium ion, and a dialkylpiperidinium ion.

Examples of the anion serving as a counter for these organic cations include PF ₆ anion, PF ₃ (C ₂ F ₅ ) ₃ anion, PF ₃ (CF ₃ ) ₃ anion, BF ₄ anion, and BF ₂ (CF ₃ ) ₂ anion. BF ₃ (CF ₃ ) anion, bisoxalatoborate anion, Tf (trifluoromethanesulfonyl) anion, Nf (nonafluorobutanesulfonyl) anion, bis (fluorosulfonyl) imide anion, bis (trifluoromethanesulfonyl) imide anion, bis (Pentafluoroethanesulfonyl) imide anion and dicyanoamine anion can be used.

  The electrolytic solution according to the present embodiment is a carbonic acid ester having a C═C bond and a fluorine-containing cyclic carbonate for the purpose of suppressing the decomposition of the raw material of the nonwoven fabric, particularly for the purpose of suppressing the decomposition of the polyester resin such as PET. At least one of them. By containing at least one of a carbonic acid ester having a C═C bond and a fluorine-containing cyclic carbonate, a protective film is formed on the negative electrode, and decomposition of the raw material of the nonwoven fabric can be suppressed. Moreover, it can replace with those compounds, or can show | play the same effect even if it uses a sulfone. Hereinafter, these compounds are also simply referred to as “additives”.

  Examples of the carbonic acid ester having a C═C bond include a cyclic carbonic acid ester and a chain carbonic acid ester. Examples of the cyclic carbonate having a C═C bond include unsaturated cyclic carbonates such as vinylene carbonate (VC), and alkenyl groups having 2 to 4 carbon atoms such as vinyl ethylene carbonate and divinyl ethylene carbonate as substituents. The cyclic carbonate which has is mentioned. Among these, vinylene carbonate is desirable from the viewpoint of battery performance.

  Examples of the chain carbonate having a C═C bond include vinyl acetate, vinyl butyrate and vinyl hexanate. Among these, vinyl acetate is desirable from the viewpoint of battery performance.

  The fluorine-containing cyclic carbonate is not particularly limited as long as it is a cyclic carbonate having a fluorine atom in the molecule. For example, monofluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,2,3-trifluoro Fluorine containing 1 to 6 fluorine atoms such as propylene carbonate, 2,3-difluoro-2,3-butylene carbonate, 1,1,1,4,4,4-hexafluoro-2,3-butylene carbonate A cyclic carbonate is mentioned. Among these, the fluorine-containing cyclic carbonate is preferably monofluoroethylene carbonate (FEC) from the viewpoint of viscosity and the solubility of lithium salt.

A sulfone is a compound having a sulfonyl group (—SO ₂ —) bonded to two carbon atoms in the molecule. Specific examples thereof include a sulfonyl group bonded to two alkyl groups such as sulfolane, 2-methylsulfolane, 3-methylsulfolane, dimethylsulfone, diethylsulfone, dipropylsulfone, methylethylsulfone, and methylpropylsulfone. The compound which has is mentioned. Of these, sulfolane is preferred from the viewpoint of battery performance.

  The carbonic acid ester, fluorine-containing cyclic carbonate and sulfone having a C═C bond contained in the electrolytic solution are used singly or in combination of two or more. It is preferable that the content rate of those compounds is 1-30 mass% with respect to the amount of electrolyte solutions in total. When the content of these compounds is 1% by mass or more, it is possible to form a protective film more fully on the negative electrode, and when the content is 30% by mass or less, an increase in film resistance due to the protective film is suppressed. Thus, the charge / discharge characteristics can be further prevented from being deteriorated. From such a viewpoint, the content ratio of these compounds is more preferably 1 to 25% by mass.

Specific examples of the lithium salt used as the electrolyte include, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{2k + 1} [k is an integer of 1 to 8], LiN (SO ₂ C _k F _{2k + 1} ) ₂ [k is an integer of 1 to 8], LiPF _n (C _k F _{2k + 1} ) _6-n [n is an integer of 1 to 5, k is an integer of 1 to 8 ] LiBF _n ((C _k F _{2k + 1} ) _4-n [n is an integer of 1 to 3, k is an integer of 1 to 8], lithium bisoxalyl borate represented by LiB (C ₂ O ₂ ) ₂ And lithium difluorooxalyl borate represented by LiBF ₂ (C ₂ O ₂ ) and lithium trifluorooxalyl phosphate represented by LiPF ₃ (C ₂ O ₂ ).

Moreover, the lithium salt represented by the following general formula (a), (b), or (c) can also be used as an electrolyte.
LiC (SO ₂ R ¹¹ ) (SO ₂ R ¹² ) (SO ₂ R ¹³ ) (a)
LiN (SO ₂ OR ¹⁴ ) (SO ₂ OR ¹⁵ ) (b)
LiN (SO ₂ R ¹⁶ ) (SO ₂ OR ¹⁷ ) (c)
Here, in the formula, R ¹¹ , R ¹² , R ¹³ , R ¹⁴ , R ¹⁵ , R ¹⁶ and R ¹⁷ may be the same or different from each other, and are a C 1-8 perfluoroalkyl group. Indicates.

These electrolytes are used singly or in combination of two or more. Among these electrolytes, LiPF ₆ , LiBF ₄ and LiN (SO ₂ C _k F _{2k + 1} ) ₂ [k is an integer of 1 to 8] are preferable from the viewpoint of battery characteristics and stability.

  The concentration of the electrolyte is arbitrary and is not particularly limited, but the electrolyte is preferably contained in the electrolytic solution at a concentration of 0.1 to 3 mol / L, more preferably 0.5 to 2 mol / L.

  The electrolyte used in this embodiment is particularly excellent in satisfying safety and battery characteristics required for a lithium ion secondary battery, and is preferably used in a lithium ion secondary battery.

<Positive electrode>
In the lithium ion secondary battery of this embodiment, the positive electrode uses a material containing one or more materials selected from the group consisting of materials capable of inserting and extracting lithium ions as the positive electrode active material. Examples of such materials include composite oxides represented by the following general formulas (d) and (e), metal chalcogenides and metal oxides having a tunnel structure and a layered structure, and olivine-type phosphate compounds.
LixMO ₂ (d)
LiyM ₂ O ₄ (e)
Here, in the formula, M represents one or more metals selected from transition metals, x represents a number from 0 to 1, and y represents a number from 0 to 2.

More specifically, for example, lithium cobalt oxide typified by LiCoO ₂ ; lithium manganese oxide typified by LiMnO ₂ , LiMn ₂ O ₄ , Li ₂ Mn ₂ O ₄ ; lithium nickel typified by LiNiO ₂ Oxide; represented by Li _z MO ₂ (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg, and z represents a number greater than 0.9 and less than 1.2). And lithium-containing composite metal oxide; and iron phosphate olivine represented by LiFePO ₄ . Further, as the positive electrode active material, for example, a metal other than lithium represented by S, MnO ₂ , FeO ₂ , FeS ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , TiS ₂ , MoS ₂ and NbSe ₂ is used. Oxides are also exemplified. Furthermore, conductive polymers represented by polyaniline, polythiophene, polyacetylene, and polypyrrole are also exemplified as the positive electrode active material.

  In the lithium ion secondary battery of this embodiment, the positive electrode preferably includes a lithium-containing compound as a positive electrode active material.

Further, it is preferable to use a lithium-containing compound as the positive electrode active material because a high voltage and a high energy density tend to be obtained. As such a lithium-containing compound, any compound containing lithium may be used. For example, a composite oxide containing lithium and a transition metal element, a phosphate compound containing lithium and a transition metal element, and lithium and a transition metal element (For example, LitMuSiO ₄ , M is as defined in the above formula (d), t is a number from 0 to 1, and u is a number from 0 to 2). From the viewpoint of obtaining a higher voltage, lithium, cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), vanadium ( V) and composite oxides containing at least one transition metal element selected from the group consisting of titanium (Ti) and phosphate compounds are preferred.

More specifically, the lithium-containing compound is preferably a metal oxide having lithium, a metal chalcogenide having lithium, and a metal phosphate compound having lithium. For example, the lithium-containing compounds are represented by the following general formulas (f) and (g), respectively. The compound which is made is mentioned.
Li _v M ^I O ₂ (f)
Li _w M ^II PO ₄ (g)
Here, in the formula, M ^I and M ^II each represent one or more transition metal elements, and the values of v and w vary depending on the charge / discharge state of the battery, but usually v is 0.05 to 1.10, w Indicates a number from 0.05 to 1.10.

  The compound represented by the general formula (f) generally has a layered structure, and the compound represented by the general formula (g) generally has an olivine structure. In these compounds, for the purpose of stabilizing the structure, a part of the transition metal element is substituted with Al, Mg, other transition metal elements or included in the crystal grain boundary, a part of the oxygen atom And those substituted with a fluorine atom or the like. Furthermore, what coat | covered the other positive electrode active material to at least one part of the positive electrode active material surface is mentioned.

  A positive electrode active material is used individually by 1 type or in combination of 2 or more types.

  The number average particle diameter (primary particle diameter) of the positive electrode active material is preferably 0.05 to 100 μm, more preferably 1 to 10 μm. The number average particle size of the positive electrode active material can be measured by a wet particle size measuring device (for example, a laser diffraction / scattering particle size distribution meter, a dynamic light scattering particle size distribution meter). Alternatively, 100 particles observed with a transmission electron microscope are randomly extracted and analyzed with image analysis software (for example, image analysis software manufactured by Asahi Kasei Engineering Co., Ltd., trade name “A Image-kun”). It can also be obtained by calculating an average. In this case, when the number average particle diameter differs between measurement methods for the same sample, a calibration curve created for the standard sample may be used.

  The positive electrode is obtained, for example, as follows. That is, first, a positive electrode mixture-containing paste is prepared by dispersing, in a solvent, a positive electrode mixture obtained by adding a conductive additive, a binder, or the like to the positive electrode active material as necessary. Next, this positive electrode mixture-containing paste is applied to a positive electrode current collector and dried to form a positive electrode mixture layer, which is pressurized as necessary to adjust the thickness, thereby producing a positive electrode.

  Here, the solid content concentration in the positive electrode mixture-containing paste is preferably 30 to 80% by mass, and more preferably 40 to 70% by mass.

  The positive electrode current collector is made of, for example, a metal foil such as an aluminum foil or a stainless steel foil.

<Negative electrode>
In the lithium ion secondary battery of the present embodiment, the negative electrode uses one or more materials selected from the group consisting of a material capable of inserting and extracting lithium ions and a metallic lithium as a negative electrode active material. In the lithium ion secondary battery of this embodiment, the negative electrode is one type selected from the group consisting of metallic lithium, a carbon material, a material containing an element capable of forming an alloy with lithium, and a lithium-containing compound as a negative electrode active material. It is preferable to contain the above materials. Examples of such materials include lithium metal, hard carbon, soft carbon, artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, glassy carbon, organic polymer compound fired body, mesocarbon microbeads. Carbon materials represented by carbon fiber, activated carbon, graphite, carbon colloid, and carbon black. Among these, examples of the coke include pitch coke, needle coke, and petroleum coke. Further, the fired body of the organic polymer compound is obtained by firing and polymerizing a polymer material such as a phenol resin or a furan resin at an appropriate temperature. In the present embodiment, a battery employing metallic lithium as the negative electrode active material is also included in the lithium ion secondary battery.

  Further, examples of the material capable of inserting and extracting lithium ions include a material containing an element capable of forming an alloy with lithium. This material may be a single metal or semi-metal, an alloy or a compound, and may have at least a part of one or more of these phases. .

  In the present specification, the “alloy” includes an alloy having one or more metal elements and one or more metalloid elements in addition to an alloy composed of two or more metal elements. In addition, the alloy may have a nonmetallic element as long as the alloy has metallic properties as a whole. In the alloy structure, a solid solution, a eutectic (eutectic mixture), an intermetallic compound, or two or more of these coexist.

  Examples of such metal elements and metalloid elements include titanium (Ti), tin (Sn), lead (Pb), aluminum, indium (In), silicon (Si), zinc (Zn), and antimony (Sb). Bismuth (Bi), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), hafnium (Hf), zirconium (Zr) and yttrium (Y).

  Among these, metal elements and metalloid elements of Group 4 or 14 in the long-period periodic table are preferable, and titanium, silicon, and tin are particularly preferable.

  As an alloy of tin, for example, as a second constituent element other than tin, silicon, magnesium (Mg), nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium (Ti), germanium, bismuth, Examples thereof include those having one or more elements selected from the group consisting of antimony and chromium (Cr).

  Examples of the silicon alloy include, as the second constituent element other than silicon, tin, magnesium, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. Those having one or more elements selected from the above are listed.

  Examples of the titanium compound, the tin compound, and the silicon compound include those having oxygen (O) or carbon (C), and in addition to titanium, tin, or silicon, the second constituent element described above is included. It may be.

  Moreover, a lithium containing compound is also mentioned as a material which can occlude and release lithium ions. As the lithium-containing compound, the same compounds as those exemplified as the positive electrode material can be used.

  A negative electrode active material is used individually by 1 type or in combination of 2 or more types.

  The number average particle diameter (primary particle diameter) of the negative electrode active material is preferably 0.1 to 100 μm, more preferably 1 to 10 μm. The number average particle size of the negative electrode active material is measured in the same manner as the number average particle size of the positive electrode active material.

  A negative electrode is obtained as follows, for example. That is, first, a negative electrode mixture-containing paste is prepared by dispersing, in a solvent, a negative electrode mixture prepared by adding a conductive additive or a binder to the negative electrode active material, if necessary. Next, the negative electrode mixture-containing paste is applied to a negative electrode current collector and dried to form a negative electrode mixture layer, which is pressurized as necessary to adjust the thickness, thereby producing a negative electrode.

  Here, the solid content concentration in the negative electrode mixture-containing paste is preferably 30 to 80% by mass, and more preferably 40 to 70% by mass.

  The negative electrode current collector is made of, for example, a metal foil such as a copper foil, a nickel foil, or a stainless steel foil.

  Examples of the conductive aid used as necessary in the production of the positive electrode and the negative electrode include graphite, carbon black typified by acetylene black and ketjen black, and carbon fiber. The number average particle size (primary particle size) of the conductive assistant is preferably 0.1 to 100 μm, more preferably 1 to 10 μm, and is measured in the same manner as the number average particle size of the positive electrode active material. Examples of the binder include polyvinylidene fluoride (PVDF), a copolymer containing polyvinylidene fluoride, polytetrafluoroethylene (PTFE), polyacrylic acid, styrene butadiene rubber, and fluorine rubber.

  The lithium ion secondary battery of the present embodiment includes a separator, a positive electrode and a negative electrode that sandwich the separator from both sides, a positive electrode current collector (disposed outside the positive electrode) that sandwiches the laminate, and a negative electrode current collector ( Arranged on the outside of the negative electrode) and a battery exterior housing them. A laminate in which the positive electrode, the separator, and the negative electrode are laminated is impregnated with the above-described electrolyte solution. As each of these members, if the electrolyte solution and the separator are combined as described above, other members can be those provided in a conventional lithium ion secondary battery, for example, those described above. Also good.

<Production method of battery>
The lithium ion secondary battery of the present embodiment may be the same as the conventional one except that it has the above-described configuration. By using the above-described separator, electrolytic solution, positive electrode, and negative electrode, a known method may be used. Produced. For example, a plurality of positive electrodes in which a positive electrode and a negative electrode are wound in a laminated state with a separator interposed therebetween to be formed into a laminate having a wound structure, or they are alternately laminated by bending or laminating a plurality of layers. Or a laminate in which a separator is interposed between the electrode and the negative electrode. Next, the laminated body is accommodated in a battery case (exterior), an electrolytic solution is injected into the case, and the laminated body is immersed in the electrolytic solution and sealed, thereby recharging the lithium ion secondary of the present embodiment. A battery can be fabricated. The shape of the lithium ion secondary battery of the present embodiment is not particularly limited, and for example, a cylindrical shape, an elliptical shape, a rectangular tube shape, a button shape, a coin shape, a flat shape, and a laminated shape are suitably employed.

  Since the electrolytic solution used in the present embodiment can achieve high conductivity, the lithium ion secondary battery including the electrolytic solution and the separator described above has high battery characteristics (for example, charge / discharge characteristics, low temperature operability, high temperature). Durability).

  In the lithium ion secondary battery according to the present embodiment, the discharge capacity maintenance rate when the charge / discharge cycle test at 25 ° C. is performed 100 cycles is preferably 80% or more, and more preferably 85% or more. More preferably, it is 90% or more. In the present embodiment, the charge / discharge cycle test indicates a case where charge / discharge of the produced battery is performed under 1C conditions. When the battery is charged and discharged once, it is counted as one cycle, and the discharge capacity maintenance rate is calculated with the discharge capacity at the second cycle as 100%.

  According to the present embodiment, even when using a separator including a non-woven fabric capable of increasing the capacity of the battery, short-circuiting is sufficiently suppressed to exhibit stable charge / discharge behavior, high heat resistance, output characteristics, and cycle characteristics. In addition, an excellent lithium ion secondary battery can be provided.

  As mentioned above, although the form for implementing this invention was demonstrated, this invention is not limited to the said embodiment. The present invention can be variously modified without departing from the gist thereof.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples. In addition, the measuring method and evaluation method of various physical properties and characteristics are as follows. Unless otherwise specified, in the nonwoven fabric, the length direction means the MD direction (machine direction), and the width direction means a direction perpendicular to the length direction.

(1) Weight per unit (g / m ² )
In accordance with the method prescribed in JIS L-1913, in a 1 m × 1 m area of a nonwoven fabric and a separator (laminated nonwoven fabric), a test piece of 20 cm in length (length direction) × 25 cm in width (width direction) A total of 9 locations were collected per 1 m × 1 m area, 3 locations per meter and 3 locations per meter in the length direction. The masses of these test pieces were measured, and the average value was converted into the mass per unit area to determine the basis weight.

(2) Thickness (mm)
The thickness of the separator was measured using a film thickness meter. As a film thickness meter, a Mitutoyo digimatic indicator (trade name) was used to measure the thickness of any three points in the separator, and the average value was taken as the thickness of the separator.

(3) Fiber diameter (μm)
Except for 10 cm of each end (nonwoven fabric) of the sample (nonwoven fabric), 1 cm square test pieces were cut out from the area of every 20 cm width of the sample. For each test piece, 30 fiber diameters were measured with a microscope, and the average value of the measured values (rounded to the first decimal place in μm) was calculated and used as the fiber diameter of the fibers contained in the sample.

(4) Porosity The porosity (%) was calculated from the basis weight (total basis weight) of the separator measured in (1) and the thickness of the separator measured in (2) by the following formula.
Porosity = [1− (Total weight / Thickness / Material density of separator)] × 100

(5) Opening hole diameter distribution (average flow hole diameter and maximum hole diameter)
A palm porometer (trade name, model: CFP-1200AEX) manufactured by PMI was used. For the measurement, Sylwick (trade name) manufactured by PMI was used as the immersion liquid, and the sample was immersed in the immersion liquid and sufficiently deaerated before measurement. The above measurement device uses a filter as a sample, immerses the filter in a liquid with a known surface tension in advance, applies pressure to the filter from a state in which all pores of the filter are covered with a liquid film, and the pressure at which the liquid film is destroyed And the pore diameter calculated from the surface tension of the liquid. The following formula is used for the calculation.
d = C · r / P
(Where d (unit: μm) is the pore size of the filter, r (unit: N / m) is the surface tension of the liquid, P (unit: Pa) is the pressure at which the liquid film of that pore size is broken, and C is a constant .)

  From the above formula, the flow rate (leakage flow rate) when the pressure P applied to the filter immersed in the liquid is continuously changed from low pressure to high pressure is measured. At the initial pressure, the flow rate is zero because the liquid film with the largest pores is not broken. As the pressure is increased, the liquid film with the largest pores is destroyed and a flow rate is generated (bubble point). As the pressure is further increased, the flow rate increases with each pressure. The flow rate at the pressure when the liquid film with the smallest pores is destroyed matches the flow rate of the dry upper body (dry flow rate).

In the measurement method using the measuring device, a value obtained by dividing the leakage flow rate at a certain pressure by the dry flow rate at the same pressure is referred to as a cumulative filter flow rate (unit:%). The pore size of the liquid film that is broken at a pressure at which the cumulative filter flow rate is 50% is referred to as the average flow pore size. In the present specification, a separator was used as the filter, and the average flow pore size was defined as the average pore size of the separator in the present specification.
The maximum pore size of the separator was the pore size of the liquid film that was destroyed at a pressure where the cumulative filter flow rate was −2σ, ie, the cumulative filter flow rate was 2.3%, using the separator as the filter.
Three points were measured for each sample by the above measurement method, and the average flow pore size and the maximum pore size were calculated as average values.

(6) Charging and discharging capacity measurement of lithium ion secondary battery The charging capacity and discharging capacity at a specific charging current and discharging current were measured as follows to evaluate the charging and discharging characteristics of the lithium ion secondary battery.
As a lithium ion secondary battery for measurement, a small battery with 1C = 3 mA was prepared and used. The charge and discharge capacity of the lithium ion secondary battery was measured using a charge / discharge device ACD-01 (trade name) manufactured by Asuka Electronics Co., Ltd. and a thermostat PLM-63S (trade name) manufactured by Futaba Kagaku Co., Ltd.
The battery was charged at a constant current of 1 mA, and after reaching 4.2 V, charging was performed for a total of 8 hours by controlling the current value so as to hold 4.2 V. Thereafter, after 10 minutes of rest, the battery was discharged to 3.0 V at 1 mA.
Subsequently, charging was performed at a constant current of 3 mA, and after reaching 4.2 V, charging was performed for a total of 3 hours by controlling the current value so as to hold 4.2 V. After 10 minutes of rest, the battery was discharged at 3 mA to 3.0 V, and the discharge capacity at that time was 1 C discharge capacity (mAh).
The battery ambient temperature at this time was set to 25 ° C. Moreover, the charge / discharge efficiency was computed from the said measurement by the following formula. If this charge / discharge efficiency was 80% or more, it was judged that there was no short circuit, and if it was less than 80%, it was judged that there was a short circuit.
Charging / discharging efficiency (%) = (capacity during discharging / capacity during charging) × 100

(7) Output performance measurement (rate characteristics)
Next, after charging at a constant current of 3 mA and reaching 4.2 V, charging was performed at a constant voltage of 4.2 V for a total of 3 hours. After 10 minutes of rest, the battery was discharged at 30 mA (10 C) to a battery voltage of 3.0 V, and the discharge capacity at that time was 10 C discharge capacity (mAh).
The ratio of 10 C discharge capacity to 1 C discharge capacity was calculated, and this value was used as the rate characteristic.
Rate characteristics (%) = 10C discharge capacity / 1C discharge capacity × 100
The battery ambient temperature at this time was set to 25 ° C.

(8) Capacity maintenance rate measurement of lithium ion secondary battery (cycle characteristics)
The capacity retention rate was measured using a charge / discharge device ACD-01 (trade name) manufactured by Asuka Electronics Co., Ltd. and a thermostat PLM-63S (trade name) manufactured by Futaba Kagaku Co., Ltd. As a lithium ion secondary battery for measurement, a battery produced in the same manner as in the above “(6) Measurement of charge and discharge capacity of lithium ion secondary battery” was used. In the charge / discharge cycle test, as the first cycle, the battery was charged at a constant current of 3 mA, reached 4.2 V, and then charged at a constant voltage of 4.2 V for a total of 3 hours. Then, after 10 minutes of rest, the battery was discharged at a constant current of 1 mA, and when it reached 3.0 V, it was rested again for 10 minutes. Subsequently, after the second cycle, the battery was charged with a constant current of 3 mA, and after reaching 4.2 V, the battery was charged with a constant voltage of 4.2 V for a total of 3 hours. Thereafter, after 10 minutes of rest, the battery was discharged at a constant current of 3 mA, and when it reached 3.0 V, it was rested again for 10 minutes. Charging and discharging were performed once each as one cycle, and 100 cycles of charging and discharging were performed. The ratio of the discharge capacity at the 100th cycle when the discharge capacity at the second cycle was 100% was taken as the capacity retention rate. The ambient temperature of the battery was set to 25 ° C.

(9) High temperature durability test of lithium ion secondary battery (high temperature cycle characteristics)
“(8) Lithium ion secondary, except that part of the positive electrode active material was peeled off to expose the Al current collector and the ambient temperature of the battery was set to 50 ° C. so that 1C = 1.8 mA. The charge / discharge cycle test was conducted up to 100 cycles in the same manner as in “Battery capacity maintenance rate measurement (cycle characteristics)”, and the capacity maintenance rate at high temperature was measured.

(10) Confirmation of separator state According to the above "(8) Capacity maintenance rate measurement of lithium ion secondary battery (cycle characteristics)" or "(9) High temperature durability test of lithium ion secondary battery (high temperature cycle characteristics)" After the test, the lithium ion secondary battery was disassembled and the state of the separator was visually confirmed. The case where no break was observed in the separator was evaluated as “no abnormality”, and the case where a break was observed in the portion of the separator facing the positive electrode current collector was evaluated as “break”.

Example 1
<Preparation of separator>
A separator was produced by the following method.
First, a nonwoven fabric layer of thermoplastic synthetic long fibers was produced by the following method. Specifically, using a general-purpose PET solution, the filament group was extruded toward the moving collection net surface by a spunbond method at a spinning temperature of 300 ° C. and spun at a spinning speed of 4500 m / min. Next, the filament group was sufficiently opened by charging about 3 μC / g by corona charging, and a thermoplastic synthetic long fiber web was formed on the collection net. The fiber diameter was adjusted by changing the traction conditions to obtain a nonwoven fabric of fibers (hereinafter simply referred to as “spunbond nonwoven fabric”) by a spunbond method having a fiber diameter of 12 μm.

Next, a nonwoven fabric containing ultrafine fibers was produced by the following method. Using a general-purpose PET solution, spinning was performed by a melt blown method under conditions of a spinning temperature of 300 ° C. and heated air of 1000 Nm ³ / hr / m, and sprayed onto the spunbonded nonwoven fabric. At this time, the distance from the meltblown nozzle to the spunbonded nonwoven fabric was set to 100 mm, the suction force at the collecting surface immediately below the meltblown nozzle was set to 0.2 kPa, and the wind speed was set to 7 m / sec. The fiber diameter is adjusted by controlling the amount of heated air, and on the spunbonded nonwoven fabric, a nonwoven fabric of fibers by a melt blown method with a fiber diameter of 0.45 μm (hereinafter simply referred to as “meltblown nonwoven fabric”). Got.

A part of the latter non-woven fabric was peeled off from the laminate of the spunbonded non-woven fabric and the meltblown non-woven fabric. Two peeled meltblown nonwoven fabrics and one spunbond nonwoven fabric prepared separately in the same manner as above were laminated so that the two meltblown nonwoven fabrics were sandwiched, and the basis weight of the separator was 11 g / m ^2. The calendering was performed so that the porosity was 63% and the thickness was 21 μm. Thus, a laminated nonwoven fabric in which a meltblown nonwoven fabric as the nonwoven fabric layer (I) and a spunbond nonwoven fabric as the nonwoven fabric layer (II) were laminated between the outermost layers was obtained.

  The obtained laminated nonwoven fabric was punched into a disk shape having a diameter of 24 mm to obtain a separator, and then the separator was used for battery evaluation. Table 1 shows various physical properties and characteristics of the separator.

<Preparation of positive electrode>
Lithium nickel, manganese and cobalt mixed oxide having a number average particle diameter of 11 μm as a positive electrode active material, graphite carbon powder having a number average particle diameter of 6.5 μm and acetylene black powder having a number average particle diameter of 48 nm as a conductive auxiliary agent, and a binder Polyvinylidene fluoride (PVDF) as a mixed oxide: graphite carbon powder: acetylene black powder: PVDF = 100: 4.2: 1.8: 4.6. N-methyl-2-pyrrolidone was added to the obtained mixture so as to have a solid content of 68% by mass and further mixed to prepare a slurry solution. This slurry-like solution was applied to one side of an aluminum foil having a thickness of 20 μm, and the solvent was dried and removed, followed by rolling with a roll press. The rolled product was punched into a disk shape having a diameter of 16 mm to obtain a positive electrode.

<Production of negative electrode>
Graphite carbon powder (III) having a number average particle diameter of 12.7 μm and graphite carbon powder (IV) having a number average particle diameter of 6.5 μm as a negative electrode active material, and a carboxymethyl cellulose solution (solid content concentration 1.83% by mass) as a binder And diene rubber (glass transition temperature: −5 ° C., number average particle size upon drying: 120 nm, dispersion medium: water, solid content concentration 40% by mass), graphite carbon powder (III): graphite carbon powder ( IV): Carboxymethylcellulose solution: Diene rubber = 90: 10: 1.44: 1.76 The solid content is mixed so that the total solid content is 45% by mass, and the slurry solution is mixed. Prepared. The slurry solution was applied to one side of a 10 μm thick copper foil, and the solvent was removed by drying, followed by rolling with a roll press. The rolled product was punched into a disk shape having a diameter of 16 mm to obtain a negative electrode.

<Preparation of electrolyte>
Vinylene carbonate was added to a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) so as to be 5% by mass with respect to the amount of the electrolytic solution, and LiPF ₆ was added at a concentration of 1.0 mol / L as a solute. The electrolyte solution was prepared by dissolving as follows.

<Battery assembly and evaluation>
The negative electrode, the separator, and the positive electrode were stacked in this order so that the active material surfaces of the positive electrode and the negative electrode were opposed to each other, and stored in a stainless steel container with a lid. The container and the lid were insulated, and the container was accommodated in contact with the negative electrode copper foil and the lid in contact with the positive electrode aluminum foil. The electrolytic solution was poured into the container and sealed to obtain a lithium ion secondary battery.
Regarding the lithium ion secondary battery assembled as described above, “(6) Lithium ion secondary battery charge and discharge capacity measurement”, “(7) Output performance measurement (rate characteristics)”, “(8) Lithium ion” The evaluation described in “Capacity maintenance rate measurement of secondary battery (cycle characteristics)” and “(10) Confirmation of separator state” was performed. The results are shown in Table 1 (the same applies hereinafter).

(Example 2)
A lithium ion secondary battery was obtained in the same manner as in Example 1 except that 5% by mass of vinylene carbonate was changed to 5% by mass of monofluoroethylene carbonate with respect to the amount of electrolytic solution. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

(Example 3)
A lithium ion secondary battery was obtained in the same manner as in Example 1 except that 5% by mass of vinylene carbonate with respect to the amount of the electrolyte was changed to 5% by mass of sulfolane with respect to the amount of the electrolyte. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

Example 4
The fiber diameter of the meltblown nonwoven fabric was changed from 0.45 μm to 0.75 μm, and the calendering conditions were changed so that the resulting separator weight was 11 g / m ² , the porosity was 64%, and the thickness was 21 μm. A lithium ion secondary battery was obtained in the same manner as in Example 1 except that. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

(Example 5)
The fiber diameter of the meltblown nonwoven fabric is changed from 0.45 μm to 2.10 μm, and the resulting nonwoven fabric is calendered so that the basis weight of the separator is 11 g / m ² , the porosity is 69%, and the thickness is 27 μm. In the same manner as in Example 1, except that 5% by mass of vinylene carbonate with respect to the amount of the electrolyte was changed to 5% by mass of monofluoroethylene carbonate with respect to the amount of the electrolyte. The next battery was obtained. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

(Example 6)
The fiber diameter of the meltblown nonwoven fabric was changed from 0.75 μm to 0.29 μm, and the separator was replaced with the above laminated nonwoven fabric, and the meltblown nonwoven fabric peeled from the laminate of the spunbond nonwoven fabric and the meltblown nonwoven fabric was calendered. In the same manner as in Example 3, except that the calendering conditions were changed so that the obtained separator was 10 g / m ² , the porosity was 70%, and the thickness was 26 μm. A secondary battery was obtained. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

(Example 7)
The fiber diameter of the meltblown nonwoven fabric was changed from 0.29 μm to 0.57 μm, and the calendering conditions were changed so that the resulting separator weight was 10 g / m ² , the porosity was 58%, and the thickness was 25 μm. A lithium ion secondary battery was obtained in the same manner as in Example 6 except that. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

(Example 8)
Regarding the lithium ion secondary battery obtained in the same manner as in Example 1, “(6) Lithium ion secondary battery charge and discharge capacity measurement”, “(7) Output performance measurement (rate characteristic)”, “(9) The evaluation described in “High temperature durability test of lithium ion secondary battery (high temperature cycle characteristic)” and “(10) Confirmation of separator state” was performed.

Example 9
Regarding the lithium ion secondary battery obtained in the same manner as in Example 7, “(6) Lithium ion secondary battery charge and discharge capacity measurement”, “(7) Output performance measurement (rate characteristic)”, “(9) The evaluation described in “High temperature durability test of lithium ion secondary battery (high temperature cycle characteristic)” and “(10) Confirmation of separator state” was performed.

(Comparative Example 1)
The fiber diameter of the spunbonded nonwoven fabric was changed from 12 μm to 9 μm, and two spunbonded nonwoven fabrics were laminated without producing a meltblown nonwoven fabric. The separator weight per unit area was 10 g / m ² , the thickness was 19 μm, and the porosity A lithium ion secondary battery was obtained in the same manner as in Example 1 except that the laminated nonwoven fabric was obtained by performing calendering so as to be 61%. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

(Comparative Example 2)
The fiber diameter of the spunbonded nonwoven fabric was changed from 12 μm to 9 μm, and three spunbonded nonwoven fabrics were laminated without producing a meltblown nonwoven fabric. The basis weight of the separator was 15 g / m ² , the thickness was 29 μm, and the porosity The laminated nonwoven fabric was obtained by calendering so as to be 61%, and 5% by mass of vinylene carbonate was changed to 5% by mass of monofluoroethylene carbonate with respect to the amount of electrolyte. Except for this, a lithium ion secondary battery was obtained in the same manner as in Example 1. The obtained lithium ion secondary battery was evaluated in the same manner as in Example 1.

(Comparative Example 3)
The fiber diameter of the meltblown nonwoven fabric was changed from 0.45 μm to 1.7 μm, one meltblown nonwoven fabric and two spunbond nonwoven fabrics were laminated so that the two spunbond nonwoven fabrics were sandwiched, A separator is 20 g / m ² , the thickness is 32 μm, the calendering is performed so that the porosity is 53% to obtain a laminated nonwoven fabric, and the vinyl acid carbonate is not added to the electrolytic solution. Similarly, a lithium ion secondary battery was obtained. The lithium ion secondary battery assembled as described above was evaluated in the same manner as in Example 6.

(Comparative Example 4)
A lithium ion secondary battery was obtained in the same manner as in Example 3 except that vinylene carbonate was not added to the electrolytic solution. The lithium ion secondary battery assembled as described above was evaluated in the same manner as in Example 6.

  As can be seen from Table 1, the lithium secondary battery of the present embodiment sufficiently suppressed short-circuiting and exhibited high heat resistance and retained its shape even when a nonwoven fabric separator capable of increasing the battery capacity was used. As it is, stable charge / discharge behavior was exhibited, and the output characteristics and cycle characteristics were also excellent.

  The lithium ion secondary battery of the present invention is expected to be used as a rechargeable battery for automobiles such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles, in addition to portable devices such as mobile phones, portable audio devices, and personal computers.

Claims (15)

A positive electrode containing one or more materials selected from the group consisting of an electrolyte solution containing a nonaqueous solvent and a lithium salt, a material capable of inserting and extracting lithium ions as a positive electrode active material, and a negative electrode active material A lithium ion secondary battery comprising a negative electrode containing a material capable of inserting and extracting lithium ions and one or more materials selected from the group consisting of metallic lithium, and a separator,
The separator includes at least one first nonwoven layer including fibers having a fiber diameter of 4 μm or less,
The electrolyte solution further includes at least one compound selected from the group consisting of a carbonic acid ester having a carbon-carbon double bond, a cyclic carbonate having a fluorine atom, and a sulfone.   The lithium ion secondary battery according to claim 1, wherein the separator includes a laminated nonwoven fabric composed of two or three or more nonwoven fabric layers having the first nonwoven fabric layer as an outermost layer.   The lithium ion secondary battery according to claim 2, wherein the nonwoven fabric layers constituting the laminated nonwoven fabric are integrated with each other by chemical bonds and / or physical bonds.   The separator has a second nonwoven fabric layer containing thermoplastic resin fibers having a fiber diameter of more than 4 μm and not more than 30 μm, and the second nonwoven fabric layer is sandwiched between two or more first nonwoven fabric layers. The lithium ion secondary battery according to any one of claims 1 to 3.   The lithium ion secondary battery according to claim 4, wherein the thermoplastic resin fiber includes a thermoplastic synthetic long fiber.   The lithium ion secondary battery according to claim 1, wherein the separator has a thickness of 10 to 60 μm.   The lithium ion secondary battery according to claim 1, wherein the first nonwoven fabric layer is formed by a melt blown method.   The lithium ion secondary battery according to claim 1, wherein the nonwoven fabric layer included in the separator is calendered.   The lithium ion secondary battery according to claim 1, wherein the carbonic acid ester having a carbon-carbon double bond includes vinylene carbonate.   The lithium ion secondary battery according to claim 1, wherein the cyclic carbonate having a fluorine atom includes monofluoroethylene carbonate.   The said separator is a lithium ion secondary battery of any one of Claims 1-10 containing the nonwoven fabric layer containing the nonwoven fabric containing a polyester-type resin.   The lithium ion secondary battery according to claim 11, wherein the polyester resin includes polyethylene terephthalate. The lithium ion secondary battery according to claim 1, wherein a total basis weight of the separator is 30 g / m ² or less. 14. The lithium ion secondary battery according to claim 1, wherein the basis weight of the first nonwoven fabric layer is 15 g / m ² or less.   The lithium ion secondary battery according to any one of claims 1 to 14, wherein the fiber included in the first nonwoven fabric layer has a fiber diameter of 0.1 to 1 µm.