WO2017014222A1

WO2017014222A1 - Lithium-ion secondary battery

Info

Publication number: WO2017014222A1
Application number: PCT/JP2016/071188
Authority: WO
Inventors: 伊藤　真吾; 学落田; 翔檜山; 葉介中林
Original assignee: 日立化成株式会社
Priority date: 2015-07-21
Filing date: 2016-07-19
Publication date: 2017-01-26
Also published as: JPWO2017014222A1

Abstract

The present invention addresses the problem of providing a lithium-ion secondary battery having excellent high-load discharge characteristics and excellent safety when overcharged. The present invention pertains to a lithium-ion secondary battery that is provided with a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. The separator has: a first layer containing a polyolefin resin; and a second layer containing at least one resin selected from the group consisting of polyolefin resins, polypropylene resins, polyethylene terephthalate resins, polyvinyl alcohol resins, polyacrylonitrile resins, and aramid resins, said second layer having a smaller air permeability than the first layer, and being disposed closer to the positive electrode than the first layer.

Description

Lithium ion secondary battery

The present invention relates to a lithium ion secondary battery.

In recent years, electronic devices have become increasingly portable and cordless, and there is an increasing demand for secondary batteries that are small and lightweight as drive power sources, and that have high capacity and high voltage. In this respect, non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, are highly expected as batteries having a high energy density. On the other hand, as the energy of lithium ion secondary batteries increases, ensuring safety is an issue.

For example, in Patent Document 1, as a lithium ion secondary battery excellent in safety in an overcharged state, a separator disposed between a positive electrode and a negative electrode uses a nonwoven fabric using a fiber containing polyethylene terephthalate, a polyethylene microfiber A lithium ion secondary battery comprising a porous membrane and having a nonwoven fabric disposed on the negative electrode side of the separator is described.

Japanese Patent Laying-Open No. 2005-293891

The inventors have clarified that the lithium ion secondary battery described in Patent Document 1 has room for improvement in high-load discharge characteristics.
An object of the present invention is to provide a lithium ion secondary battery having excellent high-load discharge characteristics and safety in an overcharged state.

Means for solving the above problems include the following embodiments.
<1> A positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,
The separator includes a first layer containing a polyolefin resin, and a second layer containing at least one selected from the group consisting of polyethylene resin, polypropylene resin, polyethylene terephthalate resin, polyvinyl alcohol resin, polyacrylonitrile resin, and aramid resin. Have
The lithium ion secondary battery in which the second layer has a lower air permeability than the first layer and is disposed closer to the positive electrode than the first layer.
<2> The lithium ion secondary battery according to <1>, wherein the second layer has a thermal contraction rate smaller than that of the first layer.
<3> The nonaqueous electrolytic solution includes a cyclic carbonate (α), a chain carbonate (β), and a lithium salt, and a volume ratio (α / β) of α and β is 20/80 to 40/60, The lithium ion secondary battery according to <1> or <2>, wherein the concentration of the lithium salt is 0.8 mol / L to 1.3 mol / L.
<4> The positive electrode includes layered lithium / nickel / manganese / cobalt composite oxide (NMC) as a positive electrode active material, and the negative electrode includes amorphous carbon as a negative electrode active material. The lithium ion secondary battery according to any one of the above.
<5> The lithium ion secondary battery according to any one of <1> to <4>, wherein the battery capacity is 20 Ah or more.

According to the present invention, a lithium ion secondary battery excellent in high load discharge characteristics and safety in an overcharged state is provided.

It is a schematic sectional drawing of the cylindrical lithium ion secondary battery of embodiment which can apply this invention. It is a schematic sectional drawing of the cylindrical lithium ion secondary battery of embodiment which can apply this invention.

Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, the constituent elements (including element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and ranges thereof, and the present invention is not limited thereto.

In this specification, the term “process” includes a process that is independent of other processes and includes the process if the purpose of the process is achieved even if it cannot be clearly distinguished from the other processes. It is.
In the present specification, numerical values indicated by using “to” include numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical range. Good. Further, in the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the values shown in the examples.
In the present specification, the content of each component in the composition is the sum of the plurality of substances present in the composition unless there is a specific indication when there are a plurality of substances corresponding to each component in the composition. It means the content rate of.
In the present specification, the particle diameter of each component in the composition is a mixture of the plurality of types of particles present in the composition unless there is a specific indication when there are a plurality of types of particles corresponding to each component in the composition. Means the value of.
In this specification, the term “layer” refers to the case where the layer is formed only in a part of the region in addition to the case where the layer is formed over the entire region. Is also included.

The lithium ion secondary battery according to the present embodiment includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the separator includes a first polyolefin resin. And a second layer containing at least one selected from the group consisting of a polyethylene resin, a polypropylene resin, a polyethylene terephthalate resin, a polyvinyl alcohol resin, a polyacrylonitrile resin, and an aramid resin, The air permeability is smaller than that of the first layer, and the gas is disposed closer to the positive electrode than the first layer.

As a result of the study, the separator of the lithium ion secondary battery includes a first layer containing a polyolefin resin, and a group consisting of a polyethylene resin, a polypropylene resin, a polyethylene terephthalate resin, a polyvinyl alcohol resin, a polyacrylonitrile resin, and an aramid resin. A second layer containing at least one selected from the second layer, the second layer has a lower air permeability than the first layer, and the second layer is a positive electrode than the first layer. It has been found that it is excellent in high load discharge characteristics and safety in an overcharged state by being arranged at a position close to.

Hereinafter, the positive electrode, the negative electrode, the electrolytic solution, the separator, and other components that are components of the lithium ion secondary battery of the present embodiment will be described.

1. Positive electrode The positive electrode (positive electrode plate) includes a current collector and a positive electrode mixture layer (positive electrode mixture layer) formed on at least one surface thereof. The positive electrode mixture layer is a layer containing a positive electrode active material, a binder, and a conductive material, a thickener, and the like used as necessary.
The positive electrode active material is not particularly limited, and examples thereof include lithium-containing composite metal oxides (including olivine-type lithium salts such as LiFePO ₄ ), chalcogen compounds, and manganese dioxide. A positive electrode active material may be used individually by 1 type, or may use 2 or more types together.

The lithium-containing composite metal oxide is a metal oxide containing lithium and a metal other than lithium. Specific examples of metals other than lithium include Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Mn, Al, Co, Ni And at least one selected from the group consisting of Mg and Mg is preferred. The metal other than lithium contained in the lithium-containing composite metal oxide may be one type or two or more types.
The lithium-containing composite metal oxide is preferably a metal oxide containing lithium and a transition metal. In the metal oxide containing lithium and a transition metal, a part of the transition metal may be substituted with a metal (heterogeneous element) other than the transition metal. Examples of the different element include metals other than lithium listed above that do not correspond to transition metals.

The lithium-containing composite metal _{_{_{_{oxides, Li x CoO 2, Li x}}}} NiO 2, Li x MnO 2, Li x Co y Ni 1-y O 2, Li x Co y M 1-y O z, Li x Ni 1 _{_{_{_{-y M y O z, Li x}}}} Mn 2 O 4, Li x Mn 2-y M y O 4, LiMPO 4, Li 2 MPO 4 F ( in the respective formulas, M is Na, Mg, Sc, Y, Mn And at least one element selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B, x = 0 to 1.2, y = 0 to 0 .9, z = 2.0 to 2.3). Here, x value which shows the molar ratio of lithium increases / decreases by charging / discharging.
Examples of the chalcogen compound include titanium disulfide and molybdenum disulfide.

In the present embodiment, it is preferable that layered lithium-nickel-manganese-cobalt composite oxide (NMC) is included as the positive electrode active material from the viewpoint of increasing capacity and extending the life.
When NMC is included as the positive electrode active material, the content is preferably 50% by mass or more of the whole positive electrode active material, more preferably 70% by mass or more, and further preferably 90% by mass or more. .

The method for forming the positive electrode mixture layer is not particularly limited. For example, it is formed by a dry method or a wet method. In the dry method, a positive electrode active material, a binder, and other materials such as a conductive material and a thickener used as needed are mixed without using a dispersion solvent to form a sheet, which is used as a current collector. Crimp. In the wet method, a positive electrode active material, a binder, and other materials such as a conductive material and a thickener used as necessary are dissolved or dispersed in a dispersion solvent to form a slurry, which is applied to a current collector. ,dry.

The positive electrode active material is generally in the form of particles, and examples of the particle shape include a lump shape, a polyhedron shape, a spherical shape, an elliptical spherical shape, a plate shape, a needle shape, and a column shape. The median diameter d50 of the positive electrode active material particles (in the case where the primary particles aggregate to form secondary particles), the median diameter d50 of the secondary particles can be adjusted within the following range. From the viewpoint of obtaining a desired tap density without the tap density (fillability) of the positive electrode active material being too low, D50 of the positive electrode active material is preferably 1 μm or more, more preferably 3 μm or more, and more preferably 5 μm or more. More preferably. In addition, from the viewpoint of suppressing the deterioration of battery performance due to the long time required for diffusion of lithium ions in the particles, and improving the coating property of the slurry for forming the positive electrode mixture layer, the positive electrode The D50 of the active material is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 15 μm or less. The median diameter D50 is a value when the integration on the small diameter side is 50% in the volume-based particle size distribution obtained by the laser diffraction / scattering method.

The BET specific surface area of the positive electrode active material particles is preferably 0.2 m ² / g or more, more preferably 0.3 m ² / g or more, from the viewpoint of suppressing a decrease in battery performance. More preferably, it is 4 m ² / g or more. Further, from the viewpoint of suppressing a decrease in miscibility with other materials such as a binder and a conductive material, it is preferably 4.0 m ² / g or less, and is 2.5 m ² / g or less. More preferably, it is still more preferably 1.5 m ² / g or less. The BET specific surface area is a specific surface area (area per unit g) determined by the BET method.

When the positive electrode mixture layer includes a conductive material, the conductive material is not particularly limited, and is a metal material such as copper or nickel; graphite such as natural graphite or artificial graphite; graphite black such as acetylene black; needle coke or the like Examples thereof include carbonaceous materials such as amorphous carbon. These conductive materials may be used alone or in combination of two or more.

The binder used for the positive electrode mixture layer is not particularly limited. Specific examples of the binder include resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene) Rubber), fluoropolymer, isoprene rubber, butadiene rubber, ethylene-propylene rubber, and other rubbery polymers; styrene / butadiene / styrene block copolymers or hydrogenated products thereof, EPDM (ethylene / propylene / diene terpolymers) ), Thermoplastic elastomeric polymers such as styrene / ethylene / butadiene / ethylene copolymers, styrene / isoprene / styrene block copolymers or hydrogenated products thereof; syndiotactic-1,2-polybutadiene, poly Soft resin polymers such as vinyl acid, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer; polyvinylidene fluoride (PVdF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene Fluorine polymers such as ethylene copolymers and polytetrafluoroethylene / vinylidene fluoride copolymers; polymer compositions having ion conductivity of alkali metal ions (particularly lithium ions), and the like. A binder may be used individually by 1 type, and may be used in combination of 2 or more type. From the viewpoint of the stability of the positive electrode, it is preferable to use a fluorine-based polymer such as polyvinylidene fluoride (PVdF) or a polytetrafluoroethylene / vinylidene fluoride copolymer.

When the positive electrode mixture layer is formed by a wet method, the dispersion solvent used for preparing the slurry is not particularly limited, and either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water, a mixed solvent of alcohol and water, and the organic solvent includes N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, Diethyltriamine, N, N-dimethylaminopropylamine, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, hexane Etc. In particular, when an aqueous solvent is used, it is preferable to use a thickener. The said dispersion solvent may be used individually by 1 type, or may be used in combination of 2 or more type.

The thickener is not particularly limited, and examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. These may be used alone or in combination of two or more.

The positive electrode mixture layer formed on the current collector is preferably consolidated by a hand press, a roller press or the like in order to improve the packing density of the positive electrode active material.
The density of compacted the positive-electrode mixture layer as described above, the input-output characteristics and in view of further improvement of ^safety, it is preferably 2.4g / cm 3 ~ 2.8g /

cm

3, 2 More preferably, it is from .45 g / cm ³ to 2.7 g / cm ³ .

The material of the positive electrode current collector is not particularly limited. Examples thereof include metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, and carbonaceous materials such as carbon cloth and carbon paper. Of these, metal materials are preferable, and aluminum is more preferable.
The shape of the current collector is not particularly limited, and materials processed into various shapes can be used.
Examples of the shape when using a metal material include a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, an expanded metal, a punch metal, and a foam metal, and the shape when using a carbonaceous material is a carbon plate, A carbon thin film, a carbon cylinder, etc. are mentioned. Among these, it is preferable to use a metal thin film. The thin film may be formed in a mesh shape as necessary.

2. Negative electrode The negative electrode (negative electrode plate) of the present embodiment is composed of a current collector and a negative electrode mixture layer (negative electrode mixture layer) formed on at least one surface thereof. The negative electrode mixture layer is a layer containing a negative electrode active material, a binder, a thickener used as necessary, and the like.

The negative electrode active material preferably contains at least one carbon material selected from the group consisting of graphite and amorphous carbon. Carbon materials are roughly classified into graphite-based carbon materials having a uniform crystal structure and non-graphite-based carbon materials having a disordered crystal structure. From the viewpoint of increasing the capacity of the lithium ion secondary battery, graphite is preferable, and from the viewpoint of safety, amorphous carbon is preferable.

Examples of graphite-based carbon materials include natural graphite and artificial graphite. Non-graphite-based carbon materials include amorphous carbon, which includes graphitizable carbon (sometimes called soft carbon) and non-graphitizable carbon (sometimes called hard carbon). are categorized. Generally, graphitizable carbon is amorphous carbon that tends to become graphite under temperature conditions of 2000 ° C. to 3000 ° C., and non-graphitizable carbon is amorphous that is difficult to become graphite under temperature conditions of 2000 ° C. to 3000 ° C. Carbon.

In the present specification, graphitizable carbon is defined as amorphous carbon having a value of the interplanar spacing d002 of less than 0.36 nm in the C-axis direction obtained by the X-ray wide angle diffraction method. Non-graphitizable carbon is defined as amorphous carbon having a surface spacing d002 in the C-axis direction obtained by an X-ray wide-angle diffraction method of 0.36 nm or more.

It is preferable that the non-graphitizable carbon has a surface spacing d002 value in the C-axis direction obtained by the X-ray wide angle diffraction method of 0.36 nm to 0.40 nm.

The graphitizable carbon has a C-axis direction plane distance d002 value obtained by an X-ray wide angle diffraction method of preferably 0.34 nm or more and less than 0.36 nm, and preferably 0.341 nm to 0.355 nm or less. More preferably, it is 0.342 nm to 0.35 nm or less.

Graphite preferably has a C-axis direction plane distance d002 value of 0.33 nm or more and less than 0.34 nm, more preferably 0.335 nm to 0.337 nm or less, obtained by the X-ray wide angle diffraction method.

Amorphous carbon can be produced, for example, by heat treating petroleum pitch, polyacene, polyparaphenylene, polyfurfuryl alcohol, or the like. Further, by changing the temperature of the heat treatment, it can be made non-graphitizable carbon or easily graphitized carbon. For example, heat treatment at about 500 ° C. to 800 ° C. is suitable for producing non-graphitizable carbon, and heat treatment at about 800 ° C. to 1000 ° C. is suitable for producing graphitizable carbon.

When the carbon material is used as the negative electrode active material, the content ratio is preferably 20% by mass or more, more preferably 50% by mass or more, and still more preferably 70% by mass or more based on the total amount of the negative electrode active material.

The average particle size of the negative electrode active material is preferably 2.0 μm to 50 μm. When the average particle size is 5 μm or more, the specific surface area can be in an appropriate range, the initial charge / discharge efficiency of the lithium ion secondary battery is excellent, and the particles are in good contact with each other and have excellent input characteristics. On the other hand, when the average particle diameter is 30 μm or less, unevenness is hardly generated on the electrode surface, and short circuit of the battery can be suppressed, and the diffusion distance of Li from the particle surface to the inside becomes relatively short, so that the lithium ion secondary battery Input characteristics tend to improve. The average particle size of the negative electrode active material is more preferably 5 μm to 30 μm, and even more preferably 10 μm to 20 μm.

In the present specification, the average particle diameter of the negative electrode active material is measured with a laser diffraction particle size distribution analyzer (for example, SALD-3000J manufactured by Shimadzu Corporation) by dispersing a sample in purified water containing a surfactant. In the volume-based particle size distribution, the value (median diameter (D50)) when the integration from the small diameter side is 50% is used.

The negative electrode active material may include a material other than the carbon material. The material other than the carbon material is not particularly limited. For example, a metal oxide such as tin oxide or silicon oxide, a metal composite oxide, a lithium simple substance, a lithium alloy such as a lithium aluminum alloy, or a material capable of forming an alloy with lithium ( Sn, Si, etc.). These materials may be used alone or in combination of two or more.

The material of the current collector for the negative electrode is not particularly limited, and examples thereof include metal materials such as copper, nickel, stainless steel, and nickel-plated steel. Among these, copper is preferable from the viewpoint of ease of processing and cost.
The shape of the current collector is not particularly limited, and materials processed into various shapes can be used.
Specific examples include metal foil, metal cylinder, metal coil, metal plate, metal thin film, expanded metal, punch metal, and foam metal. Among these, a metal thin film is preferable, and a copper foil is more preferable.

The method for forming the negative electrode mixture layer is not particularly limited. For example, it can be formed by a dry method or a wet method similarly to the positive electrode mixture layer.
The binder contained in the negative electrode mixture layer is not particularly limited. For example, it can select from what was illustrated as a binder used for a positive mix layer.
When the negative electrode mixture layer is formed by a wet method, the dispersion solvent used for preparing the slurry is not particularly limited. For example, it can select from what was illustrated as a dispersion | distribution solvent used for a positive electrode compound material layer.
When the negative electrode mixture layer includes a thickener, the thickener is not particularly limited. For example, it can select from what was illustrated as a thickener used for a positive mix layer.

3. Electrolytic Solution The electrolytic solution contains an electrolyte and a nonaqueous solvent that dissolves the electrolyte, and may contain other components such as additives as necessary.

The electrolyte is not particularly limited as long as it is a lithium salt that can be used as an electrolyte of a non-aqueous electrolyte for a lithium ion secondary battery. Examples of the electrolyte include lithium salts such as the following inorganic lithium salts, fluorine-containing organic lithium salts, and oxalatoborate salts. These lithium salts may be used alone or in combination of two or more.

Examples of the inorganic lithium _{_{_{salt, LiPF 6, LiBF 4, LiAsF}}} 6, inorganic fluoride salts LiSbF ₆ _{_such,} LiClO _{_4,} LiBrO _{_4,} perhalogenate of LiIO ₄ like, LiA
and inorganic chloride salts of LCL _4, and the like.

Examples of the fluorine-containing organic lithium salt include perfluoroalkane sulfonates such as LiCF ₃ SO ₃ ; LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C Perfluoroalkanesulfonylimide salts such as ₄ F ₉ SO ₂ ); LiC (C
Perfluoroalkanesulfonylmethide salts such as F ₃ SO ₂ ) ₃ ; Li [PF ₅ (CF ₂ CF ₂ CF ₃ )], Li [PF ₄ (CF ₂ CF ₂ CF ₃ ) ₂ ], Li [PF ₃ ( CF ₂ CF
₂ CF ₃ ) ₃ ], Li [PF ₅ (CF ₂ CF ₂ CF ₂ CF ₃ )], Li [PF ₄ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₂ ], Li [PF ₃ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₃ ] and other fluoroalkyl fluorophosphates.

Examples of the oxalatoborate salt include lithium bis (oxalato) borate and lithium difluorooxalatoborate.

When comprehensively judging solubility in a non-aqueous solvent, charge / discharge characteristics, output characteristics, cycle characteristics, and the like in the case of a secondary battery, it is preferable to include lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte.

A preferable combination when two or more lithium salts are used as the electrolyte includes a combination of LiPF ₆ and LiBF ₄ . In this case, the proportion of LiBF _{4 in} the total of both is preferably 0.01% by mass to 20% by mass, and more preferably 0.1% by mass to 5% by mass.
Another preferable combination is a combination of an inorganic fluoride salt and a perfluoroalkanesulfonylimide salt. In this case, the proportion of the inorganic fluoride salt in the total of both is preferably 70% by mass to 99% by mass, and more preferably 80% by mass to 98% by mass.
When the electrolyte includes the above-described preferable combination, deterioration of the characteristics of the lithium ion secondary battery due to high temperature storage tends to be effectively suppressed.

The concentration of the electrolyte in the electrolytic solution is not particularly limited. For example, 0.8 mol / L to 1.3 mol / L is preferable, and 0.9 mol / L to 1.2 mol / L is more preferable. When the concentration of the electrolyte in the electrolytic solution is 0.8 mol / L or more, the electric conductivity of the electrolytic solution tends to be sufficiently secured. Moreover, when the concentration of the electrolyte is 1.3 mol / L or less, an increase in the viscosity of the electrolytic solution is suppressed, and a decrease in electrical conductivity tends to be suppressed.

The non-aqueous solvent is not particularly limited. Examples include cyclic carbonates, chain carbonates, chain esters, cyclic ethers, chain ethers, and cyclic sulfones, and these may be used alone or in combination of two or more.

As the cyclic carbonate, an alkylene group constituting the cyclic carbonate preferably has 2 to 6 carbon atoms, and more preferably 2 to 4 carbon atoms. Specific examples include ethylene carbonate, propylene carbonate, butylene carbonate, and the like. Of these, ethylene carbonate and propylene carbonate are preferable.

The chain carbonate is preferably a dialkyl carbonate, more preferably a dialkyl carbonate in which the two alkyl groups each have 1 to 5 carbon atoms, and even more preferably a dialkyl carbonate in which the two alkyl groups each have 1 to 4 carbon atoms. . Dialkyl carbonates include symmetric chain carbonates such as dimethyl carbonate, diethyl carbonate and di-n-propyl carbonate in which two alkyl groups have the same carbon number, and ethyl methyl carbonate and methyl having two different alkyl groups in carbon number. Examples include asymmetric chain carbonates such as -n-propyl carbonate and ethyl-n-propyl carbonate. Of these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferable.
Examples of chain esters include methyl acetate, ethyl acetate, propyl acetate, and methyl propionate. Among them, it is preferable to use methyl acetate from the viewpoint of improving the low temperature characteristics.
Examples of the cyclic ether include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran and the like.
Examples of chain ethers include dimethoxyethane and dimethoxymethane.
Examples of the cyclic sulfone include sulfolane and 3-methylsulfolane.

The non-aqueous solvent preferably contains a cyclic carbonate that is a high dielectric constant solvent and a chain carbonate that is a low viscosity solvent.
When the non-aqueous solvent includes a cyclic carbonate and a chain carbonate, the content is preferably 85% by mass or more and 90% by mass or more based on the total amount of the non-aqueous solvent from the viewpoint of battery characteristics. More preferably, it is more preferably 95% by mass or more.

From the viewpoint of improving battery cycle characteristics and high-temperature storage characteristics (particularly, remaining capacity and high-load discharge capacity after high-temperature storage), the non-aqueous solvent includes cyclic carbonate and chain carbonate, and cyclic carbonate (α). The volume ratio (α / β) of the chain carbonate (β) is more preferably 20/80 to 40/60, more preferably 22/78 to 38/62, and 25/75 to 35 / More preferably, it is 65.

Specific examples of preferred combinations of cyclic carbonate and chain carbonate include ethylene carbonate and dimethyl carbonate, ethylene carbonate and diethyl carbonate, ethylene carbonate and ethyl methyl carbonate, ethylene carbonate and dimethyl carbonate and diethyl carbonate, ethylene carbonate and dimethyl carbonate and ethyl Examples thereof include methyl carbonate, ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

Among these combinations, a combination in which the chain carbonate includes both a symmetric chain carbonate and an asymmetric chain carbonate is preferable. Specific examples include ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, ethylene carbonate, diethyl carbonate and ethyl methyl carbonate, a combination of ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

Cycle characteristics and large current discharge characteristics of lithium ion secondary batteries tend to be further improved by combining cyclic carbonate, symmetric chain carbonate, and asymmetric chain carbonate. Among these, a combination in which the asymmetric chain carbonate is ethyl methyl carbonate is preferable. Further, a combination in which the chain carbonate is a dialkyl carbonate in which the alkyl group has 1 to 2 carbon atoms is preferable.

The nonaqueous solvent may contain an additive from the viewpoint of improving battery characteristics. The additive is not particularly limited. For example, the additive includes at least one selected from the group consisting of nitrogen and sulfur, a heterocyclic compound, a cyclic carboxylic acid ester, a cyclic sulfonic acid ester, a fluorine-containing cyclic carbonate, and other non-inorganic molecules. Examples include compounds having a saturated bond.

Examples of cyclic sulfonate esters include 1,3-propane sultone, 1-methyl-1,3-propane sultone, 3-methyl-1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, 1 , 4-butene sultone and the like. Among these, 1,3-propane sultone and 1,4-butane sultone are preferable from the viewpoint of reducing the DC resistance.

The fluorine-containing cyclic carbonate is not particularly limited, and examples thereof include fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, tetrafluoroethylene carbonate, and trifluoropropylene carbonate. Among these, fluoroethylene carbonate and the like are particularly preferable from the viewpoint of extending the life of the battery.

Other compounds having an unsaturated bond in the molecule include vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene carbonate, methyl vinyl carbonate, ethyl vinyl carbonate, propyl vinyl carbonate, divinyl carbonate, allyl methyl carbonate, allyl ethyl carbonate, allyl propyl. Carbonate compounds such as carbonate, diallyl carbonate, dimethallyl carbonate; vinyl acetate, vinyl propionate, vinyl acrylate, vinyl crotonic acid, vinyl methacrylate, allyl acetate, allyl propionate, methyl acrylate, ethyl acrylate, propyl acrylate , Ester compounds such as methyl methacrylate, ethyl methacrylate, propyl methacrylate; divinyl sulfone, methyl vinyl Sulfone compounds such as sulfone, ethyl vinyl sulfone, propyl vinyl sulfone, diallyl sulfone, allyl methyl sulfone, allyl ethyl sulfone, and allyl propyl sulfone; sulfites such as divinyl sulfite, methyl vinyl sulfite, ethyl vinyl sulfite, and diallyl sulfite Compounds; sulfonates such as vinyl methane sulfonate, vinyl ethane sulfonate, allyl methane sulfonate, allyl ethane sulfonate, methyl vinyl sulfonate, and ethyl vinyl sulfonate; sulfate compounds such as divinyl sulfate, methyl vinyl sulfate, ethyl vinyl sulfate, and diallyl sulfate It is done. Among these, vinylene carbonate, dimethacrylic carbonate, vinyl ethylene carbonate, divinyl ethylene carbonate, vinyl acetate, vinyl propionate, vinyl acrylate, divinyl sulfone, vinyl methanesulfonate, and the like are particularly preferable from the viewpoint of extending the life of the battery.

In addition to the above additives, other additives such as an overcharge inhibitor, a negative electrode film forming agent, a positive electrode protective agent, and a high input / output agent may be used depending on the required function.

4). Separator The separator is disposed between the positive electrode and the negative electrode. In the present embodiment, the separator includes at least one selected from the group consisting of a first layer containing a polyolefin resin, a polyethylene resin, a polypropylene resin, a polyethylene terephthalate resin, a polyvinyl alcohol resin, a polyacrylonitrile resin, and an aramid resin. Including a second layer. The second layer has a lower air permeability than the first layer, and is disposed closer to the positive electrode than the first layer.

The number of first layers and the number of second layers included in the separator are not particularly limited. The separator may include members other than the first layer and the second layer.

The thickness of the separator is not particularly limited. For example, the thickness is preferably 10 μm to 70 μm, more preferably 12 μm to 60 μm, and still more preferably 15 μm to 50 μm. When the thickness of the separator is 70 μm or more, the high load discharge characteristics tend to deteriorate. When the thickness of the separator is 10 μm or less, an internal short circuit due to foreign matter or the like tends to occur.

[First layer]
The first layer may be made of only a polyolefin resin, or may be made of a polyolefin resin and another resin. The content of the polyolefin resin in the total mass of the first layer is preferably 80% by mass or more, more preferably 90% by mass or more, and further preferably 95% by mass or more.

The polyolefin resin contained in the first layer is not particularly limited, and examples thereof include polyethylene and polypropylene. Of these, polyethylene is preferable.

The first layer preferably has an air permeability of 20 s to 800 s, more preferably 40 s to 750 s, and still more preferably 60 s to 700 s. In this specification, the air permeability is a value measured according to the Gurley method (JIS P8117: 2009), and is the time (seconds) for 100 ml of air to pass through a sample area of 645 mm ² .

The thickness of the first layer is not particularly limited. For example, the thickness is preferably 5 μm to 60 μm, more preferably 7 μm to 50 μm, and still more preferably 10 μm to 40 μm. When the thickness of the first layer is 60 μm or more, the high-load discharge characteristics tend to deteriorate. When the thickness of the first layer is 5 μm or less, an internal short circuit due to foreign matter or the like tends to occur.

The method for producing the first layer is not particularly limited, and can be selected from known methods. In some embodiments, the first layer is a porous sheet. In the present specification, the “porous sheet” means a sheet-like object having pores and air permeability.

[Second layer]
Even if the second layer is composed of at least one selected from the group consisting of polyethylene resin, polypropylene resin, polyethylene terephthalate resin, polyvinyl alcohol resin, polyacrylonitrile resin and aramid resin, these resins and other resins are used. It may be. The content of at least one selected from the group consisting of polyethylene resin, polypropylene resin, polyethylene terephthalate resin, polyvinyl alcohol resin, polyacrylonitrile resin, and aramid resin in the total mass of the second layer is 80% by mass or more. Is more preferable, 90 mass% or more is more preferable, and 95 mass% or more is further preferable.

The resin contained in the second layer is preferably at least one selected from the group consisting of polyethylene resin, polypropylene resin, polyethylene terephthalate resin and aramid resin, and at least selected from the group consisting of polyethylene resin and polypropylene resin. One type is more preferable.

The second layer has a lower air permeability than the first layer. If the air permeability of the second layer is smaller than the air permeability of the first layer, when gas is generated in an overcharged state, the gas tends to disperse in the battery container and tends to be superior in safety. . The air permeability of the second layer is preferably 100 s or less, and more preferably 50 s or less.

The second layer preferably has a heat shrinkage at 160 ° C. of 2% or less, and more preferably 1% or less. When the heat shrinkage rate at 160 ° C. of the second layer is 2% or less, the battery temperature rises in the overcharged state, and even when the first layer is largely heat shrunk, the second layer serves as a separator. The short circuit between the positive electrode and the negative electrode can be suppressed by maintaining the shape.

In this specification, the heat shrinkage rate at 160 ° C. is obtained by subjecting the second layer cut to a size of 70 mm (MD) in length and 58.5 mm (TD) in width to a heat treatment of 15 minutes in an oven at 160 ° C. And obtained from the measured value of the length of the second layer before and after the heat treatment as follows.
Thermal shrinkage (%) = (length before heat treatment (TD) −length after heat treatment (TD)) / length before heat treatment × 100

The thickness of the second layer is not particularly limited. The thicker the second layer, the lower the probability of occurrence of a short circuit due to the inclusion of foreign matter or the like. On the other hand, the thinner the second layer, the smaller the distance between the electrodes, and the lower the internal resistance of the battery. Specifically, for example, the thickness is preferably 10 μm to 50 μm, more preferably 12 μm to 40 μm, and still more preferably 15 μm to 35 μm.

From the viewpoint of reducing the heat shrinkage rate of the second layer at 160 ° C., an inorganic substance may be attached to the second layer by coating, impregnation or the like. Examples of inorganic substances include oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate.

The method for producing the second layer is not particularly limited, and can be selected from known methods. In certain embodiments, the second layer is a nonwoven fabric. In the present specification, the “nonwoven fabric” means a sheet-like object formed by intertwining fibers without weaving them.

5). Other components The lithium ion secondary battery may have other components other than a positive electrode, a negative electrode, electrolyte solution, and a separator as needed. For example, a cleavage valve may be provided to suppress an increase in pressure inside the battery. By opening the cleavage valve, it is possible to suppress an increase in pressure inside the battery and to improve safety.

Moreover, you may provide the structure part which discharge | releases inert gas (carbon dioxide etc.) with a temperature rise.
By providing such a component, when the temperature inside the battery rises, the cleavage valve can be opened quickly due to the generation of inert gas, and safety can be improved.

6). Configuration of Lithium Ion Secondary Battery The shape of the lithium ion secondary battery is not particularly limited, and may be any of a cylindrical shape, a square shape, a laminate type, and the like. The battery capacity of the lithium ion secondary battery is not particularly limited. From the viewpoint of battery control, it is preferably 20 Ah or more.

FIG. 1 shows a configuration example of a cylindrical lithium ion secondary battery. In the lithium ion secondary battery 1 of this configuration example, an electrode body 5 in which a strip-like positive electrode 2 and a negative electrode 3 are wound in a spiral shape with a separator 4 interposed therebetween is accommodated in a cylindrical battery container 6. Has a structured. The inside of the battery container 6 is filled with an electrolyte solution (not shown). As the cylindrical lithium ion secondary battery, an 18650 type lithium ion secondary battery is widely used as a consumer lithium ion secondary battery. The outer diameter of the 18650 type lithium ion secondary battery is about 18 mm in diameter and about 65 mm in height.

The laminate type lithium ion secondary battery can be manufactured, for example, as follows. First, a positive electrode and a negative electrode are cut into squares, and tabs are welded to the respective electrodes to produce positive and negative electrode terminals. A laminate in which the positive electrode, the separator, and the negative electrode are laminated in this order is prepared, and in that state, accommodated in an aluminum laminate pack, and the positive and negative electrode terminals are taken out of the aluminum laminate pack and sealed. Next, the nonaqueous electrolyte is poured into the aluminum laminate pack, and the opening of the aluminum laminate pack is sealed. Thereby, a lithium ion secondary battery is obtained.

(Capacity ratio of negative electrode to positive electrode of lithium ion secondary battery)
In the present invention, the capacity ratio between the negative electrode and the positive electrode (negative electrode capacity / positive electrode capacity) is preferably 1 or more and less than 1.4 from the viewpoint of safety and energy density, and is 1.05 to 1.25. Is more preferable.

Here, the negative electrode capacity means [negative electrode discharge capacity], and the positive electrode capacity means [positive charge capacity of positive electrode minus negative electrode or positive electrode, whichever is greater]. [Discharge capacity of negative electrode] is defined as a value calculated by a charge / discharge device when lithium ions inserted into the negative electrode active material are desorbed. The “initial charge capacity of the positive electrode” is defined as that calculated by the charge / discharge device when lithium ions are desorbed from the positive electrode active material.

The capacity ratio between the negative electrode and the positive electrode can be calculated from, for example, “discharge capacity of lithium ion secondary battery / discharge capacity of negative electrode”. The discharge capacity of the lithium ion secondary battery is, for example, 0.4 V, 0.1 to 0.5 C, and a constant current and constant voltage (CCCV) charge with an end time of 2 hours to 5 hours, and then is set to 0. It can be measured under conditions when a constant current (CC) is discharged to 2.7 V at 1 to 0.5 C. The discharge capacity of the negative electrode is obtained by cutting a negative electrode whose discharge capacity of a lithium ion secondary battery is measured into a predetermined area, using lithium metal as a counter electrode, and producing a single electrode cell through a separator impregnated with an electrolyte, Discharge capacity per predetermined area under the conditions of constant current (CCCV) charge at 0V, 0.1C, and final current 0.01C, and then constant current (CC) discharge to 1.5V at 0.1C Can be calculated by converting the total area when this is used as the negative electrode of a lithium ion secondary battery. In this single electrode cell, the direction in which lithium ions are inserted into the negative electrode active material is defined as charging, and the direction in which lithium ions inserted into the negative electrode active material are desorbed is defined as discharging. In the above definition, C means “current value (A) / battery discharge capacity (Ah)”.

Hereinafter, the present embodiment will be described in more detail based on examples. The present invention is not limited to the following examples.

[Production of positive electrode]
The positive electrode was produced as follows. As the positive electrode active material, a layered type lithium / nickel / manganese / cobalt composite oxide (BET specific surface area of 0.4 m ² / g, average particle diameter (D50) of 6.5 μm) was used. In this positive electrode active material, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., trade name: HS-100, average particle size 48 nm (catalog value)) as a conductive material, and N-methyl-2 of polyvinylidene fluoride as a binder -A pyrrolidone (NMP) solution was sequentially added and mixed to obtain a mixture of positive electrode materials. The mass ratio (in terms of solid content) was positive electrode active material: conductive material: binder = 90: 5: 5.

Further, N-methyl-2-pyrrolidone (NMP) as a dispersion solvent was added to the above mixture and kneaded to prepare a slurry. This slurry was applied to both surfaces of a 20 μm thick aluminum foil as a positive electrode current collector so that the thickness was substantially uniform and uniform. After applying the slurry, a drying treatment was performed. Next, a notch was formed in the uncoated portion of the aluminum foil, and the remainder of the notch was used as a lead piece. The width of the lead piece was 10 mm, and the interval between adjacent lead pieces was 20 mm. Then, it compacted with the press to the predetermined density. The density of the positive electrode mixture was 2.5 g / cm ³ . This was cut again to produce a positive electrode having a width of 195 mm.

[Production of negative electrode]
The negative electrode was produced as follows. As the negative electrode active material, graphitizable carbon (d002 is 0.35 nm, average particle diameter (D50) is 17 μm) was used. To this negative electrode active material, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride was added as a binder. The mass ratio (in terms of solid content) of these materials was negative electrode active material: binder = 92: 8. To this was added N-methyl-2-pyrrolidone (NMP) as a dispersion solvent and kneaded to prepare a slurry. This slurry was applied to both sides of a rolled copper foil having a thickness of 10 μm, which is a negative electrode current collector, so that the thickness was substantially uniform and uniform.

After applying the slurry, a drying treatment was performed. Next, a notch was made in the uncoated part of the rolled copper foil, and the remaining part of the notch was used as a lead piece. The width of the lead piece was 10 mm, and the interval between adjacent lead pieces was 20 mm. Then, it compacted with the press to the predetermined density. The negative electrode mixture density was 1.15 g / cm ³ . This was cut again to produce a negative electrode having a width of 196 mm.

[Preparation of electrolyte]
As an electrolytic solution, ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) are mixed so that the volume ratio (EC: DMC: EMC) is 30:40:30, and a mixed solution. Next, lithium hexafluorophosphate (LiPF ₆ ) was dissolved in the mixed solution to a concentration of 1.2 mol / L to prepare an electrolytic solution.

[Production of electrode body]
In Example 1, a positive electrode produced as described above, one nonwoven fabric made of polyethylene and polypropylene fibers having a thickness of 18 μm, and a polyethylene microporous film (hereinafter also referred to as a PE porous sheet) 1 having a thickness of 30 μm. The sheet and the negative electrode prepared above were superposed in this order and wound from the end to prepare a roll-shaped electrode body.

Here, the lead piece of the positive electrode and the lead piece of the negative electrode were arranged so as to be located on the opposite end surfaces of the electrode body, respectively. The lengths of the positive electrode, the negative electrode, and the separator were adjusted so that the diameter of the electrode body was 65 ± 0.1 mm. The air permeability of the PE porous sheet having a thickness of 30 μm and the PE porous sheet having a thickness of 50 μm, which were used for producing the electrode body, was 630 s and 340 s, respectively. The air permeability of a nonwoven fabric made of polyethylene and polypropylene fibers having a thickness of 18 μm was 1 s or less.

In Example 2, the positive electrode prepared above, one nonwoven fabric made of polyethylene and polypropylene fibers having a thickness of 18 μm, one PE porous sheet having a thickness of 20 μm, and the negative electrode prepared above were used. A roll-shaped electrode body was produced in the same manner as in Example 1 except that they were superposed in order.

In Example 3, the positive electrode prepared above, one nonwoven fabric made of polyethylene and polypropylene fibers having a thickness of 21 μm, one PE porous sheet having a thickness of 20 μm, and the negative electrode prepared above were used. A roll-shaped electrode body was produced in the same manner as in Example 1 except that they were superposed in order.

In Comparative Example 1, a roll-shaped electrode was prepared in the same manner as in Example 1 except that the positive electrode prepared above, one PE porous sheet having a thickness of 30 μm, and the negative electrode prepared above were superposed in this order. The body was made.

In Comparative Example 2, a roll-shaped electrode was prepared in the same manner as in Example 1 except that the positive electrode prepared above, one PE porous sheet having a thickness of 50 μm, and the negative electrode prepared above were superposed in this order. The body was made.

In Comparative Example 3, the positive electrode produced above, one PE porous sheet having a thickness of 30 μm, one nonwoven fabric made of polyethylene and polypropylene fibers having a thickness of 18 μm, and the negative electrode produced above were used. A roll-shaped electrode body was produced in the same manner as in Example 1 except that they were superposed in order.

[Production of lithium ion secondary battery]
A lithium ion secondary battery having a structure as shown in FIG. 2 was produced using the electrode body and the electrolytic solution produced above.

As shown in FIG. 2, the lead pieces 9 led out from the positive electrode were deformed, and all of them were gathered near the bottom of the flange 7 on the positive electrode side and brought into contact with each other. The flange portion 7 on the positive electrode side is integrally formed so as to protrude from the periphery of the pole column (positive electrode external terminal 1) substantially on the extension line of the axis of the electrode body 6, and has a bottom portion and a side portion. Thereafter, the lead piece 9 was connected and fixed to the bottom of the flange 7 by ultrasonic welding. Similarly, the lead piece 9 led out from the negative electrode and the bottom of the flange 7 on the negative electrode side were connected and fixed. The negative electrode side flange portion 7 is integrally formed so as to project from the periphery of the pole column (negative electrode external terminal 1 ′) substantially on the extension line of the axis of the electrode body 6, and has a bottom portion and a side portion.

Then, using an adhesive tape, an insulating coating 8 was formed by covering the side of the flange 7 on the positive electrode external terminal 1 side and the side of the flange 7 of the negative electrode external terminal 1 ′. Similarly, an insulating coating 8 was formed on the outer periphery of the electrode body 6. Specifically, the adhesive tape is stretched from the side of the flange 7 on the positive electrode external terminal 1 side to the outer peripheral surface of the electrode body 6, and further from the outer peripheral surface of the electrode body 6 to the negative electrode external terminal 1 ′ side. Insulating coating 8 was formed by winding several times over the side of 7. As an adhesive tape, the adhesive tape which apply | coated the adhesive material which consists of hexamethacrylate to the single side | surface of the polyimide base material was used. The thickness of the insulating coating 8 (the number of windings of the adhesive tape) is adjusted so that the maximum diameter portion of the electrode body 6 is slightly smaller than the inner diameter of the stainless steel battery container 5, and the electrode body 6 is placed in the battery container 5. Inserted. A battery container 5 having an outer diameter of 67 mm and an inner diameter of 66 mm was used.

Next, as shown in FIG. 2, the ceramic washer 3 ′ was fitted into a pole column whose tip constitutes the positive electrode external terminal 1 and a pole column whose tip constitutes the negative electrode external terminal 1 ′. As the ceramic washer 3 ', a ceramic washer having a thickness of 2 mm, an inner diameter of 16 mm, and an outer diameter of 25 mm made of alumina and in contact with the back surface of the battery lid 4 was used. Next, with the ceramic washer 3 placed on the battery lid 4, the positive external terminal 1 is passed through the ceramic washer 3, and with the other ceramic washer 3 placed on the other battery lid 4, the negative external terminal 1 'was passed through another ceramic washer 3. As the ceramic washer 3, a flat plate having a thickness of 2 mm, an inner diameter of 16 mm, and an outer diameter of 28 mm was used.

Thereafter, the peripheral end surface of the battery lid 4 was fitted into the opening of the battery container 5, and the entire area of both contact portions was laser welded. At this time, the positive electrode external terminal 1 and the negative electrode external terminal 1 ′ protruded through the hole (hole) in the center of the battery cover 4 and to the outside of the battery cover 4. The battery lid 4 was provided with a cleavage valve 10 that cleaves in response to an increase in the internal pressure of the battery. The cleavage pressure of the cleavage valve 10 was 13 kg / cm ² to 18 kg / cm ² .

Next, as shown in FIG. 2, the metal washer 11 was fitted into the positive external terminal 1 and the negative external terminal 1 '. Thereby, the metal washer 11 was arranged on the ceramic washer 3. As the metal washer 11, a material made of a material smoother than the bottom surface of the nut 2 was used.

Next, a metal nut 2 is screwed to the positive electrode external terminal 1 and the negative electrode external terminal 1 ′, and the battery lid 4 is connected to the flange portion 7 and the nut 2 via the ceramic washer 3, the metal washer 11, and the ceramic washer 3 ′. And fixed by tightening between. The tightening torque value at this time was 70 kgf · cm. The metal washer 11 was not rotated until the tightening operation was completed. In this state, the power generation element inside the battery container 5 is shielded from the outside air by the compression of the rubber (EPDM) O-ring 12 interposed between the back surface of the battery lid 4 and the flange 7.

Thereafter, an electrolytic solution was injected into the battery container 5 from a liquid injection port 13 provided in the battery lid 4 on the negative electrode side. Thereafter, the liquid injection port 13 was sealed to produce a cylindrical lithium ion secondary battery 20. Next, the following initialization charge / discharge cycle and aging treatment were performed to complete the manufacturing process.

[Initialization charge / discharge cycle]
The initialization charge / discharge cycle was performed in a temperature environment of 25 ° C. The current value was 20 A for both charging and discharging. Charging was constant current constant voltage (CCCV) charging with 4.1 V as the upper limit voltage, and the termination condition was 3 hours. The discharge was a constant current (CC) discharge with 2.7 V as the end condition. Further, a pause of 30 minutes was put between charge and discharge. This was carried out for 3 cycles.

[aging]
After the initialization charge / discharge cycle, the battery voltage was adjusted to 3.9 V and allowed to stand at 25 ° C. for 21 days.

[Evaluation of high load discharge characteristics]
The lithium ion secondary battery after the aging treatment was CC discharged to 2.7 V at 20A. Thereafter, CCCV charging was performed for 3 hours at 20 A with an upper limit voltage of 4.1 V. Thereafter, CC discharge was performed at 8 A to 2.7 V. Similarly, CCCV charging was performed for 3 hours at 20 A and the upper limit voltage was 4.1 V, and then discharged to 2.7 V at 200 A. A 30-minute pause was inserted between charge and discharge. Thereafter, the capacity ratio of the discharge at 200 A to the discharge at 8 A is obtained by the following formula, a value of 90% or more is designated as “A”, a value less than 90% and 85% or more is designated as “B”, and less than 85% Was designated as “C”.
Discharge capacity ratio (%) = discharge capacity at 200 A / 8 discharge capacity at 8 A × 100

[Evaluation of safety in overcharged state]
The test which evaluates the safety | security in an overcharge state was implemented using the lithium ion secondary battery (lithium ion secondary battery discharged to 2.7V by 8A) after initial capacity confirmation.
Specifically, the normal operation range of the lithium ion secondary battery used in this test was set to 2.7 to 4.1 V, and the state of charging up to a voltage exceeding this range was defined as the “overcharged state”. CC charging was performed to 5.16 V at 60 A, and the test apparatus was stopped when 5.16 V was reached. The case where explosion, ignition or smoke did not occur before reaching 5.16 V was designated as “OK”, and the case where explosion, ignition or smoke occurred was designated as “NG”.

From Table 1, Comparative Example 1 and Comparative Example 2 using only one PE porous sheet were excellent in high-load discharge characteristics, but resulted in rupture, ignition or smoke in an overcharged state. Comparative Example 3 in which the porous sheet was disposed on the positive electrode side and the non-woven fabric was disposed on the negative electrode side resulted in inferior high-load discharge characteristics although no rupture, ignition or smoke occurred in the overcharged state. Although discussion of results will be described below, the present embodiment is not limited to these considerations.

From the evaluation results of the high load discharge characteristics of Comparative Example 1 and Comparative Example 2, it can be seen that the high load discharge characteristics are slightly lowered as the thickness of the PE porous sheet is increased. This is probably because the distance between the electrodes is increased.

The reason why the high-load discharge characteristics of Comparative Example 3 are particularly inferior is not clear, but, for example, if a viscosity distribution occurs due to the concentration distribution of the electrolyte during charge and discharge, it is easily affected by the air permeability of the separator. It is possible to become. Nonwoven fabrics have a lower air permeability than porous sheets, so even electrolytes with high viscosity are less affected, but PE (polyethylene) has a higher air permeability and is strongly affected by it. It seems that this has led to a decrease in the capacity ratio.

The reason why the safety evaluation result in the overcharged state is inferior in Comparative Example 1 and Comparative Example 2 is considered as follows. It is known that in an overcharged state, the pressure inside the battery can increases due to decomposition and gasification of the electrolyte. In the case of a non-woven fabric, as described above, since the air permeability is low, the gas escape is good and the generated gas is well dispersed in the battery can, but the PE porous sheet is generated because the air permeability is high. It is considered that the positive electrode and the negative electrode were strongly pressed by the pressure of the gas and the short circuit was easily caused.

As another reason, a difference in heat shrinkage between the PE porous sheet and the nonwoven fabric constituting the separator can be considered. It is known that the battery temperature rises in an overcharged state, and when the temperature rises, the separator undergoes thermal contraction. When the separator used in this test was left in a 120 ° C. environment for 15 minutes, thermal contraction of several percent was observed in the PE porous sheet, but almost no shrinkage was observed in the nonwoven fabric. In Comparative Example 1 and Comparative Example 2 using only one PE porous sheet, it is considered that the separator contracted and smoked due to a short circuit. On the other hand, in Examples 1 to 3 and Comparative Example 3 in which the nonwoven fabric and the PE porous sheet were overlapped, the PE porous sheet caused heat shrinkage, but the nonwoven fabric did not cause heat shrinkage, and the shape of the separator as a whole was Since it was maintained, it was thought that it was not short-circuited and did not cause smoke.

From the above, it was found that the lithium ion secondary battery of the present embodiment is excellent in high-load discharge characteristics and safety in an overcharged state.

Claims

A positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,
The separator includes a first layer containing a polyolefin resin, and a second layer containing at least one selected from the group consisting of polyethylene resin, polypropylene resin, polyethylene terephthalate resin, polyvinyl alcohol resin, polyacrylonitrile resin, and aramid resin. Have
The lithium ion secondary battery in which the second layer has a lower air permeability than the first layer and is disposed closer to the positive electrode than the first layer.
The lithium ion secondary battery according to claim 1, wherein the second layer has a smaller thermal shrinkage rate than the first layer.
The non-aqueous electrolyte contains a cyclic carbonate (α), a chain carbonate (β), and a lithium salt, and a volume ratio (α / β) of α and β is 20/80 to 40/60. The lithium ion secondary battery according to claim 1 or 2, wherein the concentration is 0.8 mol / L to 1.3 mol / L.
The positive electrode includes layered lithium-nickel-manganese-cobalt composite oxide (NMC) as a positive electrode active material, and the negative electrode includes amorphous carbon as a negative electrode active material. The lithium ion secondary battery according to item.
The lithium ion secondary battery according to any one of claims 1 to 4, wherein the battery capacity is 20 Ah or more.