WO2017022731A1

WO2017022731A1 - Lithium ion secondary battery

Info

Publication number: WO2017022731A1
Application number: PCT/JP2016/072566
Authority: WO
Inventors: 美枝阿部; 賢匠星
Original assignee: 日立化成株式会社
Priority date: 2015-07-31
Filing date: 2016-08-01
Publication date: 2017-02-09
Also published as: JPWO2017022731A1; CN107534186A

Abstract

A lithium ion secondary battery which comprises a positive electrode, a negative electrode, a separator and an electrolyte solution containing an electrolyte, and wherein: the negative electrode contains an easily graphitizable carbon as a negative electrode active material; the electrolyte contains lithium bis(fluorosulfonyl)imide; and the content of lithium bis(fluorosulfonyl)imide is 70% by mass or less based on the total mass of the electrolyte.

Description

Lithium ion secondary battery

The present invention relates to a lithium ion secondary battery.

As the electrolyte contained in the electrolyte solution of the lithium ion secondary battery, lithium salts such as lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ) have been widely used.

In recent years, lithium bis (fluorosulfonyl) imide (LiFSI) has attracted attention as a lithium salt that improves the cycle characteristics, discharge load characteristics at low temperatures, capacity retention rate after storage at high temperatures, and the like of lithium ion secondary batteries. Attempts have been made to improve the characteristics of lithium ion secondary batteries by using a lithium salt together with a known lithium salt (see, for example, Patent Document 1).

Japanese Patent Laying-Open No. 2015-62154

The present invention aims to further improve the characteristics of a lithium ion secondary battery, and an object thereof is to provide a lithium ion secondary battery excellent in cycle characteristics, storage characteristics and input characteristics.

Specific means for solving the above problems include the following embodiments.
<1> a positive electrode, a negative electrode, a separator, and an electrolyte solution containing an electrolyte,
The negative electrode includes graphitizable carbon as a negative electrode active material,
The lithium ion secondary battery, wherein the electrolyte contains lithium bis (fluorosulfonyl) imide, and the content thereof is 70% by mass or less based on the total amount of the electrolyte.
<2> The lithium secondary battery according to <1>, wherein the electrolyte further includes lithium hexafluorophosphate.
<3> The lithium secondary battery according to <1> or <2>, wherein the concentration of the electrolyte in the electrolytic solution is 0.5 mol / L to 2 mol / L.
<4> The graphitizable carbon has a C-axis direction plane d002 value obtained by an X-ray wide-angle diffraction method of 0.34 nm or more and less than 0.36 nm, and any one of <1> to <3> The lithium secondary battery as described in.

According to the present invention, a lithium ion secondary battery having excellent cycle characteristics, storage characteristics, and input characteristics is provided.

It is sectional drawing of the 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 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 of the present embodiment includes a positive electrode, a negative electrode, a separator, and an electrolyte solution containing an electrolyte. The negative electrode includes graphitizable carbon as a negative electrode active material, and the electrolyte is lithium. Bis (fluorosulfonyl) imide (hereinafter also referred to as LiFSI) is included, and the content thereof is 70% by mass or less based on the total amount of the electrolyte.

As a result of studies, the present inventors have found that a lithium ion secondary battery using an electrolyte solution containing LiFSI as an electrolyte in a predetermined ratio is easily graphitized carbon as a negative electrode active material, compared with a case where graphite is used as a negative electrode active material. It has been found that the cycle characteristics, storage characteristics and input characteristics are remarkably improved when using.

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) is composed of a current collector and a 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, a conductive material used as necessary, a thickener, and the like.
The positive electrode active material preferably contains a layered lithium / nickel / manganese / cobalt composite oxide (hereinafter sometimes referred to as NMC). NMC has a high capacity and excellent safety.
From the viewpoint of further improving safety, NMC and spinel type lithium manganese oxide (hereinafter sometimes referred to as sp-Mn) may be used in combination.
The content of the positive electrode active material is preferably 65% by mass or more, more preferably 70% by mass or more, based on the total amount of the positive electrode mixture layer, from the viewpoint of increasing the capacity of the lithium ion secondary battery. 80% by mass or more is more preferable.

As said NMC, it is preferable to use what is represented by the following compositional formula (Formula 1).
Li _{(1 + δ)} Mn _x Ni _y Co _(1-xyz) M _z O ₂ (Formula 1)
In the above composition formula (Formula 1), (1 + δ) is a composition ratio of Li (lithium), x is a composition ratio of Mn (manganese), y is a composition ratio of Ni (nickel), and (1-xyz) ) Indicates the composition ratio of Co (cobalt). z represents the composition ratio of the element M. The composition ratio of O (oxygen) is 2.
The elements M are Ti (titanium), Zr (zirconium), Nb (niobium), Mo (molybdenum), W (tungsten), Al (aluminum), Si (silicon), Ga (gallium), Ge (germanium), and Sn. It is at least one element selected from the group consisting of (tin).
In the above composition formula (Formula 1), −0.15 <δ <0.15, 0.1 <x ≦ 0.5, 0.6 <x + y + z ≦ 1.0, and 0 ≦ z ≦ 0.1.

Further, as the sp-Mn, it is preferable to use one represented by the following composition formula (Formula 2).
Li _{(1 + η)} Mn _(2-λ) M ′ _λ O ₄ (Chemical formula 2)
In the above composition formula (Formula 2), (1 + η) represents the composition ratio of Li, (2-λ) represents the composition ratio of Mn, and λ represents the composition ratio of the element M ′. The composition ratio of O (oxygen) is 4.
Element M ′ includes Mg (magnesium), Ca (calcium), Sr (strontium),
It is preferably at least one element selected from the group consisting of Al, Ga, Zn (zinc) and Cu (copper).
In the above composition formula (Formula 2), 0 ≦ η ≦ 0.2 and 0 ≦ λ ≦ 0.1.
Mg or Al is preferably used as the element M ′ in the composition formula (Chemical Formula 2). By using Mg or Al, the battery tends to have a longer life. In addition, the safety of the battery tends to be improved. Furthermore, since the elution of Mn can be reduced by adding the element M ′, the storage characteristics and the charge / discharge cycle characteristics tend to be improved.

As the positive electrode active material, materials other than NMC and sp-Mn may be used.
Cathode active materials other than NMC and sp-Mn are not particularly limited, and those commonly used in this field can be used. Lithium-containing composite metal oxides other than NMC and sp-Mn, olivine type lithium salts, chalcogen compounds, dioxide dioxide Manganese etc. are mentioned.

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 shape of the particles include lumps, polyhedrons, spheres, ellipsoids, plates, needles, and columns.

The median diameter D50 of the positive electrode active material particles (when the primary particles are aggregated 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 from the small diameter side becomes 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 examples of the organic solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, and acrylic. Methyl acid, diethyltriamine, N, N-dimethylaminopropylamine, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methyl Naphthalene, hexane, etc. are mentioned. 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. Examples thereof include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. These thickeners 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, from the viewpoint of output characteristics and further improvement of ^safety, is preferably 2.4g / cm 3 ~ 2.8g / cm 3, 2.45g / More preferably, it is cm ³ to 2.7 g / cm ³ .

The material of the current collector for the positive electrode 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 in the case of using a metal material include a metal foil, a metal cylinder, a metal coil, a metal plate, an expanded metal, a punch metal, and a foam metal. Examples of the shape when using a carbonaceous material include a carbon plate, a carbon thin film, and a carbon cylinder. Among these, it is preferable to use a metal foil or a carbon thin film. The metal foil or carbon 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 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.

In the present embodiment, the negative electrode active material includes graphitizable carbon. Graphitizable carbon (sometimes called soft carbon) is classified as amorphous carbon together with non-graphitizable carbon (sometimes called hard carbon). Amorphous carbon means non-graphite carbon with a disordered crystal structure. 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 C-axis direction interplanar spacing d002 value of less than 0.36 nm obtained by an X-ray wide angle diffraction method. Non-graphitizable carbon is defined as amorphous carbon having a surface spacing d002 value in the C-axis direction of 0.36 nm or more obtained by an X-ray wide angle diffraction method.

From the viewpoint of battery characteristics, the value of the interplanar spacing d002 obtained by the X-ray wide angle diffraction method of graphitizable carbon is preferably 0.34 nm or more and less than 0.36 nm, and 0.341 nm or more and 0. More preferably, the thickness is 355 nm or less, and further preferably 0.342 nm or more and 0.35 nm or less.

Easily graphitized carbon is in the form of particles, and the average particle size is preferably 2 μm to 50 μm. When the average particle size is 2 μm or more, the specific surface area can be in an appropriate range, the initial charge / discharge efficiency of the lithium ion secondary battery tends to be excellent, and the contact between the particles is good and the input / output characteristics are good. Tend to be better. On the other hand, if the average particle size is 50 μm or less, unevenness on the electrode surface is unlikely to occur and the short circuit of the battery tends to be suppressed, and the diffusion distance of Li from the particle surface to the inside becomes relatively short, so that lithium ions The input / output characteristics of the secondary battery tend to improve. The average particle diameter of graphitizable carbon is more preferably 5 μm to 30 μm, and still more preferably 10 μm to 20 μm.

In this specification, the average particle diameter of graphitizable carbon is determined by dispersing a sample in purified water containing a surfactant, and measuring a laser diffraction particle size distribution analyzer (for example, “SALD-3000J” manufactured by Shimadzu Corporation). In the volume-based particle size distribution measured in (1), the value (median diameter (D50)) when the integration from the small diameter side is 50% is used.

The method for producing graphitizable carbon is not particularly limited. For example, it can be produced by heat-treating a substance that can be carbonized by heating, such as petroleum pitch, polyacene, polyparaphenylene, polyfurfuryl alcohol, and the like. At this time, by changing the temperature of the heat treatment, it can be made graphitizable carbon or non-graphitizable 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.

The negative electrode active material may contain only graphitizable carbon as a carbon material, or at least one selected from the group consisting of non-graphitizable carbon and graphite may be used in combination with graphitizable carbon. When at least one selected from the group consisting of non-graphitizable carbon and graphite and graphitizable carbon are used in combination, the content of graphitizable carbon is 50% by mass or more based on the total amount of the negative electrode active material. It is preferably 70% by mass or more, more preferably 90% by mass or more.

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, an oxide such as tin oxide and silicon oxide, a metal composite oxide, a lithium simple substance, a lithium alloy such as a lithium aluminum alloy, and 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, expanded metal, punch metal, and foam metal. Especially, metal foil is preferable and copper foil is more preferable.

Although there is no restriction | limiting in particular in the structure of the negative mix layer formed using the negative electrode active material, The range of the density of a negative mix layer is as follows. The lower limit of the density of the negative electrode mixture layer is preferably 0.7 g / cm ³ or more, more preferably 0.8 g / cm ³ or more, still more preferably 0.9 g / cm ³ or more, and the upper limit is preferably 2 g. / Cm ³ or less, more preferably 1.9 g / cm ³ or less, still more preferably 1.8 g / cm ³ or less, and particularly preferably 1.7 g / cm ³ or less.

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 of the present embodiment contains LiFSI as an electrolyte, and the content thereof is 70% by mass or less based on the total amount of the electrolyte. The electrolytic solution includes an electrolyte and a non-aqueous solvent that dissolves the electrolyte, and may include an additive or the like as necessary.

LiFSI is a lithium salt represented by LiN (FSO ₂ ) ₂ . The content of LiFSI is preferably 60% by mass or less, more preferably 50% by mass or less, and more preferably 40% by mass from the viewpoint of cycle characteristics, storage characteristics, and input characteristics, based on the total amount of the electrolyte. More preferably, it is as follows. The lower limit of the content of LiFSI is not particularly limited, but from the viewpoint of cycle characteristics, storage characteristics, and input characteristics, it is preferably 1% by mass or more, preferably 5% by mass or more based on the total amount of the electrolyte. More preferably, it is more preferably 10% by mass or more, and particularly preferably 20% by mass or more.

The electrolytic solution contains an electrolyte other than LiFSI, and the content thereof is 30% by mass or more based on the total amount of the electrolyte.
The electrolyte other than LiFSI is not particularly limited as long as it can be used as an electrolyte of an electrolytic solution for a lithium ion secondary battery. For example, the following inorganic lithium salt, fluorine-containing organic lithium salt, and oxalatoborate salt can be mentioned.
Examples of the inorganic lithium _{_{_{salt, LiPF 6, LiBF 4, LiAsF}}} 6, LiSbF 6 inorganic fluoride salts, such _as, LiClO _4, Libro 4, perhalogenate of LiIO _4, etc., and the like inorganic chloride salts such as LiAlCl ₄ It is done.
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 ); perfluoroalkanesulfonylmethide salts such as LiC (CF 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.
These lithium salts may be used alone or in combination of two or more.

From the viewpoint of battery characteristics, the electrolytic solution preferably includes an inorganic lithium salt as an electrolyte other than LiFSI, more preferably includes an inorganic fluoride salt, and further preferably includes LiPF ₆ .

There is no particular limitation on the concentration of the electrolyte in the electrolytic solution. For example, from the viewpoint of ensuring the electrical conductivity of the electrolytic solution, the concentration of the electrolyte (total of a plurality of types of electrolytes) in the electrolytic solution is preferably 0.5 mol / L or more, and is 0.6 mol / L or more. More preferably, it is 0.7 mol / L or more. Further, from the viewpoint of suppressing the increase in the viscosity of the electrolytic solution and suppressing the decrease in electrical conductivity, the concentration of the electrolyte in the electrolytic solution is preferably 2 mol / L or less, and is 1.8 mol / L or less. More preferred is 1.7 mol / L or less.

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. Examples thereof include ethylene carbonate, propylene carbonate, and butylene carbonate. 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.

From the viewpoint of battery characteristics, the nonaqueous solvent more preferably contains at least one selected from the group consisting of cyclic carbonates and chain carbonates.

When the nonaqueous solvent contains at least one selected from the group consisting of cyclic carbonates and chain carbonates, the content is 85% by mass or more based on the total amount of the nonaqueous solvent from the viewpoint of battery characteristics. Is more preferable, 90 mass% or more is more preferable, and 95 mass% or more is further preferable.

When the non-aqueous solvent contains at least one selected from the group consisting of cyclic carbonates and chain carbonates, the mixing ratio of the cyclic carbonates and the chain carbonates is determined by the cyclic carbonate / chain carbonate (volume ratio) from the viewpoint of battery characteristics. ) Is preferably 1/9 to 6/4, and more preferably 2/8 to 5/5.

The non-aqueous solvent may contain an additive from the viewpoint of improving battery characteristics.
The additive is not particularly limited, and examples thereof include cyclic sulfonic acid esters, cyclic carboxylic acid esters, fluorine-containing cyclic carbonates, and compounds having an unsaturated bond in the molecule. From the viewpoint of extending the life of the battery, it is preferable to include at least one selected from the group consisting of cyclic sulfonate esters, fluorine-containing cyclic carbonates, and compounds having an unsaturated bond in the molecule.

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.

Examples of the fluorine-containing cyclic carbonate include fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, tetrafluoroethylene carbonate, and trifluoropropylene carbonate.
Examples of the compound having an unsaturated bond in the molecule include vinylene carbonate.

The non-aqueous solvent may contain other additives such as an overcharge preventing material, a negative electrode film forming material, a positive electrode protective material, and a high input / output material as required.

4). Separator A separator is particularly suitable if it has ion permeability while electronically insulating between the positive electrode and the negative electrode, and has oxidizability on the positive electrode side and resistance to reducibility on the negative electrode side. Not limited. Examples of the material (material) of the separator that satisfies such characteristics include resins, inorganic substances, and glass fibers.

Examples of the resin include olefin polymer, fluoropolymer, cellulose polymer, polyimide, nylon and the like. From the viewpoint of being stable with respect to the non-aqueous electrolyte and having excellent liquid retention properties, olefin polymers are preferable, and polyolefins such as polyethylene and polypropylene are more preferable.

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.
Examples of the shape of the separator include a porous sheet and a nonwoven fabric.

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 (capacity ratio of negative electrode to positive electrode of lithium ion secondary battery)
In the present invention, the capacity ratio of the negative electrode to the positive electrode (negative electrode capacity / positive electrode capacity) is preferably 1 or more and less than 1.3 from the viewpoint of safety and energy density, and is 1.05 to 1.25. More preferably, it is more preferably 1.1 to 1.2. When the capacity ratio between the negative electrode and the positive electrode is less than 1.3, the positive electrode potential is less likely to be higher than 4.2 V when charged, and the safety tends to be further increased. In addition, the positive electrode potential at this time means a potential against Li.

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, 4.2 V, 0.1 C to 0.5 C, 0.1 C to 0.5 C after performing constant current constant voltage (CCCV) charging with a termination time of 2 to 10 hours. It can be measured under conditions when a constant current (CC) is discharged to 2.7 V at 0.5 C. For the discharge capacity of the negative electrode, the negative electrode whose discharge capacity was measured for the lithium ion secondary battery was cut into a predetermined area, lithium metal was used as the counter electrode, and a single electrode cell was prepared via a separator impregnated with an electrolyte. , 0V, 0.1C, constant current constant voltage (CCCV) charge at a final current of 0.01C, then discharge per predetermined area under the condition of constant current (CC) discharge to 1.5V at 0.1C It can be calculated by measuring the capacity and converting this to the total area used as the negative electrode of the lithium ion 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)].

(Configuration example 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 sheet shape, and the like. For example, as a 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.

FIG. 1 shows a configuration example of a cylindrical lithium ion secondary battery. The lithium ion secondary battery 1 of this configuration example has a structure in which an electrode body 5 in which a strip-like positive electrode 2 and a negative electrode 3 are wound with a separator 4 interposed therebetween is housed in a cylindrical battery container 6. Have The inside of the battery container 6 is filled with an electrolyte solution (not shown).

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 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 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. Then, the drying process was performed and it consolidated by the press to the predetermined density. The density of the positive electrode mixture layer was 2.5 g / cm ³ .

[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. Then, the drying process was performed and it consolidated by the press to the predetermined density. The density of the negative electrode mixture layer was 1.15 g / cm ³ .

In Comparative Example 4, non-graphitizable carbon (d002: 0.375 nm, average particle diameter (D50): 10 μm) was used as the negative electrode active material. An NMP solution of polyvinylidene fluoride was used as the binder, and the mass ratio (in terms of solid content) of the negative electrode active material and the binder was 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. Then, the drying process was performed and it consolidated by the press to the predetermined density. The density of the negative electrode mixture layer was 1.0 g / cm ³ .

In Comparative Example 5, artificial graphite (d002: 0.337 nm, average particle diameter (D50): 20 μm) was used as the negative electrode active material. An NMP solution of polyvinylidene fluoride was used as the binder, and the mass ratio of the negative electrode active material to the binder (in terms of solid content) was negative electrode active material: binder = 91: 9. 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. Then, the drying process was performed and it consolidated by the press to the predetermined density. The density of the negative electrode mixture layer was 1.4 g / cm ³ .

[Preparation of electrolyte]
Ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio (EC: DMC: EMC) of 2: 3: 2 to prepare a mixed solution. Subsequently, the lithium salt shown in Table 1 as an electrolyte was dissolved therein so as to have a concentration shown in Table 1. Further, 0.8% by mass of vinylene carbonate (VC) was added as an additive to prepare an electrolytic solution.

[Production of lithium ion secondary battery]
Each of the positive electrode and the negative electrode prepared above was cut into a predetermined size, and a roll-shaped electrode body was formed by sandwiching a single-layer separator of polyethylene having a thickness of 30 μm between them and winding from the end. The lengths of the positive electrode, the negative electrode, and the separator were adjusted so that the diameter of the electrode body was 17.15 mm. A current collecting lead was attached to this electrode body, inserted into a 18650 type battery case, and the electrolytic solution prepared above was injected. Finally, the battery case was sealed to complete a lithium ion secondary battery.

[Evaluation of battery characteristics]
The manufactured battery is charged at a constant current of up to 4.2 V at a current value of 0.5 C in an environment of 25 ° C., and is constant until the current value reaches 0.01 C at that voltage after reaching 4.2 V. Voltage charging was performed. Thereafter, constant current discharge was performed up to 2.7 V at a current value of 0.5 C. Further, a pause of 30 minutes was put between charge and discharge. This was carried out for 3 cycles. This state was defined as the initial state. Next, cycle characteristics, storage characteristics, and input characteristics were evaluated by the following methods using the lithium ion secondary battery in the initial state.

(Cycle characteristics)
The lithium-ion secondary battery in the initial state is charged at a constant current of up to 4.2 V at a current value of 1 C in a 25 ° C. environment, and is constant until the current value reaches 0.01 C at a constant voltage after reaching 4.2 V. The battery was fully charged by voltage charging. Then, the cycle test which repeats discharging to 2.7V was done. Further, a pause of 30 minutes was put between charge and discharge. The discharge capacity was measured during the first cycle of charge / discharge, and the discharge capacity was also measured during the charge / discharge of the 1000th cycle. And the ratio of the discharge capacity of the 1000th cycle with respect to the discharge capacity of the 1st cycle was computed by the following formula as cycle characteristics (%). The results are shown in Table 1.

Cycle characteristics (%) = [discharge capacity at 1000th cycle / discharge capacity at 1st cycle] × 100

(Storage characteristics)
For the lithium ion secondary battery in the initial state, the battery was charged to 4.2 V at a current value of 0.5 C in an environment of 25 ° C., and then by constant current discharge at a current value of 0.5 C and a final voltage of 2.7 V Discharge was performed. The capacity at the time of discharge was defined as the discharge capacity before storage at a current value of 0.5C. Thereafter, the battery was charged to 4.2 V at a current value of 0.5 C and then left for 2 months in an environment of 50 ° C. The battery after standing was charged to 4.2 V at a current value of 0.5 C in an environment of 25 ° C., and then discharged by constant current discharge at a final voltage of 2.7 V at a current value of 0.5 C. The capacity at the time of discharge was defined as the discharge capacity after storage at a current value of 0.5C. Next, the storage property (%) was calculated by the following formula. The results are shown in Table 1.

Storage characteristics (%) = [discharge capacity after storage at a current value of 0.5 C / discharge capacity before storage at a current value of 0.5 C] × 100

(Input characteristics)
The direct current resistance (DCR) was measured for the lithium ion secondary battery in the initial state and the lithium ion secondary battery after 1000 cycles of charge and discharge under the conditions described in the evaluation of the cycle characteristics. The DCR increase rate was determined. DCR indicates the resistance value of the lithium ion secondary battery. The lower the DCR increase rate before and after the cycle test, the higher the input characteristics.

DCR was measured as follows. First, after discharging the lithium ion secondary battery to 2.7 V, the battery is charged with a constant current of 0.5 C until the SOC (State of Charge, state of charge) becomes 50%, and the SOC becomes 50%. The battery was charged at a constant voltage from the time of arrival until the current value reached 0.01 C at that voltage.
Then, it is charged for 11 seconds at a current value of 0.2C, discharged to a voltage at which the SOC is 50% at a current value of 0.5C, charged for 11 seconds at a current value of 1.0C, and a current value of 0.5C. The battery was discharged to a voltage at which the SOC was 50%, charged for 11 seconds at a current value of 1.3C, and discharged to a voltage at which the SOC was 50% at a current value of 0.5C. The slope of the straight line when the charging current value at this time was plotted with the horizontal axis representing the voltage change amount for 10 seconds and the vertical axis representing the amount of change was defined as DCR (DCR) at 50% SOC. The ratio of the DCR at the 1000th cycle (DCR increase rate) to the DCR in the initial state was calculated as the input characteristic (%) by the following formula. The results are shown in Table 1.

Input characteristics (%) = [DCR at 1000th cycle / initial state DCR] × 100

As shown in Table 1, Examples 1 to 5 in which the electrolyte solution contained LiFSI as an electrolyte, the content thereof was 70% by mass or less based on the total amount of the electrolyte, and graphitized carbon was used as the negative electrode active material. The lithium ion secondary battery of the present invention was superior to the comparative examples in all evaluations of cycle characteristics, storage characteristics, and input characteristics. Therefore, it was found that the lithium ion secondary battery of the present invention was excellent in cycle characteristics, storage characteristics, and input characteristics.

DESCRIPTION OF SYMBOLS 1 ... Lithium ion secondary battery, 2 ... Positive electrode, 3 ... Negative electrode, 4 ... Separator, 5 ... Electrode body, 6 ... Battery container

Claims

A positive electrode, a negative electrode, a separator, and an electrolyte solution containing an electrolyte,
The negative electrode includes graphitizable carbon as a negative electrode active material,
The lithium ion secondary battery, wherein the electrolyte contains lithium bis (fluorosulfonyl) imide, and the content thereof is 70% by mass or less based on the total amount of the electrolyte.
The lithium secondary battery according to claim 1, wherein the electrolyte further contains lithium hexafluorophosphate.
3. The lithium secondary battery according to claim 1, wherein a concentration of the electrolyte in the electrolytic solution is 0.5 mol / L to 2 mol / L.
The graphitizable carbon has a C-axis direction plane distance d002 value obtained by an X-ray wide-angle diffraction method of 0.34 nm or more and less than 0.36 nm, according to any one of claims 1 to 3. Lithium secondary battery.