JP2016058180A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery excellent in battery characteristics and safety.SOLUTION: A lithium ion secondary battery includes a housing body composed of a material containing a metal, frame retardants arranged in the housing body, and a safety valve, where the internal pressure A of the housing body when the retardants are ejected from the housing body satisfies following formula: 0.25 MPa≤A≤working pressure of safety valve.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion secondary battery.

  Lithium ion secondary batteries are widely used as power sources for portable electronic devices and electric vehicles because of their high energy density. For example, in a lithium ion secondary battery, a winding electrode body is accommodated in a cylindrical battery can. The take-up electrode body is configured by sandwiching a microporous separator between a positive electrode and a negative electrode and winding them in a spiral shape, and the separator is impregnated with a combustible non-aqueous electrolyte. For this reason, when the temperature of the battery suddenly rises due to an abnormal situation, the electrolyte solution is vaporized, the internal pressure rises, and the lithium ion secondary battery may burst. Moreover, when the temperature of a battery rises rapidly, electrolyte solution may ignite.

  Preventing a situation where a lithium ion secondary battery ignites or ignites is important in the design of a lithium ion secondary battery. Lithium ion secondary batteries are required to further improve safety in order to further increase energy density and size in the future.

  As a method for improving the safety of a lithium ion secondary battery, for example, a lithium ion secondary battery that improves flame retardancy by adding a phosphate ester (trimethyl phosphate, triethyl phosphate, etc.) to the electrolyte is proposed. (For example, refer to Patent Document 1). In addition, a lithium ion secondary battery has been proposed that incorporates a structure containing an endothermic substance (such as Na-type BC fire extinguisher) into the battery, so that the endothermic agent is released during thermal runaway and prevents ignition due to battery temperature rise. (For example, refer to Patent Document 2).

JP-A-4-184870 JP 2009-301798 A

  However, in the lithium ion secondary battery in which a phosphoric acid ester is added to the electrolytic solution described in Patent Document 1, the flame retardance is improved, but the electrolytic solution deteriorates due to the influence of the phosphoric acid ester, and the charge / discharge efficiency Battery characteristics such as power density, energy density, and battery life may be reduced. Moreover, in the lithium ion secondary battery using the structure containing the endothermic substance described in Patent Document 2, it is effective in suppressing thermal runaway, but when igniting the jet gas of the non-aqueous electrolyte, There is a possibility that combustion cannot be suppressed. This invention is made | formed in view of the said situation, and it aims at providing the lithium ion secondary battery which is excellent in a battery characteristic and safety | security.

Means for solving the above problems are as follows.
<1> A container comprising a metal-containing material, a flame retardant disposed inside the container, and a safety valve, and the container when the flame retardant is released from the container A lithium ion secondary battery in which the internal pressure A satisfies the following formula.
0.25 MPa ≤ A ≤ Safety valve operating pressure

<2> The lithium ion secondary according to claim 1, wherein the flame retardant is released from the container when the safety valve is operated, or the flame retardant is released from the container immediately after the safety valve is operated. Next battery.

<3> A battery container, an electrode winding group obtained by winding the positive electrode, the negative electrode, and the separator, and an electrolytic solution are further provided, and the container is disposed in a gap portion of the electrode winding group. > Or <2>.

<4> The lithium ion secondary battery according to <3>, wherein the electrolytic solution includes a cyclic carbonate and a chain carbonate.

<5> The lithium ion secondary battery according to any one of <1> to <4>, wherein the container has a pressure valve.

<6> The lithium ion secondary battery according to any one of <1> to <5>, wherein the material of the container includes at least one selected from the group consisting of aluminum and steel.

<7> The lithium ion secondary according to any one of <1> to <6>, wherein at least a part of at least one selected from the group consisting of an inner wall and an outer wall of the container is covered with a resin. battery.

<8> The lithium ion secondary battery according to any one of <1> to <7>, wherein the flame retardant includes at least one selected from the group consisting of bromoform and carbon tetrabromide.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery excellent in a battery characteristic and safety | security can be provided.

It is sectional drawing of the lithium ion secondary battery of embodiment which can apply this invention. It is sectional drawing of the metal can which has a gasket which is one Embodiment of a container.

  In the present specification, a numerical range indicated using “to” indicates a range including the numerical values described before and after “to” as the minimum value and the maximum value, respectively. The content of a certain component means the total amount of the plurality of substances when there are a plurality of substances corresponding to the component, unless otherwise specified. The term “layer” includes not only the configuration of the shape formed on the entire surface when observed as a plan view, but also the configuration of the shape formed in part. The term “stacked” indicates that the layers are stacked, and two or more layers may be bonded, or two or more layers may be detachable.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention includes a container made of a metal-containing material, a flame retardant disposed inside the container, and a safety valve, and the flame retardant is released from the container. The internal pressure A of the container when being satisfied the following formula.
0.25 MPa ≤ A ≤ Safety valve operating pressure

  By providing the lithium ion secondary battery of the present invention with the above-described configuration, the flame retardant is not mixed in the electrolytic solution in the normal use temperature range of the battery, and the battery performance is not impaired. On the other hand, when the temperature of the battery rises abnormally and the internal pressure of the container reaches the internal pressure A, the flame retardant can be released into the electrolyte for the first time. Specifically, for example, when the flame retardant inside the container undergoes volume expansion or state change due to heat and the internal pressure of the container reaches the internal pressure A, the pressure valve provided in the container is opened or destroyed. Thus, the flame retardant is released from the container, and the above mixing is realized.

  On the other hand, in a cylindrical lithium ion secondary battery, in general, in order to prevent an increase in internal pressure inside the battery container, internal pressure reduction such as a safety valve that releases gas to the outside of the container when a predetermined internal pressure is reached. It has a mechanism. However, when the above-described rapid and continuous chemical reaction occurs, the battery container may be damaged (including cracks, expansion, ignition, etc.) even when the internal pressure reduction mechanism is provided. Even when the abrupt and continuous chemical reaction occurs, the operating pressure of the safety valve for effectively releasing the gas generated in the battery container without damaging the battery container is preferably 1 MPa to 2 MPa.

  Here, it is desirable that the internal pressure A of the container when the flame retardant is released from the container is equal to or lower than the operating pressure of the safety valve of the battery container. When the internal pressure A is equal to or lower than the operating pressure of the safety valve, before the safety valve of the battery container is activated and the gas derived from the electrolyte is ejected, or immediately after the gas derived from the electrolyte is ejected (for example, within 2 seconds) The flame retardant is released from the container. Even when the flame retardant is released from the container immediately after the gas derived from the electrolyte from the battery container is ejected, self-extinguishing properties are exhibited. For example, in a cylindrical lithium ion secondary battery that operates (opens, etc.) a safety valve at an internal pressure of 1.5 MPa and ejects gas derived from an electrolyte (that is, the operating pressure of the safety valve is 1.5 MPa), When the flame retardant is released from the container with the internal pressure of 1.5 MPa or less, the flame retardant is mixed into the electrolyte before the safety valve is activated or immediately after the safety valve is activated. Further, by setting the internal pressure A to 0.25 MPa or more, the flame retardant is not released from the container during normal use of the battery, and the battery performance is not deteriorated.

[Container]
The container in the lithium ion secondary battery of this invention is comprised from the material containing a metal, and the internal pressure A of the said container when the said flame retardant is discharge | released from a container satisfies the following formula.
0.25 MPa ≤ A ≤ Safety valve operating pressure

  The method for causing the internal pressure A of the container to satisfy the above formula is not particularly limited. For example, the method etc. which provide a pressure valve in a container are mentioned. In this specification, a pressure valve means a valve or structure that automatically releases pressure when the internal pressure of the container rises above a predetermined level and lowers the internal pressure. Specifically, as a pressure valve, a valve that keeps the internal pressure constant by the action of a spring (spring type pressure valve), a structure with a gasket with a lid that is cleaved when the internal pressure rises, and a container like a pull-top can Examples include a structure in which a part is more easily destroyed than the other part due to an increase in internal pressure. When using a spring type pressure valve, the internal pressure A can be adjusted depending on the strength of the spring, etc. When using a lid with a gasket, the internal pressure A can be adjusted depending on the heat resistance temperature of the resin of the gasket, etc. When making a part easier to be destroyed than the other part, the internal pressure A can be adjusted by the thickness of the part to be destroyed.

  The temperature at which the internal pressure of the container reaches the internal pressure A is preferably adjusted within the range of 80 ° C to 300 ° C depending on the use of the battery. The method for adjusting the temperature is not particularly limited. For example, a method of adjusting by the amount of inclusions in the container, a method of adjusting by the boiling point of the inclusions, a method of including a foaming agent in the inclusions, and the like can be mentioned. Furthermore, it may be prepared by any method according to the type of pressure valve described above.

  The material constituting the container is not particularly limited as long as it contains a metal. Specific examples of the metal include aluminum, iron, nickel, tin, titanium, zinc, copper, and alloys such as steel containing any of these metals. A material obtained by plating the metal such as tin or tin can also be used. Of these metals, aluminum or steel is preferred. The ratio or state of the metal contained in the material constituting the container is not particularly limited as long as the mixing of the flame retardant electrolyte during normal use of the battery is sufficiently prevented, but the flame retardant and electrolyte It is preferable that there is substantially no portion between which no metal is present.

  It is preferable that at least a part of at least one selected from the group consisting of an inner wall and an outer wall is covered with a resin. Examples of the resin include polyethylene resin, polypropylene resin, polyethylene terephthalate resin, polyamide resin, ethylene-vinyl alcohol copolymer resin, ethylene-vinyl acetate copolymer resin, acrylonitrile-butadiene copolymer resin, cellophane resin, vinylon resin, Examples include vinylidene chloride resin, epoxy resin, polyimide resin, and acrylic resin. As the polyethylene resin, those having a low density or a high density can also be used. Among the resins, polyethylene resins, polypropylene resins, and the like that are generally used as separator materials for lithium ion secondary batteries are preferable because they are stable to battery electrolytes.

  When the container has a pressure valve, the material of the pressure valve is not particularly limited, and may be selected from those exemplified as the material of the container. The thickness of the container is not particularly limited, but is preferably 100 μm to 3000 μm, more preferably 200 μm to 2000 μm, and still more preferably 300 μm to 1000 μm.

  The position of the container inside the lithium ion secondary battery is not particularly limited. In order not to impair the energy density of the battery, it is preferable to effectively use a dead space in the battery (a space where members other than the electrolyte, such as power generation elements and terminals, do not exist). As a specific example of the dead space inside the battery, there is a central void portion of an electrode winding group obtained by winding a positive electrode, a negative electrode, and a separator (hereinafter also referred to as an electrode group).

  The shape of the container is not particularly limited as long as the flame retardant can be isolated from the outside during normal use of the battery and the flame retardant is released from the container when the internal pressure of the container reaches the internal pressure A. For example, a cylindrical shape, a rectangular parallelepiped shape, a spherical shape, and the like can be cited, and the shape can be selected according to the shape of the dead space of the battery in which the container is disposed.

  When arrange | positioning a container in the space | gap part of the center of an electrode group, it is preferable to provide an axial center in the center part of an electrode group. When the shaft core is provided, it is preferable to make a hole in the side surface portion of the shaft core. By making the holes, the flame retardant released from the container can be efficiently dispersed throughout the battery.

[Flame retardants]
The flame retardant in the present invention is a general term for substances that reduce the flammability of combustible materials, and prevents flammable materials from igniting or igniting (noncombustible), extinguishing temporarily combustible combustible materials (self It means a substance having the property of extinguishing). The kind of the flame retardant is not particularly limited. For example, phosphoric acid compounds such as trimethyl phosphate, triethyl phosphate, tris (chloroethyl) phosphate, tri (dichloropropyl) phosphate, phosphazene compound having P = N bond, 2-ethyl-4 -Ionic liquid compounds such as methylimidazole, halogen compounds such as bromotrichloromethane, 1,2-dibromo-3-chloropropane, dibromomethane, bromoform, and carbon tetrabromide.

  Of the above-mentioned flame retardants, halogen compounds are preferred from the viewpoint of self-extinguishing properties. Among the halogen compounds, bromoform or carbon tetrabromide is particularly preferable because it exhibits high self-extinguishing properties with respect to an electrolytic solution containing an organic solvent. The reason why bromoform or carbon tetrabromide is particularly excellent in self-extinguishing properties is considered as follows. The electrolyte solution of a lithium ion secondary battery generally contains a mixed solvent of a cyclic carbonate and a chain carbonate, which will be described later. The boiling point of the cyclic carbonate or chain carbonate at normal pressure (1 atm) is about 90 ° C. to 261 ° C. On the other hand, the boiling point of bromoform at normal pressure is 149 ° C. to 152 ° C., and the boiling point of carbon tetrabromide at normal pressure is 190 ° C. That is, since bromoform or carbon tetrabromide is also vaporized at a temperature similar to the temperature at which the mixed solvent vaporizes, combustion of the jet gas derived from the electrolyte can be effectively suppressed (excelling in self-extinguishing properties), and Since bromoform or carbon tetrabromide remains to some extent in the non-vaporized electrolyte solution, it is considered that the ignition and combustion of the electrolyte solution can also be suppressed.

  A flame retardant may use only 1 type or may use it in combination of 2 or more type. For example, the release temperature of a flame retardant can be adjusted by combining two or more flame retardants having different boiling points. For example, when carbon tetrabromide having a boiling point of about 190 ° C. at normal pressure is used as the main component of the flame retardant, the release temperature may be lowered by combining dibromomethane having a boiling point of about 90 ° C. at normal pressure. Can be adjusted.

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

[Positive electrode]
The positive electrode (positive electrode plate) includes a positive electrode current collector and a positive electrode mixture (positive electrode mixture) disposed on the surface thereof. The positive electrode mixture is a layer (positive electrode active material layer) including at least a positive electrode active material disposed on the surface of the positive electrode current collector. The positive electrode active material layer may include a conductive material, a binder, a thickener, and the like in addition to the positive electrode active material.

The positive electrode active material is preferably a lithium-containing composite metal oxide. The lithium-containing composite metal oxide, for _{example, 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 and _{Li x Mn 2-y M y} O 4 ( in each of the formulas above, M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, It represents at least one element selected from the group consisting of Cu, Zn, Al, Cr, Pb, Sb, V and B. x = 0 to 1.2, y = 0 to 0.9, z = 2.0. -2.3). Here, the value of x indicating the molar ratio of lithium is increased or decreased by charging and discharging. Further, as the olivine type lithium salts such as LiFePO _4. An olivine type lithium salt or a chalcogen compound may be used as the positive electrode active material. Examples of the chalcogen compound include titanium disulfide and molybdenum disulfide. A positive electrode active material may be used individually by 1 type, or may use 2 or more types together.

As the positive electrode active material, from the viewpoint of _safety, it is preferred to include a lithium manganese oxide represented by Li x _Mn 2 _{O 4} or _{Li x Mn 2-y M y} O 4. When lithium manganese oxide is used as the positive electrode active material, the content is preferably 30% by mass or more, and more preferably 40% by mass or more based on the total amount of the positive electrode active material.

  Examples of the conductive material that may be used for the positive electrode active material layer include carbon black, graphite, carbon fiber, and metal fiber. Examples of carbon black include acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. Examples of graphite include natural graphite and artificial graphite. A conductive material may be used individually by 1 type, or may be used in combination of 2 or more type.

  Examples of the binder that may be used for the positive electrode active material layer include polyethylene, polypropylene, polyvinyl acetate, polymethyl methacrylate, nitrocellulose, fluororesin, and rubber particles. Examples of the fluororesin include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. Examples of the rubber particles include styrene-butadiene rubber particles and acrylonitrile rubber particles. Among these, in consideration of improving the oxidation resistance of the positive electrode active material layer, a binder containing fluorine is preferable. A binder may be used individually by 1 type, or may be used in combination of 2 or more type.

  The material of the positive electrode current collector is not particularly limited, and 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.

  There is no restriction | limiting in particular as a shape of an electrical power collector, The material processed into various shapes can be used. Specific examples of the 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 carbonaceous material includes a carbon plate, a carbon thin film, A carbon cylinder etc. are mentioned. Among these, it is preferable to use a metal thin film. In addition, you may form a thin film in mesh shape as needed. Although the thickness of the thin film is arbitrary, it is preferably 1 μm or more, more preferably 3 μm or more, and further preferably 5 μm or more. Moreover, it is preferable that it is 1 mm or less, it is more preferable that it is 100 micrometers or less, and it is still more preferable that it is 50 micrometers or less. When the thickness of the thin film is 1 μm or more, the strength required for the current collector tends to be obtained. Moreover, when it is 1 mm or less, sufficient flexibility is obtained and the processability tends to be good.

  The positive electrode active material layer can be formed, for example, by applying a positive electrode mixture paste to the surface of the positive electrode current collector, drying, and rolling as necessary. The positive electrode mixture paste can be prepared, for example, by adding a positive electrode active material to a dispersion medium together with a binder, a conductive material, a thickener, and the like as necessary. Examples of the dispersion medium include N-methyl-2-pyrrolidone (NMP) and water.

[Negative electrode]
The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the surface thereof. Examples of the negative electrode current collector include sheets and foils containing stainless steel, nickel, copper, and the like. Although the thickness of a sheet | seat and foil is not specifically limited, For example, it is preferable that they are 1 micrometer-500 micrometers, it is more preferable that they are 2 micrometers-100 micrometers, and it is still more preferable that they are 5 micrometers-50 micrometers. The negative electrode active material layer is formed on one or both surfaces in the thickness direction of the negative electrode current collector, contains the negative electrode active material, and further contains a binder, a conductive material, a thickener and the like as necessary. It may be.

  As the negative electrode active material, a material that can occlude and release lithium ions and that is commonly used in the field of lithium ion secondary batteries can be used. Examples of the negative electrode active material include lithium metal, lithium alloy, intermetallic compound, carbon material, inorganic compound, metal complex, and organic polymer compound. Examples of the inorganic compound include lithium titanium composite oxide and silicon carbide. A negative electrode active material may be used individually by 1 type, or may be used in combination of 2 or more type. Among these, a carbon material is preferable as the negative electrode active material. Examples of the carbon material include graphite, carbon black, amorphous carbon, and carbon fiber. Examples of graphite include natural graphite such as flake graphite, and artificial graphite. Examples of carbon black include acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black.

The volume average particle size of the carbon material is preferably 0.1 μm to 60 μm, and more preferably 0.5 μm to 30 μm. Further, BET specific surface area of the carbon material ^is preferably 1m ² / g~10m 2 / g.

The carbon material is preferably selected in consideration of characteristics that are important in accordance with the use of the lithium ion secondary battery. For example, from the viewpoint of further improving the discharge capacity of the battery, the distance (d ₀₀₂ ) between the carbon hexagonal planes in the X-ray wide angle diffraction method is 3.35 to 3.40 and the crystallite (Lc) in the c-axis direction is Graphite that is 100% or more is preferred. From the viewpoint of further improving the cycle characteristics and safety of the battery, amorphous carbon having an interval (d ₀₀₂ ) between carbon hexagonal planes in an X-ray wide angle diffraction method of 3.50 to 3.95 is preferable.

  As the conductive material that may be used for the negative electrode active material layer, the same conductive material as that contained in the positive electrode active material layer can be used. Moreover, as a binder which may be used for a negative electrode active material layer, what is commonly used in the field of lithium ion secondary batteries can be used. Examples thereof include polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, styrene butadiene rubber, and acrylic rubber. Examples of the thickener that may be used for the negative electrode active material layer include carboxymethylcellulose.

  The negative electrode active material layer can be formed, for example, by applying a negative electrode mixture paste to the surface of the negative electrode current collector, drying, and rolling as necessary. The negative electrode mixture paste can be prepared, for example, by adding a negative electrode active material to a dispersion medium together with a binder, a conductive material, a thickener, and the like as necessary. As the dispersion medium, N-methyl-2-pyrrolidone (NMP), water or the like can be used.

[Non-aqueous electrolyte]
The non-aqueous electrolyte (hereinafter sometimes simply referred to as electrolyte) is composed of a lithium salt (electrolyte) and a solvent that dissolves the lithium salt. You may add an additive to electrolyte solution as needed.

  The lithium salt is not particularly limited as long as it is a lithium salt that can be used as an electrolyte of an electrolytic solution for a lithium ion secondary battery. For example, the following inorganic lithium salts, fluorine-containing organic lithium salts, oxalate borate salts and the like 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] fluoroalkyl hexafluorophosphate salts such like.

  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. Among these, lithium hexafluorophosphate (LiPF ₆ ) is preferable in terms of solubility in a solvent, charge / discharge characteristics, output characteristics, cycle characteristics, and the like when a lithium ion secondary battery is used.

  The concentration of the electrolyte in the electrolytic solution is not particularly limited, but is preferably 0.5 mol / L or more, more preferably 0.6 mol / L or more, and further preferably 0.7 mol / L or more. preferable. Moreover, it is preferable that it is 2 mol / L or less, It is more preferable that it is 1.8 mol / L or less, It is still more preferable that it is 1.7 mol / L or less. When the concentration of the electrolyte is 0.5 mol / L or more, the electric conductivity of the electrolytic solution tends to be sufficient. Moreover, when it is 2 mol / L or less, the viscosity does not become too high, and good electric conductivity tends to be obtained. In addition, the performance of a lithium ion secondary battery may fall by the fall of electrical conductivity.

  The solvent is not particularly limited as long as it is a solvent that can be used as an electrolyte solvent for a lithium ion secondary battery, and examples thereof include cyclic carbonates, chain carbonates, chain esters, cyclic ethers, and chain ethers.

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

  As the chain carbonate, dialkyl carbonate is preferable, and the number of carbon atoms of the two alkyl groups is more preferably 1 to 5, and more preferably 1 to 4. Specifically, symmetrical chain carbonates such as dimethyl carbonate, diethyl carbonate, and di-n-propyl carbonate, and asymmetric chain carbonates such as ethyl methyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate Is mentioned. 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.

  Examples of the cyclic ether include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran and the like.

  Examples of chain ethers include dimethoxyethane and dimethoxymethane.

  From the viewpoint of battery characteristics, the electrolytic solution preferably contains a cyclic carbonate and a chain carbonate (a mixed solvent of a cyclic carbonate and a chain carbonate). As for the mixing ratio of the cyclic carbonate and the chain carbonate, the cyclic carbonate / chain carbonate (volume ratio) is preferably 1/9 to 6/4, and more preferably 2/8 to 5/5.

  From the viewpoint of extending the life of the battery, the electrolytic solution is preferably a mixture of a cyclic carbonate and a chain carbonate with a fluorine-containing cyclic carbonate or a compound having an unsaturated bond in the molecule. 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.

[Separator]
The separator is not particularly limited as long as it has ion permeability while electronically insulating between the positive electrode and the negative electrode, and has resistance to oxidation on the positive electrode side and reducibility on the negative electrode side. As a material (material) of the separator satisfying such characteristics, a resin, an inorganic material, glass fiber, or the like is used.

  Examples of the resin include olefin resin, fluororesin, cellulose resin, polyimide resin, and nylon resin. Among them, it is preferable to select from materials that are stable with respect to an electrolyte solution such as an olefin resin and have excellent liquid retention properties. It is more preferable to use a porous sheet, a nonwoven fabric, or the like made of an olefin resin such as polyethylene or polypropylene. preferable.

  Examples of the inorganic substance 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. For example, what made the said inorganic substance of fiber shape or particle shape adhere to thin film-shaped base materials, such as a nonwoven fabric, a woven fabric, and a microporous film, can be used as a separator. As a thin film-shaped substrate, a substrate having a pore diameter of 0.01 μm to 1 μm and a thickness of 5 μm to 50 μm is preferably used. Or what made the said inorganic substance of fiber shape or particle shape the composite porous layer using binders, such as resin, can be used as a separator. Furthermore, a composite porous layer may be formed by applying a mixture of an inorganic substance and a binder to the surface of the positive electrode or the negative electrode to form a separator. For example, a composite porous layer in which alumina particles having a 90% particle size of less than 1 μm are bound using a fluororesin as a binder may be formed on the surface of the positive electrode.

[Other components]
The lithium ion secondary battery of the present embodiment may include components other than those described above as necessary. For example, a cleavage valve may be provided. 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.

Next, an embodiment in which the present invention is applied to a 18650 type cylindrical lithium ion secondary battery will be described with reference to the drawings.
As shown in FIG. 1, the lithium ion secondary battery of this embodiment is a battery container having a bottomed cylindrical electrode group in which a strip-like positive electrode plate 2 and a negative electrode plate 3 are spirally wound through a separator 4. 6 has a structure housed inside. In a space existing in the center of the electrode group, a container 1 having a specific flame retardant disposed therein is provided. The inside of the battery container 6 is filled with an electrolyte solution (not shown). The lithium ion secondary battery shown in FIG. 1 can be manufactured according to a normal method for manufacturing a lithium ion secondary battery except that the container 1 is provided inside the battery container 6. In FIG. 1, the container is provided in the space that exists in the center of the electrode group, but is not limited to this, and is accommodated in the space between the electrode group and the battery container bottom, the space between the electrode group and the battery lid, or the like. You may have a body. A plurality of containers may be provided at different positions.

  In one embodiment, the battery container is made of nickel-plated steel, and the separator 4 is a polyethylene porous sheet. A ribbon-shaped positive electrode tab terminal made of aluminum and having one end fixed to the positive electrode plate 2 is led out on the upper end surface of the electrode group. The other end of the positive electrode tab terminal is disposed on the upper side of the electrode group, and is joined by ultrasonic welding to the lower surface of a disk-shaped battery lid serving as a positive electrode external terminal. On the other hand, a ribbon-like negative electrode tab terminal made of copper with one end fixed to the negative electrode plate 3 is led out on the lower end surface of the electrode group. The other end of the negative electrode tab terminal is joined to the inner bottom of the battery container by resistance welding. Therefore, the positive electrode tab terminal and the negative electrode tab terminal are respectively led out to the opposite sides of the both end faces of the electrode group. An insulating coating is applied to the outer peripheral surface of the electrode group. The battery lid is caulked and fixed to the upper part of the battery container via an insulating resin gasket, and the inside of the lithium ion secondary battery is sealed. A non-aqueous electrolyte is injected into the battery container.

Here, the safety valve of the lithium ion secondary battery of the present embodiment is preferably provided on the battery lid. The “safety valve” in the present embodiment includes a mechanism having a function of exhausting the gas generated in the battery container to the outside of the battery when the internal pressure in the battery container increases. The battery container is sealed with a gasket, packing, etc., and when the internal pressure in the battery container rises when the battery container is sealed with an elastic body such as resin or spring pressed against the opening, The structure etc. which discharge | emit gas outside from the clearance gap which arises by deformation | transformation of a body are mentioned. The operating pressure of the safety valve may be appropriately determined in consideration of the type of battery to be used or an assumed gas generation mode (pressure increase mode). The operating pressure of the safety valve related to gas generation is set lower than the pressure resistance of the battery container, and is preferably set in the range of 1 MPa to 2 MPa. From the viewpoint of handling and safety, it is more preferable that the operating pressure of the safety valve is set to about 1.5 MPa.

  An example of the embodiment of the container will be described with reference to FIG. A flame retardant 10 is disposed inside the bottomed cylindrical container 9. A lid 7 is disposed on the upper portion of the container 9, and the container 9 is sealed via a gasket 8. In one embodiment, the container 9 and the lid 7 are made of steel whose inner and outer walls are coated with polyethylene, and the gasket is made of a fluororesin.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. However, the present invention is not limited to these examples.

<Example 1>
(1) Production of negative electrode 91.4 parts by mass of amorphous carbon (negative electrode active material, Kureha Co., Ltd.), polyvinylidene fluoride solution (binder, trade name: Kureha KF Polymer # 9130, solid content 13% by mass, stock A negative electrode mixture paste was prepared by mixing 66.15 parts by mass of Kureha) and 27.6 parts by mass of N-methyl-2-pyrrolidone (NMP). This negative electrode mixture paste is applied to both sides of a 10 μm thick copper core material (negative electrode current collector, Hitachi Cable Ltd.), dried at 80 ° C. and rolled, and has a single-side thickness of 80 μm and a single-side coating amount of 80 g / m. ² , a negative electrode mixture layer having a mixture density of 1.15 g / cm ³ was formed to produce a negative electrode.

(2) Production of positive electrode LiMn ₂ O ₄ (positive electrode active material, Mitsui Kinzoku Co., Ltd.) 90 parts by mass, acetylene black (conductive material, trade name: HS-100, average particle size 48 nm (value described in manufacturer catalog), Electrochemical Industry Co., Ltd.) 5 parts by mass, polyvinylidene fluoride solution (binder, trade name: Kureha KF Polymer # 7305, solid content 5% by mass, Kureha Co., Ltd.) 100 parts by mass and N-methyl-2-pyrrolidone ( NMP) 28 parts by mass was mixed to prepare a positive electrode mixture paste. This positive electrode mixture paste was applied to both surfaces of a positive electrode current collector (aluminum foil having a thickness of 20 μm), dried at 120 ° C. and rolled, and then a single-side thickness of 91 μm, a single-side coating amount of 150 g / m ² , and a mixture density. A positive electrode active material layer of 2.1 g / cm ³ was formed to produce a positive electrode.

(3) Production of container A flame retardant (carbon tetrabromide) is about 2 cm ³ on a steel bottomed cylindrical metal can having a diameter of 8 mm and a height of 50 mm, and having an inner wall and an outer wall coated with polyethylene. Injected. Thereafter, a lid made of the same material as that of the metal can and having a thickness of 500 μm was disposed on the top of the can, and the top of the container was crimped through a fluororesin gasket to seal the metal can.

(4) Production of Lithium Ion Secondary Battery A product obtained by sandwiching a separator made of a polyethylene microporous film having a thickness of 30 μm and a width of 58.5 mm between the produced positive electrode plate and negative electrode plate was wound to obtain an electrode group. Produced. This electrode group was inserted into a battery container having a diameter of 18 mm and a height of 65 mm, and a negative electrode tab terminal previously welded to the negative electrode current collector was welded to the bottom of the can. Next, LiPF ₆ is dissolved to a concentration of 1.2 mol / l in a solvent in which ethylene carbonate and dimethyl carbonate are mixed at a volume ratio of 2: 3, and 0.8% by mass of vinylene carbonate is further added. Thus, an electrolytic solution was produced. 5 ml of the prepared electrolyte was injected into the battery container. Thereafter, the container produced in (3) above was placed in a cylindrical space at the center of the electrode group. Thereafter, a positive electrode tab terminal previously welded to the positive electrode current collector of the power generation element is welded so as to be electrically connected to the positive electrode external terminal, and a positive electrode cap having a safety valve with an operating pressure of about 1.5 MPa is attached to the upper part of the battery container. The battery was sealed by crimping the upper part of the battery container via an insulating gasket. A 18650 type lithium ion secondary battery was produced as described above.

[Measurement of flame retardant release pressure and flame retardant release temperature]
The internal pressure A (flame retardant release pressure) of the container when the flame retardant was released from the container was measured by a hydraulic pump connected to a pressure sensor (Keyence Corporation, trade name: pressure sensor head AP-14S). . Moreover, the container was heated and the temperature at which the flame retardant was released was measured. The results are shown in Table 1. Here, the flame retardant release temperature means a temperature at which the mass of the flame retardant in the can becomes 50% or less when left at that temperature for 1 hour.

[Battery performance]
Battery performance was evaluated using the lithium ion secondary battery produced by the above method. The discharge rate characteristics at 25 ° C. were measured under the following charge / discharge conditions using a charge / discharge device (Toyo System Co., Ltd., trade name: TOSCAT-3200), and the initial charge / discharge efficiency and the discharge rate characteristics were evaluated.

After charging to 4.2V with a charging current value of 0.5C and constant current / constant voltage (CCCV) charging until the current value of 0.01C, which is the termination condition, is constant current up to 3V with a discharging current value of 0.5C (CC) Discharge was performed, the charge capacity and discharge capacity at this time were measured, and the initial charge capacity and the initial discharge capacity were obtained, respectively. Next, charging was performed under the same conditions as described above, and the discharge capacity was measured by changing the discharge current value to 1C, 3C, and 5C. C means “current value (A) / discharge capacity (Ah)”.
The initial charge / discharge efficiency and discharge rate characteristics were calculated from the following equations. The results are shown in Table 1.
Initial charge / discharge efficiency (%) = (initial discharge capacity / initial charge capacity) × 100
Discharge rate characteristic (%) = (discharge capacity at 5 C / discharge capacity at 0.5 C) × 100

Further, as an accelerated test for evaluating the shielding property of the flame retardant of the container, the capacity retention rate after holding the battery after the test at 60 ° C. for 1 week was calculated. Specifically, the battery after the test was held at 60 ° C. for 1 week using a constant temperature dryer. After that, the battery is charged to 4.2 V at a charging current value of 0.5 C, and after performing constant current and constant voltage (CCCV) charging until the current value of 0.01 C as a termination condition is reached, to 3 V at a discharging current value of 0.5 C. A constant current (CC) discharge was performed, the discharge capacity at this time was measured, and this was defined as the discharge capacity after holding at 60 ° C. for 1 week. The capacity maintenance rate was calculated as follows. The results are shown in Table 1.
Capacity retention rate (%) = (discharge capacity after holding for 1 week at 60 ° C./initial discharge capacity) × 100

[Burner test (self-extinguishing)]
A safety test by a burner test was performed using the 18650 type lithium ion secondary battery manufactured by the above method. The burner test is a test in which the upper end (positive electrode end) of a 18650 type lithium ion secondary battery is heated by a gas burner, and verifies the combustibility and self-extinguishing properties of the gas ejected due to the increase in internal pressure. be able to. Specifically, immediately after igniting the ejected gas, heating by the gas burner was stopped, and the time taken for the fired ejected gas to extinguish was measured. It was evaluated that the shorter the time, the higher the self-extinguishing property of the electrolyte. The evaluation criteria were A if the time from ignition to extinction was less than 0.5 seconds, B if 0.5 seconds or more and less than 10 seconds, and C if 10 seconds or more. The results are shown in Table 1.

<Example 2>
The same conditions as in Example 1 except that an aluminum, bottomed cylindrical metal can with a polyethylene coat on the inner wall and outer wall, and a lid made of the same material as the metal can, with a diameter of 8 mm and a height of 50 mm. A lithium ion secondary battery was fabricated and evaluated. The results are shown in Table 1.

<Example 3>
A lithium ion secondary battery was fabricated and evaluated under the same conditions as in Example 1 except that about 2 cm ³ of bromoform was injected into a metal can as a flame retardant. The results are shown in Table 1.

<Example 4>
A lithium ion secondary battery was prepared and evaluated under the same conditions as in Example 1 except that about 2 cm ³ of a mixture of carbon tetrabromide and dibromomethane (mass ratio 1: 1) was injected into a metal can as a flame retardant. . The results are shown in Table 1.

<Example 5>
A lithium ion secondary battery was produced and evaluated under the same conditions as in Example 1 except that the thickness of the lid of the metal can was 1.5 times (0.75 mm). The results are shown in Table 1.

<Example 6>
A lithium ion secondary battery was fabricated and evaluated under the same conditions as in Example 1 except that about 2 cm ³ of trimethyl phosphate was injected into a metal can as a flame retardant. The results are shown in Table 1.

<Comparative Example 1>
A lithium ion secondary battery was produced and evaluated under the same conditions as in Example 1 except that no gasket was installed on the metal can. The results are shown in Table 1.

<Comparative Example 2>
A lithium ion secondary battery was prepared and evaluated under the same conditions as in Example 1 except that the thickness of the lid of the metal can was doubled (1 mm). The results are shown in Table 1.

<Comparative Example 3>
A lithium ion secondary battery was fabricated and evaluated under the same conditions as in Example 1 except that a polyethylene (PE) bag was used instead of the metal can. The results are shown in Table 1. The PE bag was produced as follows. Two PE films with a thickness of 100 μm are cut into a size of about 1 cm in length and about 5 cm in width, and the cut PE films are stacked to form a table sealer (trade name: T-130K, It was formed into a bag shape by heat-sealing with Fuji Impulse Co., Ltd., and 1 g of carbon tetrabromide was injected from the opening, and then the opening was sealed by heat-sealing. The four fused portions were cut out leaving a width of 0.1 cm to produce a bag in which a flame retardant having a length of about 0.5 cm and a width of about 4.5 cm was disposed.

<Comparative example 4>
A lithium ion secondary battery was prepared and evaluated under the same conditions as in Example 1 except that a polyethylene terephthalate (PET) bag was used instead of the metal can. The results are shown in Table 1. The PET bag was produced by the same method as in Comparative Example 1.

<Comparative Example 5>
A lithium ion secondary battery was produced and evaluated under the same conditions as in Example 1 except that carbon tetrabromide was not added to the metal can but added to the center of the 18650 type battery winding group. The results are shown in Table 1.

<Comparative Example 6>
A lithium ion secondary battery was produced and evaluated under the same conditions as in Example 1 except that the metal can and the flame retardant were not arranged inside the lithium ion secondary battery. The results are shown in Table 1.

  As shown in the measurement results of the flame retardant release temperature, the flame retardant release temperatures of Examples 1 to 5 using a metal can as the container exceeded 80 ° C. From this, it can be seen that the release of the flame retardant is suppressed at the normal use temperature of the battery (80 ° C. or lower), and the flame retardant is released when the battery is in a dangerous state above 80 ° C. Moreover, it turns out that a flame retardant discharge | release temperature can be adjusted by combining 2 or more types of flame retardants from which boiling points differ like Example 4. FIG.

As shown in the measurement results of the initial efficiency, rate characteristics, and discharge capacity retention rate, the batteries of Examples 1 to 6 and Comparative Examples 1 and 2 using a metal can as the container are Comparative Example 6 in which no flame retardant is added. The battery performance was equivalent to that of the battery.
The battery of Comparative Example 3 using a PE bag as the container had low initial efficiency, rate characteristics, and discharge capacity maintenance rate.
The battery of Comparative Example 4 using a PET bag as the container had the same initial efficiency and rate characteristic measurement results as Comparative Example 6, but the discharge capacity retention rate was low.
In Comparative Example 5 in which the flame retardant was added directly without being contained in the container, the discharge capacity could not be measured.
From these results, it can be seen that by disposing the flame retardant in the container made of a material containing a metal, it is possible to prevent the flame retardant from being mixed into the electrolytic solution and deteriorating the battery performance.

As shown in the results of the burner test, in Comparative Example 6 in which no flame retardant was added, the jet gas continued to burn for 10 seconds or more, whereas the flame retardant was contained and the flame retardant discharge pressure from the container was 1.5 MPa or less. In the batteries of Examples 1 to 6, the ejected gas self-extinguished in less than 10 seconds after ignition. In particular, Examples 1 to 5 containing either bromoform or carbon tetrabromide as a flame retardant had a shorter burning time than Example 6 containing trimethyl phosphate as a flame retardant.
In the battery of Comparative Example 2 in which the flame retardant discharge pressure exceeded 1.5 MPa, the ejected gas continued to burn for 10 seconds or more. This is presumably because the pressure inside the battery increased and the safety valve operated at about 1.5 MPa, so that the flame retardant was not released from the container even when the gasified electrolyte was ejected.

  As described above, according to the present invention, it was found that the expansion of damage can be suppressed even when the lithium ion secondary battery is put in a dangerous state. Further, it was also found that the performance of the lithium ion secondary battery of the present invention is not impaired because the flame retardant is not present in the electrolytic solution when a normal battery is used.

DESCRIPTION OF SYMBOLS 1 Container 2 Positive electrode 3 Negative electrode 4 Separator 5 Electrode group 6 Battery container 7 Upper lid 8 Gasket 9 Container 10 Flame retardant

Claims (8)

A container comprising a metal-containing material; a flame retardant disposed inside the container; and a safety valve; an internal pressure of the container when the flame retardant is released from the container A lithium ion secondary battery in which A satisfies the following formula.
0.25 MPa ≤ A ≤ Safety valve operating pressure   The lithium ion secondary battery according to claim 1, wherein the flame retardant is released from the container when the safety valve is operated, or the flame retardant is released from the container immediately after the safety valve is operated.   The battery container, an electrode winding group formed by winding a positive electrode, a negative electrode, and a separator, and an electrolyte solution, and the container is disposed in a gap portion of the electrode winding group. Item 3. A lithium ion secondary battery according to Item 2.   The lithium ion secondary battery according to claim 3, wherein the electrolytic solution contains a cyclic carbonate and a chain carbonate.   The lithium ion secondary battery according to claim 1, wherein the container has a pressure valve.   The lithium ion secondary battery according to any one of claims 1 to 5, wherein a material of the container includes at least one selected from the group consisting of aluminum and steel.   The lithium ion secondary battery according to any one of claims 1 to 6, wherein at least a part of at least one selected from the group consisting of an inner wall and an outer wall of the container is covered with a resin.   The lithium ion secondary battery according to any one of claims 1 to 7, wherein the flame retardant contains at least one selected from the group consisting of bromoform and carbon tetrabromide.