WO2016152718A1 - Lithium ion secondary battery - Google Patents

Abstract

This lithium ion secondary battery is provided with: a negative electrode which contains particles which are formed from at least one material (below, referred to as the metal and/or metal oxide) selected from metals that can alloy with lithium and metal oxides that can occlude and release lithium ions and which have a 0.78 or greater average value of circularity, defined by the expression below: Circularity = 4πS/L² (here, S: surface area of projected particle image, L: circumferential length of projected particle image); a positive electrode; and a single-layer, highly heat-resistant microporous resin film separator. This battery has the property of having a reduced self-discharge failure rate.

Description

Lithium ion secondary battery

The present invention relates to a lithium ion secondary battery, and more particularly, to a secondary battery including a negative electrode containing a non-carbon material and a heat-resistant separator, a manufacturing method thereof, a vehicle using the lithium ion secondary battery, and a power storage device. .

Lithium ion secondary batteries are characterized by their small size and large capacity, and they have been widely used as power sources for electronic devices such as mobile phones and laptop computers, and have contributed to improving the convenience of portable IT devices. In recent years, the use in a larger application such as a power source for driving a motorcycle or an automobile or a storage battery for a smart grid has attracted attention. As demand for lithium-ion secondary batteries increases and it is used in various fields, it is possible to use batteries with higher energy density, life characteristics that can withstand long-term use, and a wide range of temperature conditions. Such characteristics are required.

Carbon materials such as graphite are generally used for the negative electrode of lithium ion secondary batteries. However, in order to increase the energy density of batteries, metal particles such as silicon, silicon oxide, etc. are used together with carbon material particles. A negative electrode containing oxide particles has been proposed (see, for example, Patent Document 1: Japanese Patent Laid-Open No. 2003-123740).

On the other hand, high-performance secondary batteries with improved capacity and energy density require more safety considerations. As a means for enhancing the safety of the secondary battery, it is promising to improve the performance of the separator, and a high heat-resistant separator or the like has been studied. As a high heat-resistant separator, for example, Patent Document 2 describes a separator that contains fibers having a melting point of 150 ° C. or higher, such as aramid and / or polyimide, and prevents shrinkage during abnormal heat generation.

Also, Patent Document 3 (WO2012 / 070154) describes a two-layer microporous film of aramid and polyolefin as a separator.

JP 2003-123740 A JP 2006-59717 A WO2012 / 070154 JP 2014-225347 A

The silicon-based material used in Patent Document 1 is effective in improving the energy density, but the hardness is high, and the silicon-based material pulverized to obtain an appropriate particle size has a sharp angle. For this reason, when a separator that is easy to tear is used, the sharp corners of the particles penetrate the separator and cause a micro short, which causes a problem of frequent self-discharge failures. In particular, when the separator is made into a bag shape, pressure is applied to the separator due to expansion of the negative electrode accompanying charging, and self-discharge failure tends to occur. Also, when a laminate film is used for the battery outer package and sealing is performed under reduced pressure, pressure is applied to the separator, and self-discharge failure tends to occur.

As in Patent Document 2, a separator formed of a high heat resistant material such as aramid and polyimide can prevent shrinkage during abnormal heat generation. A nonwoven fabric separator formed of fibers has the advantage of being difficult to tear, but it is difficult to form a thin and uniform separator, which is disadvantageous in manufacturing a battery having a high energy density.

On the other hand, by using a microporous membrane, a thin and uniform separator is possible, but a high heat resistant material has a significantly smaller elongation at break than a polyolefin material, so it is easy to tear when a microporous membrane is formed. Self-discharge failure is likely to occur.

If a two-layer microporous membrane of aramid and polyolefin is used as in Patent Document 3, the problem of microshorts can be solved, but the heat resistance of the separator is limited by the polyolefin. That is, in a scene where heat resistance is required, in a case where heat treatment is performed on the electrode element at the time of manufacture, the heat resistance becomes insufficient, and the original heat resistance of aramid cannot be exhibited. Further, Patent Document 4 describes a technique of using silicon oxide having a high degree of circularity as a negative electrode material, but there is no description about using a separator that is easily torn.

Embodiments of the present invention have been made to solve the above-described problems, and an object of the present invention is to provide a lithium ion secondary battery having heat resistance and a reduced self-discharge failure rate.

One embodiment of the present invention is formed of at least one material selected from a metal that can be alloyed with lithium and a metal oxide that can occlude and release lithium ions (hereinafter referred to as metal and / or metal oxide). A negative electrode including particles whose average circularity defined by the following formula is 0.78 or more;
A positive electrode;
A single layer high heat resistance resin microporous membrane separator,
A lithium ion secondary battery comprising:

Circularity = 4πS / L ²
(However, S is the area of the particle projection image, and L is the circumference of the particle projection image.)

According to the embodiment of the present invention, it is possible to provide a lithium ion secondary battery having heat resistance and an improved self-discharge failure rate.

It is sectional drawing which shows an example of a laminated electrode element typically. It is a disassembled perspective view which shows the basic structure of a film-clad battery. It is sectional drawing which shows the cross section of the battery of FIG. 2 typically.

Since conventionally used metals and metal oxides are generally obtained by pulverizing lumps, the particles have sharp corners and are very hard materials. Therefore, when used with a fragile separator, the sharp corners of the particles penetrate the separator. However, in this embodiment, since the metal or metal oxide particles do not have sharp corners, the separator is damaged even if a microporous film formed of a high heat resistant resin is used as the separator. Even if there is no damage, it is estimated that the self-discharge defective rate could be reduced because it is smaller than the conventional one.

Hereinafter, embodiments of the present invention will be described for each member of a lithium secondary battery.

[Negative electrode]
The negative electrode has a structure in which a negative electrode active material is laminated on a current collector as a negative electrode active material layer integrated with a negative electrode binder. The negative electrode active material is a material capable of reversibly occluding and releasing lithium ions with charge / discharge.

The negative electrode of the present embodiment includes, as an active material, at least one material selected from a metal that can be alloyed with lithium and a metal oxide that can occlude and release lithium ions.

In the present embodiment, “a metal that can be alloyed with lithium and a metal oxide that can occlude and release lithium ions” may be one or more materials selected from either one, or one from both. The above materials may be selected and used in combination. Hereinafter, “at least one material selected from a metal that can be alloyed with lithium and a metal oxide capable of occluding and releasing lithium ions” may be referred to as “metal and / or metal oxide”. In the description of “metal that can be alloyed with lithium” and “metal oxide capable of inserting and extracting lithium ions”, both may be collectively referred to as “metal and metal oxide”.

“Metal and metal oxide” are in the form of particles and have no sharp corners. As described later, when the metal is dispersed inside the metal oxide, the metal oxide that forms the outer shape of the particles only needs to have a predetermined shape.

When the shape of the projected image of the metal and metal oxide particles is expressed using the circularity as an index, the average (number average value) circularity is 0.78 or more, preferably 0.8 or more, more preferably 0.8. 85 or more. Here, the circularity is defined by the following equation.

Circularity = 4πS / L ²
Here, S is the area of the particle projection image, and L is the circumference of the particle projection image.

The method for measuring the circularity of the particles is not particularly limited. However, before the negative electrode is manufactured, it can be obtained by, for example, performing image processing on a projected image of 500 arbitrary particles using a powder image analyzer. . As the powder image analyzing apparatus, for example, Microtrack FPA (trade name) manufactured by Nikkiso Co., Ltd., PITA-3 manufactured by Seishin Co., Ltd., or the like can be used. Moreover, if it is after negative electrode manufacture, image processing can be performed about arbitrary 100 pieces from a negative electrode cross-section photograph using SEM (scanning electron microscope).

Examples of metals that can be alloyed with lithium include Al, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, La, and alloys of two or more thereof. . In particular, silicon (Si) is preferably included as a metal that can be alloyed with lithium. The metal content in the negative electrode active material is preferably 5% by mass to 95% by mass, more preferably 10% by mass to 90% by mass, and more preferably 20% by mass to 50% by mass. More preferably.

Examples of metal oxides that can occlude and release lithium ions include aluminum oxide, silicon oxide, tin oxide, indium oxide, zinc oxide, lithium oxide, and composites thereof. In particular, silicon oxide is preferably included as a metal oxide capable of inserting and extracting lithium ions. In addition, one or more elements selected from nitrogen, boron, phosphorus and sulfur can be added to the metal oxide. By carrying out like this, the electrical conductivity of a metal oxide can be improved. The content of the metal oxide in the negative electrode active material may be 0% by mass or 100% by mass, but is preferably 5% by mass or more and 100% by mass or less, and 40% by mass or more and 95% by mass or less. Is more preferable, and it is further more preferable to set it as 50 to 90 mass%.

In this embodiment, it is preferable that at least Si and / or silicon oxide is contained as the negative electrode active material. The composition of silicon oxide is represented by SiOx (where 0 <x ≦ 2). A particularly preferred silicon oxide is SiO.

Moreover, it is preferable that all or part of the metal oxide has an amorphous structure. Since the metal oxide has an amorphous structure, it suppresses volume changes of other negative electrode active materials such as metals that can be alloyed with lithium and carbon materials that can occlude and release lithium ions, and suppresses decomposition of the electrolyte. Can be. Although this mechanism is not clear, it is presumed that the formation of a film on the interface between the carbon material and the electrolytic solution has some influence due to the amorphous structure of the metal oxide. The amorphous structure is considered to have relatively few elements due to non-uniformity such as crystal grain boundaries and defects. It can be confirmed by X-ray diffraction measurement (general XRD measurement) that all or part of the metal oxide has an amorphous structure. Specifically, when the metal oxide does not have an amorphous structure, a peak specific to the metal oxide is observed. However, the metal oxide may have a case where all or part of the metal oxide has an amorphous structure. Inherent peaks are broad and observed.

In addition, when the negative electrode active material contains a metal that can be alloyed with lithium and a metal oxide that can occlude and release lithium ions, all or a part of the alloyable metal is dispersed in the metal oxide. Is preferred. By carrying out like this, the volume change as the whole negative electrode can be suppressed, and decomposition | disassembly of electrolyte solution can also be suppressed. Note that all or part of the metal is dispersed in the metal oxide because transmission electron microscope observation (general TEM observation) and energy dispersive X-ray spectroscopy measurement (general EDX measurement). It can confirm by using together. Specifically, the cross section of the sample containing metal particles is observed, the oxygen concentration of the metal particles dispersed in the metal oxide is measured, and the metal constituting the metal particles is not an oxide. Can be confirmed.

When the negative electrode active material contains both a metal and a metal oxide, the metal oxide is preferably an oxide of a metal constituting the metal.

When the negative electrode active material contains both metal and metal oxide, there is no particular limitation on the ratio of metal and metal oxide. The metal is preferably 5% by mass or more and 90% by mass or less, and more preferably 30% by mass or more and 60% by mass or less with respect to the total of the metal and the metal oxide. The metal oxide is preferably 10% by mass or more and 95% by mass or less, and more preferably 40% by mass or more and 70% by mass or less with respect to the total of the metal and the metal oxide.

The surface of the metal and metal oxide particles may be coated with a carbon material (usually an amorphous carbon material). Examples of the coating method include a method of chemical vapor deposition (CVD) of particles in an organic gas and / or vapor. Further, the surfaces of the metal and metal oxide particles may be coated with a metal oxide film. The metal oxide film is preferably an oxide of one or more elements selected from magnesium, aluminum, titanium, and silicon, and in addition to the above elements, zirconium, hafnium, vanadium, niobium, tantalum, Chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, rhodium, iridium, nickel, palladium, cerium, indium, germanium, tin, bismuth, antimony, cadmium, copper, silver. May be included. In this case, the surface of the metal oxide film may be further coated with a carbon material (usually an amorphous carbon material).

Generally, a metal and metal oxide particles coated with a carbon material can provide a secondary battery having superior cycle characteristics.

In this embodiment, the negative electrode active material may further contain a carbon material capable of inserting and extracting lithium ions. Examples of such a carbon material include graphite, amorphous carbon, diamond-like carbon, carbon nanotube, and a composite thereof. Among these, graphite has high crystallinity, high electrical conductivity, and excellent adhesion to a current collector made of a metal such as copper and voltage flatness.

As the graphite, either natural graphite or artificial graphite may be used. The shape of graphite is not particularly limited and may be any. Examples of natural graphite include scale-like graphite, scale-like graphite, and earth-like graphite. Examples of artificial graphite include massive artificial graphite, flake-like artificial graphite, and spherical artificial graphite such as MCMB (mesophase micro beads).

In this embodiment, the surface of the carbon material may be coated. Examples of the coating material that covers the surface of the carbon material as the active material include carbon materials (usually amorphous carbon materials), metals, and metal oxides. As the coating material, amorphous carbon is preferable. Examples of the method of coating the surface of the carbon material particle as a negative electrode active material such as graphite with amorphous carbon include a method of chemical vapor deposition (CVD) in an organic gas and / or vapor. The coating amount of amorphous carbon is about 0.5 to 20% by mass based on the weight of the coated particles.

In this embodiment, the particle diameters of “metal and metal oxide” and “carbon material” are not particularly limited, but the median diameter (D50 particle diameter) of the metal and metal oxide particles is preferably about 1 to 30 μm. The median diameter (D50 particle diameter) is preferably about 5 to 50 μm.

Also, it is preferable that the median diameter of the metal and metal oxide particles is smaller than the median diameter of the carbon material. In this way, metals and metal oxides with large volume changes during charging and discharging have relatively small particle sizes, and carbon materials with small volume changes have relatively large particle sizes. Micronization is more effectively suppressed.

In the present embodiment, the content of the metal and metal oxide in the negative electrode is 1 to 100% by mass, preferably 1 to 95% by mass, based on the total amount of the metal, metal oxide and carbon material. It is preferable to change appropriately. For example, when adding a metal and / or metal oxide for the purpose of improving the disadvantages when using only a carbon material (for example, for improving energy density), the content of the metal and metal oxide is, for example, 1-30% by weight, preferably 1-20% by weight, and in certain embodiments 1-10% by weight. In applications where high energy density is required, the content of metal and metal oxide is, for example, more than 30% by mass, in a preferred embodiment of 50% by mass or more, and in a specific embodiment, 60% by mass or more. It is good.

Examples of the binder for the negative electrode include polyvinylidene fluoride, modified polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber (SBR), poly Examples thereof include tetrafluoroethylene, polypropylene, polyethylene, polyacrylic acid, polyacrylic acid metal salts, polyimide, and polyamideimide. When an aqueous binder such as an SBR emulsion is used, a thickener such as carboxymethyl cellulose (CMC) can also be used.

In this embodiment, it is preferable that a binder selected from polyimide, polyamideimide, polyacrylic acid, and a metal salt of polyacrylic acid is included as the binder for the negative electrode. The content of the binder for the negative electrode used is 0.5 to 20% by mass with respect to the total mass of the negative electrode active material from the viewpoints of “sufficient binding force” and “higher energy” which are in a trade-off relationship. Is preferred.

The negative electrode active material can be used together with a conductive auxiliary material as necessary. Specific examples of the conductive auxiliary material include the same materials as specifically exemplified in the following positive electrode, and the amount used can also be the same.

As the negative electrode current collector, aluminum, nickel, copper, silver, and alloys thereof are preferable in view of electrochemical stability. Examples of the shape include foil, flat plate, and mesh.

As a negative electrode manufacturing method, for example, a negative electrode active material, and if necessary, a conductivity imparting agent and a binder are dispersed and kneaded in a solvent such as N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode slurry. To do. The negative electrode layer can be prepared by applying the negative electrode slurry onto a negative electrode current collector such as a copper foil and drying the solvent. Examples of the coating method include a doctor blade method and a die coater method. After forming a negative electrode active material layer in advance, a thin film of aluminum, nickel, or an alloy thereof may be formed by a method such as vapor deposition or sputtering to form a negative electrode current collector. Moreover, when the heat processing above the drying temperature of a solvent, such as a polyimide precursor and a polyamideimide precursor, is required, desired heat processing can be performed as needed. It is preferable that a polyamic acid is contained as a polyamide precursor or a polyimide precursor. Alternatively, the negative electrode before lithium pre-doping may be manufactured by growing a negative electrode active material or the like on the negative electrode current collector by a vapor phase method such as vapor deposition or sputtering.

In this embodiment, since the circularity of the metal and metal oxide particles is large, even if a separator that is easy to tear is used, the damage to the separator is small, and it is considered that the battery characteristics, particularly the self-discharge failure, can be reduced.

[Positive electrode]
The positive electrode includes a positive electrode active material capable of reversibly occluding and releasing lithium ions during charge and discharge, and the positive electrode active material is laminated on the current collector as a positive electrode active material layer integrated with a positive electrode binder. It has a structure.

The positive electrode active material in the present embodiment is not particularly limited as long as it is a material capable of occluding and releasing lithium, but it is preferable to include a high capacity compound from the viewpoint of increasing the energy density. Examples of the high-capacity compound include lithium nickel oxide (LiNiO ₂ ) or a lithium nickel composite oxide obtained by substituting a part of Ni of lithium nickelate with another metal element. The layered structure is represented by the following formula (A) Lithium nickel composite oxide is preferred.

Li _y Ni _(1-x) M _x O ₂ (A)
(However, 0 ≦ x <1, 0 <y ≦ 1.2, and M is at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, and B.)

From the viewpoint of high capacity, the Ni content is high, that is, in the formula (A), x is preferably less than 0.5, and more preferably 0.4 or less. Examples of such a compound include Li _α Ni _β Co _γ Mn _δ O ₂ (0 <α ≦ 1.2, preferably 1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.7, γ ≦ 0. .2), Li _α Ni _β Co _γ Al _δ O ₂ (0 <α ≦ 1.2, preferably 1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.6, preferably β ≧ 0.7, γ ≦ 0.2), etc., especially LiNi _β Co _γ Mn _δ O ₂ (0.75 ≦ β ≦ 0.85, 0.05 ≦ γ ≦ 0.15, 0.10 ≦ δ ≦ 0.20). ). More specifically, for example, LiNi _0.8 Co _0.05 Mn _0.15 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O _{2, LiNi} 0.8 _Co 0.1 _Al can be preferably used 0.1 _{O 2} or the like.

From the viewpoint of thermal stability, it is also preferable that the Ni content does not exceed 0.5, that is, in the formula (A), x is 0.5 or more. It is also preferred that the number of specific transition metals does not exceed half. Such compounds include Li _α Ni _β Co _γ Mn _δ O ₂ (0 <α ≦ 1.2, preferably 1 ≦ α ≦ 1.2, β + γ + δ = 1, 0.2 ≦ β ≦ 0.5, 0 0.1 ≦ γ ≦ 0.4, 0.1 ≦ δ ≦ 0.4). More specifically, LiNi _0.4 Co _0.3 Mn _0.3 O ₂ (abbreviated as NCM433), LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (abbreviated as NCM523), LiNi _0.5 Co _0.3 Mn _0.2 O ₂ (abbreviated as NCM532), etc. (however, the content of each transition metal in these compounds varies by about 10%) Can also be included).

In addition, two or more compounds represented by the formula (A) may be used as a mixture. For example, NCM532 or NCM523 and NCM433 range from 9: 1 to 1: 9 (typically 2 It is also preferable to use a mixture in 1). Furthermore, in the formula (A), a material having a high Ni content (x is 0.4 or less) and a material having a Ni content not exceeding 0.5 (x is 0.5 or more, for example, NCM433) are mixed. As a result, a battery having a high capacity and high thermal stability can be formed.

Other than the above, as the positive electrode active material, for example, LiMnO ₂ , Li _x Mn ₂ O ₄ (0 <x <2), Li ₂ MnO ₃ , Li _x Mn _1.5 Ni _0.5 O ₄ (0 <x < 2) Lithium manganate having a layered structure or spinel structure such as LiCoO ₂ or a part of these transition metals replaced with another metal; Li in these lithium transition metal oxides more than the stoichiometric composition And those having an olivine structure such as LiFePO ₄ . Furthermore, a material in which these metal oxides are partially substituted with Al, Fe, P, Ti, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, La, etc. Can also be used. Any of the positive electrode active materials described above can be used alone or in combination of two or more.

As the positive electrode binder, the same as the negative electrode binder can be used. Among these, from the viewpoint of versatility and low cost, polyvinylidene fluoride or polytetrafluoroethylene is preferable, and polyvinylidene fluoride is more preferable. The amount of the positive electrode binder used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoints of “sufficient binding force” and “higher energy” which are in a trade-off relationship. .

A conductive auxiliary material may be added to the coating layer containing the positive electrode active material for the purpose of reducing impedance. Examples of the conductive auxiliary material include flaky carbonaceous fine particles such as graphite, carbon black, acetylene black, vapor grown carbon fiber (for example, VGCF manufactured by Showa Denko).

As the positive electrode current collector, the same as the negative electrode current collector can be used. In particular, the positive electrode is preferably a current collector using aluminum, an aluminum alloy, or iron / nickel / chromium / molybdenum stainless steel.

Similarly to the negative electrode, the positive electrode can be produced by forming a positive electrode active material layer containing a positive electrode active material and a positive electrode binder on a positive electrode current collector.

[Electrolyte]
Although it does not specifically limit as electrolyte solution of the lithium ion secondary battery which concerns on this embodiment, The nonaqueous electrolyte solution containing the nonaqueous solvent and supporting salt which are stable in the operating potential of a battery is preferable.

Examples of non-aqueous solvents include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC) and other cyclic carbonates; dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), Chain carbonates such as dipropyl carbonate (DPC); propylene carbonate derivatives, aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; ethers such as diethyl ether and ethyl propyl ether; trimethyl phosphate; Aprotic organic solvents such as phosphate esters such as triethyl phosphate, tripropyl phosphate, trioctyl phosphate and triphenyl phosphate, and fluorine compounds in which at least some of the hydrogen atoms of these compounds are substituted with fluorine atoms. Of aprotic organic solvents, and the like.

Among these, cyclic such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), dipropyl carbonate (DPC), etc. Or it is preferable that chain carbonates are included.

Non-aqueous solvents can be used alone or in combination of two or more.

The supporting salts include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) A lithium salt such as ₂ . The supporting salt can be used singly or in combination of two or more. LiPF ₆ is preferable from the viewpoint of cost reduction.

The electrolytic solution can further contain an additive. Although it does not specifically limit as an additive, A halogenated cyclic carbonate, an unsaturated cyclic carbonate, cyclic | annular or chain | strand-shaped disulfonic acid ester, etc. are mentioned. By adding these compounds, battery characteristics such as cycle characteristics can be improved. This is presumed to be because these additives decompose during charging / discharging of the lithium ion secondary battery to form a film on the surface of the electrode active material and suppress decomposition of the electrolytic solution and the supporting salt. In the present invention, the cycle characteristics may be further improved by the additive. The additives listed above are specifically described below.

Examples of the halogenated cyclic carbonate include compounds represented by the following formula (B).

In the formula (B), A, B, C and D are each independently a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms or a halogenated alkyl group, and at least one of A, B, C and D One is a halogen atom or a halogenated alkyl group. The number of carbon atoms of the alkyl group and the halogenated alkyl group is more preferably 1 to 4, and further preferably 1 to 3.

In one embodiment, the halogenated cyclic carbonate is preferably a fluorinated cyclic carbonate. Examples of the fluorinated cyclic carbonate include compounds in which some or all of the hydrogen atoms are substituted with fluorine atoms, such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC). -Fluoro-1,3-dioxolan-2-one (fluoroethylene carbonate: FEC) is preferred.

The content of the fluorinated cyclic carbonate is not particularly limited, but is preferably 0.01% by mass or more and 1% by mass or less in the electrolytic solution. By containing 0.01% by mass or more, a sufficient film forming effect can be obtained. Moreover, the gas generation by decomposition | disassembly of fluorinated cyclic carbonate itself can be suppressed as content is 1 mass% or less. In the present embodiment, in particular, 0.8% by mass or less is more preferable. By setting the content of the fluorinated cyclic carbonate to 0.8% by mass or less, it is possible to suppress a decrease in the activity of the negative electrode active material and maintain good cycle characteristics.

The unsaturated cyclic carbonate is a cyclic carbonate having at least one carbon-carbon unsaturated bond in the molecule. For example, vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4,5- Vinylene carbonate compounds such as diethyl vinylene carbonate; 4-vinylethylene carbonate, 4-methyl-4-vinylethylene carbonate, 4-ethyl-4-vinylethylene carbonate, 4-n-propyl-4-vinylene ethylene carbonate, 5-methyl -4-vinylethylene carbonate, 4,4-divinylethylene carbonate, 4,5-divinylethylene carbonate, 4,4-dimethyl-5-methyleneethylene carbonate, 4,4-diethyl-5-methyle Vinyl ethylene carbonate compounds such as ethylene carbonate. Among these, vinylene carbonate or 4-vinylethylene carbonate is preferable, and vinylene carbonate is particularly preferable.

The content of the unsaturated cyclic carbonate is not particularly limited, but is preferably 0.01% by mass or more and 10% by mass or less in the electrolytic solution. By containing 0.01% by mass or more, a sufficient film forming effect can be obtained. Moreover, gas generation by decomposition | disassembly of unsaturated cyclic carbonate itself can be suppressed as content is 10 mass% or less. In the present embodiment, in particular, 5% by mass or less is more preferable from the viewpoint of suppressing the decrease in activity of the negative electrode active material.

Examples of the cyclic or chain disulfonic acid ester include a cyclic disulfonic acid ester represented by the following formula (C) or a chain disulfonic acid ester represented by the following formula (D).

In the formula (C), R ₁ and R ₂ are each independently a substituent selected from the group consisting of a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, a halogen group, and an amino group. R ₃ is an alkylene group having 1 to 5 carbon atoms, a carbonyl group, a sulfonyl group, a fluoroalkylene group having 1 to 6 carbon atoms, or an alkylene group or a fluoroalkylene unit having 2 to 6 carbon atoms bonded via an ether group. A divalent group is shown.

In the formula (C), R ₁ and R ₂ are preferably each independently a hydrogen atom, an alkyl group having 1 to 3 carbon atoms or a halogen group, and R ₃ is an alkylene group having 1 or 2 carbon atoms. Or it is more preferable that it is a fluoroalkylene group.

Examples of preferable compounds of the cyclic disulfonic acid ester represented by the formula (C) include compounds represented by the following formulas (1) to (20).

In the formula (D), R ⁴ and R ⁷ are each independently a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a fluoroalkyl group having 1 to 5 carbon atoms, a carbon atom A polyfluoroalkyl group having 1 to 5 carbon atoms, —SO ₂ X ₃ (X ₃ is an alkyl group having 1 to 5 carbon atoms), —SY ₁ (Y ₁ is an alkyl group having 1 to 5 carbon atoms), —COZ (Z Represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms) and an atom or group selected from a halogen atom. R ⁵ and R ⁶ are each independently an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a phenoxy group, a fluoroalkyl group having 1 to 5 carbon atoms, or a polyalkyl having 1 to 5 carbon atoms. A fluoroalkyl group, a fluoroalkoxy group having 1 to 5 carbon atoms, a polyfluoroalkoxy group having 1 to 5 carbon atoms, a hydroxyl group, a halogen atom, —NX ₄ X ₅ (X ₄ and X ₅ are each independently a hydrogen atom; Or an alkyl group having 1 to 5 carbon atoms) and —NY ₂ CONY ₃ Y ₄ (where Y ₂ to Y ₄ are each independently a hydrogen atom or an alkyl group having 1 to 5 carbon atoms). Or a group.

In the formula (D), R ⁴ and R ⁷ are preferably each independently a hydrogen atom, an alkyl group having 1 or 2 carbon atoms, a fluoroalkyl group having 1 or 2 carbon atoms, or a halogen atom. ⁵ and R ⁶ are each independently an alkyl group having 1 to 3 carbon atoms, an alkoxy group having 1 to 3 carbon atoms, a fluoroalkyl group having 1 to 3 carbon atoms, a polyfluoroalkyl group having 1 to 3 carbon atoms, A hydroxyl group or a halogen atom is more preferred.

Examples of preferable compounds of the chain disulfonic acid ester compound represented by the formula (D) include the following compounds.

The content of the cyclic or chain disulfonic acid ester is preferably 0.005 mol / L or more and 10 mol / L or less, more preferably 0.01 mol / L or more and 5 mol / L or less in the electrolytic solution. It is particularly preferably from 05 mol / L to 0.15 mol / L. By containing 0.005 mol / L or more, a sufficient film effect can be obtained. Further, when the content is 10 mol / L or less, an increase in the viscosity of the electrolyte and an accompanying increase in resistance can be suppressed.

An additive can be used alone or in combination of two or more. When using combining 2 or more types of additives, it is preferable that the sum total of content of an additive is 10 mass% or less in an electrolyte solution, and it is more preferable that it is 5 mass% or less.

[Separator]
In the present embodiment, a microporous membrane separator made of a single layer of high heat resistance resin is used. As the high heat-resistant resin, a resin having a heat melting or decomposition temperature of 160 ° C. or higher, more preferably 180 ° C. or higher is preferable. By using a high heat resistant resin as the material of the separator, the safety of the secondary battery can be increased. The safety of the secondary battery can be evaluated by performing a high temperature heating test at 160 ° C., for example.

Particularly, a resin having a heat resistance of 300 ° C. or higher is preferable because it has a small thermal shrinkage and good shape retention. As the high heat-resistant resin, polyamide, polyimide, polyamideimide and polyphenylene sulfide are preferable. Polyamide, polyimide and polyamideimide are preferably aromatic, particularly aromatic polyamide, that is, aramid.

“Thermal melting temperature” represents a temperature measured by differential scanning calorimetry (DSC) according to JIS K 7121. “Thermal decomposition temperature” refers to a temperature from 25 ° C. to 10 ° C. in an air stream using a thermogravimetric measuring device. This represents the temperature at which the weight decreased by 10% when the temperature was raised at a rate of 10 ° C / min (10% weight loss temperature). "Heat resistance is 300 ° C or higher" indicates deformation such as softening at least at 300 ° C. Means no. In addition, “the thermal melting or thermal decomposition temperature is 160 ° C. or higher” means that the lower one of the thermal melting temperature and the thermal decomposition temperature is 160 ° C. or higher. In the case of a resin that decomposes, it means that the thermal decomposition temperature is 160 ° C. or higher.

Aramid is an aromatic polyamide in which one or more aromatic groups are directly connected by an amide bond. Examples of the aromatic group include a phenylene group, and two aromatic rings may be bonded with oxygen, sulfur, or an alkylene group (for example, a methylene group, an ethylene group, a propylene group, etc.). . These aromatic groups may have a substituent. Examples of the substituent include an alkyl group (for example, a methyl group, an ethyl group, a propyl group, etc.), an alkoxy group (for example, a methoxy group, an ethoxy group, Propoxy group, etc.), halogen (chloro group, etc.) and the like. A material in which part or all of the hydrogen atoms on the aromatic ring are substituted with a halogen group such as fluorine, bromine or chlorine is a preferable material because it has high oxidation resistance and does not cause oxidative deterioration at the positive electrode. . The aramid may be either a para type or a meta type.

It is particularly preferable to use an aramid microporous membrane as a separator because it has high heat resistance, does not deteriorate even under a high energy density, maintains insulation against Li precipitation, and prevents a complete short circuit.

Examples of the aramid that can be preferably used in the present embodiment include polymetaphenylene isophthalamide, polyparaphenylene terephthalamide, copolyparaphenylene 3,4′-oxydiphenylene terephthalamide, and hydrogen on these phenylene groups. And the like.

In order to prevent ignition due to thermal runaway of the battery, the separator used preferably has an oxygen index of 25 or more. The oxygen index means a minimum oxygen concentration at which a vertically supported small test piece maintains combustion in a mixed gas of nitrogen and acidity at room temperature, and a higher value represents a flame retardant material. The oxygen index can be measured according to JIS K7201. Examples of the material used for the separator having an oxygen index of 25 or more include polyphenylene sulfide, polyimide, and aramid.

The film thickness of the heat resistant resin microporous film is not particularly limited, but is usually about 3 μm to 40 μm, preferably 30 μm or less, more preferably 25 μm or less, and particularly preferably 20 μm or less. The air permeability (Gurley value) of the heat-resistant resin microporous membrane is not particularly limited, but is usually selected from the range of 1 second to 300 seconds.

[Secondary battery]
In the lithium ion secondary battery according to the present embodiment, an electrode body in which at least a pair of positive and negative electrodes are arranged to face each other, and an electrolytic solution are included in the exterior body. The shape of the secondary battery may be any of a cylindrical type, a flat wound square type, a laminated square type, a coin type, a flat wound laminated type, and a laminated laminate type, and a laminated laminate type is preferable. Hereinafter, a laminated laminate type secondary battery will be described.

FIG. 1 shows a schematic cross-sectional view of an example of a laminated electrode body 1 included in a laminated laminate type secondary battery. A plurality of positive electrodes 2 and a plurality of negative electrodes 3 are alternately stacked with the separator 4 interposed therebetween. At one end of each positive electrode 2 and each negative electrode 3, an active material uncoated portion where the positive electrode current collector 5 and the negative electrode current collector 6 are not covered with the active material is provided. The positive electrode 2 and the negative electrode 3 are stacked with the active material uncoated portions facing in opposite directions.

The positive electrode current collector 5 is electrically connected to each other at an active material uncoated portion, and a positive electrode lead terminal 7 is further connected to the connection portion. The negative electrode current collector 6 is electrically connected to each other at an active material uncoated portion, and a negative electrode lead terminal 8 is further connected to the connection portion.

A laminated laminate type secondary battery is manufactured by wrapping a laminated electrode body 1 with an exterior body such as an aluminum laminated film, injecting an electrolyte into the inside, and then sealing under reduced pressure.

As another aspect, a secondary battery having a structure as shown in FIGS. 2 and 3 may be used. The secondary battery includes a battery element 20, a film outer package 10 that houses the battery element 20 together with an electrolyte, and a positive electrode tab 51 and a negative electrode tab 52 (hereinafter also simply referred to as “electrode tabs”). .

As shown in FIG. 3, the battery element 20 is formed by alternately laminating a plurality of positive electrodes 30 and a plurality of negative electrodes 40 with a separator 25 interposed therebetween. In the positive electrode 30, the electrode material 32 is applied to both surfaces of the metal foil 31. Similarly, in the negative electrode 40, the electrode material 42 is applied to both surfaces of the metal foil 41.

The secondary battery in FIG. 1 has electrode tabs drawn out on both sides of the outer package. However, in the secondary battery to which the present invention can be applied, the electrode tab is drawn out on one side of the outer package as shown in FIG. It may be a configuration. Although detailed illustration is omitted, each of the positive and negative metal foils has an extension on a part of the outer periphery. The extensions of the negative electrode metal foil are collected together and connected to the negative electrode tab 52, and the extensions of the positive electrode metal foil are collected together and connected to the positive electrode tab 51 (see FIG. 3). The portions gathered together in the stacking direction between the extension portions in this way are also called “current collecting portions”.

The film outer package 10 is composed of two films 10-1 and 10-2 in this example. The films 10-1 and 10-2 are heat sealed to each other at the periphery of the battery element 20 and sealed. In FIG. 3, the positive electrode tab 51 and the negative electrode tab 52 are drawn out in the same direction from one short side of the film outer package 10 sealed in this way.

Of course, electrode tabs may be drawn from two different sides. As for the film configuration, FIGS. 2 and 3 show examples in which a cup portion is formed on one film 10-1 and a cup portion is not formed on the other film 10-2. In addition, a configuration in which a cup portion is formed on both films (not shown) or a configuration in which neither cup portion is formed (not shown) may be employed.

[Method for producing lithium ion secondary battery]
The lithium ion secondary battery according to the present embodiment can be produced according to a normal method. Taking a laminated laminate type lithium ion secondary battery as an example, an example of a method for producing a lithium ion secondary battery will be described. First, in the dry air or inert atmosphere, the above-mentioned electrode element is formed by arranging the positive electrode and the negative electrode opposite to each other with a separator interposed therebetween. Next, this electrode element is accommodated in an exterior body (container), and an electrolytic solution is injected to impregnate the electrode with the electrolytic solution. Then, the opening part of an exterior body is sealed and a lithium ion secondary battery is completed.

[Battery]
A plurality of lithium ion secondary batteries according to this embodiment can be combined to form an assembled battery. For example, the assembled battery may have a configuration in which two or more lithium ion secondary batteries according to the present embodiment are used and connected in series, in parallel, or both. Capacitance and voltage can be freely adjusted by connecting in series and / or in parallel. About the number of the lithium ion secondary batteries with which an assembled battery is provided, it can set suitably according to battery capacity or an output.

[vehicle]
The lithium ion secondary battery or its assembled battery according to this embodiment can be used in a vehicle. Vehicles according to this embodiment include hybrid vehicles, fuel cell vehicles, and electric vehicles (all include four-wheel vehicles (passenger cars, trucks, buses and other commercial vehicles, light vehicles, etc.), motorcycles (motorcycles), and tricycles. ). Note that the vehicle according to the present embodiment is not limited to an automobile, and may be used as various power sources for other vehicles, for example, moving bodies such as trains.

[Power storage device]
The lithium ion secondary battery or its assembled battery according to this embodiment can be used for a power storage device. As the power storage device according to the present embodiment, for example, a power source connected to a commercial power source supplied to a general household and a load such as a home appliance, and used as a backup power source or auxiliary power at the time of a power failure, Examples include photovoltaic power generation, which is also used for large-scale power storage for stabilizing power output with a large time fluctuation due to renewable energy.

Next, this embodiment will be specifically described by way of examples. The following examples illustrate preferred forms of the present embodiment, and the present invention is not limited to the following examples.

[Example 1]
(Adjustment and measurement of SiO circularity)
SiO (Catalog No. SIO02PB manufactured by Kojundo Chemical Co., Ltd., 75 μm mesh passing product) was pulverized using a planetary ball mill (Fritsch Classic Line P-5) to adjust the particle size distribution and circularity. The median diameter (d50) of the adjusted SiO particles and the circularity of 500 arbitrary SiO particles were measured with a powder measuring instrument (Seishin company: PITA-3). Table 1 shows the average values of d50 and circularity.

(Preparation of negative electrode)
Each of the above-mentioned SiO, scale-like natural graphite, and a mixed solution of polyamic acid and N-methyl-2-pyrrolidone (NMP) (trade name: U-Varnish A, manufactured by Ube Industries, Ltd., solid content: 18 wt%) each 60 Was mixed at a mass ratio of 25:15 (where the polyamic acid solution had a solid mass), and n-methylpyrrolidone (NMP) was further added to adjust the viscosity to obtain a slurry. This slurry was applied on a copper foil having a thickness of 10 μm with a doctor blade, and then dried by heating at 130 ° C. for 7 minutes. Thereafter, the obtained negative electrode was heated in a vacuum at 180 ° C. for 15 minutes to imidize polyamic acid to complete the negative electrode.

(Preparation of positive electrode)
Lithium nickelate, carbon black (trade name: “# 3030B”, manufactured by Mitsubishi Chemical Corporation), and polyvinylidene fluoride (trade name: “W # 7200”, manufactured by Kureha Corporation) are each 95: Weighed at a mass ratio of 2: 3. These and NMP were mixed to form a slurry. The mass ratio of NMP to solid content was 54:46. This slurry was applied to an aluminum foil having a thickness of 15 μm using a doctor blade. The aluminum foil coated with this slurry was heated at 120 ° C. for 5 minutes to dry the NMP, thereby producing a positive electrode.

(Assembly of secondary battery)
An aluminum terminal and a nickel terminal were welded to each of the produced positive electrode and negative electrode. These were overlapped via a separator to produce an electrode element. The electrode element was covered with a laminate film, and an electrolyte solution was injected into the laminate film. Thereafter, the laminate film was heat-sealed and sealed while reducing the pressure inside the laminate film. As a result, a plurality of flat-type secondary batteries before the first charge were produced. A polyimide microporous film having a thickness of 15 μm was used as the separator. As the laminate film, a polypropylene film on which aluminum was deposited was used. As the electrolytic solution, a solution containing 1.0 mol / l LiPF ₆ as an electrolyte and a mixed solvent of ethylene carbonate and diethyl carbonate (7: 3 (volume ratio)) as a nonaqueous electrolytic solvent was used. Ten identical batteries were made.

(Self-discharge test of secondary battery)
The produced battery was charged to 4.15V. Charging was performed by the CCCV method, and the voltage was kept constant for one hour after reaching 4.15V. The charged battery was stored in a thermostat kept at 45 ° C. for 19 days and subjected to a self-discharge test. Self-discharge failure was determined when the voltage after storage was 4.00 V or less. The defect rate (n / 10) is shown in Table 1.

[Example 2]
A secondary battery was produced in the same manner as in Example 1 except that the particle size and circularity of the ground SiO in Example 1 were adjusted as shown in Table 1, and a self-discharge test was performed.

[Example 3]
A secondary battery was produced in the same manner as in Example 1 except that the particle size and circularity of the ground SiO in Example 1 were adjusted as shown in Table 1, and a self-discharge test was performed.

[Example 4]
The ratio of the negative electrode slurry is a mixed solution of SiO, flaky natural graphite, polyamic acid and N-methyl-2-pyrrolidone (NMP) (trade name: U-Varnish A, manufactured by Ube Industries, Ltd., solid content 18 wt% ) Was made into a mass ratio of 82: 3: 15 (however, the polyamic acid solution was a solid content mass), and a secondary battery was produced in the same manner as in Example 1 and subjected to a self-discharge test.

[Example 5]
The ratio of the negative electrode slurry is a mixed solution of SiO, flaky natural graphite, polyamic acid and N-methyl-2-pyrrolidone (NMP) (trade name: U-Varnish A, manufactured by Ube Industries, Ltd., solid content 18 wt% ) Was set to 70:15:84, respectively, and a secondary battery was produced in the same manner as in Example 1 and subjected to a self-discharge test.

[Example 6]
The ratio of the negative electrode slurry is a mixed solution of SiO, flaky natural graphite, polyamic acid and N-methyl-2-pyrrolidone (NMP) (trade name: U-Varnish A, manufactured by Ube Industries, Ltd., solid content 18 wt% ) Was made into a mass ratio of 70:15:15 (however, the polyamic acid solution was a solid content mass), and a secondary battery was produced in the same manner as in Example 1 and subjected to a self-discharge test.

[Example 7]
A secondary battery was produced in the same manner as in Example 1 except that Si (catalog No. SIE07PB, manufactured by Kojundo Chemical Co., Ltd., 300 μm or less) was used instead of SiO in Example 1, and a self-discharge test was performed. .

[Example 8]
A secondary battery was produced in the same manner as in Example 1 except that SnO (catalog No SNO01PB, manufactured by Kojundo Chemical Co., Ltd.) was used instead of SiO in Example 1, and a self-discharge test was performed.

[Comparative Example 1]
A secondary battery was produced in the same manner as in Example 1 except that the particle size and circularity of the ground SiO in Example 1 were adjusted as shown in Table 1, and a self-discharge test was performed.

[Comparative Example 2]
A secondary battery was produced in the same manner as in Example 7 except that the particle size and circularity of Si after pulverization in Example 7 were adjusted as shown in Table 1, and a self-discharge test was performed.

[Comparative Example 3]
A secondary battery was produced in the same manner as in Example 8 except that the particle size and circularity of SnO after pulverization in Example 8 were adjusted as shown in Table 1, and a self-discharge test was performed.

[Comparative Example 4]
The ratio of the negative electrode slurry is a mixed solution of SiO, flaky natural graphite, polyamic acid and N-methyl-2-pyrrolidone (NMP) (trade name: U-Varnish A, manufactured by Ube Industries, Ltd., solid content 18 wt% ) Was made into a mass ratio of 82: 3: 15 (wherein the polyamic acid solution was a solid mass), a secondary battery was prepared in the same manner as in Comparative Example 1, and a self-discharge test was performed.

[Example 7]
The ratio of the negative electrode slurry is a mixed solution of SiO, flaky natural graphite, polyamic acid and N-methyl-2-pyrrolidone (NMP) (trade name: U-Varnish A, manufactured by Ube Industries, Ltd., solid content 18 wt% ) Was set to 50:35:84, respectively, and a secondary battery was produced in the same manner as in Example 1 and subjected to a self-discharge test.

The secondary battery provided in the present invention can be used in all industrial fields that require a power source and in industrial fields related to the transport, storage, and supply of electrical energy. Specifically, it can be used for a power source of a mobile device, a power source of a moving / transport medium, a backup power source, a solar power generation, a wind power generation, and a power storage facility for storing power generated by the power generation.

DESCRIPTION OF SYMBOLS 1 Laminated electrode body 2 Positive electrode 3 Negative electrode 4 Separator 5 Positive electrode collector 6 Negative electrode collector 7 Positive electrode lead terminal 8 Negative electrode lead terminal 10 Film exterior body 20 Battery element 25 Separator 30 Positive electrode 40 Negative electrode

Claims (9)

1. A circle formed of at least one material selected from a metal that can be alloyed with lithium and a metal oxide capable of occluding and releasing lithium ions (hereinafter referred to as metal and / or metal oxide) and defined by the following formula: A negative electrode containing particles having an average degree of 0.78 or more;
   A positive electrode;
   A single layer high heat resistance resin microporous membrane separator,
   A lithium ion secondary battery comprising:
   Circularity = 4πS / L ²
   (However, S is the area of the particle projection image, and L is the circumference of the particle projection image.)
2. 2. The lithium ion secondary battery according to claim 1, wherein a heat melting or thermal decomposition temperature of the high heat resistance resin constituting the single layer high heat resistance resin microporous film is 160 ° C. or more.
3. The lithium ion secondary battery according to claim 2, wherein the high heat resistant resin contains aramid.
4. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the metal and / or metal oxide contains at least Si and / or silicon oxide.
5. 5. The lithium ion secondary battery according to claim 1, further comprising a carbon material as the negative electrode active material.
6. The median diameter of the metal and / or metal oxide particles is 1 to 30 μm, the median diameter of the surface-coated carbon material particles is 5 to 50 μm, and the median diameter of the metal and / or metal oxide particles is The lithium ion secondary battery according to any one of claims 1 to 5, wherein the lithium ion secondary battery is smaller than the median diameter of the surface-coated carbon material.
7. A vehicle equipped with the lithium ion secondary battery according to any one of claims 1 to 6.
8. A power storage device using the lithium ion secondary battery according to claim 6.
9. A method for producing a lithium secondary battery, comprising: a step of laminating a positive electrode and a negative electrode via a separator to produce an electrode element; and a step of encapsulating the electrode element and an electrolytic solution in an exterior body,
   The negative electrode is formed of at least one material selected from a metal that can be alloyed with lithium and a metal oxide that can occlude and release lithium ions, and the average value of circularity defined by the following formula is 0.78. Including particles that are
   A method for producing a lithium ion secondary battery, wherein the separator is formed of a single-layer high-heat-resistant resin microporous film.
   Circularity = 4πS / L ²
   (However, S is the area of the particle projection image, and L is the circumference of the particle projection image.)

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