WO2014002561A1 - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

Provided is a non-aqueous electrolyte secondary battery capable of improving both input characteristics and stability. A lithium ion secondary battery (100) comprising a wound electrode body (55) comprising a positive electrode (10) and a negative electrode (20) wound having a separator (30) therebetween; having a negative electrode mixture layer (22) formed on the surface of the negative electrode (20); and including a negative electrode active substance in the negative electrode mixture layer (22). The average particle diameter (Dm) of the negative electrode active substance is 5-20 µm and the fine powder amount (P) being the cumulative frequency of negative electrode active substance having a particle diameter (D) of no more than 3 µm is greater than 10% and no more than 50%.

Description

Nonaqueous electrolyte secondary battery

The present invention relates to the technology of a non-aqueous electrolyte secondary battery.

As the non-aqueous electrolyte secondary battery, for example, a lithium ion secondary battery is well known. In recent years, lithium ion secondary batteries have become increasingly important as on-vehicle power sources mounted on hybrid vehicles and electric vehicles, or as power sources mounted on personal computers, portable terminals, and other electrical products.

Lithium ion secondary batteries include, for example, a box-shaped battery case, an electrode body housed inside the battery case, and a sealing body that seals the opening of the battery case by being joined to the battery case by laser welding ( Lid). In addition, the electrode body of the lithium ion secondary battery is configured as a wound electrode body that is wound in a state in which a negative electrode, a separator, and a positive electrode are stacked and further formed into a flat shape.

For example, Patent Document 1 discloses a lithium ion secondary battery for improving capacity reduction of a negative electrode, wherein the negative electrode active material has an average particle diameter of 7 μm or more and 20 μm or less, and a particle size distribution of the negative electrode active material. Discloses a lithium ion secondary battery in which the ratio of the particle size of 3 μm or less is 1% or more and 10% or less. However, if the ratio of the particle size of 3 μm or less in the particle size distribution of the negative electrode active material is 10% or less, the charging resistance increases and the input characteristics as a lithium ion secondary battery cannot be satisfied.

JP 2007-287622 A

The problem to be solved by the present invention is to provide a nonaqueous electrolyte secondary battery capable of improving both input characteristics and safety.

The problems to be solved by the present invention are as described above. Next, means for solving the problems will be described.

That is, according to the first aspect of the present invention, a wound electrode body configured by winding a positive electrode and a negative electrode through a separator is provided, and a negative electrode mixture layer is formed on a surface of the negative electrode. Is a non-aqueous electrolyte secondary battery containing a negative electrode active material, wherein the negative electrode active material has an average particle diameter of 5 μm or more and 20 μm or less, and a cumulative frequency of the negative electrode active material having a particle diameter of 3 μm or less. A certain fine powder amount is larger than 10% and 50% or less.

Claim 2 is the nonaqueous electrolyte secondary battery according to claim 1, wherein a heat-resistant layer is formed on at least one surface of the separator, and the thickness of the heat-resistant layer is 2 μm or more and 10 μm or less.

According to the nonaqueous electrolyte secondary battery of the present invention, both input characteristics and safety can be improved.

The schematic diagram which showed the whole structure of the lithium ion secondary battery. The cross-sectional schematic diagram which showed the electrode body. The graph which showed the amount of fine powder. The graph which showed the characteristic of the amount of fine powder. The graph which showed the characteristic of the thickness of the HRL layer in a separator.

The configuration of the lithium ion secondary battery 100 will be described with reference to FIG.
In FIG. 1, the battery case 40, the wound electrode body 55, and the lid body 60 are separated and schematically shown for easy understanding.

The lithium ion secondary battery 100 is an embodiment according to the non-aqueous electrolyte secondary battery of the present invention. The lithium ion secondary battery 100 includes a battery case 40, a wound electrode body 55, and a lid body 60.

The battery case 40 is configured as a substantially rectangular parallelepiped box having an upper surface opened. The opened upper surface of the battery case 40 is sealed by the lid body 60. A wound electrode body 55 is accommodated in the battery case 40.

The wound electrode body 55 is obtained by winding an electrode body 50 (see FIG. 2) in which the negative electrode 20, the positive electrode 10, and the separator 30 are laminated so that the separator 30 is interposed between the negative electrode 20 and the positive electrode 10. It is formed into a shape.

The wound electrode body 55 is accommodated in the battery case 40 so that the axial direction of the wound electrode body 55 and the sealing direction of the opening of the battery case 40 by the lid body 60 are orthogonal to each other.

A positive electrode current collector 51 (in which only the current collector foil 11 described later is wound) is exposed at the end on one side in the axial direction of the wound electrode body 55. On the other hand, a negative electrode current collector 52 (only a current collector foil 21 to be described later is wound) is exposed at the end of the wound electrode body 55 on the other side in the axial direction.

The lid 60 seals the upper surface of the battery case 40. More specifically, the lid 60 seals the upper surface of the battery case 40 by being joined to the upper surface of the battery case 40 by laser welding. That is, in the lithium ion secondary battery 100, the opening of the battery case 40 is sealed by joining the lid 60 to the opening of the battery case 40 by laser welding.

A positive electrode current collector terminal 61 and a negative electrode current collector terminal 62 are provided on the upper surface of the lid 60. The positive current collecting terminal 61 is formed with a leg portion 71 extending downward. Similarly, the negative electrode current collecting terminal 62 is formed with a leg portion 72 extending downward.

When the wound electrode body 55 is accommodated in the battery case 40, the positive electrode current collector 51 of the wound electrode body 55 is joined to the leg portion 71 of the positive electrode current collector terminal 61. Similarly, when the wound electrode body 55 is accommodated in the battery case 40, the negative electrode current collector 52 of the wound electrode body 55 is joined to the leg portion 72 of the negative electrode current collector terminal 62. That is, the wound electrode body 55 is accommodated in the battery case 40 in a state where the wound electrode body 55 is joined to the lid body 60 including the positive electrode current collector terminal 61 and the negative electrode current collector terminal 62.

A liquid injection hole 63 is provided on the upper surface of the lid 60, and the wound electrode body 55 is attached to the battery case 40 in a state where the wound electrode body 55 is joined to the lid 60 having the positive electrode current collector terminal 61 and the negative electrode current collector terminal 62. The battery is completed by injecting the electrolytic solution from the liquid injection hole 63 after being accommodated and joining the lid 60 and the upper surface of the battery case 40 by laser welding.

The electrode body 50 will be described with reference to FIG.
In FIG. 2, a part of the electrode body 50 is schematically shown in a cross-sectional view.

The electrode body 50 is obtained by stacking the negative electrode 20, the positive electrode 10, and the separator 30 so that the separator 30 is interposed between the negative electrode 20 and the positive electrode 10.

The positive electrode 10 includes a current collector foil 11 and a positive electrode mixture layer 12. The positive electrode mixture layer 12 is formed on both surfaces of the current collector foil 11. Positive electrode mixture layer 12, the positive electrode active material (for _{example, Li 1. 14 Ni 0.} 34 Co 0. 33 Mn 0. 33 O 2) and a conductive agent (e.g., acetylene black (AB)) and, a binder A positive electrode mixture prepared by kneading (for example, polyvinylidene fluoride (PVDF)) with a solvent (for example, N-methyl-2-pyrrolidone (NMP)) at a predetermined ratio is placed on the current collector foil 11. It was pressed after being applied and dried.

[Positive electrode active material]
The positive electrode mixture forming the positive electrode mixture layer 12 of the positive electrode 10 includes a positive electrode active material that inserts and desorbs lithium ions. Examples of the positive electrode active material include lithium transition metal composite oxides (LiNiO ₂ , LiCoO ₂ , LiNiCoMnO _2, etc.) typically having a layered crystal structure (typically a layered rock salt structure belonging to a hexagonal system). Part W, Cr, Mo, Zr, Mg, Ca, Na, Fe, Zn, Si, Sn, Al and the like, and a lithium transition metal composite oxide having a spinel crystal structure (LiMn ₂ O ₄ , LiNiMn ₂ O ₄ , and the like) and lithium transition metal complex oxides (LiFePO _4, etc.) having a crystal structure of an olivine structure.

[Positive electrode mixture]
In addition to the positive electrode active material, additives such as a conductive material and a binder (binder) are added to the positive electrode mixture as necessary.
As the conductive material, carbon powder (carbon black such as acetylene black (AB), furnace black and ketjen black, graphite powder, graphite powder, etc.), and conductive substances such as conductive carbon fiber are used alone. Or it can contain as a mixture of 2 or more types.

Bind materials include various polymer materials. For example, when a solvent mainly composed of water is used as the dispersion medium, a polymer material that is dissolved or dispersed in water can be preferably used as the binder. Examples of water-soluble or water-dispersible polymer materials include cellulose polymers such as carboxymethyl cellulose (CMC), fluorine resins such as polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), vinyl acetate polymer, and styrene butadiene. Examples thereof include rubbers such as rubber (SBR). When a solvent mainly composed of an organic solvent such as N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium, a polymer material such as polyvinylidene fluoride (PVDF) or polyalkylene oxide such as polyethylene oxide (PEO) Can be used as a binder. The aforementioned binders may be used in combination of two or more, and may be used as a thickener and other additives.

The ratio of each constituent component of the positive electrode active material, the conductive material, the binder, and the like in the positive electrode mixture is determined from the viewpoint of the retention of the positive electrode mixture layer 12 on the current collector foil 11 and the battery performance. Typically, for example, the positive electrode active material is preferably about 75 to 95 wt%, the conductive material is about 3 to 18 wt%, and the binder is about 2 to 7 wt%.

[Production method of positive electrode]
First, a positive electrode active material, a conductive material, a binder and the like are mixed with an appropriate solvent to prepare a positive electrode mixture. This mixing preparation can be performed, for example, using a kneader such as a planetary mixer, a homodisper, Claremix (registered trademark), and Fillmix (registered trademark).

The positive electrode mixture thus prepared is applied to the current collector foil 11 by a coating device such as a slit coater, a die coater, a gravure coater, and a comma coater (registered trademark), and the solvent is evaporated by drying and then pressed. Through the above steps, the positive electrode 10 in which the positive electrode mixture layer 12 is formed on the current collector foil 11 is obtained.

The basis weight per unit area (mg / cm ² ) of the positive electrode mixture on the current collector foil 11 is not limited to energy in high output applications such as hybrid vehicles, and the electron conductivity in the positive electrode mixture layer 12 and in view of the lithium ion diffusibility, it is preferable that the per side ^{6mg / cm 2 ~ 20mg / cm} 2 of collector foil 11. For the same reason also the density of the positive electrode mixture layer ^12, it is preferable to 1.7g / cm 3 ~ 2.8g / cm 3.

For the current collector foil 11, a conductive member made of a highly conductive metal is preferably used, and aluminum or an alloy mainly composed of aluminum can be used. The shape and thickness of the current collector foil 11 are not particularly limited, and the shape may be a sheet shape, a foil shape, a mesh shape, or the like, and the thickness may be, for example, 10 μm to 30 μm.

The negative electrode 20 includes a current collector foil 21 and a negative electrode mixture layer 22. The negative electrode mixture layer 22 is formed on both surfaces of the current collector foil 21. The negative electrode mixture layer 22 kneads a negative electrode active material, a thickener (for example, carboxymethyl cellulose (CMC)), and a binder (for example, styrene butadiene rubber (SBR)) with water at a predetermined ratio. The negative electrode mixture prepared in this manner was applied onto the current collector foil 21 and dried, and then pressed. The negative electrode active material of the present embodiment is prepared by mixing and impregnating a predetermined proportion of pitch with spheroidized natural graphite coated with low crystalline carbon and firing it in an inert atmosphere.

[Negative electrode active material]
The negative electrode mixture forming the negative electrode mixture layer 22 of the negative electrode 20 includes a negative electrode active material that inserts and desorbs lithium ions. Examples of the negative electrode active material include various oxides such as lithium titanate, simple substances such as silicon materials and tin materials, alloys, compounds, and composite materials using the above materials in combination. However, when the viewpoints of cost, productivity, energy density, and long-term reliability are combined, it is most preferable to employ a carbon material mainly composed of graphite as the negative electrode active material. In particular, in high-power applications such as hybrid vehicles, a composite material in which the surface of particles having graphite as a core is coated with amorphous carbon, which can improve lithium ion insertion / extraction, is more preferable. Further, carbon materials other than graphite such as non-graphitizable amorphous carbon and easily graphitizable amorphous carbon may be mixed.

In the negative electrode active material according to the present invention, the cumulative frequency of a particle diameter of 3 μm or less obtained by measurement with a flow type particle image analyzer is preferably 10% or more and 50% or less.

Among the above graphites, for example, spheroidized natural graphite can be used as the negative electrode active material. The spheronization treatment is usually performed by applying stress in a direction parallel to the graphite crystal basal surface (AB surface) of the scaly graphite particles or the like by mechanical treatment, so that the graphite crystal basal surface is concentrically or folded. It is made spherical while taking a fold structure. By performing pulverization or grinding, and sieving or classification, spheroidized natural graphite having a desired particle size can be obtained. Classification can be performed by a method such as air classification, wet classification, or specific gravity classification, but it is preferable to use an air classifier. In this case, the target particle size distribution can be adjusted by controlling the air volume and the wind speed.

The graphite may be low crystalline carbon-coated natural graphite in which spheroidized graphite as a core is coated with an amorphous carbon material. Since low crystalline carbon-coated natural graphite contains spheroidized graphite as a core, a high energy density can be obtained. In general, spheroidized graphite has an edge portion (typically, the end of the hexagonal mesh surface (basal surface) of graphite) and a non-aqueous electrolyte (typically a non-aqueous solvent contained in the electrolyte). Although it is known that the reaction causes a decrease in battery capacity and an increase in resistance, the surface is coated with an amorphous carbon material, so the reactivity with the non-aqueous electrolyte is relatively low. Yes. Therefore, in the lithium ion secondary battery including the low crystalline carbon-coated natural graphite as the negative electrode active material, the irreversible capacity is suppressed and high durability can be exhibited.

The low crystalline carbon-coated natural graphite can be produced by, for example, a general gas phase method (dry method) or a liquid phase method (wet method). Thereby, a carbon material having low reactivity with the electrolytic solution can be suitably imparted to a part of the spheroidized graphite (typically, part of the outer surface). As an example, spheroidized graphite as a core and carbonizable materials such as pitch and tar as precursors of amorphous carbon are mixed in an appropriate solvent, and the carbon material is mixed with the surface of spheroidized graphite. Adhere to. And the said low crystalline carbon covering natural graphite can be produced by baking the carbon material adhering to this surface. The mixing ratio of the spheroidized graphite and the carbon material can be appropriately determined depending on the type and properties of the carbon material used. The firing temperature can be set at, for example, 800 ° C. to 1300 ° C.

[Negative electrode mix]
In addition to the negative electrode active material, additives such as a thickener and a binder are added to the negative electrode mixture.
Various polymer materials are mentioned as a thickener and a binder. For example, when a solvent mainly composed of water is used as the dispersion medium, a polymer material that dissolves or disperses in water can be preferably used as the thickener and the binder. Examples of water-soluble or water-dispersible polymer materials include cellulose polymers such as carboxymethyl cellulose (CMC), fluorine resins such as polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), vinyl acetate polymer, and styrene butadiene. Examples thereof include rubbers such as rubber (SBR). When a solvent mainly composed of an organic solvent such as N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium, a polymer material such as polyvinylidene fluoride (PVDF) or polyalkylene oxide such as polyethylene oxide (PEO) Can be used as a thickener and a binder. The aforementioned thickener and binder may be used in combination of two or more.

The proportions of the respective constituent components such as the negative electrode active material, the thickener and the binder in the negative electrode mixture are determined from the viewpoint of the retention of the negative electrode mixture layer 22 on the current collector foil 21 and the battery performance. . Typically, it is preferable that the negative electrode active material is, for example, about 90 to 99 wt%, and the thickener and the binder are about 1 to 10 wt%.

[Production method of negative electrode]
First, a negative electrode active material, a thickener, a binder and the like are mixed with an appropriate solvent to prepare a negative electrode mixture. This mixing preparation can be performed, for example, using a kneader such as a planetary mixer, a homodisper, Claremix (registered trademark), and Fillmix (registered trademark).

The negative electrode mixture thus prepared is applied to the current collector foil 21 by a coating device such as a slit coater, a die coater, a gravure coater, and a comma coater (registered trademark), and the solvent is evaporated by drying and then pressed. Through the above steps, the negative electrode 20 in which the negative electrode mixture layer 22 is formed on the current collector foil 21 is obtained.

The basis weight per unit area (mg / cm ² ) of the negative electrode mixture on the current collector foil 21 is not only energy, but also the electronic conductivity in the negative electrode mixture layer 22 in high output applications such as hybrid vehicles. in view of the lithium ion diffusibility, it is preferable that one surface per ^{3mg / cm 2 ~ 10mg / cm} 2 of collector foil 21. For the same reason also the density of the negative electrode mixture layer ^22, it is preferable to 1.0g / cm 3 ~ 1.4g / cm 3.

For the current collector foil 21, a conductive member made of a metal having good conductivity is preferably used, and copper or an alloy containing copper as a main component can be used. The shape and thickness of the current collector foil 21 are not particularly limited, and the shape may be a sheet shape, a foil shape, a mesh shape, or the like, and the thickness may be, for example, 5 μm to 20 μm.

The separator 30 includes a base material layer 31 and a Heat Resistance layer (HRL) layer 32 as a heat resistant layer. The HRL layer 32 is formed on both surfaces of the base material layer 31. The HRL layer 32 of the present embodiment is formed from a porous inorganic filler. Here, the thickness of the HRL layer 32 is defined as a thickness T.

[Separator]
The separator 30 insulates the positive electrode mixture layer 12 and the negative electrode mixture layer 22 and allows the electrolyte to move during normal use. When the inside of the battery becomes a high temperature (eg, 130 ° C. or higher) due to an abnormal phenomenon. A mechanism for blocking the movement of the electrolyte is provided. As the base material layer 31 of the separator 30, a porous resin can be adopted. For example, polyolefin resin such as polyethylene (PE) and polypropylene (PP) can be suitably employed as the base material layer 31. In particular, it is preferable to employ a separator having a three-layer structure in which PP, PE, and PP are sequentially laminated.

The base material layer 31 can be made porous by, for example, uniaxial stretching or biaxial stretching. In particular, uniaxial stretching in the longitudinal direction is suitable as an element of the separator 30 constituting the wound electrode body 55 because there is little thermal contraction in the width direction.

The thickness of the separator 30 is not particularly limited, but is preferably about 10 to 30 μm, typically about 15 to 25 μm. When the thickness of the separator 30 is within the above range, the ion permeability of the separator 30 is further improved, and film breakage due to shrinkage at high temperature and melting is particularly difficult to occur.

The HRL layer 32 is configured on at least one surface of the base material layer 31, and suppresses shrinkage of the base material layer 31 when the inside of the battery becomes high temperature. Even so, a short circuit due to direct contact between the positive electrode 10 and the negative electrode 20 is suppressed. The HRL layer 32 is mainly composed of inorganic fillers such as inorganic oxides such as alumina, boehmite, silica, titania, zirconia, calcia, and magnesia, inorganic nitrides, carbonates, sulfates, fluorides, and covalent crystals. Include as. In addition, it is preferable that alumina, boehmite, silica, titania, zirconia, zirconia, calcia or magnesia is included in the HRL layer 32, and that boehmite or alumina is included in the HRL layer 32 because it is excellent in heat resistance and cycle characteristics. preferable.

The shape of the inorganic filler is not particularly limited, but is preferably a plate-like (flaky) particle from the viewpoint of suppressing a short circuit between the positive electrode 10 and the negative electrode 20 when the base material layer 31 is broken. The average particle size of the inorganic filler is not particularly limited, but it is suitably from 0.1 μm to 5 μm from the viewpoint of smoothness of the film surface, input / output performance, and function at high temperature.

The HRL layer 32 preferably contains an additive such as a binder from the viewpoint of retention on the base material layer 31. The HRL layer 32 is generally formed by preparing a paste by dispersing an inorganic filler and an additive in a solvent, applying the paste onto the base material layer 31, and drying the paste. The dispersion solvent is not particularly limited, such as an aqueous solvent or an organic solvent, but it is preferable to use an aqueous solvent in consideration of cost and handleability. As an additive when the aqueous solvent is used, a polymer that is dispersed or dissolved in the aqueous solvent can be used. For example, polyolefin resins such as styrene butadiene rubber (SBR) and polyethylene (PE), cellulose polymers such as carboxymethyl cellulose (CMC), fluororesins such as polyvinyl alcohol (PVA), or polyethylene oxide (PEO) Alkylene oxide can be used. In addition, acrylics such as homopolymers obtained by polymerizing monomers such as acrylic acid, methacrylic acid, acrylamide, methacrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, methyl methacrylate, 2-ethylhexyl acrylate and butyl acrylate in one kind Based resins. The additive may be a copolymer obtained by polymerizing two or more of the monomers. Further, a mixture of two or more of the above homopolymers and copolymers may be used.

The proportion of the inorganic filler in the entire HRL layer 32 is not particularly limited, but is preferably 90% by mass or more, and typically 95% by mass or more from the viewpoint of securing the function at high temperature.

The thickness of the HRL layer 32 is preferably 2 μm to 10 μm.

The HRL layer 32 can be formed by the following method, for example.
First, the above-described inorganic filler and additive are dispersed in a dispersion medium to produce a paste. For paste preparation, dispersyl (registered trademark), Claremix (registered trademark), fillmix (registered trademark), a kneader such as a ball mill, a homodisper, and an ultrasonic disperser can be used. The obtained paste is coated on the surface of the base material layer 31 with a coating apparatus such as a gravure coater, slit coater, die coater, comma coater (registered trademark), dip coater, and the like, and dried to form the HRL layer 32. It is preferable that the drying temperature is equal to or lower than the temperature at which the separator 30 contracts (for example, 110 ° C. or lower).

[Non-aqueous electrolyte]
As the non-aqueous solvent and the electrolyte salt constituting the electrolytic solution injected into the lithium ion secondary battery 100, those used in conventional lithium ion secondary batteries can be used without any particular limitation. Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2 -Diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ- Butyrolactone can be used, and one of these can be used alone or in admixture of two or more. In particular, a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) is preferable.

Examples of the electrolyte salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ). One or two or more lithium compounds (lithium salts) such as ₃ and LiI can be used. The concentration of the electrolyte salt is not particularly limited, but can typically be 0.8 mol / L to 1.5 mol / L.

The electrolytic solution may contain an additive as long as the object of the present invention is not significantly impaired. Examples of the additive include fluorophosphate (preferably difluorophosphate. For example, lithium difluorophosphate represented by LiPO ₂ F ₂ ), lithium bisoxalate borate (LiBOB), and the like. And improvement of durability reliability (capacity reduction, suppression of resistance rise, etc.) and the like. The concentration of each additive in the electrolytic solution can be appropriately changed according to the purpose, and is, for example, 0.01 to 0.1 mol / L.

Since the lithium ion secondary battery 100 having such a configuration is excellent in both input / output characteristics and thermal stability during overcharge, the hybrid vehicle (HV), plug-in hybrid vehicle (PHV), and electric vehicle (EV) are particularly preferred. ), Or a power source for a drive source such as a drive motor of an automobile equipped with an electric motor such as a fuel cell automobile (typically, a battery pack formed by connecting a plurality of them in series).

(Preparation of positive electrode)
A mixed solution of Ni sulfate, Co sulfate, and Mn sulfate solution was neutralized with Na hydroxide, and Ni ₀ . ₃₄ Co ₀ . ₃₃ Mn ₀ . A precursor based on ₃₃ (OH) ₂ was prepared. The obtained precursor was mixed with Li carbonate, and optionally calcined at 800 to 950 ° C. for 5 to 15 hours in an air atmosphere to obtain Li ₁ . ₁₄ Ni ₀ . ₃₄ Co ₀ . ₃₃ Mn ₀ . ₃₃ O ₂ was produced. This positive electrode active material was adjusted so that the particle diameter D50 was 3 to 8 μm and the specific surface area was 0.5 to 1.9 m ² / g.

The positive electrode active material, AB (conductive material), and PVDF (binder) are mixed with NMP (dispersion medium) so that the mass ratio of these materials is 90: 8: 2, An agent was prepared. This positive electrode mixture was applied to both surfaces of an aluminum foil (current collector foil) having a thickness of 15 μm. The coating amount of the positive electrode mixture on both sides was adjusted to be about 11.3 mg / cm ² (on a solid basis after drying). The coated positive electrode mixture was dried and then pressed with a rolling press to adjust the density of the positive electrode mixture layer to 1.8 to 2.4 g / cm ³ . The obtained electrode was slit to produce a strip-shaped positive electrode having a length of 3000 mm and a width of 98 mm.

[Production of negative electrode]
The particle size of natural graphite powder was adjusted using an air classifier to obtain natural graphite powder having different particle sizes. The obtained natural graphite powder was mixed with pitch (mass ratio of natural graphite powder and pitch = 96: 4) and fired at 800 to 1300 ° C. for 10 hours in an N ₂ atmosphere. Through the above steps, negative electrode active materials having different amounts of fine powder and different surface areas were obtained. This negative electrode active material, SBR, and CMC were mixed with ion-exchanged water at a weight ratio of 97.0: 1.5: 1.5, and sheared with a planetary mixer to prepare a negative electrode mixture. This negative electrode mixture was applied to both sides of a 10 μm thick copper foil. The coating amount of the negative electrode mixture on both sides was adjusted to about 7.0 mg / cm ² (after drying, based on solid content). The coated negative electrode mixture was dried and then pressed by a rolling press to adjust the density of the negative electrode mixture layer to about 0.9 g / cm ³ to 1.3 g / cm ³ . The obtained electrode was slit to produce a strip-shaped negative electrode having a length of 3200 mm and a width of 102 mm.

[Production of heat-resistant separator]
Alumina powder (Al ₂ O ₃ ) as an inorganic filler, an acrylic binder, and CMC as a thickener, and the blend ratio of Al ₂ O ₃ : binder: CMC is 98: 1.3: 0.7 Thus, a paste was prepared by kneading with ion-exchanged water as a solvent. This paste was applied to one side of a polyethylene single-layer porous sheet having a thickness of 20 μm and dried at 70 ° C. to form an inorganic porous layer (heat-resistant layer) to obtain a heat-resistant separator. The amount of paste applied (weight per unit area) was adjusted to 0.7 mg / cm ² on a solid basis. The thickness of the inorganic porous layer after drying was 4 μm.

[Preparation of electrolyte]
The electrolyte was prepared by dissolving 1.1 mol / L LiPF ₆ in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at 3: 3: 4, and an additive. It was prepared by dissolving difluorophosphate (LiPO ₂ F ₂ ) and lithium bisoxalate borate (LiBOB). The mixing ratio of the additive is as described above.

[Production of cell]
The positive electrode and the negative electrode were overlapped via the two heat-resistant separators to produce a flat wound electrode body.
This wound electrode body was sealed in a box-type battery case together with the electrolyte.
Cell evaluation was performed after implementing first time charge / discharge with respect to the battery cell produced as mentioned above.

[Particle size distribution measurement method]
The amount of fine powder was measured using a flow type particle image analyzer (manufactured by Sysmex Corporation: FPIA (registered trademark) -3000). Dispersion conditions were performed using RO water and a surfactant (Naroacti (registered trademark)) at a stirring speed of 300 rpm.

[Leakage current measurement method]
The cell was adjusted to SOC 30% at −10 ° C., charged at a current value of 40 A, and the maximum current value 10 minutes after the separator substrate was shut down was measured.

[Charging resistance measurement method]
The charging resistance at 25 ° C. of the cell adjusted to SOC 60% was measured by the following procedures (1) to (4).
(1) In a temperature environment of 25 ° C., charge from 3 V to SOC 60% (CC charge) at a constant current of 1 C, and then charge at the same voltage for 2.5 hours (CV charge).
(2) The battery after (1) is charged at 25 ° C., and the number of seconds from the start of charging until the voltage reaches 4.15 V (charge cut voltage) is measured.
(3) Charge input for constant wattage charge in (2) (charge power amount for constant wattage charge) is varied between 80 W and 200 W (more specifically, charge input for constant watt discharge in step (3)) The above steps (1) to (3) are repeated while increasing the first 80W, the second 90W, the third 100W.
(4) From the approximate curve in which the horizontal axis represents the number of seconds up to a voltage of 2.0 V measured in each constant watt discharge of the procedure (3) and the vertical axis represents the constant wattage charge input at that time, the voltage is 4.15 V. An input value (SOC 60% input) when the number of seconds until is 2 seconds is obtained.

The amount P of fine powder is demonstrated using FIG.
In FIG. 3, the horizontal axis represents the particle diameter D of the negative electrode active material, and the vertical axis represents the cumulative frequency of the amount of the negative electrode active material having a particle diameter D or less with respect to the total amount of the negative electrode active material.

As shown in FIG. 3, the particle diameter D of the negative electrode active material shows non-uniform variation between 0 μm and 10 μm. Here, the negative electrode active material having a particle diameter D of 3 μm or less is referred to as fine powder, and the cumulative frequency of the negative electrode active material having a particle diameter D of 3 μm or less is defined as the fine powder amount P. That is, if the fine powder amount P is 15%, the cumulative frequency with a particle diameter D of 3 μm or less is 15%. In addition, about the particle diameter D of the negative electrode active material of this embodiment, the average particle diameter Dm (particle diameter D50) is 5 micrometers or more and 20 micrometers or less.

The characteristics of the fine powder amount P will be described with reference to FIG.
In FIG. 4A, the horizontal axis is the fine powder amount P of the negative electrode active material, and the vertical axis is the charging resistance ratio R indicating the input characteristics of the lithium ion secondary battery 100, and the relationship between the fine powder quantity P and the input characteristics. Represents. FIG. 4B shows the relationship between the fine powder amount P and safety, where the horizontal axis is the fine powder amount P of the negative electrode active material and the vertical axis is the leakage current J indicating the safety of the lithium ion secondary battery 100. Represents.

As shown in FIG. 4A, there is a correlation between the fine powder amount P of the negative electrode active material and the charge resistance ratio R, and it is known that the charge resistance ratio R decreases as the fine powder amount P increases. The reason for this is that in the negative electrode with a small amount of fine powder, the gap between the negative electrode active materials in the negative electrode mixture layer is large, so that the conductivity is lowered. In the negative electrode with a large amount of fine powder, the negative electrode active material having a relatively large particle diameter D This is because fine powder enters the gap and the conductivity increases.

The charging resistance ratio R indicates a charging resistance value with respect to another fine powder amount P when the charging resistance value of the lithium ion secondary battery 100 with respect to a certain fine powder amount P is 100, and each fine powder amount P Is a dimensionless charge resistance. Here, the fine powder amount P is required to be 10% or more when the charge resistance ratio R criterion (determination condition for satisfying the standard) indicating the input characteristics of the lithium ion secondary battery 100 is R1 or less. The

As shown in FIG. 4B, it is known that there is a correlation between the fine powder amount P of the negative electrode active material and the leakage current J. The leakage current J is the maximum value of the leakage current value measured under a predetermined condition. Here, the fine powder amount P is required to be 50% or less when the leakage current J criteria (determination condition for satisfying the criteria) indicating the safety of the lithium ion secondary battery 100 is E1 or less. .

Based on the above, considering the input characteristics and safety criteria, the fine powder amount P of the negative electrode active material of the present embodiment is 10% or more and 50% or less.

It has been found that for the negative electrode active material having a fine powder amount P of 10% or more and 50% or less, the specific surface area measured by the Kr gas adsorption method is 2.0 to 5.0 m ² / g.
The Kr gas adsorption method is a method for adsorbing molecules (Kr) whose occupied area is known on the surface of powder particles and obtaining the specific surface area of the sample powder from the amount of adsorption. The specific surface area is the sum of the surface areas of all particles contained in the powder of unit mass.

The characteristics of the thickness T will be described with reference to FIG.
5A shows the relationship between the thickness T and the input characteristics, where the horizontal axis is the thickness T of the HRL layer 32 and the vertical axis is the charging resistance ratio R indicating the input characteristics of the lithium ion secondary battery 100. ing. FIG. 5B shows the relationship between the thickness T and safety, where the horizontal axis is the thickness T of the HRL layer 32 and the vertical axis is the leakage current J indicating the safety of the lithium ion secondary battery 100. Yes.

As shown in FIG. 5A, there is a correlation between the thickness T of the HRL layer 32 and the charging resistance ratio R, and it is known that the charging resistance ratio R increases as the thickness T increases.

Here, when the criterion of the charging resistance ratio R (determination condition for satisfying the standard) is R1 or less, the thickness T is required to be 10 μm or less.

As shown in FIG. 5B, it is known that the thickness T of the HRL layer 32 and the leakage current J have a correlation. Here, when the criterion of the leakage current J (determination condition for satisfying the standard) is E1 or less, the thickness T is required to be 2 μm or more.

Based on the above, considering the input characteristics and safety criteria, the thickness T of the separator 30 of this embodiment is 2 μm or more and 10 μm or less.

The effect of the lithium ion secondary battery 100 will be described.
According to the lithium ion secondary battery 100, the input characteristics and the safety can be improved at the same time.
That is, there is a correlation between the fine powder amount P of the negative electrode active material and the charging resistance ratio R, and there is a correlation between the fine powder amount P and the leakage current J. By defining the amount P of fine powder that satisfies the criteria for the leakage current J, which is an index of safety, it is possible to improve both input characteristics and safety.

Further, since there is a correlation between the thickness T of the HRL layer 32 and the charging resistance ratio R, and there is a correlation between the thickness T of the HRL layer 32 and the leakage current J, the charging resistance ratio R, which is an index of input characteristics, By defining the thickness T that satisfies the criteria for the leakage current J, which is an index of safety, it is possible to improve both input characteristics and safety.

The present invention can be used for a non-aqueous electrolyte secondary battery.

DESCRIPTION OF SYMBOLS 10 Positive electrode 11 Current collection foil 12 Positive electrode mixture layer 20 Negative electrode 21 Current collection foil 22 Negative electrode mixture layer 30 Separator 55 Winding electrode body 100 Lithium ion secondary battery

Claims (2)

A wound electrode body configured by winding a positive electrode and a negative electrode through a separator is provided, a negative electrode mixture layer is formed on the surface of the negative electrode, and the negative electrode mixture layer contains a negative electrode active material. A water electrolyte secondary battery,
The average particle diameter of the negative electrode active material is 5 μm or more and 20 μm or less,
The amount of fine powder, which is the cumulative frequency of the negative electrode active material having a particle size of 3 μm or less, is greater than 10% and 50% or less.
Non-aqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1,
A heat-resistant layer is formed on at least one surface of the separator;
The heat-resistant layer has a thickness of 2 μm or more and 10 μm or less.
Non-aqueous electrolyte secondary battery.

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