Negative electrode and secondary battery including the same

Abstract

To provide a negative electrode that can suppress both capacity deterioration and resistance increase when a battery is stored at a high temperature.SOLUTION: A negative electrode disclosed herein includes a negative electrode current collector, and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and a binder. The binder includes a polycarboxylic acid binder. The ratio of the average particle diameter (D50B) of the polycarboxylic acid binder when dispersed in water to the average particle diameter (D50A) of the negative electrode active material is 0.19 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a negative electrode. The present invention also relates to a secondary battery including the negative electrode.

Background Art

[0002] In recent years, secondary batteries such as lithium ion secondary batteries have been suitably used for portable power sources such as personal computers and mobile terminals, and power sources for driving vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] Typical electrodes of secondary batteries, particularly electrodes of lithium ion secondary batteries, generally have a configuration in which an active material layer containing an active material is supported by a current collector. In this active material layer, a binder is used to join the active material particles to each other and the active material particles to the current collector.

[0004] For the purpose of reducing environmental impact, water may be used as a solvent in the electrode paste used for forming the active material layer. As a binder suitable for an electrode paste containing water as a solvent (that is, an aqueous electrode paste), a polycarboxylic acid-based binder, which is a polymer of an unsaturated carboxylic acid such as acrylic acid, is known. For example, Patent Document 1 discloses that by using a polymer polycarboxylic acid as a binder for a negative electrode, the energy density, high-temperature storage characteristics, and durability during cycle driving of a lithium ion secondary battery can be improved.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In Patent Document 1, the capacity retention rate after high-temperature storage is evaluated as a high-temperature storage characteristic. On the other hand, when a battery is stored at a high temperature, it is difficult to achieve both suppression of resistance increase and suppression of capacity degradation. As a result of intensive studies by the present inventors, when a negative electrode is produced using a conventional polycarboxylic acid-based binder and a battery is constructed using the negative electrode, as described in Patent Document 1, the resistance increase suppression when the battery is stored at a high temperature is insufficient, although the resistance increase suppression when the battery is stored at a high temperature is insufficient. It was found that there is a problem that the resistance increase suppression when the battery is stored at a high temperature is insufficient.

[0007] Therefore, an object of the present invention is to provide a negative electrode capable of suppressing both capacity degradation and resistance increase when a battery is stored at a high temperature.

Means for Solving the Problems

[0008] The negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material and a binder. The binder includes a polycarboxylic acid-based binder. The ratio of the average particle diameter (D50 A ) of the polycarboxylic acid-based binder when dispersed in water to the average particle diameter (D50 B ) of the negative electrode active material is 0.19 or less. According to such a configuration, it is possible to provide a negative electrode capable of suppressing both capacity degradation and resistance increase when a battery is stored at a high temperature.

[0009] Here, it is more advantageous that the average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water is 0.1 μm or more and 5.0 μm or less.

[0010] Here, it is more advantageous that the polymer constituting the polycarboxylic acid-based binder contains a crosslinkable monomer unit.

[0011] Further, it is more advantageous that the negative electrode disclosed herein is a negative electrode of a lithium ion secondary battery.

[0012] From another aspect, the secondary battery disclosed herein includes a positive electrode, the negative electrode described above, and a non-aqueous electrolyte. According to such a configuration, a battery capable of suppressing both capacity degradation and resistance increase when stored at high temperatures can be provided.

[0013] Here, it is more advantageous that the non-aqueous electrolyte contains an oxalato complex coordinated with B, P, or Si as a film-forming agent.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that matters not mentioned in this specification but necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. In the following drawings, members and parts having the same function are denoted by the same reference numerals and described. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

[0016] In this specification, the term "secondary battery" refers to a rechargeable power storage device, and is a term including so-called storage batteries and power storage elements such as electric double layer capacitors. Also, in this specification, the term "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0017] The negative electrode disclosed herein is typically used in a secondary battery, preferably in a lithium-ion secondary battery. FIG. 1 is a cross-sectional view schematically showing a negative electrode 60 according to an exemplary embodiment of the negative electrode disclosed herein, and is a cross-sectional view perpendicular to the thickness direction. The negative electrode 60 according to the exemplary embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0018] As shown in the drawing, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0019] In the illustrated example, a negative electrode active material layer non-formation portion 62a where the negative electrode active material layer 64 is not provided is provided at one end in the width direction of the negative electrode 60. In the negative electrode active material layer non-formation portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-formation portion 62a can function as a current collection portion. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0020] The shape of the negative electrode current collector 62 is, in the illustrated example, in the form of a foil (or sheet), but is not limited thereto. The negative electrode current collector 62 may be in various forms such as a rod shape, a plate shape, or a mesh shape. As the material of the negative electrode current collector 62, a metal having good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be used as in a conventional lithium-ion secondary battery, and among them, copper is preferable. As the negative electrode current collector 62, a copper foil is particularly preferable.

[0021] The dimensions of the negative electrode current collector 62 are not particularly limited and may be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0022] The negative electrode active material layer 64 contains a negative electrode active material and a binder. As the negative electrode active material, for example, carbon materials such as graphite, hard carbon, and soft carbon can be used. Among them, graphite is preferable because the effects of the present invention can be obtained more highly. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which the graphite is coated with an amorphous carbon material.

[0023] In the present embodiment, a polycarboxylic acid-based binder is used as the binder contained in the negative electrode active material layer 64. In this specification, the "polycarboxylic acid-based binder" refers to a binder composed of a polymer (hereinafter also referred to as "polycarboxylic acid-based polymer") containing an unsaturated carboxylic acid or its salt as a monomer unit. Examples of the unsaturated carboxylic acid or its salt include acrylic acid or its salt, methacrylic acid or its salt, maleic acid or its salt, fumaric acid or its salt, etc., and acrylic acid or its salt, and methacrylic acid or its salt are preferable, and acrylic acid or its salt is preferable. Therefore, in the present embodiment, the polycarboxylic acid-based binder is preferably an acrylic acid-based binder or a methacrylic acid-based binder, and more preferably an acrylic acid-based binder. Regarding the salt, an alkali metal salt (e.g., Li, Na, K, etc.) is preferable.

[0024] The polycarboxylic acid-based polymer may be a homopolymer or a copolymer. In the case of a copolymer, examples of the monomer copolymerized with the unsaturated carboxylic acid or its salt include acrylic acid ester, methacrylic acid ester, etc.

[0025] In the present embodiment, a polycarboxylic acid-based binder that satisfies a specific relationship with respect to the average particle diameter (D50 B ) when dispersed in water is used with respect to the average particle diameter (D50 A ) of the negative electrode active material. That is, in the present embodiment, the average particle diameter (D50 AThe ratio (D50 B ) of the average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water to that (D50 A ) is 0.19 or less. When the ratio (D50 B / D50 A ) is 0.19 or less, both capacity degradation and resistance increase when the battery is stored at high temperature can be suppressed. The reason is considered as follows.

[0026] When the negative electrode active material layer 64 is formed using an aqueous negative electrode paste, the polycarboxylic acid-based binder dispersed in the paste swells with water, which is the solvent of the paste. Due to this swelling, the polycarboxylic acid-based binder becomes soft, so when the polycarboxylic acid-based binder joins the negative electrode active material particles, it spreads on the surface of the negative electrode active material particles. Here, if the polycarboxylic acid-based binder spreads excessively on the surface of the negative electrode active material particles, the surface area of the negative electrode active material capable of battery reaction becomes small, leading to an increase in reaction resistance. When the ratio (D50 B / D50 A ) is 0.19 or less, the particle diameter of the swollen polycarboxylic acid-based binder becomes sufficiently smaller than the particle diameter of the negative electrode active material particles, and the surface area of the negative electrode active material capable of battery reaction can be sufficiently ensured. Also, when the ratio (D50 B / D50 A ) is 0.19 or less, capacity degradation can be suppressed. This is considered to be because the permeation amount of Li ions through the binder film decreases, and thus the decomposition amount of the non-aqueous electrolyte at the interface between the negative electrode active material and the binder decreases. Therefore, when the ratio (D50 B / D50 A ) is 0.19 or less, both capacity degradation and resistance increase when the battery is stored at high temperature can be suppressed.

[0027] The lower limit of the ratio (D50 B / D50 A ) is not particularly limited, but the ratio (D50 B / D50 A) can be 0.05 or more, 0.07 or more, or 0.10 or more.

[0028] The average particle diameter (D50 A ) of the negative electrode active material is not particularly limited as long as it satisfies the above ratio (D50 B / D50 A ). The average particle diameter (D50 A ) of the negative electrode active material is typically 50 μm or less, preferably 1 μm or more and 25 μm or less, and more preferably 5 μm or more and 20 μm or less. The average particle diameter (D50 A ) of the negative electrode active material can be obtained by measuring the particle size distribution of the negative electrode active material based on volume by the laser diffraction scattering method and obtaining the particle diameter (D50) at which the cumulative frequency is 50% in volume percentage in the particle size distribution.

[0029] Also, the BET specific surface area of the negative electrode active material is not particularly limited and is usually 1.5 m 2 / g or more, preferably 2.5 m 2 / g or more. On the other hand, the BET specific surface area is usually 10 m 2 / g or less, preferably 6 m 2 / g or less. In this specification, the "BET specific surface area" refers to the value obtained by analyzing the gas adsorption amount measured by the gas adsorption method (constant volume adsorption method) using nitrogen (N 2 ) gas as the adsorbate by the BET method.

[0030] The average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water is not particularly limited as long as it satisfies the above ratio (D50 B / D50 A ). To reduce the above ratio (D50 B / D50 A ), it is advantageous from the viewpoint of ease of design of the negative electrode 60 to reduce the average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water. From this viewpoint, the average particle diameter (D50 B) is preferably 0.1 μm or more, more preferably 0.5 μm or more, still more preferably 0.7 μm or more, and most preferably 1.0 μm or more. The average particle diameter (D50 B ) of the polycarboxylic acid-based binder is preferably 5.0 μm or less, more preferably 4.5 μm or less, and still more preferably 4.0 μm or less. Note that the negative electrode 60 according to the present embodiment is defined using the average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water, but in the negative electrode 60, it is not required that the polycarboxylic acid-based binder is actually dispersed in water.

[0031] The average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water can be effectively reduced by using a crosslinked polycarboxylic acid-based binder. Further, according to the crosslinked polycarboxylic acid-based binder, the decomposition of the non-aqueous electrolyte at the interface between the negative electrode active material and the binder at high temperature is particularly difficult to occur. Therefore, the polycarboxylic acid polymer is preferably a crosslinked polymer. In other words, the polycarboxylic acid polymer preferably contains a crosslinkable monomer unit. The crosslinkable monomer unit is, for example, a unit of a monomer having two or more polymerizable groups (that is, a polyfunctional monomer), and specific examples include a diacrylate monomer unit, a triacrylate monomer unit, and the like. From the viewpoint of effectively reducing the average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water, the content of the crosslinkable monomer unit in the polycarboxylic acid polymer is preferably 0.1% by mass or more and 10% by mass or less.

[0032] The average particle diameter (D50 B) can be obtained as follows. Disperse the polycarboxylic acid binder in water to prepare a sample of the polycarboxylic acid binder sufficiently swollen in water. Prepare a particle size distribution measuring device based on the laser diffraction / scattering method equipped with a sample circulator. While circulating the aqueous solvent, introduce the sample into the device, measure the particle size distribution based on volume, and obtain the particle diameter (D50) at the integrated value of 50% in the volume-based particle size distribution as the average particle diameter (D50 B )

[0033] The content of the negative electrode active material in the negative electrode active material layer 64 (that is, the content of the negative electrode active material with respect to the total mass of the negative electrode active material layer 64) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more and 99.5% by mass or less, and still more preferably 85% by mass or more and 99% by mass or less.

[0034] The content of the polycarboxylic acid binder in the negative electrode active material layer 64 is not particularly limited, but is, for example, 0.1% by mass or more and 8% by mass or less, preferably 0.2% by mass or more and 5% by mass or less.

[0035] The negative electrode active material layer 64 may contain components other than the negative electrode active material and the polycarboxylic acid binder. Examples of such components include thickeners.

[0036] As the thickener, for example, carboxymethyl cellulose (CMC) or the like can be used. The content of the thickener in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.3% by mass or more and 3% by mass or less, more preferably 0.4% by mass or more and 2% by mass or less.

[0037] The thickness per side of the negative electrode active material layer 64 is not particularly limited, but is usually 20 μm or more, preferably 50 μm or more. On the other hand, the thickness is usually 300 μm or less, preferably 200 μm or less.

[0038] The negative electrode 60 can be produced, for example, as follows. A step of preparing an aqueous electrode paste containing a negative electrode active material, a binder, water as a solvent, and an optional component, a step of applying this aqueous electrode paste to a current collector, and a step of drying the applied aqueous electrode paste are performed. In order to adjust the thickness, density, etc. of the active material layer, a step of pressing the active material layer formed by drying may be further performed. Specific operations of each step can be performed according to known methods.

[0039] In addition, in this specification, the "paste" refers to a dispersion liquid in which the active material is dispersed as a solid content. Therefore, the "paste" includes "slurry", "ink", etc.

[0040] When a battery is produced using the negative electrode 60 according to this embodiment, both capacity degradation and resistance increase can be suppressed when stored at a high temperature.

[0041] Therefore, from another aspect, the secondary battery disclosed herein includes a positive electrode, the negative electrode 60 according to this embodiment, and a non-aqueous electrolyte. Hereinafter, taking a lithium-ion secondary battery as an example, embodiments of the secondary battery disclosed herein will be described with reference to FIGS. 2 and 3.

[0042] The lithium-ion secondary battery 100 shown in FIG. 2 is a sealed lithium-ion secondary battery 100 constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight and highly thermally conductive metal material such as aluminum is used.

[0043] As shown in FIGS. 2 and 3, the wound electrode body 20 has a form in which the positive electrode sheet 50 and the negative electrode sheet 60 are overlapped via two long separator sheets 70 and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of a long negative electrode current collector 62. The positive electrode active material layer non-formation portion 52a (that is, the portion where the positive electrode current collector 52 is exposed without the formation of the positive electrode active material layer 54) and the negative electrode active material layer non-formation portion 62a (that is, the portion where the negative electrode current collector 62 is exposed without the formation of the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction of the wound electrode body 20 (that is, the sheet width direction orthogonal to the longitudinal direction). A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined to the positive electrode active material layer non-formation portion 52a and the negative electrode active material layer non-formation portion 62a, respectively.

[0044] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in a lithium ion secondary battery may be used. Examples thereof include a sheet or foil made of a metal having good conductivity (for example, aluminum, nickel, titanium, stainless steel, etc.). As the positive electrode current collector 52, an aluminum foil is preferable.

[0045] The dimensions of the positive electrode current collector 52 are not particularly limited and may be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0046] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material having a known composition used in a lithium ion secondary battery may be used. Specifically, for example, as the positive electrode active material, a lithium composite oxide, a lithium transition metal phosphate compound, or the like can be used. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0047] As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable. Specific examples thereof include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel manganese-based composite oxides, lithium nickel cobalt manganese-based composite oxides, lithium nickel cobalt aluminum-based composite oxides, lithium iron nickel manganese-based composite oxides, and the like. These positive electrode active materials may be used alone or in combination of two or more.

[0048] In the present specification, the term "lithium nickel cobalt manganese-based composite oxide" includes oxides containing one or more additional elements other than those in addition to the oxides composed of Li, Ni, Co, Mn, and O as constituent elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn, etc. Further, the additional element may be a semi-metal element such as B, C, Si, P, etc. or a non-metal element such as S, F, Cl, Br, I, etc. This also applies to the above-mentioned lithium nickel-based composite oxide, lithium cobalt-based composite oxide, lithium manganese-based composite oxide, lithium nickel manganese-based composite oxide, lithium nickel cobalt aluminum-based composite oxide, lithium iron nickel manganese-based composite oxide, and the like.

[0049] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO 4 ), lithium manganese phosphate (LiMnPO 4) Examples include lithium iron manganese phosphate and the like.

[0050] The average particle size of the positive electrode active material is not particularly limited and may be of the same degree as the average particle size employed in conventional lithium ion secondary batteries. The average particle size of the positive electrode active material is typically 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less. Note that the average particle size (D50) of the positive electrode active material can be determined by the same method as the average particle size (D50 A ) of the negative electrode active material.

[0051] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, and the like.

[0052] As the conductive material, for example, carbon black such as acetylene black (AB) and other carbon materials (e.g., graphite) can be preferably used.

[0053] As the binder, for example, polyvinylidene fluoride (PVDF), a polycarboxylic acid-based binder, and the like can be used. Examples of the polycarboxylic acid-based binder are the same as those used in the negative electrode active material layer 64. From the viewpoint of higher high-temperature storage characteristics, the binder of the positive electrode active material layer 54 is preferably a polycarboxylic acid-based binder.

[0054] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more and 97% by mass or less, and still more preferably 85% by mass or more and 96% by mass or less. The content of lithium tripolyphosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.5% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 15% by mass or less, and still more preferably 2% by mass or more and 10% by mass or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.5% by mass or more and 15% by mass or less, more preferably 1% by mass or more and 10% by mass or less, and still more preferably 1.5% by mass or more and 8% by mass or less.

[0055] The thickness per side of the positive electrode active material layer 54 is not particularly limited, but is usually 20 μm or more, preferably 50 μm or more. On the other hand, the thickness is usually 300 μm or less, preferably 200 μm or less.

[0056] As the negative electrode sheet 60, the above-described negative electrode 60 is used.

[0057] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0058] The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt. As the non-aqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, lactones, etc. that are used in the electrolytes of general lithium-ion secondary batteries can be used without particular limitation. Among them, carbonates are preferable, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), etc. Such non-aqueous solvents can be used alone or in appropriate combinations of two or more kinds.

[0059] As the supporting salt, for example, lithium salts such as LiPF 6 , LiBF 4 , LiClO 4 etc. (preferably LiPF 6 ) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0060] Further, the non-aqueous electrolyte may contain an oxalato complex coordinated with B, P, or Si as a film-forming agent. When the non-aqueous electrolyte contains the oxalato complex, the high-temperature storage characteristics of the battery can be further improved. Examples of the oxalato complex include LiB(C 2 O 4 ) 2 , LiBF 2 (C 2 O 4 ), LiPF 2 (C 2 O 4 ) 2 , LiPF 4 (C 2 O 4 ), (CH 3 ) 2 Si(C 2 O 4 ), (CH3 CH 2 ) 2 Si(C 2 O 4 ) and the like can be mentioned. As the concentration of the oxalato complex in the non-aqueous electrolyte, for example, it is 0.010 mol / kg or more and 0.600 mol / kg or less, preferably 0.020 mol / kg or more and 0.050 mol / kg or less.

[0061] In addition, as long as the effects of the present invention are not significantly impaired, the non-aqueous electrolyte may contain components other than the above-described components, for example, gas generators such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and various additives such as these.

[0062] In the lithium ion secondary battery 100 configured as described above, both capacity degradation and resistance increase are suppressed when stored at high temperatures. The lithium ion secondary battery 100 can be used for various applications. Suitable applications include driving power sources mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Further, the lithium ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium ion secondary battery 100 can typically be used in the form of a battery pack in which a plurality of cells are connected in series and / or in parallel.

[0063] As an example, a rectangular lithium ion secondary battery 100 including a flat wound electrode body 20 has been described. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery including a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). Further, the lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated lithium ion secondary battery, or the like.

[0064] In addition, the negative electrode 60 according to the present embodiment is suitable for the negative electrode of a lithium ion secondary battery, but can be used as the negative electrode of other batteries, and other batteries can be configured according to known methods.

[0065] Hereinafter, examples related to the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

[0066] <Examples 1 to 3 and Comparative Examples 1 to 3> Natural graphite (C) having an average particle diameter (D50 A ) shown in Table 1 and a polycarboxylic acid-based binder having an average particle diameter (D50 B ) when dispersed in water shown in Table 1 were mixed with ion-exchanged water at a mass ratio of C:binder = 96:3 to prepare a negative electrode paste. This negative electrode paste was applied in a strip shape to both sides of a long copper foil, dried, and then pressed to produce a negative electrode sheet.

[0067] LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 (LNCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of LNCM:AB:PVdF = 90:8:2 to prepare a positive electrode paste. This slurry was applied in a strip shape to both sides of a long aluminum foil, dried, and then pressed to produce a positive electrode sheet.

[0068] Also, as a separator, a porous polyolefin sheet having a three-layer structure of PP / PE / PP with HRL provided thereon was prepared. The positive electrode sheet, the negative electrode sheet, and two of the prepared separator sheets described above were laminated and wound, and then pressed from the side direction and pulled to produce a flat wound electrode body.

[0069] Next, the wound electrode body was connected to a positive electrode terminal and a negative electrode terminal, and was housed in a rectangular battery case having an electrolyte injection port. Subsequently, a non-aqueous electrolyte was injected from the electrolyte injection port of the battery case, and the injection port was hermetically sealed. The non-aqueous electrolyte contains ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a mixed solvent with a volume ratio of EC:DMC:EMC = 3:3:4, and LiPF 6 was dissolved at a concentration of 1.1 mol / L, and further LiBOB was added so as to be 0.02 mol / kg. Then, an aging treatment was performed to obtain lithium ion secondary batteries for evaluation of each example and each comparative example.

[0070] <The average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water> The average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water shown in Table 1 was measured as follows. The polycarboxylic acid-based binder was dispersed in water to prepare a sample of the binder sufficiently swollen in water. The sample was prepared using a rotation-revolution mixer ("Awatori Rentaro" manufactured by Shinki Co., Ltd.) so that no lumps (binder aggregates not containing water inside) were generated. As a laser diffraction / scattering type particle size distribution measuring device, "Microtrac MT300II series" manufactured by Microtrac Bell Co., Ltd. was prepared. Water was circulated in the attached sample circulator, the sample was put into the device, the particle size distribution based on volume was measured, and the particle diameter (D50) at the integrated value of 50% in the particle size distribution based on volume was determined as the average particle diameter (D50 B ).

[0071] <High-temperature storage test - reaction resistance> After charging each of the above-prepared lithium ion secondary batteries for evaluation to 3.7 V, impedance measurement was performed while applying an AC voltage with a frequency of 0.01 Hz to 100,000 Hz and a voltage amplitude of 5 mV at -10°C. Then, the diameter of the arc of the obtained Cole-Cole plot was measured as the initial reaction resistance.

[0072] After adjusting each lithium-ion secondary battery for evaluation to 80% SOC, it was stored in a temperature environment of 70°C for 40 days. Thereafter, the reaction resistance was measured by the same method as above. The resistance increase rate (%) = (reaction resistance after high-temperature storage / initial reaction resistance) × 100 was used to determine the resistance increase rate. When the resistance increase rate of Comparative Example 1-2 was set to 100, the ratio of the resistance increase rates of Comparative Example 1-1, Example 1-1, and Example 1-2 was determined. When the resistance increase rate of Comparative Example 2-1 was set to 100, the ratio of the resistance increase rates of Example 2-1 and Example 2-2 was determined. When the resistance increase rate of Comparative Example 3-1 was set to 100, the ratio of the resistance increase rates of Example 3-1 and Example 3-2 was determined. The results are shown in Table 1.

[0073] <High-temperature storage test - Capacity retention rate> Each of the above-prepared lithium-ion secondary batteries for evaluation was placed in an environment of 25°C. This was charged at a constant current - constant voltage up to 4.1V at a current value of 1 / 5C (cut-off current: 1 / 50C), rested for 10 minutes, and then discharged at a constant current of 1 / 5C down to 3.0V. The discharge capacity at this time was measured and used as the initial capacity.

[0074] After adjusting each lithium-ion secondary battery for evaluation to 80% SOC, it was stored in a temperature environment of 70°C for 40 days. Thereafter, the capacity was measured by the same method as above. The capacity retention rate (%) = (capacity after high-temperature storage / initial capacity) × 100 was used to determine the capacity retention rate. When the capacity retention rate of Comparative Example 1-2 was set to 100, the ratio of the capacity retention rates of Comparative Example 1-1, Example 1-1, and Example 1-2 was determined. When the capacity retention rate of Comparative Example 2-1 was set to 100, the ratio of the capacity retention rates of Example 2-1 and Example 2-2 was determined. When the capacity retention rate of Comparative Example 3-1 was set to 100, the ratio of the capacity retention rates of Example 3-1 and Example 3-2 was determined. The results are shown in Table 1.

[0075]

Table 1

[0076] From the results in Table 1, the average particle size (D50) of the negative electrode active material AThe ratio of the average particle diameter (D50 B ) of the polycarboxylic acid-based binder when dispersed in water to

[0077] is 0.19 or less, it can be seen that while maintaining high capacity deterioration resistance, an increase in resistance during high-temperature storage can be suppressed. In particular, from the results of Comparative Examples 1-1, 1-2 and Examples 1-1, 1-2, it can be seen that when the ratio is 0.19 or less, the effect of suppressing the increase in resistance after high-temperature storage is rapidly exerted.

[0078] From the above results, it can be seen that according to the negative electrode disclosed herein, it is possible to achieve both suppression of capacity deterioration and suppression of increase in resistance when the battery is stored at high temperature.

Explanation of Reference Numerals

[0079] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Portion where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 100 Lithium ion secondary battery

Claims

1. a positive electrode, a negative electrode, and a non-aqueous electrolyte, the secondary battery comprising: the negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector; the negative electrode active material layer containing a negative electrode active material and a binder; the binder including a polycarboxylic acid-based binder; The average particle diameter (D50 Ａ ) of the polycarboxylic acid-based binder when dispersed in water with respect to the average particle diameter (D50 Ｂ ) of the negative electrode active material is 0.19 or less, the non-aqueous electrolyte containing an oxalato complex coordinated with B, P, or Si as a film-forming agent; the negative electrode active material layer being formed by applying an aqueous electrode paste containing the negative electrode active material, the binder, and water as a solvent to the negative electrode current collector and drying; a secondary battery.

2. The average particle diameter (D50 Ｂ ) of the polycarboxylic acid-based binder when dispersed in water is 0.1 μm or more and 5.0 μm or less. The secondary battery according to claim 1.

3. The secondary battery according to claim 1 or 2, wherein the polymer constituting the polycarboxylic acid-based binder contains a crosslinkable monomer unit.

4. The secondary battery according to any one of claims 1 to 3, which is a lithium ion secondary battery.

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