JP7685960B2 - Negative electrode and secondary battery including the same - Google Patents

Description

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

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

A typical secondary battery electrode, particularly an electrode for a lithium-ion secondary battery, generally has a configuration in which an active material layer containing an active material is supported on a current collector. This active material layer contains a binder to bond the active material particles together and to bond the active material particles to the current collector.

In order to reduce the environmental load, water may be used as a solvent in the electrode paste used to form the active material layer. Polycarboxylic acid-based binders, which are polymers of unsaturated carboxylic acids such as acrylic acid, are known as binders suitable for electrode pastes containing water as a solvent (i.e., aqueous electrode pastes). For example, Patent Document 1 discloses that the energy density, high-temperature storage characteristics, and durability during cycle operation of lithium-ion secondary batteries can be improved by using polymeric polycarboxylic acid as a binder for the negative electrode.

International Publication No. 2013/124950

In Patent Document 1, the capacity retention rate after high-temperature storage is evaluated as a high-temperature storage characteristic. However, it is difficult to simultaneously suppress the increase in resistance and the decrease in capacity when the battery is stored at high temperatures. As a result of intensive research, the inventors have found that when a negative electrode is produced using a polycarboxylic acid-based binder of the conventional technology and a battery is constructed using the negative electrode, as described in Patent Document 1, the battery has high resistance to capacity decrease when stored at high temperatures, but there is a problem in that the suppression of the increase in resistance when the battery is stored at high temperatures is insufficient.

Therefore, the present invention aims to provide a negative electrode that can suppress both capacity degradation and resistance increase when a battery is stored at high temperatures.

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 binder. The ratio of the average particle diameter (D50 _B ) of the polycarboxylic acid binder when dispersed in water to the average particle diameter (D50 _A ) of the negative electrode active material is 0.19 or less. With this configuration, it is possible to provide a negative electrode that can suppress both capacity deterioration and resistance increase when a battery is stored at high temperatures.

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

Here, it is more advantageous for the polymer constituting the polycarboxylic acid-based binder to contain a crosslinkable monomer unit.

Moreover, the negative electrode disclosed herein is more advantageously a negative electrode for a lithium-ion secondary battery.

From another perspective, the secondary battery disclosed herein comprises a positive electrode, the above-mentioned negative electrode, and a non-aqueous electrolyte. With this configuration, it is possible to provide a battery that can suppress both capacity degradation and resistance increase when stored at high temperatures.

Here, it is more advantageous for the non-aqueous electrolyte to contain an oxalate complex coordinated with B, P, or Si as a film-forming agent.

FIG. 1 is a cross-sectional view illustrating a schematic diagram of a negative electrode according to one embodiment of the present invention. FIG. 1 is a cross-sectional view showing a schematic internal structure of a lithium-ion secondary battery using a negative electrode according to one embodiment of the present invention. 3 is a schematic diagram showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. 2.

Below, the embodiments of the present invention will be described with reference to the drawings. Note that matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. In addition, in the following drawings, components and parts that perform the same function are explained by using the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and includes so-called storage batteries and electricity storage elements such as electric double-layer capacitors. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and realizes charging and discharging by the transfer of charge associated with lithium ions between the positive and negative electrodes.

The negative electrode disclosed herein is typically used in a secondary battery, and is preferably used in a lithium ion secondary battery. FIG. 1 is a cross-sectional view showing a cross-section perpendicular to the thickness direction of a negative electrode 60 according to the present embodiment, which is an example of the negative electrode disclosed herein. The negative electrode 60 according to the present embodiment shown in FIG. 1 is a negative electrode for a lithium ion secondary battery.

As shown in the figure, 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 on only 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. The negative electrode active material layer 64 is preferably provided on both sides of the negative electrode current collector 62.

In the illustrated example, a negative electrode active material layer non-forming 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-forming portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-forming portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

In the illustrated example, the shape of the negative electrode collector 62 is a foil (or sheet), but is not limited to this. The negative electrode collector 62 may be in various forms such as a rod, a plate, or a mesh. As with conventional lithium-ion secondary batteries, the material of the negative electrode collector 62 can be a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.), and copper is particularly preferred. Copper foil is particularly preferred as the negative electrode collector 62.

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

The negative electrode active material layer 64 contains a negative electrode active material and a binder. For example, carbon materials such as graphite, hard carbon, and soft carbon can be used as the negative electrode active material. Among these, graphite is preferred because it provides a higher effect of the present invention. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

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

The polycarboxylic acid polymer may be a homopolymer or a copolymer. In the case of a copolymer, examples of monomers that are copolymerized with the unsaturated carboxylic acid or its salt include acrylic acid esters and methacrylic acid esters.

In this embodiment, a polycarboxylic acid binder is used whose average particle diameter (D50 _B ) when dispersed in water satisfies a specific relationship with the average particle diameter (D50 _A ) of the negative electrode active material. That is, in this embodiment, the ratio (D50 _B /D50 _{A ) of the average particle diameter (D50 B ) of the polycarboxylic acid binder when dispersed in water to the average particle diameter (D50 A ) of} the negative electrode active material is 0.19 or less. By making the ratio (D50 _B /D50 _A ) 0.19 or less, both the capacity deterioration and the resistance increase when the battery is stored at high temperatures can be suppressed. The reason for this is considered to be as follows.

When the negative electrode active material layer 64 is prepared using an aqueous negative electrode paste, the polycarboxylic acid binder dispersed in the paste swells due to water, which is the solvent of the paste. This swelling softens the polycarboxylic acid binder, so that when the polycarboxylic acid binder bonds the negative electrode active material particles together, it spreads on the surface of the negative electrode active material particles. Here, if the polycarboxylic acid 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 is reduced, leading to an increase in reaction resistance. By making the ratio (D50 _B /D50 _A ) 0.19 or less, the particle diameter of the swollen polycarboxylic acid 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 secured. In addition, by making the ratio (D50 _B /D50 _A ) 0.19 or less, capacity deterioration can be suppressed. This is believed to be because the amount of Li ions permeating through the binder film is reduced, which reduces the amount of decomposition of the nonaqueous electrolyte at the interface between the negative electrode active material and the binder. Therefore, by making the ratio (D50 _B /D50 _A ) 0.19 or less, both the capacity deterioration and the resistance increase when the battery is stored at high temperatures can be suppressed.

Although the lower limit of the ratio (D50 _B /D50 _A ) is not particularly limited, the ratio (D50 _B /D50 _A ) may be 0.05 or more, 0.07 or more, or 0.10 or more.

The average particle diameter (D50 _A ) of the negative electrode active material is not particularly limited as long as it satisfies the range of the 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 on a volume basis by a laser diffraction scattering method and determining the particle diameter (D50) at which the cumulative frequency in the particle size distribution is 50% by volume percentage.

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, and preferably 2.5 m ² /g or more. Meanwhile, the BET specific surface area is usually 10 m ² /g or less, and preferably 6 m ² /g or less. In this specification, the "BET specific surface area" refers to a value obtained by analyzing the gas adsorption amount measured by a gas adsorption method (constant volume adsorption method) using nitrogen (N ₂ ) gas as an adsorbate, by the BET method.

The average particle diameter (D50 _B ) of the polycarboxylic acid binder when dispersed in water is not particularly limited as long as the above ratio (D50 _B /D50 _A ) is satisfied. In order 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 binder when dispersed in water. From this viewpoint, the average particle diameter (D50 _B ) of the polycarboxylic acid binder is preferably 0.1 μm or more, more preferably 0.5 μm or more, even 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 binder is preferably 5.0 μm or less, more preferably 4.5 μm or less, and even more preferably 4.0 μm or less. Although the negative electrode 60 according to this embodiment is defined using the average particle diameter (D50 _B ) of the polycarboxylic acid-based binder when dispersed in water, it is not required that the polycarboxylic acid-based binder in the negative electrode 60 is actually dispersed in water.

The average particle size (D50 _B ) of the polycarboxylic acid-based binder when dispersed in water can be effectively reduced by using a crosslinked polycarboxylic acid-based binder. In addition, the crosslinked polycarboxylic acid-based binder makes it particularly difficult for the nonaqueous electrolyte to decompose at the interface between the negative electrode active material and the binder under high temperatures. 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 (i.e., a polyfunctional monomer), and specific examples include a diacrylate monomer unit and a triacrylate monomer unit. From the viewpoint of effectively reducing the average particle size (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.

The average particle diameter (D50 _B ) of a polycarboxylic acid binder when dispersed in water can be determined as follows. A polycarboxylic acid binder is dispersed in water to prepare a sample of the polycarboxylic acid binder that is sufficiently swollen in water. A particle size distribution measuring device based on a laser diffraction/scattering method, equipped with a sample circulator, is prepared. While circulating the water solvent, the sample is introduced into the device, the particle size distribution on a volume basis is measured, and the particle diameter (D50) at an integrated value of 50% in the volume-based particle size distribution is determined as the average particle diameter (D50 _B ).

The content of the negative electrode active material in the negative electrode active material layer 64 (i.e., the content of the negative electrode active material relative to the total mass of the negative electrode active material layer 64) is not particularly limited, but is preferably 70 mass% or more, more preferably 80 mass% or more and 99.5 mass% or less, and even more preferably 85 mass% or more and 99 mass% or less.

The content of the polycarboxylic acid-based 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, and preferably 0.2% by mass or more and 5% by mass or less.

The negative electrode active material layer 64 may contain components other than the negative electrode active material and the polycarboxylic acid binder. Such components include a thickener, etc.

As a 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, and more preferably 0.4% by mass or more and 2% by mass or less.

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

The negative electrode 60 can be produced, for example, as follows: a step of preparing an aqueous electrode paste containing an aqueous electrode active material, a binder, water as a solvent, and optional components; a step of applying the aqueous electrode paste to a current collector; and a step of drying the applied aqueous electrode paste. 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. The specific operations of each step can be performed according to known methods.

In this specification, the term "paste" refers to a dispersion liquid in which active materials are dispersed as solids, and therefore includes "slurry," "ink," etc.

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

Therefore, from another perspective, the secondary battery disclosed herein comprises a positive electrode, the negative electrode 60 according to this embodiment, and a non-aqueous electrolyte. Below, an embodiment of the secondary battery disclosed herein will be described with reference to Figures 2 and 3, taking a lithium ion secondary battery as an example.

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 nonaqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The battery case 30 is also provided with an injection port (not shown) for injecting the nonaqueous electrolyte. The positive terminal 42 is electrically connected to the positive electrode current collector 42a. The negative terminal 44 is electrically connected to the negative electrode current collector 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum.

As shown in Figures 2 and 3, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are stacked with two long separator sheets 70 interposed therebetween 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 (both sides here) of a long positive electrode 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 (both sides here) of a long negative electrode collector 62. The positive electrode active material layer non-forming portion 52a (i.e., the portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer non-forming portion 62a (i.e., the portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends of the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. The positive electrode active material layer non-forming portion 52a and the negative electrode active material layer non-forming portion 62a are respectively joined to the positive electrode current collector 42a and the negative electrode current collector 44a.

The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current collector used in lithium ion secondary batteries, examples of which include a sheet or foil made of a metal with good electrical conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

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

The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material of a known composition used in lithium ion secondary batteries may be used. Specifically, for example, a lithium composite oxide, a lithium transition metal phosphate compound, or the like may be used as the positive electrode active material. 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.

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, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more.

In this specification, the term "lithium nickel cobalt manganese composite oxide" includes oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements other than those. 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, and Sn. The additional element may also be a semimetal element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the above-mentioned lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

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

The average particle diameter of the positive electrode active material is not particularly limited, and may be the same as the average particle diameter used in conventional lithium ion secondary batteries. The average particle diameter 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. The average particle diameter (D50) of the positive electrode active material can be determined by the same method as the average particle diameter (D50 _A ) of the negative electrode active material.

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, etc.

As a conductive material, for example, carbon black such as acetylene black (AB) or other carbon materials (e.g., graphite) can be suitably used.

As the binder, for example, polyvinylidene fluoride (PVDF), polycarboxylic acid-based binders, etc. can be used. An example of the polycarboxylic acid-based binder is the same as the polycarboxylic acid-based binder used in the negative electrode active material layer 64. From the viewpoint of higher high-temperature storage characteristics, it is preferable that the binder of the positive electrode active material layer 54 is a polycarboxylic acid-based binder.

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 relative 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 even more preferably 85% by mass or more and 96% by mass or less. The content of trilithium phosphate 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 even 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 even more preferably 1.5% by mass or more and 8% by mass or less.

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

The above-mentioned negative electrode 60 is used as the negative electrode sheet 60.

The separator 70 may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or 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 a PP layer is laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

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, and lactones that are used in general electrolytes of lithium ion secondary batteries can be used without any 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 combination of two or more.

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

The non-aqueous electrolyte may also contain an oxalate complex coordinated with B, P, or Si as a film-forming agent. When the non-aqueous electrolyte contains the oxalate complex, the high-temperature storage characteristics of the battery can be further improved. Examples of the oxalate 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 ), (CH ₃ CH ₂ ) ₂ Si (C ₂ O ₄ ), etc. The concentration of the oxalato complex in the nonaqueous electrolyte is, for example, 0.010 mol/kg or more and 0.600 mol/kg or less, and preferably 0.020 mol/kg or more and 0.050 mol/kg or less.

The nonaqueous electrolyte may contain various additives, such as gas generators such as biphenyl (BP) and cyclohexylbenzene (CHB), thickeners, and other components other than those mentioned above, as long as they do not significantly impair the effects of the present invention.

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 a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of multiple batteries connected in series and/or parallel.

As an example, a rectangular lithium ion secondary battery 100 having 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 having a laminated electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are alternately laminated). The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated lithium ion secondary battery, etc.

The negative electrode 60 according to this embodiment is suitable for use as a negative electrode in a lithium ion secondary battery, but can also be used as a negative electrode in other batteries, and the other batteries can be constructed according to known methods.

The following describes examples of the present invention, but is not intended to limit the present invention to those examples.

<Examples 1 to 3 and Comparative Examples 1 to 3>
A negative electrode paste was prepared by mixing natural graphite (C) having an average particle size (D50 _A ) shown in Table 1 and a polycarboxylic acid-based binder having an average particle size (D50 _B ) when dispersed in water shown in Table 1 with ion-exchanged water in a mass ratio of C:binder = 96:3. This negative electrode paste was applied in strips on both sides of a long copper foil, dried, and then pressed to produce a negative electrode sheet.

A positive electrode paste was prepared by mixing LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder with N-methylpyrrolidone (NMP) in a mass ratio of LNCM:AB:PVdF = 90:8:2. This slurry was applied in strips on both sides of a long aluminum foil, dried, and then pressed to produce a positive electrode sheet.

As a separator, a porous polyolefin sheet with a three-layer structure of PP/PE/PP with an HRL was prepared. The positive electrode sheet and negative electrode sheet prepared above and two of the separator sheets prepared above were stacked and wound, and then pressed from the side direction to bend the stack, to produce a flat wound electrode body.

Next, the wound electrode body was connected to a positive electrode terminal and a negative electrode terminal, and housed in a square battery case having an electrolyte injection port. Next, a nonaqueous electrolyte was injected from the electrolyte injection port of the battery case, and the injection port was sealed airtight. The nonaqueous electrolyte was prepared by dissolving LiPF 6 as a supporting salt at a concentration of _1.1 mol/L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC=3:3:4, and further adding LiBOB to the mixture at a concentration of 0.02 mol/kg. Then, an aging treatment was performed to obtain lithium ion secondary batteries for evaluation in each of the examples and comparative examples.

<Average particle size ( _D50B ) of polycarboxylic acid-based binder when dispersed in water>
The average particle diameter (D50 _B ) of the polycarboxylic acid binder when dispersed in water shown in Table 1 was measured as follows. A polycarboxylic acid binder was dispersed in water and a binder sample was prepared by sufficiently swelling the binder in water. The sample was prepared using a rotation/revolution mixer (Thinky Corporation's "Awatori Rentaro") to prevent the formation of lumps (binder aggregates that do not contain water inside). As a laser diffraction/scattering type particle size distribution measuring device, Microtrac-Bell's "Microtrac MT300II Series" was prepared. Water was circulated in the attached sample circulator, the sample was put into the device, the particle size distribution on a volume basis was measured, and the particle diameter (D50) at an integrated value of 50% in the volume-based particle size distribution was calculated as the average particle diameter (D50 _B ).

<High temperature storage test - reaction resistance>
Each of the lithium ion secondary batteries for evaluation prepared above was charged to 3.7 V, and then impedance measurement was performed while applying an AC voltage of 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.

After adjusting the SOC of each evaluation lithium-ion secondary battery to 80%, it was stored in a temperature environment of 70°C for 40 days. Thereafter, the reaction resistance was measured in the same manner as above. The resistance increase rate was calculated from the resistance increase rate (%) = (reaction resistance after high-temperature storage/initial reaction resistance) x 100. The ratios of the resistance increase rates of Comparative Example 1-1, Example 1-1, and Example 1-2 were calculated when the resistance increase rate of Comparative Example 1-2 was set to 100. The ratios of the resistance increase rates of Example 2-1 and Example 2-2 were calculated when the resistance increase rate of Comparative Example 2-1 was set to 100. The ratios of the resistance increase rates of Example 3-1 and Example 3-2 were calculated when the resistance increase rate of Comparative Example 3-1 was set to 100. The results are shown in Table 1.

<High temperature storage test - capacity retention rate>
Each of the lithium ion secondary batteries for evaluation prepared above was placed in an environment of 25° C. It was charged at a constant current and constant voltage (cut current: 1/50 C) at a current value of 1/5 C to 4.1 V, and after a 10-minute pause, it was discharged at a constant current value of 1/5 C to 3.0 V. The discharge capacity at this time was measured and this was taken as the initial capacity.

Each evaluation lithium-ion secondary battery was adjusted to an SOC of 80%, and then stored in a temperature environment of 70°C for 40 days. The capacity was then measured in the same manner as above. The capacity retention rate was calculated from the formula: Capacity retention rate (%) = (Capacity after high-temperature storage/Initial capacity) x 100. The ratios of the capacity retention rates of Comparative Example 1-1, Example 1-1, and Example 1-2 were calculated when the capacity retention rate of Comparative Example 1-2 was set to 100. The ratios of the capacity retention rates of Examples 2-1 and 2-2 were calculated when the capacity retention rate of Comparative Example 2-1 was set to 100. The ratios of the capacity retention rates of Examples 3-1 and 3-2 were calculated when the capacity retention rate of Comparative Example 3-1 was set to 100. The results are shown in Table 1.

From the results in Table 1, it can be seen that when the ratio of the average particle diameter (D50 _B ) of the polycarboxylic acid binder when dispersed in water to the average particle diameter (D50 _A ) of the negative electrode active material is 0.19 or less, it is possible to suppress the increase in resistance during high-temperature storage while maintaining high resistance to capacity degradation. In particular, from the results of Comparative Examples 1-1 and 1-2 and Examples 1-1 and 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.

These results show that the negative electrode disclosed herein can simultaneously suppress capacity degradation and resistance increase when the battery is stored at high temperatures.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

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 Positive electrode active material layer non-forming portion 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-forming portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 Lithium-ion secondary battery

Claims (4)

A positive electrode and
A negative electrode;
A secondary battery comprising:
the negative electrode comprises 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 binder,
a ratio of an average particle size (D50 _B ) of the polycarboxylic acid binder when dispersed in water to an average particle size (D50 _A ) of the negative electrode active material is 0.19 or less;
the non-aqueous electrolyte contains an oxalato complex in which B, P, or Si is coordinated as a film-forming agent;
The negative electrode active material layer is 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 the aqueous electrode paste.
Secondary battery. The secondary battery according to claim 1 , wherein the polycarboxylic acid-based binder has an average particle diameter (D50 _B ) of 0.1 μm or more and 5.0 μm or less when dispersed in water. The secondary battery according to claim 1 or 2, wherein the polymer constituting the polycarboxylic acid binder contains a crosslinkable monomer unit. The secondary battery according to any one of claims 1 to 3, which is a lithium ion secondary battery.

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