JP2024122207A - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

To provide a lithium ion secondary battery that has a high capacity and excellent cycle characteristics.SOLUTION: A negative electrode for a lithium ion secondary battery includes a negative electrode active material that contains silicon and has ridges on its surface, and a binder, and the binder has a COOX group, X is any one selected from the group consisting of Li, Na and K, and a specific surface area S (m2/g) of the negative electrode active material, a weight wa of the negative electrode active material, and a weight wb of the binder satisfy the relationship 0.5≤{wb/(wa+wb)}/S×100≤5.0.SELECTED DRAWING: Figure 2

Description

The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery.

Lithium-ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptops, as well as hybrid cars.

The capacity of a lithium-ion secondary battery depends mainly on the active material of the electrodes. Graphite is generally used as the negative electrode active material, but there is a demand for negative electrode active materials with higher capacity. For this reason, silicon (Si), which has a theoretical capacity much larger than that of graphite (372 mAh/g), has attracted attention.

Negative electrode active materials that contain Si undergo a large volume expansion during charging. The volume expansion of the negative electrode active material causes a decrease in the cycle characteristics of the battery. When the negative electrode active material expands in volume, for example, the conductive paths between the negative electrode active materials are cut, peeling occurs at the interface between the negative electrode active material layer and the current collector, cracks occur in the SEI (Solid Electrolyte Interphase) coating, and electrolyte decomposition occurs. These reduce the cycle characteristics of the battery.

To improve the cycle characteristics of the battery, a binder is used in the negative electrode active material layer. For example, Patent Document 1 discloses a lithium ion secondary battery that uses a binder having a carboxyl group.

JP 2018-067555 A

There is a demand for higher capacity and further improvements in cycle characteristics.

This disclosure has been made in consideration of the above problems, and aims to provide a lithium-ion secondary battery that has high capacity and excellent cycle characteristics.

To solve the above problems, the following measures are provided:

(1) A negative electrode for a lithium ion secondary battery according to a first aspect includes a negative electrode active material and a binder. The negative electrode active material contains silicon and has a ridge line on its surface. The binder has a COOX group, where X is any one selected from the group consisting of Li, Na, and K. A specific surface area S ( ^m2 /g) of the negative electrode active material, a weight w _a of the negative electrode active material, and a weight w _b of the binder satisfy the relationship 0.5≦{w _b /(w _a +w _b )}/S×100≦5.0.

(2) In the negative electrode for a lithium ion secondary battery according to the above aspect, X in the COOX group may be Li.

(3) The lithium ion secondary battery according to the above aspect has a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte. The negative electrode is the negative electrode for the lithium ion secondary battery according to the above aspect.

The lithium-ion secondary battery according to the above aspect has high capacity and excellent cycle characteristics.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment; 1 is a scanning electron microscope image of the negative electrode active material according to the first embodiment.

The following describes the embodiments in detail, with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of clarity, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them. Appropriate modifications may be made without departing from the spirit of the invention.

"Lithium-ion secondary battery"
Fig. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. The lithium ion secondary battery 100 shown in Fig. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of terminals 60, 62 connected to the power generating element 40. The non-aqueous electrolyte is contained in the exterior body 50. Although Fig. 1 illustrates a case in which one power generating element 40 is provided in the exterior body 50, a plurality of power generating elements 40 may be stacked.

(Power generation element)
The power generating element 40 includes a separator 10 , a positive electrode 20 , and a negative electrode 30 .

<Positive electrode>
The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[Positive electrode current collector]
The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[Positive electrode active material layer]
The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive assistant and a binder as necessary.

The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, remove and insert lithium ions (intercalation), or dope and dedope lithium ions and counter anions.

The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a compound of the general formula: LiNi _x Co _y Mn _z M _a O ₂ (wherein x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithium vanadium compound (LiV ₂ O ₅ ), an olivine-type LiMPO ₄ (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and a lithium titanate (Li _{4 Ti5O12} ), _{LiNixCoyAlzO2} (0.9<x+y+z< _1.1 ). The positive _electrode active material _may be an organic material. For example, the positive electrode active material _may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

The positive electrode active material may be a lithium-free material. Examples of the lithium-free material include FeF ₃ , conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. The lithium-free material may be any one of the materials or a combination of a plurality of materials. When the positive electrode active material is a lithium-free material, for example, discharge is performed first. Lithium is inserted into the positive electrode active material by discharging. In addition, lithium may be pre-doped chemically or electrochemically into the lithium-free positive electrode active material.

The conductive additive enhances the electronic conductivity between the positive electrode active materials. The conductive additive is, for example, carbon powder, carbon nanotubes, carbon material, metal powder, a mixture of carbon material and metal powder, or conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, etc. The metal powder is, for example, copper, nickel, stainless steel, iron, etc.

The content of the conductive assistant in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive assistant relative to the total mass of the positive electrode active material, the conductive assistant, and the binder is 0.5% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 5% by mass or less.

The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Any known binder can be used. The binder may be the same as that used in the negative electrode active material layer 34 described later. The binder is preferably one that does not dissolve in the electrolyte, has oxidation resistance, and has adhesive properties. The binder is, for example, a fluororesin. The binder is, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, polypropylene (PP) or polyethylene (PE) grafted with maleic anhydride, or a mixture thereof. PVDF is particularly preferable as the binder used in the positive electrode active material layer.

The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content relative to the total mass of the positive electrode active material, conductive assistant, and binder is 1% by mass or more and 15% by mass or less, and preferably 1.5% by mass or more and 5% by mass or less. If the binder content is low, the adhesive strength of the positive electrode 20 is weakened. If the binder content is high, the binder is electrochemically inactive and does not contribute to the discharge capacity, so that the energy density of the lithium ion secondary battery 100 is low.

<Negative Electrode>
The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer 34 may contain a conductive assistant as necessary.

The negative electrode active material includes, for example, silicon. Silicon may be a simple substance or a silicon compound. Silicon compounds include, for example, silicon alloys, silicon oxide, and the like. For example, silicon or silicon compounds may be crystalline or amorphous. Amorphous silicon or silicon compounds can be produced by a melt-spun method, a gas atomization method, or the like. The negative electrode active material may include an active material other than silicon-based materials.

The silicon alloy is represented by XnSi. X is a cation. X is, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K, etc. n satisfies 0≦n≦0.5. Silicon oxide is represented by SiO _x . x satisfies, for example, 0.8≦x≦2. Silicon oxide may be composed of only SiO ₂ , may be composed of only SiO, or may be a mixture of SiO and SiO _2. Silicon oxide may also be partially deficient in oxygen.

The negative electrode active material may be a composite of silicon or a silicon compound. The composite is a silicon or silicon compound particle having at least a part of its surface coated with a conductive material. The conductive material is, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or the like. For example, a silicon carbon composite material (Si-C) is an example of a composite. The amount of the conductive material coated on the silicon or silicon compound particle is, for example, 0.01% by mass to 30% by mass, and preferably 0.1% by mass to 20% by mass, based on the total mass of the composite. The composite can be produced, for example, by mechanical alloying, chemical vapor deposition, wet method, or a method of coating a polymer and then pyrolyzing the polymer to carbonize it.

Figure 2 is a scanning electron microscope (SEM) image of the negative electrode active material 1 according to this embodiment. The negative electrode active material 1 is amorphous and has ridges on its surface. The surface of the negative electrode active material 1 is made up of multiple faces, and the boundaries between these faces are the ridges.

The average length of the ridge lines 2 is, for example, 0.5 μm or more, and preferably 1.0 μm or more and 5.0 μm or less. The average length of the ridge lines 2 can be calculated by dividing the total length of the ridge lines 2 present in 10 scanning electron microscope images taken at a magnification of 10,000 times, by the number of negative electrode active materials 1 that include the ridge lines 2.

The angle between the two faces that sandwich the ridge line 2 is, for example, 150° or less, and preferably 60° or more and 120° or less.

The average number of ridges 2 contained in one negative electrode active material 1 is, for example, 20 or less, and preferably 8 to 15.

The specific surface area S (m ² /g) of the negative electrode active material determined by the BET method satisfies, for example, the relationship 0.5≦{w _b /( _wa + w _b )}/S×100≦5.0. In this relational expression, _wa is the weight of the negative electrode active material, and w _b is the weight of the binder. When the weight of the negative electrode active material and the binder is determined from the negative electrode, the following procedure is performed. First, the weight of the negative electrode is measured. Next, the negative electrode is fired at a temperature of about 500° C. in a nitrogen atmosphere to remove the binder. Next, the weight of the negative electrode after firing is measured. The weight of the binder can be determined by calculating the weight of the negative electrode before and after firing. In addition, the completed negative electrode can be crushed by ultrasonic waves in a solvent such as water and filtered to separate the negative electrode active material and the binder dissolved in the solvent. The weight of the negative electrode active material and the binder can be determined by determining the weight of each of the separated materials.

The smaller the amount of binder or the larger the specific surface area of the negative electrode active material, the smaller the above value. If the value is too small, the amount of binder is insufficient relative to the specific surface area of the negative electrode active material, and the binding force between the binder and the negative electrode active material becomes insufficient. Also, the larger the amount of binder is or the smaller the specific surface area of the negative electrode active material, the larger the above value. If the value is too large, the proportion of binder that does not contribute to charging and discharging in the negative electrode increases, and the capacity per unit volume of the lithium-ion secondary battery decreases.

The binder in the negative electrode active material layer 34 binds the negative electrode active materials together. The binder has a COOX group. X is any one selected from the group consisting of Li, Na, and K. X is preferably Li. When a part of the carboxyl group is substituted with X, the electrolyte resistance is increased compared to when the carboxyl group is not substituted, the penetration of the electrolyte between the active material and the binder is suppressed, and the amount of binding between the negative electrode active material and the binder is improved. In addition, Li has a small ionic radius, so the negative electrode active material and the binder are bound at a high density, and the binding force between them is further increased.

Examples of binders include polyacrylic acid, poly(ethylene-maleic acid), and carboxymethyl cellulose that have a COOX group. Binders that do not have a COOX group may also include polyethylene oxide, polyamide, and polyimide.

The conductive additive in the negative electrode active material layer 34 enhances the electronic conductivity between the negative electrode active materials. The conductive additive can be the same as that in the positive electrode active material layer 24.

The content of the conductive additive in the negative electrode active material layer 34 is not particularly limited. For example, the content of the conductive additive relative to the total mass of the negative electrode active material, the conductive additive, and the binder is 5% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 12% by mass or less.

<Separator>
The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched membrane of a mixture of polyethylene, polypropylene, etc. The separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 may be an inorganic-coated separator. The inorganic-coated separator is a separator in which a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica is applied to the surface of the above-mentioned film. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode on the negative electrode surface.

<Electrolyte>
The electrolytic solution is sealed in the exterior body 50 and impregnates the power generating element 40. The non-aqueous electrolytic solution includes, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

The electrolyte may be a known electrolyte. The electrolyte may contain, for example, a non-aqueous solvent and an electrolyte salt.

The electrolytic salt is, for example, a lithium salt. The electrolyte is, for example, _LiPF6 , _{LiClO4 , LiBF4 , LiCF3SO3 , LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2 ) 2 , LiN ( CF3SO2 ) ( C4F9SO2 ) , LiN} ( _CF3CF2CO ) ₂ , LiBOB, LiN( _FSO2 ) ₂ , _etc. The lithium salt may _be used alone or in combination of _two or more. From the viewpoint of the degree _{of ionization} , it is preferable that the electrolyte contains _LiPF6 .

The non-aqueous solvent is, for example, an aprotic organic solvent. The organic solvent is, for example, a cyclic carbonate, a chain carbonate, an ether, a mixture thereof, etc.

The cyclic carbonate solvates the electrolyte. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and fluoroethylene carbonate. The cyclic carbonate preferably contains at least fluoroethylene carbonate. Fluoroethylene carbonate (FEC) has a high redox potential and is easily reduced and decomposed. By reducing and decomposing a portion of the fluoroethylene carbonate (FEC), the electrolyte and remaining solvent in the electrolyte are less likely to decompose. In addition, fluoroethylene carbonate (FEC) forms a thin and stable coating (SEI coating) on the entire surface of the negative electrode active material at the beginning of use of the lithium ion secondary battery. The SEI coating prevents direct contact between the negative electrode active material and the electrolyte and prevents decomposition of the electrolyte.

The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of the chain carbonate are diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent may also contain other solvents such as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, and 1,2-diethoxyethane.

<Exterior body>
The power generating element 40 and the non-aqueous electrolyte are sealed inside the exterior body 50. The exterior body 50 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and the like from entering the lithium ion secondary battery 100 from the outside.

As shown in FIG. 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

The metal foil 52 may be, for example, aluminum foil. The resin layer 54 may be a polymer film such as polypropylene. The materials constituting the resin layer 54 may be different on the inside and outside. For example, the material for the outside may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the material for the polymer film on the inside may be polyethylene (PE), polypropylene (PP), etc.

<Terminals>
The terminals 60 and 62 are connected to the negative electrode 30 and the positive electrode 20, respectively. The terminal 60 connected to the negative electrode 30 is a negative electrode terminal, and the terminal 62 connected to the positive electrode 20 is a positive electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. The terminals 60 and 62 are preferably protected with insulating tape to prevent short circuits.

"Method of manufacturing lithium-ion secondary batteries"
The lithium ion secondary battery 100 is produced by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50. An example of a method for producing the lithium ion secondary battery 100 will be described below.

The negative electrode 30 is produced, for example, by carrying out a slurry production process, an electrode application process, a drying process, and a rolling process in that order.

The slurry preparation process is a process in which a negative electrode active material (silicon or silicon compound), a binder, a conductive assistant, and a solvent are mixed to prepare a slurry. First, for example, a commercially available negative electrode active material is selected that has a predetermined ridge line to prepare a predetermined negative electrode active material. A binder having a COOX group is prepared. A binder having a carboxyl group may be ion-exchanged.

The solvent is, for example, water. The conductive assistant is, for example, carbon powder, carbon nanotubes, carbon material, metal powder, a mixture of carbon material and metal powder, conductive oxide, etc. The carbon powder is, for example, carbon black, acetylene black, ketjen black. The metal powder is, for example, a powder made of copper, nickel, stainless steel, iron, etc.

Next, the negative electrode active material and the binder are mixed in a ratio such that the specific surface area S (m ² /g) of the negative electrode active material, the weight w _a of the negative electrode active material, and the weight w _b of the binder satisfy the relationship 0.5≦{w _b /(w _a +w _b )}/S×100≦5.0. The composition ratio of the negative electrode active material, the conductive additive, and the binder is, for example, 70 wt % to 99 wt %: 0 wt % to 10 wt %: 1 wt % to 20 wt % in mass ratio. These mass ratios are adjusted to be 100 wt % in total.

The negative electrode active material may be a composite obtained by mixing active material particles and a conductive material while applying a shear force. When the active material particles are mixed with a shear force to the extent that the active material particles are not altered, the surfaces of the active material particles are coated with the conductive material. The particle size of the negative electrode active material can be adjusted by adjusting the degree of mixing. The negative electrode active material may also be sieved after preparation to make the particle size uniform.

The electrode coating process is a process of coating the surface of the negative electrode current collector 32 with a slurry. There are no particular limitations on the method of coating the slurry. For example, the slit die coating method or the doctor blade method can be used as a method of coating the slurry.

The drying process is a process for removing the solvent from the slurry. For example, the negative electrode current collector 32 on which the slurry is applied is dried in an atmosphere of 60°C or higher and 200°C or lower. In this manner, the negative electrode active material layer 34 is formed on the negative electrode current collector 32.

The rolling process is performed as necessary. The rolling process is a process in which pressure is applied to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling process is performed, for example, with a roll press device.

The positive electrode 20 can be produced in the same manner as the negative electrode 30. The separator 10 and the outer casing 50 can be commercially available.

Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. If the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

Finally, the power generating element 40 is enclosed in the exterior body 50. The non-aqueous electrolyte is injected into the exterior body 50. After the non-aqueous electrolyte is injected, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte is impregnated into the power generating element 40. The exterior body 50 is sealed by applying heat, etc., to obtain the lithium ion secondary battery 100. Note that instead of injecting the electrolyte into the exterior body 50, the power generating element 40 may be impregnated with the electrolyte.

The lithium ion secondary battery 100 according to the first embodiment has a high capacity and excellent cycle characteristics. The negative electrode active material of the lithium ion secondary battery 100 according to the first embodiment is amorphous with ridges, and the negative electrode active material is not easily packed closely together. Therefore, gaps are secured for volume expansion during charging and discharging, and the electrode is not easily destroyed.

In addition, in the lithium ion secondary battery 100 according to the first embodiment, the binder has a COOX group, and a predetermined amount of binder is added relative to the specific surface area of the negative electrode active material, so that sufficient binding strength can be ensured even at the ridgeline portion. If the negative electrode active material is amorphous, for example, the electrolyte may easily penetrate between the binder attached to the ridgeline of the negative electrode active material and the negative electrode active material, and the binding strength between the negative electrode active material and the binder may not be sufficiently ensured. If the binder has a COOX group and a predetermined amount of binder is added relative to the specific surface area of the negative electrode active material, the binding strength between the negative electrode active material and the binder can be sufficiently ensured.

The lithium ion secondary battery 100 according to the first embodiment has a high capacity with a smaller amount of binder added relative to the specific surface area of the negative electrode active material than expected. The binding strength between the negative electrode active material and the binder can be increased by increasing the amount of binder. However, since the binder does not contribute to charging and discharging, the capacity of the lithium ion secondary battery decreases when the amount of binder in the negative electrode increases. When an amorphous negative electrode active material is used, it was expected that the amount of binder added would increase, but the cycle characteristics of the lithium ion secondary battery 100 could be ensured even with a smaller amount of binder than expected.

The above describes the embodiments of the present invention in detail with reference to the drawings, but each configuration and their combinations in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible without departing from the spirit of the present invention.

"Example 1"
A positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm. The positive electrode slurry was prepared by mixing a positive electrode active material, a conductive assistant, a binder, and a solvent.

The positive electrode active material was Li _x CoO _2. The conductive assistant was acetylene black. The binder was polyvinylidene fluoride (PVDF). The solvent was N-methyl-2-pyrrolidone. 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive assistant, 2 parts by mass of the binder, and 70 parts by mass of the solvent were mixed to prepare a positive electrode slurry. The amount of the positive electrode active material carried in the positive electrode active material layer after drying was set to 25 mg/cm ^2. The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer. The positive electrode active material layer was pressed with a roll press to prepare a positive electrode.

Next, the negative electrode slurry was applied to one side of the 10 μm thick copper foil. The negative electrode slurry was prepared by mixing the negative electrode active material, the conductive additive, the binder, and the solvent.

The negative electrode active material was silicon with an average particle size of 3 μm and ridges on the surface. Carbon black was used as the conductive additive. The binder was a mixture of polyethylene oxide and polyacrylic acid in which the H in the carboxyl group was replaced with Li in a ratio of 2:8.

Water was used as the solvent. 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive assistant, and 5 parts by mass of the binder were mixed with water to prepare a negative electrode slurry. The amount of the negative electrode active material in the negative electrode active material layer after drying was 2.5 mg/cm ^2. The solvent was removed from the negative electrode slurry in a drying furnace to prepare a negative electrode active material layer. The negative electrode active material layer was pressed with a roll press, and then treated at 150° C. or higher for 5 hours in a nitrogen atmosphere and dried. The specific surface area S (m ² /g) of the negative electrode active material, the weight w _a of the negative electrode active material, and the weight w _b of the binder were {w _b /(w _a +w _b )}/S×100=0.50.

Next, an electrolyte solution was prepared. The solvent was a mixture of fluoroethylene carbonate (FEC):diethyl carbonate (DEC) = 11:89 by mass ratio. LiPF ₆ was used as the electrolyte salt. The concentration of LiPF ₆ was 1 mol/L.

(Preparation of Lithium-Ion Secondary Battery for Evaluation)
The prepared negative electrode and positive electrode were laminated with a separator (porous polyethylene sheet) between them so that the positive electrode active material layer and the negative electrode active material layer faced each other to obtain a laminate. A nickel negative electrode lead was attached to the negative electrode of the laminate. An aluminum positive electrode lead was attached to the positive electrode of the laminate. The positive electrode lead and the negative electrode lead were welded by an ultrasonic welding machine. This laminate was inserted into an exterior body of an aluminum laminate film and heat-sealed except for one place around the periphery to form a closed portion. Finally, the above-mentioned electrolyte was injected into the exterior body, and the remaining one place was sealed by heat sealing while reducing the pressure with a vacuum sealer to produce a lithium ion secondary battery.

(Measurement of capacity retention rate after 500 cycles)
The cycle characteristics of the lithium ion secondary battery were measured using a secondary battery charge/discharge tester (manufactured by Hokuto Denko Corporation).

The battery was charged at a constant current of 0.5 C (a current value at which charging at a constant current of 25° C. is completed in 1 hour) at a constant current rate of 0.5 C (CC-CV charging) until the battery voltage reached 4.2 V, and then discharged at a constant current rate of 1.0 C until the battery voltage reached 2.5 V (CC discharge). The discharge capacity after charging and discharging was detected to determine the battery capacity _Q1 before the cycle test.

The battery whose battery capacity _Q1 was determined above was charged again using a secondary battery charge/discharge tester at a constant current and constant voltage charge rate of 0.5 C until the battery voltage reached 4.2 V, and discharged at a constant current discharge rate of 1.0 C until the battery voltage reached 2.5 V. The above charge/discharge was counted as one cycle, and 500 charge/discharge cycles were performed. Thereafter, the discharge capacity after the completion of 500 charge/discharge cycles was detected, and the battery capacity _Q2 after 500 cycles was determined.

The capacity retention rate E after 500 cycles was calculated from the capacities _Q1 and _Q2 obtained above. The capacity retention rate E is calculated by E = _Q2 / _Q1 x 100. The capacity retention rate of Example 1 was 71%.

In addition, the battery capacity _Q1 before the cycle test was divided by the weight of the negative electrode active material layer to obtain the capacity (mAh) per 1 g of the negative electrode active material layer.

"Examples 2 to 4"
Examples 2 to 4 differ from Example 1 in that the relationship between the specific surface area S (m ² /g) of the negative electrode active material, the weight w _a of the negative electrode active material, and the weight w _b of the binder was changed. These relationships were changed by changing the specific surface area of the negative electrode active material, and the amounts of the negative electrode active material and binder added. In Examples 2 to 4, the capacity (mAh) per 1 g of the negative electrode active material layer and the capacity retention rate were obtained under the same other conditions as in Example 1.

"Examples 5 and 6"
Examples 5 and 6 differ from Example 1 in that the negative electrode binder used was a mixture of polyethylene oxide and polyacrylic acid in which H in the carboxyl group was substituted with Na or K in a ratio of 2:8. The substituting element in Example 5 was Na, and the substituting element in Example 6 was K. In Examples 5 and 6, the other conditions were the same as in Example 1, and the capacity (mAh) and capacity retention rate per 1 g of the negative electrode active material layer were obtained.

"Example 7"
Example 7 differs from Example 1 in that the negative electrode binder used was a mixture of polyethylene oxide and poly(ethylene-maleic acid) in which H of a carboxyl group was substituted with Li at a ratio of 2:8. In Example 7, the other conditions were the same as in Example 1, and the capacity (mAh) and capacity retention rate per 1 g of the negative electrode active material layer were determined.

"Examples 8 to 13"
Examples 8 to 13 differ from Examples 1 to 7 in that a mixture of polyethylene oxide and carboxymethyl cellulose in which H of a carboxyl group is substituted, in a ratio of 2:8, was used as the negative electrode binder. In Examples 8 to 13, the other conditions were the same as in Example 1, and the capacity (mAh) and capacity retention rate per 1 g of the negative electrode active material layer were determined.

"Comparative Examples 1 and 2"
Comparative Examples 1 and 2 differ from Example 1 in that the relationship between the specific surface area S ( ^m2 /g) of the negative electrode active material, the weight w _a of the negative electrode active material, and the weight w _b of the binder was changed. These relationships were changed by changing the specific surface area of the negative electrode active material, and the amounts of the negative electrode active material and binder added. In Comparative Examples 1 and 2, the capacity (mAh) per 1 g of the negative electrode active material layer and the capacity retention rate were obtained under the same other conditions as in Example 1.

"Comparative Example 3"
Comparative Example 3 differs from Example 2 in that a negative electrode binder was used that was a mixture of polyethylene oxide and polyacrylic acid not substituted with a carboxyl group in a ratio of 2:8. In Comparative Example 3, the other conditions were the same as in Example 2, and the capacity (mAh) and capacity retention rate per 1 g of the negative electrode active material layer were determined.

"Comparative Example 4"
Comparative Example 4 differs from Example 2 in that silicon having no ridges on its surface was used as the negative electrode active material. In Comparative Example 3, the other conditions were the same as in Example 2, and the capacity (mAh) and capacity retention rate per 1 g of the negative electrode active material layer were determined.

The results of Examples 1 to 13 and Comparative Examples 1 to 4 are summarized in Table 1. X in Table 1 indicates the substitution ion that replaces the H of the COOH group.

All of Examples 1 to 13 had a higher capacity retention rate than Comparative Examples 1 to 4. In addition, even with a smaller binder amount than expected, the cycle characteristics were good and the capacity per unit mass was high.

Reference Signs List 1 Negative electrode active material 2 Ridge line 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layers 60, 62 Terminal 100 Lithium ion secondary battery

Claims (3)

a negative electrode active material containing silicon and having a ridge on its surface;
A binder;
The binder has a COOX group,
X is any one selected from the group consisting of Li, Na, and K;
a specific surface area S ( ^m2 /g) of the negative electrode active material, a weight w _a of the negative electrode active material, and a weight w _b of the binder satisfy the relationship: 0.5≦{w _b /(w _a +w _b )}/S×100≦5.0. The negative electrode for a lithium ion secondary battery according to claim 1, wherein X in the COOX group is Li. A battery comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte;
The negative electrode is the negative electrode for lithium ion secondary batteries according to claim 2 .

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