Negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

To secure binding strength and reduce resistance when soft carbon is used as an active material.SOLUTION: The negative electrode 1 for the lithium ion secondary battery has a negative electrode active material layer 5 formed on a surface of a negative electrode current collector 3. The negative electrode active material layer 5 contains soft carbon 7 and polyacrylic acid 9.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery. [Background technology]

[0002] A technology has been disclosed for lithium-ion secondary batteries that uses an aqueous binder in the negative electrode to improve vibration resistance (improve peel strength) (see Patent Document 1). This technology also improves battery characteristics (and suppresses capacity loss) by using an aqueous binder in the negative electrode. Specifically, this technology uses SBR (styrene butadiene rubber) as the aqueous binder in the negative electrode, with the binder amount in the negative electrode set to 2 to 4 wt%. This technology also uses graphite (artificial) as the negative electrode active material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-106345 Summary of the Invention [Problem to be solved by the invention]

[0004] In negative electrodes that use aqueous binders, graphite is generally used as the active material, with SBR and CMC (carboxymethyl cellulose) as the binder and dispersant. When soft carbon is used as the active material, SBR is difficult to adsorb to the active material due to its hydrophobicity, and a large amount must be added to ensure binding strength. When SBR is used, CMC is required to act as a dispersant and thickener, but CMC becomes a resistive component. Therefore, it has been difficult to ensure both binding strength and low resistance. The present disclosure has been made in view of the above circumstances, and aims to achieve both ensuring binding strength and reducing resistance when soft carbon is used as an active material. The present disclosure can be realized in the following forms. [Means for solving the problem]

[0005] [1] A negative electrode for a lithium ion secondary battery, comprising a negative electrode active material layer formed on the surface of a negative electrode current collector, The negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer contains soft carbon and polyacrylic acid. [Effects of the Invention]

[0006] According to the present disclosure, when soft carbon is used as the substance, it is possible to ensure binding strength and reduce resistance at the same time. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of an example of a negative electrode for a lithium ion secondary battery. [Figure 2] FIG. 1 is a schematic diagram illustrating an example of a lithium ion secondary battery. [Figure 3] FIG. 2 is a schematic diagram illustrating a negative electrode according to an example. [Figure 4] This is the chemical formula for polyacrylic acid. DETAILED DESCRIPTION OF THE INVENTION

[0008] Here are examples of the present disclosure. [1] A negative electrode for a lithium ion secondary battery, comprising a negative electrode active material layer formed on the surface of a negative electrode current collector, The negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer contains soft carbon and polyacrylic acid. [2] [1] The negative electrode for a lithium ion secondary battery according to [1], wherein the amount of the polyacrylic acid-based binder contained in the negative electrode active material layer is 8% by mass or more and 10% by mass or less, relative to 100% by mass of the total amount of the negative electrode active material layer. [3] The negative electrode for a lithium ion secondary battery according to [1] or [2], wherein the weight average molecular weight of the polyacrylic acid is 450,000 or less. [4] [1] or [2], and the negative electrode for a lithium ion secondary battery; a positive electrode for a lithium ion secondary battery having a positive electrode active material; an ion-conductive medium interposed between the negative electrode for the lithium ion secondary battery and the positive electrode for the lithium ion secondary battery, and conducting lithium ions; A lithium-ion secondary battery comprising:

[0009] The present disclosure will be described in detail below. In this specification, when a numerical range is indicated using "to" it is intended to include both the lower limit and the upper limit unless otherwise specified. For example, the expression "10 to 20" includes both the lower limit "10" and the upper limit "20". In other words, "10 to 20" has the same meaning as "10 or more and 20 or less". In addition, in this specification, the upper limit and lower limit of each numerical range can be combined in any way.

[0010] 1. Negative electrode for lithium-ion secondary battery 1 The negative electrode 1 for a lithium ion secondary battery (hereinafter also simply referred to as "negative electrode 1") is formed by forming a negative electrode active material layer 5 on the surface of a negative electrode current collector 3. The negative electrode active material layer 5 contains soft carbon and polyacrylic acid.

[0011] (1) Negative electrode current collector 3 The negative electrode current collector 3 is a chemically inactive, highly electron-conductive material that allows current to flow continuously through the electrode during charging or discharging of the lithium-ion secondary battery 11. Materials that can be used for the negative electrode current collector 3 include metal materials such as stainless steel, titanium, nickel, aluminum, and copper, as well as conductive resins. The negative electrode current collector 3 can also be in the form of a foil, sheet, film, or the like. Therefore, metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil are suitable for use as the negative electrode current collector 3. The thickness of the current collector is not particularly limited, but is preferably 10 μm or more and 100 μm or less.

[0012] (2) Soft carbon Soft carbon is amorphous carbon (carbon with an amorphous structure). Soft carbon is easily graphitized carbon that can be graphitized by heat treatment at temperatures above 2000°C. Soft carbon can be obtained by carbonizing a precursor such as a resin or a resin composition. The resin or resin composition that serves as the raw material (precursor) of soft carbon is not particularly limited, and examples include coal-based pitch (e.g., coal tar pitch), petroleum-based pitch, mesophase pitch, coke, and low-molecular-weight heavy oil. The shape of the soft carbon precursor is not particularly limited, and any shape such as powder, plate, granule, fiber, lump, or sphere can be used. The weight average molecular weight of the soft carbon precursor used is preferably 1,000 or more, and more preferably 1,000,000 or more.

[0013] (3) Polyacrylic acid The weight-average molecular weight of the polyacrylic acid is not particularly limited. From the viewpoint of improving the coating properties of the slurry on the negative electrode current collector 3, the weight-average molecular weight of the polyacrylic acid is preferably 5,000 to 1,000,000, more preferably 100,000 to 1,000,000, and even more preferably 250,000 to 450,000. Note that the weight-average molecular weight of the polyacrylic acid is measured by GPC (gel permeation chromatography) analysis.

[0014] There are no particular limitations on the amount of polyacrylic acid contained in the negative electrode active material layer 5. From the viewpoint of suppressing the resistance value while ensuring the binding strength of the soft carbon, the amount of polyacrylic acid is preferably 8% by mass or more and 10% by mass or less relative to the total amount (100% by mass) of the negative electrode active material layer.

[0015] (4) Conductive additive The negative electrode active material layer 5 may contain a conductive additive.

[0016] 2. Lithium-ion secondary battery11 The lithium ion secondary battery 11 includes a positive electrode 13 for a lithium ion secondary battery (hereinafter also simply referred to as "positive electrode 13") having a positive electrode active material, a negative electrode 1, and an ion conductive medium (not shown) interposed between the positive electrode 13 and the negative electrode 1 and conducting lithium ions.

[0017] (1) Positive electrode 13 The positive electrode 13 has a positive electrode active material layer formed by binding a positive electrode active material with a binder, and the positive electrode active material layer is disposed on the surface of a positive electrode current collector. The positive electrode active material layer may further contain a conductive additive as necessary.

[0018] The current collector can be the same as that described for the negative electrode 1.

[0019] The positive electrode active material may be a known lithium-containing compound or other metal compound. Examples of the lithium-containing compound include a layered lithium-cobalt composite oxide, a layered lithium-nickel composite oxide, a spinel lithium-manganese composite oxide, and a lithium-cobalt composite oxide represented by the general formula: Li a Co p Ni q Mn r D s O x(D is at least one selected from Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, and Na. p + q + r + s = 1, 0 < p < 1, 0 ≤ q < 1, 0 ≤ r < 1, 0 ≤ s < 1, 0.8 ≤ a < 2.0, -0.2 ≤ x - (a + p + q + r + s) ≤ 0.2), a lithium cobalt-containing composite metal oxide having a layered structure, a general formula: an olivine-type lithium phosphate composite oxide represented by LiMPO4 (M is at least one of Mn, Fe, Co, and Ni), a general formula: a fluorinated olivine-type lithium phosphate composite oxide represented by Li2MPO4F (M is at least one of Mn, Fe, Co, and Ni), a general formula: a silicate-based lithium composite oxide represented by Li2MSiO4 (M is at least one of Mn, Fe, Co, and Ni) can be used. Further, as other metal compounds, for example, oxides such as titanium oxide, vanadium oxide, or manganese dioxide, or disulfides such as titanium disulfide or molybdenum disulfide can be mentioned.)

[0020] As the lithium-containing oxide, for example, LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiCoO2, LiNi 0.8 Co 0.2 O2, LiCoMnO2 can be used. LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiNi[[ID=3३]] 0.5 Co 0.2 Mn 0.3 O2 is preferable in terms of thermal stability.

[0021] The binder serves to bind the positive electrode active material and the conductive additive to the positive electrode current collector. Examples of binders that can be used include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluorine rubber, thermoplastic resins such as polypropylene, polyethylene, and polyvinyl acetate resins, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, and rubbers such as styrene-butadiene rubber (SBR). The positive electrode active material layer may contain a conductive additive.

[0022] (2) Separator 15 The lithium ion secondary battery 11 typically includes a separator 15. The separator 15 separates the positive electrode 13 from the negative electrode 1, preventing short circuits due to contact between the electrodes while allowing lithium ions to pass through. The separator 15 may be a porous membrane made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous membrane made of ceramics.

[0023] (3) Ion-conducting medium The lithium ion secondary battery 11 includes an electrolytic solution as an ion-conducting medium. The electrolytic solution contains a solvent and an electrolyte dissolved in the solvent. Examples of solvents that can be used include cyclic esters, chain esters, and ethers. Examples of cyclic esters that can be used include ethylene carbonate, propylene carbonate, butylene carbonate, gamma-butyrolactone, vinylene carbonate, 2-methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone, and gamma valerolactone. Examples of chain esters that can be used include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, alkyl propionate, dialkyl malonate, and alkyl acetate. Examples of ethers that can be used include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. Furthermore, as the electrolyte to be dissolved in the electrolytic solution, for example, lithium salts such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, and LiN(CF3SO2)2 can be used.

[0024] As the electrolyte, for example, a solution in which a lithium salt such as LiClO4, LiPF6, LiBF4, or LiCF3SO3 is dissolved in a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, or dimethyl carbonate at a concentration of about 0.5 mol / L to 1.7 mol / L can be used. [Example]

[0025] 1. Fabrication of Lithium-ion Secondary Batteries (1) Preparation of the positive electrode The positive electrode active material was LiNiCoAlO2 (average particle size: 4-8 μm), the conductive additives acetylene black and carbon black, and the binder polyvinylidene fluoride (PVDF) were mixed in a predetermined mass ratio, and an appropriate amount of viscosity-adjusting solvent N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode slurry. This paste is applied to both sides of aluminum foil with a thickness of 10 to 15 μm with a coating weight of 3 to 6 mg / cm on one side. 2 The positive electrode was fabricated by applying the solution so as to form a positive electrode.

[0026] (2) Preparation of the negative electrode In Examples 1 to 8, a negative electrode slurry was prepared by mixing soft carbon (average particle diameter: 9 to 15 μm) as a negative electrode active material and an aqueous solution of an acrylic acid-based polymer (aqueous solution of polyacrylic acid) as a binder, and adding an appropriate amount of ion-exchanged water as a viscosity adjusting solvent. The binder was added in a predetermined amount so that the total amount of the soft carbon and the binder was 100% by mass. This paste is applied to one side of a copper foil with a thickness of 6 to 10 μm, with a specified basis weight of 2 to 5 mg / cm 2 The mixture was then dried and pressed to prepare a negative electrode. A schematic diagram of the structure of the negative electrode of this example is shown in Figure 3. A negative electrode active material layer 5 is formed on a negative electrode current collector 3. In the negative electrode active material layer 5, soft carbon 7 is bound to each other by an acrylic acid-based binder 9. The negative electrode active material layer 5 is also bound to the negative electrode current collector 3 by the acrylic acid-based binder 9. In the comparative examples, various binders were used as listed in Table 1. The SBR in comparative example 1 is a water-based binder, while the PVDF is an organic solvent-based binder.

[0027] [Table 1]

[0028] (3) Method for fabricating evaluation lithium-ion secondary batteries Tabs were welded to each of the positive electrode and two negative electrodes prepared above, and these were alternately stacked with nonwoven fabric separators to prepare an electrode assembly. The electrode body was sealed in an exterior material made of an aluminum laminate film, along with an electrolyte solution prepared by dissolving LiPF6 as a supporting salt in a solvent containing EC and EMC, to obtain a lithium ion battery for evaluation. The battery thus obtained was charged at a constant current and constant voltage up to 4.2 V in an environment of 25° C., and then discharged at a constant current and constant voltage down to 2.7 V to complete a lithium-ion battery for evaluation.

[0029] 2. Evaluation Method (1) Viscosity measurement of slurry The shear rate dependence of viscosity of the negative electrode slurry prepared in the above section "1.(2) Preparation of the negative electrode" was measured using a rheometer (Anton Paar MCR301). The measurement was performed using a 50 mmφ 1° cone and plate as a measuring jig, with a gap of 0.1 mm and a shear rate of 0.01 to 1000 s -1 The shear rate was measured in both the increasing and decreasing directions within the range of 100 s -1 The viscosity at this shear rate was used as an index of coatability. The evaluation was as follows: "A": The viscosity of the slurry was less than 3.0 mPa·s, and the coating properties were very good. "B": The viscosity of the slurry was 3.0 mPa·s or more and less than 6.0 mPa·s, and the coating properties were good. "C": The viscosity of the slurry was 6.0 mPa·s or more, and the coating properties were poor.

[0030] (2) Peel strength measurement The peel strength of the electrode to the copper foil obtained in the above section "1.(2) Preparation of the negative electrode" was measured by a 90°C peel test. The measurement was carried out using an electric test stand (Imada MX2-500N) equipped with a digital force gauge (Imada ZTA-50N) and a 90° peel test jig (Imada P90-200N). The electrode was cut into a strip 20 mm wide and 150 mm long (80 mm coated area) and attached to a metal plate with double-sided tape, with the coated side facing down. The copper foil at the end of the attached strip electrode was attached to the film chuck of a digital force gauge, and the force gauge was moved upward at a speed of 100 mm / min to measure the strength at which the coated electrode peeled from the copper foil. The peel strength was calculated by dividing the tensile load measured by the digital force gauge by the electrode width of 20 mm. The evaluation was as follows: "A": Peel strength was 6 N / m or more. "B": Peel strength was 3 N / m or more and less than 6 N / m. "C": Peel strength was less than 3 N / m.

[0031] (3) Resistance measurement method The lithium ion secondary battery for evaluation was adjusted to 3.6 V. It was placed in a 25°C environment and discharged at a current value of 20 C for 10 seconds, and the voltage drop ΔV was determined. The voltage drop ΔV was divided by the discharge current value (20 C) to calculate the battery resistance, which was used as the resistance value. The evaluation was as follows: "A"...The resistance value was less than 2.3 mΩ. "B": The resistance value was 2.3 mΩ or more and less than 3.0 mΩ. "C"...The resistance value was 3.0 mΩ or more.

[0032] 3. Evaluation Results The evaluation results are shown in Table 1. All of Examples 1 to 8 were evaluated as either "A" or "B" in the categories of "peel strength" and "resistance," confirming that it was possible to ensure both binding strength and low resistance. In addition, Examples 1 to 8 also had good "slurry viscosity" and excellent coatability. Focusing on the "weight average molecular weight," it was confirmed that Examples 1 to 7, which used polyacrylic acid with a weight average molecular weight of 450,000 or less, could achieve a lower resistance than Example 8. Focusing on "peel strength," sufficient binding strength was ensured when the amount of polyacrylic acid added was 8 mass % or more, regardless of the weight average molecular weight of the polyacrylic acid used. When SBR of Comparative Example 1 was used, the resistance value was high (evaluated as "C"). Furthermore, the PVDF of Comparative Example 2 is not a water-based binder because an organic solvent is used when preparing the negative electrode slurry.

[0033] The foregoing examples are for illustrative purposes only and are not to be construed as limiting the present disclosure. While the present disclosure has been described with reference to exemplary embodiments, the words used in describing and illustrating the present disclosure are understood to be descriptive and exemplary, rather than limiting. As detailed herein, changes may be made within the purview of the appended claims without departing from the scope or spirit of the present disclosure in its form. While specific structures, materials, and examples have been referenced herein in the detailed description of the present disclosure, it is not intended that the present disclosure be limited to the specifics disclosed herein; rather, the present disclosure extends to all functionally equivalent structures, methods, and uses within the scope of the appended claims.

[0034] The present disclosure is not limited to the above-described embodiments, and various modifications and variations are possible within the scope of the claims. [Explanation of symbols]

[0035] 1...Anode for lithium-ion secondary battery 3...Negative electrode current collector 5...Negative electrode active material layer 7...Soft carbon 9...Polyacrylic acid 11...Lithium-ion secondary battery 13...Positive electrodes for lithium-ion secondary batteries 15...Separator

Claims

1. A negative electrode for a lithium ion secondary battery, comprising a negative electrode active material layer formed on the surface of a negative electrode current collector, The negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer contains soft carbon and polyacrylic acid.

2. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the amount of the polyacrylic acid contained in the negative electrode active material layer is 8% by mass or more and 10% by mass or less, relative to 100% by mass of the total amount of the negative electrode active material layer.

3. 3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the weight average molecular weight of the polyacrylic acid is 450,000 or less.

4. The negative electrode for a lithium ion secondary battery according to claim 1 or 2, a positive electrode for a lithium ion secondary battery having a positive electrode active material; an ion-conductive medium interposed between the negative electrode for the lithium ion secondary battery and the positive electrode for the lithium ion secondary battery, and conducting lithium ions; A lithium-ion secondary battery comprising:

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