Battery

Abstract

Provided is a battery comprising an electrode winding assembly which is a charge / discharge body that includes a positive electrode, a negative electrode, and a separator disposed therebetween, wherein: the positive electrode has a configuration in which a positive electrode active material layer is formed on both surfaces of a positive electrode collector; the positive electrode active material layer contains a positive electrode active material and a binder; the binder contains a first binder that has a relatively high peel strength and a second binder that has a relatively low peel strength; and the second binder is contained in a greater amount than the first binder. Thus, it is possible to prevent breakage of a bend part of the electrode winding assembly and to improve the safety of the battery.

Description

battery

[0001] The present disclosure relates to batteries.

[0002] In the automotive industry, fuel efficiency and environmental regulations are being strengthened in each country and region. In order to comply with these regulations, attention is being paid to the development of battery-powered electric vehicles and hydrogen-fueled fuel cell vehicles, which do not emit carbon dioxide.

[0003] However, electric vehicles face problems such as an insufficient charging infrastructure and the longer charging time required compared to refueling, while fuel cell vehicles face problems such as the huge cost required to build hydrogen station infrastructure and the high cost of fuel cells.

[0004] For this reason, PHEVs (Plug-in Hybrid Electric Vehicles) and HEVs (Hybrid Electric Vehicles), which are powered by both an internal combustion engine and a battery and emit low amounts of carbon dioxide, have become promising candidates for complying with fuel economy regulations and environmental regulations.

[0005] In a PHEV or HEV, a lithium ion secondary battery is used.

[0006] Patent Document 1 describes a positive electrode plate for a lithium ion secondary battery in which the positive electrode current collector plate and the positive electrode active material layer are well bonded together and the resistance of the positive electrode active material layer is low, the positive electrode plate having a positive electrode active material layer containing, in a predetermined ratio, a first binder made of polyacrylic acid having a molecular weight of 50,000 or less and a second binder made of polyacrylic acid having a molecular weight of 300,000 or more.

[0007] Patent Document 2 describes a positive electrode for a non-aqueous electrolyte secondary battery that can improve the characteristics of the non-aqueous electrolyte secondary battery while maintaining the flexibility of the positive electrode active material layer, and includes a positive electrode active material and a mixed binder including a first binder, a second binder, and a third binder, where the first binder includes at least one selected from polyvinylidene fluoride, acid-modified polyvinylidene fluoride, and a copolymer containing acid-modified polyvinylidene fluoride, and the proportion of the first binder and the tensile modulus are within a predetermined range. Patent Document 2 also describes that the second binder is preferably hydrogenated acrylonitrile butadiene rubber, and the third binder is preferably a copolymer containing vinylidene fluoride.

[0008] Patent Document 3 describes a positive electrode for an electric device that improves the discharge voltage and rate characteristics of the electric device, which includes: a first binder that is in direct contact with a positive electrode active material and is made of a vinylidene fluoride copolymer having a weight-average molecular weight of 300,000 to 400,000 and an inherent viscosity of 1.2 dL / g to 1.5 dL / g; and a second binder that binds together the first binders and is made of a vinylidene fluoride polymer having a weight-average molecular weight of 800,000 to 1,000,000 and an inherent viscosity of 2.0 dL / g to 3.0 dL / g.

[0009] JP 2017-022020 A JP 2017-147206 A JP 2018-101472 A

[0010] The positive electrodes described in Patent Documents 1 and 3 do not contain a mixture of multiple types of binders in order to prevent breakage when bent.

[0011] On the other hand, the positive electrode described in Patent Document 2 uses a mixture of three types of binders and is evaluated from the viewpoints of capacity retention rate and flexibility. However, Patent Document 2 also describes that, of the three types of binders, the second binder has a function of increasing flexibility, but may also degrade battery characteristics.

[0012] An object of the present disclosure is to prevent breakage of bent portions in an electrode wound group and improve the safety of the battery.

[0013] The battery of the present disclosure has an electrode wound group, which is a charge / discharge body including a positive electrode, a negative electrode, and a separator disposed therebetween. The positive electrode has a configuration in which a positive electrode active material layer is formed on both sides of a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and a binder. The binder includes a first binder having a relatively high peel strength and a second binder having a relatively low peel strength, and the second binder is contained in a larger amount than the first binder.

[0014] According to the present disclosure, breakage of bent portions in an electrode wound group can be prevented, and the safety of the battery can be improved.

[0015] Fig. 3 is an external perspective view showing a lithium ion secondary battery of an embodiment. Fig. 4 is a perspective view showing a charge / discharge body built into the lithium ion secondary battery 1 of Fig. 1. Fig. 5 is a perspective view showing a partially developed charge / discharge body 100 of Fig. 2. Fig. 6 is a cross-sectional view partially showing a charge / discharge body according to an embodiment produced by winding. Fig. 7 is a partial cross-sectional view showing a charge / discharge body of a comparative example. Fig. 8 is a partial cross-sectional view showing a charge / discharge body of an example.

[0016] The present disclosure relates to batteries used in various industrial equipment, and in particular to lithium ion secondary batteries.

[0017] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the size and proportions of components may be exaggerated to facilitate understanding of the embodiments. In each drawing, the same components are assigned the same reference numerals. In each drawing, the width direction X, depth direction Y, and height direction Z of the lithium ion secondary battery and its components are indicated by arrows. However, in each drawing, the width direction X, depth direction Y, and height direction Z indicate relative directional relationships. That is, for example, if the lithium ion secondary battery is rotated 180 degrees and placed with its top and bottom faces reversed, or if the lithium ion secondary battery is rotated 90 degrees and placed with its top face facing sideways, the width direction X, depth direction Y, and height direction Z of the lithium ion secondary battery will change.

[0018] (Configuration of Lithium-Ion Secondary Battery Having Positive Electrode of the Embodiment) The configuration of a lithium-ion secondary battery having a positive electrode of the embodiment will be described with reference to FIGS.

[0019] FIG. 1 is a perspective view showing the appearance of a lithium ion secondary battery according to an embodiment.

[0020] In this figure, the lithium-ion secondary battery 1 has a configuration in which a charging / discharging element (not shown) is housed in a container formed by a case 201 and a lid 202. A positive electrode terminal 301 and a negative electrode terminal 302 are attached to the lid 202 in an insulated state. The lid 202 is joined to the opening of the case 201 and, together with the case 201, seals the charging / discharging element.

[0021] When a battery pack is constructed using multiple lithium ion secondary batteries 1, the positive electrode terminal 301 of adjacent lithium ion secondary batteries 1 is joined to the negative electrode terminal 302 of another adjacent lithium ion secondary battery 1 via a bus bar.

[0022] FIG. 2 is a perspective view showing a charging / discharging element built into the lithium ion secondary battery 1 of FIG.

[0023] The charge / discharge unit 100 shown in Fig. 2 has a configuration in which a positive electrode and a negative electrode are wound with a separator sandwiched between them. That is, the charge / discharge unit 100 is an electrode wound group. The charge / discharge unit 100 has a positive electrode tab 111b and a negative electrode tab 121b. The positive electrode tab 111b is connected to the positive electrode terminal 301 in Fig. 1. The negative electrode tab 121b is connected to the negative electrode terminal 302 in Fig. 1.

[0024] The charging / discharging unit 100 is sealed in the lithium-ion secondary battery 1 of FIG. 1 , and an electrolytic solution (electrolyte) is poured into the charging / discharging unit 100 through a pouring port provided in the lid 202. This causes the charging / discharging unit 100 to be immersed in the electrolytic solution. After the electrolytic solution is poured, the pouring port is sealed with a pouring stopper. The electrolytic solution contains an organic solvent, a supporting salt, and an additive. For example, a carbonate ester is used as the organic solvent. For example, a lithium salt is used as the supporting salt. The additive contains a material for forming a negative electrode film. For example, the material for forming the negative electrode film is vinylene carbonate.

[0025] In one example of a lithium ion secondary battery, the minimum value of the radius of curvature of the innermost periphery of the positive electrode of the electrode wound group is 0.46 mm.

[0026] FIG. 3 is a perspective view showing the charge / discharge body 100 of FIG. 2 in a partially developed state.

[0027] 3, the positive electrode 110, negative electrode 120, and separator 130 that constitute the charge / discharge body are each formed in a long strip shape extending in the X-axis direction. The positive electrode 110 is provided with a positive electrode tab 111b. The negative electrode 120 is provided with a negative electrode tab 121b. The positive electrode tab 111b and the negative electrode tab 121b protrude in the same direction (positive direction of the Z-axis).

[0028] A separator 130 is sandwiched between the positive electrode 110 and the negative electrode 120. Furthermore, to prevent direct contact between the positive electrode 110 and the negative electrode 120 when they are wound, another separator 130 is disposed on the opposite side of the negative electrode 120. In other words, in this figure, the negative electrode 120 is sandwiched between two separators 130. The separator 130 insulates the positive electrode 110 from the negative electrode 120. The separator 130 allows lithium ions to pass through via the electrolyte.

[0029] The negative electrode current collecting portion of the negative electrode 120 has a greater width in the short side direction (Z-axis direction) than the positive electrode current collecting portion of the positive electrode 110. The separator 130 has a greater width in the short side direction (Z-axis direction) than the positive electrode 110 and the negative electrode 120.

[0030] FIG. 4 is a cross-sectional view partially showing the charge / discharge element according to the embodiment produced by winding.

[0031] This figure shows a charge / discharge element produced by laminating strip-shaped positive electrode 110, negative electrode 120, and separator 130 and winding them around a shaft core 40. The negative electrode 120 is in direct contact with the shaft core 40. On the outside of the negative electrode 120, the separator 130 and positive electrode 110 are arranged in this order. On the outside of the positive electrode 110, the separator 130 and negative electrode 120 are arranged in this order.

[0032] The positive electrode 110 has a positive electrode active material layer formed on both sides of a positive electrode current collector 115. The negative electrode 120 has a negative electrode active material layer formed on both sides of a negative electrode current collector.

[0033] The positive electrode active material layer contains a positive electrode active material and a binder, and preferably further contains a conductive additive and a dispersant.

[0034] Here, a problem with the charging / discharging body of the lithium ion secondary battery of the comparative example will be described.

[0035] FIG. 5A is a partial cross-sectional view showing a charge / discharge body of a comparative example.

[0036] In this figure, other components are omitted and the positive electrode 110 is shown in a fractured state.

[0037] The positive electrode 110 wound around the axial core is subjected to tensile stress in the longitudinal direction, which may ultimately cause both the positive electrode current collector and the positive electrode active material layer that constitute the positive electrode 110 located outside the end of the axial core to break.

[0038] FIG. 5B is a partial cross-sectional view showing the charge / discharge body of the example.

[0039] In this figure, similarly to FIG. 5A, components other than the positive electrode 110 are omitted.

[0040] 5B , in the charge / discharge body of the example, compressive stress due to bending is applied to the positive electrode active material layer on the axial side of the positive electrode 110, causing the positive electrode active material layer on the axial side to buckle and partially detach. This reduces the tensile stress applied to the positive electrode current collector and the outer positive electrode active material layer, making it possible to prevent fracture of the entire positive electrode 110.

[0041] Next, the positive electrode, negative electrode, and separator used in the battery of the example will be explained.

[0042] For the positive electrode, NCM622 (Li １．０５ Ni ０．６ Co ０．２ Mn ０．２ O ２The conductive particles were acetylene black, and the current collector was aluminum foil (thickness: 13.3 μm). The binder was Kureha KF Polymer W#9700 (Modified polymer, Mw=8.8×10) manufactured by Kureha Corporation. ５ g / mol) and Kureha KF Polymer W#1100 (Homopolymer, Mw=2.8×10 ５ Two types of ethylenediaminetetraacetic acid (ethylenediaminetetraacetic acid) were used.

[0043] The particle size (median diameter D50) of the positive electrode active material NCM622 is 13.3 μm. The positive electrode active material NCM622 is a mixture of a large particle size (median diameter D50) of 17 μm and a small particle size (median diameter D50) of 4.6 μm in a mass ratio of 7:3. The particle size was measured by a laser diffraction scattering method.

[0044] Mixing positive electrode active materials with different particle sizes is one method for increasing the density of a positive electrode. However, when positive electrode active materials with different particle sizes are mixed and used, when the positive electrode is fabricated by pressing to increase the density, deformation such as undulation of the aluminum foil of the current collector may occur. Such deformation is thought to result in a decrease in the strength of the foil. Therefore, the present inventors conducted extensive research to prevent this and found that using two types of binders according to the present disclosure is effective.

[0045] The peel strength of Kureha KF Polymer W#9700 is 5.2 to 5.5 gf / mm at an electrode density of 3.0 to 3.4 g / ml, and that of Kureha KF Polymer W#1100 is 0.7 to 1.5 gf / mm at an electrode density of 3.0 to 3.8 g / ml. Here, Kureha KF Polymer W#9700, which has a relatively high peel strength, corresponds to the "first binder," and Kureha KF Polymer W#1100, which has a relatively low peel strength, corresponds to the "second binder." Here, electrode density refers to the density of the positive electrode active material layer.

[0046] The range of the molecular weight Mw of the binder is 7.8×10 for the first binder. ５ ~9.8 x 10 ５ g / mol is desirable, and 8.3 × 10 ５ ~9.3 x 10５ g / mol is more preferable. In the case of the second binder, 1.8×10 ５ ~3.8 x 10 ５ g / mol is preferable, and 2.7×10 ５ ~2.9 x 10 ５ g / mol is more preferable.

[0047] The peel strength was measured by applying an adhesive tape (width: 18 mm) conforming to JIS Z 1522 to the surface of a binder that was in close contact with an aluminum foil or copper foil serving as a current collector using a rubber roller, and then performing a peel test using a force gauge (force gauge moving speed: 5 mm / sec, load angle: 180 degrees).

[0048] The above two types of binders are vinylidene fluoride resins, which are said to cause little deterioration in battery characteristics.

[0049] For the negative electrode, graphite was used as the negative electrode active material, an acrylic binder, and copper foil (thickness 8 μm) as the current collector.

[0050] The separator used was a polypropylene resin sheet, but the separator may be made of other materials such as polyethylene resin.

[0051] Typically, a binder with high flexibility has low adhesion, and a binder with high adhesion has low flexibility. Therefore, it is desirable to use both. By using both, it is possible to achieve high density of the positive electrode, suppress deformation of the aluminum foil current collector, and improve peel strength.

[0052] The density of the positive electrode active material layer is 3.30 g / cm ３ 3.72g / cm or more ３ It is desirable that the following:

[0053] The binder includes a first binder having a relatively high peel strength and a second binder having a relatively low peel strength.

[0054] The second binder is contained in a larger amount than the first binder. If the proportion of the second binder is equal to or less than the proportion of the first binder, fracture of the positive electrode 110 shown in FIG. 5A may occur.

[0055] The mixing ratio of the first binder to the second binder is preferably 1:9 to 3:7 by weight, and more preferably 1.5:8.5 to 2.5:7.5. In other words, the mixing ratio is preferably in the range of 1 part by weight of the first binder to 9 parts by weight of the second binder to 3 parts by weight of the first binder to 7 parts by weight of the second binder, and more preferably in the range of 1.5 parts by weight of the first binder to 8.5 parts by weight of the second binder to 2.5 parts by weight of the first binder to 7.5 parts by weight of the second binder.

[0056] The positive electrode active material layer preferably contains 98.2 to 98.7 parts by weight of the positive electrode active material and 0.5 to 1.0 part by weight of the binder.

[0057] Finally, the effects of the present disclosure will be summarized.

[0058] According to the present disclosure, by using a binder that causes little deterioration in battery characteristics, it is possible to prevent breakage of the bent portion of the positive electrode located at the innermost side of the electrode wound group, thereby improving the safety of the battery.

[0059] Furthermore, according to the present disclosure, minor buckling, i.e., partial detachment of the positive electrode active material layer on the inner periphery of the positive electrode, can be caused by a relatively weak force at the bent portion of the positive electrode located at the innermost side of the electrode wound group, thereby preventing major fracture.

[0060] 1: Lithium ion secondary battery, 40: shaft core, 100: charging / discharging body, 110: positive electrode, 111b: positive electrode tab, 115: positive electrode current collector, 120: negative electrode, 121b: negative electrode tab, 130: separator, 201: case, 202: lid, 301: positive electrode terminal, 302: negative electrode terminal.

Claims

1. A battery comprising an electrode wound assembly which is a charge / discharge body including a positive electrode, a negative electrode, and a separator disposed therebetween, wherein the positive electrode has a configuration in which a positive electrode active material layer is formed on both sides of a positive electrode current collector, the positive electrode active material layer includes a positive electrode active material and a binder, the binder includes a first binder having a relatively high peel strength and a second binder having a relatively low peel strength, and the second binder is contained in a larger amount than the first binder.

2. The battery according to claim 1, wherein the mixing ratio of said first binder to said second binder is 1:9 to 3:7 by weight.

3. The battery according to claim 1 or 2, wherein the positive electrode active material layer contains 98.2 to 98.7 parts by weight of the positive electrode active material and 0.5 to 1.0 part by weight of the binder.

4. The battery according to claim 1 or 2, wherein the positive electrode active material layer further contains a conductive additive and a dispersant.

5. The battery according to claim 1 or 2, wherein the first binder and the second binder are polyvinylidene fluoride resins.

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