Secondary battery

Abstract

This secondary battery (10) is provided with an electrode body (14) in which a positive electrode (11) and a negative electrode (12) are wound with a separator (13) interposed therebetween. The positive electrode (11) comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material and a binder. The secondary battery (10) is characterized in that the content of the binder is 0.6 mass% or more and less than 1.5 mass% with respect to the total mass of the positive electrode mixture layer, and the ratio of the area where the intensity of the binder component is 30 or more in an elemental mapping image of a cross section of the positive electrode mixture layer acquired using an electron probe microanalyzer (EPMA) is more than 10% and less than 40%.

Description

secondary battery

[0001] The present invention relates to secondary battery technology.

[0002] BACKGROUND ART In recent years, secondary batteries that have high output and high energy density have been widely used, and that include an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and that perform charging and discharging by transferring ions such as lithium ions between the positive electrode and the negative electrode.

[0003] In a secondary battery having a wound electrode assembly, if the flexibility of the positive electrode is low, the positive electrode may break when the electrode is wound, for example, at a portion of the electrode assembly with a high curvature at the innermost periphery. Note that the thicker the positive electrode composite layer constituting the positive electrode, the lower the flexibility of the positive electrode, and the more likely the positive electrode to break.

[0004] As a method for suppressing the decrease in flexibility of the positive electrode, for example, a method of heat treating the positive electrode can be mentioned (for example, Patent Document 1).

[0005] JP 2012-033381 A

[0006] However, when a secondary battery is produced using a heat-treated positive electrode, problems such as an increase in the initial battery resistance and a decrease in charge-discharge cycle characteristics may occur.

[0007] Therefore, an object of the present disclosure is to provide a secondary battery that can suppress the loss of a positive electrode, the increase in initial battery resistance, and the deterioration of charge / discharge cycle characteristics.

[0008] A secondary battery according to one aspect of the present disclosure includes an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, the positive electrode including a positive electrode current collector and a positive electrode composite layer disposed on the positive electrode current collector and containing a positive electrode active material and a binder, the content of the binder being 0.6 mass% or more and less than 1.5 mass% with respect to a total mass of the positive electrode composite layer, and the proportion of an area where the intensity of the binder component is 30 or more in an elemental mapping image of a cross section of the positive electrode composite layer obtained using an electron beam microanalyzer (EPMA) is more than 10% and less than 40%.

[0009] According to one aspect of the present disclosure, a secondary battery can be provided that can suppress the occurrence of a positive electrode failure, an increase in the initial battery resistance, and a decrease in charge-discharge cycle characteristics.

[0010] 1 is a cross-sectional view of a secondary battery according to an embodiment of the present invention;

[0011] Hereinafter, an example of an embodiment of a secondary battery according to the present disclosure will be described with reference to the drawings. Note that the secondary battery according to the present disclosure is not limited to the embodiment described below.

[0012] Fig. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 10 shown in Fig. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, an electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing body 17 that closes the opening of the case body 16. Examples of the battery case 15 include a cylindrical or rectangular metal case, a resin case (so-called pouch-type) formed by laminating a resin sheet, and the like.

[0013] The electrolyte has, for example, ion conductivity (for example, lithium ion conductivity). The electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0014] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0015] Furthermore, examples of the solid electrolyte that can be used include solid or gel-like polymer electrolytes, inorganic solid electrolytes, and the like. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. For example, a polymer material that absorbs a non-aqueous solvent and gels is used as the matrix polymer. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. For example, the inorganic solid electrolyte can be a material known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, and the like). While the above-exemplified electrolytes are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may be an aqueous electrolyte.

[0016] The case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0017] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. If the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 may deform and rupture, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure further increases, the upper valve body 26 may rupture, and gas may be discharged from the opening of the cap 27.

[0018] In the secondary battery 10 shown in Fig. 1, a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the case body 16, and the case body 16 serves as the negative electrode terminal.

[0019] The positive electrode 11, the negative electrode 12, and the separator 13 will be described in detail below.

[0020] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode composite layer provided on the positive electrode current collector. The positive electrode composite layer includes a positive electrode active material and a binder. The positive electrode composite layer may also include an additive such as a conductive material. The positive electrode composite layer may be provided on one side of the positive electrode current collector or on both sides of the positive electrode current collector.

[0021] The positive electrode current collector may be a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode, or a film having such a metal disposed on the surface thereof.

[0022] The positive electrode active material uses a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination.

[0023] The lithium transition metal composite oxide has, for example, a layered rock salt structure. Examples of the layered rock salt structure include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of high capacity and stability of the crystal structure. The content of the positive electrode active material is, for example, 90% by mass or more and 99% by mass or less with respect to the mass of the positive electrode mixture layer. From the viewpoint of increasing the capacity of the battery, the density of the positive electrode mixture layer is preferably 3.3 g / cc or more, and, for example, 3.3 cc or more and 3.8 g / cc or less.

[0024] Examples of conductive materials contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. One type of conductive material may be used alone, or multiple types may be used in combination. The content of the conductive material is, for example, 0.1% by mass or more and 5% by mass or less with respect to the total mass of the positive electrode mixture layer.

[0025] Examples of the binder contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer and ethylene-propylene-butadiene copolymer, acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide and ethylene-acrylic acid copolymer, styrene-butadiene copolymer (SBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), and polyethylene oxide (PEO).

[0026] The binder may be used alone or in combination of two or more types. In terms of the binding ability between the positive electrode mixture layer and the positive electrode current collector, the binding ability between particles of the positive electrode active material, etc., the binder preferably contains polyvinylidene fluoride (PVDF).

[0027] The content of the binder is 0.6% by mass or more and less than 1.5% by mass, preferably 0.6% by mass or more and 1.0% by mass or less, and more preferably 0.8% by mass or more and 1% by mass or less, relative to the total mass of the positive electrode mixture layer. By setting the content of the binder within this range, the binding between the positive electrode mixture layer and the positive electrode current collector and the binding between particles of the positive electrode active material and the like are ensured, thereby suppressing, for example, peeling of the positive electrode mixture layer and shedding of positive electrode active material particles due to charge and discharge, and suppressing deterioration of charge and discharge cycle characteristics. Furthermore, by setting the content of the binder within this range, for example, the amount of binder covering the positive electrode active material is reduced, which leads to suppression of an increase in initial battery resistance.

[0028] In the positive electrode of this embodiment, in an elemental mapping image of the cross section of the positive electrode composite layer obtained using an electron probe microanalyzer (EPMA), the ratio of the area where the binder component has an intensity of 30 or more (hereinafter sometimes referred to as the area ratio of the binder component with an intensity of 30 or more) is greater than 10% and less than 40%, preferably 15% to 35%. The binder component to be analyzed by EPMA may be any component that allows the amount of binder present in the cross section of the positive electrode composite layer to be confirmed, such as C or F. Note that the positive electrode composite layer may contain the same component as the binder component, but the influence is within the margin of error. The area ratio of the binder component with an intensity of 30 or more corresponds to the dispersion state of the binder. When this area ratio satisfies the above range, it is presumed that the binder in the positive electrode composite layer is adequately dispersed as aggregates. As a result, the coverage area of ​​the binder on the positive electrode active material is kept small, thereby suppressing an increase in initial battery resistance. Furthermore, the nonaqueous electrolyte may easily penetrate into the positive electrode mixture layer, which may lead to suppression of deterioration in charge-discharge cycle characteristics. On the other hand, when the area ratio of the binder component with a strength of 30 or more is 40% or more, it is presumed that the binder is widely distributed in the positive electrode mixture layer as aggregates or a thin film. This increases the area covered by the binder on the positive electrode active material, leading to an increase in initial battery resistance. Furthermore, when the area ratio of the binder component with a strength of 30 or more is 10% or less, the binder is poorly dispersed in the positive electrode mixture layer, which may deteriorate the adhesion between particles of the positive electrode active material, etc., and between the positive electrode mixture layer and the positive electrode current collector, resulting in powder falling off of the positive electrode active material during winding. Therefore, as in this embodiment, it is desirable to set the area ratio of the binder component with a strength of 30 or more to more than 10%.

[0029] A method for measuring the area ratio of binder component strength of 30 or more is described below. A positive electrode cut to a predetermined size (15 x 10 mm) is embedded in resin, and the cross-section of the positive electrode composite layer is exposed by cross-section polishing (CP). A conductive coating is then applied to the processed cross-section of the positive electrode composite layer by sputtering, vacuum deposition, or the like. The cross-section of the conductively coated positive electrode composite layer is analyzed by EPMA. The EPMA analysis conditions are an acceleration voltage of 10 kV, a probe current of 30 nA, and a beam diameter of <1 μm. Multiple measurement points are set at a vertical and horizontal pitch of 0.5 μm within a square analysis area with sides of 250 μm. An electron beam is then irradiated onto the measurement points, and an elemental mapping image is created based on the obtained binder component strength. In the obtained element mapping image, the number of measurement points where the intensity of the binder component is 30 or more is counted, and the ratio of measurement points with an intensity of 30 or more to the total number of measurement points in the analysis area is calculated, and this is defined as the area ratio of the binder component with an intensity of 30 or more.

[0030] In order to increase the capacity of the secondary battery, it is desirable to increase the thickness of the positive electrode composite layer. Specifically, it is preferable to set the ratio (T2 / T1) of the thickness (T2) of the positive electrode composite layer to the thickness (T1) of the positive electrode current collector to 7 or more. However, in this case, the risk of the positive electrode breaking when the positive electrode 11 is wound increases. However, with the positive electrode of this embodiment, in which the area ratio of the binder component having a strength of 30 or more is greater than 10% and less than 40%, the flexibility of the positive electrode is ensured, and therefore, positive electrode breakage when the positive electrode is wound can be suppressed even if the T2 / T1 is 7 or more. In terms of increasing the capacity of the secondary battery and suppressing positive electrode breakage, the T2 / T1 is preferably 7 or more, and more preferably 8 or more and 12 or less. Note that when positive electrode composite layers are provided on both sides of the positive electrode current collector, the thickness (T2) of the positive electrode composite layer is the total thickness of both positive electrode composite layers.

[0031] An example of a method for fabricating the positive electrode 11 will be described. First, a positive electrode mixture slurry containing predetermined amounts of a positive electrode active material, a conductive material, and a binder is prepared. The binder content is 0.6 mass% or more and less than 1.5 mass% relative to the total mass of the solids. The prepared positive electrode mixture slurry is applied to both sides or one side of a positive electrode current collector and dried to form a positive electrode mixture layer on the positive electrode current collector, and the positive electrode mixture layer is then rolled. In this manner, a positive electrode having a binder content of 0.6 mass% or more and less than 1.5 mass% relative to the total mass of the positive electrode mixture layer is obtained. Next, the positive electrode is heat-treated and then rapidly cooled. By heat-treating a positive electrode having a binder content within the above range and then rapidly cooling it, a positive electrode can be obtained in which the area ratio of the binder component with a strength of 30 or more is more than 10% and less than 40%.

[0032] The temperature of the heat treatment is, for example, preferably equal to or higher than the melting point of the binder, and more preferably in the range of 20°C to 40°C higher than the melting point of the binder. The temperature rise rate during the heat treatment is not particularly limited, but is, for example, 10°C / sec or higher. The heat treatment is performed, for example, by bringing the positive electrode into contact with a roller heated to a predetermined temperature.

[0033] During rapid cooling, the heat-treated positive electrode is cooled to 40° C. or to room temperature, for example, at a cooling rate of 20° C. / sec or more and 40° C. / sec or less. Rapid cooling is performed, for example, by bringing the positive electrode into contact with a roller cooled to a predetermined temperature.

[0034] The heating treatment and cooling treatment may be performed before rolling the positive electrode mixture layer or while rolling the positive electrode mixture layer. However, the heating treatment and cooling treatment are preferably performed after rolling the positive electrode mixture layer, since this further increases the flexibility of the positive electrode and further suppresses breakage of the positive electrode.

[0035] [Negative Electrode] The negative electrode 12 includes a negative electrode current collector such as a metal foil and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode current collector can be a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film with such a metal disposed on the surface layer. The negative electrode composite layer includes, for example, a negative electrode active material, a binder, etc.

[0036] The negative electrode 12 can be obtained, for example, by applying a negative electrode composite slurry containing a negative electrode active material, a binder, etc., onto a negative electrode current collector, drying the slurry, and then rolling the negative electrode composite layer. The negative electrode composite layer may be provided on one side or both sides of the negative electrode current collector.

[0037] The negative electrode active material may be, for example, a carbon material that reversibly absorbs and releases lithium ions. Examples of carbon materials that function as the negative electrode active material include graphite, such as natural graphite, artificial graphite, and mixtures thereof. The negative electrode active material may be an element that alloys with Li, such as Si or Sn, or a material containing such an element. Among these, Si-containing materials are preferred. Furthermore, lithium titanate, which has a higher charge / discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. The content of the negative electrode active material is, for example, 90% by mass or more and 99.5% by mass or less, based on the mass of the negative electrode mixture layer.

[0038] Examples of the binder include the same binders as those used in the positive electrode 11. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder is, for example, 0.1 mass % or more and 5 mass % or less with respect to the total mass of the negative electrode mixture layer. Note that the negative electrode mixture layer may also contain a conductive material.

[0039] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator whose surface is coated with a material such as an aramid-based resin or ceramic may be used.

[0040] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to the following examples.

[0041] Example 1 Preparation of Positive Electrode Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride, which are positive electrode active materials, were mixed in a solids mass ratio of 98.4:1.0:0.6, and a positive electrode mixture slurry was prepared using N-methylpyrrolidone (NMP) as a dispersion medium. The slurry was applied to both sides of a positive electrode current collector made of a long aluminum foil with a thickness of 0.015 mm, and the coating was dried and then rolled using a rolling roller. In this way, a positive electrode was obtained in which a positive electrode mixture layer with a thickness of 0.075 mm was formed on each side of the positive electrode current collector. The obtained positive electrode was passed through a roller heated to 220 ° C., and the positive electrode was heat-treated until the positive electrode surface temperature reached 220 ° C., and then passed through a roller controlled at 20 ° C. to rapidly cool the positive electrode. The cooling rate during rapid cooling was 30 ° C. / sec from the maximum temperature to 40 ° C.

[0042] <Area ratio of binder component with strength of 30 or more> A cross section of the positive electrode composite layer was prepared from the positive electrode of Example 1 by the above-described cross-section polisher (CP) processing. Then, EPMA analysis was performed on the cross section of the positive electrode composite layer under the above-described conditions. Element F was used as the binder component, and the area ratio of the binder component (element F) with strength of 30 or more was calculated to be 30%. The EPMA device used was a JXA8530F manufactured by JEOL Corporation.

[0043] <Stiffness Test> Fig. 2 is a diagram illustrating the method of the stiffness test. The stiffness test described below was performed on the positive electrode of Example 1. The stiffness test is a method for evaluating the flexibility of a test piece by measuring the stress when a test piece rolled into a ring is pressed at a predetermined speed. A lower stiffness value obtained by the stiffness test indicates higher flexibility.

[0044] First, the positive electrode of Example 1 was cut to a width of 55 mm and a length of 80 mm, and the ends were butted together and rolled into a ring to create a test piece 30 with a diameter of 26 mm and an outer circumference of 80 mm. Next, the butted portion of the test piece 30 was fixed on the lower flat plate 32 and sandwiched between the upper flat plate 34 and the lower flat plate 32. The upper flat plate 34 was then moved downward at a speed of 1.6 mm / sec to press the outer circumference of the test piece 30, and the repulsive force of the test piece 30 when the gap L of the test piece 30 reached 11 mm was measured with the load cell 36. In this way, the repulsive forces of three test pieces 30 were measured, and the average value was calculated from the obtained repulsive forces, and this average value was used as the stiffness value. The stiffness value of the positive electrode of Example 1 was 0.13 N.

[0045] [Fabrication of Negative Electrode] Graphite, which is the negative electrode active material, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a solids mass ratio of 98:1:1, and water was used as a dispersion medium to prepare a negative electrode composite slurry. The slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and then rolled using a rolling roller. In this way, a negative electrode was fabricated in which a negative electrode composite layer was formed on both sides of the negative electrode current collector.

[0046] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 20:75:5. LiPF ６ was dissolved in the above solution to a concentration of 1.3 mol / L to prepare a non-aqueous electrolyte.

[0047] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] A positive electrode lead was attached to the positive electrode, and a negative electrode lead was attached to the negative electrode. The positive electrode, negative electrode, and polyethylene separator were then spirally wound around a cylindrical winding core, and the winding core was then removed to obtain a wound electrode assembly. After insulating plates were placed above and below the electrode assembly, the negative electrode lead was welded to the inner bottom of a bottomed cylindrical outer can, and the positive electrode lead was welded to the internal terminal plate of the sealing member, and the electrode assembly was housed in the outer can. A non-aqueous electrolyte was then injected into the outer can under reduced pressure, and the opening of the outer can was sealed with the sealing member via a gasket to obtain a non-aqueous electrolyte secondary battery.

[0048] Example 2 A positive electrode was fabricated in the same manner as in Example 1, except that lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 98.0:1.0:1.0. The resulting positive electrode of Example 2 was subjected to EPMA analysis and stiffness testing in the same manner as in Example 1. As a result, the area ratio of the binder component with a strength of 30 or more was 32%, and the stiffness value was 0.13 N. Then, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode of Example 2.

[0049] Example 3 A positive electrode was fabricated in the same manner as in Example 1, except that lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 98.2:1.0:0.8, a long aluminum foil with a thickness of 0.020 mm was used as the positive electrode current collector, and the thickness of one side of the positive electrode composite layer was 0.070 mm. The resulting positive electrode of Example 3 was subjected to EPMA analysis and stiffness testing in the same manner as in Example 1. The results showed that the area ratio of the binder component with a strength of 30 or more was 31%, and the stiffness value was 0.11 N. A nonaqueous electrolyte secondary battery was then fabricated in the same manner as in Example 1 using the positive electrode of Example 3.

[0050] Example 4 A positive electrode was fabricated in the same manner as in Example 1, except that lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 98.2:1.0:0.8, and the thickness of one side of the positive electrode mixture layer was 0.080 mm. The resulting positive electrode of Example 4 was subjected to EPMA analysis and stiffness testing in the same manner as in Example 1. The results showed that the area ratio of the binder component with a strength of 30 or more was 31%, and the stiffness value was 0.16 N. A nonaqueous electrolyte secondary battery was then fabricated in the same manner as in Example 1, using the positive electrode of Example 4.

[0051] Comparative Example 1 A positive electrode was fabricated in the same manner as in Example 1, except that the heat treatment and rapid cooling were not performed on the positive electrode. EPMA analysis and stiffness testing were performed on the obtained positive electrode of Comparative Example 1 in the same manner as in Example 1. As a result, the area ratio of the binder component with a strength of 30 or more was 10%, and the stiffness value was 0.45 N. Then, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode of Comparative Example 1.

[0052] Comparative Example 2 A positive electrode was fabricated in the same manner as in Example 1, except that a 0.020 mm thick long aluminum foil was used as the positive electrode current collector, the thickness of one side of the positive electrode composite layer was 0.070 mm, and the positive electrode was not subjected to heat treatment or rapid cooling. The resulting positive electrode of Comparative Example 2 was subjected to EPMA analysis and stiffness testing in the same manner as in Example 1. As a result, the area ratio of the binder component with a strength of 30 or more was 10%, and the stiffness value was 0.25 N. Then, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode of Comparative Example 2.

[0053] Comparative Example 3 A positive electrode was fabricated in the same manner as in Example 1, except that the positive electrode was subjected to a heat treatment and then allowed to cool naturally. EPMA analysis and stiffness testing were performed on the obtained positive electrode of Comparative Example 3 in the same manner as in Example 1. As a result, the area ratio of the binder component with a strength of 30 or more was 40%, and the stiffness value was 0.13 N. Then, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode of Comparative Example 3.

[0054] Comparative Example 4 A positive electrode was fabricated in the same manner as in Example 1, except that lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 97.5:1.0:1.5. The obtained positive electrode of Comparative Example 4 was subjected to EPMA analysis and stiffness testing in the same manner as in Example 1. As a result, the area ratio of the binder component with a strength of 30 or more was 42%, and the stiffness value was 0.13 N. Then, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode of Comparative Example 4.

[0055] Comparative Example 5 A positive electrode was fabricated in the same manner as in Example 1, except that lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 98.6:1.0:0.4. EPMA analysis and stiffness testing were performed on the obtained positive electrode of Comparative Example 5 in the same manner as in Example 1. As a result, the area ratio of the binder component with a strength of 30 or more was 15%, and the stiffness value was 0.13 N. Then, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode of Comparative Example 5.

[0056] [Evaluation of Positive Electrode Breakage] The positive electrode was removed from the wound electrode body of each Example and Comparative Example, and visually inspected to see if the positive electrode had broken. If no positive electrode breakage occurred, it was marked "O", and if positive electrode breakage occurred, it was marked "X". The results are summarized in Table 1.

[0057] [Measurement of Initial Battery Resistance] In an environment of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.5 C until the battery reached a 100% state of charge, and then rested for 2 hours. Subsequently, the batteries were discharged at a current of 0.5 C for 30 seconds. The voltage drop 30 seconds after the start of discharge was measured, and the initial battery resistance was calculated by dividing the voltage drop by the discharge current. The initial battery resistance of each Example and other Comparative Examples was expressed relative to the initial battery resistance of Comparative Example 1, where a value less than 100 was marked with "◎", a value between 100 and 104 was marked with "◯", and a value 104 or more was marked with "X". The results are summarized in Table 1.

[0058] [Charge-Discharge Cycle Test] In an environment of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.5 C until the voltage reached 4.2 V, and then charged at a constant voltage of 0.05 C at 4.2 V until the voltage reached 0.05 C. Thereafter, the batteries were discharged at a constant current of 0.5 C until the voltage reached 2.5 V. This cycle of charge and discharge was counted as one cycle, and 500 cycles were performed. The capacity retention rate during the charge-discharge cycles was calculated using the following formula: Capacity retention rate = (discharge capacity at 500th cycle / discharge capacity at 1st cycle) x 100

[0059] Table 1 summarizes the results of stiffness value, positive electrode disconnection, initial battery resistance, and capacity retention rate for each example and comparative example.

[0060]

[0061] As can be seen from the results of each Example and Comparative Example in Table 1, the positive electrodes of Examples 1 to 4, in which the binder content was 0.6% by mass or more but less than 1.5% by mass and the area ratio of the binder component with a strength of 30 or more was more than 10% but less than 40%, had low stiffness values ​​and high flexibility, so even when the positive electrodes were wound to produce an electrode assembly, no positive electrode breakage occurred. Furthermore, the nonaqueous electrolyte secondary batteries using the positive electrodes of Examples 1 to 4 exhibited a more suppressed increase in initial battery resistance than Comparative Examples 3 and 4, which were subjected to heat treatment. Furthermore, the nonaqueous electrolyte secondary batteries using the positive electrodes of Examples 1 to 4 exhibited a high capacity retention rate in a charge-discharge cycle test, and the deterioration of charge-discharge cycle characteristics was also suppressed.

[0062] The content of the binder was in the range of 0.6 mass % or more and less than 1.5 mass %, but the positive electrodes of Comparative Examples 1 and 2, in which the area ratio of the binder component with a strength of 30 or more was below the range of more than 10% and less than 40%, had high stiffness values ​​and low flexibility, and therefore, when the positive electrode was wound to prepare an electrode body, breakage of the positive electrode occurred.

[0063] The content of the binder was within the range of 0.6 mass % or more and less than 1.5 mass %, but the nonaqueous electrolyte secondary battery using the positive electrode of Comparative Example 3, in which the area ratio of the binder component with a strength of 30 or more was greater than 10% and less than 40%, showed an increase in initial battery resistance.

[0064] The nonaqueous electrolyte secondary battery using the positive electrode of Comparative Example 4, in which the binder content exceeded the range of 0.6 mass% or more and less than 1.5 mass%, and further the area ratio of the binder component with an intensity of 30 or more exceeded the range of more than 10% and less than 40%, showed an increase in initial battery resistance. Furthermore, the nonaqueous electrolyte secondary battery using the positive electrode of Comparative Example 4 showed a capacity retention rate of less than 70% in a charge-discharge cycle test, and a significant decrease in charge-discharge cycle characteristics.

[0065] The nonaqueous electrolyte secondary battery using the positive electrode of Comparative Example 5, in which the area ratio of the binder component with a strength of 30 or more was within the range of more than 10% and less than 40%, but the binder content was below the range of 0.6 mass % or more and less than 1.5 mass %, showed a capacity retention rate of 50% in the charge-discharge cycle test, and the deterioration of the charge-discharge cycle characteristics was more significant.

[0066] The present disclosure is further described by the following embodiments. Aspect 1: A secondary battery comprising an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material and a binder, wherein the binder content is 0.6 mass% or more and less than 1.5 mass% relative to the total mass of the positive electrode mixture layer, and wherein in an elemental mapping image of a cross section of the positive electrode mixture layer obtained using an electron probe microanalyzer (EPMA), the proportion of an area where the binder component has an intensity of 30 or more is more than 10% and less than 40%. Aspect 2: The secondary battery according to Aspect 1, wherein the binder content is 0.6 mass% or more and 1 mass% or less relative to the total mass of the positive electrode mixture layer. Aspect 3: The secondary battery according to Aspect 1 or 2, wherein the ratio of the thickness (T2) of the positive electrode mixture layer to the thickness (T1) of the positive electrode current collector is 7 or more. Configuration 4: The secondary battery according to any one of configurations 1 to 3, wherein the binder includes polyvinylidene fluoride (PVDF).

[0067] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding portion, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Test piece, 32 Lower flat plate, 34 Upper flat plate, 36 Load cell.

Claims

1. A secondary battery comprising an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, the positive electrode comprising a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material and a binder, the content of the binder being 0.6 mass% or more and less than 1.5 mass% relative to the total mass of the positive electrode mixture layer, and the proportion of the area where the intensity of the binder component is 30 or more in an elemental mapping image of a cross section of the positive electrode mixture layer obtained using an electron beam microanalyzer (EPMA) is more than 10% and less than 40%.

2. The secondary battery according to claim 1, wherein the content of the binder is 0.6 mass % or more and 1 mass % or less with respect to the total mass of the positive electrode mixture layer.

3. The secondary battery according to claim 1 or 2, wherein the ratio of the thickness (T2) of the positive electrode mixture layer to the thickness (T1) of the positive electrode current collector is 7 or more.

4. The secondary battery according to claim 1 or 2, wherein the binder contains polyvinylidene fluoride (PVDF).

Images