Positive electrode for secondary battery, and secondary battery

Abstract

A positive electrode for a secondary battery according to the present disclosure comprises a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector, wherein: the positive electrode mixture layer contains a positive electrode active material, a conductive auxiliary agent, and a fluorine-containing resin serving as a binding material; the positive electrode mixture layer contains 0.4-1.2 mass% of the binding material; the positive electrode mixture layer has, on a peeled surface obtained by peeling from the positive electrode current collector, a fine powder region configured to have an elliptical long diameter of less than 1 μm by means of a part of the binding material present on the peeled surface, and an aggregation region configured to have an elliptical long diameter of at least 1 μm by means of the remainder of the binding material present on the peeled surface; and when an observation region having a length of at least 200 μm and a width of at least 200 μm is placed on the peeled surface, the total area of the fine powder region accounts for 4.2-5.6% and the total area of the aggregation region accounts for 4.5-6.5%, on the basis of the area of the observation region.

Description

Positive electrode for secondary battery and secondary battery

[0001] The present invention relates to an electrode for a secondary battery and a secondary battery. More particularly, the present invention relates to an electrode for a secondary battery and a secondary battery including the electrode for a secondary battery.

[0002] A secondary battery has been known that includes a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, a non-aqueous electrolyte, and a battery can that accommodates the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte. The positive electrode of the secondary battery (hereinafter also referred to as a secondary battery positive electrode) includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, a binder, and a conductive additive.

[0003] In recent years, there has been an increasing demand for secondary batteries with higher capacity and improved cycle characteristics. For example, Patent Document 1 below describes that, in order to achieve both higher capacity and improved cycle characteristics, a fluororesin (such as polyvinylidene fluoride) having a crystallinity of 15 to 35% is used as the binder in a positive electrode composite layer of a positive electrode for a secondary battery, and a carbonate component having a specific structure is used as the non-aqueous electrolyte.

[0004] Furthermore, Patent Document 2 below describes that in order to suppress peeling of the positive electrode mixture layer from the positive electrode current collector, a fluororesin (such as polyvinylidene fluoride) having a crystallinity of 30 to 40% and a mass average molecular weight of 700,000 to 1,400,000 is used as the binder in the positive electrode mixture layer, and that such a binder is contained in an appropriate range.

[0005] JP 2016-186921 A International Publication No. 2011 / 114626

[0006] To achieve high capacity in a secondary battery, it is important to reduce the content of binder and conductive additive in the positive electrode composite layer and increase the content of positive electrode active material. On the other hand, to improve cycle characteristics in a secondary battery, it is important to reduce the positive electrode composite resistance and improve the adhesion between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector. Therefore, to achieve both high capacity and improved cycle characteristics in a secondary battery, it is necessary to satisfy all three requirements: reducing the content of binder and conductive additive in the positive electrode composite layer, reducing the positive electrode composite resistance, and improving the adhesion between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector (hereinafter simply referred to as adhesion of the positive electrode active material).

[0007] However, it is difficult to say that the previously proposed methods have necessarily achieved sufficient improvements in cycle characteristics. For example, in the method described in Patent Document 1, although the cycle characteristics are improved to a certain extent by adjusting the structural design of the binder (e.g., adjusting the crystallinity), there is a concern that the high content of binder in the positive electrode composite layer may increase the positive electrode composite resistance. Therefore, it is difficult to say that the cycle characteristics have been sufficiently improved. Furthermore, in the method described in Patent Document 2, in addition to adjusting the structural design of the binder (e.g., adjusting the crystallinity and the mass average molecular weight), the content of binder in the positive electrode composite layer is reduced, thereby reducing the positive electrode composite resistance and achieving high capacity. However, due to the reduced content of binder, it is difficult to say that the adhesion of the positive electrode active material is sufficiently ensured. Therefore, it is difficult to say that the cycle characteristics have been sufficiently improved.

[0008] Therefore, an object of the present disclosure is to provide a positive electrode for a secondary battery that achieves reduced composite resistance and high adhesion of a positive electrode active material despite a small binder content, and further that can obtain sufficient cycle characteristics, and a secondary battery including the electrode for a secondary battery.

[0009] One aspect of the present invention relates to a positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode composite layer formed on the positive electrode current collector, the positive electrode composite layer including a positive electrode active material and a fluorine-containing resin as a binder, the binder content being 0.4% by mass or more and 1.2% by mass or less, and the positive electrode composite layer has, on a peeled surface obtained by peeling the positive electrode composite layer from the positive electrode current collector, fine powder regions formed by part of the binder present on the peeled surface and having an ellipse major axis of less than 1 μm, and agglomerated regions formed by remaining parts of the binder present on the peeled surface and having an ellipse major axis of 1 μm or more, when an observation region 50 μm or more in length and 50 μm or more in width is placed on the peeled surface, the total area of ​​the fine powder regions accounts for 4.2% or more and 5.6% or less, and the total area of ​​the agglomerated regions accounts for 4.5% or more and 6.5% or less, based on the area of ​​the observation region.

[0010] Another aspect of the present invention relates to a secondary battery including a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, a non-aqueous electrolyte, and a battery case that accommodates the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte, wherein the positive electrode is the above-described positive electrode for a secondary battery.

[0011] According to the present disclosure, it is possible to provide a positive electrode for a secondary battery that achieves reduced composite resistance and high adhesion of a positive electrode active material despite a small binder content, and further that can obtain sufficient cycle characteristics, and a secondary battery including the electrode for a secondary battery.

[0012] 1 is a cross-sectional view schematically showing a lithium secondary battery according to a first embodiment.

[0013] To solve the above-mentioned problems, the inventors focused on the dispersion state of the binder in the positive electrode mixture layer. Specifically, they focused on optimizing the dispersion state of the binder in the positive electrode mixture layer to minimize the binder content in the positive electrode mixture layer to reduce the positive electrode mixture resistance and maximize the adhesion between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector. Here, if the binder is sufficiently dispersed in the positive electrode mixture layer, the number of contact points between the positive electrode active material and the positive electrode current collector increases, but the volume of each binder aggregate in the dispersed state decreases. In such a case, the binder cannot maintain its shape against elastic deformation. Therefore, sufficient adhesion strength cannot be achieved between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector. On the other hand, if the binder is not sufficiently dispersed in the positive electrode mixture layer, the volume of each binder aggregate in the dispersed state increases. In such a case, although the binder can maintain its shape against elastic deformation, the portion that is not in contact with the positive electrode active material and the positive electrode current collector increases, and therefore it is difficult to say that sufficient adhesion strength is necessarily exhibited between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector.

[0014] Therefore, the present inventors believed that by appropriately controlling the dispersion state of the binder in the positive electrode mixture layer, improving the dispersibility of the binder while maintaining large aggregates, and reducing the portions that are not in contact with the positive electrode active material and the positive electrode current collector, it would be possible to obtain maximum adhesion between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector.

[0015] Below, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that known components may be applied to components characteristic of the present disclosure. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0016] In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties, conditions, etc. are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of them can be selected and used alone, or two or more can be used in combination, unless otherwise specified.

[0017] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0018] [Positive Electrode for Secondary Battery] A positive electrode for secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material and a fluorine-containing resin as a binder. The positive electrode mixture layer contains 0.4 mass % to 1.2 mass % of the binder.

[0019] The positive electrode current collector preferably has a strip shape (long shape) in a planar view. As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of materials for the positive electrode current collector include metal materials such as Al, Al alloys, Ti, Ti alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm, and even more preferably 10 to 20 μm.

[0020] When the positive electrode current collector has an elongated shape in a planar view and a thickness of 10 to 20 μm, the positive electrode for a secondary battery preferably has an elongation percentage in the longitudinal direction of the elongated shape of 2.5% or more, more preferably 2.8% or more, and even more preferably 3.0% or more. When the positive electrode for a secondary battery has the above-described elongation percentage, the positive electrode current collector can be appropriately elongated in the longitudinal direction when a laminate of the positive electrode current collector and the positive electrode composite layer is rolled to obtain a positive electrode for a secondary battery. This improves the adhesion of the positive electrode composite layer to the positive electrode current collector. The upper limit of the elongation percentage of such a positive electrode for a secondary battery may be 10% or less, 5.0% or less, or 4.0% or less. In the positive electrode for a secondary battery, the elongation percentage in the longitudinal direction of the elongated shape can be adjusted by changing the thickness of the positive electrode current collector and the positive electrode composite layer. Specifically, the elongation rate can be reduced by increasing the thickness of the positive electrode current collector and the positive electrode composite layer, and the elongation rate can be increased by decreasing the thickness of the positive electrode current collector and the positive electrode composite layer.

[0021] The elongation percentage here means the rate of change in the dimension of a long rectangular test piece in the longitudinal direction. More specifically, the elongation percentage is the length of the longitudinal dimension at which the test piece breaks when the test piece is pulled in the longitudinal direction with a constant force at 25°C. Ｂ The initial dimension of the test piece in the longitudinal direction (the dimension before applying force) is L Ｉ When this is done, L Ｉ L based on Ｂ means the rate of change.

[0022] The elongation of the secondary battery electrode can be measured by a tensile test. A long rectangular secondary battery positive electrode (12.5 mm x 30 mm) with a longitudinal dimension of 30 mm and a transverse dimension (width direction) of 15 mm is prepared as a test specimen. The test specimen has a rating distance of 25 mm. An Instron universal testing machine (Model 4505) is used as the tensile tester. The tensile speed is 0.5 mm / min, and the elongation is determined by the change in rating distance.

[0023] The positive electrode mixture layer may be formed on both main surfaces of the positive electrode current collector, or may be formed on only one main surface. When the positive electrode current collector is a porous conductive substrate as described above, the positive electrode mixture layer may be formed in a state where at least a portion of the positive electrode mixture layer is embedded in the pores of the porous substrate.

[0024] The positive electrode active material is a material that electrochemically absorbs and releases lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Representative examples of lithium-containing transition metal oxides include lithium cobalt oxide and lithium nickel oxide, which have a layered crystal structure and are classified as rock salt type.

[0025] The positive electrode active material may be, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn. Examples of such composite oxides include LiCoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO ４ Examples of the metals include F. In the above composite oxide, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. In the above composite oxide, a, b, and c satisfy 0<a≦1.2, 0<b≦0.9, and 2.0≦c≦2.3. The value of a, which represents the molar ratio of lithium, increases or decreases with charge and discharge.

[0026] As the positive electrode active material, it is preferable to use a lithium nickel composite oxide. The lithium nickel composite oxide is, for example, represented by the formula (1): Li ａ Ni ｂ M １－ｂ O ２ In formula (1), M is at least one selected from the group consisting of Mn, Co, and Al, and a and b satisfy 0<a≦1.2 and 0.3≦b<1, respectively. From the viewpoint of increasing capacity, b preferably satisfies 0.85≦b<1. From the viewpoint of stabilizing the crystal structure, the lithium nickel composite oxide contains Co and Al as M and can be represented by formula (2): Li ａ Ni ｂ Co ｃ Al ｄ O ２ In formula (2), a, b, c, and d satisfy the following conditions: 0<a≦1.2, 0.85≦b<1, 0<c<0.15, 0<d≦0.1, and b+c+d=1.

[0027] The positive electrode active material preferably has an average particle diameter of 15 μm or less, more preferably 12 μm or less. The positive electrode active material preferably has an average particle diameter of 5 μm or more, more preferably 10 μm or more. Because the positive electrode active material has the above average particle diameter, even if the binder contained in the positive electrode mixture layer is as small as 0.4 mass% or more and 1.2 mass% or less, the binder can sufficiently bind the positive electrode active material together. This allows the positive electrode active material to be stably held in the positive electrode mixture layer. As a result, the secondary battery can exhibit good cycle characteristics. Furthermore, the positive electrode active material having an average particle diameter of 15 μm or less is preferably used in combination with a binder containing a fluorine-containing resin obtained by emulsion polymerization. As a result, even if the amount of binder contained in the positive electrode composite layer is small, at 0.4 mass % or more and 1.2 mass % or less, it is possible to further reduce the composite resistance, and to further achieve high adhesion between the positive electrode active materials themselves and between the positive electrode active material and the positive electrode current collector, and it is also possible to obtain more satisfactory cycle characteristics.

[0028] The average particle size of the positive electrode active material is the cumulative 50% particle size (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the positive electrode active material is incorporated into the positive electrode composite layer.

[0029] The average particle diameter of the positive electrode active material may be measured from a cross section obtained by cutting the laminate of the positive electrode composite layer and the positive electrode current collector in the thickness direction. The cross section may be formed using a cross-section polisher (CP). In this case, the positive electrode composite layer may be embedded in a thermosetting resin (e.g., epoxy resin). The average particle diameter from the cross section can be measured using a scanning electron microscope (SEM) image of the cross section. An SEM image can be used in which 10 or more positive electrode active material particles are observed. Then, image processing is performed to determine the circular equivalent diameters of the cross sections of 10 or more positive electrode active material particles, and the average value is calculated as the average particle diameter. Here, the circular equivalent diameter refers to the diameter of a circle having the same area as the cross section of the positive electrode active material particle (the area of ​​the positive electrode active material particle observed in the cross section of the positive electrode composite layer). Note that the average particle diameter of the positive electrode active material measured using a particle size distribution measurement device is equivalent to the average particle diameter determined from the cross section.

[0030] The positive electrode active material preferably has an oil absorption of 12 mL / 100 g or more and 20 mL / 100 g or less. Here, oil absorption refers to the amount of oil that penetrates into the voids present in the positive electrode active material. Therefore, the larger the oil absorption value, the greater the amount of solvent (specifically, the solvent contained in the non-aqueous electrolyte) that is incorporated into the positive electrode active material, and accordingly, the less solvent is present around the binder. As a result, the mechanical strength of the binder increases. On the other hand, the smaller the oil absorption value, the less solvent is incorporated into the positive electrode active material, and accordingly, the more solvent is present around the binder. As a result, the mechanical strength of the binder decreases. As described below, the positive electrode active material is kneaded with a conductive additive, a binder, and the like to form a positive electrode mixture slurry. Therefore, by setting the oil absorption of the positive electrode active material to 20 mL / 100 g or less, excessive crushing of the binder can be suppressed when kneading the binder with the positive electrode active material, conductive additive, etc. to obtain a positive electrode mixture slurry. Furthermore, by setting the oil absorption to 12 mL / 100 g or more, excessive aggregation of the binder in the positive electrode mixture slurry can be suppressed. As a result, even if the binder content in the positive electrode mixture layer is as small as 0.4 mass % or more and 1.2 mass % or less, a binder dispersion state that ensures sufficient adhesion can be obtained.

[0031] The oil absorption can be determined as DBP oil absorption in accordance with JIS K 6217-4 (method for determining oil absorption).

[0032] Examples of fluorine-containing resins used as binders include polytetrafluoroethylene and polyvinylidene fluoride (PVDF). The binder preferably has an average particle diameter of 10 μm or more, more preferably 20 μm or more. The binder preferably has an average particle diameter of 150 μm or less, more preferably 120 μm or less. By having the average particle diameter of the binder within the above range, even if the amount of binder contained in the positive electrode mixture layer is as small as 0.4 mass% or more and 1.2 mass% or less, a sufficient contact area between the binder and the positive electrode active material in the positive electrode mixture layer can be ensured. This allows the binder to sufficiently bind the positive electrode active materials together in the positive electrode mixture layer.

[0033] Like the average particle diameter of the positive electrode active material, the average particle diameter of the binder is the cumulative 50% particle diameter (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the binder is incorporated into the positive electrode composite layer.

[0034] The binder preferably has a mass average molecular weight Mw of 300,000 or more, more preferably a mass average molecular weight Mw of 500,000 or more, and even more preferably a mass average molecular weight Mw of 1,000,000 or more. Furthermore, the binder preferably has a mass average molecular weight Mw of 1,500,000 or less, and even more preferably a mass average molecular weight Mw of 1,200,000 or less. When the binder has a mass average molecular weight Mw within the above range, the positive electrode mixture slurry can have an appropriate viscosity when kneaded with the binder together with the positive electrode active material, conductive additive, etc. This facilitates coating the positive electrode mixture slurry onto the positive electrode current collector. Furthermore, since the positive electrode active material can be appropriately dispersed in the positive electrode mixture slurry, when the positive electrode mixture slurry is formed into a positive electrode mixture layer, the binder can sufficiently bind the positive electrode active materials together in the positive electrode mixture layer. This allows the positive electrode active material to be stably held in the positive electrode mixture layer.

[0035] The weight average molecular weight Mw of the binder can be measured, for example, by ultra-high temperature gel permeation chromatography (GPC), where Mw is the weight average molecular weight based on polystyrene.

[0036] Ultra-high temperature GPC measurements can be performed using, for example, an Ultra-High Temperature GPC SSC-7110 (manufactured by Senshu Scientific Co., Ltd.) as the measuring device, one TSKguard column HHR(S) (manufactured by Tosoh Corporation) and two TSKgel GMHHR-H(S)HT (7.8 mm I.D. x 30 cm) (manufactured by Tosoh Corporation) as the columns, and a differential refractometer (RI detector) as the detector. A solution with a sample (binder) concentration of 2 mg / mL is used as the measurement sample. The sample injection volume is 500 μL, the flow rate is 1.0 mL / min, the pre-oven temperature is set to 250 ° C, and the column temperature is set to 210 ° C. Polystyrene is also used as the molecular weight standard. The measurement sample can be obtained by dissolving a predetermined amount of binder in 1-chloronaphthalene (1-CN) by heating at 250°C for 1 hour to obtain a solution, then filtering the solution with heat using a PTFE filter with a pore size of 0.5 µm to obtain a filtrate, and adjusting the concentration of the sample (binder) in the filtrate to 2 mg / mL.

[0037] The binder can be obtained by various known polymerization methods, such as suspension polymerization, emulsion polymerization, solution polymerization, and bulk polymerization. From the viewpoints of easy adjustment of the polymerization temperature and easy production of a polymer with a high degree of polymerization, the polymerization method is preferably suspension polymerization or emulsion polymerization. That is, the binder is preferably a copolymer obtained by suspension polymerization or emulsion polymerization. Furthermore, when the binder is a copolymer with a high branching structure, the entanglement between the binders is strong, making it easier to maintain the shape (shape of the binder) against elastic deformation. This allows for strong adhesion between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector. Here, since emulsion polymerization easily produces a copolymer with a high branching structure, when a copolymer with a high branching structure is used as the binder as described above, the binder is preferably a copolymer obtained by emulsion polymerization.

[0038] The positive electrode mixture layer may contain a conductive additive in addition to the positive electrode active material and the binder. For example, a conductive carbonaceous material can be used as the conductive additive. Examples of the conductive carbonaceous material include carbon black, carbon nanotubes, and graphite. Examples of carbon black include acetylene black and ketjen black. The conductive additive may be used alone or in combination of two or more.

[0039] In a positive electrode for a secondary battery according to an embodiment of the present disclosure, the positive electrode mixture layer has, on a peeled surface obtained by peeling it from the positive electrode current collector, a fine powder region constituted by a portion of the binder present on the peeled surface and having an elliptical major axis of less than 1 μm, and an aggregate region constituted by the remaining portion of the binder present on the peeled surface and having an elliptical major axis of 1 μm or more. That is, the positive electrode mixture layer has, on the peeled surface, two groups of regions constituted by the binder and having different size ranges.

[0040] In an embodiment of the present disclosure, when an observation area of ​​50 μm or more in length and width is placed on the peeled surface, it is important that the total area of ​​the fine powder regions accounts for 4.2% to 5.6% and the total area of ​​the agglomeration regions accounts for 4.5% to 6.5% based on the area of ​​the observation area. That is, it is important that two groups of regions of different size ranges, each composed of the binder, are present on the peeled surface in a balanced manner with an appropriate area ratio. This allows for improved bonding of the positive electrode mixture layer to the positive electrode current collector, even when the binder content in the positive electrode mixture layer is as low as 0.4% to 1.2% by mass. The area ratio of the fine powder regions (the ratio of the total area of ​​the fine powder regions) and the area ratio of the agglomeration regions (the ratio of the total area of ​​the agglomeration regions) can be adjusted by heat-treating the positive electrode mixture layer or by adjusting the oil absorption of the positive electrode active material to a preferred range. Heat-treating the positive electrode mixture layer changes the area ratio of the fine powder regions and the area ratio of the agglomeration regions to a preferred ratio. Specifically, when there are many agglomerated regions where the binder is largely aggregated, the binder contained in the agglomerated regions can be dissolved by heating to produce fine powder regions. Also, when there are many fine powder regions, the binder contained in the fine powder regions can be dissolved by heating, and then adjacent binders can be aggregated to produce aggregate regions.

[0041] Furthermore, by increasing the oil absorption of the positive electrode active material, the amount of solvent (specifically, the solvent contained in the non-aqueous electrolyte) incorporated into the positive electrode active material in the positive electrode mixture slurry can be increased. Accordingly, the amount of solvent present around the binder can be reduced, thereby increasing the mechanical strength of the binder. As a result, the amount of binder in the form of coarse agglomerates in the positive electrode mixture slurry can be reduced, and the amount of binder in the form of fine powder can be increased. On the other hand, by decreasing the oil absorption of the positive electrode active material, the amount of solvent incorporated into the positive electrode active material in the positive electrode mixture slurry can be reduced. Accordingly, the amount of solvent present around the binder can be increased, thereby decreasing the mechanical strength of the binder. As a result, the amount of binder in the form of coarse agglomerates in the positive electrode mixture slurry can be increased, and the amount of binder in the form of fine powder can be reduced. In this way, by adjusting the ratio of the binder in the form of agglomerates to the binder in a fine powder state in the positive electrode composite slurry, the area ratio of the fine powder region and the area ratio of the aggregated region can also be adjusted.

[0042] It is more preferable that the total area of ​​the fine powder regions be 4.4% to 5.0% and the total area of ​​the agglomeration regions be 4.9% to 6.0% based on the area of ​​the observation region. By having the fine powder regions and agglomeration regions occupy such area ratios, two groups of regions with different size ranges, each composed of the binder, can be present in a more appropriate ratio on the peeling surface. This can further enhance the bonding strength of the positive electrode mixture layer to the positive electrode current collector. That is, even if the binder content in the positive electrode mixture layer is as small as 0.4% by mass to 1.2% by mass, peeling of the positive electrode mixture layer from the positive electrode current collector can be more sufficiently suppressed.

[0043] The area ratio of the fine powder regions (the ratio of the total area of ​​the fine powder regions) and the area ratio of the agglomerated regions (the ratio of the total area of ​​the agglomerated regions) can be determined by analyzing the peeled surface using SEM-EDX (scanning electron microscope-energy dispersive X-ray analysis). Specifically, first, the peeled surface is observed using an SEM at a magnification of, for example, 100 to 500 times. The peeled surface can be obtained by attaching a single-sided adhesive tape (31B manufactured by Nitto Denko Corporation) to one surface (exposed surface) of the positive electrode composite layer, and then peeling the other surface (the surface that contacts the positive electrode current collector) of the positive electrode composite layer from the positive electrode current collector. Then, an arbitrary region (a rectangular region with a length of 50 μm or more and a width of 50 μm or more) along the peeled surface is imaged to obtain an SEM image. Next, elemental analysis by EDX analysis is performed using the captured SEM image. Note that the arbitrary region may be large enough to observe at least 10 fine powder regions and at least 10 agglomerated regions. A binder-derived component (fluorine) is extracted from the EDX SEM image analysis data to obtain a binder mapping image for any one region. Next, the mapping image is analyzed using image analysis software (ImageJ / developed by the National Institutes of Health (NIH)). The fine powder region, which is formed by a portion of the binder present on the release surface and has an elliptical major axis of less than 1 μm, and the aggregated region, which is formed by the remaining portion of the binder present on the release surface and has an elliptical major axis of 1 μm or more, are identified, and the area of ​​each region is calculated. After calculating the area of ​​each of the six non-overlapping regions on the release surface, the sum of the areas of the fine regions (S1) and the aggregated region (S2) are calculated. The total area (Sa) of the six mapping images (six images) is then calculated. The area ratio of the fine powder region (S1 / Sa×100) and the area ratio of the aggregated region (S2 / Sa×100) are then calculated.

[0044] Desirable measurement conditions for SEM-EDX analysis are shown below. Measuring device (SEM): Hitachi SU8220 (field emission scanning electron microscope) Measuring device (EDX): BRUKER XFlash5060FQ / XFlash6 Combine System Acceleration voltage: 5 kV Emission: 25 μm Probe current: High Condenser lens: 1.0 Note that the mapping image of the binder is obtained by processing the analysis data of the SEM image by EDX so that the binder appears red and everything other than the binder appears black. Furthermore, in order to clearly separate the colors of the mapping image, measurement may be performed in deconvolution mode.

[0045] The following is a recommended analysis procedure using image analysis software (ImageJ). (1) Open the SEM image using ImageJ. At this time, select Set Scale and align the pixel scale obtained in the SEM image with the actual length. (2) Convert the SEM image to 8 bits. (3) Run Sharpen twice on the SEM image to emphasize the image contours. (4) Binarize using Threshold's Dark Background mode Auto. (5) Analyze the particle size of the binder using Analyze Particles. At this time, select Ellipses in the show tab and perform elliptical approximation. (6) Areas with a Major of less than 1 μm are considered to be the fine powder region, and areas with a Major of 1 μm or more are considered to be the agglomerated region, and the sum of the respective Areas is calculated. (7) (1) to (6) are performed on six SEM images, and the total of the Area of ​​the fine powder region and the Area of ​​the agglomeration region is divided by the area of ​​the six SEM images, and the result is multiplied by 100 to calculate the area ratio of the differential region and the area ratio of the agglomeration region.

[0046] The positive electrode can be obtained, for example, by applying a slurry containing the components of the positive electrode mixture layer and a dispersion medium onto a positive electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the positive electrode mixture layer include a positive electrode active material, a binder, and a conductive additive.

[0047] [Secondary Battery] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, a non-aqueous electrolyte, and a battery case that accommodates the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte.

[0048] In the secondary battery according to the embodiment of the present disclosure, the positive electrode for secondary batteries according to the embodiment of the present disclosure described above is used as the positive electrode.

[0049] (Separator) A porous sheet having ion permeability and insulating properties is used for the separator. Examples of the form of the porous sheet include a microporous film, a woven fabric, and a nonwoven fabric. The separator may be made of a polymer material. Examples of the polymer material include an olefin resin, a polyamide resin, and cellulose. Examples of the olefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The separator may contain an additive as needed. Examples of the additive include an inorganic filler.

[0050] The separator may include multiple layers differing in at least one of form and composition, such as a laminate of a microporous polyethylene film and a microporous polypropylene film, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers.

[0051] (Negative Electrode) The negative electrode includes at least a negative electrode current collector. The negative electrode may include a negative electrode mixture layer, lithium foil, lithium alloy foil, etc., on the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material and a binder.

[0052] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials for the negative electrode current collector include metal materials such as Ni, Ni alloys, Cu, Cu alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0053] The negative electrode mixture layer may be formed on both main surfaces of the negative electrode current collector, or may be formed on only one main surface. When the negative electrode current collector is a porous conductive substrate as described above, the negative electrode mixture layer may be formed in a state where at least a portion of the layer is embedded in the pores of the porous conductive substrate.

[0054] The negative electrode active material is a material that electrochemically absorbs and releases lithium ions, such as an alloy material or a carbon material.

[0055] Examples of alloy materials include silicon (Si)-containing materials, tin (Sn)-containing materials, etc. From the viewpoint of achieving high capacity, it is preferable to use a silicon (Si)-containing material as the alloy material.

[0056] The silicon-containing material includes a silicon phase. Silicon can reversibly form an alloy with lithium. Therefore, the silicon-containing material can reversibly absorb and release lithium ions. The silicon-containing material may be a composite material including a silicon phase and a matrix phase in which the silicon phase is dispersed. The matrix phase may be composed of a material having lithium ion conductivity. The matrix phase may include, for example, at least one selected from the group consisting of a silicon oxide phase and a carbon phase.

[0057] The silicon oxide phase may contain, in addition to Si and O, elements other than Si and O. The silicon oxide phase may contain silicon dioxide (SiO ２ ) phase, or may be constituted as a lithium silicate phase, or may be constituted as both of these phases.

[0058] When the silicon-containing material is the above-mentioned composite material, the composite material may be composed of any one of the following (a) to (c):

[0059] (a) a silicon phase and silicon dioxide (SiO ２(b) a structure including a silicon phase and a lithium silicate phase in which the silicon phase is dispersed (second composite material); and (c) a structure including a silicon phase and a carbon phase in which the silicon phase is dispersed (third composite material).

[0060] When the silicon-containing material is the first composite material (case (a) above), there is an advantage that the volume change accompanying the absorption and desorption of lithium ions is small. It is presumed that one of the reasons for this advantage is that the silicon dioxide phase has a relatively large number of sites that irreversibly trap lithium ions, making it difficult for the first composite material to shrink in volume accompanying the desorption of lithium ions.

[0061] The first composite material can be synthesized, for example, by heating silicon oxide, which is a raw material, in a non-oxidizing atmosphere (inert atmosphere) to cause a disproportionation reaction.

[0062] When the silicon-containing material is the second composite material (case (b) above), the advantage of being able to reduce the irreversible capacity is obtained. Therefore, when the second composite material is used as the silicon-containing material, excellent charge / discharge efficiency can be obtained. This effect is particularly noticeable in the early stages of charge / discharge.

[0063] The lithium silicate phase contained in the second composite material may contain elements other than Si, O, and Li. Such elements may be at least one selected from the group consisting of Group 1 elements (other than Li) and Group 2 elements of the long periodic table. The Group 1 elements and Group 2 elements may be, for example, K, Na, Mg, Ca, Sr, Ba, etc. The lithium silicate phase may also contain Al, B, La, P, Zr, Ti, Fe, Cr, Ni, Mn, Cu, Mo, Zn, etc.

[0064] The ratio of the number of O atoms to the number of Si atoms in the lithium silicate phase (O / Si) is, for example, greater than 2 and less than 4. In this case, in addition to being advantageous in terms of the stability of the lithium silicate phase, it is also advantageous in terms of lithium ion conductivity. The O / Si ratio may be greater than 2 and less than 3. The ratio of the number of Li atoms to the number of Si atoms in the lithium silicate phase (Li / Si) is, for example, greater than 0 and less than 4.

[0065] The lithium silicate, which is the raw material for obtaining the lithium silicate phase, has the formula Li ２ｚ SiO ２＋ｚ (0<z<2). It is preferable that z satisfies 0<z<1. When z is in this range, the stability of the lithium silicate is increased and the lithium silicate is easily produced. Furthermore, when the lithium silicate is made into a lithium silicate phase, the lithium ion conductivity can be increased. It is more preferable that z is 1 / 2.

[0066] The second composite material can be obtained, for example, by mixing and stirring the raw materials, lithium silicate and silicon, while crushing them in a mixer such as a ball mill to obtain a mixture, and then firing the mixture under pressure in an inert atmosphere. Note that the second composite material may also be obtained by heating the mixture to a predetermined temperature, necking at least one of the lithium silicate and silicon in the mixture to obtain a sintered body, and then pulverizing the sintered body.

[0067] Even when the silicon-containing material is the third composite material (case (c) above), the advantage of being able to reduce the irreversible capacity is obtained. In addition, the carbon phase exhibits capacity through a Faraday reaction with lithium ions, which is advantageous in realizing a high capacity.

[0068] The carbon phase may contain crystalline carbon (graphite) or amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, non-graphitizable carbon, easily graphitizable carbon, or other.

[0069] The third composite material can be obtained in the same manner as the second composite material, except that a carbon source and silicon are used as raw materials.

[0070] Examples of carbon sources that can be used include sugars, water-soluble resins, etc. Examples of carbon sources that can be used include carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, sucrose, etc. The mixture may be obtained by dispersing the carbon source and silicon in an organic solvent such as alcohol.

[0071] The negative electrode composite layer may contain a carbon material instead of an alloy material, or may contain a carbon material in addition to an alloy material. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Among the above carbon materials, graphite is preferred because it has excellent charge / discharge stability and can reduce irreversible capacity.

[0072] Graphite is a carbon material having a (002) plane spacing d002 of, for example, 0.340 nm or less as measured by X-ray diffraction. The crystallite size Lc(002) of graphite as measured by X-ray diffraction may be, for example, 5 nm or more, 5 nm or more to 300 nm or less, or 10 nm or more to 200 nm or less.

[0073] When graphite and a silicon-containing material are used in combination, the proportion of the silicon-containing material in the negative electrode active material (the total of graphite and the silicon-containing material) is, for example, 1% by mass or more and 20% by mass or less. The proportion may be 3% by mass or more and 15% by mass or less, or 3% by mass or more and 10% by mass or less. By having the proportion in the above range, it is possible to achieve a good balance between improved cycle characteristics and high capacity.

[0074] In the negative electrode mixture layer, materials other than the carbon material may be used in combination with the alloy material, such as spinel-type lithium titanium oxide and spinel-type lithium manganese oxide.

[0075] Examples of binders include resin materials. Examples of resin materials include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl acetate; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0076] The negative electrode mixture layer may contain a conductive additive in addition to the negative electrode active material and binder. As described for the positive electrode, a conductive carbonaceous material can be used as the conductive additive. Furthermore, in the negative electrode, in addition to the conductive carbonaceous material, metal fibers, metal powder such as aluminum, and the like can also be used. The conductive additive may be used alone or in combination of two or more.

[0077] The negative electrode mixture layer may contain a thickener as needed. Examples of thickeners that can be used include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC), its modified forms, and methyl cellulose. Examples of modified forms of CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0078] The negative electrode can be obtained, for example, by applying a slurry containing the components of the negative electrode mixture layer and a dispersion medium onto a negative electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the negative electrode mixture layer include a negative electrode active material, a binder, a conductive additive, and a thickener.

[0079] As described above, when a lithium foil or lithium alloy foil is provided on the negative electrode current collector, the thickness of the foil may be, for example, in the range of 5 μm to 25 μm. The lithium alloy foil may contain elements such as aluminum, magnesium, indium, and zinc in addition to lithium. By providing the negative electrode with a lithium foil or lithium alloy foil, lithium metal can be precipitated on the foil during charging. This effectively prevents dendrites (needle-shaped metallic lithium) from being precipitated on the negative electrode.

[0080] (Non-aqueous electrolyte) The non-aqueous electrolyte (non-aqueous electrolyte solution) contains a solvent (non-aqueous solvent) and a solute dissolved in the solvent. Examples of the solute include lithium salt. Various additives may be added to the non-aqueous electrolyte.

[0081] As the solvent, various known organic solvents can be used, such as cyclic carbonate esters, chain carbonate esters, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, cyclic ethers, fluorinated chain ethers, and fluorinated cyclic ethers.

[0082] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0083] Examples of the chain carbonate ester include diethylene carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0084] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0085] Examples of the chain carboxylic acid ester include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0086] Examples of chain ethers include dimethyl ether, ethyl methyl ether, diethyl ether, ethyl propyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, and o-dimethoxybenzene. The chain ether may be a chain ether having two or more ether bonds. Examples of such chain ethers include 1,1-dimethoxymethane, 1,1-diethoxyethane, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol ethyl methyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, and tetraethylene glycol ethyl methyl ether.

[0087] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0088] The fluorinated chain ether has a structure in which one or more hydrogen atoms of the chain ethers described above are substituted with fluorine atoms. Examples of the fluorinated chain ether include bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0089] Fluorinated cyclic ethers are those in which one or more hydrogen atoms of the above-mentioned cyclic ethers have been substituted with fluorine atoms. Examples of fluorinated cyclic ethers include 3,3,4,4-tetrafluorotetrahydrofuran.

[0090] The above-mentioned various solvents (non-aqueous solvents) may be used alone or in combination of two or more.

[0091] Examples of the lithium salt include lithium salts of chlorine-containing acids, lithium salts of fluorine-containing acids, lithium salts of fluorine-containing acid imides, lithium halides, and lithium salts containing oxalate complexes. Examples of the lithium salts of chlorine-containing acids include LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ Examples of lithium salts of fluorine-containing acids include LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ Examples of lithium salts of fluorine-containing acid imides include LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (FSO ２ ), LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ Examples of lithium halides include LiCl, LiBr, and LiI. Examples of lithium salts containing oxalate complexes include LiB(C ２ O ４ ) ２, LiBF ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), LIPF ２ (C ２ O ４ ) ２ The above lithium salts may be used alone or in combination of two or more.

[0092] The concentration of the lithium salt in the nonaqueous electrolyte (nonaqueous electrolytic solution) may be 1 mol / L or more and 5 mol / L or less, or 1 mol / L or more and 3 mol / L or less. By setting the lithium salt concentration within the above range, a nonaqueous electrolyte (nonaqueous electrolytic solution) having excellent ionic conductivity and appropriate viscosity can be obtained.

[0093] The nonaqueous electrolyte (nonaqueous electrolyte solution) may contain various known additives. Examples of such additives include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, and ethylene sulfite (ES). Cyclic carbonate esters such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), which are exemplified as solvents, may also function as additives. When the nonaqueous electrolyte contains such additives, if the negative electrode composite layer contains graphite, the additives can form a high-quality SEI (Solid Electrolyte Interface) coating on the surface of the graphite. This can reduce the resistance of the secondary battery. Furthermore, the durability of the SEI against the expansion and contraction of graphite during charge-discharge cycles can be improved.

[0094] A specific configuration of a secondary battery according to an embodiment of the present disclosure will be described below with reference to Fig. 1. Note that, hereinafter, the secondary battery according to an embodiment of the present disclosure will be simply referred to as a secondary battery according to a first embodiment.

[0095] FIG. 1 is a longitudinal cross-sectional view schematically illustrating a secondary battery (lithium secondary battery 10) according to a first embodiment. The lithium secondary battery 10 is a cylindrical battery. The lithium-ion battery 10 includes a cylindrical battery case (battery can), a wound electrode group 14 housed within the battery case, and a nonaqueous electrolyte (not shown). The battery case includes a cylindrical case body 15 with a bottom and a sealing body 16 that seals the opening of the case body 15. The case body 15 is made of metal. A gasket 27 is disposed between the case body 15 and the sealing body 16. The gasket 27 ensures the hermeticity of the battery case. The case body 15, the sealing body 16, and the gasket 27 form an exterior body. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the direction of the winding axis.

[0096] The case body 15 has a step portion 21. The step portion 21 supports the sealing body 16. The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. The above-mentioned components constituting the sealing body 16 are electrically connected to each other except for the insulating member 24. The cap 26 functions as a positive electrode terminal. The case body 15 functions as a negative electrode terminal.

[0097] The electrode group 14 is a wound electrode group composed of a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The lithium secondary battery 10 according to the first embodiment includes the positive electrode according to the embodiment of the present disclosure described above as the positive electrode 11. The negative electrode 12, the separator 13, and the non-aqueous electrolyte can be configured as described above.

[0098] The positive electrode 11 is electrically connected to the cap 26 via a positive electrode lead 19. One end of the positive electrode lead 19 is connected to the positive electrode 11. The other end of the positive electrode lead 19 is connected to the sealing body 16 (filter 22). The negative electrode 12 is electrically connected to the case body 15 via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to the negative electrode 12. The other end of the negative electrode lead 20 is connected to the case body 15.

[0099] In the above example, a secondary battery configured by housing a wound electrode group in a cylindrical battery case (battery can) has been described, but the configuration of the secondary battery is not limited to this. The secondary battery may also be configured by housing a wound electrode group in a rectangular battery case (battery can). Furthermore, the secondary battery may be configured by housing a stacked electrode group in a battery case such as a film exterior (e.g., a pouch).

[0100] (Additional Note) The above description discloses the following techniques. (Technology 1) A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode composite layer formed on the positive electrode current collector, wherein the positive electrode composite layer includes a positive electrode active material and a fluorine-containing resin as a binder, and the positive electrode composite layer contains 0.4 mass % to 1.2 mass % of the binder, and the positive electrode composite layer has, on a peeled surface obtained by peeling the positive electrode composite layer from the positive electrode current collector, fine powder regions formed by part of the binder present on the peeled surface so as to have an elliptical major axis of less than 1 μm, and agglomerated regions formed by the remainder of the binder present on the peeled surface so as to have an elliptical major axis of 1 μm or more, and when an observation region 50 μm or more in length and width is placed on the peeled surface, the total area of ​​the fine powder regions accounts for 4.2% to 5.6% and the total area of ​​the agglomerated regions accounts for 4.5% to 6.5% based on the area of ​​the observation region. (Technology 2) The positive electrode for a secondary battery according to Technology 1, wherein the positive electrode current collector has a thickness of 10 μm or more and 20 μm or less and has an elongated shape in a plan view, and wherein an elongation rate in the longitudinal direction of the elongated shape is 2.5% or more. (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, wherein the positive electrode active material has an oil absorption of 12 mL / 100 g or more and 20 mL / 100 g or less. (Technology 4) The positive electrode for a secondary battery according to any one of Technology 1 to 3, wherein the positive electrode active material has an average particle diameter of 15 μm or less. (Technology 5) The positive electrode for a secondary battery according to any one of Technology 1 to 4, wherein the binder has a mass average molecular weight of 300,000 or more and 1,500,000 or less. (Technology 6) The positive electrode for a secondary battery according to any one of Technology 1 to 5, wherein the binder has an average particle diameter of 10 μm or more and 150 μm or less. (Technology 7) The positive electrode for a secondary battery according to any one of Technologies 1 to 6, wherein the binder contains a fluorine-containing resin obtained by emulsion polymerization. (Technology 8) A secondary battery comprising: a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, a non-aqueous electrolyte, and a battery can accommodating the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte, wherein the positive electrode is the positive electrode for a secondary battery according to any one of Technologies 1 to 7.

[0101] While the present invention has been described with respect to presently preferred embodiments, such disclosure should not be interpreted as limiting. Various variations and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all variations and modifications that do not depart from the true spirit and scope of the invention.

[0102] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0103] [Example 1] (1) Preparation of Positive Electrode A positive electrode active material (active material A), a conductive additive (conductive additive A), and a binder (binder A) were mixed in a mass ratio of 98:1.4:0.6 to obtain a positive electrode mixture. A suitable amount of N-methyl-2-pyrrolidone as a dispersion medium was added to the positive electrode mixture and stirred to obtain a positive electrode mixture slurry. As the active material A, a lithium nickel composite oxide (Li １．０５ Ni ０．８０ Co ０．１５ Al ０．０５ O ２ ) was used. Carbon black was used as the conductive additive A. Polyvinylidene fluoride (PVDF) having the average particle size and mass average molecular weight Mw shown in Table 1 below was used as the binder A. In Example 1, the binder A was not heat-treated. The binder A was PVDF obtained by suspension polymerization. Furthermore, the average particle size and oil absorption of the active material A, as well as the average particle size and mass average molecular weight Mw of the binder A, were measured by the methods described in the embodiment section above.

[0104] The positive electrode composite slurry according to Example 1 was applied to both sides of an aluminum foil (thickness: 15 μm) serving as a positive electrode current collector to form a coating film. The coating film was then dried, and the dried coating film was compressed in the thickness direction using a roller to form a positive electrode composite layer on the positive electrode current collector. The laminate of the positive electrode current collector and the positive electrode composite layer was then cut to a predetermined size. In this manner, a positive electrode according to Example 1 was obtained.

[0105] (2) Negative Electrode A negative electrode active material, a binder, and a thickener were mixed in a mass ratio of 98:1:1 to obtain a negative electrode mixture. An appropriate amount of water as a dispersion medium was added to the negative electrode mixture and stirred to obtain a negative electrode mixture slurry. SiO ｘ (x=1.0) and graphite (the blending mass ratio is SiO ｘ A mixture of styrene-butadiene copolymer rubber (SBR) was used as the binder, and carboxymethyl cellulose (CMC) was used as the thickener.

[0106] The negative electrode composite slurry according to Example 1 was applied to both sides of a copper foil serving as a negative electrode current collector to form a coating film, which was then dried. The dried coating film was then compressed in the thickness direction using a roller to form a negative electrode composite layer on the negative electrode current collector. The laminate of the negative electrode current collector and the negative electrode composite layer was then cut to a predetermined size. In this manner, a negative electrode according to Example 1 was obtained.

[0107] (3) Nonaqueous Electrolyte (Nonaqueous Electrolyte Solution) Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:3 (EC:DMC), and vinylene carbonate (VC) was further added to obtain a nonaqueous mixed solvent. LiPF ６ The VC content in the nonaqueous electrolyte was 5 mass %. ６ The concentration of VC was set to 1.5 mol / L. VC functions as a non-aqueous solvent and also functions as an additive for forming a good SEI film on the surface of the graphite contained in the negative electrode active material.

[0108] (4) Fabrication of Secondary Battery One end of an aluminum positive electrode lead was attached to an aluminum foil positive electrode current collector by welding. One end of a nickel negative electrode lead was attached to a copper foil negative electrode current collector by welding. The positive and negative electrodes were then stacked in an inert gas atmosphere with a separator interposed therebetween to obtain a laminate, which was then wound to obtain a wound electrode group. A polyethylene microporous film was used as the separator.

[0109] Next, as shown in FIG. 1 , a secondary battery is completed by housing a wound electrode group (electrode group 14) and a non-aqueous electrolyte (nonaqueous electrolyte solution) in a cylindrical battery case. As shown in FIG. 1 , in the wound electrode group (electrode group 14), the positive electrode 11 is connected to a sealing body 16 via a positive electrode lead 19, and the negative electrode 12 is connected to a case body 15 via a negative electrode lead 20. The other end of the positive electrode lead 19 is connected to the sealing body 16, and the other end of the negative electrode lead 20 is connected to the case body 15. As described in the embodiment section, the cap 26 provided on the sealing body 16 functions as a positive electrode terminal, and the case body 15 functions as a negative electrode terminal.

[0110] [Example 2] The positive electrode obtained in Example 1 was subjected to heat treatment (at 200°C for 10 seconds) to obtain a positive electrode according to Example 2. A secondary battery according to Example 2 was produced in the same manner as in Example 1 except for the positive electrode.

[0111] [Example 3] A secondary battery according to Example 3 was fabricated in the same manner as in Example 2, except that the positive electrode composite layer was formed using binder B. Binder B was polyvinylidene fluoride (PVDF) having the average particle size and mass average molecular weight Mw shown in Table 1 below. Binder B was PVDF obtained by emulsion polymerization.

[0112] [Example 4] As shown in Table 2, a secondary battery according to Example 4 was fabricated in the same manner as in Example 2, except that an aluminum foil having a thickness of 22 μm was used as the positive electrode current collector instead of the aluminum foil having a thickness of 15 μm.

[0113] Example 5 A secondary battery according to Example 5 was fabricated in the same manner as Example 2, except that the positive electrode composite layer was formed using binder C. Binder C was polyvinylidene fluoride (PVDF) having the average particle size and mass average molecular weight Mw shown in Table 1 below. Binder C was PVDF obtained by emulsion polymerization.

[0114] Example 6 A secondary battery according to Example 6 was fabricated in the same manner as in Example 2, except that the positive electrode composite layer was formed using active material B instead of active material A. Active material B was a lithium nickel composite oxide (Li１．０５ Ni ０．８０ Co ０．１５ Al ０．０５ O ２ ) was.

[0115] Example 7 A secondary battery according to Example 7 was fabricated in the same manner as in Example 3, except that the positive electrode composite layer was formed using active material C instead of active material A. Active material C was a lithium nickel composite oxide (Li １．０５ Ni ０．８０ Co ０．１５ Al ０．０５ O ２ ) was.

[0116] Comparative Example 1 A secondary battery according to Comparative Example 1 was fabricated in the same manner as in Example 1, except that binder B was used to form the positive electrode mixture layer.

[0117] Comparative Example 2 A secondary battery according to Comparative Example 2 was fabricated in the same manner as in Example 1, except that the positive electrode composite layer was formed using active material B instead of active material A. Active material B was a lithium nickel composite oxide (Li １．０５ Ni ０．８０ Co ０．１５ Al ０．０５ O ２ ) was.

[0118] Comparative Example 3 A secondary battery according to Comparative Example 3 was fabricated in the same manner as in Comparative Example 2, except that binder B was used to form the positive electrode mixture layer.

[0119] [Comparative Example 4] The positive electrode obtained in Comparative Example 3 was subjected to heat treatment (at 200°C for 10 seconds) to obtain a positive electrode according to Comparative Example 4. A secondary battery according to Comparative Example 4 was produced in the same manner as in Comparative Example 3 except for the positive electrode.

[0120] Comparative Example 5 A secondary battery according to Comparative Example 5 was fabricated in the same manner as in Example 1, except that active material C was used instead of active material A to form the positive electrode mixture layer.

[0121] [Comparative Example 6] The positive electrode obtained in Comparative Example 5 was subjected to heat treatment (at 200°C for 10 seconds) to obtain a positive electrode according to Comparative Example 6. A secondary battery according to Comparative Example 6 was fabricated in the same manner as in Comparative Example 5 except for the positive electrode.

[0122] Comparative Example 7 A secondary battery according to Comparative Example 7 was fabricated in the same manner as in Comparative Example 5, except that binder B was used to form the positive electrode mixture layer.

[0123] Comparative Example 8 A secondary battery according to Comparative Example 8 was fabricated in the same manner as in Example 2, except that the positive electrode composite layer was formed with a mass ratio of the positive electrode active material (active material A), the conductive additive (conductive additive A), and the binder (binder A) of 98.3:1.4:0.3.

[0124] Comparative Example 9 A secondary battery according to Comparative Example 8 was fabricated in the same manner as in Example 2, except that the positive electrode composite layer was formed with a mass ratio of the positive electrode active material (active material A), the conductive additive (conductive additive A), and the binder (binder A) of 97.1:1.4:1.5.

[0125] [Comparative Example 10] A secondary battery according to Comparative Example 10 was fabricated in the same manner as in Example 2, except that the positive electrode composite layer was formed using binder D. Binder D was an acrylic polymer having the average particle size and mass average molecular weight Mw shown in Table 1 below. Binder D was also a polymer obtained by suspension polymerization.

[0126]

[0127]

[0128] <Evaluation> Positive Electrode (Elongation Rate) According to the procedure described in the above embodiment section, the elongation rate in the longitudinal direction was measured for the positive electrodes of Examples 1 to 7 and Comparative Examples 1 to 10. The results are shown in Table 2 above.

[0129] (Area Ratio of Fine Powder Region and Area Ratio of Aggregate Region) According to the procedure described in the above embodiment section, for the peeled surface (the surface peeled from the positive electrode current collector) of the positive electrode composite layer of Examples 1 to 7 and Comparative Examples 1 to 10, the area ratio of the fine powder region (a region configured to have an ellipse major axis of less than 1 μm by a portion of the binder present on the peeled surface) (the ratio of the total area of ​​the fine powder region based on the area of ​​the observation region), and the area ratio of the aggregate region (a region configured to have an ellipse major axis of 1 μm or more by the remainder of the binder present on the peeled surface) (the ratio of the total area of ​​the aggregate region based on the area of ​​the observation region) were determined. The results are shown in Table 3 below. In addition, for the peeled surface of the positive electrode composite layer of Examples 1 to 7 and Comparative Examples 1 to 10, the presence or absence of fluorine peaks detected in the fine powder region and aggregate region are also shown in Table 3 below.

[0130]

[0131] (Peel Strength) The peel strength of the positive electrode composite layer from the positive electrode current collector was measured by a "peel test in accordance with JIS K 6854-1." A rectangular piece cut out from a positive electrode for a secondary battery with planar dimensions of 1.5 cm × 12 cm was used as a test specimen. The average peel strength was measured at a gripping speed of 50 mm / min. The results are shown in Table 4 below.

[0132] Secondary Battery (Charge-Discharge Cycle Test) A charge-discharge cycle test was conducted on the secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 10. The charge-discharge cycle test was conducted by charging the secondary battery according to each example under the charging conditions below, resting for 20 minutes, and then discharging under the discharging conditions below, and repeating this cycle 500 times. The charge-discharge cycle test was conducted while the secondary battery according to each example was left in a thermostatic chamber at 25°C.

[0133] ・Charging: Constant current charging is performed at a current of 0.2 C until the battery voltage reaches 4.1 V, and then constant voltage charging is performed at a voltage of 4.1 V until the charging current reaches 0.02 C. ・Discharging: Constant current discharging is performed at a current of 0.2 C until the battery voltage reaches 3.0 V.

[0134] (Capacity Retention) The capacity retention of the secondary batteries according to each example subjected to the charge-discharge cycle test was evaluated. Specifically, the capacity retention rate of the secondary batteries according to each example was calculated using the following formula, and the calculated value was evaluated according to the following criteria. In the formula, E0 is the battery capacity of the secondary battery measured before the charge-discharge cycle test, and E1 is the battery capacity of the secondary battery measured after the charge-discharge cycle test (after 500 cycles). Capacity retention rate (%) = E1 / E0 x 100 Evaluation criteria: Excellent: The capacity retention rate exceeds 80%. Good: The capacity retention rate is greater than 70% and not more than 80%. Fair: The capacity retention rate is greater than 60% and not more than 70%. Bad: The capacity retention rate is not more than 60%. The results of evaluating the capacity retention rate of the secondary batteries according to each example are shown in Table 4 below.

[0135] (Positive Electrode Composite Resistance) The positive electrode composite resistance (unit: Ω cm) was measured for the secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 10. The positive electrode composite resistance was measured using an electrode resistance measuring device (device name: RM2610) manufactured by Hioki E.E. Corporation. The measurement current was set to 100 μA, and the voltage range was set to 0.5 V. The results are shown in Table 4 below.

[0136]

[0137] From Table 4, it can be seen that the secondary batteries according to Examples 1 to 7 had good evaluation results for capacity retention, either ⊚ or ◯, and had high peel strengths (peel strength of the positive electrode composite layer from the positive electrode current collector) of 6.0 N / m or more. In contrast, the secondary batteries according to Comparative Examples 1 to 10 had poor evaluation results for capacity retention, either Δ or ×, and all except Comparative Example 9 had low peel strengths of less than 6.0 N / m.

[0138] Furthermore, in the secondary batteries according to Examples 1 to 7, the composite resistance of the positive electrode was less than 50 Ω·cm, which was confirmed to be at a level that poses no practical problems.

[0139] The positive electrode for a secondary battery and the secondary battery according to the present disclosure can be used in applications where a low composite resistance and high adhesion of the positive electrode active material are achieved despite a small binder content, and further where sufficient cycle characteristics are required to be obtained.

[0140] 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 14: Electrode group 15: Case body 16: Sealing body 27: Gasket

Claims

1. A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode mixture layer formed on the positive electrode current collector, wherein the positive electrode mixture layer contains a positive electrode active material and a fluorine-containing resin as a binder, and the positive electrode mixture layer contains 0.4% by mass or more and 1.2% by mass or less of the binder, and the positive electrode mixture layer has, on a peeled surface obtained by peeling the positive electrode mixture layer from the positive electrode current collector, a fine powder region formed by a part of the binder present on the peeled surface so as to have an ellipse major axis of less than 1 μm, and an agglomeration region formed by a remainder of the binder present on the peeled surface so as to have an ellipse major axis of 1 μm or more, wherein when an observation region with a length of 50 μm or more and a width of 50 μm or more is placed on the peeled surface, based on the area of ​​the observation region, a total area of ​​the fine powder region accounts for 4.2% or more and 5.6% or less, and a total area of ​​the agglomeration region accounts for 4.5% or more and 6.5% or less.

2. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode collector has a thickness of 10 μm or more and 20 μm or less and has an elongated shape in a planar view, and an elongated shape has a longitudinal elongation rate of 2.5% or more.

3. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode active material has an oil absorption of 12 mL / 100 g or more and 20 mL / 100 g or less.

4. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode active material has an average particle size of 15 μm or less.

5. The positive electrode for a secondary battery according to claim 1, wherein the binder has a mass average molecular weight of 300,000 or more and 1,500,000 or less.

6. The positive electrode for a secondary battery according to claim 1, wherein the binder has an average particle size of 10 μm or more and 150 μm or less.

7. The positive electrode for a secondary battery according to claim 1, wherein the binder contains a fluorine-containing resin obtained by emulsion polymerization.

8. A secondary battery comprising: a positive electrode; a separator; a negative electrode facing the positive electrode with the separator interposed therebetween; a non-aqueous electrolyte; and a battery case accommodating the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte, wherein the positive electrode is the positive electrode for a secondary battery according to any one of claims 1 to 7.

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