Positive electrode for secondary batteries, and secondary battery

Abstract

A positive electrode for secondary batteries, the positive electrode being provided with a positive electrode collector and a positive electrode mixture layer that is arranged on the surface of the positive electrode collector, wherein: the positive electrode mixture layer contains first positive electrode active material particles that have a first average particle diameter D1 and second positive electrode active material particles that have a second average particle diameter D2, while satisfying D1 > D2, with D1 being 10 µm or more; if the positive electrode mixture layer is divided into a first region and a second region, which have the same thickness, the second region contains a larger amount of the second positive electrode active material particles than the first region; the first region is closer to the positive electrode collector than the second region; 50% or more of the surface of the positive electrode collector is a press fit region to which the first positive electrode active material particles are press fitted; and in the press fit region, the first positive electrode active material particles are press fitted to a depth of 10% or more of D1.

Description

Positive electrode for secondary battery and secondary battery

[0001] The present disclosure relates to a positive electrode for a secondary battery and a secondary battery.

[0002] BACKGROUND ART Secondary batteries, especially lithium-ion secondary batteries, have high output and high energy density, and are therefore expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles.

[0003] Patent Document 1 aims to provide a positive electrode plate that can ensure the capacity of a battery while reducing the diffusion resistance of lithium ions when a positive electrode is used in a lithium ion secondary battery. The positive electrode plate includes a metal foil having a first main surface and a second main surface, and a laminated active material layer formed on at least one of the first main surface and the second main surface, the laminated active material layer being a stack of multiple positive electrode active material layers containing positive electrode active material particles made of a lithium compound, wherein the content of the positive electrode active material particles in the laminated active material layer is uniform when viewed in the stacking direction, and the positive electrode active material layers containing the positive electrode active material particles with smaller average particle diameters are arranged in higher layers.

[0004] Japanese Patent Application Laid-Open No. 2009-026599

[0005] As in Patent Document 1, reducing the ion diffusion resistance can increase the output of a secondary battery, but it is not sufficient to reduce the internal resistance of the secondary battery. This tendency becomes more pronounced as the thickness of the positive electrode is increased to increase the energy density of the secondary battery. On the other hand, from the viewpoint of improving charge-discharge cycle characteristics, it is desirable to reduce the internal resistance as much as possible.

[0006] One aspect of the present disclosure relates to a positive electrode for a secondary battery, including: a positive electrode current collector; and a positive electrode mixture layer provided on a surface of the positive electrode current collector, the positive electrode mixture layer including first positive electrode active material particles having a first average particle size D1 and second positive electrode active material particles having a second average particle size D2, wherein D1 > D2 are satisfied and D1 is 10 μm or greater, when the positive electrode mixture layer is divided into a first region and a second region having the same thickness, the second region contains more of the second positive electrode active material particles than the first region, the first region is closer to the positive electrode current collector than the second region, 50% or more of a surface of the positive electrode current collector is a pressed-in region into which the first positive electrode active material particles are pressed, and the pressed-in region is a region into which the first positive electrode active material particles are pressed to a depth of 10% or more of D1.

[0007] Another aspect of the present disclosure relates to a secondary battery including the above-described positive electrode for secondary batteries, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolyte solution.

[0008] According to the present disclosure, it is possible to realize a secondary battery that has high output, is advantageous in terms of increasing capacity, and improving charge / discharge cycle characteristics.

[0009] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] 1 is a cross-sectional view schematically showing the structure of a positive electrode according to an embodiment of the present disclosure; FIG. 2 is a cross-sectional view schematically showing the structure of a positive electrode according to another embodiment of the present disclosure; FIG. 3 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away.

[0011] The following describes embodiments of the present disclosure 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 ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions 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 the materials may be selected and used alone, or two or more materials may be used in combination.

[0012] 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.

[0013] In the following description, the terms "contain" or "comprise" encompass "contain (or include)," "consist essentially of," and "consist of."

[0014] The secondary battery includes at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries.

[0015] A positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector. The positive electrode current collector is made of a sheet-like conductive material. The positive electrode mixture layer is supported on one or both surfaces of the positive electrode current collector. The positive electrode mixture layer is typically a layer (including a membrane or film) made of a positive electrode mixture. The positive electrode mixture contains a positive electrode active material as an essential component.

[0016] The positive electrode mixture layer includes first positive electrode active material particles having a first average particle diameter D1 and second positive electrode active material particles having a second average particle diameter D2. D1 and D2 can be measured from a cross section in the thickness direction obtained by simultaneously cutting the positive electrode mixture layer and the positive electrode current collector. The cross section may be formed using a cross-section polisher (CP). At this time, a thermosetting resin may be filled into the positive electrode mixture layer and cured. Next, a scanning electron microscope (SEM) image of the cross section is taken. The SEM image is taken so that at least 10 first positive electrode active material particles and at least 10 second positive electrode active material particles are observed. Image processing is performed to determine the first circle-equivalent diameters of the cross sections of at least 10 first positive electrode active material particles, and the average value is determined as D1. Image processing is performed to determine the second circle-equivalent diameters of the cross sections of at least 10 second positive electrode active material particles, and the average value is determined as D2. Here, the circle-equivalent diameter refers to the diameter of a circle having the same area as the cross-sectional area of ​​the particle (the area of ​​the particle observed in the cross-section of the positive electrode mixture layer).

[0017] When the first positive electrode active material particles and the second positive electrode active material particles can be separated and recovered from the positive electrode mixture layer, the median diameters (particle diameters at 50% cumulative volume) in the volume-based particle size distributions of the first positive electrode active material particles and the second positive electrode active material particles may be determined as D1 and D2, respectively. The volume-based particle size distribution can be measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by HORIBA, Ltd. can be used as the measuring device.

[0018] The first positive electrode active material particles and the second positive electrode active material particles may be the same type or different types. For example, the first positive electrode active material particles and the second positive electrode active material particles may have different average particle sizes but may be formed from compounds having the same chemical structure or composition.

[0019] D1 and D2 satisfy D1 > D2, and D1 is 10 μm or more. First positive electrode active material particles with D1 of 10 μm or more are dense, less susceptible to deterioration, and advantageous for achieving high capacity. On the other hand, second positive electrode active material particles with D1 > D2 have a large surface area and are advantageous for achieving high output. Using the first positive electrode active material particles and the second positive electrode active material particles in combination makes it easier to control the balance between capacity and output. D1 may be 11 μm or more, 12 μm or more, or 15 μm or more. From the viewpoint of ensuring uniform reactivity, D1 may be 30 μm or less, or 25 μm or less.

[0020] When the positive electrode mixture layer is divided into a first region and a second region having the same thickness (wherein the first region is closer to the positive electrode current collector than the second region), the second region contains more second positive electrode active material particles than the first region. That is, the mass of the second positive electrode active material particles contained per unit volume of the second region is greater than the mass of the second positive electrode active material particles contained per unit volume of the first region.

[0021] In other words, the proportion of the first positive electrode active material particles is large at the interface between the positive electrode current collector and the positive electrode mixture layer, and the proportion of the first positive electrode active material particles is small at the surface side of the positive electrode mixture layer. In other words, the proportion of the second positive electrode active material particles is small at the interface between the positive electrode current collector and the positive electrode mixture layer, and the proportion of the second positive electrode active material particles is large at the surface side of the positive electrode mixture layer.

[0022] The second region may be substantially free of first positive electrode active material particles, or may contain a mixture of first and second positive electrode active material particles. From the viewpoint of increasing capacity, the second region desirably contains first positive electrode active material particles. This is because the small-sized second positive electrode active material particles are filled to fill gaps formed between the large-sized first positive electrode active material particles. However, it is desirable that more first positive electrode active material particles are contained in the first region than in the second region. In other words, it is desirable that the mass of the first positive electrode active material particles contained per unit volume of the first region is greater than the mass of the first positive electrode active material particles contained per unit volume of the second region.

[0023] The first region may be substantially free of second positive electrode active material particles, but a region of the first region close to the second region may contain the second active material. From the viewpoint of increasing capacity, the region of the first region close to the second region desirably contains second positive electrode active material particles, and may contain a mixture of the first positive electrode active material particles and the second positive electrode active material particles.

[0024] When the effect of increasing capacity is enhanced by sufficiently filling the gaps formed between the large-diameter first positive electrode active material particles with the small-diameter second positive electrode active material particles, the D1 / D2 ratio may be, for example, 2 or more and 6 or less, or 3 or more and 5 or less.

[0025] D2 may be less than 10 μm, may be 8 μm or less, may be 6 μm or less, or may be 5 μm or less, etc. From the viewpoint of enhancing the effect of increasing capacity and improving charge / discharge cycle characteristics, D2 may be 1 μm or more, or may be 3 μm or more.

[0026] In the region of the first region closest to the positive electrode current collector, the proportion of small-particle-size second positive electrode active material particles is small, making it easier for the large-particle-size first positive electrode active material to be pressed into the positive electrode current collector. That is, the surface of the positive electrode current collector has a sufficient pressed-in region into which the first positive electrode active material particles are pressed. The pressed-in region improves the current collecting ability of the positive electrode and, as a result, contributes to improving the charge-discharge cycle characteristics of the secondary battery. In the pressed-in region, the large-particle-size first positive electrode active material particles are pressed into the positive electrode current collector, and the positive electrode current collector is plastically deformed to conform to the particle shape. This increases the contact area between the first positive electrode active material particles and the positive electrode current collector, and increases the bonding strength between the positive electrode mixture layer and the positive electrode current collector. As a result, the conductivity and load characteristics of the positive electrode are improved, positive electrode degradation is suppressed, and charge-discharge cycle characteristics are significantly improved. The region of the first region closest to the positive electrode current collector may be substantially free of second positive electrode active material particles.

[0027] The region of the first region closest to the positive electrode current collector refers to, for example, a boundary region having a thickness of 2×D1 from the surface of the positive electrode current collector. It is desirable for as many first positive electrode active material particles as possible to be present in the boundary region. In the boundary region, the volume ratio of the first positive electrode active material particles having a circle-equivalent diameter of D1 or more to the total of the first positive electrode active material particles and the second positive electrode active material particles may be 50% or more, 60% or more, 70% or more, or 80% or more. Here, the volume ratio may be considered to be the area ratio of the first positive electrode active material particles having a circle-equivalent diameter of D1 or more to the total of the first positive electrode active material particles and the second positive electrode active material particles in the boundary region of a cross section taken through the positive electrode mixture layer and the positive electrode current collector in the thickness direction. When counting the area of ​​the first positive electrode active material particles, not only the first positive electrode active material particles and the second positive electrode active material particles contained in the boundary region with a thickness of 2 × D1 from the surface of the positive electrode current collector are counted, but also the area of ​​the portion of the first positive electrode active material particles embedded in the positive electrode current collector.

[0028] Furthermore, in the first region where the proportion of the second positive electrode active material particles is small, many large voids are formed between the first positive electrode active material particles. The porosity of the first region may be larger than the porosity of the second region. In this case, the liquid circulation (ion transport) in the deep portion of the positive electrode mixture layer close to the positive electrode current collector is improved. The improved liquid circulation further promotes higher output and improved charge / discharge cycle characteristics of the secondary battery.

[0029] Here, "having a sufficient pressed-in region" refers to a region where 50% or more of the surface of the positive electrode current collector is a pressed-in region in a cross section taken through the positive electrode mixture layer and the positive electrode current collector simultaneously in the thickness direction. The pressed-in region is a region where the first positive electrode active material particles are pressed into the positive electrode current collector to a depth of 10% or more of D1. The depth refers to the distance from the surface of the positive electrode current collector to each portion of the first positive electrode active material particles embedded in the positive electrode current collector. The proportion of the pressed-in region is calculated as follows.

[0030] First, a first reference line is drawn in a cross section of the positive electrode mixture layer and the positive electrode current collector cut simultaneously in the thickness direction, indicating the level of the surface of the positive electrode current collector that has not undergone plastic deformation. A second reference line is drawn at a distance D1 / 10 from the first reference line, crossing the cross section of the first positive electrode active material particles and the positive electrode current collector. The length L of the second reference line is 100 μm or more. The ratio of the total length Lp of the line segments crossing the first positive electrode active material particles to the length L of the second reference line is the proportion of the pressed-in region. When the total length Lc of the line segments crossing the cross section of the positive electrode current collector is L = Lp + Lc.

[0031] The thickness of the positive electrode mixture layer may be 80 μm or more, or may be 100 μm or more, per side of the positive electrode current collector. To increase the capacity of a secondary battery, it is desirable to make the positive electrode mixture layer as thick as possible. On the other hand, generally, the thicker the positive electrode mixture layer, the more likely it is that the output characteristics and load characteristics will deteriorate, and the charge / discharge cycle characteristics will also deteriorate. In this embodiment, by arranging the first positive electrode active material particles and the second positive electrode active material particles as described above and providing a press-fit region over 50% or more of the surface of the positive electrode current collector, a secondary battery with high output, excellent load characteristics, and excellent charge / discharge cycle characteristics can be obtained, even when a thick positive electrode mixture layer is provided as described above. The upper limit of the thickness of the positive electrode mixture layer is, for example, about 300 μm per side of the positive electrode current collector, but is not limited thereto.

[0032] The positive electrode mixture layer may have a multilayer structure having a first layer and one or more other layers with different morphologies. When the first layer is closest to the positive electrode current collector, the thickness of the first layer may be 10 μm or more and 40 μm or less, or 10 μm or more and 30 μm or less. The first layer may include a boundary region having a thickness of 2×D1 from the surface of the positive electrode current collector. Conversely, a boundary region having a thickness of 2×D1 from the surface of the positive electrode current collector may include the first layer. Examples of different morphologies include when the particle size of the positive electrode active material is different, or when the type or composition of the positive electrode active material is different. Whether the positive electrode mixture layer has a multilayer structure having a first layer and one or more other layers with different morphologies can be easily determined by observing a cross section in the thickness direction obtained by simultaneously cutting the positive electrode mixture layer and the positive electrode current collector.

[0033] The first layer or boundary region may be, for example, a layer that contains substantially only first positive electrode active material particles as the positive electrode active material, or may be a layer in which the first positive electrode active material particles account for 80 mass % or more of the positive electrode active material.

[0034] The second layer may be, for example, a layer containing substantially only second cathode active material particles as the cathode active material, or a layer containing a mixture of first and second cathode active material particles as the cathode active material. In the mixture of the first and second cathode active material particles, the second cathode active material particles may account for, for example, 20% by mass or more, or even 40% by mass or more, or 50% by mass or more, with the remainder being the first cathode active material particles.

[0035] The first positive electrode active material particles may contain a first lithium transition metal composite oxide containing Ni. The first lithium transition metal composite oxide may be a lithium transition metal composite oxide containing lithium and Ni and having a layered rock salt crystal structure. The proportion of Ni to the metal elements other than Li contained in the first lithium transition metal composite oxide may be 50 atomic % or more. The first lithium transition metal composite oxide may contain Co. However, from the viewpoints of cost reduction and high capacity, the proportion of Co to the metal elements other than Li contained in the first lithium transition metal composite oxide is preferably 0 atomic % or more and 20 atomic % or less, and more preferably 0 atomic % or more and 15 atomic % or less. Co is thought to increase the conductivity of the positive electrode active material particles. When the first positive electrode active material particles pressed into the surface of the positive electrode current collector contain Co, the current collecting properties of the positive electrode can be further improved.

[0036] The second positive electrode active material particles may contain a second lithium transition metal composite oxide containing Ni. The second lithium transition metal composite oxide may be a lithium transition metal composite oxide containing lithium and Ni and having a layered rock salt crystal structure. The proportion of Ni in the metal elements other than Li contained in the second lithium transition metal composite oxide may be 50 atomic % or more. The second lithium transition metal composite oxide may contain Co. However, from the viewpoints of cost reduction and high capacity, the proportion of Co in the metal elements other than Li contained in the second lithium transition metal composite oxide is preferably 0 atomic % or more and 10 atomic % or less, and more preferably 0 atomic % or more and 5 atomic % or less.

[0037] In general, lithium transition metal composite oxides in which the proportion of Ni among metal elements other than Li is 50 atomic % or more are thought to be prone to change in crystal structure with repeated charge and discharge, and prone to high resistance. In this embodiment, by arranging the first positive electrode active material particles and the second positive electrode active material particles as described above and providing a pressed-in region covering 50% or more of the surface of the positive electrode current collector, it is possible to obtain a secondary battery that has high output, excellent load characteristics, and excellent charge and discharge cycle characteristics, even when using a lithium transition metal composite oxide that is prone to high resistance.

[0038] FIG. 1 is a cross-sectional view schematically illustrating the structure of a positive electrode for a secondary battery according to this embodiment. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode mixture layer 12 provided on the surface of the positive electrode current collector 11. FIG. 1 illustrates a portion of a cross section in the thickness direction obtained by simultaneously cutting the positive electrode mixture layer 12 and the positive electrode current collector 11. The positive electrode mixture layer 12 can be formed on both main surfaces of the positive electrode current collector 11. FIG. 1 illustrates a portion of one main surface of the positive electrode current collector 11 and a portion of the positive electrode mixture layer 12 formed on that main surface, and does not illustrate the other main surface of the positive electrode current collector 11.

[0039] The positive electrode mixture layer 12 includes first positive electrode active material particles P1 having a first average particle size D1 and second positive electrode active material particles P2 having a second average particle size D2. D1 and D2 satisfy D1 > D2, D1 is 10 μm or greater, and the D1 / D2 ratio is in the range of 2 to 6. The thickness T of the positive electrode mixture layer 12 is, for example, 80 μm or greater.

[0040] Line L0 divides the positive electrode mixture layer 12 into a first region R1 and a second region R2 having the same thickness. However, the first region R1 is closer to the positive electrode current collector 11 than the second region R2. The second region R2 contains more second positive electrode active material particles P2 than the first region R1. The first positive electrode active material particles P1 are not contained in the second region R2, but are contained only in the first region R1.

[0041] Line L1 is a first reference line indicating the level of the surface of the positive electrode current collector 11 that has not been plastically deformed. Line L2 is a second reference line that is spaced a distance of D1 / 10 from the first reference line and is drawn to cross the cross sections of the first positive electrode active material particles P1 and the positive electrode current collector 11. The length L of the second reference line is set to 100 μm or more. The dashed line segments of the second reference line are line segments that cross the first positive electrode active material particles P1, and their total length is Lp. The solid line segments of the second reference line are line segments that cross the cross section of the positive electrode current collector 11, and their total length is Lc. The ratio of Lp to L is the proportion of the pressed-in region, and L = Lp + Lc is satisfied. The proportion of the pressed-in region into which the first positive electrode active material particles P1 are pressed is significantly greater than 50% of the surface of the positive electrode current collector 11.

[0042] Line L3 is a third reference line located 2×D1 away from the surface of the positive electrode current collector 11 (i.e., the first reference line) toward the second region R2. The region sandwiched between the first reference line and the third reference line is the boundary region. The boundary region significantly affects the formation process of the press-fit region and the liquid circulation performance. The boundary region is included in the first region R1 and is substantially occupied only by the first positive electrode active material particles P1. That is, the volume ratio of the first positive electrode active material particles P1 having a circle equivalent diameter of D1 or more to the total volume of the first positive electrode active material particles P1 and the second positive electrode active material particles is 100%. The more large-diameter first positive electrode active material particles P1 occupying the boundary region, the more effectively the pressure applied when pressing the first positive electrode active material particles P1 into the positive electrode current collector 11 acts without being dispersed, allowing the proportion of the press-fit region to be significantly increased. Compared to the second region R2, the boundary region contains many large voids, which significantly improve the liquid circulation performance. The porosity of the first region R1 may be greater than the porosity of the second region R2.

[0043] Line L4 is a fourth reference line indicating the boundary between the first layer and the second layer, which are different in form from each other and are included in the positive electrode mixture layer 12. The fourth reference line may be, for example, a line furthest from the surface of the positive electrode current collector 11, where the total length of line segments crossing the first positive electrode active material particles P1 is 20% or less. The boundary between the first layer and the second layer (fourth reference line) can be clearly identified by observing a cross section of the positive electrode mixture layer. The thickness of the first layer is desirably half or less of the thickness T of the positive electrode mixture layer 12 (i.e., equal to or less than the thickness of the first region R1), and may be in the range of 10 μm to 40 μm, or may be thicker than the boundary region having a thickness of 2 × D1.

[0044] 1 shows the positive electrode mixture layer 12 having a two-layer structure consisting of a first layer and a second layer, one or more layers of a different structure may be interposed between the first layer and the second layer. Also, one or more layers of a different structure may be present that are farther from the positive electrode current collector 11 than the second layer. However, the layer is designed so that the more distant from the positive electrode current collector 11 the layer is, the smaller the average particle size of the positive electrode active material particles contained in that layer becomes.

[0045] 2 is a cross-sectional view schematically illustrating the structure of a positive electrode for a secondary battery according to another embodiment. The positive electrode 10A includes a positive electrode current collector 11 and a positive electrode mixture layer 12A provided on the surface of the positive electrode current collector 11. The positive electrode mixture layer 12 includes first positive electrode active material particles P1 having a first average particle size D1 and second positive electrode active material particles P2 having a second average particle size D2, where D1 > D2 and D1 is 10 μm or greater.

[0046] 2, the second region R2 or second layer contains a mixture of the first positive electrode active material particles P1 and the second positive electrode active material particles P2. In this second region R2, the second positive electrode active material particles P2 fill the gaps formed between the first positive electrode active material particles P1, which is advantageous in terms of increasing capacity. Apart from the different configuration of the second region R2, the positive electrode 10A shown in FIG. 2 has a structure similar to that of the positive electrode 10 shown in FIG. 1.

[0047] Next, a secondary battery according to an embodiment of the present disclosure will be described in detail. The secondary battery includes, for example, the positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolyte solution.

[0048] [Positive Electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material layer contains the positive electrode active material as an essential component and may contain optional components such as a binder and a conductive agent. Known materials can be used as the binder, conductive agent, and thickener.

[0049] As described above, the positive electrode active material includes first positive electrode active material particles having a first average particle size D1 and second positive electrode active material particles having a second average particle size D2. The positive electrode active material may include one or more types of positive electrode active material particles in addition to the first positive electrode active material particles and the second positive electrode active material particles, and the total of the first positive electrode active material particles and the second positive electrode active material particles desirably accounts for 70 mass% or more of the positive electrode active material, and may also account for 90 mass% or more.

[0050] When the positive electrode mixture layer has the first and second layers described above, the first and second layers preferably account for 70% by mass or more of the positive electrode mixture layer, and may account for 90% by mass or more. The first layer can be formed, for example, by a method including a process of applying a first positive electrode slurry, in which a first positive electrode mixture containing first positive electrode active material particles, a binder, and the like is dispersed in a dispersion medium, to the surface of a positive electrode current collector. The second layer can be formed, for example, by a method including a process of applying a second positive electrode slurry, in which a second positive electrode mixture containing second positive electrode active material particles, a binder, and the like is dispersed in a dispersion medium, to the surface of the first positive electrode slurry. The laminated coating film after drying may be rolled as necessary. The first positive electrode slurry and the second positive electrode slurry may be simultaneously applied to the surface of the positive electrode current collector using a two-fluid nozzle.

[0051] As described above, the first and second positive electrode active material particles may be made of a first or second lithium transition metal composite oxide (hereinafter also referred to as composite oxide N) containing lithium and Ni and having a layered rock salt crystal structure. Nickel is advantageous for achieving high capacity and low cost. The proportion of Ni among the metal elements other than Li contained in composite oxide N may be 50 atomic % or more, 68.5 atomic % or more, 80 atomic % or more, or 90 atomic % or more.

[0052] The composite oxide N is, for example, a compound represented by the general formula: LiαNiCo x1Mn x2 Al y Me z O 2+ However, the general formula satisfies 0.95≦α≦1.05, 0≦x1≦0.1, 0≦x2≦0.5, 0≦y≦0.1, 0≦z≦0.1, 0.5≦1-x1-x2-y-z, and -0.05≦β≦0.05, and Me is an element other than Li, Ni, Mn, Al, Co, and oxygen. The α value, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0053] As Me, from the viewpoint of stabilizing the crystal structure of the composite oxide N, at least one selected from the group consisting of Nb, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Si, Ti, Fe, and Cr can be used.

[0054] When the composite oxide N is used as the first lithium transition metal composite oxide, x1 may satisfy 0<x1≦0.1, or even 0.005≦x1≦0.015, from the viewpoint of improving the load characteristics. Specific examples of such composite oxides include lithium-cobalt-aluminum composite oxides (e.g., LiNi 0.8 Co 0.15 Al 0.05 Cobalt is advantageous for extending the life of the battery. Aluminum is advantageous for improving thermal stability.

[0055] When the composite oxide N is used as the first or second lithium transition metal composite oxide, from the viewpoint of cost reduction and stability of the crystal structure, x2 may satisfy 0<x2≦0.3, or even 0.15≦x2≦0.25. Specific examples of such composite oxides include lithium-nickel-manganese composite oxides (e.g., LiNi 0.8 Mn 0.2 O2).

[0056] The shape and thickness of the positive electrode current collector may be, for example, 5 μm or more and 20 μm or less. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0057] [Negative Electrode] The negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry, in which a negative electrode mixture containing a negative electrode active material, a binder, etc. is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the slurry. The dried coating may be rolled as necessary. In other words, the negative electrode active material may be a negative electrode mixture layer. Alternatively, a lithium metal foil or a lithium alloy foil may be attached to the negative electrode current collector as the negative electrode active material layer. The negative electrode active material layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0058] The negative electrode active material layer contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. Known materials can be used as the binder, conductive agent, and thickener.

[0059] Negative electrode active materials include materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, etc. Materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxides and tin oxides formed by bonding these with oxygen may also be used.

[0060] Examples of alloy-based materials containing silicon include a lithium ion conductive phase and a silicon composite material in which silicon particles are dispersed in the lithium ion conductive phase. Examples of lithium ion conductive phases that can be used include a silicon oxide phase, a silicate phase, and a carbon phase. The silicon oxide phase may be primarily composed of silicon dioxide (e.g., 95 to 100% by mass). Among these, composite materials composed of a silicate phase and silicon particles dispersed in the silicate phase are preferred because of their high capacity and low irreversible capacity. Furthermore, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred because of its low irreversible capacity and high initial charge / discharge efficiency.

[0061] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li 2z SiO 2+z (0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2. Examples of elements other than Li, Si, and O that can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0062] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other.

[0063] 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 the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0064] [Electrolyte] The electrolyte contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that ionically dissociates in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and the solute are additives. The electrolyte may contain various additives.

[0065] The solvent may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0066] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts that can be used include lithium salts of fluorine-containing acids (LiPF, LiPF0, LiBF, LiSbF, LiAsF, LiCFSO, LiCFCO, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO), LiN(CFSO), LiN(CFSO)(CFSO), LiN(CFSO)), and lithium halides (LiCl, LiBr, LiI, etc.). One type of lithium salt may be used alone, or two or more types may be used in combination.

[0067] The concentration of the lithium salt in the electrolyte solution may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0068] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0069] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an exterior body together with an electrolyte solution. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0070] Hereinafter, the structure of a prismatic non-aqueous secondary battery will be described as an example of the secondary battery according to the present invention with reference to FIG.

[0071] The battery includes a bottomed, rectangular battery case 4, and an electrode group 1 and an electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0072] 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.

[0073] Example 1 [Fabrication of Positive Electrode] The first positive electrode active material particles and the second positive electrode active material particles were made of a lithium transition metal composite oxide (lithium-nickel-manganese composite oxide (LiNi)) having different average particle sizes and a layered rock salt crystal structure and containing lithium and Ni. 0.8 Mn 0.2 O2) was prepared.

[0074] The median diameters D1 and D2 of the first and second positive electrode active material particles measured by a laser diffraction scattering method in the volume-based particle size distribution were 17 μm and 4 μm, respectively.

[0075] 100 parts by mass of the first positive electrode active material particles, 1.1 parts by mass of acetylene black, 0.9 parts by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were mixed together to prepare a first positive electrode slurry.

[0076] 100 parts by mass of the second positive electrode active material particles, 1.1 parts by mass of acetylene black, 0.9 parts by mass of polyvinylidene fluoride, and an appropriate amount of NMP were mixed together to prepare a second positive electrode slurry.

[0077] The first positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, the coating was dried, and then the second positive electrode slurry was applied to cover the first positive electrode slurry coating. The coating was then dried and rolled to form a positive electrode mixture layer on the aluminum foil, comprising a first layer 40 μm thick and a second layer 40 μm thick. That is, the thickness of the first region R1 was equal to the thickness of the first layer, and the thickness of the second region R2 was equal to the thickness of the second layer. The first region R1 contained only first positive electrode active material particles P1, and the second region R2 contained only second positive electrode active material particles P2. The overall density of the positive electrode mixture layer after rolling was 3.45 g / cm. 3 It was adjusted so that

[0078] A cross-section was formed by simultaneously cutting the positive electrode mixture layer and the positive electrode current collector using a cross-section polisher (CP), an SEM image of the cross-section was taken, and the average first circle-equivalent diameter of 10 or more first positive electrode active material particles was determined as D1, and the average second circle-equivalent diameter of the cross-section of 10 or more second positive electrode active material particles was determined as D2 using the method described above. As a result, the average values ​​D1 and D2 were almost identical to the median diameters D1 and D2 (i.e., 17 μm and 4 μm) determined as the volume-based particle size distribution, respectively.

[0079] On the obtained cross section, a first reference line and a second reference line spaced a distance of D1 / 10 (=1.7 μm) from the first reference line were drawn, and the ratio (proportion of the pressed-in area) of the total length Lp of the line segments of the second reference line that crossed the first positive electrode active material particles P1 to the length L (=120 μm) of the second reference line was calculated using the method described above, and was found to be 83%.

[0080] Large voids that significantly affect liquid circulation were observed in the boundary region between the surface (first reference line) of positive electrode current collector 11 and a third reference line that is a distance of 2 × D1 away from the first reference line toward second region R2. In the boundary region, the volume ratio of the first positive electrode active material particles having a circle-equivalent diameter of D1 or more to the total volume of the first positive electrode active material particles and the second positive electrode active material particles was 100%.

[0081] The positive electrode was cut into a predetermined shape to obtain a positive electrode for evaluation. The positive electrode had a 20 mm × 20 mm region to function as a positive electrode and a 5 mm × 5 mm region to connect to the tab lead. The positive electrode mixture layer formed on the connection region was then scraped off to expose the positive electrode current collector. The exposed portion of the positive electrode current collector was then connected to the positive electrode tab lead, and a predetermined region around the periphery of the positive electrode tab lead was covered with an insulating tab film.

[0082] [Fabrication of Negative Electrode] A lithium metal foil (thickness: 300 μm) was attached to one side of an electrolytic copper foil serving as a negative electrode current collector to fabricate a negative electrode.

[0083] The negative electrode was cut into the same shape as the positive electrode to obtain a negative electrode for evaluation. The lithium metal foil formed on the connection area was peeled off in the same way as the positive electrode to expose the negative electrode current collector. The exposed portion of the negative electrode current collector was then connected to a negative electrode tab lead, and a predetermined area around the periphery of the negative electrode tab lead was covered with an insulating tab film, just like the positive electrode.

[0084] [Preparation of Electrolyte Solution] An electrolyte solution was prepared by adding LiPF as a lithium salt to a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 4:1:15. The concentration of LiPF in the electrolyte solution was 1.0 mol / L.

[0085] [Preparation of Secondary Battery] A battery for evaluation was prepared using a positive electrode and a negative electrode for evaluation. First, the positive electrode and the negative electrode were opposed to each other with a separator interposed between them so that the positive electrode mixture layer and the lithium metal foil of the negative electrode overlapped to obtain an electrode plate assembly. Next, an Al laminate film (thickness 100 μm) cut into a rectangle of 60 × 90 mm was folded in half, and the end of the 60 mm long side was heat-sealed at 230 ° C to form a 60 × 45 mm cylindrical shape. Then, the prepared electrode plate assembly was placed in the cylinder, and the end face of the Al laminate film was aligned with the insulating tab film of each tab lead and heat-sealed at 230 ° C. Next, nonaqueous electrolyte was poured 0.3 cm from the short side of the Al laminate film that was not heat-sealed. 3 After the injection, the mixture was left standing for 5 minutes under a reduced pressure of 0.06 MPa to allow the electrolyte to penetrate into each mixture layer. Finally, the end face of the Al laminate film on the injected side was heat-sealed at 230°C to prepare Battery A1 for evaluation. The evaluation cell was prepared in a dry environment with a dew point of -50°C or lower.

[0086] Example 2 A first positive electrode slurry containing the first positive electrode active material particles and the second positive electrode active material particles was prepared by mixing 50 parts by mass of first positive electrode active material particles, 50 parts by mass of second positive electrode active material particles, 1.1 parts by mass of acetylene black, 0.9 parts by mass of polyvinylidene fluoride, and an appropriate amount of NMP.

[0087] A positive electrode mixture layer including a first layer having a thickness of 40 μm and a second layer having a thickness of 40 μm was formed on an aluminum foil in the same manner as in Example 1, except that a first positive electrode slurry containing first positive electrode active material particles and second positive electrode active material particles was used, and a positive electrode and a secondary battery (battery A2) were produced.

[0088] The overall density of the positive electrode mixture layer after rolling was 3.47 g / cm 3 The ratio of the pressed-in region in the positive electrode was 81%.

[0089] Comparative Example 1 A secondary battery (B1) was produced in the same manner as in Example 1, except that only the first slurry prepared in the same manner as the first positive electrode slurry of Example 2 was applied to produce a positive electrode having a positive electrode mixture layer of the same thickness of 80 μm as in Example 1 and the same theoretical capacity as in Example 1. The overall density of the positive electrode mixture layer after rolling was 3.47 g / cm 3 The ratio of the pressed-in region in the obtained positive electrode was 21%.

[0090] <Comparative Example 2> A second positive electrode slurry prepared in the same manner as in Example 1 was first applied to the surface of an aluminum foil serving as a positive electrode current collector, the coating was dried, and then a first positive electrode slurry prepared in the same manner as in Example 1 was applied to cover the coating of the second positive electrode slurry. The coating was then dried and rolled to form a positive electrode mixture layer on the aluminum foil, each having a first layer with a thickness of 40 μm and a second layer with a thickness of 40 μm. A positive electrode and a secondary battery (B2) were fabricated in the same manner as in Example 1. That is, the distribution state of the first positive electrode active material particles and the second positive electrode active material particles was reversed from that in Example 1. The proportion of the pressed-in region in the resulting positive electrode was 39%.

[0091] Comparative Example 3 A first positive electrode slurry prepared in the same manner as in Example 1 was applied to the surface of an aluminum foil serving as a positive electrode current collector. Next, after drying the coating, a second positive electrode slurry prepared in the same manner as in Example 1 was applied so as to cover the coating of the first positive electrode slurry. Thereafter, the coating was dried and rolled to form a positive electrode mixture layer having a first layer having a thickness of 40 μm and a second layer having a thickness of 40 μm on each side of the aluminum foil. A positive electrode and a secondary battery (B3) were produced in the same manner as in Example 1. However, the pressure during rolling was reduced and the rolling was performed in two stages, and the overall density of the positive electrode mixture layer after rolling was 3.46 g / cm. 3 The ratio of the pressed-in region in the obtained positive electrode was 47%.

[0092] [Evaluation] (Initial Charge / Discharge) Each completed battery was clamped between a pair of 80 x 80 cm stainless steel clamps (2 mm thick) and pressurized to 0.2 MPa. Each battery was placed in a 25°C environment and subjected to constant current charging at a current of 0.2 It until the voltage reached 4.3 V. Thereafter, constant voltage charging at a constant voltage of 4.3 V until the current reached 0.05 It. Subsequently, constant current discharging was performed at a current of 0.2 It until the voltage reached 2.5 V, and the initial capacity C was determined. Charging and discharging were performed in a 25°C environment.

[0093] (Capacity Retention Rate) The rest period between charge and discharge was 20 minutes, and 30 cycles of charge and discharge were repeated under the above charge and discharge conditions in an environment of 25°C. The discharge capacity C1 at the 30th cycle was determined. The ratio R1 = C1 / C0 of the discharge capacity C1 to the initial discharge capacity C0 was evaluated as the capacity retention rate. (DCIR Measurement) After the initial charge and discharge test, each battery was placed in an environment of 25°C and subjected to constant current charging at a current of 0.2 It until the state of charge (SOC) reached 50%. After a 2-hour rest, the battery was discharged at a current of 0.3 It for 30 seconds. The resistance value was calculated from the voltage drop before and after the test, and this was used as the initial DCIR.

[0094] Table 1 shows the evaluation results of the capacity retention rate R1 and DCIR for batteries A1, A2, B1, B2, and B3. The values ​​in Table 1 are relative values ​​when the result for battery B1 of Comparative Example 1 is set to 100. A larger value for the capacity retention rate R1 is more desirable, and a smaller value for the DCIR is more desirable.

[0095]

[0096] As shown in Table 1, when the proportion of the pressed-in region is 50% or more (especially 70% or more), not only is the DCIR significantly reduced, but the capacity retention rate is also significantly improved. From the perspective of increasing capacity, the positive electrode mixture layer of Comparative Example 1 is considered to be suitable. However, to achieve not only high capacity but also high output and good charge / discharge cycle characteristics, it is important to have a greater abundance of large-diameter positive electrode active material particles in the first region R1 or the first layer than in the second region R1 or the second layer. Such a structure significantly increases the proportion of the pressed-in region and is advantageous for ensuring sufficient liquid circulation in the boundary region between the positive electrode current collector and the positive electrode mixture layer.

[0097] The secondary battery according to the present disclosure can provide a secondary battery that has high output, is advantageous in terms of increasing capacity, and improving charge / discharge cycle characteristics, and is useful as a main power source for mobile communication devices, electric vehicles, hybrid vehicles, portable electronic devices, etc.

[0098] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes 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 changes and modifications that do not depart from the true spirit and scope of the invention.

[0099] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 10: positive electrode, 11: positive electrode current collector, 12: positive electrode mixture layer, P1: first positive electrode active material particles, P2: second positive electrode active material particles

Claims

1. A positive electrode current collector, and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer contains first positive electrode active material particles having a first average particle diameter D1 and second positive electrode active material particles having a second average particle diameter D2, D1 > D2 is satisfied, and D1 is 10 μm or more, when the positive electrode mixture layer is divided into a first region and a second region having the same thickness, the second positive electrode active material particles are contained more in the second region than in the first region, the first region is closer to the positive electrode current collector than the second region, 50% or more of the surface of the positive electrode current collector is a press-fitting region where the first positive electrode active material particles are press-fitted, the press-fitting region is a region where the first positive electrode active material particles are press-fitted to a depth of 10% or more of D1, a positive electrode for a secondary battery.

2. The positive electrode for a secondary battery according to claim 1, wherein the first positive electrode active material particles are contained more in the first region than in the second region.

3. In a boundary region having a thickness of 2 × D1 from the surface of the positive electrode current collector, the volume ratio of the first positive electrode active material particles having a circular equivalent diameter of D1 or more to the total of the first positive electrode active material particles and the second positive electrode active material particles is 50% or more, the positive electrode for a secondary battery according to claim 1.

4. The positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the D1 / D2 ratio is 2 or more and 6 or less.

5. The positive electrode for a secondary battery according to any one of claims 1 to 3, wherein D2 is less than 10 μm.

6. The positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the thickness of the positive electrode mixture layer is 80 μm or more.

7. The positive electrode mixture layer is distinguished into a first layer having a different form from each other and other layers, and when the first layer is closest to the positive electrode current collector, the thickness of the first layer is 10 μm or more and 40 μm or less, the positive electrode for a secondary battery according to any one of claims 1 to 3.

8. The first positive electrode active material particles contain a first lithium transition metal composite oxide containing Ni, the ratio of Ni to the metal elements other than Li contained in the first lithium transition metal composite oxide is 50 atomic% or more, the second positive electrode active material particles contain a second lithium transition metal composite oxide containing Ni, the positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the ratio of Ni to the metal elements other than Li contained in the second lithium transition metal composite oxide is 50 atomic% or more and the ratio of Co is 0 atomic% or more and 10 atomic% or less.

9. A secondary battery comprising the positive electrode for a secondary battery according to any one of claims 1 to 3, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolytic solution.