Energy storage element

Abstract

One aspect of the present invention is a power storage element that comprises a flat electrode body and a container that houses the electrode body. The electrode body is formed by winding a positive electrode and a negative electrode with a separator therebetween and has two curved surface parts that are opposite each other. At least one of the positive electrode and the negative electrode has an active material layer that contains active material particles and a fibrous conducting agent, and the power storage element satisfies expression 1. In expression 1, X is the distance from the tip of one curved surface part to the inside surface of the container opposite the tip of said curved surface part in a winding axis direction view of the electrode body. R is the length of the outer circumference of the one curved surface part in a winding axis direction view of the electrode body. D is the average particle diameter D50 of the active material particles. A is the surface roughness Ra of the active material layer.　1: (R / D)A≥2X.

Description

[Technical Field]

[0001] The present invention relates to an energy storage element. [Background technology]

[0002] Chargeable and dischargeable energy storage elements (secondary batteries, capacitors, etc.) are used in a variety of devices, including electric vehicles and other vehicles, home appliances, and mobile phones. Known energy storage elements include a wound electrode assembly in which a strip-shaped positive electrode having a positive electrode active material layer containing positive electrode active material particles and a strip-shaped negative electrode having a negative electrode active material layer containing negative electrode active material particles are stacked together with a strip-shaped separator interposed therebetween and wound together (see Patent Documents 1 and 2). Such an electrode assembly is housed in a container together with an electrolyte to form an energy storage element. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-16440 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-191467 Summary of the Invention [Problem to be solved by the invention]

[0004] Electrodes (positive and negative electrodes) expand with repeated charge and discharge due to a decrease in the adhesive strength between active material particles and the generation of gas. When the electrodes expand, the active material particles separate from each other, reducing the conductivity of the active material layer and causing a decrease in discharge capacity. The expansion of the electrodes leads to the expansion of the electrode body. In particular, in the case of an energy storage device in which a flat, wound electrode body is housed in a prismatic container, the flat portions of the electrode body are generally not prone to expansion because they are in contact with the inner surface of the container, but the curved portions are prone to expansion, especially if they are not in contact with the inner surface of the container. This expansion of the curved portions can cause a decrease in capacity.

[0005] The present invention has been made based on the above circumstances, and its object is to provide an energy storage element having a flat, wound electrode body, which has a high capacity retention rate after charge / discharge cycles. [Means for solving the problem]

[0006] One aspect of the present invention is an energy storage element comprising a flat electrode body having two curved surfaces facing each other, the flat electrode body being formed by winding a positive electrode and a negative electrode with a separator interposed therebetween, and a container for accommodating the electrode body, wherein at least one of the positive electrode and the negative electrode has an active material layer containing active material particles and a fibrous conductive agent, and the energy storage element satisfies the following formula 1: (R / D)A≧2X 1 In Equation 1, X is the distance from the tip of one of the curved portions to the inner surface of the container opposite the tip of the curved portion when viewed in the direction of the winding axis of the electrode body. R is the outer periphery of one of the curved portions when viewed in the direction of the winding axis of the electrode body. D is the average particle diameter D50 of the active material particles. A is the surface roughness Ra of the active material layer.

[0007] Another aspect of the present invention is a storage element comprising a flat electrode body having two curved surfaces facing each other, the flat electrode body being formed by winding a positive electrode and a negative electrode with a separator interposed therebetween, and a container for accommodating the electrode body, wherein at least one of the positive electrode and the negative electrode has an active material layer containing active material particles and a fibrous conductive agent, and the element satisfies the following formula 2: A / D ≥ 0.2 2 In formula 2, D is the average particle diameter D50 of the active material particles, and A is the surface roughness Ra of the active material layer. [Effects of the Invention]

[0008] According to one embodiment of the present invention, it is possible to provide an energy storage element including a flat wound electrode body, which has a high capacity retention rate after charge / discharge cycles. [Brief explanation of the drawings]

[0009] [Figure 1]FIG. 1 is a schematic perspective view showing an energy storage device according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view of the energy storage device of FIG. 1 taken along the line II. [Figure 3A] FIG. 3A is a first schematic diagram showing the state of the surface of an active material layer of an energy storage element according to one embodiment of the present invention. [Figure 3B] FIG. 3B is a second schematic diagram showing the state of the surface of the active material layer of an energy storage element according to one embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram showing an energy storage device configured by assembling a plurality of energy storage elements according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, an overview of the energy storage element disclosed in this specification will be described.

[0011] A storage element according to one embodiment of the present invention is a storage element (α) that is formed by winding a positive electrode and a negative electrode with a separator interposed therebetween, and includes a flat electrode body having two curved surfaces facing each other, and a container that houses the electrode body, and at least one of the positive electrode and the negative electrode has an active material layer containing active material particles and a fibrous conductive agent, and satisfies the following formula 1. (R / D)A≧2X 1 In Equation 1, X is the distance from the tip of one of the curved portions to the inner surface of the container opposite the tip of the curved portion when viewed in the direction of the winding axis of the electrode body. R is the outer periphery of one of the curved portions when viewed in the direction of the winding axis of the electrode body. D is the average particle diameter D50 of the active material particles. A is the surface roughness Ra of the active material layer.

[0012] An energy storage element (α) according to one embodiment of the present invention is an energy storage element including a flat, wound electrode assembly, and exhibits a high capacity retention rate after charge-discharge cycling. The reason for this effect is unclear, but the following reason is presumed. One of the reasons for the low capacity retention rate after charge-discharge cycling in energy storage elements including a conventional flat, wound electrode assembly, as described above, is a decrease in the conductivity of the active material layer due to expansion of the curved surface of the electrode assembly. In contrast, in the energy storage element (α), the active material layer contains a fibrous conductive agent along with active material particles, and satisfying formula 1 suppresses the decrease in conductivity of the active material layer in the curved surface, presumably resulting in an improved capacity retention rate. Formula 1 and other factors are described in detail below. Note that the following length and other factors are described as the length and other factors when viewed in the direction of the winding axis of the electrode assembly (see FIG. 2). In the left side of Equation 1, (R / D)A, R is the perimeter (unit: mm) of the curved portion 8a (see FIG. 2) of the electrode body 1. D is the average particle diameter D50 (unit: μm) of the active material particles in the active material layer. Therefore, R / D represents the number of active material particles in the active material layer along the length of the perimeter of the curved portion. A is the surface roughness Ra (unit: μm) of the active material layer, which represents the degree of unevenness on the active material layer surface. Therefore, the product (R / D)A of R / D and A represents the degree of the length of the active material layer surface along the surface unevenness on the perimeter of the curved portion. In other words, even if the perimeter length of the curved portion is the same, if the height of the unevenness on the active material layer surface (A: surface roughness Ra) is large or if the spacing between the unevenness on the active material layer surface (D: average particle diameter D50 of the active material particles) is small, the length of the active material layer surface along the surface unevenness will be longer. As shown schematically in FIG. 3A, the fibrous conductive agent 12 present on the active material layer surface 11 contacts the active material particles 13 along their surfaces, electrically connecting the active material particles 13. It is believed that the fibrous conductive agent 12 bends along the irregularities of the active material layer surface 11, i.e., along the shapes of the active material particles 13 present on the active material layer surface 11, to contact the active material particles 13, thereby ensuring electrical conductivity between the active material particles 13. Even when the curved surface of the electrode body expands and the spacing between the active material particles 13 widens as shown schematically in FIG. 3B, the fibrous conductive agent 12 can stretch to a straight state and contact the active material particles 13, thereby ensuring electrical conductivity between the active material particles 13. In other words, it is believed that the fibrous conductive agent 12 can stretch like a bellows while in contact with the active material particles 13. In this case, the longer the length of the active material layer surface along the surface irregularities at the outer periphery of the curved portion of the electrode body described above, i.e., (R / D)A, the longer the length that the fibrous conductive agent can stretch, i.e., the more the decrease in conductivity between the active material particles can be suppressed even if the fibrous conductive agent expands. Meanwhile, with regard to the right-hand side 2X in Equation 1, X is the distance (unit: mm) from the tip of the curved portion 8a of the electrode body 1 to the inner surface of the container 2 facing the tip of this curved portion 8a, as shown in Figure 2. Then, when the curved portion 8a expands and the periphery of the curved portion 8a extends by 2X, it can be assumed that the tip of the curved portion 8a comes into contact with the inner surface of the container 2, and further expansion is suppressed. In this way, 2X indicates a rough guide to the upper limit of extension of the periphery of the curved portion. In other words, the above formula 1 indicates that even when the curved surface of the electrode body expands to the point where it contacts the inner surface of the container due to repeated charge and discharge, the fibrous conductive agent can suppress a decrease in conductivity between the active material particles.For these reasons, it is presumed that the storage element (α) will have an increased capacity retention rate after charge and discharge cycles.

[0013] Here, the "curved surface portion" of the electrode body refers to the approximately semicircular portions located at both ends when viewed in the direction of the winding axis. Specifically, when the thickness of the electrode body is T, the curved surface portion is the region extending from both ends of the electrode body to a length of T / 2 when viewed in the direction of the winding axis (see FIG. 2). In this case, R (the outer periphery of one of the curved surface portions) is πT / 2. The term "fibrous conductive agent" refers to a conductive agent having a deformable, elongated shape. The ratio of the length to the diameter of the fibrous conductive agent is, for example, 10 or more. The diameter and length of the fibrous conductive agent are values ​​measured in an image of the surface of the active material layer taken with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Regarding the above X, if the distances from the tips of the two curved portions to the inner surface of the container opposite these tips are different, the shorter distance is defined as X. For example, in the energy storage device 100 of FIG. 2, the distance from the tip of the upper curved portion 8a to the inner surface of the container 2 opposite the tip of the curved portion 8a is defined as X. Furthermore, if there is another member (such as a spacer) between the tip of the curved portion of the electrode assembly and the container, X is defined as the distance from the tip of the curved portion to the surface of the other member. In other words, if the energy storage device contains other members other than the electrode assembly inside the container, the "container" in the definition of X above also includes other members other than the electrode assembly inside the container. Note that the other members do not include the electrolyte. "Average particle diameter D50 (D) of active material particles" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50%, based on the particle size distribution measured by laser diffraction / scattering method in accordance with JIS-Z-8825 (2013) for a solution in which particles are diluted with a solvent. "Surface roughness Ra (A) of the active material layer" refers to the arithmetic mean roughness measured in accordance with JIS B0601:2013. The sample used to measure "surface roughness Ra (A) of the active material layer" is prepared according to the following procedure. First, the energy storage element (secondary battery) is charged at a constant current of 0.05 C until the end-of-charge voltage for normal use is reached, and the battery is fully charged. After a 30-minute rest, the battery is discharged at a constant current of 0.05 C until the lower limit voltage for normal use is reached. The battery is then disassembled, the electrodes are removed, and a test battery is assembled using metallic Li as the counter electrode. If the electrode is a positive electrode, the current is 10 mA per 1 g of positive electrode active material, and the positive electrode potential is 2.0 V vs. Li / Li. + If the electrode is a negative electrode, discharge at a constant current until the negative electrode potential reaches 1.5 V vs. Li / Li at a current value of 10 mA per 1 g of negative electrode active material. + Discharge at a constant current until the value reaches 0.05. Disassemble the battery again and remove the electrode. Use dimethyl carbonate to thoroughly wash off any non-aqueous electrolyte adhering to the removed electrode, then dry it at room temperature for 24 hours to use as a sample for measuring surface roughness Ra.

[0014] Another aspect of the present invention provides a storage element (β) that is formed by winding a positive electrode and a negative electrode with a separator interposed therebetween, and includes a flat electrode body having two curved surfaces facing each other, and a container that houses the electrode body, and at least one of the positive electrode and the negative electrode has an active material layer containing active material particles and a fibrous conductive agent, and satisfies the following formula 2: A / D ≥ 0.2 2 In formula 2, D is the average particle diameter D50 of the active material particles, and A is the surface roughness Ra of the active material layer.

[0015] An energy storage device (β) according to one embodiment of the present invention includes a flat, wound electrode assembly and exhibits a high capacity retention rate after charge-discharge cycling. While the reason for this effect is unclear, the following is presumed. A / D, calculated by dividing the surface roughness Ra of the active material layer by the average particle diameter D50 of the active material particles, represents the extent of the length of the active material layer surface along the surface irregularities per unit length of the outer periphery of the curved portion of the electrode assembly. In other words, the larger A / D, the more the fibrous conductive agent in contact with the surface irregularities of the active material layer is sufficiently bent and therefore has a longer stretchable length. Furthermore, as shown in Equation 2 above, if A / D is 0.2 or greater, even when the curved portion of the electrode assembly expands and the spacing between the active material particles widens, the fibrous conductive agent sufficiently stretches like a bellows, sufficiently suppressing a decrease in conductivity between the active material particles. Therefore, it is presumed that the energy storage device (β) exhibits a high capacity retention rate after charge-discharge cycling.

[0016] It is preferable that the electricity storage element (β) further satisfies the following formula 1. (R / D)A≧2X 1 In Equation 1, X is the distance from the tip of one of the curved portions to the inner surface of the container opposite the tip of the curved portion when viewed in the direction of the winding axis of the electrode body. R is the outer periphery of the one of the curved portions when viewed in the direction of the winding axis of the electrode body. D and A are synonymous with D and A in Equation 2.

[0017] When the electricity storage element (β) further satisfies the above formula 1, the capacity retention rate after charge-discharge cycling is further increased for the same reason as in the case of the electricity storage element (α).

[0018] It is preferable that the electric storage element (α) and the electric storage element (β) further satisfy the following formula 3. 0.06≦X / R≦0.12 3 In formula 3, X and R have the same meanings as X and R in formula 1.

[0019] When X / R is small, i.e., when the distance X from the tip of the curved portion of the electrode body to the inner surface of the container is relatively short, the curved portion of the electrode body tends to come into contact with the inner surface of the container due to expansion during charge / discharge cycles, and the curved portion is likely to be compressed. Therefore, when the energy storage element (α) and the energy storage element (β) are used in a lithium ion energy storage element, the separator is likely to be compressed at the curved portion, resulting in an increase in resistance. On the other hand, when the distance X from the tip of the curved portion to the inner surface of the container is relatively long, the space within the container that does not contribute to charge / discharge becomes large, and the volumetric energy density of the energy storage element decreases. Therefore, when the energy storage element (α) and the energy storage element (β) further satisfy the above formula 3, the performance as an energy storage element is further improved.

[0020] The content of the fibrous conductive agent in the active material layer is preferably 0.01% by mass or more. In this case, even when the curved surface of the electrode body expands, the presence of a sufficient amount of the fibrous conductive agent can particularly sufficiently suppress a decrease in conductivity between the active material particles, thereby further improving the capacity retention rate after charge-discharge cycling.

[0021] The average length of the fibrous conductive agent is preferably greater than the average particle diameter D50 of the active material particles. In this case, when the curved surface of the electrode body expands and the spacing between the active material particles increases, the fibrous conductive agent having a sufficient length can particularly sufficiently suppress a decrease in conductivity between the active material particles, thereby further increasing the capacity retention rate after charge-discharge cycling.

[0022] The "average length" of the fibrous conductive agent is the average length of any 10 pieces of fibrous conductive agent observed with an SEM or TEM.

[0023] The active material particles preferably exist in the form of secondary particles having a ratio of the average particle diameter D50 to the primary particle diameter of 3 or less, or in the form of substantially unagglomerated primary particles. When such active material particles are used, an active material layer having a relatively large surface roughness Ra relative to the average particle diameter D50 is formed, making it easier to obtain an energy storage element that satisfies Formulas 1 and 2.

[0024] The "primary particle diameter" of an active material particle is the average particle diameter of any 50 primary particles that constitute the active material particle observed under SEM. A primary particle is a particle that does not have visible grain boundaries when observed under SEM. The particle diameter of a primary particle is determined as follows: The shortest diameter passing through the center of the smallest circumscribing circle of the primary particle is defined as the minor diameter, and the diameter passing through the center and perpendicular to the minor diameter is defined as the major diameter. The particle diameter of the primary particle is defined as the average of the major and minor diameters. If there are two or more shortest diameters, the longest diameter that intersects at right angles is defined as the minor diameter. The phrase "active material particles exist in the form of substantially unagglomerated primary particles" means that when the active material layer is observed with an SEM, or when active material particles are collected from the active material layer and the binder and conductive agent are removed and the active material particles are observed with an SEM, a plurality of primary particles exist independently without agglomeration, or the primary particles are not generally directly bonded to other primary particles.

[0025] The positive electrode preferably contains the active material particles and the fibrous conductive agent. The active material particles of the positive electrode usually have lower conductivity than the active material particles of the negative electrode, and a decrease in conductivity in the positive electrode has a large effect on the capacity retention rate. Therefore, by suppressing the decrease in conductivity in the positive electrode, the capacity retention rate after charge-discharge cycling can be effectively improved.

[0026] An energy storage device according to one embodiment of the present invention will be described in detail below.

[0027] <Energy storage element> An energy storage element 100 according to one embodiment of the present invention, shown in FIGS. 1 and 2, includes an electrode assembly 1, an electrolyte (not shown), and a container 2 for accommodating these. Note that FIGS. 1 and 2 do not limit the orientation of the energy storage element 100 when in use. For example, the energy storage element 100 of FIGS. 1 and 2 may be used upside down or upside down. The energy storage element 100 is a secondary battery, which is an example of an energy storage element. The electrode assembly 1 is a flat, wound electrode assembly including a positive electrode, a negative electrode, and a separator. The specific structure of the electrode assembly 1 will be described later. The energy storage element 100 further includes a positive electrode connecting member 3, a positive electrode external terminal 4, a negative electrode connecting member 5, and a negative electrode external terminal 6. The positive electrode of the electrode assembly 1 is electrically connected to the positive electrode external terminal 4 via the positive electrode connecting member 3. The negative electrode of the electrode assembly 1 is electrically connected to the negative electrode external terminal 6 via the negative electrode connecting member 5.

[0028] (electrode body) The electrode assembly 1 has a positive electrode, a negative electrode, and a separator, and the positive electrode and the negative electrode are stacked together with the separator interposed between them. The electrode assembly 1 is a flat, wound electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are stacked together with the strip-shaped separator interposed between them and wound.

[0029] The electrode assembly 1 is flat and has a flat portion 7 where the positive electrode, negative electrode, and separator are stacked substantially parallel to one another, and two curved portions 8 (8a, 8b) where the positive electrode, negative electrode, and separator are stacked in a curved state (see FIG. 2). The curved portion 8 may also be referred to as a curved portion, etc. The two curved portions 8a, 8b are positioned so as to face each other.

[0030] The outer circumferential length R of curved surface portion 8a and the outer circumferential length R' of curved surface portion 8b when viewed in the direction of the winding axis of electrode body 1 (FIG. 2) are set appropriately depending on the size of energy storage element 100, and may be, for example, 5 mm or more and 100 mm or less, or 10 mm or more and 50 mm or less. Typically, outer circumferential length R of curved surface portion 8a and outer circumferential length R' of curved surface portion 8b are substantially equal.

[0031] (container) The container 2 is a sealed container that houses the electrode assembly 1 and the like and has an electrolyte sealed therein. The container 2 may be made of any material, such as resin or metal, as long as it has the sealing properties to seal in the electrolyte and the strength to protect the electrode assembly 1.

[0032] The container 2 is a rectangular container, and the outer surface of the flat portion 7 of the electrode assembly 1 is in contact with the inner surface of the container 2. That is, the thickness T of the electrode assembly 1 and the inner dimensions of the container 2 may be substantially the same (see FIG. 2).

[0033] 2 of the electrode body 1 is spaced apart from the inner surface of the container 2. The distance X from the tip of the curved surface 8a of the electrode body 1 as viewed in the direction of the winding axis (FIG. 2) to the inner surface of the container 2 facing the tip of the curved surface 8a is set appropriately depending on the size of the energy storage element 100, etc., and may be, for example, 0.1 mm or more and 10 mm or less, or 0.5 mm or more and 3 mm or less.

[0034] There is also a distance between the lower curved surface portion 8b of the electrode body 1 in FIG. 2 and the inner surface of the container 2. In this embodiment, the distance X' from the tip of the curved surface portion 8b to the opposing inner surface of the container 2 is greater than the above-mentioned distance X. The distance X' may be, for example, within the same range as the above-mentioned distance X.

[0035] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer laminated on the positive electrode substrate directly or via an intermediate layer.

[0036] The positive electrode substrate is electrically conductive. Whether or not it is electrically conductive is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 7The threshold value is Ω·cm. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum and aluminum alloys are preferred because of their balance of high potential resistance, high conductivity, and cost. The positive electrode substrate may be formed in the form of foil or vapor-deposited film, with foil being preferred from a cost perspective. In other words, aluminum foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0037] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the energy storage device. The "average thickness" of the positive electrode substrate and the negative electrode substrate described below refers to the value obtained by dividing the punched mass when a substrate of a predetermined area is punched out by the true density and punched area of ​​the substrate.

[0038] The intermediate layer is a coating layer on the surface of the positive electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles.

[0039] The positive electrode active material layer is formed from a so-called positive electrode mixture containing positive electrode active material particles. The positive electrode active material layer preferably contains the above-mentioned positive electrode active material particles and a fibrous conductive agent. The positive electrode mixture forming the positive electrode active material layer further contains optional components such as a binder, a thickener, and a filler as needed. Note that, in this embodiment, the positive electrode active material layer will be described mainly as containing a fibrous conductive agent, but the positive electrode active material layer may not contain a fibrous conductive agent and the negative electrode active material layer may contain a fibrous conductive agent.

[0040] The material (type) of the positive electrode active material constituting the positive electrode active material particles can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, lithium transition metal composite oxides having a spinel type crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. Examples of the lithium transition metal composite oxides having an α-NaFeO2 type crystal structure include Li[Li x Ni 1-x ]O2(0≦x<0.5), Li[Li x Ni γ Co 1-x-γ ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co 1-x ]O2(0≦x<0.5), Li[Li x Ni γ Mn 1-x-γ ]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co 1-x-γ-β ]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li x Ni γ Co β Al 1-x-γ-β ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Lithium transition metal composite oxides with spinel crystal structure include Li x Mn2O4, Li x Ni γ Mn 2-γ Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements.

[0041] The positive electrode active material is preferably a lithium transition metal composite oxide, more preferably a lithium transition metal composite oxide containing nickel, cobalt, and manganese or aluminum, and even more preferably a lithium transition metal composite oxide containing nickel, cobalt, and manganese. This lithium transition metal composite oxide preferably has an α-NaFeO2 crystal structure. Use of such a lithium transition metal composite oxide can increase energy density. Furthermore, high-energy-density energy storage devices equipped with a wound electrode assembly using such a positive electrode active material are prone to expansion of the curved surface due to repeated charge and discharge. Therefore, when the present invention is applied to an energy storage device equipped with a wound electrode assembly having such a positive electrode active material, the advantage of increased capacity retention after charge and discharge cycles is particularly effectively obtained.

[0042] The positive electrode active material may be used singly or in combination of two or more. In particular, the positive electrode active material preferably contains a lithium transition metal composite oxide in a proportion of 50 mass % or more (preferably 70 to 100 mass %, more preferably 80 to 100 mass %) of the total positive electrode active material used, and it is more preferable to use a positive electrode active material consisting essentially of a lithium transition metal composite oxide.

[0043] The average particle diameter D50 of the positive electrode active material particles is, for example, preferably 0.1 μm or more and 20 μm or less, more preferably 1 μm or more and 12 μm or less, and even more preferably 3 μm or more and 8 μm or less. By setting the average particle diameter D50 of the positive electrode active material particles within the above range, the conductivity of the positive electrode active material layer is improved, and the capacity retention rate after charge-discharge cycling tends to be further increased. In particular, by setting the average particle diameter D50 of the positive electrode active material particles (D in Equations 1 and 2) to the above upper limit or less, (R / D)A and A / D increase, and even if the positive electrode active material particles are spaced apart, a decrease in the conductivity between the positive electrode active material particles can be sufficiently suppressed, thereby further increasing the capacity retention rate after charge-discharge cycling of the energy storage device.

[0044] The positive electrode active material particles preferably exist in the form of secondary particles having a ratio of the average particle diameter D50 to the primary particle diameter of 3 or less, or substantially unagglomerated primary particles. Hereinafter, "secondary particles having a ratio of the average particle diameter D50 to the primary particle diameter of 3 or less, or substantially unagglomerated primary particles" will also be referred to as "single particle particles." The ratio of the average particle diameter D50 to the primary particle diameter of the secondary particles is preferably 2 or less, more preferably 1.5 or less, and even more preferably 1.2 or less. The lower limit of the ratio of the average particle diameter D50 to the primary particle diameter of the secondary particles may be 1. Note that, due to differences in the methods for measuring the primary particle diameter and the average particle diameter D50, the lower limit of the ratio of the average particle diameter D50 to the primary particle diameter of the secondary particles may be less than 1, for example, 0.9.

[0045] The positive electrode active material particles may contain positive electrode active material particles other than the monoparticle particles. However, the content of the monoparticle particles relative to all the positive electrode active material particles contained in the positive electrode active material layer is preferably 80 mass% or more, more preferably 90 mass% or more, even more preferably 99 mass% or more, and even more preferably substantially 100 mass%. That is, it is particularly preferable to use substantially only monoparticle particles as the positive electrode active material particles in the positive electrode. In such a case, it becomes easier to obtain an energy storage element that satisfies Formula 1 and Formula 2, and as a result, the capacity retention rate of the energy storage element after charge / discharge cycling can be further increased.

[0046] Positive electrode active material particles, which are single-particle particles, can be produced by known methods, and the primary particle size and average particle size D50 can be controlled by the production conditions. Alternatively, commercially available positive electrode active material particles may be used. In the production process of the active material, the particle size can be increased by growing multiple primary particles, for example, by increasing the firing temperature or prolonging the firing time. Alternatively, primary particles can be obtained by crushing secondary particles.

[0047] The content of the positive electrode active material particles in the positive electrode active material layer is preferably 80% by mass to 99% by mass, more preferably 85% by mass to 98% by mass, and even more preferably 90% by mass to 97% by mass. By setting the content of the positive electrode active material particles in the positive electrode active material layer within this range, the conductivity and energy density of the positive electrode active material layer can be increased in a balanced manner.

[0048] Examples of the fibrous conductive agent include fibrous metal, fibrous conductive resin, and fibrous carbon, with fibrous carbon being preferred. Fibrous carbon refers to a fibrous conductive agent that is a carbonaceous material. Examples of fibrous carbon (a fibrous conductive agent that is a carbonaceous material) include carbon nanofiber, pitch-based carbon fiber, and carbon nanotube (CNT), with graphene-based carbon being preferably used as CNT.

[0049] The average diameter of the fibrous conductive agent is preferably 1 nm or more and 300 nm or less, more preferably 3 nm or more and 100 nm or less, even more preferably 4 nm or more and 50 nm or less, and even more preferably 5 nm or more and 30 nm or less. When the average diameter of the fibrous conductive agent is within the above range, the conductivity of the positive electrode active material layer is improved, and the capacity retention rate after charge / discharge cycling is further increased. Note that the "average diameter" of the fibrous conductive agent is the average value of the diameters of any 10 fibrous conductive agents observed by SEM or TEM.

[0050] The average length of the fibrous conductive agent is preferably greater than the average particle diameter D50 of the positive electrode active material particles, and more preferably at least two, three, or even five times the average particle diameter D50 of the positive electrode active material particles. By using a fibrous conductive agent that is sufficiently long compared to the size of the positive electrode active material particles, even when the curved surface portion 8a of the electrode body 1 expands and the spacing between the positive electrode active material particles increases, the fibrous conductive agent can effectively suppress a decrease in conductivity between the positive electrode active material particles, thereby further improving the capacity retention rate after charge-discharge cycling. Specifically, the average length of the fibrous conductive agent is preferably from 1 μm to 1,000 μm, more preferably from 10 μm to 600 μm, and even more preferably from 20 μm to 300 μm.

[0051] The average aspect ratio of the fibrous conductive agent (ratio of average length to average diameter) is, for example, preferably 10 or more and 10,000 or less, more preferably 100 or more, and even more preferably 1,000 or more. By using a fibrous conductive agent with an average aspect ratio within the above range, the capacity retention rate after charge-discharge cycles can be further improved.

[0052] When the fibrous conductive material is fibrous carbon, the average lattice spacing (d 002 ) of the fibrous carbon is preferably less than 0.340 nm. 002 When the average lattice spacing (d 002 The lower limit of the (002) angle can be, for example, 0.330 nm. The half width (002) of the peak corresponding to the (002) plane of the fibrous carbon measured by X-ray diffraction is, for example, 0.5° or more. The half width (002) of the fibrous carbon is preferably less than 0.7°.

[0053] The fibrous carbon can be obtained by, for example, forming a polymer into fibers by a spinning method or the like and then heat treating the fibers in an inert atmosphere, or by a vapor growth method in which an organic compound is reacted at high temperature in the presence of a catalyst. The fibrous carbon is preferably fibrous carbon obtained by a vapor growth method (vapor-grown fibrous carbon). Commercially available fibrous carbon and other fibrous conductive agents can be used.

[0054] The content of the fibrous conductive agent in the positive electrode active material layer is, for example, preferably 0.01% by mass or more and 5% by mass or less, preferably 0.1% by mass or more and 4% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less, and even more preferably 1.0% by mass or more and 2.5% by mass or less. By setting the content of the fibrous conductive agent to the above lower limit or more, the capacity retention rate after charge / discharge cycling can be further increased. On the other hand, by setting the content of the fibrous conductive agent to the above upper limit or less, the capacity retention rate after charge / discharge cycling can be sufficiently increased while the content of the positive electrode active material particles can be relatively increased, thereby increasing the energy density. Furthermore, by setting the content of the fibrous conductive agent to the above upper limit or less, production costs can also be reduced.

[0055] The fibrous conductive agent preferably exists along the entire length of the curved surface portion 8a of the electrode assembly 1 as viewed in the direction of the winding axis, forming a network of fibrous conductive agent along the shape of the positive electrode active material particles present on the surface of the positive electrode active material layer. That is, the surface of the positive electrode active material layer in the curved surface portion 8a of the electrode assembly 1 is preferably provided with conductivity by a plurality of fibrous conductive agents from one end of the periphery of the curved surface portion 8a to the other end as viewed in the direction of the winding axis. Furthermore, the fibrous conductive agent is preferably oriented in the length direction of the strip-shaped positive electrode (positive electrode active material layer), i.e., in the length direction of the periphery of the curved surface portion 8a. This facilitates elongation of the fibrous conductive agent when the curved surface portion 8a expands, thereby maintaining sufficient conductivity. The fibrous conductive agent can be oriented in a desired direction by adjusting the coating and pressing directions.

[0056] The positive electrode active material layer preferably further contains a conductive agent other than the fibrous conductive agent. Examples of the other conductive agent include a granular conductive agent. The granular conductive agent refers to a conductive agent having a shape in which the ratio of the major axis to the minor axis is, for example, 1 or more and less than 10. The minor axis and major axis of the granular conductive agent are values ​​measured in an SEM or TEM image of the surface of the positive electrode active material layer. The shortest diameter passing through the center of the smallest circumscribing circle of the granular conductive agent is defined as the minor axis, and the diameter passing through the center and perpendicular to the minor axis is defined as the major axis. When there are two or more shortest diameters, the diameter perpendicular to the minor axis is defined as the longest diameter. The granular conductive agent may be a granular conductive agent that does not substantially deform.

[0057] Examples of granular conductive agents include granular metals, granular conductive resins, granular conductive ceramics, and granular carbon, with granular carbon being preferred. Granular carbon refers to a granular conductive agent that is a carbonaceous material. Examples of granular carbon (a granular conductive agent that is a carbonaceous material) include graphite carbon, non-graphite carbon, and graphene-based carbon. Examples of non-graphite carbon include carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene and fullerene. Among these, non-graphite carbon is preferred, and carbon black is more preferred.

[0058] The granular conductive agent is preferably composed of primary particles, and these primary particles are preferably present in an aggregated state. The average particle diameter of the primary particles of the granular conductive agent is, for example, 10 nm or more and 500 nm or less, and more preferably 20 nm or more and 100 nm or less. By using a granular conductive agent of this size, the gaps between the positive electrode active material particles can be effectively filled, further increasing the conductivity of the positive electrode active material layer and the capacity retention rate after charge-discharge cycling. The "average particle diameter" of the granular conductive agent is the average particle diameter of any 10 granular conductive agent particles observed by SEM or TEM. The particle diameter of the granular conductive agent is the average of the major axis and the minor axis.

[0059] The content of the granular conductive agent in the positive electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 6% by mass or less. By setting the content of the granular conductive agent in the positive electrode active material layer to be equal to or greater than the above-mentioned lower limit, the conductivity of the positive electrode active material layer can be further increased, and the capacity retention rate after charge-discharge cycling can be further increased. On the other hand, by setting the content of the granular conductive agent to be equal to or less than the above-mentioned upper limit, the content of the positive electrode active material particles can be relatively increased, and the energy density can be increased.

[0060] The binder may be a solvent-based binder or a water-based binder, with the solvent-based binder being preferred. The solvent-based binder is a binder that disperses or dissolves in an organic solvent.

[0061] Examples of solvent-based binders include fluororesins (such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)), thermoplastic resins such as polyethylene, polypropylene, and polyimide, copolymers of vinylidene fluoride and hexafluoropropylene, copolymers of ethylene and vinyl alcohol, polyacrylonitrile, polyphosphazene, polysiloxane, polyvinyl acetate, polymethyl methacrylate, polystyrene, polycarbonate, polyamide, polyamideimide, crosslinked polymers of cellulose and chitosan pyrrolidone carboxylate, and derivatives of chitin or chitosan, with fluororesins being preferred and PVDF being more preferred. One or more types of binders can be used.

[0062] The content of the binder in the positive electrode active material layer is preferably 0.3% by mass to 10% by mass, more preferably 0.5% by mass to 8% by mass, and even more preferably 5% by mass or less. By setting the content of the binder to the above lower limit or more, the positive electrode active material particles can be stably held. Furthermore, by setting the content of the binder to the above upper limit or less, the content of the positive electrode active material particles can be increased, thereby increasing the energy density.

[0063] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. Furthermore, when the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like. When a thickener is used, the content of the thickener in the positive electrode active material layer is preferably 5% by mass or less, and more preferably 1% by mass or less. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a thickener.

[0064] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene; inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; carbonates such as calcium carbonate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate; nitrides such as aluminum nitride and silicon nitride; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and artificial products thereof. When a filler is used, the content of the filler in the positive electrode active material layer is preferably 5% by mass or less, and more preferably 1% by mass or less. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0065] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material particles, conductive agent (such as a fibrous conductive agent), binder, thickener, and filler.

[0066] The surface roughness Ra of the positive electrode active material layer is preferably 0.5 μm or more and 3.0 μm or less, more preferably 1.0 μm or more and 2.0 μm or less, and even more preferably 1.3 μm or more. By setting the surface roughness Ra (A in Equations 1 and 2) of the positive electrode active material layer at or above the lower limit, (R / D)A and A / D become large, and even if the positive electrode active material particles are spaced apart, a decrease in conductivity between the positive electrode active material particles can be sufficiently suppressed, thereby further improving the capacity retention rate after charge / discharge cycling of the energy storage device. The surface roughness Ra of the positive electrode active material layer can be adjusted by the type and size of the positive electrode active material particles, as well as by the pressing and pressure applied to the positive electrode active material layer during the manufacturing process.

[0067] (Regarding Equation 1) In the energy storage device 100 according to one embodiment of the present invention, the following formula 1 is satisfied. (R / D)A≧2X 1 In Equation 1, X is the distance from the tip of one curved surface portion 8a to the inner surface of the container 2 facing the tip of the curved surface portion 8a when viewed in the direction of the winding axis of the electrode body 1 (FIG. 2). R is the outer periphery of one curved surface portion 8a when viewed in the direction of the winding axis of the electrode body 1. D is the average particle diameter D50 of the positive electrode active material particles. A is the surface roughness Ra of the positive electrode active material layer.

[0068] (R / D)A is preferably 3X or greater, more preferably 3.5X or greater, and even more preferably 3.8X or greater. By making (R / D)A larger relative to X, even when the tip of the curved portion 8a of the electrode body 1 expands to the point where it contacts the inner surface of the container 2, the fibrous conductive agent can be sufficiently stretched to suppress a decrease in conductivity between the positive electrode active material particles, thereby increasing the capacity retention rate of the energy storage element 100 after charge-discharge cycles.

[0069] On the other hand, (R / D)A is preferably, for example, 20X or less, and may be more preferably 15X or less or 10X or less. By setting (R / D)A to the above upper limit or less, the space in the container 2 where the electrode body 1 is not disposed becomes relatively small, and the energy density of the energy storage element 100 can be increased, for example.

[0070] (Regarding Equation 2) In the energy storage device 100 according to one embodiment of the present invention, the following formula 2 is satisfied. A / D ≥ 0.2 2 In formula 2, D is the average particle diameter D50 of the positive electrode active material particles, and A is the surface roughness Ra of the positive electrode active material layer.

[0071] A / D is preferably 0.25 or more, and more preferably 0.26 or more. By increasing A / D, even when the curved surface portion 8a of the electrode body 1 expands, the fibrous conductive agent is sufficiently stretched, thereby suppressing a decrease in conductivity between the positive electrode active material particles, thereby improving the capacity retention rate after charge / discharge cycles of the energy storage device 100. On the other hand, A / D may be, for example, 0.5 or less, 0.4 or less, or 0.30 or less.

[0072] (Regarding Equation 3) In the energy storage device 100 according to one embodiment of the present invention, it is preferable to satisfy the following formula 3. 0.06≦X / R≦0.12 3 In Equation 3, X is the distance from the tip of one curved surface portion 8a to the inner surface of the container 2 facing the tip of the curved surface portion 8a when viewed in the direction of the winding axis of the electrode body 1. R is the length of the outer periphery of one curved surface portion 8a when viewed in the direction of the winding axis of the electrode body 1.

[0073] X / R is more preferably 0.07 or more. By setting X / R to the above-mentioned lower limit or more, even when the curved surface portion 8a expands, it becomes difficult for the curved surface portion 8a to come into contact with the inner surface of the container 2, and it is possible to suppress an increase in resistance due to compression of the separator at the curved surface portion 8a.

[0074] On the other hand, X / R is more preferably 0.10 or less, and even more preferably 0.08 or less. By setting X / R to the above upper limit or less, the space within the container 2 that does not contribute to charging and discharging is reduced, and the energy density is increased.

[0075] Furthermore, with respect to the relationship between the distance X' from the tip of the other curved surface portion 8b to the inner surface of the container 2 facing the tip of the curved surface portion 8b as viewed in the direction of the winding axis of the electrode body 1 and the outer circumferential length R' of the other curved surface portion 8b as viewed in the direction of the winding axis of the electrode body 1, X' / R' is preferably 0.12 or less, more preferably 0.10 or less, and even more preferably 0.08 or less. By setting X' / R' to the above upper limit or less, the space within the container 2 that does not contribute to charge and discharge is reduced, thereby increasing the energy density. Furthermore, even when X is replaced with X' and R is replaced with R' in the above formula 1 or 3, the above formula 1 or 3 is preferably satisfied. In this case, both of the two curved surface portions 8a, 8b are in a favorable state, and the capacity retention rate after charge and discharge cycles of the energy storage device 100 is improved.

[0076] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode active material layer laminated on the negative electrode substrate directly or via an intermediate layer, which may have the same structure as the intermediate layer of the positive electrode.

[0077] The negative electrode substrate can have the same structure as the positive electrode substrate, but is made of a metal such as copper, nickel, stainless steel, or nickel-plated steel, or an alloy thereof, with copper or a copper alloy being preferred. That is, copper foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0078] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the energy storage element.

[0079] The negative electrode active material layer is generally formed from a so-called negative electrode mixture containing a negative electrode active material. The negative electrode mixture forming the negative electrode active material layer also contains optional components such as a conductive agent, binder, thickener, and filler as needed. The optional components, such as the conductive agent, binder, thickener, and filler, can be the same as those used in the positive electrode active material layer. In one embodiment of the present invention, the conductive agent used in the negative electrode active material layer may be one or both of a fibrous conductive agent and a granular conductive agent, or may be other conductive agents, but may also be a fibrous conductive agent. The negative electrode active material layer may also be a layer essentially consisting of a negative electrode active material, such as metallic Li.

[0080] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0081] The negative electrode active material can be appropriately selected from known negative electrode active materials. For example, materials capable of absorbing and releasing lithium ions are usually used as negative electrode active materials for lithium ion secondary batteries. Examples of negative electrode active materials include metal Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、 Examples of the material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0082] "Graphite" refers to a graphite material that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0083] "Non-graphitic carbon" refers to carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0084] Here, the "discharged state" of a carbon material refers to a state in which lithium ions that can be absorbed and released during charging and discharging have been sufficiently released from the carbon material, which is the negative electrode active material. For example, this refers to a state in which the open circuit voltage of a half cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode is 0.7 V or higher.

[0085] "Non-graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.36 nm or more and 0.42 nm or less.

[0086] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0087] The negative electrode active material may be present as negative electrode active material particles. In this case, the average particle size of the negative electrode active material particles may be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is, for example, a carbon material, a titanium-containing oxide, a polyphosphate compound, or the like, the average particle size D50 may be preferably 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size D50 may be preferably 1 nm or more and 1 μm or less. By setting the average particle size D50 of the negative electrode active material particles to the above lower limit or more, the production and handling of the negative electrode active material particles becomes easy. By setting the average particle size D50 of the negative electrode active material particles to the above upper limit or less, the conductivity of the negative electrode active material layer is improved. A pulverizer, a classifier, or the like is used to obtain negative electrode active material particles with a predetermined particle size. Furthermore, when the negative electrode active material is metallic Li, the form may be a foil or a plate.

[0088] For example, when the negative electrode active material layer is formed from a negative electrode mixture, the content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved. When the negative electrode active material is metallic Li, the content of the negative electrode active material in the negative electrode active material layer may be 99% by mass or more, or may be 100% by mass.

[0089] (separator) The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the form of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0090] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in air, and more preferably a mass loss of 5% or less when heated from room temperature to 800°C in air. Examples of materials exhibiting a specific mass loss or less upon heating include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0091] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0092] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0093] (electrolyte) The electrolyte can be appropriately selected from known electrolytes. The following description will mainly focus on non-aqueous electrolytes. The non-aqueous electrolyte may be a non-aqueous electrolytic solution. The non-aqueous electrolytic solution contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0094] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0095] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0096] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, DMC and EMC are preferred.

[0097] As the non-aqueous solvent, it is preferable to use at least one of a cyclic carbonate and a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0098] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0099] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0100] The content of the electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm 3 More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0101] The non-aqueous electrolyte may contain an additive. Examples of the additive include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexyl benzene. hexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, etc. These additives may be used alone or in combination of two or more.

[0102] The content of the additive in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or charge / discharge cycle performance after high-temperature storage, and further improve safety.

[0103] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0104] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc. and is solid at room temperature (e.g., 20° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

[0105] As the sulfide solid electrolyte, for example, Li2S-P2S5, LiI-Li2S-P2S5, Li 10 Ge-P2S 12 etc.

[0106] (Applications, etc.) The energy storage element of this embodiment is a high-energy storage element and can be particularly suitably applied to energy storage elements used in a manner in which charging and discharging are repeated at a high current density. This is because such energy storage elements are prone to expansion of the curved surface portions of the electrode body due to repeated charging and discharging, resulting in a significant decrease in the capacity retention rate. Specifically, the energy storage element of this embodiment can be particularly suitably used as a power source for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0107] The energy storage element of this embodiment can be suitably applied to an energy storage element in which expansion in the thickness direction of the container (Y direction in FIGS. 1 and 2) is restricted or the container is pressed in the thickness direction. This is because in such an energy storage element, expansion of the flat portions of the electrode body is particularly unlikely to occur, and therefore the capacity retention rate decreases significantly due to expansion of the curved portions of the electrode body. Pressing of the container in the thickness direction can be performed, for example, by a pressure member that presses the container 2 from the outside. That is, the energy storage element of this embodiment can further include a pressure member. Examples of the pressure member include a restraining band and a metal frame. Note that multiple energy storage elements may be arranged in the thickness direction and fixed using a frame or the like while being pressed from both ends in the thickness direction.

[0108] (Manufacturing method) The energy storage element 100 can be manufactured by a conventionally known method, for example, by a manufacturing method including preparing a positive electrode, preparing a negative electrode, preparing a separator, preparing an electrolyte, stacking the positive electrode and the negative electrode with the separator interposed therebetween and rolling them up to form a flat electrode assembly 1, housing the electrode assembly 1 in a container 2, and injecting the electrolyte into the container 2.

[0109] <Configuration of the power storage device> The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) configured by assembling a plurality of energy storage elements in a power source for an automobile such as an EV, HEV, or PHEV, a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, it is sufficient that the technology according to one embodiment of the present invention is applied to at least one energy storage element included in the energy storage unit.

[0110] 4 shows an example of a power storage device 300 in which power storage units 200, each of which is an assembly of two or more electrically connected power storage elements 100, are further assembled. The power storage device 300 may include a bus bar (not shown) that electrically connects two or more power storage elements 100, a bus bar (not shown) that electrically connects two or more power storage units 200, etc. The power storage unit 200 or the power storage device 300 may include a status monitoring device (not shown) that monitors the status of one or more power storage elements.

[0111] <Other embodiments> The present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, or part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0112] In the above embodiment, the positive electrode active material layer contains a fibrous conductive agent, and D in Formulas 1 and 2 represents the average particle diameter D50 of the positive electrode active material particles, and A represents the surface roughness Ra of the positive electrode active material layer. However, the present invention is not limited to this. That is, the scope of the present invention also includes an energy storage element in which the negative electrode active material layer contains a fibrous conductive agent, D in Formulas 1 and 2 represents the average particle diameter D50 of the negative electrode active material particles, and A represents the surface roughness Ra of the negative electrode active material layer, and at least one of Formulas 1 and 2 is satisfied. Furthermore, the technology according to one embodiment of the present invention may be applied to both the positive electrode active material layer and the negative electrode active material layer.

[0113] 2, the distance X from the tip of the upper curved surface portion 8a to the inner surface of the container 2 facing the tip of the curved surface portion 8a is shorter than the distance X' from the tip of the lower curved surface portion 8b to the inner surface of the container 2 facing the tip of the curved surface portion 8b, but this is not limited to a specific shape. However, if the distance from the tip of the lower curved surface portion 8b to the inner surface of the container 2 facing the tip of the curved surface portion 8b is shorter, the distance from the tip of the lower curved surface portion 8b to the inner surface of the container 2 facing the tip of the curved surface portion 8b is defined as X, and the periphery of the curved surface portion 8b as viewed in the direction of the winding axis of the electrode body 1 is defined as R, and Equations 1 and 3 are applied. Furthermore, the distance X from the tip of the upper curved surface portion 8a to the inner surface of the container 2 facing the tip of the curved surface portion 8a and the distance X' from the tip of the lower curved surface portion 8b to the inner surface of the container 2 facing the tip of the curved surface portion 8b may be equal.

[0114] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable secondary battery (for example, a lithium ion secondary battery) has been described, but the type, shape, size, capacity, etc. of the energy storage element are arbitrary. The energy storage element of the present invention can also be applied to capacitors such as electric double layer capacitors or lithium ion capacitors. In addition, the shape of the container of the energy storage element is not particularly limited. [Example]

[0115] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. All measurements in the following examples were measured by the methods described above.

[0116] The positive electrode active material particles used are shown below. Positive electrode active material particles A: Positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2 O2 particles Primary particle diameter 5.0μm, average particle diameter (D50) 5.0μm, average particle diameter (D50) / primary particle diameter = 1.0 Positive electrode active material particles B: Positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2 O2 particles Primary particle diameter 0.6μm, average particle diameter (D50) 8.5μm, average particle diameter (D50) / primary particle diameter = 14

[0117] [Example 1] (Preparation of positive electrode) A positive electrode mixture paste containing the positive electrode active material particles A, the fibrous conductive agent CNT (average diameter 7 nm, average length approximately 60 to 100 μm), the granular conductive agent acetylene black (AB), and the binder polyvinylidene fluoride (PVDF) in a mass ratio (solid content equivalent) of 94.5:1.5:2.5:1.5, and N-methyl-pyrrolidone (NMP) as a dispersion medium, was prepared. This positive electrode mixture paste was applied to the surface of aluminum foil, the positive electrode substrate, and dried to prepare a positive electrode active material layer. Thereafter, roll pressing was performed to obtain the positive electrode of Example 1. The surface roughness Ra of the positive electrode active material layer of the obtained positive electrode of Example 1 was 1.3 μm.

[0118] (Preparation of negative electrode) A negative electrode mixture paste containing graphite as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener in a mass ratio (solid content equivalent) of 96:3:1, with water as the dispersion medium, was prepared. This negative electrode mixture paste was applied to the surface of copper foil as the negative electrode substrate and dried to prepare a negative electrode active material layer. A negative electrode was then obtained by roll pressing.

[0119] (Secondary battery assembly) A flat, wound electrode body was fabricated using the positive electrode, the negative electrode, and a polyolefin microporous membrane separator. This electrode body was placed in a square container, and a non-aqueous electrolyte was poured into it to assemble the secondary battery (energy storage element) of Example 1. The non-aqueous electrolyte was a non-aqueous solvent made by mixing EC (ethylene carbonate), EMC (ethyl methyl carbonate), and dimethyl carbonate (DMC) in a volume ratio of 30:35:35, and lithium hexafluorophosphate (LiPF6) was added as an electrolyte salt at a concentration of 1.0 mol / dm 3 A solution containing the compound dissolved at a concentration of 100 mg / kg was used. The outer periphery length (R) of the curved portion of the obtained electrode body of the secondary battery of Example 1, as viewed in the winding axis direction, was 25.8 mm, and the distance (X) from the tip of the curved portion to the opposing inner surface of the container was 1.7 mm.

[0120] [Comparative Example 1] A positive electrode and a secondary battery of Comparative Example 1 were obtained in the same manner as in Example 1, except that the CNTs were not used when preparing the positive electrode mixture paste, and the mass ratio of the positive electrode active material particles A, AB, and the binder was set to 94.5:4.0:1.5. The surface roughness Ra of the positive electrode active material layer of the obtained positive electrode of Comparative Example 1 was 1.2 μm.

[0121] Comparative Example 2 A positive electrode and a secondary battery of Comparative Example 2 were obtained in the same manner as in Example 1, except that when preparing the positive electrode mixture paste, positive electrode active material particles B were used instead of positive electrode active material particles A. The surface roughness Ra of the positive electrode active material layer of the obtained positive electrode of Comparative Example 2 was 1.0 μm.

[0122] Comparative Example 3 A positive electrode and a secondary battery of Comparative Example 3 were obtained in the same manner as in Comparative Example 1, except that when preparing the positive electrode mixture paste, positive electrode active material particles B were used instead of positive electrode active material particles A. The surface roughness Ra of the positive electrode active material layer of the obtained positive electrode of Comparative Example 3 was 1.0 μm.

[0123] The outer periphery length (R) of the curved portion of the electrode body of each of the secondary batteries obtained in Comparative Examples 1 to 3, as viewed in the direction of the winding axis, was 25.8 mm, and the distance (X) from the tip of the curved portion to the inner surface of the opposing container was 1.7 mm.

[0124] [evaluation] (Charge-discharge cycle test) The following charge-discharge cycle test was performed on each of the obtained secondary batteries. After storing them in a thermostatic chamber at 60°C for 3 hours, they were charged at a constant current of 1.0 C to 4.20 V, and then charged at a constant voltage of 4.2 V. The charging was terminated when the total charging time reached 3 hours. A 10-minute rest period was then provided. A constant current discharge was performed at a current of 1.0 C to 2.50 V, followed by a 10-minute rest period. These charge and discharge steps constitute one cycle, and this cycle was repeated 700 times. These charge, discharge, and rest periods were performed in a thermostatic chamber at 60°C.

[0125] (Capacity maintenance rate) The ratio of the discharge capacity at the 700th cycle to the discharge capacity at the 1st cycle in the charge-discharge cycle test was calculated as the discharge capacity retention rate. The results are shown in Table 1.

[0126] [Table 1]

[0127] As shown in Table 1, the secondary battery of Example 1, in which (R / D)A was 2X (=3.4 mm) or more and A / D was 0.2 or more and the positive electrode active material layer contained CNTs as a fibrous conductive agent, exhibited a high capacity retention rate after the charge-discharge cycle test. In contrast, the secondary batteries of Comparative Example 1, in which the positive electrode active material layer did not contain a fibrous conductive agent despite having large (R / D)A and A / D values, and Comparative Examples 2 and 3, in which the (R / D)A and A / D values ​​were small, exhibited low capacity retention rates after the charge-discharge cycle test. [Industrial Applicability]

[0128] The present invention can be applied to electric storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles. [Explanation of symbols]

[0129] 100 Energy storage element 1 Electrode body 2 containers 3 Positive electrode connecting member 4 Positive external terminal 5. Negative electrode connecting member 6 Negative external terminal 7 Flat area 8(8a, 8b) Curved part 11 Active material layer surface 12 Fibrous conductive agent 13 Active material particles 200 Energy Storage Unit 300 Electricity storage device

Claims

1. a flat electrode body formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and having two curved surface portions facing each other; A container for accommodating the electrode assembly Equipped with At least one of the positive electrode and the negative electrode has an active material layer containing active material particles and a fibrous conductive agent, An energy storage element that satisfies the following formula 1. (R / D)A≧2X...1 In Equation 1, X is the distance (unit: mm) from the tip of one of the curved portions to the inner surface of the container opposite the tip of the curved portion when viewed in the direction of the winding axis of the electrode body. R is the outer periphery length (unit: mm) of one of the curved portions when viewed in the direction of the winding axis of the electrode body. D is the average particle diameter D50 (unit: μm) of the active material particles. A is the surface roughness Ra (unit: μm) of the active material layer.

2. a flat electrode body formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and having two curved surface portions facing each other; A container for accommodating the electrode assembly Equipped with At least one of the positive electrode and the negative electrode has an active material layer containing active material particles and a fibrous conductive agent, A storage element that satisfies the following formula 2. A / D≧0.2...2 In Equation 2, D is the average particle diameter D50 (unit: μm) of the active material particles, and A is the surface roughness Ra (unit: μm) of the active material layer.

3. The energy storage element according to claim 2 , further satisfying the following formula 1: (R / D)A≧2X...1 In Equation 1, X is the distance (unit: mm) from the tip of one of the curved portions to the inner surface of the container opposite the tip of the curved portion when viewed in the direction of the winding axis of the electrode body. R is the outer periphery length (unit: mm) of the one of the curved portions when viewed in the direction of the winding axis of the electrode body. D and A are synonymous with D and A in Equation 2.

4. The energy storage element according to claim 1 or 3, further satisfying the following formula 3: 0.06≦X / R≦0.12 ...3 In Formula 3, X and R have the same meanings as X and R in Formula 1.

5. 5. The energy storage element according to claim 1, wherein the content of the fibrous conductive agent in the active material layer is 0.01% by mass or more.

6. 6. The energy storage element according to claim 1, wherein an average length of the fibrous conductive agent is greater than an average particle diameter D50 of the active material particles.

7. 7. The energy storage element according to claim 1, wherein the active material particles are present in the form of secondary particles having a ratio of average particle diameter D50 to primary particle diameter of 3 or less, or in the form of substantially unagglomerated primary particles.

8. The energy storage element according to claim 1 , wherein the positive electrode has an active material layer containing the active material particles and the fibrous conductive agent.

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