Power storage element

Abstract

A power storage element according to one aspect of the present invention comprises: a flat electrode body in which a positive electrode and a negative electrode are wound with a separator therebetween; an electrolytic solution; and a container that accommodates the electrode body and the electrolytic solution. The winding axis of the electrode body is horizontally disposed. An excess electrolytic solution, which is a part of said electrolytic solution, is present between the electrode body and the container. In a cross-section that is perpendicular to the winding axis of the electrode body, two or more gaps are present in the electrode body at positions below the liquid surface of the excess electrolytic solution and on a vertical line passing through the winding axis.

Description

Energy storage element

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

[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 and a strip-shaped negative electrode are wound together with a strip-shaped separator interposed therebetween (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.

[0003] Japanese Patent Application Publication No. 2013-16440 Japanese Patent Application Publication No. 2013-191467

[0004] In an energy storage device, the electrolyte impregnated in the electrode body may flow out of the electrode body with the charge / discharge cycle, causing the amount of electrolyte impregnated in the electrode body to decrease or become depleted. Depletion of the electrolyte in the electrode body causes a decrease in the performance of the energy storage device, such as a decrease in discharge capacity. In particular, in a wound electrode body, the electrolyte can substantially only flow in from the end face perpendicular to the winding axis, making it difficult for the electrolyte that has flowed out of the electrode body to return to the electrode body, making the above phenomenon more likely to occur.

[0005] An object of the present invention is to provide an energy storage element including a wound electrode body, which has a high capacity retention rate after charge / discharge cycles.

[0006] An energy storage element according to one aspect of the present invention comprises a flat electrode body formed by winding a positive electrode and a negative electrode with a separator interposed therebetween, an electrolyte, and a container for accommodating the electrode body and the electrolyte, wherein the winding axis of the electrode body is arranged horizontally, and excess electrolyte, which is a part of the electrolyte, is present between the electrode body and the container, and in a cross section perpendicular to the winding axis of the electrode body, there are two or more voids within the electrode body that are located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis.

[0007] According to one aspect of the present invention, it is possible to provide an energy storage element including a wound electrode body, which has a high capacity retention rate after charge / discharge cycles.

[0008] Fig. 1 is a schematic perspective view showing an energy storage element according to one embodiment of the present invention. Fig. 2 is a schematic cross-sectional view of the energy storage element of Fig. 1 taken along the line II. Fig. 3 is a schematic view showing an energy storage device formed by assembling a plurality of energy storage elements according to one embodiment of the present invention. Fig. 4 is a schematic cross-sectional view showing an energy storage element according to another embodiment of the present invention.

[0009] First, an overview of the energy storage element disclosed in this specification will be described.

[0010] [1] An energy storage element according to one aspect of the present invention comprises a flat electrode body formed by winding a positive electrode and a negative electrode with a separator interposed therebetween, an electrolyte, and a container for accommodating the electrode body and the electrolyte, wherein the winding axis of the electrode body is arranged horizontally, and excess electrolyte, which is a part of the electrolyte, is present between the electrode body and the container, and in a cross section perpendicular to the winding axis of the electrode body, there are two or more voids within the electrode body that are located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis.

[0011] The energy storage element described in [1] above is an energy storage element including a wound electrode assembly and exhibits a high capacity retention rate after charge / discharge cycles. While the reason for this effect is unclear, the following reason is presumed. One of the reasons for the low capacity retention rate after charge / discharge cycles in energy storage elements including conventional flat, wound electrode assemblies as described above is that the electrolyte in the electrode assembly flows out of the electrode assembly with each charge / discharge cycle, resulting in depletion of the electrolyte within the electrode assembly. In contrast, in the energy storage element described in [1] above, in a cross section perpendicular to the winding axis of the electrode assembly, two or more voids exist within the electrode assembly that are located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis. These two or more voids allow the electrolyte to be sufficiently retained within the electrode assembly. Therefore, it is presumed that the energy storage element described in [1] above suppresses depletion of the electrolyte in the electrode assembly with repeated charge / discharge cycles, thereby improving the capacity retention rate after charge / discharge cycles.

[0012] The voids in the electrode body refer to voids identified in images obtained by binarizing X-ray computed tomography (CT) images. Specifically, they are voids identified in images obtained by the following analysis method. The position of the excess electrolyte surface is also confirmed using the same X-ray CT images. (1) Acquisition of a Cross-Sectional Image of the Energy Storage Element A microfocus X-ray CT (Shimadzu Corporation's inspeXio SMX-225CT FPD HR) is used to acquire a cross-sectional image of the energy storage element to be measured, perpendicular to the winding axis of the electrode body. The conditions for acquiring the cross-sectional image are as follows: 3D CT imaging mode, image size of 512 pixels in both directions, slice thickness of 77 μm, inter-slice distance of 77 μm, voxel size of 77 μm, field of view of 39.3 mmφ, and number of views of 1800. A cross-sectional image of the energy storage element to be measured is obtained on a plane perpendicular to the winding axis of the electrode body (the YZ plane in FIG. 2 ), and the obtained cross-sectional image is saved as an image file. In the obtained cross-sectional image, the position of the intersection of a vertical line passing through the winding axis of the electrode body and the liquid level of the excess electrolyte is determined as the liquid level of the excess electrolyte. (2) Cutting out the outline of the electrode body: Using the image cutout function of the image editing software Adobe Photoshop Elements 11, the outline of the electrode body is cut out from the obtained cross-sectional image of the electrode body. (3) Binarization: For the cross-sectional image of the electrode body with the outline cut out, the number of voids within the electrode body is counted using image processing software WinROOF2013. Here, the void count is performed on the central region of the cross-sectional image of the electrode body with the outline cut out, located on the vertical line passing through the winding axis. Specifically, binarization is performed by first setting the threshold value to a density 80% lower than the density at which the brightness intensity is maximized. On the low brightness side, pixel groups of 10 pixels or less are considered noise, and pixel groups of 11 pixels or more are counted as voids.

[0013] The winding axis of the electrode body can be the center point in the height direction (Z-axis direction in FIG. 2 ) and thickness direction (Y-axis direction in FIG. 2 ) of the electrode body. Furthermore, the two or more voids may be located not only completely on a vertical line passing through the winding axis of the electrode body, but also substantially on a vertical line passing through the winding axis of the electrode body. For example, when the positive electrode, negative electrode, and separator components constituting the electrode body are stacked in a curved or meandering manner, some or all of the two or more voids may deviate from the vertical line passing through the winding axis of the electrode body. For example, the two or more voids may be located inside a component (e.g., a separator) constituting the innermost periphery of the electrode body.

[0014] [2] In the energy storage element described in [1] above, in the cross section perpendicular to the winding axis of the electrode body, there may be substantially no voids in the electrode body at positions other than on the vertical line passing through the winding axis.

[0015] If voids exist in the electrode body at positions other than on the vertical line passing through the winding axis, these voids are usually located between the positive electrode and the negative electrode, and increase the distance between the electrodes, which may result in a decrease in the charge / discharge performance of the energy storage element. Therefore, if there are substantially no voids in the electrode body at positions other than on the vertical line passing through the winding axis, as in the energy storage element described in [2] above, the charge / discharge performance of the energy storage element can be improved.

[0016] The phrase "substantially no voids exist" means that no voids are observed in the image obtained by binarizing the X-ray CT image.

[0017] [3] In the energy storage element according to [1] or [2] above, the two or more voids may be an area surrounded by the separator or a porous member other than the separator.

[0018] According to the energy storage element described in the above [3], the electrolyte stored in the voids is effectively supplied to the positive electrode and the negative electrode, so that the capacity retention rate after charge-discharge cycles can be further increased.

[0019] [4] In the energy storage element described in any one of [1] to [3] above, in the cross section perpendicular to the winding axis of the electrode body, the ratio of the total area of ​​the two or more voids to the area of ​​the electrode body below the liquid level of the excess electrolyte may be greater than 4%.

[0020] According to the energy storage element described in [4] above, a particularly sufficient amount of electrolyte can be stored in the two or more voids, and therefore the capacity retention rate after charge-discharge cycles can be further increased.

[0021] The ratio of the total area of ​​the two or more voids to the area of ​​the electrode body below the liquid level of the excess electrolyte is a value determined based on an image obtained by binarizing the X-ray CT image, and the area of ​​the electrode body below the liquid level of the excess electrolyte does not include the total area of ​​the two or more voids.

[0022] [5] In the energy storage element described in any one of [1] to [4] above, the thickness of the electrode body in a dry state and under no load may be 0.94 or less relative to the inner dimension of the container in the thickness direction of the electrode body.

[0023] The energy storage element according to the above item [5] also allows a particularly sufficient amount of electrolyte to be stored in the two or more voids, thereby further increasing the capacity retention rate after charge-discharge cycles.

[0024] The "thickness of the electrode body in a dry state and with no load" refers to the thickness of the center (the center in the width and height directions perpendicular to the thickness direction of the electrode body) of the electrode body in a dry state and with no load. Furthermore, the "internal dimensions of the electrode body in the thickness direction of the container" refer to the internal dimensions of the center of the container (the center in the width and height directions perpendicular to the thickness direction of the container). The thickness of the electrode body in a dry state and with no load is measured using the following procedure. The energy storage element is discharged at a constant current of 0.05 C to 0% SOC, followed by a constant voltage discharge for two hours to achieve a discharged state. The energy storage element is disassembled, and the electrode body is removed. The electrolyte adhering to the removed electrode body is thoroughly washed using dimethyl carbonate, and the electrode body is vacuum-dried at room temperature for 24 hours to obtain a measurement sample. The thickness of this dried measurement sample (electrode body) in the stacking direction of the positive and negative electrodes is measured in an unloaded state. Furthermore, if multiple electrode bodies are housed in the container, the thickness is the sum of the thicknesses of the multiple electrode bodies stacked in the thickness direction. The inner dimension of the container in the thickness direction of the electrode body is measured by observing a cross section of the energy storage element using an X-ray CT image or the like.

[0025] An energy storage element, an energy storage device, a method for manufacturing an energy storage element, and other embodiments according to an embodiment of the present invention will be described in detail. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0026] <Energy Storage Element> An energy storage element 100 according to one embodiment of the present invention, shown in FIG. 1 , includes an electrode assembly 1, an electrolyte (not shown), and a container 2 for accommodating these. In this embodiment, one electrode assembly 1 is accommodated in the container 2. The energy storage element 100 is a secondary battery, which is an example of an energy storage element. The secondary battery may be a nonaqueous electrolyte secondary battery such as a lithium-ion battery. The electrode assembly 1 is a flat electrode assembly formed by winding a strip-shaped positive electrode and a strip-shaped negative electrode with a strip-shaped separator interposed therebetween. The electrode assembly 1 is disposed in the container 2 such that the winding axis W of the electrode assembly 1 is horizontal (X-axis direction) and the thickness direction of the electrode assembly 1 is horizontal (Y-axis direction). That is, the longitudinal direction of the electrode assembly 1 in a cross section ( FIG. 2 ) perpendicular to the winding axis W of the electrode assembly 1 is disposed so that the height direction (Z-axis direction) is the same as the longitudinal direction. In the energy storage element 100, the winding axis W of the electrode assembly 1 is parallel to the bottom surface 11 of the container 2. 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 body 1 is electrically connected to the positive electrode external terminal 4 via the positive electrode connecting member 3. The negative electrode of the electrode body 1 is electrically connected to the negative electrode external terminal 6 via the negative electrode connecting member 5.

[0027] 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 shown in Fig. 1 is a substantially rectangular parallelepiped container. 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.

[0028] As described above, the container 2 contains an electrolyte together with the electrode assembly 1. A portion of the electrolyte is impregnated into the electrode assembly 1, and the remainder of the electrolyte is accumulated on the bottom side of the container 2 as excess electrolyte 7, as shown in Fig. 2. In other words, excess electrolyte 7 exists between the electrode assembly 1 and the container 2.

[0029] A portion of the electrode body 1 is in contact with the excess electrolyte 7. The position of the liquid level 8 of the excess electrolyte 7 can be, for example, in a range of 2 / 10 to 9 / 10 based on the height (length in the Z-axis direction) of the electrode body 1. In other words, the electrode body 1 can be immersed in the excess electrolyte 7 in a range of 2 / 10 to 9 / 10 in its height direction. The position of the liquid level 8 of the excess electrolyte 7 can be 3 / 10 or more, or 4 / 10 or more (e.g., 5 / 10 or more or 6 / 10 or more) based on the height of the electrode body 1. The position of the liquid level 8 of the excess electrolyte 7 can be 8 / 10 or less, or 7 / 10 or less based on the height of the electrode body 1. The position of the liquid level 8 of the excess electrolyte 7 can be above any of the lower limits described above and below any of the upper limits described above.

[0030] In the energy storage element 100, as shown in Fig. 2 , in a cross section perpendicular to the winding axis W of the electrode body 1, there are two or more voids 9 (specifically, two voids 9 in Fig. 2 ) within the electrode body 1 that are located below the liquid level 8 of the excess electrolyte 7 and on a vertical line V that passes through the winding axis W. The electrolyte is usually present within these voids 9. In the energy storage element 100, the presence of these two or more voids 9 within the electrode body 1 increases the capacity retention rate after charge / discharge cycles.

[0031] The number of two or more voids 9 located below the liquid level 8 of the excess electrolyte 7 in one electrode body 1 and on the vertical line V passing through the winding axis W is not limited to two. The lower limit of the number of voids 9 may be 2, 3 (see FIG. 4 ), or 4. The number of voids 9 is preferably 3 or more. On the other hand, the upper limit of the number of voids 9 may be, for example, 5, 4, or 3. The number of voids 9 may be equal to or greater than any of the above-mentioned lower limits and equal to or less than any of the above-mentioned upper limits. The number of voids 9 can be adjusted by the position of the liquid level 8 of the excess electrolyte 7, the ratio of the thickness of the electrode body 1 in a dry state and with no load (described later) to the inner dimension T of the electrode body 1 in the thickness direction (Y-axis direction) of the container 2, and the like.

[0032] The two or more voids 9 are preferably regions surrounded by a separator or a porous member other than a separator. The separator is typically formed of a porous member. When the two or more voids 9 are regions surrounded by a separator or a porous member other than a separator, the electrolyte stored in the two or more voids 9 permeates the separator or the porous member other than a separator and is effectively supplied to the positive electrode and the negative electrode, thereby further improving the capacity retention rate after charge-discharge cycles. For example, when the electrode assembly 1 is obtained by winding a first separator, a first electrode (one of the positive electrode and the negative electrode), a second separator, and a second electrode (the other of the positive electrode and the negative electrode) stacked in this order with the first separator side facing inward, the first separator is located at the innermost periphery of the electrode assembly 1. Therefore, in such a case, two or more voids 9 may be formed in the region surrounded by the innermost first separator. Furthermore, when a winding core (also referred to as a central core, etc.) is formed by winding a strip-shaped porous member or the like, and a wound-type electrode assembly 1 is fabricated around this winding core, the strip-shaped porous member forming the winding core is located at the innermost periphery of the electrode assembly 1. Therefore, in such a case, two or more voids 9 may be formed inside the innermost winding core, i.e., in the area surrounded by the strip-shaped porous member. The porous member forming the winding core may be made of the same material as the separator, which will be described later.

[0033] In a cross section ( FIG. 2 ) perpendicular to the winding axis W of the electrode assembly 1, the ratio (S2 / S1) of the total area (S2) of the two or more voids 9 to the area (S1) of the electrode assembly 1 below the liquid level 8 of the excess electrolyte 7 is preferably greater than 4%. The lower limit of the ratio (S2 / S1) is, for example, more preferably 5%, and even more preferably 6%. In some embodiments, the lower limit of the ratio (S2 / S1) is more preferably 10%, and even more preferably 20%. When the ratio (S2 / S1) is greater than 4%, or even equal to or greater than the lower limit, a particularly sufficient amount of electrolyte can be stored in the two or more voids 9, thereby further improving the capacity retention rate after charge / discharge cycling. On the other hand, the upper limit of the ratio (S2 / S1) is preferably 50%, more preferably 40%, and even more preferably 35%. In some embodiments, the upper limit of the ratio (S2 / S1) may be 20% or 10% (e.g., 8%). When the ratio (S2 / S1) is equal to or less than the upper limit, it is possible to increase the energy density of the energy storage element 100. The ratio (S2 / S1) may be equal to or greater than any of the lower limits and equal to or less than any of the upper limits.

[0034] In a cross section ( FIG. 2 ) perpendicular to the winding axis W of the electrode body 1, it is preferable that there are substantially no voids in the electrode body 1 at positions other than on a vertical line V passing through the winding axis W. In such a case, the distance between the positive electrode and the negative electrode in the electrode body 1 is maintained relatively uniform, thereby improving the charge / discharge performance of the energy storage element 100.

[0035] In addition, in a cross section (Figure 2) perpendicular to the winding axis W of the electrode body 1, there may be one or more voids 10 within the electrode body 1 located above the liquid level 8 of the excess electrolyte 7 and on a vertical line V passing through the winding axis W.

[0036] The thickness of the electrode body 1 in a dry state and with no load (the length in the Y-axis direction in FIG. 2 ) relative to the inner dimension T of the electrode body 1 in the thickness direction (the Y-axis direction in FIG. 2 ) of the container 2 is preferably 0.94 or less, and more preferably 0.92 or less or 0.90 or less (e.g., 0.88 or less) in some cases. Even in such cases, a particularly sufficient amount of electrolyte can be stored in two or more voids 9, thereby further increasing the capacity retention rate after charge / discharge cycles. On the other hand, the thickness of the electrode body 1 in a dry state and with no load is preferably 0.80 or more, and more preferably 0.85 or more or 0.90 or more, relative to the inner dimension T of the electrode body 1 in the thickness direction of the container 2. In such cases, the energy density of the energy storage element 100 can be increased, etc. The thickness of the electrode body 1 in a dry state and with no load may be equal to or greater than any of the above-mentioned lower limits and equal to or less than any of the above-mentioned upper limits.

[0037] The specific configurations of the positive electrode, negative electrode, separator, and electrolyte that constitute the electrode assembly will be described in detail below.

[0038] (Positive Electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0039] The positive electrode substrate has electrical conductivity. Whether or not it has electrical conductivity is determined by whether or not the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 ７ The resistance is determined using Ω cm as a threshold value. 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 or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy 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).

[0040] 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, it is possible to increase the strength of the positive electrode substrate and also increase the energy density per volume of the energy storage element.

[0041] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent 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 may contain, for example, a binder and a conductive agent.

[0042] The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0043] The positive electrode active material can be appropriately selected from known positive electrode active materials. As the positive electrode active material for a lithium ion secondary battery, a material capable of absorbing and releasing lithium ions is usually used. Examples of the positive electrode active material include α-NaFeO ２ Examples of suitable lithium transition metal composite oxides include those having a α-type crystal structure, those having a spinel type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. ２ As the lithium transition metal composite oxide having a crystalline structure, for example, Li[Li ｘ Ni （１－ｘ） ]O ２ (0≦x<0.5), Li[Li ｘ Ni γ Co （１－ｘ－γ） ]O ２ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li ｘ Co （１－ｘ） ]O ２ (0≦x<0.5), Li[Li ｘ Ni γ Mn （１－ｘ－γ） ]O ２ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li ｘ Ni γ Mn β Co （１－ｘ－γ－β）]O ２ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1−x−γ−β), Li[Li ｘ Ni γ Co β Al （１－ｘ－γ－β） ]O ２ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1−x−γ−β), etc. Examples of lithium transition metal composite oxides having a spinel crystal structure include Li ｘ Mn ２ O ４ , Li ｘ Ni γ Mn （２－γ） O ４ Examples of polyanion compounds include LiFePO ４ , LiMnPO ４ , LiNiPO ４ , LiCoPO ４ , Li ３ V ２ (P.O. ４ ) ３ , Li ２ MnSiO ４ , Li ２ CoPO ４ Examples of the chalcogen compound 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. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more of them may be used in combination.

[0044] In one embodiment of the present invention, the positive electrode active material is preferably a lithium transition metal composite oxide, such as α-NaFeO ２ A lithium transition metal composite oxide having a crystalline structure is more preferred. Furthermore, the lithium transition metal composite oxide preferably contains nickel as the transition metal element. The content ratio (Ni / Me) of nickel (Ni) to the transition metal element (Me) in the lithium transition metal composite oxide is preferably 0.5 or more, more preferably 0.6 or more, in terms of molar ratio. Meanwhile, this content ratio (Ni / Me) may be 1 or less, or may be 0.9 or less.

[0045] The use of such a lithium transition metal composite oxide can increase the energy density. On the other hand, such lithium transition metal composite oxides, particularly those with a high nickel content, are prone to particle cracking during charge-discharge cycles, and generally suffer from a decrease in capacity retention. Therefore, when the present invention is applied to an energy storage device including a wound electrode body containing such a lithium transition metal composite oxide as a positive electrode active material, the advantage of an increased capacity retention rate after charge-discharge cycles is particularly effectively obtained.

[0046] The lithium transition metal composite oxide containing nickel preferably further contains at least one of cobalt and manganese in addition to nickel, and more preferably both cobalt and manganese. The ratio of the total content of nickel (Ni), cobalt (Co), and manganese (Mn) to the transition metal element (Me) in the lithium transition metal composite oxide ((Ni + Co + Mn) / Me) is preferably 0.9 or more, more preferably 0.95 or more, in terms of molar ratio. On the other hand, this ratio ((Ni + Co + Mn) / Me) may be 1 or less, or may be 0.99 or less. The lithium transition metal composite oxide may further contain an element other than lithium, oxygen, and transition metal elements (e.g., aluminum).

[0047] 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 an amount 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.

[0048] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above-mentioned lower limit or more, the positive electrode active material is easily manufactured or handled. By setting the average particle size of the positive electrode active material to the above-mentioned upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. "Average particle size" 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 in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

[0049] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0050] The positive electrode active material may be a primary particle (single particle) or a secondary particle formed by aggregation of a plurality of primary particles. In a preferred embodiment of the present invention, the positive electrode active material is a secondary particle. When the positive electrode active material is a secondary particle, particle cracking is likely to occur with charge-discharge cycles, and the capacity retention rate is generally likely to decrease. Therefore, when the present invention is applied to an energy storage device including a wound electrode body having a positive electrode active material that is a secondary particle, the advantage of an increased capacity retention rate after charge-discharge cycles is particularly effectively obtained.

[0051] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0052] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNT may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0053] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent in this range, the energy density of the energy storage element can be increased.

[0054] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0055] The content of the binder in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass, by which the positive electrode active material can be stably maintained.

[0056] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be deactivated 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.

[0057] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, 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 even 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.

[0058] 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 metallic 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, conductive agent, binder, thickener, and filler.

[0059] The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0060] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0061] 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, it is possible to increase the strength of the negative electrode substrate and also increase the energy density per volume of the energy storage element.

[0062] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0063] 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.

[0064] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for a lithium ion secondary battery. Examples of the negative electrode active material 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; Li ４ Ti ５ O １２ , LiTiO ２、 TiNb ２ O ７Examples of the material include titanium-containing oxides such as titanium dioxide, 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.

[0065] "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.

[0066] "Non-graphitic carbon" refers to a carbon having an average lattice spacing (d ００２ ) is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitizable carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitizable 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.

[0067] Here, the "discharged state" refers to a state in which the battery is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released from the carbon material serving as 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.

[0068] "Non-graphitizable carbon" refers to the above-mentioned d ００２ The carbon material has a particle size of 0.36 nm or more and 0.42 nm or less.

[0069] "Graphitizable carbon" refers to the above-mentioned d ００２ The term "carbon material" refers to a carbon material having a particle size of 0.34 nm or more and less than 0.36 nm.

[0070] In one embodiment of the present invention, the negative electrode active material is preferably a material that exhibits a large volume change upon charge and discharge, specifically a material that exhibits a volume change of 5% or more upon charge and discharge. Examples of such materials include carbon materials such as graphite and non-graphitic carbon, silicon materials such as Si and Si oxides, and tin materials such as Sn and Sn oxides. When such a negative electrode active material exhibits a large volume change upon charge and discharge, the electrolyte tends to leak out of the electrode body and become depleted within the electrode body during charge and discharge cycles, resulting in a decrease in capacity retention. Therefore, when the present invention is applied to an energy storage device including a wound electrode body containing such a negative electrode active material, the advantage of an increased capacity retention rate after charge and discharge cycles is particularly effectively achieved.

[0071] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 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 may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the production or handling of the negative electrode active material becomes easier. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the negative electrode active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and classification method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material layer may be in the form of a foil.

[0072] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0073] (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 shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous 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.

[0074] 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 under an air atmosphere at 1 atmosphere, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less 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 aluminosilicate; 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.

[0075] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0076] 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.

[0077] (Electrolyte) The electrolyte can be appropriately selected from known electrolytes. A non-aqueous electrolyte may be used as the electrolyte. The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0078] 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.

[0079] 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. Of these, EC is preferred.

[0080] 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, EMC is preferred.

[0081] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a cyclic carbonate and a chain carbonate in combination. 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.

[0082] 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.

[0083] The lithium salt is LiPF ６ , LiPO ２ F ２ , LiBF ４ , LiClO ４ , LiN(SO ２ F) ２ inorganic lithium salts such as lithium oxalate salts, lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); ３ CF ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) (SO２ C ４ F ９ ), LiC(SO ２ CF ３ ) ３ , LiC(SO ２ C ２ F ５ ) ３ Among these, inorganic lithium salts are preferred, and LiPF ６ is more preferred.

[0084] The content of the electrolyte salt in the non-aqueous electrolyte solution is 0.1 mol / dm at 20°C and 1 atmosphere. ３ 2.5mol / dm or more ３ It is preferably 0.3 mol / dm or less. ３ 2.0mol / dm or more ３ It 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 1.5mol / dm or more ３ By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0085] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, and silyl. Aromatic compounds such as 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; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, and the like. Carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, 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, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives may be used alone or in combination of two or more.

[0086] The content of the additive contained 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, based on the total mass of the non-aqueous electrolyte. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0087] <Electricity Storage Device> The energy storage element of this embodiment can be mounted as an automobile power source for an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or the like, or as a power storage power source, etc., as an energy storage unit (battery module) configured by assembling a plurality of energy storage elements 100. In this case, it is sufficient that the technology of the present invention is applied to at least one energy storage element included in the energy storage unit.

[0088] 3 shows an example of an energy storage device 300 in which energy storage units 200, each of which is an assembly of two or more electrically connected energy storage elements 100, are further assembled. The energy storage device 300 may include a bus bar (not shown) that electrically connects two or more energy storage elements 100, a bus bar (not shown) that electrically connects two or more energy storage units 200, etc. The energy storage unit 200 or the energy storage device 300 may include a status monitoring device (not shown) that monitors the status of one or more energy storage elements.

[0089] <Method for Manufacturing Energy Storage Element> The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing a flat, wound electrode assembly, preparing an electrolyte solution, and housing the electrode assembly and the electrolyte solution in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by winding the positive electrode and the negative electrode with a separator interposed therebetween. In this manufacturing method, when housing the electrode assembly in the container, the electrode assembly is housed in an orientation such that the winding axis and the thickness direction of the electrode assembly are substantially parallel to the bottom surface of the container.

[0090] The method for placing the electrolyte in the container can be appropriately selected from known methods. For example, the electrolyte may be poured into the container through an inlet formed therein, and then the inlet may be sealed. As described above, the number of voids in the electrode body located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis of the electrode body can be adjusted by the relative thickness of the electrode body with respect to the inner dimensions of the container, the amount of electrolyte poured in, etc.

[0091] <Other Embodiments> The energy storage device of 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, and 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.

[0092] In the above embodiment, the energy storage element has been described in which one electrode body is housed in a container, but the number of electrode bodies is not limited. An energy storage element 101 according to one embodiment of the present invention shown in FIG. 4 includes two electrode bodies 1a, 1b, an electrolyte, and a container 102 that houses them. In this embodiment, the two electrode bodies 1a, 1b are housed in the container 102, and the electrode bodies 1a, 1b are arranged in the container 102 so that their winding axes Wa, Wb are horizontal (X-axis direction) and the thickness directions of the electrode bodies 1a, 1b are horizontal (Y-axis direction). The two electrode bodies 1a, 1b are arranged in a stacked state in the thickness direction (Y-axis direction in FIG. 4 ). In energy storage element 101, in a cross section ( FIG. 4 ) perpendicular to winding axes Wa, Wb of each electrode body 1a, 1b, there are three voids 9a in one electrode body 1a that are located below liquid level 108 of excess electrolyte 107 and on a vertical line Va that passes through the winding axis Wa of this electrode body 1a. In addition, in a cross section ( FIG. 4 ) perpendicular to winding axes Wa, Wb of each electrode body 1a, 1b, there are three voids 9b in the other electrode body 1b that are located below liquid level 108 of excess electrolyte 107 and on a vertical line Vb that passes through the winding axis Wb of this electrode body 1b.

[0093] When the energy storage element of the present invention includes two or more electrode bodies, at least one electrode body may have two or more voids located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis of the electrode body. In addition, at least one electrode body may have a winding core made of a non-porous material disposed in the center thereof.

[0094] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery) has been described, but the type, shape, size, capacity, etc. of the energy storage element are arbitrary. The present invention can also be applied to various secondary batteries and capacitors such as electric double layer capacitors and lithium ion capacitors. The present invention can also be applied to energy storage elements using electrolytes other than nonaqueous electrolytes.

[0095] 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.

[0096] [Example 1] Positive electrode active material LiNi ０．６ Co ０．２ Mn ０．２ O ２ A positive electrode mixture paste was prepared using acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, AB, and PVDF was 93:4:3 (solid content equivalent). This positive electrode mixture paste was applied to both sides of a strip-shaped aluminum foil positive electrode substrate and dried to form a positive electrode active material layer. This resulted in a strip-shaped positive electrode.

[0097] A negative electrode mixture paste was prepared using graphite as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of the negative electrode active material, SBR, and CMC was 97.8:1:1.2 (solid content equivalent). This negative electrode mixture paste was applied to both sides of a strip-shaped copper foil negative electrode substrate and dried to form a negative electrode active material layer. This resulted in a strip-shaped negative electrode.

[0098] A strip-shaped separator made of a microporous polyethylene film was prepared as the separator.

[0099] The strip-shaped positive electrode and the strip-shaped negative electrode were stacked together with the strip-shaped separator interposed therebetween, and then wound in the longitudinal direction to prepare two flat electrode bodies.

[0100] A solvent containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 30:70 was added to the solution containing 1.2 mol / dm ３ At a concentration of LiPF ６ was dissolved to obtain a non-aqueous electrolyte solution.

[0101] The two electrode bodies thus fabricated were stacked and housed in an aluminum rectangular container with the winding axis of each electrode body aligned horizontally (in the X-axis direction) and the thickness direction of each electrode body aligned horizontally (in the Y-axis direction) (see FIG. 4 ), and a positive electrode terminal and a negative electrode terminal were attached. The nonaqueous electrolyte solution was poured into the container, which was then sealed to obtain the energy storage element of Example 1. In the energy storage element of Example 1, the ratio of the thickness of the electrode body in a dry state and under no load to the inner dimension of the container in the thickness direction of the electrode body (hereinafter also referred to as the "insertion coefficient") was 0.871. Furthermore, for the energy storage element of Example 1, the number of voids (hereinafter also referred to as the "void number") present within one electrode body in a cross section perpendicular to the winding axis of the electrode body, located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis of the electrode body, was measured using the above-described method, and was found to be three. For the energy storage element of Example 1, the ratio of the total area of ​​two or more voids located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis of the electrode body to the area of ​​the electrode body below the liquid level of the excess electrolyte in a cross section perpendicular to the winding axis of the electrode body (hereinafter also referred to as the "void area ratio") was measured using the method described above, and was found to be 6%.

[0102] [Example 2, Comparative Examples 1 and 2] The energy storage elements of Example 2 and Comparative Examples 1 and 2 were obtained in the same manner as Example 1, except that the number of windings of the positive electrode, negative electrode, and separator when preparing the electrode body was changed to the insertion coefficient shown in Table 1, and the amount of electrolyte injected was appropriately changed. Specifically, the amount of electrolyte injected into the energy storage element of Example 2 was less than that of Example 1, and the amount of electrolyte injected into the energy storage element of Comparative Example 2 was less than that of Example 2. Furthermore, the amount of electrolyte injected into the energy storage element of Comparative Example 1 was approximately the same as that of Example 1. The number of voids and the void area ratio measured by the above-mentioned method for each of the obtained energy storage elements are shown in Table 1.

[0103] [Evaluation] (Initial Charge / Discharge) Each of the obtained energy storage elements was subjected to initial charge / discharge under the following conditions. In a thermostatic chamber at 25°C, the element was charged at a constant current of 1.0 C up to 4.10 V. Thereafter, a constant voltage charge was performed at 4.10 V. The charge was terminated until the total charge time reached 3 hours. A 10-minute rest period was then provided. The element was discharged at a constant current of 1.0 C up to a discharge cut-off voltage of 2.50 V. The quantity of electricity discharged at this time was defined as the initial discharge capacity. (Charge / Discharge Cycle Test) Each of the energy storage elements after the initial charge / discharge was subjected to the following charge / discharge cycle test. After storage in a thermostatic chamber at 45°C for 3 hours, the element was charged at a constant current / constant voltage of 1.0 C up to 4.1 V. The charge was terminated until the total charge time reached 3 hours. A 10-minute rest period was then provided. The element was discharged at a constant current of 1.0 C up to a discharge cut-off voltage of 2.50 V. Thereafter, a 10-minute rest period was provided. This cycle of charging and discharging was counted as one cycle, and this cycle was repeated 1000 times. These charging, discharging, and resting periods were carried out in a thermostatic chamber at 45°C.

[0104] (Capacity Retention Rate) In the charge-discharge cycle test, the percentage of the discharge capacity at the 1000th cycle to the discharge capacity at the 1st cycle was calculated as the discharge capacity retention rate. The results are shown in Table 1.

[0105]

[0106] As shown in Table 1, in each of the energy storage elements of Examples 1 and 2, which have two or more voids (the number of voids present in one electrode body in a cross section perpendicular to the winding axis of the electrode body, which are located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis of the electrode body), the capacity retention rate after charge / discharge cycles exceeded 90%, confirming that the capacity retention rate was high.

[0107] 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.

[0108] REFERENCE SIGNS LIST 100, 101 Energy storage element 1, 1a, 1b Electrode body 2, 102 Container 3 Positive electrode connecting member 4 Positive electrode external terminal 5 Negative electrode connecting member 6 Negative electrode external terminal 7, 107 Excess electrolyte 8, 108 Liquid surface 9, 9a, 9b Gap 10 Gap 11 Bottom surface 200 Energy storage unit 300 Energy storage device W, Wa, Wb Winding axis V, Va, Vb Vertical line T Internal dimension

Claims

1. An energy storage element comprising: a flat electrode body formed by winding a positive electrode and a negative electrode with a separator interposed therebetween; an electrolyte; and a container for accommodating the electrode body and the electrolyte; wherein the winding axis of the electrode body is arranged horizontally; excess electrolyte, which is a part of the electrolyte, is present between the electrode body and the container; and in a cross section perpendicular to the winding axis of the electrode body, there are two or more voids within the electrode body that are located below the liquid level of the excess electrolyte and on a vertical line passing through the winding axis.

2. The energy storage element according to claim 1, wherein in the cross section perpendicular to the winding axis of the electrode body, there are substantially no voids within the electrode body at any position other than on the vertical line passing through the winding axis.

3. The energy storage element according to claim 1 or 2, wherein the two or more voids are an area surrounded by the separator or a porous member other than the separator.

4. A storage element as described in claim 1 or claim 2, wherein in the cross section perpendicular to the winding axis of the electrode body, the ratio of the total area of ​​the two or more voids to the area of ​​the electrode body below the liquid level of the excess electrolyte is greater than 4%.

5. An energy storage element as set forth in claim 1 or 2, wherein the thickness of said electrode body in a dry state and under no load is 0.94 or less relative to the inner dimension of said electrode body in the thickness direction of said container.

Images