Power storage element and power storage device

Abstract

A power storage element according to one aspect of the present invention is provided with: a flattened wound electrode body which includes a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and which has a pair of curved parts facing each other; an electrolyte solution; and a container which houses the electrode body and the electrolyte solution. In at least one of the pair of curved parts, a slit which extends along the winding axis of the electrode body is substantially provided only in the positive electrode active material layer among the positive electrode active material layer and the negative electrode active material layer.

Description

Energy storage element and energy storage device

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

[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] JP 2013-16440 A JP 2013-191467 A

[0004] In energy storage devices, the electrolyte impregnated within the electrode body leaks out due to factors such as expansion of the negative electrode active material during charging. Since not all of the leaked electrolyte returns to the electrode body during discharge, localized depletion of the electrolyte within the electrode body may occur. The depleted area within the electrode body does not contribute to charging and discharging. Therefore, depletion of the electrolyte within the electrode body leads to a decrease in the performance of the energy storage device, such as a decrease in discharge capacity. In particular, in the case of a wound-type 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 leaked out of the electrode body to return to the electrode body, making the above phenomenon more likely to occur. Furthermore, from the perspective of maintaining performance, it is desirable for energy storage devices to have a structure that suppresses the deposition of charge-transporting ions as metal on the negative electrode surface during charging.

[0005] The object of the present invention is to provide an energy storage element having a wound electrode body, in which the electrolyte in the electrode body is unlikely to be depleted during charging and discharging, and in which metal deposition on the negative electrode surface is suppressed, and an energy storage device using such an energy storage element.

[0006] An energy storage element according to one aspect of the present invention comprises a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and comprises a flat, wound electrode body having a pair of opposing curved portions, an electrolyte, and a container containing the electrode body and the electrolyte, and in at least one of the pair of curved portions, a slit along the winding axis of the electrode body is provided in substantially only the positive electrode active material layer out of the positive electrode active material layer and the negative electrode active material layer.

[0007] An electricity storage device according to another aspect of the present invention is configured by assembling a plurality of electricity storage elements, at least one of which is the electricity storage element according to the aspect of the present invention.

[0008] According to any one aspect of the present invention, it is possible to provide an energy storage element having a wound electrode body, in which the electrolyte in the electrode body is less likely to run out during charging and discharging, and in which metal deposition on the negative electrode surface is suppressed, and an energy storage device using such an energy storage element.

[0009] 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 as viewed from the arrows I-I. FIG. 3 is a schematic diagram showing the configuration of an electrode body included in the energy storage element of FIG. 1. FIG. 4 is a schematic cross-sectional view showing one embodiment of a curved portion of the electrode body. FIG. 5 is a schematic cross-sectional view showing an embodiment of the curved portion of the electrode body different from that shown in FIG. 4. FIG. 6 is a schematic cross-sectional view showing an embodiment of the curved portion of the electrode body different from that shown in FIGS. 4 and 5. FIG. 7 is a schematic partial plan view showing one embodiment of a positive electrode. FIG. 8 is a schematic partial cross-sectional view of the positive electrode of FIG. 7. FIG. 9 is a schematic diagram showing an energy storage device formed by assembling a plurality of energy storage elements according to one embodiment of the present invention.

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

[0011] [1] An energy storage element according to one aspect of the present invention includes a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and includes a flat, wound electrode body having a pair of opposing curved portions, an electrolyte, and a container that contains the electrode body and the electrolyte, and in at least one of the pair of curved portions, a slit that is aligned with the winding axis of the electrode body is provided in substantially only the positive electrode active material layer of the positive electrode active material layer and the negative electrode active material layer.

[0012] The energy storage element described in [1] above is less likely to experience depletion of the electrolyte within the electrode body during charging and discharging, and metal deposition on the negative electrode surface is suppressed. The reason for this effect is unclear, but the following reason is presumed. In a wound-type electrode body, the pores of the porous negative electrode active material layer are likely to contract, particularly at curved portions, due to expansion of the negative electrode active material, etc. Therefore, in the case of a conventional wound-type electrode body, the electrolyte is likely to flow out, particularly at the curved portions of the electrode body, and the flowed-out electrolyte is difficult to return to the electrode body, resulting in depletion of the electrolyte. In contrast, at least one of the pair of curved portions of the wound-type electrode body in the energy storage element described in [1] above has a slit along the winding axis. This slit serves as a flow path for the electrolyte, allowing the electrolyte to easily permeate into the electrode body through the slit. Therefore, the energy storage element described in [1] is less likely to experience depletion of the electrolyte within the electrode body. Furthermore, if the slits are provided mainly in the negative electrode active material layer, metal precipitation originating from charge transport ions is likely to occur on the negative electrode surface during charging. In contrast, in the energy storage element described in [1] above, the slits are provided substantially only in the positive electrode active material layer out of the positive electrode active material layer and the negative electrode active material layer, thereby making it possible to suppress metal precipitation on the negative electrode surface.

[0013] The phrase "a slit along the winding axis of the electrode body is provided substantially only in the positive electrode active material layer of the positive electrode active material layer and the negative electrode active material layer" means that a slit is provided in at least the positive electrode active material layer, and a slit may also be provided in the negative electrode active material layer to the extent that the effect is not impaired. For example, the total width of all slits provided in the negative electrode active material layer may be 1 / 10 or less, or 1 / 100 or less, of the total width of all slits provided in the positive electrode active material layer. The slit along the winding axis of the electrode body may be provided only in the positive electrode active material layer of the positive electrode active material layer and the negative electrode active material layer.

[0014] [2] In the energy storage element described in [1] above, in a normal state of use, the winding axis of the electrode body may be arranged in a horizontal direction, and the slit may be provided at a position below the liquid surface of the electrolyte in the normal state of use.

[0015] In the energy storage element described in [2] above, the slits are provided at a position below the liquid surface of the electrolyte, which makes it easier for the electrolyte to penetrate through the slits, making it less likely that the electrolyte will run out in the electrode body during charging and discharging.

[0016] The "electrolyte liquid level" refers to the liquid level when the energy storage element is charged to the end-of-charge voltage during normal use. Here, the "end-of-charge voltage during normal use" refers to the end-of-charge voltage when the energy storage element is used under the charging conditions recommended or specified for the energy storage element, and if dedicated equipment for using the energy storage element is available, refers to the end-of-charge voltage when the energy storage element is used with that equipment. The electrolyte liquid level and the position of the slit can be confirmed, for example, using an X-ray computed tomography (CT) device.

[0017] [3] In the energy storage element according to [2] above, the slits may be provided only at positions below the liquid surface of the electrolyte in the normal state of use.

[0018] When slits are provided in the positive electrode active material layer, the amount of positive electrode active material is reduced by the amount of the slits, so it is preferable to provide as few slits as possible from the viewpoint of discharge capacity, etc. On the other hand, from the viewpoint of electrolyte penetration, it is sufficient to provide slits only at positions below the liquid level of the electrolyte. In the energy storage element described in [3] above, since the slits are provided only at positions below the liquid level of the electrolyte, the discharge capacity is sufficiently large and the electrolyte in the electrode body is unlikely to run out during charge and discharge.

[0019] [4] In the energy storage element according to any one of [1] to [3] above, the slit may be provided in only one of the pair of curved portions.

[0020] In the energy storage element described in [4] above, the slit is provided in only one of the pair of curved portions, and therefore the discharge capacity is sufficiently large.

[0021] [5] In the energy storage element according to any one of [1] to [4] above, the slit may be provided around the entire circumference of the positive electrode active material layer in the curved portion of the pair of curved portions in which the slit is provided.

[0022] In the energy storage element described in [5] above, the electrolyte can penetrate through the slits provided around the entire circumference of the positive electrode active material layer in the curved portion, making it less likely that the electrolyte will run out in the electrode body during charging and discharging.

[0023] With regard to "the slits being provided around the entire periphery of the positive electrode active material layer," for example, in the case of a positive electrode in which a positive electrode active material layer is provided on each of both sides of a positive electrode substrate, slits may be provided around the entire periphery of the positive electrode active material layer on at least one of the surfaces. In this case, slits may or may not be provided in the positive electrode active material layer on the other surface. In another embodiment, in the case of a positive electrode in which a positive electrode active material layer is provided on each of both sides of a positive electrode substrate, slits may be provided around the entire periphery of the positive electrode active material layer on both surfaces.

[0024] [6] In the energy storage element according to any one of [1] to [4] above, in the curved portion of the pair of curved portions in which the slit is provided, the slit may be provided only around a portion of the periphery of the positive electrode active material layer.

[0025] In the energy storage element described in [6] above, the number of slits can be reduced, so that the discharge capacity is sufficiently large and the decrease in strength at the curved portion is suppressed.

[0026] With regard to "the slits being provided only on a portion of the periphery of the positive electrode active material layer," for example, in the case of a positive electrode in which a positive electrode active material layer is provided on each of both surfaces of a positive electrode substrate, the slits may be provided only on a portion of the periphery of the positive electrode active material layer on at least one surface. In this case, the positive electrode active material layer on the other surface may or may not have a slit. In another embodiment, in the case of a positive electrode in which a positive electrode active material layer is provided on each of both surfaces of a positive electrode substrate, the slits may be provided only on a portion of the periphery of the positive electrode active material layer on both surfaces.

[0027] [7] In the energy storage element described in any one of [1] to [6] above, in the curved portion of the pair of curved portions where the slit is provided, the slit may be provided on multiple different circumferences of the positive electrode active material layer, and at least one set of the slits provided on different circumferences may be arranged so as not to overlap when viewed in the depth direction of the slits.

[0028] The energy storage element described in [7] above excludes an embodiment in which all of the slits provided on multiple different circumferences in the curved portion are arranged so as to overlap when viewed in the depth direction of the slits. In the energy storage element described in [7] above, since all of the slits provided on multiple different circumferences do not overlap, it is possible to suppress the occurrence of wrinkles and a decrease in strength in the curved portion.

[0029] [8] In the energy storage element according to any one of [1] to [7] above, the positive electrode may have an insulating layer provided linearly so as to cover a boundary between the slit and other regions in the positive electrode active material layer.

[0030] In the energy storage element described in the above [8], the insulating layer can prevent the positive electrode active material layer from peeling off from the boundary with the slit.

[0031] [9] In the energy storage element according to any one of [1] to [8] above, at least one of the positive electrode and the negative electrode may not have a through-hole.

[0032] In the energy storage element described in [9] above, since at least one of the positive electrode and the negative electrode does not have a through-hole, the discharge capacity and strength are sufficiently large. From this viewpoint, it is preferable that neither the positive electrode nor the negative electrode has a through-hole.

[0033]

[10] In the energy storage element according to any one of [1] to [9] above, the electrode body and the container may be in contact with each other directly or via another member in the thickness direction of the electrode body.

[0034] Typically, when a wound electrode body is in contact with a container in the thickness direction of the electrode body, either directly or via another member, the expansion of the electrode body in the thickness direction is suppressed, resulting in more likely shrinkage of the voids in the negative electrode active material layer at the curved portion and depletion of the electrolyte. Therefore, the energy storage element described in

[10] above has the particularly effective advantage of being less susceptible to depletion of the electrolyte within the electrode body during charging and discharging. Furthermore, according to the energy storage element described in

[10] above, the electrode body is sufficiently fixed within the container, so that it can exhibit good charge / discharge performance even in an environment where vibration occurs. The contact between the electrode body and the container in the thickness direction of the electrode body, either directly or via another member, can be confirmed, for example, using an X-ray computed tomography (CT) device.

[0035]

[11] In the energy storage element according to

[10] above, the container may be constrained or pressed in the thickness direction of the electrode body.

[0036] When the container is constrained or pressed in the thickness direction of the electrode body, the electrode body is pressed in the thickness direction via the container, and expansion of the electrode body in the thickness direction is further suppressed, making it more likely that the voids in the negative electrode active material layer at the curved portion will shrink and the electrolyte will be depleted. Therefore, in the energy storage element described in

[11] above, the advantage of being less likely to cause depletion of the electrolyte in the electrode body during charge and discharge is more effectively exhibited. Furthermore, according to the energy storage element described in

[11] above, the electrode body is more sufficiently fixed within the container, making it possible to improve charge and discharge performance, for example, in an environment where vibration occurs.

[0037]

[12] In the energy storage element according to any one of [1] to

[11] above, the ratio (L / W) of the length (L) of the electrode body in the direction of the winding axis to the width (W) in the direction perpendicular to the winding axis may be 2 or more.

[0038] As with the energy storage element described in

[12] above, an energy storage element including an electrode body that is long in the direction of the winding axis may be required depending on the application. On the other hand, when the electrode body is long in the direction of the winding axis, it is difficult for the electrolyte to penetrate to the center portion of the electrode body, making it particularly susceptible to depletion of the electrolyte. Therefore, the energy storage element described in

[12] above also effectively exhibits the advantage that the electrolyte in the electrode body is less likely to be depleted during charging and discharging.

[0039] The "width (W)" of the electrode body refers to the maximum length in the direction perpendicular to the winding axis of the electrode body.

[0040]

[13] In the energy storage element according to any one of [1] to

[12] above, the length (L) of the electrode body in the direction of the winding axis may be 60 mm or more.

[0041] In the energy storage element described in

[13] above, when the length (L) of the electrode body in the direction of the winding axis is as long as 60 mm or more, the electrolyte is less likely to penetrate into the central part of the electrode body, making it particularly susceptible to depletion of the electrolyte. Therefore, the advantage of being less likely to deplete the electrolyte in the electrode body during charging and discharging is particularly effectively exhibited in the energy storage element described in

[13] above.

[0042]

[14] Another aspect of the present invention provides an energy storage device comprising a plurality of energy storage elements, at least one of which is the energy storage element described in any one of [1] to

[13] above.

[0043] The energy storage device described in

[14] above includes at least one energy storage element described in any one of [1] to

[13] above, and therefore is less likely to experience depletion of the electrolyte in the electrode body during charging and discharging, and is suppressed from depositing metal on the surface of the negative electrode.

[0044] 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.

[0045] <Electricity storage element> An energy storage element 100 according to one embodiment of the present invention, shown in Figures 1 and 2, comprises an electrode body 1, an electrolyte solution 10 (not shown in Figure 1; see Figure 2), and a container 2 for accommodating these. The energy storage element 100 further comprises 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 electrode body 1 comprises a positive electrode and a negative electrode, as described below. 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. 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.

[0046] The container 2 is a sealed container that contains the electrode assembly 1 and the electrolyte solution 10. The container 2 shown in Fig. 1 etc. is a square container having a substantially rectangular parallelepiped shape. The material of the container 2 may be any material that has the sealability to seal in the electrolyte solution 10 and the strength to protect the electrode assembly 1, and may be, for example, a resin or a metal.

[0047] As described above, the container 2 contains the electrolyte solution 10 together with the electrode assembly 1. A portion of the electrolyte solution 10 permeates the electrode assembly 1, and the remainder of the electrolyte solution 10 accumulates on the bottom surface 25 side of the container 2 (see FIG. 2). That is, the electrolyte solution 10 is present between the electrode assembly 1 and the container 2 on the bottom surface 25 side. The electrolyte solution 10 usually contains an electrolyte salt and a solvent. Details of the electrolyte solution 10 will be described later.

[0048] The electrode assembly 1 is a flat, wound electrode assembly. As shown in Fig. 3, the electrode assembly 1 includes a strip-shaped positive electrode 11, a strip-shaped first separator 12a, a strip-shaped negative electrode 13, and a strip-shaped second separator 12b. The electrode assembly 1 is formed by winding the positive electrode 11, the first separator 12a, the negative electrode 13, and the second separator 12b, which are stacked in this order, in the longitudinal direction with the second separator 12b on the outside.

[0049] This flat, wound electrode assembly 1 has a pair of curved portions 14a, 14b that face each other across the winding axis A, and a flat portion 15 located between the pair of curved portions 14a, 14b. In the curved portions 14a, 14b, the positive electrode 11, the negative electrode 13, and the separators 12a, 12b are arranged in a semicircular curve in the width direction (Z-axis direction) of the electrode assembly 1 when viewed from the direction of the winding axis A (when viewed from the X-axis direction) (see FIG. 2, etc.). In the flat portion 15, the positive electrode 11, the negative electrode 13, and the separators 12a, 12b are arranged in a planar manner and parallel to each other. Here, the "curved portion" of the electrode body 1 refers to the approximately semicircular portion located at both ends of the width direction (Z-axis direction) of the electrode body 1 when viewed in the direction of the winding axis A (when viewed in the direction of the X-axis). Specifically, when the maximum thickness of the electrode body 1 in the thickness direction (Y-axis direction) is T, the "curved portion" refers to the region from both ends of the width direction (Z-axis direction) of the electrode body 1 to a length of T / 2 when viewed in the direction of the winding axis A (see Figure 2).

[0050] The lower limit of the ratio (L / W) of the length (L) of the electrode body 1 along the winding axis A to the width (W) perpendicular to the winding axis A is not particularly limited and may be, for example, 1.5, 2, or 3. For example, increasing the length (L) of the electrode body 1 along the winding axis A can relatively reduce the size of the positive electrode active material layer non-laminated region 18 and the negative electrode active material layer non-laminated region 21 in the electrode body 1, which will be described later, thereby increasing the energy density. Furthermore, by employing the present invention in an energy storage device 100 including an electrode body 1 having a relatively large ratio (L / W), the advantage of less depletion of the electrolyte solution 10 in the electrode body 1 can be particularly effectively enjoyed. The upper limit of the ratio (L / W) may be 5, 4, 3, or 2. The ratio (L / W) may be within a range that is a combination of any of the above-mentioned lower limits and any of the above-mentioned upper limits (provided that the upper limit is greater than the lower limit).

[0051] The lower limit of the length (L) of the electrode body 1 in the direction of the winding axis A is not particularly limited and may be, for example, 40 mm, 60 mm, or 80 mm. For example, increasing the length (L) of the electrode body 1 in the direction of the winding axis A allows the positive electrode active material layer non-laminated region 18 and the negative electrode active material layer non-laminated region 21, described below, in the electrode body 1 to be relatively small, thereby increasing the energy density, etc. Furthermore, by employing the present invention in an energy storage device 100 including an electrode body 1 having a relatively large length (L), the advantage of less depletion of the electrolyte solution 10 in the electrode body 1 can be particularly effectively enjoyed. The upper limit of the length (L) may be 200 mm, 180 mm, 160 mm, or 140 mm. The length (L) may be within a range that is a combination of any of the above-mentioned lower limits and any of the above-mentioned upper limits.

[0052] As shown in FIGS. 3 and 4 , the positive electrode 11 has a strip-shaped positive electrode substrate 16 and positive electrode active material layers 17 (17a, 17b) laminated on both sides of the positive electrode substrate 16. At one end in the width direction of the strip-shaped positive electrode substrate 16 (the lower side in FIG. 3 ), the positive electrode active material layer 17 is not laminated, and is exposed as a positive electrode active material layer non-laminated region 18. The positive electrode active material layer non-laminated region 18 of the positive electrode 11 is the portion connected to the positive electrode connecting member 3. The negative electrode 13 has a strip-shaped negative electrode substrate 19 and negative electrode active material layers 20 (20a, 20b) laminated on both sides of the negative electrode substrate 19. At one end in the width direction of the strip-shaped negative electrode substrate 19 (the upper side in FIG. 3 ), the negative electrode active material layer 20 is not laminated, and is exposed as a negative electrode active material layer non-laminated region 21. The negative electrode active material layer non-laminated region 21 of the negative electrode 13 is the portion that is connected to the negative electrode connecting member 5. Details of each component of the positive electrode 11, the negative electrode 13, and the separators 12a and 12b will be described later.

[0053] At least one of the pair of curved portions 14 a, 14 b has a slit 22 formed in the positive electrode active material layer 17 along the winding axis A of the electrode body 1. That is, the slit 22 is formed in the width direction of the strip-shaped positive electrode active material layer 17 from one end to the other end of the positive electrode active material layer 17. Substantially, no slit 22 is formed in the negative electrode active material layer 20. In other words, of the positive electrode active material layer 17 and the negative electrode active material layer 20, the slit 22 is formed substantially only in the positive electrode active material layer 17. In the energy storage element 100, because the electrode body 1 has such a slit 22, depletion of the electrolyte 10 in the electrode body 1 during charging and discharging is unlikely to occur, and metal deposition on the surface of the negative electrode 13 is suppressed.

[0054] The slits 22 may be provided in the positive electrode active material layer 17 so that the positive electrode substrate 16 is exposed, or may be provided so that the positive electrode substrate 16 is not exposed. However, from the viewpoint of achieving good permeability of the electrolyte solution 10, the average depth of the slits 22 is preferably at least half, and more preferably at least two-thirds, of the average thickness of one positive electrode active material layer 17. It is particularly preferable that the slits 22 are provided so that the positive electrode substrate 16 is exposed. The average depth of the slits 22 is the average value of the depths of the slits 22 measured at any five positions in one slit 22. When multiple slits 22 are provided, the average depth of each slit 22 is a value determined for each individual slit 22. When multiple slits 22 are provided, the average depths of the slits 22 may be the same or different. The average thickness of one positive electrode active material layer 17 is the average value of the thicknesses of one positive electrode active material layer 17 measured at any five positions. The average thickness of one positive electrode active material layer 17 may be, for example, 5 μm or more and 200 μm or less, or 10 μm or more and 100 μm or less.

[0055] The width of the slit 22 is not particularly limited, but is preferably 0.5 mm or more and 20 mm or less. The lower limit of this width may be 1 mm. The upper limit of this width may be 2 mm. When the width of the slit 22 is equal to or greater than the above-mentioned lower limit, it is possible to increase the permeability of the electrolyte solution 10, etc. On the other hand, when the width of the slit 22 is equal to or less than the above-mentioned upper limit, it is possible to suppress the occurrence of wrinkles in the curved portion 14, etc. When multiple slits 22 are provided, the widths of the multiple slits 22 may be the same or different.

[0056] The number of slits 22 is also not particularly limited. The number of slits provided in one bending portion 14 may be one or more. The lower limit of the total number of slits 22 provided in one bending portion 14 may be 1, or may be 3, 5, or 10. The upper limit of the total number of slits 22 provided in one bending portion 14 may be 1,0000, or may be 200, 100, 50, or 20. The upper limit of the total number of slits 22 provided in one bending portion 14 may be 8, 4, 2, or 1 times the number of turns. The total number of slits 22 provided in one bending portion 14 can be within a range that is a combination of any of the above-mentioned lower limits and any of the above-mentioned upper limits.

[0057] In the curved portion 14 shown in FIG. 4 , slits 22 are provided in both of the two positive electrode active material layers 17a, 17b stacked on both sides of the positive electrode substrate 16. Furthermore, the slits 22 are provided around the entire circumference of each of the positive electrode active material layers 17a, 17b. When the slits 22 are provided around the entire circumference of each of the two positive electrode active material layers 17a, 17b in the curved portion 14, as in this embodiment, the permeability of the electrolyte 10 is particularly high. Furthermore, when the slits 22 are provided in both of the two positive electrode active material layers 17a, 17b stacked on both sides of the positive electrode substrate 16, as in this embodiment, the permeability of the electrolyte 10 is also particularly high. As in this embodiment, the same number of slits 22 may be provided per turn (e.g., one per turn). While FIG. 4 shows the positive electrode 11 and other components wound three times, this is merely a schematic representation, and typically, a wound-type electrode body 1 has a greater number of turns (the same applies to FIG. 5 and other figures described later).

[0058] In the embodiment shown in FIG. 4 , the slits 22 provided in the inner positive electrode active material layer 17a and the outer positive electrode active material layer 17b are arranged in the same position (overlapping positions when viewed in the depth direction of the slits via the positive electrode substrate 16) relative to the positive electrode substrate 16. However, other arrangements are also possible. That is, the slits 22 provided in the inner positive electrode active material layer 17a and the slits 22 provided in the outer positive electrode active material layer 17b may be arranged in positions that do not overlap when viewed in the depth direction of the slits. By arranging the inner slits 22 and the outer slits 22 so that they are offset when viewed in the depth direction of the slits, it is possible to suppress the occurrence of wrinkles in the curved portion 14. The number, width, etc. of slits 22 provided on the inner and outer sides may be different. Alternatively, the slits 22 may be provided in only one of the two positive electrode active material layers 17a, 17b. In this case, the slits 22 may be provided only in the inner positive electrode active material layer 17a, or only in the outer positive electrode active material layer 17b.

[0059] In another form of curved portion 14A shown in FIG. 5 , slits 22 are provided only on a portion of the circumference of the positive electrode active material layers 17a, 17b. Specifically, slits 22 are provided in the positive electrode active material layers 17a, 17b on the first and third circumferences counting from the inside, while no slits 22 are provided in the positive electrode active material layers 17a, 17b on the second circumference. By not providing slits 22 on a portion of the circumference in this way, the discharge capacity is sufficiently large and reduction in strength and occurrence of wrinkles in the curved portion 14A can be suppressed. Alternatively, slits 22 may be provided in both of the two positive electrode active material layers 17a, 17b on a portion of the circumference, and slits 22 may be provided in only one of the two positive electrode active material layers 17a, 17b on another portion of the circumference.

[0060] In another embodiment of the curved portion 14B shown in FIG. 6 , one slit 22 is provided in the positive electrode active material layers 17a, 17b in the first and third turns counting from the inside, and two slits 22 are provided in the positive electrode active material layers 17a, 17b in the second turn. The slits 22 in the positive electrode active material layers 17a, 17b in the second turn are positioned so as not to overlap with the slits 22 in the positive electrode active material layers 17a, 17b in the first and third turns as viewed in the depth direction of the slits 22. In this embodiment, the slits 22 provided in multiple different turns do not all overlap, thereby preventing wrinkles and a decrease in strength in the curved portion 14B. As in the embodiments shown in FIGS. 5 and 6 , different numbers of slits 22 may be provided in each turn.

[0061] 4 to 6, no through-holes are provided in the positive electrode 11 or the negative electrode 13. A through-hole refers to a hole that penetrates from one surface to the other surface in the thickness direction of the positive electrode 11 or the negative electrode 13. By not providing a through-hole in at least one of the positive electrode 11 or the negative electrode 13, the energy storage element 100 has the advantage of having sufficiently large discharge capacity and strength.

[0062] As another example, as shown in FIGS. 7 and 8 , a linear insulating layer 24 may be provided to cover the boundary 23 between the slits 22 and the remaining area in the positive electrode active material layer 17, as in the positive electrode 11A. In the positive electrode 11A, the positive electrode active material layer 17 is provided only on one side of the positive electrode substrate 16. The insulating layer 24 is provided to cover the end surface of the positive electrode active material layer 17 resulting from the provision of the slits 22 in the positive electrode active material layer 17. By providing such an insulating layer 24 in the positive electrode 11A, peeling of the positive electrode active material layer 17 from the boundary with the slits 22 can be suppressed. Furthermore, the provision of the insulating layer 24 can deepen the slits 22, which is expected to improve the permeability of the electrolyte solution 10. The insulating layer 24 can be a porous layer composed of insulating particles, such as inorganic oxide particles, and a binder. Examples of the insulating particles include fillers, which are exemplified as optional components of the positive electrode active material layer, as described below. As the binder contained in the insulating layer 24, binders exemplified as optional components of the positive electrode active material layer described later can be used.

[0063] 2 , the electrode body 1 is placed in the container 2 so that the winding axis A of the electrode body 1 is horizontal (X-axis direction) and the thickness direction of the electrode body 1 is horizontal (Y-axis direction). In the energy storage element 100, the winding axis A of the electrode body 1 is parallel to the bottom surface 25 of the container 2.

[0064] The energy storage element 100 may be arranged such that, in normal use, the winding axis A of the electrode body 1 is horizontal. The energy storage element 100 may be arranged such that, in normal use, the width direction of the electrode body 1 is vertical (Z-axis direction). The normal use state may be, for example, a state in which the energy storage element 100 is used under conditions recommended or specified for the energy storage element 100. Furthermore, if there are no recommended or specified conditions, the normal use state may be a state in which the surface on which the positive electrode external terminal 4 and the negative electrode external terminal 6 are provided is arranged facing upward.

[0065] As shown in Fig. 2, in the energy storage element 100, a slit 22 is provided only in one curved portion 14a of a pair of curved portions 14a, 14b of the electrode body 1. The slit 22 is provided at a position below a liquid level 26 of the electrolyte solution 10 in normal use. In this embodiment, the slit 22 is not provided at a position above the liquid level 26 of the electrolyte solution 10 in normal use. That is, the slit 22 is provided only at a position below the liquid level 26 of the electrolyte solution 10 in normal use. By providing the slit 22 at such a position, it is possible to sufficiently prevent the depletion of the electrolyte solution 10 in the electrode body 1 during charging and discharging while ensuring sufficient discharge capacity, etc.

[0066] In one embodiment, slits 22 may be provided in both of the pair of curved portions 14a, 14b of the electrode body 1, or in only one of them. Furthermore, slits 22 may be provided both below and above the liquid level 26 of the electrolyte solution 10 in normal use, or different numbers, widths, etc. of slits 22 may be provided below and above the liquid level 26 of the electrolyte solution 10 in normal use. The total width of all slits 22 present below the liquid level 26 of the electrolyte solution 10 in normal use is preferably greater than the total width of all slits 22 present above the liquid level 26 of the electrolyte solution 10 in normal use. For example, the total width of all slits 22 present in the positive electrode active material layer 17 of one curved portion 14a is preferably greater than the total width of all slits 22 present in the positive electrode active material layer 17 of the other curved portion 14b. The total width of all slits 22 present in the positive electrode active material layer 17 of one curved portion 14a is preferably at least twice, and more preferably at least three times, the total width of all slits 22 present in the positive electrode active material layer 17 of the other curved portion 14b. In this case, one curved portion 14a is preferably positioned below the liquid level 26 of the electrolyte 10 in normal use.

[0067] In the energy storage element 100 of the embodiment shown in FIG. 2 , the electrode body 1 and the container 2 are in direct contact with each other in the thickness direction of the electrode body 1. Alternatively, in the energy storage element 100, the electrode body 1 and the container 2 may be in contact with each other in the thickness direction of the electrode body 1 via another member (e.g., an insulating sheet, etc.). In such a case, the electrode body 1 is sufficiently fixed within the container 2, and therefore, for example, good charge / discharge performance can be exhibited even in an environment where vibration occurs. Furthermore, in the past, when the electrode body 1 and the container 2 were in contact with each other in the thickness direction of the electrode body 1 directly or via another member, the electrolyte 10 in the electrode body 1 was likely to be depleted. Therefore, by employing the present invention in such an energy storage element 100, the advantage of less depletion of the electrolyte 10 in the electrode body 1 can be particularly effectively enjoyed.

[0068] In the energy storage element 100, the container 2 may be constrained or pressed in the thickness direction of the electrode body 1. Specifically, the container 2 may be constrained in the thickness direction of the electrode body 1 so as to prevent it from expanding in the thickness direction (Y-axis direction). Furthermore, the container 2 may be pressed in the thickness direction of the electrode body 1 so as to prevent it from expanding in the thickness direction. For example, a constraining member or pressing member that performs such constraining or pressing may be provided for the energy storage element 100. When the container 2 is constrained or pressed in the thickness direction of the electrode body 1, the electrode body 1 is sufficiently fixed within the container, thereby improving charge / discharge performance, for example, in an environment where vibration occurs. On the other hand, in a conventional case where the container is constrained or pressed in the thickness direction of the electrode body 1, the electrode body is pressed in the thickness direction via the container, which suppresses expansion of the electrode body in the thickness direction, making it particularly susceptible to depletion of the electrolyte solution within the electrode body. Therefore, by adopting the present invention in such an energy storage element 100, the advantage of less depletion of the electrolyte solution 10 within the electrode body 1 can be particularly effectively enjoyed.

[0069] The specific configurations of the positive electrode, negative electrode, separator, and electrolyte that constitute the electrode assembly will be described in detail below.

[0070] (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.

[0071] 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 －２ 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).

[0072] 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.

[0073] 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.

[0074] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer is usually porous. The positive electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed.

[0075] 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 ２Examples 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 ２CoPO 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.

[0076] 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).

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0084] The filler is not particularly limited, and examples of the filler 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, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0085] 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.

[0086] The porosity of the positive electrode active material layer may be, for example, 15% or more and 50% or less, 20% or more and 45% or less, or 25% or more and 40% or less. The present invention is particularly suitable for application to a storage element in which the positive electrode active material layer has such a porosity. The "porosity (%)" of the positive electrode active material layer and the negative electrode active material layer described later is calculated by dividing the apparent volume (volume including voids) of the positive (negative) electrode active material layer by V. １ The sum of the actual volumes of the materials constituting the positive (negative) electrode active material layer is V ２ In this case, (1-V ２ / V １ The sum of the actual volumes of the materials constituting the positive (negative) electrode active material layer, V ２can be calculated from the content of each material in the positive (negative) electrode active material layer and the true density of each material.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer is usually porous. 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 a conductive agent, a binder, a thickener, and a filler can be selected from the materials exemplified for the positive electrode above.

[0091] 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.

[0092] 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 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 ７ Examples 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.

[0093] "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.

[0094] "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.

[0095] 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.

[0096] "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.

[0097] "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.

[0098] 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 the electrolyte tends to run out of the electrode body during charge and discharge cycles. Therefore, when the present invention is applied to an energy storage device equipped with a wound electrode body having such a negative electrode active material, the advantage of being less likely to cause electrolyte runout within the electrode body can be particularly effectively enjoyed.

[0099] 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 Si or Sn, the negative electrode active material layer may be in the form of a foil.

[0100] 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.

[0101] The porosity of the negative electrode active material layer may be, for example, 30% to 70%, 35% to 60%, or 40% to 50%. The present invention is particularly suitable for use with an energy storage device in which the negative electrode active material layer has such a porosity.

[0102] (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 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.

[0103] 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.

[0104] 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.

[0105] The separator may be a polymer gel composed of a polymer and an 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.

[0106] (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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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 ) (SO ２ 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.

[0113] 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 ３It 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.

[0114] 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.

[0115] 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.

[0116] <Electricity Storage Device> The energy storage element of the present embodiment can be mounted as an energy storage unit (battery module) configured by assembling a plurality of energy storage elements in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), or in a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one energy storage element included in the energy storage unit.

[0117] 9 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.

[0118] <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 body, preparing an electrolyte solution, and housing the electrode body and the electrolyte solution in a container. Preparing the electrode body includes preparing a positive electrode and a negative electrode, and forming the electrode body by winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0119] Preparing a positive electrode may also mean manufacturing a positive electrode. The positive electrode can be manufactured by a known method. The positive electrode can be manufactured, for example, by applying a paste-like positive electrode mixture (positive electrode mixture paste) to a positive electrode substrate directly or via an intermediate layer, and drying the paste to form a positive electrode active material layer. In this case, slits can be formed by intermittently applying the positive electrode mixture paste. The positive electrode mixture paste usually contains a positive electrode active material, other optional components, and a dispersion medium. After drying, the positive electrode active material layer may be pressed, for example. The insulating layer described above can also be formed, for example, by applying and drying an insulating layer paste containing insulating particles and a binder.

[0120] Preparing the negative electrode may also mean manufacturing the negative electrode. The negative electrode can be manufactured by a known method. The negative electrode can be manufactured, for example, in the same manner as in the above-described method for manufacturing the positive electrode, by applying a paste-like negative electrode mixture (negative electrode mixture paste) to the negative electrode substrate directly or via an intermediate layer, and drying the mixture to form a negative electrode active material layer. After drying, the negative electrode active material layer may be pressed, etc.

[0121] The electrolyte solution can be placed in the container by any known method, for example, by injecting the electrolyte solution through an inlet formed in the container and then sealing the inlet.

[0122] <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.

[0123] 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.

[0124] In the above embodiment, the description has been given of an energy storage element in which slits are provided only in the curved portions of the electrode body, but slits may also be provided in the flat portions. The orientation of the energy storage element is not limited to the above-described embodiment, and the present invention may be applied to an energy storage element in which the orientation of the electrode body in normal use is not fixed. Furthermore, the present invention may be applied to an energy storage element in which the electrode body is housed in a container with the winding axis of the electrode body oriented perpendicular to the bottom surface of the container.

[0125] 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.

[0126] REFERENCE SIGNS LIST 100 Energy storage element 1 Electrode body 2 Container 3 Positive electrode connecting member 4 Positive electrode external terminal 5 Negative electrode connecting member 6 Negative electrode external terminal 10 Electrolyte 11, 11A Positive electrode 12a, 12b Separator 13 Negative electrode 14, 14a, 14b, 14A, 14B Curved portion 15 Flat portion 16 Positive electrode substrate 17, 17a, 17b Positive electrode active material layer 18 Positive electrode active material layer non-laminated region 19 Negative electrode substrate 20, 20a, 20b Negative electrode active material layer 21 Negative electrode active material layer non-laminated region 22 Slit 23 Boundary 24 Insulating layer 25 Bottom surface 26 Liquid surface 200 Energy storage unit 300 Energy storage device A Winding axis L Length of electrode body W Width of electrode body T Maximum thickness of electrode body

Claims

1. An energy storage element comprising: a flat, wound electrode body having a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, the electrode body having a pair of opposing curved portions; an electrolyte; and a container for accommodating the electrode body and the electrolyte, wherein a slit is provided in at least one of the pair of curved portions along a winding axis of the electrode body, substantially only in the positive electrode active material layer of the positive electrode active material layer and the negative electrode active material layer.

2. The energy storage element as described in claim 1, wherein, in normal use, the winding axis of the electrode body is arranged horizontally, and the slit is provided at a position below the liquid surface of the electrolyte in the normal use state.

3. The energy storage element according to claim 2, wherein the slits are provided only at positions below the liquid level of the electrolyte in the normal operating condition.

4. The energy storage element according to claim 1 or 2, wherein the slit is provided in only one of the pair of curved portions.

5. The energy storage element according to claim 1 or 2, wherein in the curved portion of the pair of curved portions in which the slits are provided, the slits are provided around the entire circumference of the positive electrode active material layer.

6. The energy storage element according to claim 1 or 2, wherein in the curved portion of the pair of curved portions in which the slit is provided, the slit is provided only around a portion of the circumference of the positive electrode active material layer.

7. An energy storage element as described in claim 1 or 2, wherein in the curved portion of the pair of curved portions in which the slits are provided, the slits are provided on multiple different circumferences of the positive electrode active material layer, and at least one set of the slits provided on different circumferences are arranged so as not to overlap when viewed in the depth direction of the slits.

8. The energy storage element according to claim 1 or 2, wherein the positive electrode has an insulating layer provided linearly so as to cover the boundary between the slit and the other area in the positive electrode active material layer.

9. The energy storage element according to claim 1 or 2, wherein at least one of the positive electrode and the negative electrode has no through-hole.

10. The energy storage element according to claim 1 or 2, wherein the electrode body and the container are in contact with each other directly or via another member in the thickness direction of the electrode body.

11. The energy storage element according to claim 10, wherein the container is restrained or pressed in the thickness direction of the electrode body.

12. The energy storage element according to claim 1 or 2, wherein the ratio (L / W) of the length (L) of the electrode body in the direction of the winding axis to the width (W) in the direction perpendicular to the winding axis is 2 or greater.

13. The energy storage element according to claim 1 or 2, wherein the length (L) of the electrode body in the direction of the winding axis is 60 mm or more.

14. An electricity storage device comprising a plurality of electricity storage elements, at least one of which is the electricity storage element according to claim 1 or 2.

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