Secondary batteries

Abstract

The present invention provides a secondary battery which has higher reliability. This secondary battery is provided with: a battery element which is obtained by stacking a first electrode and a second electrode upon each other, with a separator being interposed therebetween, and subsequently winding the stacked electrodes around a winding axis that extends in a first direction; and an outer package member which contains the battery element. The second electrode comprises: a second electrode collector which has a second electrode inner surface that faces the winding axis side, and a second electrode outer surface that is on the reverse side of the second electrode inner surface; a second electrode inner active material layer which is provided on the second electrode inner surface; and a second electrode outer active material layer which is provided on the second electrode outer surface. From the inner periphery-side end part of the battery element to the outer periphery-side end part of the battery element, the area density of the second electrode outer active material layer is higher than the area density of the second electrode inner active material layer, which faces the second electrode outer active material layer with the second electrode collector being interposed therebetween.

Description

Technical Field

[0001] The present disclosure relates to secondary batteries.

Background Art

[0002] Due to the widespread use of various electronic devices such as mobile phones, the development of secondary batteries is underway as a power source that is small and lightweight and can obtain a high energy density. This secondary battery includes a positive electrode, a negative electrode, and an electrolyte housed inside an exterior member, and various studies have been made regarding the configuration of the secondary battery (see, for example, Patent Document 1).

[0003] For example, Patent Document 1 describes a sealed power storage device including an electrode body in which a positive electrode body and a negative electrode body are laminated or wound via a separator, and an exterior case that houses the electrode body.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] Various studies have been made to improve the performance of secondary batteries. However, there is still room for improvement in the performance of secondary batteries.

[0006] Therefore, it is desirable to provide a secondary battery with higher reliability.

[0007] A secondary battery according to a first embodiment of one of the present disclosure comprises a battery element in which a first electrode and a second electrode are stacked with a separator and wound around a winding shaft extending in a first direction, and an outer casing member housing the battery element. The second electrode includes a second electrode current collector including an inner surface of the second electrode facing the winding shaft and an outer surface of the second electrode opposite to the inner surface of the second electrode, a second electrode inner active material layer provided on the inner surface of the second electrode, and a second electrode outer active material layer provided on the outer surface of the second electrode. From the inner circumference end of the battery element to the outer circumference end of the battery element, the area density of the second electrode outer active material layer is greater than the area density of the second electrode inner active material layer facing the second electrode outer active material layer across the second electrode current collector.

[0008] A secondary battery according to a second embodiment of one embodiment of the present disclosure comprises a battery element in which a first electrode and a second electrode are stacked with a separator and wound around a winding shaft extending in a first direction, and an outer casing member housing the battery element. The second electrode includes a second electrode current collector including an inner surface of the second electrode facing the winding shaft and an outer surface of the second electrode opposite to the inner surface of the second electrode, a second electrode inner active material layer provided on the inner surface of the second electrode, and a second electrode outer active material layer provided on the outer surface of the second electrode. The area density of the second electrode outer active material layer is highest at the inner circumference end of the battery element and decreases as it approaches the outer circumference end of the battery element from the inner circumference end. The area density of the second electrode inner active material layer is lowest at the inner circumference end of the battery element and increases as it approaches the outer circumference end of the battery element from the inner circumference end.

[0009] According to the secondary battery of the first and second embodiments of one embodiment of this disclosure, in the relationship between the first and second electrodes facing each other across a separator, the capacity of the second electrode is greater than the capacity of the first electrode. Therefore, the formation of precipitates during the battery reaction can be suppressed, and the deterioration of battery performance can be suppressed. Thus, it has high reliability.

[0010] Furthermore, the effects of this disclosure are not necessarily limited to those described herein, but may include any of the series of effects related to this technology described later. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a perspective view showing the configuration of a secondary battery as a first embodiment of the present disclosure. [Figure 2] Figure 2 is a cross-sectional view showing the configuration of the secondary battery shown in Figure 1. [Figure 3] Figure 3 is a cross-sectional view showing the configuration of the battery element shown in Figure 2. [Figure 4] Figure 4 is a cross-sectional view showing one example of the cross-sectional structure of the battery element shown in Figure 2. [Figure 5] Figure 5 is a schematic unfolded view of the positive and negative electrodes of the battery element shown in Figure 2. [Figure 6] Figure 6 is a perspective view showing the configuration of the outer casing used in the manufacturing process of the secondary battery shown in Figure 1. [Figure 7] Figure 7 is an explanatory diagram illustrating the relationship between the capacitance of the positive electrode and the capacitance of the negative electrode in the battery element shown in Figure 2. [Figure 8] Figure 8 is a schematic unfolded view showing the positive and negative electrodes of a battery element of a secondary battery as a second embodiment of the present disclosure. [Figure 9] Figure 9 is an explanatory diagram illustrating the relationship between the capacitance of the positive electrode and the capacitance of the negative electrode in the battery element shown in Figure 8. [Figure 10] Figure 10 is a schematic unfolded view showing the positive and negative electrodes of a battery element of a secondary battery as a third embodiment of the present disclosure. [Figure 11] Figure 11 is an explanatory diagram illustrating the relationship between the capacitance of the positive electrode and the capacitance of the negative electrode in the battery element shown in Figure 10. [Modes for carrying out the invention]

[0012] Hereinafter, one embodiment of this disclosure will be described in detail with reference to the drawings. The order of description is as follows. 1. Secondary battery of the first embodiment 1-1. Composition 1-2.Operation 1-3. Manufacturing method 1-4. Function and effect 2. Secondary battery of the second embodiment 2-1. Structure 2-2. Operation 2-3. Manufacturing method 2-4. Function and effect 3. Secondary battery of the third embodiment 3-1. Structure 3-2. Operation 3-3. Manufacturing method 3-4. Function and effect 4. Examples

[0013] <1. Secondary battery of the first embodiment> First, the secondary battery of the first embodiment of the present disclosure will be described.

[0014] The secondary battery described here has a flat and columnar three-dimensional shape and is called a so-called coin type or button type. As will be described later, this secondary battery has a pair of bottoms facing each other and a side wall portion located between the pair of bottoms. In this secondary battery, the height is smaller than the outer diameter. The "outer diameter" mentioned here is the maximum diameter (maximum outer diameter) of the bottom. In this secondary battery, the maximum diameters of each of the pair of opposing bottoms are substantially equal to each other. Also, the "height" mentioned here is the maximum distance from the upper surface of one bottom to the lower surface of the other bottom. In the present embodiment, the direction in which the pair of bottoms face each other is defined as the height direction Z.

[0015] The charging and discharging principle of a secondary battery is not particularly limited, but the following description will focus on a case where battery capacity is obtained by utilizing the intercalation and deintercalation of electrode reactive materials. This secondary battery includes an electrolyte along with a positive electrode and a negative electrode. In this secondary battery, in order to prevent the deposition of electrode reactive materials on the surface of the negative electrode during charging, the charging capacity of the negative electrode is greater than the discharging capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is set to be greater than the electrochemical capacity per unit area of ​​the positive electrode. Furthermore, the secondary battery of this embodiment is a high-charging-voltage secondary battery that can exhibit good cycle characteristics without reducing energy density even when charging is performed at a high voltage of 4.38V or higher.

[0016] The types of electrode reactants are not particularly limited, but specifically, they are light metals such as alkali metals and alkaline earth metals. Alkali metals include lithium, sodium, and potassium, while alkaline earth metals include beryllium, magnesium, and calcium.

[0017] In the following example, we will consider the case where lithium is the electrode reactant. A secondary battery that obtains battery capacity by utilizing the intercalation and deintercalation of lithium is a so-called lithium-ion secondary battery. In this lithium-ion secondary battery, lithium is intercalated and deintercalated in an ionic state.

[0018] <1-1. Structure> Figure 1 shows a perspective view of the secondary battery. Figure 2 shows a cross-sectional view of the secondary battery shown in Figure 1. Figure 3 shows a cross-sectional view of the battery element 40 shown in Figure 2. However, in Figure 3, only a portion of the cross-sectional view of the battery element 40 is magnified.

[0019] For convenience, in the following explanation, the upper side in Figure 1 and Figure 2 will be described as the upper side of the secondary battery, and the lower side in Figure 1 and Figure 2 will be described as the lower side of the secondary battery.

[0020] The secondary battery described here has a three-dimensional shape in which the height H is smaller than the outer diameter D, as shown in Figure 1; that is, a flat and columnar three-dimensional shape. Here, the three-dimensional shape of the secondary battery is flat and cylindrical. In this embodiment, the vertical direction of the paper in Figures 1 and 2 is defined as the height direction Z. Therefore, the height H represents the dimension in the height direction Z of the secondary battery in this embodiment. The outer diameter D represents the dimension in the direction perpendicular to the height direction Z of the secondary battery in this embodiment.

[0021] The dimensions of a secondary battery are not particularly limited, but as an example, the outer diameter D is 3mm to 30mm and the height H is 0.5mm to 70mm. However, the ratio of the outer diameter D to the height H (D / H) is greater than 1. That is, the outer diameter D is greater than the height H. There is no particular upper limit to this ratio (D / H), but it is preferably 25 or less.

[0022] As shown in Figures 1 to 3, this rechargeable battery comprises an outer casing 10, external terminals 20, a battery element 40, and a positive electrode lead 51. In this case, the rechargeable battery further comprises a gasket 30, a negative electrode lead 52, a sealant 61, and insulating films 62 and 63.

[0023] [Outer can] As shown in Figures 1 and 2, the outer casing 10 is a hollow outer component that houses the battery element 40 and the like. The outer casing 10 is made of a conductive material.

[0024] Here, the outer casing 10 has a flat and approximately cylindrical three-dimensional shape, corresponding to the flat and cylindrical three-dimensional shape of the secondary battery. For this reason, the outer casing 10 has a pair of opposing bottoms M1 and M2, and a side wall M3 located between the bottoms M1 and M2. That is, the side wall M3 connects the bottoms M1 and M2 and surrounds the battery element 40. The upper end of the side wall M3 is connected to the bottom M1. The lower end of the side wall M3 is connected to the bottom M2. As described above, the outer casing 10 is approximately cylindrical. The planar shapes of the bottoms M1 and M2 are circular, and the surface of the side wall M3 is a convex curved surface.

[0025] Furthermore, the outer can 10 includes a storage section 11 and a lid section 12 that are welded to each other. That is, the internal space of the outer can 10 is sealed by welding the lid section 12 to the storage section 11. In this embodiment, the bottom section M1 constitutes the lid section 12, and the bottom section M2 and the side wall section M3 together constitute the storage section 11. Therefore, the outer edge of the lid section 12 is welded to the upper end of the side wall section M3.

[0026] The storage section 11 is a flat, cylindrical storage member that houses the battery element 40 and the like. The storage section 11 has a hollow structure with an open upper end and a closed lower end. That is, the storage section 11 has an opening 11K (Figure 2) at its upper end, which serves as a passage through which the battery element 40 can be inserted in the height direction Z.

[0027] The lid portion 12 is a substantially disc-shaped lid member that closes the opening 11K of the storage portion 11 and has a through-hole 12K. The through-hole 12K is used as a connection path for connecting the battery element 40 and the external terminal 20 to each other. As described above, the lid portion 12 is welded to the storage portion 11 at the opening 11K. The external terminal 20 is attached to the lid portion 12 via a gasket 30. That is, the lid portion 12 supports the external terminal 20 via the gasket 30. The external terminal 20 is attached to the lid portion 12 via the gasket 30 so as to close the through-hole 12K. The external terminal 20 is electrically insulated from the outer casing 10.

[0028] Furthermore, in the completed secondary battery, as described above, the lid 12 is welded to the storage section 11. As mentioned above, the opening 11K is closed by the lid 12. Therefore, it is possible that by looking at the exterior of the secondary battery, it is not possible to confirm whether the storage section 11 had an opening 11K.

[0029] However, if the lid 12 is welded to the storage section 11, welding marks will remain on the surface of the outer can 10, more specifically at the boundary between the storage section 11 and the lid 12. Based on the presence or absence of these welding marks, it is possible to retrospectively confirm whether the storage section 11 had an opening 11K.

[0030] In other words, if welding marks remain on the surface of the outer can 10, it means that the storage compartment 11 had an opening 11K. On the other hand, if no welding marks remain on the surface of the outer can 10, it means that the storage compartment 11 did not have an opening 11K.

[0031] The lid portion 12 is bent so as to partially protrude along the height direction Z toward the interior of the storage portion 11, forming a recessed portion 12H. That is, when viewed from the outside of the outer can 10, the lid portion 12 has a shape that is partially recessed in the height direction Z toward the battery element 40 housed inside the outer can 10. The recessed portion 12H includes a through-hole 12K that penetrates in the height direction Z, a bottom portion 12HB that surrounds the through-hole 12K along a horizontal plane perpendicular to the height direction Z, and a wall portion 12HW that is erected along the outer edge of the bottom portion 12HB.

[0032] Furthermore, the portion of the lid 12 other than the recessed portion 12H is the peripheral portion 12R. The peripheral portion 12R is an annular shape that surrounds the recessed portion 12H in a horizontal plane perpendicular to the height direction Z of the secondary battery. The peripheral portion 12R is a portion that surrounds the recessed portion 12H and protrudes away from the battery element 40 along the height direction Z. Therefore, in the height direction Z, the surface 12HS of the bottom 12HB of the recessed portion 12H is lower toward the interior of the storage portion 11 than the surface 12RS of the peripheral portion 12R. That is, in the height direction Z, the distance between the surface 12HS of the bottom 12HB of the recessed portion 12H and the battery element 40 is shorter than the distance between the surface 12RS of the peripheral portion 12R and the battery element 40.

[0033] The plan view shape of the recess 12H, that is, the shape defined by the outer edge of the recess 12H when the secondary battery is viewed from above, is not particularly limited. Here, the plan view shape of the recess 12H is approximately circular. The inner diameter and depth of the recess 12H are not particularly limited and can be set arbitrarily. However, when the external terminal 20 is attached to the recess 12H via the gasket 30, the depth of the recess 12H is set such that the height position of the surface 20S of the external terminal 20 is lower than the height position of the surface 12RS of the peripheral portion 12R.

[0034] As described above, the outer can 10 is a can in which the storage section 11 and the lid section 12, which were previously physically separated from each other, are welded together; in other words, it is a welded can. As a result, the outer can 10 after welding is a single, physically integrated component, and therefore cannot be separated into the storage section 11 and the lid section 12 afterward.

[0035] The outer can 10, which is a welded can, is a different type of can from a crimped can formed using a crimping process, and is a so-called crimp-pressed can. This is because the element space volume increases inside the outer can 10, thus increasing the energy density per unit volume. This "element space volume" refers to the volume (effective volume) of the internal space of the outer can 10 that can be used to house the battery elements 40.

[0036] Furthermore, the outer can 10, which is a welded can, does not have any overlapping parts, nor does it have any overlapping parts between two or more components.

[0037] "Having no overlapping parts" means that no part of the outer casing 10 is processed (bent) in a way that allows it to overlap with other parts. Furthermore, "Having no overlapping parts between two or more components" means that, after the secondary battery is completed, the outer casing 10 is physically a single component, and therefore cannot be subsequently separated into two or more components. In other words, the state of the outer casing 10 in the completed secondary battery is not one in which two or more components are combined while overlapping each other in a way that would allow for subsequent separation.

[0038] Here, the outer casing 10 is conductive. More specifically, the storage section 11 and the lid section 12 are both conductive. The outer casing 10 is electrically connected to the negative electrode 42 of the battery element 40 via the negative electrode lead 52. Therefore, the outer casing 10 also serves as an external connection terminal for the negative electrode 42. In this embodiment, the secondary battery does not need to have an external connection terminal for the negative electrode 42 separate from the outer casing 10, thus suppressing the reduction in element space volume caused by the presence of an external connection terminal for the negative electrode 42. As a result, the element space volume increases, and therefore the energy density per unit volume increases.

[0039] Specifically, the outer can 10 is a metal can containing one or more types of conductive materials, such as metal materials and alloy materials. The conductive materials that make up the metal can include iron, copper, nickel, stainless steel, iron alloys, copper alloys, and nickel alloys. The type of stainless steel is not particularly limited, but specifically, it includes SUS304 and SUS316. However, the forming material of the storage section 11 and the forming material of the lid section 12 may be the same or different from each other.

[0040] The lid portion 12 is insulated from the external terminal 20, which serves as the external connection terminal for the positive electrode 41, via a gasket 30. This is to prevent contact, i.e., short circuits, between the outer casing 10, which is the external connection terminal for the negative electrode 42, and the external terminal 20, which is the external connection terminal for the positive electrode 41.

[0041] [External terminals] As shown in Figures 1 and 2, the external terminal 20 is a connection terminal used to connect to electronic equipment when the secondary battery is mounted on that electronic equipment. As described above, the external terminal 20 is attached to and supported by the lid 12 of the outer casing 10. The external terminal 20 is located on the opposite side of the bottom M2 when viewed from the lid 12, and overlaps with the through-hole 12K in the height direction Z.

[0042] Here, the external terminal 20 is connected to the positive electrode 41 of the battery element 40 via the positive electrode lead 51. Therefore, the external terminal 20 functions as an external connection terminal for the positive electrode 41. As a result, when the secondary battery is used, the secondary battery is connected to the electronic device via the external terminal 20 (external connection terminal for the positive electrode 41) and the outer casing 10 (external connection terminal for the negative electrode 42). Thus, the electronic device can operate using the secondary battery as a power source.

[0043] The external terminal 20 is a flat, roughly plate-shaped member that extends along a horizontal plane perpendicular to the height direction Z of the secondary battery, and is located inside the recessed portion 12H via a gasket 30. The external terminal 20 is insulated from the lid portion 12 via the gasket 30. Here, as shown in Figure 2, in the height direction Z, the position of the surface 20S of the external terminal 20 is lower toward the battery element 40 than the position of the surface 12RS of the peripheral portion 12R of the outer casing 10. That is, the external terminal 20 is housed inside the recessed portion 12H such that its upper end surface 20S is recessed toward the battery element 40 than the surface 12RS. In the secondary battery of this embodiment, the height of the secondary battery is reduced compared to the case where the external terminal 20 protrudes above the lid portion 12. As a result, the energy density per unit volume of the secondary battery increases. In addition, it is possible to prevent short circuits between the outer casing 10 and the external terminal 20 via other conductive members. Furthermore, in this embodiment, the peripheral portion of the external terminal 20 overlaps with the bottom portion 12HB of the recessed portion 12H in the height direction Z. By having an overlapping portion between the external terminal 20 and the cover portion 12, the overall mechanical strength of the secondary battery can be improved. Here, the length of the overlapping portion between the external terminal 20 and the peripheral portion along the horizontal plane perpendicular to the height direction Z is preferably greater than the thickness of the external terminal 20 and greater than the thickness of the bottom portion 12HB.

[0044] The outer diameter of the external terminal 20 is smaller than the inner diameter of the recess 12H. Therefore, the outer edge 20T of the external terminal 20 is separated from the cover 12. The gasket 30 is placed only in a portion of the area between the external terminal 20 and the cover 12 (recess 12H). More specifically, it is placed only in the areas where the external terminal 20 and the cover 12 would come into contact with each other if the gasket 30 were not present. However, it is preferable that the gasket 30 also be provided between the inner wall surface of the wall portion 12HW of the recess 12H and the outer edge 20T of the external terminal 20. Furthermore, it is preferable that the cover 12 and the external terminal 20 are fixed together by the gasket 30.

[0045] Furthermore, the external terminal 20 contains one or more conductive materials, such as metal materials and alloy materials, and the conductive materials are aluminum and aluminum alloys. However, the external terminal 20 may be formed of a clad material. This clad material contains an aluminum layer and a nickel layer in order from the side closest to the gasket 30, and in this clad material, the aluminum layer and the nickel layer are roll-bonded to each other.

[0046] [gasket] As shown in Figure 2, the gasket 30 is an insulating member positioned between the outer can 10 (lid portion 12) and the external terminal 20. The external terminal 20 is fixed to the lid portion 12 via the gasket 30. The gasket 30 has a ring-shaped planar form with a through-hole at a location corresponding to the through-hole 12K. The gasket 30 also contains one or more types of insulating materials, such as insulating polymer compounds, and these insulating materials are resins such as polypropylene and polyethylene.

[0047] The installation range of the gasket 30 is not particularly limited and can be set arbitrarily. Here, the gasket 30 is positioned in the gap between the upper surface of the lid 12 and the lower surface of the external terminal 20 inside the recess 12H. However, as mentioned above, it is preferable that the gasket 30 also be provided between the inner wall surface of the wall 12HW of the recess 12H and the outer edge 20T of the external terminal 20. Furthermore, it is preferable that the lid 12 and the external terminal 20 are fixed together by the gasket 30.

[0048] [Battery element] As shown in Figures 2 and 3, the battery element 40 is a power generation element that carries out a charge-discharge reaction and is housed inside the outer casing 10. The battery element 40 includes a positive electrode 41 and a negative electrode 42. Here, the battery element 40 further includes a separator 43 and an electrolyte (not shown) which is a liquid electrolyte.

[0049] The center line PC shown in Figure 2 is a line segment corresponding to the center of the battery element 40 in the direction along the outer diameter D of the secondary battery (casing 10). In other words, the position P0 of the center line PC corresponds to the position of the center of the battery element 40.

[0050] The battery element 40 is a so-called wound electrode body. That is, in the battery element 40, the positive electrode 41 and the negative electrode 42 are stacked on top of each other via a separator 43. Furthermore, as shown in Figure 4, the stacked positive electrode 41, negative electrode 42, and separator 43 are wound around the center line PC as the winding axis. The positive electrode 41 and the negative electrode 42 are wound while maintaining a state of facing each other via the separator 43. For this reason, a winding center space 40K is formed at the center of the battery element 40. Note that Figure 4 shows one configuration example along a horizontal cross section perpendicular to the height direction Z of the battery element 40. However, in Figure 4, the separator 43 is omitted from the illustration to ensure visibility.

[0051] Here, the positive electrode 41, negative electrode 42, and separator 43 are wound such that the separator 43 is positioned at the outermost circumference and innermost circumference of the wound electrode body, respectively. The number of turns for each of the positive electrode 41, negative electrode 42, and separator 43 is not particularly limited and can be set arbitrarily. Also, at the outermost circumference of the battery element 40, the negative electrode 42 is positioned outside the positive electrode 41. That is, as shown in Figure 4, the outermost positive electrode portion 41out, which is located at the outermost circumference of the positive electrode 41 included in the battery element 40, is located inside the outermost negative electrode portion 42out, which is located at the outermost circumference of the negative electrode 42 included in the battery element 40. Here, the outermost positive electrode portion 41out is the outermost one turn of the positive electrode 41 in the battery element 40. The outermost negative electrode portion 42out is the outermost one turn of the negative electrode 42 in the battery element 40. On the other hand, at the innermost circumference of the battery element 40, it is preferable that the negative electrode 42 is positioned inside the positive electrode 41. In other words, as shown in Figure 4, the innermost negative electrode portion 42in, located at the innermost circumference of the negative electrode 42 included in the battery element 40, is preferably located inside the innermost positive electrode portion 41in, located at the innermost circumference of the positive electrode 41 included in the battery element 40. Here, the innermost positive electrode portion 41in is the innermost one-turn portion of the positive electrode 41 in the battery element 40. The innermost negative electrode portion 42in is the innermost one-turn portion of the negative electrode 42 in the battery element 40.

[0052] The battery element 40 has a three-dimensional shape similar to the three-dimensional shape of the outer casing 10. Specifically, the battery element 40 has a flattened and substantially cylindrical three-dimensional shape. Compared to the case where the battery element 40 has a three-dimensional shape different from the three-dimensional shape of the outer casing 10, when the battery element 40 is housed inside the outer casing 10, so-called dead space, specifically the gap between the outer casing 10 and the battery element 40, is less likely to occur. As a result, the internal space of the outer casing 10 is effectively utilized. Consequently, the volume of the element space increases, and the energy density per unit volume of the secondary battery increases.

[0053] (positive electrode) The positive electrode 41 is the first electrode used to carry out the charge-discharge reaction, and as shown in Figures 3 and 4, it includes a positive electrode current collector 41A and a positive electrode active material layer 41B.

[0054] The positive electrode current collector 41A has a pair of surfaces on which the positive electrode active material layer 41B is provided. More specifically, the positive electrode current collector 41A includes an inner surface 41A1 facing the winding center side of the battery element 40, i.e., position P0, and an outer surface 41A2 facing the opposite side of the winding center side of the battery element 40, i.e., the side opposite the inner surface 41A1. The positive electrode current collector 41A contains a conductive material such as a metal material, and the metal material is such as aluminum.

[0055] The positive electrode 41 has a positive electrode active material layer 41B, which includes an inner positive electrode active material layer 41B1 covering at least a portion of the inner surface 41A1 of the positive electrode current collector, and an outer positive electrode active material layer 41B2 covering at least a portion of the outer surface 41A2 of the positive electrode current collector. The inner positive electrode active material layer 41B1 and the outer positive electrode active material layer 41B2 may be made of the same constituent material and have the same thickness. In this specification, the inner positive electrode active material layer 41B1 and the outer positive electrode active material layer 41B2 may be referred to collectively as the positive electrode active material layer 41B without distinction. The positive electrode active material layer 41B contains one or more types of positive electrode active materials capable of intercalating and deintercalating lithium. The positive electrode active material layer 41B may also further contain a positive electrode binder and a positive electrode conductive agent. The method for forming the positive electrode active material layer 41B is not particularly limited, but specifically, it may be a coating method.

[0056] The positive electrode active material contains a lithium compound. This lithium compound is a general term for compounds that contain lithium as a constituent element, and more specifically, it is a compound that contains lithium along with one or more transition metal elements as constituent elements. This is because a high energy density can be obtained. However, the lithium compound may further contain one or more of any other elements (excluding lithium and transition metal elements). The type of lithium compound is not particularly limited, but specifically, it includes oxides, phosphoric acid compounds, silicate compounds, and borate compounds. Specific examples of oxides include LiNiO2, LiCoO2, and LiMn2O4, and specific examples of phosphoric acid compounds include LiFePO4 and LiMnPO4.

[0057] The positive electrode binder contains one or more of the following: synthetic rubber and polymer compounds. Synthetic rubber is styrene-butadiene rubber, while polymer compounds are polyvinylidene fluoride. The positive electrode conductive agent contains one or more of the following: carbon materials, and carbon materials are graphite, carbon black, acetylene black, and Ketjen black. However, the conductive material may also be a metal material or a polymer compound.

[0058] (Negative electrode) The negative electrode 42 is a second electrode used to advance the charge-discharge reaction, and as shown in Figure 3, it includes a negative electrode current collector 42A and a negative electrode active material layer 42B.

[0059] The negative electrode current collector 42A has a pair of surfaces on which the negative electrode active material layer 42B is provided. More specifically, the negative electrode current collector 42A includes an inner surface 42A1 facing the winding center side of the battery element 40, i.e., position P0, and an outer surface 42A2 facing the opposite side of the winding center side of the battery element 40, i.e., the side opposite to the inner surface 42A1. The negative electrode current collector 42A contains a conductive material such as a metal material, and the metal material is such as copper.

[0060] The negative electrode 42 has a negative electrode active material layer 42B, which includes an inner negative electrode active material layer 42B1 covering at least a portion of the inner surface 42A1 of the negative electrode current collector, and an outer negative electrode active material layer 42B2 covering at least a portion of the outer surface 42A2 of the negative electrode current collector. From the inner circumference end 40E1 to the outer circumference end 40E2 of the battery element 40, the area density of the outer negative electrode active material layer 42B2 is greater than that of the inner negative electrode active material layer 42B1. For example, when the area density of the outer negative electrode active material layer 42B2 is 101.8%, the area density of the inner negative electrode active material layer 42B1 is 98.2%. In the negative electrode 42, for example, the inner active material layer 42B1 and the outer active material layer 42B2 are made of the same constituent material, and as shown in Figure 5, the thickness T2 of the outer active material layer 42B2 is thicker than the thickness T1 of the inner active material layer 42B1 from the inner peripheral end 40E1 to the outer peripheral end 40E2 of the battery element 40. Figure 5 is a schematic unfolded view of the positive electrode 41 and negative electrode 42 of the battery element 40. The dashed lines in Figure 5 represent the inner active material layer 42B1 and the outer active material layer 42B2 when their thicknesses T1 and T2 are equal. In this specification, the inner active material layer 42B1 and the outer active material layer 42B2 are sometimes referred to collectively as the negative electrode active material layer 42B without distinction. In this specification, the inner circumference end 40E1 refers to the innermost end of the portion of the battery element 40 where the positive electrode active material layer 41B and the negative electrode active material layer 42B face each other. In this specification, the outer circumference end 40E2 refers to the outermost end of the portion of the battery element 40 where the positive electrode active material layer 41B and the negative electrode active material layer 42B face each other.

[0061] The negative electrode active material layer 42B contains one or more negative electrode active materials capable of intercalating and deintercalating lithium. However, the negative electrode active material layer 42B may further contain a negative electrode binder and a negative electrode conductive agent. Details regarding the negative electrode binder and negative electrode conductive agent are the same as those regarding the positive electrode binder and positive electrode conductive agent. The method for forming the negative electrode active material layer 42B is not particularly limited, but specifically, it is one or more of the following: coating, gas phase, liquid phase, thermal spraying, and firing (sintering).

[0062] The negative electrode active material contains either or both carbon materials and metallic materials because they allow for high energy density. Carbon materials include easily graphitizable carbon, poorly graphitizable carbon, and graphite (natural graphite and artificial graphite). Metallic materials are materials that contain one or more metallic elements and metalloid elements capable of forming alloys with lithium, such as silicon and tin, either or both. However, metallic materials may be elements, alloys, compounds, mixtures of two or more of these, or materials containing two or more of these phases. Specific examples of metallic materials include TiSi2 and SiOx (0 <x≦2、または0.2<x<1.4)などである。

[0063] Here, the height of the negative electrode 42 is greater than the height of the positive electrode 41. That is, the negative electrode 42 protrudes both above and below the positive electrode 41. This is to prevent lithium released from the positive electrode 41 from being deposited. This "height" is the dimension corresponding to the height H of the secondary battery described above, that is, the vertical dimension in Figures 1 and 2, respectively. The definition of height explained here will remain the same throughout.

[0064] (Separator) As shown in Figures 2 and 3, the separator 43 is an insulating porous membrane placed between the positive electrode 41 and the negative electrode 42. The separator 43 allows lithium ions to pass through while preventing a short circuit between the positive electrode 41 and the negative electrode 42. The separator 43 contains a polymer compound such as polyethylene.

[0065] Here, as shown in Figure 2, the height of the separator 43 is greater than the height of the negative electrode 42. In other words, the separator 43 should protrude both above and below the negative electrode 42.

[0066] (electrolyte) The electrolyte is impregnated into the positive electrode 41, the negative electrode 42, and the separator 43, and contains a solvent and an electrolyte salt. The solvent contains one or more non-aqueous solvents (organic solvents) such as carbonate ester compounds, carboxylic acid ester compounds, and lactone compounds, and the electrolyte containing such a non-aqueous solvent is a so-called non-aqueous electrolyte. The electrolyte salt contains one or more light metal salts such as lithium salts.

[0067] [Positive lead] The positive electrode lead 51 is housed inside the outer casing 10, as shown in Figure 2. The positive electrode lead 51 is a connecting wire connected to the positive electrode 41 and the external terminal 20, respectively. The secondary battery shown in Figure 2 has one positive electrode lead 51. However, the secondary battery may have two or more positive electrode leads 51.

[0068] The positive lead 51 is connected to the upper end of the positive electrode 41. Specifically, the positive lead 51 is connected to the upper end of the positive electrode current collector 41A. The positive lead 51 is also connected to a part of the surface 20S of the external terminal 20 via a through-hole 12K provided in the cover portion 12. The method of connecting the positive lead 51 is not particularly limited, but specifically, it is one or more types of welding methods such as resistance welding and laser welding. Details regarding the welding methods described here will also be described hereafter.

[0069] A portion of the positive electrode lead 51 is electrically insulated from the lid 12 of the outer casing 10 and the negative electrode 42 of the battery element 40, and is sandwiched between the lid 12 and the battery element 40 in the height direction of the secondary battery. As shown in Figure 2, the positive electrode lead 51 includes a first portion 511, a second portion 512, and a folded portion 513. The first portion 511 and the second portion 512 extend along a horizontal plane perpendicular to the height direction Z of the secondary battery. The first portion 511 and the second portion 512 also overlap each other in the height direction Z of the secondary battery via the sealant 61. The folded portion 513 is curved to connect the first portion 511 and the second portion 512. The first portion 511 and the second portion 512 are sandwiched between the battery element 40 and the recessed portion 12H of the lid 12 in the height direction Z of the secondary battery.

[0070] In this way, a portion of the positive electrode lead 51 extends along the lower surface of the lid 12 and the upper surface of the battery element 40, respectively, and is held in place by the lid 12 and the battery element 40. Therefore, the positive electrode lead 51 is fixed inside the outer casing 10. Even if the secondary battery is subjected to external forces such as vibration and shock, the positive electrode lead 51 is less likely to move, thus reducing the likelihood of damage to the positive electrode lead 51. Damage to the positive electrode lead 51 here refers to cracks occurring in the positive electrode lead 51, the positive electrode lead 51 being cut, or the positive electrode lead 51 being detached from the positive electrode 41.

[0071] In other words, "a portion of the positive electrode lead 51 is sandwiched between the outer casing 10 and the battery element 40" means that the positive electrode lead 51 is insulated from both the outer casing 10 and the battery element 40, and is held in place from above and below by the outer casing 10 and the battery element 40. Therefore, even if the secondary battery is subjected to external forces such as vibration and shock, the positive electrode lead 51 is unlikely to move inside the outer casing 10. The fact that the positive electrode lead 51 is unlikely to move inside the outer casing 10 means that the battery element 40 is also unlikely to move inside the outer casing 10. Consequently, when the secondary battery is subjected to vibration or shock, problems such as unwinding of the wound electrode body of the battery element 40 can be avoided.

[0072] Furthermore, it is preferable that the positive electrode lead 51 is pressed against the battery element 40 and thus embedded in the battery element 40. More specifically, as described above, the height of the separator 43 is greater than the heights of the positive electrode 41 and the negative electrode 42, so it is preferable that the positive electrode lead 51 is embedded in the upper end of the separator 43. In this case, a recess is formed at the upper end of the separator 43 due to the pressure of the positive electrode lead 51. Since part or all of the positive electrode lead 51 is housed inside this recess, the positive electrode lead 51 is held in place by the separator 43. This makes it more difficult for the positive electrode lead 51 to move inside the outer casing 10, thus making it less likely to be damaged.

[0073] Here, as described above, the lid portion 12 includes a recessed portion 12H, and a portion of the positive electrode lead 51 is sandwiched between the recessed portion 12H and the battery element 40. That is, a portion of the positive electrode lead 51 extends along the lower surface of the recessed portion 12H and the upper surface of the battery element 40, respectively, so that it is held by the recessed portion 12H and the battery element 40. Since the positive electrode lead 51 is more easily held by utilizing the recessed portion 12H, the positive electrode lead 51 becomes less prone to damage.

[0074] Furthermore, a portion of the positive electrode lead 51 is insulated from the lid portion 12 and the negative electrode 42 via the separator 43, sealant 61, and insulating films 62 and 63, respectively.

[0075] Specifically, as described above, the height of the separator 43 is greater than the height of the negative electrode 42. As a result, a portion of the positive electrode lead 51 is separated from the negative electrode 42 via the separator 43, and is therefore insulated from the negative electrode 42 via the separator 43. This prevents a short circuit between the positive electrode lead 51 and the negative electrode 42.

[0076] Furthermore, the positive lead 51 is surrounded by an insulating sealant 61. This insulates a portion of the positive lead 51 from the cover 12 and the negative electrode 42 via the sealant 61. This prevents short circuits between the positive lead 51 and the cover 12, as well as between the positive lead 51 and the negative electrode 42.

[0077] Furthermore, an insulating film 62 is placed between the lid 12 and the positive lead 51. This insulates a portion of the positive lead 51 from the lid 12 via the insulating film 62, thereby preventing a short circuit between the positive lead 51 and the lid 12.

[0078] Furthermore, an insulating film 63 is placed between the battery element 40 and the positive electrode lead 51. This insulates a portion of the positive electrode lead 51 from the negative electrode 42 via the insulating film 63, thereby preventing a short circuit between the positive electrode lead 51 and the negative electrode 42.

[0079] Details regarding the forming material of the positive electrode lead 51 are the same as details regarding the forming material of the positive electrode current collector 41A. However, the forming material of the positive electrode lead 51 and the forming material of the positive electrode current collector 41A may be the same or different.

[0080] Here, the positive lead 51 is connected to the positive electrode 41 in the region in front of the center line PC, that is, the region to the right of the center line PC in Figure 2. The positive lead 51 has a folded portion 513 on its way to the external terminal 20 in order to connect to the external terminal 20. The folded portion 513 is located in the region behind the center line PC, that is, the region to the left of the center line PC in Figure 2. The positive lead 51 has a first portion 511, which is the part that extends from the point where it is connected to the positive electrode 41, through the center position P0, to the folded portion 513. The first portion 511 extends along the upper surface of the battery element 40 in a direction perpendicular to the height direction Z. Furthermore, the positive lead 51 has a second portion 512, which is the part that extends from the folded portion 513 to the point where it is connected to the external terminal 20. The second portion 512 extends along the upper surface of the battery element 40 in a direction perpendicular to the height direction Z, so as to overlap the first portion 511. Thus, a portion of the positive lead 51 extends toward the external terminal 20, sandwiched between the cover portion 12 and the battery element 40 in both the region in front of the center line PC and the region behind the center line PC.

[0081] Here, the "region in front of the center line PC" is, as is clear from Figure 2, the region in which the battery element 40 is divided into two regions with respect to the center line PC in the direction along the outer diameter D, and the connection point of the positive electrode lead 51 to the positive electrode 41 exists. In Figure 2, the "region in front of the center line PC" is the region to the right of the center line PC. In contrast, the "region behind the center line PC" is, as is clear from Figure 2, the other of the two regions described above, and in Figure 2, the region to the left of the center line PC. That is, the "region behind the center line PC" is the other region in which the battery element 40 is divided into two regions with respect to the center line PC in the direction along the outer diameter D, and the connection point of the positive electrode lead 51 to the positive electrode 41 does not exist.

[0082] The connection position of the positive electrode lead 51 to the positive electrode 41 is not particularly limited and can be set arbitrarily. In particular, it is preferable that the positive electrode lead 51 is connected to the positive electrode 41 on the inner side of the positive electrode 41 rather than on the outermost side of the positive electrode 41. This is because, unlike when the positive electrode lead 51 is connected to the positive electrode 41 on the outermost side of the positive electrode 41, corrosion of the outer casing 10 caused by electrolyte creep is prevented. This "electrolyte creep" refers to the phenomenon where, when the positive electrode lead 51 is positioned close to the inner wall surface of the outer casing 10, the electrolyte in the battery element 40 creeps up the positive electrode lead 51 and reaches the inner wall surface of the outer casing 10. When the electrolyte comes into contact with the outer casing 10 due to this "electrolyte creep," the outer casing 10 may dissolve or discolor.

[0083] Here, the positive lead 51 is folded back at least once between the positive electrode 41 and the external terminal 20, and therefore overlaps at least once. The number of times the positive lead 51 is folded back is not particularly limited, as long as it is at least once. "The positive lead 51 is folded back" means that the direction of extension of the positive lead 51 changes at some point so that it forms an angle greater than 90°. The folded portion of the positive lead 51 may have a curved shape without bending, such as the folded portion 513. Also, although Figure 2 illustrates the case where the positive lead 51 includes one folded portion 513, it may also include multiple folded portions 513.

[0084] The positive electrode lead 51 is folded back at a folded portion 513 located midway between the positive electrode 41 and the external terminal 20. Specifically, as shown in Figure 2, the first portion 511 extends in a horizontal plane perpendicular to the height direction of the secondary battery from a first position P1 other than the center position P0 of the outer casing 10 to a second position P2 opposite to the first position P1 when viewed from the center position. The second portion 512 extends from the second position P2 toward the center position P0. In the positive electrode lead 51, the overlapping portion of the first portion 511 and the second portion 512 is the excess portion. In other words, the positive electrode lead 51 has a length margin in its longitudinal direction.

[0085] As a result, as will be described later, when forming the outer casing 10 using the storage section 11 and the lid section 12 in the secondary battery manufacturing process, there is room to change the orientation of the lid section 12 relative to the storage section 11. In addition, when the secondary battery is subjected to external forces such as vibration and shock, these external forces are mitigated by utilizing the length margin of the positive electrode lead 51, making the positive electrode lead 51 less susceptible to damage. Furthermore, by utilizing the length margin of the positive electrode lead 51, the connection position of the positive electrode lead 51 to the positive electrode 41 can be arbitrarily changed without changing the length of the positive electrode lead 51.

[0086] In this case, the length of the positive electrode lead 51 (total length including length margin) is not particularly limited and can be set arbitrarily. In particular, it is preferable that the length of the positive electrode lead 51 be at least half the outer diameter D of the outer can 10. This is because the length of the positive electrode lead 51 ensures a length margin for standing the lid 12 upright relative to the storage section 11, making it easier to stand the lid 12 upright relative to the storage section 11.

[0087] The connection range of the positive lead 51 to the external terminal 20 is not particularly limited. In particular, it is preferable that the connection range of the positive lead 51 to the external terminal 20 be wide enough to prevent the positive lead 51 from easily falling off the external terminal 20, and narrow enough to provide a sufficient length margin for the positive lead 51. The reason why a sufficiently narrow connection range of the positive lead 51 to the external terminal 20 is preferable is that the portion of the positive lead 51 not connected to the external terminal 20 becomes the length margin, thus making the length margin of the positive lead 51 sufficiently large.

[0088] The positive electrode lead 51 is provided separately from the positive electrode current collector 41A. However, since the positive electrode lead 51 is physically continuous with the positive electrode current collector 41A, it may be integrated with the positive electrode current collector 41A.

[0089] [Negative lead] The negative electrode lead 52 is housed inside the outer casing 10, as shown in Figure 2. The negative electrode lead 52 is electrically connected to both the negative electrode 42 and the outer casing 10 (storage section 11). Therefore, the storage section 11 (bottom M2) is electrically connected to the negative electrode 42 via the negative electrode lead 52. Here, the secondary battery has one negative electrode lead 52. However, the secondary battery may have two or more negative electrode leads 52.

[0090] The negative electrode lead 52 is connected to the lower end of the negative electrode 42, and more specifically, to the lower end of the negative electrode current collector 42A. The negative electrode lead 52 is also connected to the bottom surface of the housing 11. The method of connecting the negative electrode lead 52 is not particularly limited, but specifically, it is one or more welding methods such as resistance welding and laser welding.

[0091] Details regarding the forming material of the negative electrode lead 52 are the same as details regarding the forming material of the negative electrode current collector 42A. However, the forming material of the negative electrode lead 52 and the forming material of the negative electrode current collector 42A may be the same or different.

[0092] The connection position of the negative electrode lead 52 to the negative electrode 42 is not particularly limited and can be set arbitrarily. Here, the negative electrode lead 52 is connected to the outermost part of the negative electrode 42 that constitutes the wound electrode body.

[0093] The negative electrode lead 52 is provided separately from the negative electrode current collector 42A. However, since the negative electrode lead 52 is physically continuous with the negative electrode current collector 42A, it may be integrated with the negative electrode current collector 42A.

[0094] [Sealant] As shown in Figure 2, the sealant 61 is a first insulating member that covers the periphery of the positive electrode lead 51. The sealant 61 is constructed by attaching two insulating tapes to the front and back surfaces of the positive electrode lead 51, respectively. Here, the sealant 61 covers the periphery of the middle portion of the positive electrode lead 51 in order to connect the positive electrode lead 51 to the positive electrode 41 and the external terminal 20, respectively. Note that the sealant 61 is not limited to having a tape-like structure, but may also have a tubular structure, for example.

[0095] The sealant 61 contains one or more insulating materials, such as insulating polymer compounds, and these insulating materials include polyimide.

[0096] [Insulating film] As shown in Figure 2, the insulating film 62 is an insulating member positioned between the lid 12 and the battery element 40 in the height direction Z. Here, the insulating film 62 has a ring-shaped planar form with an opening 62K at a location corresponding to the through-hole 12K in the height direction Z.

[0097] In this case, the insulating film 62 may be bonded to the lid portion 12 via an adhesive layer.

[0098] Furthermore, the insulating film 62 may contain one or more insulating materials, such as insulating polymer compounds. The insulating materials contained in the insulating film 62 are polyimide, etc.

[0099] As shown in Figure 2, the insulating film 63 is an insulating member placed between the battery element 40 and the positive electrode lead 51. Here, the insulating film 63 has a flat, planar shape. The insulating film 63 is positioned to shield the winding center space 40K and to cover the battery element 40 around the winding center space 40K.

[0100] Details regarding the material for forming the insulating film 63 are the same as those regarding the material for forming the insulating film 62. However, the material for forming the insulating film 63 and the material for forming the insulating film 62 may be the same or different from each other.

[0101] [others] Furthermore, the secondary battery may also comprise one or more other components.

[0102] Specifically, the secondary battery is equipped with a safety valve mechanism. This safety valve mechanism disconnects the electrical connection between the outer casing 10 and the battery element 40 when the internal pressure of the outer casing 10 reaches a certain level. The causes of the internal pressure of the outer casing 10 reaching a certain level include a short circuit occurring inside the secondary battery and the secondary battery being heated from the outside. The location of the safety valve mechanism is not particularly limited, but it is preferable that the safety valve mechanism be provided on either the bottom M1 or M2, and more preferably on the bottom M2 where the external terminal 20 is not attached.

[0103] Furthermore, the secondary battery may have an insulator other than the insulating films 62 and 64 between the outer casing 10 and the battery element 40. This insulator includes one or more types of insulating films and insulating sheets, etc., and prevents short circuits between the outer casing 10 and the battery element 40. The installation range of the insulator is not particularly limited and can be set arbitrarily.

[0104] The outer container 10 is provided with an opening valve. This opening valve ruptures when the internal pressure of the outer container 10 reaches a certain level, thereby releasing the internal pressure. The location of the opening valve is not particularly limited, but, similar to the location of the safety valve mechanism described above, either the bottom M1 or M2 is preferred, with the bottom M2 being particularly preferred.

[0105] <1-2. Operation> During charging of the secondary battery, lithium is released from the positive electrode 41 in the battery element 40, and this lithium is absorbed into the negative electrode 42 via the electrolyte. Conversely, during discharging of the secondary battery, lithium is released from the negative electrode 42 in the battery element 40, and this lithium is absorbed into the positive electrode 41 via the electrolyte. During these charging and discharging processes, lithium is absorbed and released in an ionic state.

[0106] <1-3. Manufacturing method> Figure 6 shows a perspective view of the outer casing 10 used in the manufacturing process of secondary batteries, and corresponds to Figure 1.

[0107] Figure 6 shows the state in which the lid portion 12 is separated from the storage portion 11, before the lid portion 12 is welded to the storage portion 11.

[0108] In the following explanation, we will refer to Figures 1 through 5, which have already been explained, along with Figure 6, as needed.

[0109] Here, in order to form the outer can 10, a storage section 11 and a lid section 12 that are physically separated from each other are prepared, as shown in Figure 6. The storage section 11 is a roughly container-shaped member in which the bottom section M2 and the side wall section M3 are integrated with each other, and has an opening 11K. The lid section 12 is a roughly plate-shaped member corresponding to the bottom section M1, and the external terminals 20 are pre-attached to the recessed section 12H provided in the lid section 12 via a gasket 30.

[0110] However, the storage section 11 may be formed by preparing a bottom section M2 and a side wall section M3 that are physically separated from each other, and welding the side wall section M3 to the bottom section M2.

[0111] [Fabrication of the positive electrode] First, a positive electrode mixture is prepared by mixing positive electrode active material, positive electrode binder, and positive electrode conductive agent. Next, a paste-like positive electrode mixture slurry is prepared by adding the prepared positive electrode mixture to an organic solvent. Subsequently, the positive electrode mixture slurry is applied to both sides of the positive electrode current collector 41A to form a positive electrode active material layer 41B. Finally, the positive electrode active material layer 41B is compressed and molded using a roll press or the like. In this case, the positive electrode active material layer 41B may be heated, or the compression molding may be repeated multiple times. This produces the positive electrode 41.

[0112] [Fabrication of the negative electrode] The negative electrode 42 is manufactured using the same procedure as that used for manufacturing the positive electrode 41. Specifically, a negative electrode mixture, which consists of a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent, is added to an organic solvent to prepare a paste-like negative electrode mixture slurry. Then, the negative electrode mixture slurry is applied to both sides of the negative electrode current collector 42A to form the negative electrode active material layer 42B. At this time, the thickness T2 of the negative electrode outer active material layer 42B2 covering the outer surface 42A2 of the negative electrode current collector is made thicker than the thickness T1 of the negative electrode inner active material layer 42B1 covering the inner surface 42A1 of the negative electrode current collector. After this, the negative electrode active material layer 42B is compressed and molded using a roll press or the like. This completes the manufacturing of the negative electrode 42.

[0113] [Preparation of electrolyte solution] The electrolyte salt is added to the solvent. This disperses or dissolves the electrolyte salt in the solvent, thus preparing the electrolyte solution.

[0114] [Assembly of rechargeable batteries] First, using a welding method such as resistance welding, the positive lead 51, which is covered with sealant 61, is connected to the positive electrode 41 (positive electrode current collector 41A), and the negative lead 52 is connected to the negative electrode 42 (negative electrode current collector 42A).

[0115] Next, the positive electrode 41 and the negative electrode 42 are stacked with a separator 43 in between, and then the stacked material including the positive electrode 41, the negative electrode 42, and the separator 43 is wound up to produce a wound body 40Z as shown in Figure 6. The wound body 40Z has the same configuration as the battery element 40, except that the positive electrode 41, the negative electrode 42, and the separator 43 are not impregnated with electrolyte. Note that the positive electrode lead 51 and the negative electrode lead 52 are not shown in Figure 6.

[0116] Next, the winding body 40Z, to which the positive lead 51 and the negative lead 52 are connected, is housed inside the storage section 11 through the opening 11K. In this case, the negative lead 52 is connected to the storage section 11 using a welding method such as resistance welding. Subsequently, an insulating film 63 is placed on top of the winding body 40Z.

[0117] Next, a cover portion 12 is prepared, which already has the external terminals 20 attached via a gasket 30 and an insulating film 62 provided. Then, the positive lead 51 is connected to the external terminals 20 via the through-hole 12K using a welding method such as resistance welding.

[0118] As a result, the wound body 40Z (positive electrode 41) housed inside the storage section 11 and the external terminal 20 attached to the lid section 12 are connected to each other via the positive electrode lead 51.

[0119] Next, electrolyte is injected into the storage section 11 through the opening 11K. In this case, as described above, even though the battery element 40 and the external terminal 20 are connected to each other via the positive electrode lead 51, the lid 12 does not block the opening 11K, so the electrolyte can be easily injected into the storage section 11 through the opening 11K. As a result, the electrolyte is impregnated into the wound body 40Z, which includes the positive electrode 41, the negative electrode 42, and the separator 43, and the battery element 40, which is a wound electrode body, is fabricated.

[0120] Next, the lid 12 is tilted down so as to approach the storage section 11, thereby closing the opening 11K with the lid 12, and then the lid 12 is welded to the storage section 11 using a welding method such as laser welding. In this case, as shown in Figure 2, a part of the positive electrode lead 51 is sandwiched between the lid 12 and the battery element 40, and a curved folded portion 513 is formed in front of the connection point to the external terminal 20 of the positive electrode lead 51. As a result, the outer casing 10 is formed, and the battery element 40 and other components are housed inside the outer casing 10, completing the assembly of the secondary battery.

[0121] [Stabilization of secondary batteries] The rechargeable battery is charged and discharged after assembly. Ambient temperature, number of charge / discharge cycles and charge Various conditions, such as discharge conditions, can be set arbitrarily. As a result, a coating is formed on the surface of the negative electrode 42, etc., which electrochemically stabilizes the state of the secondary battery. Thus, the secondary battery is completed.

[0122] <1-4. Mechanism and Effects> Thus, in the secondary battery of this embodiment, the area density of the negative electrode outer active material layer 42B2 is greater than the area density of the negative electrode inner active material layer 42B1, from the inner peripheral end 40E1 to the outer peripheral end 40E2 in the battery element 40. Therefore, in the relationship between the positive electrode 41 and the negative electrode 42 facing each other across the separator 43, the capacity of the negative electrode 42 is greater than the capacity of the positive electrode 41. Specifically, in the relationship between the positive electrode inner active material layer 41B1 and the negative electrode outer active material layer 42B2 facing each other across the separator 43, the capacity of the negative electrode outer active material layer 42B2 can be made greater than the capacity of the positive electrode inner active material layer 41B1. As a result, the secondary battery of this embodiment can suppress the formation of precipitates such as lithium metal in conjunction with the battery reaction during charging, and can suppress the deterioration of battery performance. Therefore, it has high reliability.

[0123] The effects of the secondary battery of this embodiment will be explained in more detail with reference to Figure 7. Figure 7 is an explanatory diagram showing the relationship between the capacity of the positive electrode 41 and the capacity of the negative electrode 42 in the battery element 40. The horizontal axis of Figure 7 represents the element diameter d, and the vertical axis of Figure 7 represents N / P, which is the ratio of the negative electrode capacity N to the positive electrode capacity P. Specifically, the element diameter d is the distance from the center position P0 of the battery element 40 to any position on the separator 43, as shown in Figure 3. N / P represents the ratio of capacities between the positive electrode active material layer 41B and the negative electrode active material layer 42B that are facing each other across the separator 43 at the position of element diameter d. As shown in Figure 7, N / P changes continuously according to the size of the element diameter d. More specifically, among the four curves shown in Figure 7, the two curves C7-1 and C7-2, where N / P increases with increasing element diameter d, represent the ratio Nout / Pin of the capacity of the negative electrode outer active material layer 42B2 to the capacity of the positive electrode inner active material layer 41B1. Of the two curves C7-1 and C7-2, the solid curve C7-1 represents the Nout / Pin of the secondary battery of this embodiment, and the dashed curve C7-2 represents the Nout / Pin of a comparative secondary battery in which the area density of the negative electrode outer active material layer 42B2 and the area density of the negative electrode inner active material layer 42B1 are substantially equal. Furthermore, among the four curves shown in Figure 7, the two curves C7-3 and C7-4, where N / P decreases with increasing element diameter d, represent the ratio Nin / Pout of the capacity of the negative electrode inner active material layer 42B1 to the capacity of the positive electrode outer active material layer 41B2. Of the two curves C7-3 and C7-4, the solid curve C7-3 represents the Nin / Pout of the secondary battery of this embodiment, while the dashed curve C7-4 represents the Nin / Pout of a comparative secondary battery in which the area density of the negative electrode outer active material layer 42B2 and the area density of the negative electrode inner active material layer 42B1 are substantially equal.

[0124] As is clear from FIG. 7, the smaller the element diameter d, the wider the interval between Nout / Pin and Nin / Pout. In the region where the element diameter d is small, Nout / Pin is particularly small. Therefore, in the secondary battery of the comparative example, Nout / Pin becomes less than 1 in the region where the element diameter d < d1 (see curve C7-2). In that case, deposits such as lithium metal due to the battery reaction are likely to be generated particularly during charging at a high voltage. On the other hand, in the secondary battery of the present embodiment shown by the solid line, the decrease in Nout / Pin in the region where the element diameter d is small can be suppressed, and Nout / Pin can be made 1 or more in the region where the element diameter d ≧ d0 (see curve C7-1). Therefore, in the secondary battery of the present embodiment, the generation of deposits such as lithium metal due to the battery reaction during charging can be suppressed, and the deterioration of battery performance can be suppressed. Further, according to the secondary battery of the present embodiment, the deviation between Nout / Pin and Nin / Pout can be reduced from the inner peripheral side end portion 40E1 to the outer peripheral side end portion 40E2, which is advantageous for improving the cycle characteristics.

[0125] Further, in the secondary battery of the present embodiment, a recessed portion 12H is provided in the lid portion 12, and the external terminal 20 is arranged in the recessed portion 12H. Therefore, while securing the battery capacity, the height dimension of the secondary battery can be reduced.

[0126] Further, if the secondary battery is flat and columnar, that is, if the secondary battery is a secondary battery called a coin type or a button type, etc., the positive electrode lead 51 is less likely to be damaged even in a small secondary battery with large restrictions in terms of size, so a higher effect can be obtained from the perspective of physical durability.

[0127] Further, if the secondary battery is a lithium ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the absorption and release of lithium.

[0128] <2. Secondary Battery of the Second Embodiment> Next, a secondary battery as a second embodiment of the present disclosure will be described with reference to Figure 8. Figure 8 is a schematic unfolded view showing the positive electrode 41 and negative electrode 42 of the battery element 40A of the secondary battery as a second embodiment of the present disclosure. Figure 8 corresponds to Figure 5, which shows an unfolded view of the battery element 40 of the secondary battery of the first embodiment described above.

[0129] <2-1. Structure> In the battery element 40 of the secondary battery according to the first embodiment described above, the area density of the negative electrode outer active material layer 42B2 is greater than the area density of the negative electrode inner active material layer 42B1 from the inner circumference end 40E1 to the outer circumference end 40E2. Specifically, the negative electrode inner active material layer 42B1 and the negative electrode outer active material layer 42B2 are made of the same constituent material, and the thickness T2 of the negative electrode outer active material layer 42B2 is greater than the thickness T1 of the negative electrode inner active material layer 42B1 from the inner circumference end 40E1 to the outer circumference end 40E2 of the battery element 40. In other words, both thickness T1 and thickness T2 are substantially constant in the longitudinal direction of the negative electrode 42 (winding direction of the battery element 40). In contrast, in the battery element 40A of the secondary battery of this embodiment shown in Figure 8, the area density of the negative electrode active material layer 42B gradually changes from the inner circumference end 40E1 to the outer circumference end 40E2. In the battery element 40A of the secondary battery of this embodiment, for example, the thickness T1 and thickness T2 are each gradually changing. However, the area density of the negative electrode outer active material layer 42B2 of the battery element 40A as a whole is substantially equal to the area density of the negative electrode inner active material layer 42B1 of the battery element 40A as a whole. That is, if the constituent materials of the negative electrode inner active material layer 42B1 and the constituent materials of the negative electrode outer active material layer 42B2 are substantially the same, and the occupied area of ​​the negative electrode inner active material layer 42B1 and the occupied area of ​​the negative electrode outer active material layer 42B2 are substantially the same, then the weight of the negative electrode inner active material layer 42B1 and the weight of the negative electrode outer active material layer 42B2 are substantially equal. Note that the dashed lines in Figure 8 represent the inner active material layer 42B1 and the outer active material layer 42B2 of the negative electrode when thickness T1 and thickness T2 are equal to each other.

[0130] Specifically, the areal density of the negative electrode outer active material layer 42B2 at the inner peripheral side end portion 40E1 of the battery element 40A is higher than the areal density of the negative electrode outer active material layer 42B2 at the outer peripheral side end portion 40E2 of the battery element 40A. More specifically, in the example of FIG. 8, the areal density of the negative electrode outer active material layer 42B2 is highest at the inner peripheral side end portion 40E1, and decreases as it approaches the outer peripheral side end portion 40E2 from the inner peripheral side end portion 40E1. Furthermore, the negative electrode outer active material layer 42B2 is made of a substantially homogeneous constituent material, and the thickness T2 of the negative electrode outer active material layer 42B2 is thickest at the inner peripheral side end portion 40E1, becomes thinner as it approaches the outer peripheral side end portion 40E2 from the inner peripheral side end portion 40E1, and is thinnest at the outer peripheral side end portion 40E2. That is, if the thickness of the negative electrode outer active material layer 42B2 at the inner peripheral side end portion 40E1 is T2S and the thickness of the negative electrode outer active material layer 42B2 at the outer peripheral side end portion 40E2 is T2E, then T2S > T2E.

[0131] Furthermore, in the secondary battery of the present embodiment, the areal density of the negative electrode inner active material layer 42B1 at the inner peripheral side end portion 40E1 of the battery element 40A is higher than the areal density of the negative electrode inner active material layer 42B1 at the outer peripheral side end portion 40E2 of the battery element 40A. More specifically, in the example of FIG. 8, the areal density of the negative electrode inner active material layer 42B1 is lowest at the inner peripheral side end portion 40E1, and increases as it approaches the outer peripheral side end portion 40E2 from the inner peripheral side end portion 40E1. Furthermore, the negative electrode inner active material layer 42B1 is made of a substantially homogeneous constituent material, and the thickness T1 of the negative electrode inner active material layer 42B1 is thinnest at the inner peripheral side end portion 40E1, becomes thicker as it approaches the outer peripheral side end portion 40E2 from the inner peripheral side end portion 40E1, and is thickest at the outer peripheral side end portion 40E2. That is, if the thickness of the negative electrode inner active material layer 42B1 at the inner peripheral side end portion 40E1 is T1S and the thickness of the negative electrode inner active material layer 42B1 at the outer peripheral side end portion 40E2 is T1E, then T1S < T1E.

[0132] The secondary battery of the present embodiment has substantially the same configuration as the secondary battery of the first embodiment except for the above points.

[0133] <2-2. Operation> The operation of the secondary battery in this embodiment is the same as the operation of the secondary battery in the first embodiment described above.

[0134] <2-3. Manufacturing method> The method for manufacturing the secondary battery of this embodiment is the same as the method for manufacturing the secondary battery of the first embodiment, except that the thickness T2 of the negative electrode outer active material layer 42B2 and the thickness T1 of the negative electrode inner active material layer 42B1 gradually change from the inner circumference end 40E1 to the outer circumference end 40E2.

[0135] <2-4. Action and Effects> Thus, in the secondary battery of this embodiment, the area density of the negative electrode active material layer 42B gradually changes from the inner circumference end 40E1 to the outer circumference end 40E2. Specifically, the area density of the negative electrode outer active material layer 42B2 at the inner circumference end 40E1 of the battery element 40A is higher than the area density of the negative electrode outer active material layer 42B2 at the outer circumference end 40E2 of the battery element 40A. Therefore, similar to the secondary battery of the first embodiment described above, in the relationship between the positive electrode 41 and the negative electrode 42 facing each other across the separator 43, the capacity of the negative electrode 42 is greater than the capacity of the positive electrode 41. That is, in the relationship between the positive electrode inner active material layer 41B1 and the negative electrode outer active material layer 42B2 facing each other across the separator 43, the capacity of the negative electrode outer active material layer 42B2 can be made greater than the capacity of the positive electrode inner active material layer 41B1. As a result, the secondary battery of this embodiment can suppress the formation of precipitates such as lithium metal during the battery reaction during charging, thereby suppressing the deterioration of battery performance. Therefore, it has high reliability.

[0136] The effects and benefits of the secondary battery of this embodiment will be explained in more detail with reference to Figure 9. Figure 9 is an explanatory diagram showing the relationship between the capacity of the positive electrode 41 and the capacity of the negative electrode 42 in the battery element 40A of this embodiment, and corresponds to Figure 7 described in the first embodiment above. The horizontal axis of Figure 9 represents the element diameter d, and the vertical axis of Figure 9 represents N / P, which is the ratio of the negative electrode capacity N to the positive electrode capacity P. In Figure 9, the meaning of element diameter d and N / P is the same as in Figure 7. As shown in Figure 9, N / P changes continuously according to the size of the element diameter d. More specifically, of the four curves shown in Figure 9, two curves C9-1 and C9-2 represent the ratio Nout / Pin, which is the ratio of the capacity of the negative electrode outer active material layer 42B2 to the capacity of the positive electrode inner active material layer 41B1. Of the two curves C9-1 and C9-2, the solid curve C9-1 represents the Nout / Pin of the secondary battery of this embodiment, and the dashed curve C9-2 represents the Nout / Pin of a comparative secondary battery in which the area density of the negative electrode outer active material layer 42B2 and the area density of the negative electrode inner active material layer 42B1 are substantially equal. Also, of the four curves shown in Figure 9, two curves C9-3 and C9-4 represent the ratio Nin / Pout of the capacity of the negative electrode inner active material layer 42B1 to the capacity of the positive electrode outer active material layer 41B2. Of the two curves C9-3 and C9-4, the solid curve C9-3 represents the Nin / Pout of the secondary battery of this embodiment, and the dashed curve C9-4 represents the Nin / Pout of a comparative secondary battery in which the area density of the negative electrode outer active material layer 42B2 and the area density of the negative electrode inner active material layer 42B1 are substantially equal.

[0137] As is clear from Figure 9, in the secondary battery of this embodiment, the gap between Nout / Pin and Nin / Pout can be narrowed even in the region where the element diameter d is small compared to the comparative example. That is, from element diameter d=d0 to element diameter d=d2, the variation in the gap between curve C9-1 and curve C9-3 can be kept smaller than the variation in the gap between curve C9-2 and curve C9-4. Therefore, good cycle characteristics can be obtained.

[0138] In contrast, in the secondary battery of the comparative example, the variation in the interval between curve C9-2 and curve C9-4 is large from the element diameter d = d0 to the element diameter d = d2, which is disadvantageous for obtaining good cycle characteristics. Further, in the secondary battery of the comparative example, as shown by curve C9-2, Nout / Pin is less than 1 in the region where the element diameter d < d1. Therefore, in the region where the element diameter d < d1, deposits such as lithium metal due to the battery reaction are likely to be generated particularly during charging at a high voltage, and the battery performance is likely to deteriorate. In order to avoid the generation of such deposits, it is desirable to set the element diameter d = d1 at the inner peripheral side end portion 40E1 in the secondary battery of the comparative example. As a result, the winding center space 40K of the battery element 40 has to be increased, which is disadvantageous for increasing the capacity.

[0139] In the secondary battery of the present embodiment, regardless of the size of the element diameter d, both Nout / Pin and Nin / Pout are greater than 1 from the element diameter d = d0 to the element diameter d = d2 (see curves C9-1 and C9-3). Therefore, the element diameter d = d0 can be set at the inner peripheral side end portion 40E1, and the element diameter d = d2 can be set at the outer peripheral side end portion 40E2, and a battery element 40 having a smaller winding center space 40K can be realized. As a result, the secondary battery of the present embodiment is advantageous for increasing the capacity.

[0140] <3. Secondary battery of the third embodiment> Subsequently, a secondary battery as the third embodiment of the present disclosure will be described with reference to FIG. 10. FIG. 10 is an exploded view schematically showing the positive electrode 41 and the negative electrode 42 of the battery element 40B of the secondary battery as the third embodiment of the present disclosure. FIG. 10 corresponds to FIG. 8 showing the exploded view of the battery element 40A of the secondary battery of the second embodiment described above.

[0141] <3-1. Configuration> In the battery element 40A of the secondary battery according to the second embodiment described above, the area density of the negative electrode outer active material layer 42B2 of the battery element 40A as a whole is substantially equal to the area density of the negative electrode inner active material layer 42B1 of the battery element 40A as a whole. In contrast, in the battery element 40B of the secondary battery according to this embodiment shown in Figure 10, the area density of the negative electrode outer active material layer 42B2 of the battery element 40B as a whole is greater than the area density of the negative electrode inner active material layer 42B1 of the battery element 40B as a whole. Note that the dashed lines in Figure 10 represent the negative electrode inner active material layer 42B1 and the negative electrode outer active material layer 42B2 when thickness T1 and thickness T2 are equal to each other.

[0142] Except for the points mentioned above, the secondary battery of this embodiment has substantially the same configuration as the secondary battery of the second embodiment.

[0143] <3-2. Operation> The operation of the secondary battery in this embodiment is the same as the operation of the secondary battery in the second embodiment described above.

[0144] <3-3. Manufacturing method> The method for manufacturing a secondary battery in this embodiment is the same as the method for manufacturing a secondary battery in the second embodiment, except that the area density of the outer active material layer 42B2 of the negative electrode in the entire battery element 40B is greater than the area density of the inner active material layer 42B1 of the negative electrode in the entire battery element 40B. That is the case.

[0145] <3-4. Action and Effects> Thus, in the secondary battery of this embodiment, the area density of the outer negative electrode active material layer 42B2 of the battery element 40B is greater than the area density of the inner negative electrode active material layer 42B1 of the battery element 40B, and the area density of the negative electrode active material layer 42B gradually changes from the inner peripheral end 40E1 to the outer peripheral end 40E2. For this reason, similar to the secondary battery of the first embodiment described above, in the relationship between the positive electrode 41 and the negative electrode 42 facing each other across the separator 43, the capacity of the negative electrode 42 is greater than the capacity of the positive electrode 41. That is, in the relationship between the inner positive electrode active material layer 41B1 and the outer negative electrode active material layer 42B2 facing each other across the separator 43, the capacity of the outer negative electrode active material layer 42B2 can be made greater than the capacity of the inner positive electrode active material layer 41B1. As a result, the secondary battery of this embodiment can suppress the formation of precipitates such as lithium metal in conjunction with the battery reaction during charging, and can suppress the deterioration of battery performance. Therefore, it has high reliability.

[0146] The effects and benefits of the secondary battery of this embodiment will be explained in more detail with reference to Figure 11. Figure 11 is an explanatory diagram showing the relationship between the capacity of the positive electrode 41 and the capacity of the negative electrode 42 in the battery element 40B of this embodiment, and corresponds to Figure 7 described in the first embodiment above. The horizontal axis of Figure 11 represents the element diameter d, and the vertical axis of Figure 11 represents N / P, which is the ratio of the negative electrode capacity N to the positive electrode capacity P. In Figure 11, the meaning of element diameter d and N / P is the same as in Figure 7. As shown in Figure 11, N / P changes continuously according to the size of the element diameter d. More specifically, of the four curves shown in Figure 11, two curves C11-1 and C11-2 represent the ratio Nout / Pin, which is the ratio of the capacity of the negative electrode outer active material layer 42B2 to the capacity of the positive electrode inner active material layer 41B1. Of the two curves C11-1 and C11-2, the solid curve C11-1 represents the Nout / Pin of the secondary battery of this embodiment, while the dashed curve C11-2 represents the Nout / Pin of a comparative secondary battery in which the area density of the negative electrode outer active material layer 42B2 and the area density of the negative electrode inner active material layer 42B1 are substantially equal. In addition, of the four curves shown in Figure 11, two curves C11-3 and C11-4 represent the ratio Nin / Pout of the capacity of the negative electrode inner active material layer 42B1 to the capacity of the positive electrode outer active material layer 41B2. Of the two curves C11-3 and C11-4, the solid curve C11-3 represents the Nin / Pout of the secondary battery of this embodiment, while the dashed curve C11-4 represents the Nin / Pout of a comparative secondary battery in which the area density of the negative electrode outer active material layer 42B2 and the area density of the negative electrode inner active material layer 42B1 are substantially equal.

[0147] As is clear from Figure 11, in the secondary battery of this embodiment, the gap between Nout / Pin and Nin / Pout can be narrowed even in the region where the element diameter d is small compared to the comparative example. That is, from element diameter d=d0 to element diameter d=d2, the variation in the gap between curve C11-1 and curve C11-3 can be kept smaller than the variation in the gap between curve C11-2 and curve C11-4, and furthermore, it can be kept smaller than the variation in the gap between curve C9-1 and curve C9-3 described in the second embodiment above. Therefore, better cycle characteristics can be obtained than in the secondary battery of the second embodiment above.

[0148] Furthermore, in the secondary battery of this embodiment, regardless of the size of the element diameter d, both Nout / Pin and Nin / Pout are greater than 1 from element diameter d=d0 to element diameter d=d2 (see curves C11-1 and C11-3). Therefore, the element diameter d=d0 can be set to the inner circumference end 40E1 and the element diameter d=d2 can be set to the outer circumference end 40E2, making it possible to realize a battery element 40 with a smaller winding center space 40K. As a result, the secondary battery of this embodiment is advantageous for increasing capacity.

[0149] <4. Examples> Examples of the present disclosure will be described below.

[0150] [Example 1] The secondary batteries (lithium-ion secondary batteries) shown in Figures 1-5 were fabricated. Specifically, a coin-type secondary battery was fabricated in which the area density of the negative electrode outer active material layer 42B2 was greater than the area density of the negative electrode inner active material layer 42B1, from the inner peripheral end 40E1 to the outer peripheral end 40E2 of the battery element 40, as described below.

[0151] (Fabrication of the positive electrode) First, a positive electrode mixture was prepared by mixing 91 parts by mass of positive electrode active material (LiCoO2), 3 parts by mass of positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of positive electrode conductive agent (graphite). Next, the positive electrode mixture was added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like positive electrode mixture slurry. Subsequently, the positive electrode mixture slurry was applied to both sides of the positive electrode current collector 41A (a strip of aluminum foil with a thickness of 12 μm) using a coating apparatus, and the positive electrode mixture slurry was dried to form a positive electrode active material layer 41B. Finally, the positive electrode active material layer 41B was compressed and molded using a roll press. This produced a positive electrode 41 (width = 3.3 mm). The thickness of the positive electrode inner active material layer 41B1 and the positive electrode outer active material layer 41B2 after compression molding were set to 0.037 mm, respectively.

[0152] (Fabrication of the negative electrode) First, a negative electrode mixture was prepared by mixing 95 parts by mass of negative electrode active material (graphite) and 5 parts by mass of negative electrode binder (polyvinylidene fluoride). Next, the negative electrode mixture was added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like negative electrode mixture slurry. Subsequently, the positive electrode mixture slurry was applied to both sides of the negative electrode current collector 42A (a strip of copper foil with a thickness of 15 μm) using a coating apparatus, and then the negative electrode mixture slurry was dried to form the negative electrode active material layer 42B. Finally, the negative electrode active material layer 42B was compressed and molded using a roll press. This produced the negative electrode 42 (width = 3.8 mm). In this process, the inner active material layer 42B1 and the outer active material layer 42B2 were formed such that the thickness T1 of the inner active material layer 42B1 and the thickness T2 of the outer active material layer 42B2 were constant, and the thickness T2 of the outer active material layer 42B2 was greater than the thickness T1 of the inner active material layer 42B1. Specifically, the thickness T1 after compression molding was set to 0.046 mm, and the thickness T2 after compression molding was set to 0.049 mm. In other words, a difference was created between the area density of the outer active material layer 42B2 and the area density of the inner active material layer 42B1. In detail, when the average area density of the negative electrode outer active material layer 42B2 and the negative electrode inner active material layer 42B1 of the battery element 40 is taken as 100%, the area density of the negative electrode outer active material layer 42B2 of the battery element 40 was 101.8%, and the area density of the negative electrode inner active material layer 42B1 of the battery element 40 was 98.2%. However, in this embodiment, there is no longitudinal gradient in the area density of the negative electrode outer active material layer 42B2 and the negative electrode inner active material layer 42B1 of the negative electrode, and they are substantially constant.

[0153] (Preparation of electrolyte solution) An electrolyte salt (LiPF6) was added to a solvent (ethylene carbonate and diethyl carbonate), and the solvent was then stirred. In this case, the solvent mixing ratio (by weight) was ethylene carbonate:diethyl carbonate = 30:70, and the electrolyte salt content was 1 mol / kg relative to the solvent. As a result, the electrolyte salt was dissolved or dispersed in the solvent, and an electrolyte solution was prepared.

[0154] (Assembly of secondary batteries) First, using resistance welding, the aluminum was partially coated with a tubular sealant 61 (polypropylene film, outer diameter = 9.0 mm, inner diameter = 3.0 mm). A nickel-plated positive electrode lead 51 (thickness = 0.1 mm, width = 2.0 mm, protrusion length from positive electrode 41 = 11.7 mm) was welded to the positive electrode 41 (positive electrode current collector 41A). Additionally, a nickel-plated negative electrode lead 52 (thickness = 0.1 mm, width = 2.0 mm, protrusion length from negative electrode 42 = 6.0 mm) was welded to the negative electrode 42 (negative electrode current collector 42A) using resistance welding. In this case, the welding position of the positive electrode lead 51 was adjusted so that it was located in the middle of the winding of the positive electrode 41.

[0155] Next, the positive electrode 41 and the negative electrode 42 were stacked on top of each other via a separator 43 (a microporous polyethylene film with a thickness of 25 μm and a width of 4.0 mm). Then, the positive electrode 41, the negative electrode 42, and the separator 43 were wound together to produce a cylindrical wound body 40Z (outer diameter = 11.6 mm) having a winding center space 40K (inner diameter = 1.5 mm).

[0156] Next, a ring-shaped insulating film (polyimide film, outer diameter = 11.6 mm, inner diameter = 2.2 mm, thickness = 0.05 mm) for use as a base was placed inside a cylindrical storage section 11 (wall thickness = 0.15 mm, outer diameter = 12.0 mm, height = 5.0 mm) made of stainless steel (SUS316) through the opening 11K, and then the wound body 40Z was placed inside the storage section 11. In this case, the negative electrode lead 52 was welded to the storage section 11 using resistance welding. Next, using resistance welding, a positive lead 51 was welded to the external terminal 20 of a stainless steel (SUS316) disc-shaped lid portion 12 (wall thickness = 0.15 mm, outer diameter = 11.7 mm), which has a recessed portion 12H (inner diameter = 9.0 mm, step height = 0.3 mm) with a through-hole 12K (inner diameter = 3.0 mm) and an aluminum disc-shaped external terminal 20 (wall thickness = 0.3 mm, outer diameter = 7.2 mm) attached via a gasket 30 (polyimide film, outer diameter = 9.2 mm, inner diameter = 3.2 mm).

[0157] Next, with the lid 12 positioned upright relative to the storage section 11, electrolyte was injected into the storage section 11 through the opening 11K. As a result, the winding body 40Z (positive electrode 41, negative electrode 42, and separator 43) was impregnated with electrolyte, and the battery element 40 was fabricated.

[0158] Finally, after closing the opening 11K with the lid 12, the lid 12 was welded to the storage section 11 using laser welding. When closing the opening 11K with the lid 12, a folded portion 513 was formed on a part of the positive electrode lead 51 to form a curved shape, and the folded portion 513 was positioned on the peripheral portion 12R. Specifically, the distance between the folded portion 513 and the inner surface of the side wall M3 was adjusted to 0.5 mm. In addition, a ring-shaped insulating film 62 (polyimide film, outer diameter = 9.2 mm, inner diameter = 3.2 mm) was placed between the lid 12 and the positive electrode lead 51, and a disc-shaped insulating film 63 (polyimide film, outer diameter = 3.2 mm) was placed between the battery element 40 and the positive electrode lead 51. As a result, the storage section 11 and the lid section 12 formed the outer can 10, and the battery element 40 was sealed inside the outer can 10, thus assembling a secondary battery (outer diameter = 12.0 mm, height = 5.0 mm).

[0159] (Stabilization of secondary batteries) The assembled secondary battery was subjected to one charge-discharge cycle in a normal temperature environment (temperature = 23°C). During charging, constant current charging was performed at a current of 0.1C until the voltage reached 4.2V, and then constant voltage charging was performed at that voltage of 4.2V until the current reached 0.05C. During discharging, constant current discharge was performed at a current of 0.1C until the voltage reached 3.0V. 0.1C is the current value required to completely discharge the battery capacity (theoretical capacity) in 10 hours, and 0.05C is the current value required to completely discharge the battery capacity in 20 hours.

[0160] As a result, a coating was formed on the surface of the negative electrode 42 and other components, which electrochemically stabilized the state of the secondary battery. Thus, the secondary battery was completed.

[0161] Next, the performance of the secondary battery prepared as described above was evaluated. The results are shown in Table 1.

[0162] [Table 1]

[0163] Here, the minimum negative electrode potential (d < 4 mm) [mV] in the region of the battery element 40 where the element diameter d is less than 4 mm, the minimum negative electrode potential (d ≥ 4 mm) [mV] in the region of the battery element 40 where the element diameter d is 4 mm or more, the discharge capacity [mAh], the energy density increase rate [%], and the cycle capacity maintenance rate [%] were measured.

[0164] The discharge capacity [mAh] was measured by conducting a discharge test under the following test conditions. The test conditions for the discharge test are as follows: (Discharge test conditions) (1) Operating environment temperature: 23℃ (2) Charging conditions: Constant current constant voltage (CC-CV) charging was performed. The battery was charged with a constant current of 0.5C up to a voltage of 4.38V, and then charged with a constant voltage of 4.38V. The cutoff current was set to 0.025C. (3) Rest period after charging: 30 minutes (4) Discharge conditions: Constant current (CC) discharge was performed with a constant current of 0.5C. The cutoff voltage was set to 3.0V.

[0165] The cycle capacity retention rate [%] was determined by performing charge-discharge cycle tests under the following test conditions.

[0166] In the charge-discharge cycle test, the discharge capacity (discharge capacity in the first cycle) was first measured by charging and discharging the secondary battery in a normal temperature environment (temperature = 25°C). Subsequently, the discharge capacity (discharge capacity in the 501st cycle) was measured by repeatedly charging and discharging the secondary battery in the same environment until the total number of cycles reached 501. Finally, the capacity retention rate (%) was calculated as (discharge capacity in the 501st cycle / discharge capacity in the 1st cycle) × 100.

[0167] In the charge-discharge cycle test, during charging, the battery was charged with a constant current of 2C until the battery voltage reached 4.38V, and then charged with a constant voltage of 0.025C at that voltage. During discharging, the battery was discharged with a constant current of 0.7C until the battery voltage reached 3.0V. 2C is the current value that completely discharges the battery capacity (theoretical capacity) in 0.5 hours, while 0.7C is the current value that completely discharges the battery capacity in 1.43 hours.

[0168] The minimum negative electrode potential (d < 4 mm) [mV] is the open-circuit potential (relative to lithium metal) of the negative electrode 42 measured in the region of the battery element 40 of a fully charged secondary battery where the element diameter d is less than 4 mm.

[0169] The minimum negative electrode potential (d≧4mm) [mV] is the open-circuit potential (relative to lithium metal) of the negative electrode 42 measured in the region of the battery element 40 of a fully charged secondary battery where the element diameter d is 4 mm or more.

[0170] Furthermore, the discharge capacity was obtained by a discharge test based on the discharge test conditions described above, and assuming that the volume of the secondary battery is constant, the capacity increase rate was defined as the energy density increase rate [%] based on the discharge capacity of Comparative Example 2 described later.

[0171] [Example 2] A secondary battery for Example 2 was fabricated in the same manner as in Example 1. However, the battery voltage during charging in the charge-discharge cycle test (charging voltage) was set to 4.45V. Except for this point, the secondary battery for Example 2 was evaluated in the same manner as the secondary battery for Example 1. The results are shown in Table 1.

[0172] [Example 3] The inner diameter of the winding center space at 40K was set to 1.0 mm. Except for this point, the secondary battery of Example 3 was fabricated in the same manner as the secondary battery of Example 1, and evaluated in the same manner as the secondary battery of Example 1. The results are shown in Table 1.

[0173] [Example 4] A secondary battery for Example 4 was fabricated in the same manner as in Example 3. However, the battery voltage during charging in the charge-discharge cycle test (charging voltage) was set to 4.45V. Except for this point, the secondary battery for Example 4 was evaluated in the same manner as the secondary battery for Example 1. The results are shown in Table 1.

[0174] [Example 5] The inner diameter of the winding center space 40K was set to 1.0 mm. Furthermore, as shown in Figure 10, the thickness T1 of the negative electrode inner active material layer 42B1 gradually increased from the inner circumference end 40E1 to the outer circumference end 40E2, while the thickness T2 of the negative electrode outer active material layer 42B2 gradually decreased. In addition, the battery voltage (charging voltage) during charging in the charge-discharge cycle test was set to 4.45 V. Except for these points, the secondary battery of Example 5 was manufactured in the same manner as in Example 1, and evaluated in the same manner as the secondary battery of Example 1. The results are shown in Table 1. At that time, when the average value of thickness T1 was set to 100%, the minimum value of thickness T1 was set to 94% and the maximum value of thickness T1 was set to 106%. Similarly, when the average value of thickness T2 was set to 100%, the minimum value of thickness T2 was set to 94% and the maximum value of thickness T2 was set to 106%.

[0175] [Comparative Example 1] The inner diameter of the winding center space 40K was set to 1.0 mm. Furthermore, the thickness T1 of the negative electrode inner active material layer 42B1 and the thickness T2 of the negative electrode outer active material layer 42B2 were both set to 0.047 mm. Except for these points, the secondary battery of Comparative Example 1 was fabricated in the same manner as the secondary battery of Example 1, and evaluated in the same manner as the secondary battery of Example 1. The results are shown in Table 1.

[0176] [Comparative Example 2] The secondary battery of Comparative Example 2 was fabricated in the same manner as the secondary battery of Comparative Example 1, except that the inner diameter of the winding center space of 40K was set to 4.0 mm. In addition, the battery voltage during charging (charging voltage) in the charge-discharge cycle test was set to 4.45 V. Except for this point, the same evaluation as the secondary battery of Comparative Example 1 was performed. The results are shown in Table 1.

[0177] [Comparative Example 3] A secondary battery for Comparative Example 3 was fabricated in the same manner as for Comparative Example 1. However, the battery voltage during charging in the charge-discharge cycle test (charging voltage) was set to 4.45V. Except for this point, the secondary battery for Comparative Example 3 was evaluated in the same manner as the secondary battery for Comparative Example 1. The results are shown in Table 1.

[0178] As shown in Table 1, in Examples 1 and 3, where the charging voltage was 4.38V, the cycle capacity retention rate was found to be significantly improved compared to Comparative Example 1, which also used a charging voltage of 4.38V. This is because, in Comparative Example 1, the minimum negative electrode potential in the region where the element diameter d is less than 4mm was significantly lower than the minimum negative electrode potential in the region where the element diameter d is 4mm or more, whereas in Examples 1 and 3, no decrease in the minimum negative electrode potential occurred in the region where the element diameter d is less than 4mm. In other words, in Examples 1 and 3, it is considered that the capacity of the negative electrode outer active material layer 42B2 was made larger than the capacity of the positive electrode inner active material layer 41B1, even in the region where the element diameter d of the battery element 40 is less than 4mm.

[0179] In Examples 2, 4, and 5, where the charging voltage was 4.45V, it was found that the cycle capacity retention rate improved compared to Comparative Example 3, which also used a charging voltage of 4.45V. This is because, in Comparative Example 3, the minimum negative electrode potential in the region where the element diameter d is less than 4mm decreased significantly compared to the minimum negative electrode potential in the region where the element diameter d is 4mm or more, whereas in Examples 2, 4, and 5, no decrease in the minimum negative electrode potential in the region where the element diameter d is less than 4mm occurred. In Comparative Example 2, where the charging voltage was also 4.45V, the inner diameter of the winding center space 40K was set to 4.0mm, thus avoiding deterioration of the cycle capacity retention rate. However, in Comparative Example 2, the discharge capacity was lower compared to Examples 2, 4, 5, and Comparative Example 3.

[0180] Furthermore, a comparison of Example 4 and Example 5 confirmed that the cycle capacity maintenance rate can be further improved by making the area density of the negative electrode outer active material layer 42B2 greater than the area density of the negative electrode inner active material layer 42B1 from the inner circumferential end 40E1 to the outer circumferential end 40E2, and by making both the area density of the negative electrode outer active material layer 42B2 and the area density of the negative electrode inner active material layer 42B1 gradually change as one moves from the inner circumferential end 40E1 to the outer circumferential end 40E2.

[0181] Although the present technology has been described above with reference to one embodiment and one example, the configuration of the present technology is not limited to the configuration described in the one embodiment and one example, and can be modified in various ways.

[0182] Specifically, the explanation described the case where the outer can is a welded can (crimped can), but the structure of the outer can is not particularly limited, and a crimped can is also acceptable. In this crimped can, the storage section and the lid section, which are separate from each other, are crimped together via a gasket.

[0183] Furthermore, while we have described the case where the electrode reactant is lithium, the electrode reactant is not particularly limited. Therefore, as mentioned above, the electrode reactant may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. In addition, the electrode reactant may be other light metals such as aluminum.

[0184] The effects described herein are illustrative only, and the effects of this disclosure are not limited to those described herein. Therefore, other effects may be obtained with respect to this disclosure.

[0185] Furthermore, this disclosure may take the following forms: <1> A battery element comprising a first electrode and a second electrode stacked with a separator in between and wound around a winding axis extending in a first direction, An outer casing member housing the aforementioned battery element and Equipped with, The second electrode includes a second electrode current collector including an inner surface of the second electrode facing the winding axis and an outer surface of the second electrode opposite to the inner surface of the second electrode, an inner active material layer of the second electrode provided on the inner surface of the second electrode, and an outer active material layer of the second electrode provided on the outer surface of the second electrode. From the inner circumference end of the battery element to the outer circumference end of the battery element, the area density of the outer active material layer of the second electrode is greater than the area density of the inner active material layer of the second electrode facing the outer active material layer of the second electrode across the second electrode current collector. Secondary battery. <2> The area density of the second electrode outer active material layer at the inner circumference end of the battery element is higher than the area density of the second electrode outer active material layer at the outer circumference end of the battery element. the above <1> The rechargeable battery described. <3> The area density of the active material layer on the outer side of the second electrode is highest at the inner circumference end of the battery element and decreases as it approaches the outer circumference end of the battery element from the inner circumference end. the above <1> or <2> The rechargeable battery described. <4> The thickness of the second electrode outer active material layer is thickest at the inner circumference end of the battery element and thins as it approaches the outer circumference end of the battery element from the inner circumference end. the above <1> from <3> A rechargeable battery as described in one of the following. <5> The area density of the active material layer inside the second electrode at the inner circumference end of the battery element is lower than the area density of the active material layer inside the second electrode at the outer circumference end of the battery element. the above <1> from <4> A rechargeable battery as described in one of the following. <6> The area density of the active material layer inside the second electrode is lowest at the inner circumference end of the battery element and increases as you move from the inner circumference end towards the outer circumference end of the battery element. the above <1> from <5> A rechargeable battery as described in one of the following. <7> The thickness of the active material layer inside the second electrode is thinnest at the inner circumference end of the battery element and increases as it approaches the outer circumference end of the battery element. the above <1> from <6> A rechargeable battery as described in one of the following. <8> A battery element comprising a first electrode and a second electrode stacked with a separator in between and wound around a winding axis extending in a first direction, An outer casing member housing the aforementioned battery element and Equipped with, The second electrode includes a second electrode current collector including an inner surface of the second electrode facing the winding axis and an outer surface of the second electrode opposite to the inner surface of the second electrode, an inner active material layer of the second electrode provided on the inner surface of the second electrode, and an outer active material layer of the second electrode provided on the outer surface of the second electrode. The area density of the second electrode outer active material layer is highest at the inner circumference end of the battery element and decreases as it approaches the outer circumference end of the battery element from the inner circumference end. The area density of the active material layer inside the second electrode is lowest at the inner circumference end of the battery element and increases as you move from the inner circumference end towards the outer circumference end of the battery element. Secondary battery. <9> The thickness of the second electrode outer active material layer is thickest at the inner circumference end of the battery element and thins as it approaches the outer circumference end of the battery element from the inner circumference end. the above <8> The rechargeable battery described. <10> The thickness of the active material layer inside the second electrode is thinnest at the inner circumference end of the battery element and increases as it approaches the outer circumference end of the battery element. the above <8> or <9> The secondary battery described above. <11> The first electrode is the positive electrode, and the second electrode is the negative electrode. the above <1> from <10> A rechargeable battery as described in one of the following. <12> The outer diameter in the second direction perpendicular to the first direction of the exterior member is greater than the height of the exterior member in the first direction. the above <1> from <11> A rechargeable battery as described in one of the following.

Claims

1. A battery element comprising a positive electrode and a negative electrode stacked with a separator in between and wound around a winding axis extending in a first direction, An outer casing member housing the aforementioned battery element and Equipped with, The positive electrode comprises a positive electrode current collector including a positive electrode inner surface facing the winding shaft and a positive electrode outer surface opposite to the positive electrode inner surface, a positive electrode inner active material layer provided on the positive electrode inner surface, and a positive electrode outer active material layer provided on the positive electrode outer surface. The negative electrode comprises a negative electrode current collector including an inner negative electrode surface facing the winding axis and an outer negative electrode surface opposite to the inner negative electrode surface, an inner negative electrode active material layer provided on the inner negative electrode surface, and an outer negative electrode active material layer provided on the outer negative electrode surface. From the inner circumference end of the battery element to the outer circumference end of the battery element, the area density of the negative electrode outer active material layer is greater than the area density of the negative electrode inner active material layer facing the negative electrode outer active material layer across the negative electrode current collector. The area density of the negative electrode outer active material layer at the inner circumference end of the battery element is higher than the area density of the negative electrode outer active material layer at the outer circumference end of the battery element. The negative electrode outer active material layer faces the positive electrode inner active material layer with the separator in between. The capacity of the outer active material layer of the negative electrode is greater than the capacity of the inner active material layer of the positive electrode. Secondary battery.

2. A battery element comprising a positive electrode and a negative electrode stacked with a separator in between and wound around a winding axis extending in a first direction, An outer casing member housing the aforementioned battery element and Equipped with, The positive electrode comprises a positive electrode current collector including a positive electrode inner surface facing the winding shaft and a positive electrode outer surface opposite to the positive electrode inner surface, a positive electrode inner active material layer provided on the positive electrode inner surface, and a positive electrode outer active material layer provided on the positive electrode outer surface. The negative electrode comprises a negative electrode current collector including an inner negative electrode surface facing the winding axis and an outer negative electrode surface opposite to the inner negative electrode surface, an inner negative electrode active material layer provided on the inner negative electrode surface, and an outer negative electrode active material layer provided on the outer negative electrode surface. From the inner circumference end of the battery element to the outer circumference end of the battery element, the area density of the negative electrode outer active material layer is greater than the area density of the negative electrode inner active material layer facing the negative electrode outer active material layer across the negative electrode current collector. The area density of the negative electrode inner active material layer at the inner peripheral end of the battery element is lower than the area density of the negative electrode inner active material layer at the outer peripheral end of the battery element. The negative electrode outer active material layer faces the positive electrode inner active material layer with the separator in between. The capacity of the outer active material layer of the negative electrode is greater than the capacity of the inner active material layer of the positive electrode. Secondary battery.

3. The area density of the negative electrode outer active material layer is highest at the inner circumference end of the battery element and decreases as it approaches the outer circumference end of the battery element. A secondary battery according to claim 1 or claim 2.

4. The thickness of the negative electrode outer active material layer is thickest at the inner circumference end of the battery element and thins as it approaches the outer circumference end of the battery element from the inner circumference end. A secondary battery according to claim 1 or claim 2.

5. The area density of the negative electrode inner active material layer is lowest at the inner circumference end of the battery element and increases as it approaches the outer circumference end of the battery element. A secondary battery according to claim 1 or claim 2.

6. The thickness of the negative electrode inner active material layer is thinnest at the inner circumference end of the battery element and increases as it approaches the outer circumference end of the battery element. A secondary battery according to claim 1 or claim 2.

7. A battery element comprising a positive electrode and a negative electrode stacked with a separator in between and wound around a winding axis extending in a first direction, An outer casing member housing the aforementioned battery element and Equipped with, The positive electrode comprises a positive electrode current collector including a positive electrode inner surface facing the winding shaft and a positive electrode outer surface opposite to the positive electrode inner surface, a positive electrode inner active material layer provided on the positive electrode inner surface, and a positive electrode outer active material layer provided on the positive electrode outer surface. The negative electrode comprises a negative electrode current collector including an inner negative electrode surface facing the winding axis and an outer negative electrode surface opposite to the inner negative electrode surface, an inner negative electrode active material layer provided on the inner negative electrode surface, and an outer negative electrode active material layer provided on the outer negative electrode surface. The area density of the negative electrode outer active material layer is highest at the inner circumference end of the battery element and decreases as it approaches the outer circumference end of the battery element from the inner circumference end. The area density of the negative electrode inner active material layer is lowest at the inner circumference end of the battery element and increases as it approaches the outer circumference end of the battery element from the inner circumference end. The negative electrode outer active material layer faces the positive electrode inner active material layer with the separator in between. The capacity of the outer active material layer of the negative electrode is greater than the capacity of the inner active material layer of the positive electrode. Secondary battery.

8. The thickness of the negative electrode outer active material layer is thickest at the inner circumference end of the battery element and thins as it approaches the outer circumference end of the battery element from the inner circumference end. The secondary battery according to claim 7.

9. The thickness of the negative electrode inner active material layer is thinnest at the inner circumference end of the battery element and increases as it approaches the outer circumference end of the battery element. The secondary battery according to claim 7.

10. The outer diameter in the second direction perpendicular to the first direction of the exterior member is greater than the height of the exterior member in the first direction. A secondary battery according to claim 1, claim 2, or claim 7.

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