Secondary battery

Abstract

To suppress deterioration of cycle characteristics in a secondary battery.SOLUTION: The secondary battery 1 includes a sheet-shaped positive electrode 11 and a sheet-shaped negative electrode 12 stacked on the positive electrode 11 via a separator 13. The positive electrode 11 and the negative electrode 12 are wound. The negative electrode 12 includes a sheet-shaped negative electrode current collector 12a, a first negative electrode mixture layer 12a disposed on an inner surface 12a1 of the negative electrode current collector 12b on the inner side in the stacking direction L, and a second negative electrode mixture layer 12a disposed on an outer surface 12a2 of the negative electrode current collector 12c on the outer side in the stacking direction L. Regarding the expansion rate in the thickness direction of the negative electrode 12 in the charged state with respect to the discharged state, the first negative electrode mixture layer 12b is larger than the second negative electrode mixture layer 12c.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to secondary batteries. [Background technology]

[0002] Patent Document 1 discloses, as an example of a secondary battery, a nonaqueous electrolyte secondary battery including a wound electrode assembly in which a sheet-shaped positive electrode and a sheet-shaped negative electrode are wound with a separator interposed therebetween. In the secondary battery of Patent Document 1, graphite and a Si-containing material are used as the negative electrode active material.

[0003] The amount of lithium stored per unit area in Si-containing materials is greater than that in carbon materials such as graphite, so using Si-containing materials as the negative electrode active material can increase the capacity of batteries. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-178913 Summary of the Invention [Problem to be solved by the invention]

[0005] However, Si-containing materials undergo a relatively large volume change during charge and discharge of a secondary battery. Therefore, the electrode body may deform toward the center of the electrode body, especially at the start of winding of a wound electrode body, and the electrode body may buckle. If the electrode body buckles, the secondary battery's characteristics related to deterioration due to repeated charge and discharge (so-called cycle characteristics) may deteriorate.

[0006] The present disclosure has been made in view of the above, and aims to suppress deterioration in cycle characteristics of a secondary battery. [Means for solving the problem]

[0007] The secondary battery of the present disclosure comprises a sheet-shaped positive electrode and a sheet-shaped negative electrode stacked on the positive electrode via a separator, wherein the positive electrode and the negative electrode are wound together, and the negative electrode comprises a sheet-shaped negative electrode current collector, a first negative electrode composite layer disposed on an inner surface of the negative electrode current collector that is on the inner side in the stacking direction, and a second negative electrode composite layer disposed on an outer surface of the negative electrode current collector that is on the outer side in the stacking direction, wherein the first negative electrode composite layer has a larger expansion coefficient in the thickness direction of the negative electrode in a charged state relative to a discharged state than the second negative electrode composite layer. [Effects of the Invention]

[0008] According to the secondary battery of the present disclosure, it is possible to suppress deterioration in cycle characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is an enlarged partial cross-sectional view of the electrode body taken along line II-II shown in FIG. [Figure 3] FIG. 3 is a partially enlarged cross-sectional view showing the degree of bending of the electrode body of the secondary battery of Example 1 when it is charged. [Figure 4] FIG. 4 is a partially enlarged cross-sectional view showing the degree of bending of the electrode body of the secondary battery of Comparative Example 1 when it is charged. [Figure 5] FIG. 5 is a partially enlarged cross-sectional view showing the degree of bending of the electrode body of the secondary battery of Comparative Example 2 when it is charged. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments will be described in detail with reference to the drawings. Note that the present disclosure is not limited to these embodiments. Each embodiment is an example, and it goes without saying that partial substitution or combination of the configurations shown in different embodiments is possible.

[0011] FIG. 1 is a cross-sectional view of a secondary battery 1 according to an embodiment of the present disclosure.

[0012] The secondary battery 1 is, for example, a lithium battery. The secondary battery 1 includes an electrode assembly 10 and a housing 20.

[0013] The electrode body 10 is a wound type electrode body. The electrode body 10 is formed by stacking sheet-like positive electrodes 11 and negative electrodes 12 with separators 13 interposed therebetween and winding them. The electrode body 10 is cylindrical. The electrode body 10 may also be flat. The positive electrodes 11 and negative electrodes 12 will be described in detail later.

[0014] The electrode assembly 10 includes a strip-shaped positive electrode terminal 14 that is electrically connected to the positive electrode 11 , and a strip-shaped negative electrode terminal 15 that is electrically connected to the negative electrode 12 .

[0015] The housing 20 includes a main body 21 and a lid 22. The main body 21 and the lid 22 are made of electrically conductive materials such as iron, stainless steel, and aluminum.

[0016] The main body 21 is cylindrical and has an opening 21a at one end. The negative electrode terminal 15 is electrically connected to the inner surface of the main body 21.

[0017] The lid portion 22 covers the opening 21a of the main body portion 21. The lid portion 22 is disposed on the main body portion 21 while being electrically insulated from the main body portion 21. The positive electrode terminal 14 is electrically connected to the lid portion 22.

[0018] Figure 2 is an enlarged partial cross-sectional view of the electrode assembly 10 taken along line II-II in Figure 1. The electrode assembly 10 shown in Figure 2 is in a discharged state. In Figure 2, the separator 13 is indicated by a dashed line. Arrows indicating the winding direction R and the stacking direction L are shown in Figure 2.

[0019] The side indicated by the arrow in the winding direction R is the winding end side of the electrode body 10, and the opposite side is the winding start side of the electrode body 10. In the electrode body 10, the winding start end is located on the inside, and the winding end end is located on the outside.

[0020] The stacking direction L is the direction in which the positive electrode 11, the negative electrode 12, and the separator 13 are stacked, and is perpendicular to the winding direction R. In the stacking direction L, the side indicated by the arrow is the side facing the outside of the electrode body 10, and the opposite side is the direction facing the inside of the electrode body 10.

[0021] FIG. 2 also shows a portion of the electrode body 10, including the end portion on the winding start side.

[0022] Positive electrode 11 is in a sheet shape. Positive electrode 11 includes positive electrode current collector 11a, first positive electrode composite layer 11b, and second positive electrode composite layer 11c. Positive electrode current collector 11a, first positive electrode composite layer 11b, and second positive electrode composite layer 11c are each in a sheet shape.

[0023] The positive electrode current collector 11a is a conductive layer made of, for example, a metal, and is specifically made of aluminum or the like.

[0024] The first positive electrode composite layer 11b and the second positive electrode composite layer 11c are disposed on opposite sides of the positive electrode current collector 11a. The first positive electrode composite layer 11b is located more inward in the stacking direction L than the second positive electrode composite layer 11c. The first positive electrode composite layer 11b and the second positive electrode composite layer 11c contain a positive electrode active material. The positive electrode active material is, for example, a metal oxide containing lithium ions, specifically, lithium cobalt oxide, lithium nickel oxide, or the like.

[0025] Negative electrode 12 is in a sheet shape. Negative electrode 12 includes a negative electrode current collector 12a, a first negative electrode composite material layer 12b, and a second negative electrode composite material layer 12c. Negative electrode current collector 12a, first negative electrode composite material layer 12b, and second negative electrode composite material layer 12c are each in a sheet shape.

[0026] The negative electrode current collector 12a is a conductive layer made of, for example, a metal, and is specifically made of copper.

[0027] The first negative electrode composite layer 12b and the second negative electrode composite layer 12c are disposed on opposite sides of the negative electrode current collector 12a. The first negative electrode composite layer 12b is located more inward in the stacking direction L than the second negative electrode composite layer 12c. Specifically, the first negative electrode composite layer 12b is disposed on an inner surface 12a1 of the negative electrode current collector 12a that is located on the inner side in the stacking direction L. The second negative electrode composite layer 12c is disposed on an outer surface 12a2 of the negative electrode current collector 12a that is located on the outer side in the stacking direction L.

[0028] First negative electrode composite layer 12b and second negative electrode composite layer 12c contain a negative electrode active material. The negative electrode active material includes both graphite and a silicon-containing material. Specifically, the silicon-containing material is a silicon oxide represented by the general formula: SiOx (where x is a real number satisfying 0≦x≦2). Note that the silicon-containing material may be a material containing at least one of the above silicon oxides, a mixture of silicon and a carbon material, a mixture of a silicon compound and a carbon material, and silicon.

[0029] Furthermore, the ratio of the first weight to the total weight of the first weight of the silicon-containing material and the second weight of graphite (=first weight×100 / (first weight+second weight): hereinafter referred to as the "silicon ratio") is greater in first negative electrode composite layer 12b than in second negative electrode composite layer 12c. As a result, the expansion rate in the thickness direction of negative electrode 12 when electrode body 10 is charged relative to when electrode body 10 is discharged is greater in first negative electrode composite layer 12b than in second negative electrode composite layer 12c.

[0030] That is, when the electrode body 10 changes from a discharged state to a charged state, the expansion coefficient in the thickness direction of the first negative electrode composite layer 12b is greater than the expansion coefficient in the thickness direction of the second negative electrode composite layer 12c. Therefore, when the electrode body 10 changes from a discharged state to a charged state, the negative electrode 12 is prevented from bending inward in the stacking direction L, and the negative electrode 12 is prevented from buckling.

[0031] 2, the winding start end of positive electrode 11 is closer to the winding end of negative electrode 12 in winding direction R than the winding start end of negative electrode 12. In other words, first negative electrode composite layer 12b is stacked on the outer side of the winding start end of positive electrode 11 in stacking direction L (hereinafter referred to as the "inner end E1 of positive electrode 11"), with separator 13 interposed therebetween. Also, as described above, negative electrode 12 is prevented from bending inward in stacking direction L. Therefore, when negative electrode 12 bends toward the inner end E1 of positive electrode 11, separator 13 is pressed by the inner end E1 of positive electrode 11 and negative electrode 12, and the inner end E1 of positive electrode 11 does not break through separator 13. Therefore, short-circuiting between positive electrode 11 and negative electrode 12 is prevented.

[0032] Furthermore, the winding start end of the negative electrode 12 (hereinafter referred to as "inner end E2 of the negative electrode 12") overlaps the outer side of the inner end E1 of the positive electrode 11 in the stacking direction L, with the separator 13 interposed therebetween. The inner end E2 of the negative electrode 12 is more likely to bend inward in the stacking direction L than other parts of the negative electrode 12 due to the presence of a space in the center of the electrode body 10. Meanwhile, as described above, the negative electrode 12 is prevented from bending inward in the stacking direction L. Therefore, even when the inner end E2 of the negative electrode 12 overlaps the inner end E1 of the positive electrode 11 in the stacking direction L, the inner end E1 of the positive electrode 11 is prevented from breaking through the separator 13, thereby preventing a short circuit between the positive electrode 11 and the negative electrode 12.

[0033] The negative electrode 12 may be stacked on the inner side of the positive electrode 11 in the stacking direction L. Alternatively, the negative electrode 12 may be stacked on both sides of the positive electrode 11 in the stacking direction L.

[0034] Next, the results of repeated charge and discharge cycles of the secondary battery 1 of the above embodiment and a secondary battery of a comparative example will be compared. In this comparison, the silicon-containing material is silicon oxide.

[0035] In Table 1 below, the "Active material composition ratio" column shows the weight ratio of silicon oxide and the weight ratio of graphite for first negative electrode composite layer 12b and second negative electrode composite layer 12c in the "Weight ratio of silicon oxide" and "Weight ratio of graphite" columns, respectively. Also, the "Silicon ratio" column shows the silicon ratio (=first weight × 100 / (first weight + second weight)) for first negative electrode composite layer 12b and second negative electrode composite layer 12c.

[0036] The "internal / external silicon ratio" column indicates the ratio of the silicon ratio of first negative electrode composite layer 12b to the silicon ratio of second negative electrode composite layer 12c (=silicon ratio of first negative electrode composite layer 12b / silicon ratio of second negative electrode composite layer 12c).

[0037] Furthermore, the "Thickness before charge" column indicates the thickness of the electrode body 10 before charging (discharged state) for each of the first negative electrode composite layer 12b and the second negative electrode composite layer 12c. The "Expansion rate" column indicates the ratio of the increase in thickness after charging to the thickness before charging for each of the first negative electrode composite layer 12b and the second negative electrode composite layer 12c (=increase in thickness after charging × 100 / thickness before charging). The "Internal / External Expansion Ratio" column indicates the ratio of the expansion rate of the first negative electrode composite layer 12b to the expansion rate of the second negative electrode composite layer 12c (=expansion rate of the first negative electrode composite layer 12b / expansion rate of the second negative electrode composite layer 12c).

[0038] The column "presence or absence of short circuit in cycle test" indicates the presence or absence of short circuit between the positive electrode 11 and the negative electrode 12 in a cycle test in which the electrode assembly 10 is repeatedly charged and discharged.

[0039] [Table 1]

[0040] The secondary batteries 1 of Examples 1, 2, and 3 are the secondary batteries 1 of the above embodiment, and the silicon ratio of the first negative electrode composite layer 12b is greater than the silicon ratio of the second negative electrode composite layer 12c. Therefore, in the secondary batteries of Examples 1, 2, and 3, the "internal / external silicon abundance ratio" is greater than 1.

[0041] Specifically, in the secondary battery 1 of Example 1, the weight ratio of silicon oxide in the first negative electrode composite layer 12b is 18.0%; the weight ratio of graphite is 82.0%; and the silicon ratio is 18.0%. The weight ratio of silicon oxide in the second negative electrode composite layer 12c is 12.0%; the weight ratio of graphite is 88.0%; and the silicon ratio is 12.0%. The internal / external silicon abundance ratio is 1.50.

[0042] The thickness of first negative electrode composite layer 12b before charging is 33.3 μm, and the expansion coefficient is 41.0%. The thickness of second negative electrode composite layer 12c before charging is 36.3 μm, and the expansion coefficient is 32.1%. The internal / external expansion ratio is 1.28.

[0043] In the secondary battery 1 of Example 2, the weight ratio of silicon oxide in the first negative electrode composite layer 12b is 16.0%; the weight ratio of graphite is 84.0%; and the silicon ratio is 16.0%. In the second negative electrode composite layer 12c, the weight ratio of silicon oxide is 14.0%; the weight ratio of graphite is 86.0%; and the silicon ratio is 14.0%. Furthermore, the internal / external silicon abundance ratio is 1.14.

[0044] The thickness of first negative electrode composite layer 12b before charging is 34.0 μm, and the expansion rate is 36.3%. The thickness of second negative electrode composite layer 12c before charging is 35.7 μm, and the expansion rate is 34.6%. The internal / external expansion ratio is 1.05.

[0045] In the secondary battery 1 of Example 3, the weight ratio of silicon oxide in the first negative electrode composite layer 12b is 15.5%; the weight ratio of graphite is 84.5%; and the silicon ratio is 15.5%. In the second negative electrode composite layer 12c, the weight ratio of silicon oxide is 14.5%; the weight ratio of graphite is 85.5%; and the silicon ratio is 14.5%. Furthermore, the internal / external silicon abundance ratio is 1.07.

[0046] The thickness of first negative electrode composite layer 12b before charging is 34.7 μm, and the expansion rate is 35.6%. The thickness of second negative electrode composite layer 12c before charging is 35.3 μm, and the expansion rate is 34.9%. The internal / external expansion ratio is 1.02.

[0047] In the secondary batteries 1 of Examples 1, 2, and 3, no short circuit occurred between the positive electrode 11 and the negative electrode 12 until the number of repeated charge and discharge cycles reached a predetermined number (for example, 2000 cycles).

[0048] The secondary battery of Comparative Example 1 differs from secondary battery 1 of the above embodiment only in that the silicon ratio of first negative electrode composite material layer 12b is equal to the silicon ratio of second negative electrode composite material layer 12c.

[0049] Specifically, in the secondary battery of Comparative Example 1, the weight ratio of silicon oxide in first negative electrode composite layer 12b is 15.0%; the weight ratio of graphite is 85.0%; and the silicon ratio is 15.0%. In addition, the weight ratio of silicon oxide in second negative electrode composite layer 12c is 15.0%; the weight ratio of graphite is 85.0%; and the silicon ratio is 15.0%. Furthermore, the internal / external silicon abundance ratio is 1.00.

[0050] The thickness of first negative electrode composite layer 12b before charging is 35.0 μm, and the expansion coefficient is 35.2%. The thickness of second negative electrode composite layer 12c before charging is 34.7 μm, and the expansion coefficient is 35.6%. The internal / external expansion ratio is 0.99.

[0051] In the secondary battery of Comparative Example 1, a short circuit occurred between the positive electrode 11 and the negative electrode 12 when the number of charge / discharge cycles was 310, which was less than the predetermined number of cycles.

[0052] The secondary battery of Comparative Example 2 differs from secondary battery 1 of the above embodiment in that the silicon ratio of second negative electrode composite material layer 12c is greater than the silicon ratio of first negative electrode composite material layer 12b.

[0053] Specifically, in the secondary battery of Comparative Example 2, the weight ratio of silicon oxide in first negative electrode composite layer 12b is 12.0%; the weight ratio of graphite is 88.0%; and the silicon ratio is 12.0%. Furthermore, the weight ratio of silicon oxide in second negative electrode composite layer 12c is 18.0%; the weight ratio of graphite is 82.0%; and the silicon ratio is 18.0%. Furthermore, the internal / external silicon abundance ratio is 0.67.

[0054] The thickness of first negative electrode composite layer 12b before charging is 36.3 μm, and the expansion rate is 32.1%. The thickness of second negative electrode composite layer 12c before charging is 33.3 μm, and the expansion rate is 39.0%. The internal / external expansion ratio is 0.82.

[0055] In the secondary battery of Comparative Example 2, a short circuit occurred between the positive electrode 11 and the negative electrode 12 when the number of charge / discharge cycles was 290, which was less than the predetermined number of cycles.

[0056] Fig. 3 is a partially enlarged cross-sectional view showing the degree of bending when the electrode body 10 of the secondary battery 1 of Example 1 is charged. Fig. 4 is a partially enlarged cross-sectional view showing the degree of bending when the electrode body 10 of the secondary battery 1a of Comparative Example 1 is charged. Fig. 5 is a partially enlarged cross-sectional view showing the degree of bending when the electrode body 10 of the secondary battery 1b of Comparative Example 2 is charged.

[0057] 3, 4, and 5 show the cross-sectional shape of the electrode assembly 10 at the start of winding, similar to FIG. 2. In the electrode assembly 10 shown in FIGS. 3, 4, and 5, a negative electrode 112 is further stacked on the inside of the positive electrode 11 in the stacking direction L. The information shown in Table 1 relates to the negative electrode 12 stacked on the outside of the positive electrode 11 in the stacking direction L. The negative electrode 112 has the same configuration as the negative electrode 12. In addition, in FIGS. 3, 4, and 5, the shape of the electrode assembly 10 before charging is shown by a dashed line, and the shape of the electrode assembly 10 after charging is shown by a solid line.

[0058] 3, 4, and 5, the degree of bending of the negative electrode 12 after charging increases in the order of Example 1, Comparative Example 1, and Comparative Example 2. That is, the force with which the inner end E1 of the positive electrode 11 presses the separator 13 increases in the order of Example 1, Comparative Example 1, and Comparative Example 2, making it more likely that a short circuit will occur between the positive electrode 11 and the negative electrode 12. Thus, in Table 1, there is no short circuit between the positive electrode 11 and the negative electrode 12 in Example 1, while a short circuit occurs between the positive electrode 11 and the negative electrode 12 in Comparative Examples 1 and 2, and the short circuit occurs earlier in Comparative Example 2 than in Comparative Example 1.

[0059] Furthermore, the degree of bending of negative electrode 12 after charging increases in the order of Example 1, Comparative Example 1, and Comparative Example 2, which corresponds to the fact that the internal / external silicon abundance ratios shown in Table 1 decrease in the order of Example 1, Comparative Example 1, and Comparative Example 2. That is, in Example 1, the silicon ratio in first negative electrode composite layer 12b is greater than the silicon ratio in second negative electrode composite layer 12c, which prevents bending of negative electrode 12 after charging and prevents short-circuiting between positive electrode 11 and negative electrode 12. Therefore, secondary battery 1 of the above embodiment can prevent deterioration in cycle characteristics.

[0060] The above-described embodiments are intended to facilitate understanding of the present disclosure and are not intended to limit the present disclosure. The present disclosure may be modified or improved without departing from the spirit thereof, and equivalents thereof are also included in the present disclosure.

[0061] For example, the material of the first negative electrode composite layer 12b may be graphite, and the material of the second negative electrode composite layer 12c may be lithium titanate. Even in this case, the expansion coefficient of the first negative electrode composite layer 12b is greater than that of the second negative electrode composite layer 12c. Alternatively, the material of the first negative electrode composite layer 12b may contain graphite and a silicon-containing material, and the material of the second negative electrode composite layer 12c may be graphite. Even in this case, the expansion coefficient of the first negative electrode composite layer 12b is greater than that of the second negative electrode composite layer 12c, and the silicon ratio of the first negative electrode composite layer 12b is greater than that of the second negative electrode composite layer 12c.

[0062] Furthermore, at least at inner end E2 of negative electrode 12, first negative electrode composite material layer 12b may have a higher expansion coefficient than second negative electrode composite material layer 12c.

[0063] The present disclosure may also be implemented as a combination of the following configurations.

[0064] (1) a sheet-like positive electrode; a sheet-like negative electrode laminated on the positive electrode via a separator, the positive electrode and the negative electrode are wound, The negative electrode is a sheet-like negative electrode current collector; a first negative electrode mixture layer disposed on an inner surface of the negative electrode current collector that is on the inner side in the stacking direction; a second negative electrode mixture layer disposed on an outer surface of the negative electrode current collector that is on the outer side in the stacking direction, The expansion rate of the first negative electrode composite layer in the thickness direction of the negative electrode in a charged state relative to a discharged state is greater than that of the second negative electrode composite layer. Secondary battery.

[0065] (2) the first negative electrode mixture layer and the second negative electrode mixture layer each contain graphite and a silicon-containing material, a ratio of the first weight of the silicon-containing material and the second weight of the graphite to a total weight of the first weight of the silicon-containing material and the second weight of the graphite is greater in the first negative electrode mixture layer than in the second negative electrode mixture layer; The secondary battery according to (1).

[0066] (3) The silicon-containing material is a silicon oxide represented by the general formula: SiOx (wherein x is a real number satisfying 0≦x≦2). (2) The secondary battery according to (2).

[0067] (4) the first negative electrode composite layer is laminated on the outer side of the inner end of the positive electrode in the lamination direction, with the separator interposed therebetween; A secondary battery according to any one of (1) to (3).

[0068] (5) an inner end portion of the negative electrode overlaps with an outer side of the inner end portion of the positive electrode in the stacking direction via the separator; the expansion coefficient of the first negative electrode composite layer is greater than the expansion coefficient of the second negative electrode composite layer at least at an inner end portion of the negative electrode; (4) The secondary battery according to (4). [Explanation of symbols]

[0069] 1 Secondary battery 10 Electrode body 11 Positive electrode 11a Positive electrode current collector 11b First positive electrode composite layer 11c Second positive electrode composite layer 12 Negative electrode 12a Negative electrode current collector 12a1 Inside surface 12a2 outer surface 12b First negative electrode composite layer 12c 2nd negative electrode composite layer 13 Separator E1 Inner end of positive electrode E2 Inner end of negative electrode L Lamination direction R Winding direction

Claims

1. a sheet-like positive electrode; a sheet-like negative electrode laminated on the positive electrode via a separator, the positive electrode and the negative electrode are wound, The negative electrode is a sheet-like negative electrode current collector; a first negative electrode mixture layer disposed on an inner surface of the negative electrode current collector that is located on the inner side in the stacking direction; a second negative electrode mixture layer disposed on an outer surface of the negative electrode current collector that is on the outer side in the stacking direction, The expansion rate of the first negative electrode composite layer in the thickness direction of the negative electrode in a charged state relative to a discharged state is greater than that of the second negative electrode composite layer. Secondary battery.

2. the first negative electrode mixture layer and the second negative electrode mixture layer each contain graphite and a silicon-containing material, a ratio of the first weight of the silicon-containing material to a total weight of the second weight of the graphite is greater in the first negative electrode mixture layer than in the second negative electrode mixture layer; The secondary battery according to claim 1 .

3. The silicon-containing material is a silicon oxide represented by the general formula: SiOx (wherein x is a real number satisfying 0≦x≦2). The secondary battery according to claim 2 .

4. the first negative electrode composite layer is laminated on the outer side of the inner end of the positive electrode in the lamination direction, with the separator interposed therebetween; The secondary battery according to claim 1 .

5. an inner end portion of the negative electrode overlaps with an outer side of the inner end portion of the positive electrode in the stacking direction via the separator; the expansion coefficient of the first negative electrode composite layer is greater than the expansion coefficient of the second negative electrode composite layer at least at an inner end portion of the negative electrode; The secondary battery according to claim 4 .

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