Secondary battery

Abstract

The invention relates to a secondary battery. The purpose of the present invention is to suppress deterioration in cycle characteristics in a secondary battery. A secondary battery (1) is provided with a sheet-like positive electrode (11) and a sheet-like negative electrode (12) laminated on the positive electrode (11) with a separator (13) interposed therebetween. The positive electrode (11) and the negative electrode (12) are wound. The negative electrode (12) is provided with: a sheet-shaped negative electrode current collector (12a); a first negative electrode mixture layer (12b) disposed on an inner surface (12a1) of the negative electrode current collector (12a), the inner surface (12a1) being located on the inner side in the lamination direction (L); and a second negative electrode mixture layer (12c) disposed on an outer surface (12a2) of the negative electrode current collector (12a), the outer surface (12a2) being located on the outside in the lamination direction (L). The first negative electrode composite material layer (12b) is larger than the second negative electrode composite material layer (12c) with respect to the expansion rate in the thickness direction of the negative electrode (12) in the charged state than in the discharged state.

Description

Technical Field

[0001] This disclosure relates to a secondary battery. Background Technology

[0002] Patent Document 1 discloses a non-aqueous electrolyte secondary battery as an example of a secondary battery, comprising a wound electrode body consisting of a sheet-like positive electrode and a negative electrode wound together with a separator. In the secondary battery of Patent Document 1, the negative electrode active material uses graphite and a Si-containing material.

[0003] Regarding lithium storage per unit area, silicon-containing materials provide more lithium than carbon-based materials such as graphite. Therefore, by using silicon-containing materials as negative electrode active materials, it is possible to achieve higher battery capacity.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2013-178913 Summary of the Invention

[0007] The technical problem that the invention aims to solve

[0008] However, Si-containing materials undergo significant volume changes due to the charging and discharging of secondary batteries. Therefore, especially at the beginning of the winding process of wound electrodes, the electrode may deform towards its center, resulting in buckling. When buckling occurs, characteristics related to the degradation of the secondary battery due to repeated charging and discharging (so-called cycle characteristics) may decrease.

[0009] This disclosure is made in view of the above circumstances, and its purpose is to suppress the degradation of cycle characteristics in secondary batteries.

[0010] Technical solutions for solving technical problems

[0011] The disclosed secondary battery comprises: a sheet-shaped positive electrode; and a sheet-shaped negative electrode, which are stacked on the positive electrode with a separator in between. The positive electrode and the negative electrode are wound together. The negative electrode comprises: a sheet-shaped negative electrode current collector; a first negative electrode composite material layer disposed on the inner side of the negative electrode current collector in the stacking direction; and a second negative electrode composite material layer disposed on the outer side of the negative electrode current collector in the stacking direction. The first negative electrode composite material layer is larger than the second negative electrode composite material layer in the thickness direction with respect to the expansion rate of the negative electrode in the charging state relative to the discharging state.

[0012] The effects of the invention

[0013] The secondary battery according to this disclosure can suppress the deterioration of cycle characteristics. Attached Figure Description

[0014] Figure 1 This is a cross-sectional view of a secondary battery according to an embodiment of this disclosure.

[0015] Figure 2 It is along Figure 1 An enlarged partial sectional view of the electrode body of line II-II shown.

[0016] Figure 3 This is a partially enlarged cross-sectional view showing the degree of bending of the electrode body of the secondary battery in Example 1 when it is being charged.

[0017] Figure 4 This is a partially enlarged cross-sectional view showing the degree of bending of the electrode body of the secondary battery in Comparative Example 1 when it is being charged.

[0018] Figure 5 This is a partially enlarged cross-sectional view showing the degree of bending of the electrode body of the secondary battery in Comparative Example 2 when it is being charged. Detailed Implementation

[0019] The embodiments will now be described in detail with reference to the accompanying drawings. It should be noted that this disclosure is not limited to these embodiments. The embodiments are illustrative, and of course, partial substitutions or combinations can be made to the structures shown in different embodiments.

[0020] Figure 1 This is a cross-sectional view of the secondary battery 1 according to an embodiment of this disclosure.

[0021] The secondary battery 1 is, for example, a lithium battery. The secondary battery 1 includes an electrode body 10 and a casing 20.

[0022] Electrode body 10 is a wound type electrode body. Electrode body 10 is formed by stacking and winding sheet-shaped positive electrode 11 and negative electrode 12 with a separator 13 in between. Electrode body 10 is cylindrical. It should be noted that electrode body 10 can also be flat. Details of positive electrode 11 and negative electrode 12 will be described below.

[0023] The electrode body 10 has a strip-shaped positive terminal 14 electrically connected to the positive electrode 11 and a strip-shaped negative terminal 15 electrically connected to the negative electrode 12.

[0024] The housing 20 includes a main body 21 and a cover 22. The main body 21 and the cover 22 are made of conductive materials. The materials of the main body 21 and the cover 22 are, for example, iron, stainless steel, and aluminum.

[0025] The main body 21 is a cylindrical shape with an opening 21a at one end. A negative terminal 15 is electrically connected to the inner surface of the main body 21.

[0026] The cover 22 covers the opening 21a of the main body 21. The cover 22 is disposed on the main body 21 in a state of electrical insulation from the main body 21. A positive terminal 14 is electrically connected to the cover 22.

[0027] Figure 2 It is along Figure 1 An enlarged partial cross-sectional view of the electrode body 10 of line II-II shown. Figure 2 The electrode 10 shown represents the state of being discharged. Figure 2 In the middle, diaphragm 13 is represented by a dashed line. Figure 2 The image shows arrows indicating the winding direction R and the stacking direction L.

[0028] In the winding direction R, the side indicated by the arrow 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 inner side, and the winding end end is located on the outer side.

[0029] Furthermore, the stacking direction L is the direction in which the positive electrode 11, negative electrode 12, and separator 13 are stacked, and it is orthogonal 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.

[0030] In addition, Figure 2 The figure shows a portion of the end of the electrode body 10 that includes the winding start side.

[0031] The positive electrode 11 is sheet-shaped. The positive electrode 11 includes a positive current collector 11a, a first positive composite material layer 11b, and a second positive composite material layer 11c. The positive current collector 11a, the first positive composite material layer 11b, and the second positive composite material layer 11c are all sheet-shaped.

[0032] The positive current collector 11a is a conductive layer, such as a metal. Specifically, the material of the positive current collector 11a is aluminum or the like.

[0033] The first positive electrode composite material layer 11b and the second positive electrode composite material layer 11c are disposed on opposite sides of each other, separated by the positive electrode current collector 11a. The first positive electrode composite material layer 11b is located inside the second positive electrode composite material layer 11c in the lamination direction L. Both the first positive electrode composite material layer 11b and the second positive electrode composite material layer 11c contain a positive electrode active material. The positive electrode active material is, for example, a lithium-ion-containing metal oxide, specifically lithium cobalt oxide and lithium nickel oxide.

[0034] The negative electrode 12 is sheet-shaped. The 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. The negative electrode current collector 12a, the first negative electrode composite material layer 12b, and the second negative electrode composite material layer 12c are all sheet-shaped.

[0035] The negative current collector 12a is a conductive layer, such as a metal. Specifically, the material of the negative current collector 12a is copper.

[0036] The first negative electrode composite material layer 12b and the second negative electrode composite material layer 12c are disposed on opposite sides of each other, separated by the negative electrode current collector 12a. The first negative electrode composite material layer 12b is located inside the second negative electrode composite material layer 12c in the lamination direction L. Specifically, the first negative electrode composite material layer 12b is disposed on the inner surface 12a1 of the negative electrode current collector 12a, located inside the lamination direction L. The second negative electrode composite material layer 12c is disposed on the outer surface 12a2 of the negative electrode current collector 12a, located outside the lamination direction L.

[0037] The first negative electrode composite material layer 12b and the second negative electrode composite material layer 12c contain negative electrode active materials. The negative electrode active materials contain both graphite and silicon-containing materials. 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).

[0038] It should be noted that silicon-containing materials can be materials containing at least one of the above-mentioned silicon oxides, mixtures of silicon and carbon materials, mixtures of silicon compounds and carbon materials, and silicon.

[0039] Furthermore, the ratio of the first weight of the silicon-containing material to the total weight of the first weight and the second weight of the graphite (= first weight × 100 / (first weight + second weight): hereinafter referred to as the "silicon ratio") is greater in the first negative electrode composite material layer 12b than in the second negative electrode composite material layer 12c. Therefore, regarding the expansion rate of the negative electrode 12 in the thickness direction relative to the state in which the electrode body 10 is charged compared to the state in which the electrode body 10 is discharged, the first negative electrode composite material layer 12b is greater than the second negative electrode composite material layer 12c.

[0040] That is, when the electrode body 10 changes from a discharged state to a charged state, the expansion rate of the first negative electrode composite material layer 12b in the thickness direction is greater than that of the second negative electrode composite material layer 12c in the thickness direction. Therefore, when the electrode body 10 changes from a discharged state to a charged state, the bending of the negative electrode 12 in the lamination direction L can be suppressed, and the buckling of the negative electrode 12 can be suppressed.

[0041] In addition, Figure 2In this configuration, the starting end of the winding of the positive electrode 11 is located on the ending side of the winding of the negative electrode 12 in the winding direction R, compared to the starting end of the winding of the negative electrode 12. In other words, in the positive electrode 11, a first negative electrode composite material layer 12b is stacked on the outer side of the lamination direction L at one end on the winding start side (hereinafter referred to as "the inner end E1 of the positive electrode 11"), separated by a separator 13. Furthermore, as described above, bending of the negative electrode 12 inward in the lamination direction L can be suppressed. Therefore, bending of the negative electrode 12 towards the inner end E1 of the positive electrode 11 can be suppressed, preventing the separator 13 from being punctured by the inner end E1 of the positive electrode 11 due to pressure from the inner end E1 of the positive electrode 11 and the negative electrode 12. Therefore, short circuit between the positive electrode 11 and the negative electrode 12 can be suppressed.

[0042] Furthermore, on the outer side of the inner end E1 of the positive electrode 11 in the stacking direction L, the end of the negative electrode 12 that overlaps and is wound around the starting side with the diaphragm 13 (hereinafter referred to as "inner end E2 of the negative electrode 12"). Since the inner end E2 of the negative electrode 12 has a space in the center of the electrode body 10, it is easier to bend inward in the stacking direction L compared to other parts of the negative electrode 12. On the other hand, as described above, bending of the negative electrode 12 inward in the stacking direction L can be suppressed. Therefore, even when the inner end E2 of the negative electrode 12 overlaps with the inner end E1 of the positive electrode 11 in the stacking direction L, the situation where the diaphragm 13 is punctured by the inner end E1 of the positive electrode 11, resulting in a short circuit between the positive electrode 11 and the negative electrode 12, can be suppressed.

[0043] It should be noted that the negative electrode 12 can also be stacked inside the stacking direction L of the positive electrode 11. Alternatively, the negative electrode 12 can be stacked on both sides of the stacking direction L of the positive electrode 11.

[0044] Next, the results of repeated charge-discharge cycles in the secondary battery 1 of the above-described embodiment and the secondary battery of the comparative example were compared. It should be noted that in this comparison, the silicon-containing material was silicon oxide.

[0045] In Table 1 below, under the "Active Material Composition Ratio" column, the weight ratios of silicon oxide and graphite are indicated in the "Weight Ratio of Silicon Oxide" and "Weight Ratio of Graphite" columns, respectively, for the first negative electrode composite material layer 12b and the second negative electrode composite material layer 12c. Additionally, the "Silicon Ratio" column indicates the aforementioned silicon ratio (= first weight × 100 / (first weight + second weight)) for each of the first negative electrode composite material layer 12b and the second negative electrode composite material layer 12c.

[0046] In addition, the "Inner and outer silicon ratio" column indicates the ratio of the silicon ratio of the first negative electrode composite material layer 12b to the silicon ratio of the second negative electrode composite material layer 12c (= silicon ratio of the first negative electrode composite material layer 12b / silicon ratio of the second negative electrode composite material layer 12c).

[0047] Furthermore, the "Thickness before charging" column indicates the thickness of the first negative electrode composite material layer 12b and the second negative electrode composite material layer 12c before the electrode body 10 is charged (in the discharged state). The "Expansion Rate" column indicates the ratio of the increase in thickness after charging to the thickness before charging for both the first negative electrode composite material layer 12b and the second negative electrode composite material layer 12c (=Increase in thickness after charging × 100 / Thickness before charging). The "Internal and External Expansion Ratio" column indicates the ratio of the expansion rate of the first negative electrode composite material layer 12b to the expansion rate of the second negative electrode composite material layer 12c (=Expansion rate of the first negative electrode composite material layer 12b / Expansion rate of the second negative electrode composite material layer 12c).

[0048] In addition, the column "Presence or absence of short circuit in cyclic test" indicates whether there is a short circuit between the positive electrode 11 and the negative electrode 12 in the cyclic test, which is a test of repeatedly charging and discharging the electrode body 10.

[0049] [Table 1]

[0050]

[0051] The secondary battery 1 in Examples 1, 2, and 3 is the secondary battery 1 of the above-described embodiments, wherein the silicon ratio of the first negative electrode composite material layer 12b is greater than the silicon ratio of the second negative electrode composite material layer 12c. Therefore, in the secondary batteries of Examples 1, 2, and 3, the "internal and external silicon presence ratio" is greater than 1.

[0052] Specifically, in the secondary battery 1 of Example 1, the weight percentage of silicon oxide in the first negative electrode composite material layer 12b is 18.0%, the weight percentage of graphite is 82.0%, and the silicon percentage is 18.0%. Furthermore, the weight percentage of silicon oxide in the second negative electrode composite material layer 12c is 12.0%, the weight percentage of graphite is 88.0%, and the silicon percentage is 12.0%. Additionally, the internal and external silicon content ratio is 1.50.

[0053] Furthermore, the thickness of the first negative electrode composite material layer 12b before charging is 33.3 μm, and the expansion rate is 41.0%. The thickness of the second negative electrode composite material layer 12c before charging is 36.3 μm, and the expansion rate is 32.1%. In addition, the internal-to-external expansion ratio is 1.28.

[0054] Furthermore, in the secondary battery 1 of Example 2, the weight percentage of silicon oxide in the first negative electrode composite material layer 12b is 16.0%, the weight percentage of graphite is 84.0%, and the silicon percentage is 16.0%. Similarly, the weight percentage of silicon oxide in the second negative electrode composite material layer 12c is 14.0%, the weight percentage of graphite is 86.0%, and the silicon percentage is 14.0%. Additionally, the internal and external silicon content ratio is 1.14.

[0055] Furthermore, the thickness of the first negative electrode composite material layer 12b before charging is 34.0 μm, and its expansion rate is 36.3%. The thickness of the second negative electrode composite material layer 12c before charging is 35.7 μm, and its expansion rate is 34.6%. In addition, the internal-to-external expansion ratio is 1.05.

[0056] Furthermore, in the secondary battery 1 of Example 3, the weight percentage of silicon oxide in the first negative electrode composite material layer 12b is 15.5%, the weight percentage of graphite is 84.5%, and the silicon percentage is 15.5%. Similarly, the weight percentage of silicon oxide in the second negative electrode composite material layer 12c is 14.5%, the weight percentage of graphite is 85.5%, and the silicon percentage is 14.5%. Additionally, the internal and external silicon content ratio is 1.07.

[0057] Furthermore, the thickness of the first negative electrode composite material layer 12b before charging is 34.7 μm, and the expansion rate is 35.6%. The thickness of the second negative electrode composite material layer 12c before charging is 35.3 μm, and the expansion rate is 34.9%. In addition, the internal-to-external expansion ratio is 1.02.

[0058] In the secondary battery 1 of Examples 1, 2, and 3, there is no short circuit between the positive electrode 11 and the negative electrode 12 before the number of charge-discharge cycles reaches a predetermined number (e.g., 2000 times).

[0059] The secondary battery of Comparative Example 1 differs from the secondary battery 1 of the above-described embodiment only in that the silicon ratio of the first negative electrode composite material layer 12b is equal to the silicon ratio of the second negative electrode composite material layer 12c.

[0060] Specifically, in the secondary battery of Comparative Example 1, the weight percentage of silicon oxide in the first negative electrode composite material layer 12b is 15.0%, the weight percentage of graphite is 85.0%, and the silicon percentage is 15.0%. Similarly, the weight percentage of silicon oxide in the second negative electrode composite material layer 12c is 15.0%, the weight percentage of graphite is 85.0%, and the silicon percentage is 15.0%. Furthermore, the internal and external silicon content ratio is 1.00.

[0061] Furthermore, the thickness of the first negative electrode composite material layer 12b before charging is 35.0 μm, and the expansion rate is 35.2%. The thickness of the second negative electrode composite material layer 12c before charging is 34.7 μm, and the expansion rate is 35.6%. In addition, the internal-to-external expansion ratio is 0.99.

[0062] 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 less than 310 times, which was less than the specified number.

[0063] The difference between the secondary battery of Comparative Example 2 and the secondary battery 1 of the above embodiment is that the silicon ratio of the second negative electrode composite material layer 12c is greater than the silicon ratio of the first negative electrode composite material layer 12b.

[0064] Specifically, in the secondary battery of Comparative Example 2, the weight percentage of silicon oxide in the first negative electrode composite material layer 12b is 12.0%, the weight percentage of graphite is 88.0%, and the silicon percentage is 12.0%. Furthermore, the weight percentage of silicon oxide in the second negative electrode composite material layer 12c is 18.0%, the weight percentage of graphite is 82.0%, and the silicon percentage is 18.0%. In addition, the internal and external silicon content ratio is 0.67.

[0065] Furthermore, the thickness of the first negative electrode composite material layer 12b before charging is 36.3 μm, and the expansion rate is 32.1%. The thickness of the second negative electrode composite material layer 12c before charging is 33.3 μm, and the expansion rate is 39.0%. In addition, the internal-to-external expansion ratio is 0.82.

[0066] 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 less than 290 times, which was the specified number.

[0067] Figure 3 This is a partially enlarged cross-sectional view showing the degree of bending of the electrode body 10 of the secondary battery 1 in Embodiment 1 when it is being charged. Figure 4This is a partially enlarged cross-sectional view showing the degree of bending of the electrode body 10 of the secondary battery 1a of Comparative Example 1 when it is being charged. Figure 5 This is a partially enlarged cross-sectional view showing the degree of bending of the electrode body 10 of the secondary battery 1b of Comparative Example 2 when it is being charged.

[0068] Figure 3 , 4 5 and Figure 2 Similarly, the cross-sectional shape of the portion on the winding start side of the electrode body 10 is also shown. Figure 3 , 4 In the electrode body 10 shown in Figures 5 and 6, a negative electrode 112 is further stacked on the inner side of the stacking direction L than the positive electrode 11. Table 1 shows the contents regarding the negative electrode 12 stacked on the positive electrode 11 on the outer side of the stacking direction L. The negative electrode 112 is constructed in the same manner as the negative electrode 12. Furthermore, in... Figure 3 , 4 In Figure 5, the shape of the electrode body 10 before charging is represented by a dashed line, and the shape of the electrode body 10 after charging is represented by a solid line.

[0069] like Figure 3 , 4 As shown in Figure 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, in the order of Example 1, Comparative Example 1, and Comparative Example 2, the force of the inner end E1 of the positive electrode 11 pressing against the diaphragm 13 increases, making it easier for a short circuit to occur between the positive electrode 11 and the negative electrode 12. Therefore, in Table 1, there is no short circuit between the positive electrode 11 and the negative electrode 12 in Example 1, while short circuits occur 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 compared to Comparative Example 1.

[0070] Furthermore, 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, which corresponds to the decrease in the ratio of internal and external silicon content shown in Table 1 in the order of Example 1, Comparative Example 1, and Comparative Example 2. That is, in Example 1, by making the silicon ratio of the first negative electrode composite material layer 12b greater than the silicon ratio of the second negative electrode composite material layer 12c, the bending of the negative electrode 12 after charging can be suppressed, thereby suppressing the short circuit between the positive electrode 11 and the negative electrode 12. Therefore, the secondary battery 1 of the above-described embodiment can suppress the reduction of cycle characteristics.

[0071] It should be noted that the above embodiments are for the purpose of facilitating understanding of this disclosure, and are not intended to limit the interpretation of this disclosure. This disclosure can be modified / improved without departing from its spirit, and its equivalents are also included in this disclosure.

[0072] For example, the first negative electrode composite layer 12b may be made of graphite, and the second negative electrode composite layer 12c may be made of lithium titanate. In this case, the expansion rate of the first negative electrode composite layer 12b is also greater than that of the second negative electrode composite layer 12c. Alternatively, the first negative electrode composite layer 12b may contain both graphite and silicon-containing materials, and the second negative electrode composite layer 12c may be made of graphite. In this case, the expansion rate of the first negative electrode composite layer 12b is also greater than that of the second negative electrode composite layer 12c, and the silicon ratio of the first negative electrode composite layer 12b is also greater than that of the second negative electrode composite layer 12c.

[0073] In addition, at least at the inner end E2 of the negative electrode 12, the expansion rate of the first negative electrode composite material layer 12b can also be greater than the expansion rate of the second negative electrode composite material layer 12c.

[0074] It should be noted that this disclosure may be a combination of the following components.

[0075] (1) A secondary battery, comprising:

[0076] A sheet-like positive electrode; and

[0077] A sheet-like negative electrode is stacked on top of the positive electrode, separated by a separator.

[0078] The positive and negative electrodes are wound together.

[0079] The negative electrode has the following characteristics:

[0080] A sheet-like negative electrode current collector;

[0081] The first negative electrode composite material layer is disposed on the inner side of the negative electrode current collector located on the inner side in the stacking direction; and

[0082] The second negative electrode composite material layer is disposed on the outer surface of the negative electrode current collector located on the outer side in the stacking direction.

[0083] Regarding the expansion rate of the negative electrode in the thickness direction in the charging state relative to the discharging state, the first negative electrode composite material layer is greater than the second negative electrode composite material layer.

[0084] (2) According to the secondary battery described in (1),

[0085] The first negative electrode composite material layer and the second negative electrode composite material layer respectively contain graphite and silicon-containing materials.

[0086] Regarding the ratio of the first weight of the silicon-containing material to the total weight of the first weight and the second weight of the graphite, the first negative electrode composite material layer is greater than the second negative electrode composite material layer.

[0087] (3) According to the secondary battery described in (2),

[0088] The silicon-containing material is a silicon oxide represented by the general formula: SiOx (where x is a real number satisfying 0≤x≤2).

[0089] (4) The secondary battery according to any one of (1) to (3) has the first negative electrode composite material layer stacked on the outer side of the stacking direction of the inner end of the positive electrode, separated by the separator.

[0090] (5) The secondary battery according to (4),

[0091] On the outer side of the stacking direction of the inner end of the positive electrode, the inner end of the negative electrode overlaps with the diaphragm in between.

[0092] At least at the inner end of the negative electrode, the expansion rate of the first negative electrode composite material layer is greater than the expansion rate of the second negative electrode composite material layer.

[0093] Explanation of reference numerals in the attached figures

[0094] 1: Secondary battery; 10: Electrode body; 11: Positive electrode; 11a: Positive electrode current collector; 11b: First positive electrode composite material layer; 11c: Second positive electrode composite material layer; 12: Negative electrode; 12a: Negative electrode current collector; 12a1: Inner surface; 12a2: Outer surface; 12b: First negative electrode composite material layer; 12c: Second negative electrode composite material layer; 13: Separator; E1: Inner end of positive electrode; E2: Inner end of negative electrode; L: Lamination direction; R: Winding direction.

Claims

1. A secondary battery, comprising: A sheet-like positive electrode; and A sheet-like negative electrode is stacked on top of the positive electrode, separated by a separator. The positive and negative electrodes are wound together. The negative electrode has the following characteristics: A sheet-like negative electrode current collector; The first negative electrode composite material layer is disposed on the inner side of the negative electrode current collector located on the inner side in the stacking direction; and The second negative electrode composite material layer is disposed on the outer surface of the negative electrode current collector located on the outer side in the stacking direction. Regarding the expansion rate of the negative electrode in the thickness direction in the charging state relative to the discharging state, the first negative electrode composite material layer is greater than the second negative electrode composite material layer.

2. The secondary battery according to claim 1, wherein, The first negative electrode composite material layer and the second negative electrode composite material layer respectively contain graphite and silicon-containing materials. Regarding the ratio of the first weight of the silicon-containing material to the total weight of the first weight and the second weight of the graphite, the first negative electrode composite material layer is greater than the second negative electrode composite material layer.

3. The secondary battery according to claim 2, wherein, The silicon-containing material is a silicon oxide represented by the general formula: SiOx. In the formula, x is a real number that satisfies 0 ≤ x ≤ 2.

4. The secondary battery according to claim 1, wherein, On the outer side of the inner end of the positive electrode in the stacking direction, the first negative electrode composite material layer is stacked with the diaphragm in between.

5. The secondary battery according to claim 4, wherein, On the outer side of the stacking direction of the inner end of the positive electrode, the inner end of the negative electrode overlaps with the membrane in between. At least at the inner end of the negative electrode, the expansion rate of the first negative electrode composite material layer is greater than the expansion rate of the second negative electrode composite material layer.

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