Negative pole piece and secondary battery

Abstract

The invention relates to the field of batteries, and discloses a negative pole piece and a secondary battery, the negative pole piece comprises: a negative current collector; the first negative electrode active material layer is located on at least one side of the negative electrode current collector, the first negative electrode active material layer comprises a first negative electrode active material, the first negative electrode active material comprises graphite, and the particle size of the first negative electrode active material meets the condition that the difference value between Dv90 and Dv50 is 0.1-1 mu m; and the difference value between the Dv50 and the Dv10 is 0.1 [mu] m-2 [mu] m. The first negative electrode active material of the negative electrode plate provided by the invention has more uniform surface energy distribution, the diffusion of lithium ions is smoother, the precipitation of copper ions is slowed down, and the short circuit of the battery caused by piercing the diaphragm by generated copper dendrites is favorably reduced.

Description

Technical Field

[0001] The present application relates to the field of batteries, and in particular, to negative electrode plates and secondary batteries. Background Art

[0002] The market for portable consumer electronics has experienced rapid expansion, and with it comes increasing requirements for batteries, a key power component. Lithium-ion batteries are widely used and have become the mainstream electrochemical energy storage method. Graphite, as a common negative electrode active material, has the advantages of low cost and high theoretical capacity. However, batteries using graphite as the negative electrode active material still have the problem of poor cycle performance, and there are still many problems to be solved in the actual production and application process. Summary of the invention

[0003] In the first aspect of the present application, the present application proposes a negative electrode plate, comprising: a negative electrode current collector; a first negative electrode active material layer, the first negative electrode active material layer is located on at least one side of the negative electrode current collector, the first negative electrode active material layer includes a first negative electrode active material, the first negative electrode active material includes graphite, and the particle size of the first negative electrode active material satisfies: the difference between Dv90 and Dv50 is 0.1μm-1μm; the difference between Dv50 and Dv10 is 0.1μm-2μm.

[0004] Therefore, the first negative electrode active material of the negative electrode plate proposed in the present application has a more uniform surface energy distribution, the diffusion of lithium ions is smoother, and the concentrated precipitation of metal ions, such as copper ions, at the negative electrode can be slowed down, which is beneficial to reduce the battery short circuit caused by the formation of copper dendrites piercing the diaphragm, and improve the safety performance and cycle performance of the battery.

[0005] In some embodiments, the Dv50 of the first negative electrode active material is 0.1 μm-5 μm. Thus, the first negative electrode active material particles can form a first negative electrode active material layer surface that is relatively flat at the microscopic level.

[0006] In some embodiments, the Dv50 of the first negative electrode active material is 1 μm-3 μm. Thus, the gaps between the particles of the first negative electrode active material are small, which can reduce the probability of copper ion aggregation and deposition, thereby reducing the formation of copper dendrites.

[0007] In some embodiments, the thickness of the first negative electrode active material layer is less than or equal to 12 μm. Thus, the lithium ions can maintain a high diffusion speed.

[0008] In some embodiments, the first negative electrode active material layer includes 93wt%-98wt% of graphite, 0.5wt%-1.5wt% of acetylene black, 0.5wt%-2.5wt% of carbon fiber, and 1wt%-3wt% of a binder, thereby providing the first negative electrode active material layer with better conductivity and higher specific capacity.

[0009] In some embodiments, the present invention further comprises: a second negative electrode active material layer, the second negative electrode active material layer is located between the negative electrode current collector and the first negative electrode active material layer, the second negative electrode active material layer comprises a second negative electrode active material, the second negative electrode active material comprises graphite, and the mass fraction of the conductive agent in the second negative electrode active material layer is less than the mass fraction of the conductive agent in the first negative electrode active material layer. Thus, the proportion of active material in the second negative electrode active material layer is relatively high, and the energy density of the negative electrode sheet can be improved.

[0010] In some embodiments, the Dv50 of the second negative electrode active material is 3 μm-12 μm, which helps to increase the compaction density of the second negative electrode active material layer, thereby increasing the energy density of the negative electrode sheet.

[0011] In some embodiments, the ratio of the thickness of the first negative electrode active material layer to the thickness of the second negative electrode active material layer is 1:(9-150). Thus, the negative electrode sheet has both a higher energy density and a better ability to inhibit the growth of copper dendrites.

[0012] In some embodiments, the thickness of the second negative electrode active material layer is 50 μm-150 μm. Thus, the negative electrode sheet has a higher energy density.

[0013] In the second aspect of the present application, the present application proposes a secondary battery, including the negative electrode sheet proposed in the present application. Thus, the secondary battery has all the characteristics and advantages of the aforementioned negative electrode sheet, and can effectively improve the battery self-discharge problem caused by foreign matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0015] Figure 1 This is a schematic diagram of the structure of the negative electrode plate of one embodiment of the present application.

[0016] Description of reference numerals:

[0017] First negative electrode active material layer 1: second negative electrode active material layer 2; current collector 3. DETAILED DESCRIPTION

[0018] The embodiments of the present application are described in detail below. Examples of the embodiments are shown in the drawings, but unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of actually identical structures may be omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0019] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0020] The terms "including" and "having" in the specification and claims of the present application and any modifications thereof are open expressions, that is, including the contents specified in the present application but not excluding other contents.

[0021] In the description of this application, regardless of whether the words "about" or "approximately" are used, all the numbers disclosed herein are approximate values. The value of each number may differ by less than 10% or a reasonable difference considered by those skilled in the art, such as 1%, 2%, 3%, 4% or 5%.

[0022] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0023] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0024] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0025] During the battery manufacturing process, the introduction of impurities in the production environment or the corrosion of battery components may produce metallic copper foreign matter. When these copper foreign matter come into contact with the surface of the negative electrode active material, the copper foreign matter may further form copper dendrites during the battery charging and discharging process. Specifically, during the charging process, the copper metal on the positive electrode side is oxidized into copper ions. Under the action of the electric field, the copper ions migrate to the negative electrode and are reduced to copper elemental substance by electrons at the negative electrode. Copper may first nucleate at a single point or several points on the graphite surface. As copper continues to deposit, these nucleation points gradually grow into a dendrite-like structure. The growth of copper dendrites may lead to local stress concentration, thereby further promoting the growth of dendrites, and ultimately causing copper dendrites to pierce the battery's diaphragm, resulting in direct contact between the positive and negative electrodes, causing the battery to short-circuit, and even causing thermal runaway and safety accidents. In addition, the formation of copper dendrites will also increase the internal resistance of the battery, leading to an increase in the self-discharge phenomenon of the battery and affecting the cycle performance of the battery. The present application proposes that the negative electrode plate can achieve a more uniform electric field distribution and inhibit ion deposition, thereby reducing the formation of copper dendrites, reducing the risk of battery short circuit and improving the safety and cycle stability of the battery.

[0026] In the first aspect of the present application, the present application proposes a negative electrode plate, comprising: a negative electrode current collector; a first negative electrode active material layer, the first negative electrode active material layer is located on at least one side of the negative electrode current collector, the first negative electrode active material layer includes a first negative electrode active material, the first negative electrode active material includes graphite, and the particle size of the first negative electrode active material satisfies: the difference between Dv90 and Dv50 is 0.1μm-1μm; the difference between Dv50 and Dv10 is 0.1μm-2μm.

[0027] As an example, the difference between Dv90 and Dv50 of the particle size of the first negative active material may be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1.0 μm.

[0028] As an example, the difference between Dv50 and Dv10 of the particle size of the first negative active material may be 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, 2.0μm.

[0029] When the particle size of the first negative electrode active material meets the aforementioned requirements, its particle size distribution is relatively concentrated, and the formed first negative electrode active material layer has a more regular microstructure, so that the obtained negative electrode plate has a more uniform surface energy distribution. The electron distribution of the first negative electrode active material layer is also relatively uniform, so that the copper ions can form more uniform deposition nuclei on the surface of the negative electrode active material layer, avoiding the aggregation phenomenon of copper ion nucleation, making the subsequent copper ion deposition morphology thinner and smoother, reducing the formation of sharp dendrites, thereby reducing the puncture of the diaphragm and reducing the occurrence of short circuits. Furthermore, the generation of copper dendrites that cause diaphragm puncture and battery short circuits are also effectively reduced, improving the safety and cycle performance of the battery.

[0030] In some embodiments, the Dv50 of the first negative electrode active material is 0.1 μm-5 μm.

[0031] As an example, the Dv50 of the first negative active material may be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2 .4μm, 2.5μm, 2.6μm, 2.7μm, 2.8μm, 2.9μm, 3.0μm, 3.1μm, 3.2μm, 3.3μm, 3.4μm, 3.5μm, 3.6μm, 3.7 μm, 3.8μm, 3.9μm, 4.0μm, 4.1μm, 4.2μm, 4.3μm, 4.4μm, 4.5μm, 4.6μm, 4.7μm, 4.8μm, 4.9μm, 5.0μm.

[0032] Within the aforementioned particle size range, the first negative electrode active material forms a first negative electrode active material layer.

[0033] In some embodiments, the Dv50 of the first negative electrode active material is 1 μm-3 μm. As a result, the gaps between the first negative electrode active material particles are small, and the probability of large pores and cracks on the surface of the first negative electrode active material layer is small, which reduces the probability of copper ions being deposited in a relatively concentrated area, thereby reducing the copper crystals that can be formed.

[0034] In some embodiments, the thickness of the first negative electrode active material layer is less than or equal to 12 μm. Therefore, the diffusion resistance of lithium ions in the first negative electrode active material layer is small, and the diffusion of lithium ions is relatively smooth during the charge and discharge process, and the lithium ions can be relatively evenly distributed in the negative electrode sheet.

[0035] In some embodiments, the first negative electrode active material layer includes 93wt%-98wt% of graphite, 0.5wt%-1.5wt% of acetylene black, 0.5wt%-2.5wt% of carbon fiber, and 1wt%-3wt% of a binder. Thus, the first negative electrode active material may contain acetylene black and carbon fiber to form a conductive path, so that the first negative electrode active material layer has a higher conductivity. The carbon element in graphite can react with lithium ions to produce an electrode, so that the first negative electrode active material has a higher specific capacity.

[0036] In some embodiments, the negative electrode plate proposed in the present application further includes: the second negative electrode active material layer is located between the negative electrode current collector and the first negative electrode active material layer, the second negative electrode active material layer includes a second negative electrode active material, the second negative electrode active material includes graphite, and the mass fraction of the conductive agent in the second negative electrode active material layer is less than the mass fraction of the conductive agent in the first negative electrode active material layer.

[0037] Therefore, the proportion of active material in the second negative electrode active material layer is relatively high, which can improve the energy density of the negative electrode plate.

[0038] Therefore, the content of the conductive agent in the second negative electrode active material layer is relatively small, and the proportion of the active material capable of storing sodium ions is relatively high, so that the overall energy density of the two negative electrode active material layers is relatively high, thereby improving the energy density of the negative electrode sheet.

[0039] In some embodiments, the Dv50 of the second negative electrode active material is 3 μm-12 μm. Thus, the particle size of the second negative electrode active material is larger than that of the first negative electrode active material, which helps to increase the compaction density of the second negative electrode active material layer, thereby increasing the energy density of the negative electrode sheet.

[0040] In some embodiments, the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:(9-150). Within the aforementioned thickness ratio range, the prepared negative electrode sheet has a more suitable energy density and lithium ion capacity, and can make copper ions deposit more evenly, reduce the concentrated precipitation of metallic copper on the negative electrode, and inhibit the formation and growth of copper dendrites.

[0041] In some embodiments, the thickness of the second negative electrode active material layer is 50 μm-150 μm. When the thickness of the second negative electrode active material layer is within the aforementioned range, the negative electrode sheet has a higher energy density.

[0042] In the second aspect of the present application, the present application proposes a secondary battery, including the negative electrode plate proposed in the present application. Therefore, the secondary battery has all the characteristics and advantages of the aforementioned negative electrode plate, can effectively improve the battery self-discharge problem caused by foreign matter, reduce the dissolution of copper foreign matter in the secondary battery causing positive and negative short circuits and battery failure, and improve the battery life.

[0043] The scheme of the present application is described below by specific examples. It should be noted that the following examples are only used to illustrate the present application and should not be considered as limiting the scope of the present application. If no specific technology or conditions are specified in the examples, the technology or conditions described in the literature in this area or the product instructions are used. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained commercially.

[0044] Example 1

[0045] The first negative electrode active material includes: 97wt% graphite, 1wt% acetylene black, 1wt% vapor-grown carbon fiber, and 1wt% binder (including: 0.5wt% sodium carboxymethyl cellulose and 0.5wt% styrene-butadiene rubber); wherein the difference between Dv90 and Dv50 is 0.5μm, the difference between Dv50 and Dv10 is 0.5μm, and the particle size is 1μm;

[0046] The second negative electrode active material includes: 97wt% graphite, 0.5wt% acetylene black, 0.5wt% vapor-grown carbon fiber, and 2wt% binder (including 1wt% sodium carboxymethyl cellulose and 1wt% styrene-butadiene rubber); wherein the particle size is 9μm;

[0047] A double-layer coating device is used to sequentially coat the second negative electrode active material and the first negative electrode active material on the current collector copper foil to form a second negative electrode active material layer and a first negative electrode active material layer. The thickness of the current collector copper foil is 10 μm, the thickness of the second negative electrode active material layer is 81 μm, the thickness of the first negative electrode active material layer is 9 μm, and the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:9.

[0048] Example 2

[0049] Example 2 is consistent with Example 1, except that, for the first negative electrode active material, the difference between Dv90 and Dv50 is 0.1 μm.

[0050] Example 3

[0051] Example 3 is consistent with Example 1, except that, for the first negative electrode active material, the difference between Dv90 and Dv50 is 1 μm.

[0052] Example 4

[0053] Example 4 is consistent with Example 1, except that, for the first negative electrode active material, the difference between Dv50 and Dv10 is 0.1 μm.

[0054] Example 5

[0055] Example 5 is consistent with Example 1, except that, for the first negative electrode active material, the difference between Dv50 and Dv10 is 1 μm.

[0056] Example 6

[0057] Example 6 is consistent with Example 1, except that, for the first negative electrode active material, the difference between Dv50 and Dv10 is 2 μm.

[0058] Example 7

[0059] Example 7 is consistent with Example 1, except that the Dv50 of the first negative electrode active material is 0.1 μm.

[0060] Example 8

[0061] Example 8 is consistent with Example 1, except that the Dv50 of the first negative electrode active material is 0.5 μm.

[0062] Example 9

[0063] Example 9 is consistent with Example 1, except that the Dv50 of the first negative electrode active material is 2 μm.

[0064] Example 10

[0065] Example 10 is consistent with Example 1, except that the Dv50 of the first negative electrode active material is 3 μm.

[0066] Embodiment 11

[0067] Example 11 is consistent with Example 1, except that the Dv50 of the first negative electrode active material is 5 μm.

[0068] Example 12

[0069] Example 12 is consistent with Example 1, except that the thickness of the first negative electrode active material layer is 12 μm.

[0070] Example 13

[0071] Example 13 is consistent with Example 1, except that the thickness of the first negative electrode active material layer is 6 μm.

[0072] Embodiment 14

[0073] Example 14 is consistent with Example 1, except that the thickness of the first negative electrode active material layer is 3 μm.

[0074] Embodiment 15

[0075] Example 15 is consistent with Example 1, except that the ratio of the thickness of the first negative electrode active material layer to the second negative electrode active material layer is 1:11.

[0076] Example 16

[0077] Example 16 is consistent with Example 1, except that the ratio of the thickness of the first negative electrode active material layer to the second negative electrode active material layer is 1:13.

[0078] Embodiment 17

[0079] Example 17 is consistent with Example 1, except that the ratio of the thickness of the first negative electrode active material layer to the second negative electrode active material layer is 1:15.

[0080] Embodiment 18

[0081] Example 18 is consistent with Example 1, except that the Dv50 of the second negative electrode active material is 3 μm.

[0082] Embodiment 19

[0083] Example 19 is consistent with Example 1, except that the Dv50 of the second negative electrode active material is 6 μm.

[0084] Embodiment 20

[0085] Example 20 is consistent with Example 1, except that the Dv50 of the second negative electrode active material is 12 μm.

[0086] Comparative Example 1

[0087] The second negative electrode active material includes: 97wt% graphite, 0.5wt% acetylene black, 0.5wt% vapor-grown carbon fiber, and 2wt% binder (including 1wt% sodium carboxymethyl cellulose and 1wt% styrene-butadiene rubber); wherein the particle size is 9μm;

[0088] The second negative electrode active material was coated on the current collector copper foil by using a double-layer coating device to form a second negative electrode active material layer. The current collector copper foil had a thickness of 10 μm and the second negative electrode active material layer had a thickness of 100 μm.

[0089] Comparative Example 2

[0090] Comparative Example 2 is consistent with Example 1, except that, for the first negative electrode active material, the difference between Dv90 and Dv50 is 2 μm.

[0091] Comparative Example 3

[0092] Comparative Example 2 is consistent with Example 1, except that, for the first negative electrode active material, the difference between Dv50 and Dv10 is 3 μm.

[0093] Test method:

[0094] Lithium iron phosphate and the negative electrode sheet are selected to assemble into a battery, wherein a 1mm×1mm copper foil is placed on the surface of the positive electrode sheet, and the copper scraps are selected from 1mm 2 Left and right dimensions (copper foil foreign matter of this size is sufficient to cause a battery short circuit);

[0095] The battery is charged at 0.02C, and the upper protection voltage is 3.8V. Most of the dissolution process of the metallic foreign matter copper is completed during the first charge. The copper dissolution ratio is calculated as (copper dissolution in the negative electrode + copper dissolution in the diaphragm) / (copper dissolution in the positive electrode + copper dissolution in the negative electrode + copper dissolution in the diaphragm) × 100%. Among them, the copper in the positive and negative electrode sides and the diaphragm is accurately measured by ICP test;

[0096] The termination condition of the experiment is when the voltage drops suddenly or becomes 0V during the charging process.

[0097] Test results: See Table 1.

[0098] Table 1

[0099]

[0100]

[0101] From the test results, we can see that:

[0102] Compared with Comparative Examples 1 to 3, the mass of metallic copper dissolved in the positive electrode of Example 1 to Example 20 is relatively small, and the sum of the mass of metallic copper dissolved in the negative electrode and the diaphragm is relatively large, indicating that the mass of copper that forms dendrites in the negative electrode and the diaphragm and pierces the diaphragm is larger; and the copper dissolution ratio is also higher. The higher the copper dissolution ratio, the more the negative electrode and the diaphragm can accommodate deposited copper when the positive electrode contains the same amount of copper foreign matter, and therefore the safety margin of the battery is wider.

[0103] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. "First feature" and "second feature" may include one or more of the features.

[0104] In the description of the present application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for examples and may be any technical feature connected by "and / or" in the present application.

[0105] In the present application, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of each step should be determined by its function and possible internal logic. If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) can be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0106] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A negative electrode plate, characterized in that: include: Anode current collector; A first negative electrode active material layer, the first negative electrode active material layer is located on at least one side of the negative electrode current collector, the first negative electrode active material layer includes a first negative electrode active material, the first negative electrode active material includes graphite, and the particle size of the first negative electrode active material satisfies: The difference between Dv90 and Dv50 is 0.1μm-1μm; the difference between Dv50 and Dv10 is 0.1μm-2μm.

2. The negative electrode sheet according to claim 1, characterized in that: The Dv50 of the first negative electrode active material is 0.1 μm-5 μm.

3. The negative electrode sheet according to claim 2, characterized in that: The Dv50 of the first negative electrode active material is 1 μm-3 μm.

4. The negative electrode sheet according to claim 1, characterized in that: The thickness of the first negative electrode active material layer is less than or equal to 12 μm.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The first negative electrode active material layer includes 93wt%-98wt% of graphite, 0.5wt%-2.5wt% of acetylene black, 0.5wt%-1.5wt% of carbon fiber, and 1wt%-3wt% of a binder.

6. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: Further including: A second negative electrode active material layer, wherein the second negative electrode active material layer is located between the negative electrode current collector and the first negative electrode active material layer, the second negative electrode active material layer includes a second negative electrode active material, the second negative electrode active material includes graphite, and the mass fraction of the conductive agent in the second negative electrode active material layer is less than the mass fraction of the conductive agent in the first negative electrode active material layer.

7. The negative electrode sheet according to claim 6, characterized in that: The Dv50 of the second negative electrode active material is 3 μm-12 μm.

8. The negative electrode sheet according to claim 6, characterized in that: The thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:(9-150).

9. The negative electrode sheet according to claim 8, characterized in that: The thickness of the second negative electrode active material layer is 50 μm-150 μm.

10. A secondary battery, characterized in that: The invention comprises the negative electrode sheet as described in any one of claims 1 to 9.

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