Negative plate, secondary battery and electric equipment

Abstract

The utility model discloses a negative pole piece, a secondary battery and electric equipment. The negative pole piece comprises a current collector, a positive pole piece and a negative pole piece, the first graphite layer is arranged on at least one surface of the current collector; the second graphite layer is arranged on the surface, deviating from the current collector, of the first graphite layer; the third graphite layer is arranged on the surface, deviating from the first graphite layer, of the second graphite layer; wherein a plurality of silicon particles are distributed in the second graphite layer, and no silicon particles are overlapped or part of silicon particles are overlapped in the same thickness direction. The silicon particles are uniformly distributed in the graphite layer of the negative plate, and no overlapped silicon particles or only part of the silicon particles are overlapped in the same thickness direction, so that the expansion of the silicon particles in the charge-discharge cycle of the battery cell cannot cause the large expansion of the negative plate, thereby ensuring that the consistency of the negative plate is increased after the cycle; and the thickness expansion of the battery cell is reduced by reducing the thickness of the thickest point.

Description

Technical Field

[0001] The utility model relates to the technical field of secondary batteries, in particular to a negative electrode sheet, a secondary battery and electrical equipment. Background Art

[0002] As mobile phones continue to increase their requirements for battery cell ED (Energy Density), the proportion of silicon negative electrodes in battery cell usage continues to increase. However, since silicon itself expands significantly in thickness during the charging and discharging process, batteries with a high silicon content are very likely to expand beyond the specification during cycling. Utility Model Content

[0003] The main purpose of the utility model is to provide a negative electrode sheet, a secondary battery and an electrical device, aiming to solve the problem that the thickness of the existing negative electrode sheet easily expands beyond the specification during circulation.

[0004] To achieve the above-mentioned purpose, the present invention provides a negative electrode sheet, which includes:

[0005] current collector;

[0006] a first graphite layer, the first graphite layer being disposed on at least one surface of the current collector;

[0007] a second graphite layer, the second graphite layer being disposed on a surface of the first graphite layer facing away from the current collector;

[0008] a third graphite layer, the third graphite layer being disposed on a surface of the second graphite layer facing away from the first graphite layer;

[0009] Wherein, a plurality of silicon particles are distributed in the second graphite layer, and no silicon particles overlap or some silicon particles overlap in the same thickness direction.

[0010] Preferably, the thickness of the second graphite layer is greater than the particle size of the silicon particles and less than 2 times the particle size of the silicon particles.

[0011] Preferably, the thickness of the second graphite layer is less than or equal to 30% of the total thickness of the first graphite layer, the second graphite layer, and the third graphite layer.

[0012] Preferably, the thickness of the third graphite layer is greater than the particle size of the silicon particles.

[0013] Preferably, the probability of some silicon particles overlapping in the same thickness direction is less than 10%.

[0014] Preferably, the particle size of the silicon particles is 4 μm to 7 μm;

[0015] And / or, the first graphite layer, the second graphite layer, and the third graphite layer all contain graphite particles, and the particle size of the graphite particles is 7 μm to 14 μm.

[0016] Preferably, the particle size of the silicon particles is 5 μm.

[0017] Preferably, the silicon particles are selected from at least one of Si, SiC, and SiO.

[0018] The present invention further provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet and a separator between the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet as described above or the negative electrode sheet prepared by the preparation method as described above.

[0019] The present invention further provides an electrical device comprising the secondary battery as described above.

[0020] The beneficial effect of the technical solution of the present invention is that silicon particles are evenly distributed in the graphite layer of the negative electrode sheet of the present invention, and there are no overlapping silicon particles or only some silicon particles overlap in the same thickness direction, so that the expansion of silicon particles during the charge and discharge cycle of the battery cell will not cause a large expansion of the thickness of the negative electrode sheet, thereby ensuring that the consistency of the negative electrode sheet increases after the cycle, reducing the thickness of the thickest point to reduce the thickness expansion of the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the structure of the negative electrode sheet according to an embodiment of the present utility model;

[0022] Figure 2 This is a schematic diagram of coating according to an embodiment of the present utility model;

[0023] Figure 3 This is a schematic structural diagram of the negative electrode sheet of Comparative Example 1 before charge and discharge cycle testing;

[0024] Figure 4 This is a schematic diagram of the structure of the negative electrode sheet of comparative example 1 after charge and discharge cycle testing.

[0025] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the schemes in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0027] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, top, bottom, side...) are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0028] It should also be noted that when an element is referred to as being "fixed on" or "disposed on" another element, it may be directly on the other element or there may be an intermediate element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an intermediate element.

[0029] In addition, the descriptions involving "first", "second", etc. in the embodiments of the present invention are only for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features.

[0030] In view of the technical defects existing in the related art, the present invention proposes a negative electrode sheet, referring to Figure 1 , the negative electrode sheet comprises:

[0031] Current collector 10;

[0032] A first graphite layer 20 , the first graphite layer 20 is disposed on at least one surface of the current collector 10 ;

[0033] A second graphite layer 30 , the second graphite layer 30 is disposed on a surface of the first graphite layer 20 facing away from the current collector 10 ;

[0034] A third graphite layer 40 is provided on a surface of the second graphite layer 30 facing away from the first graphite layer 20 ;

[0035] A plurality of silicon particles 31 are distributed in the second graphite layer 30 , and no silicon particles 31 overlap or some silicon particles 31 overlap in the same thickness direction.

[0036] Specifically, silicon particles 31 are evenly distributed in the graphite layer of the negative electrode sheet of the present application, and there are no overlapping silicon particles 31 in the same thickness direction or only some silicon particles 31 overlap, so that the expansion of the silicon particles 31 during the charge and discharge cycle of the battery cell will not cause a large expansion of the thickness of the negative electrode sheet, thereby ensuring that the consistency of the negative electrode sheet increases after the cycle, reducing the thickness of the thickest point to reduce the thickness expansion of the battery cell; compared with the primer silicon layer, the negative electrode sheet structure of the present application can freely adjust the silicon content, and prevent silicon particles from contacting the current collector, effectively preventing the current collector from being damaged.

[0037] In some embodiments, the thickness D2 of the second graphite layer 30 is greater than the particle size d of the silicon particles 31 and less than twice the particle size d of the silicon particles 31 .

[0038] In this embodiment, by limiting the thickness of the second graphite layer 30, it is ensured that the second graphite layer 30 can accommodate the silicon particles 31, but a large number of silicon particles 31 will not overlap in the second graphite layer 30, so that after the battery cell undergoes multiple cycles, the expansion of the silicon particles 31 will not cause the thickness of the battery cell to expand beyond the specification.

[0039] Furthermore, the thickness of the second graphite layer 30 can also be less than or equal to 30% of the total thickness of the first graphite layer 20, the second graphite layer 30, and the third graphite layer 40. The specific value can be adjusted according to the selection of silicon material and actual needs. The thickness of the second graphite layer 30 determines whether the silicon particles 31 are easy to overlap and whether the expansion of the silicon particles 31 after the battery cell undergoes multiple cycles will cause the thickness expansion of the battery cell to not exceed the specification.

[0040] In some embodiments, the thickness D1 of the third graphite layer 40 is greater than the particle size d of the silicon particles 31 .

[0041] In this embodiment, after the negative electrode sheet is coated and dried as a whole, it is necessary to perform laser drilling on the negative electrode sheet. Laser drilling forms a porous structure in the negative electrode sheet, and the electrolyte can more effectively infiltrate into the interior of the electrode material, thereby improving the lithium ion transmission efficiency. However, if the thickness of the third graphite layer 40 is small, the silicon particles 31 in the second graphite layer 30 will be broken during the laser drilling process. Therefore, it is necessary to limit the thickness of the third graphite layer 40 to be greater than the particle size of the silicon particles 31 to prevent the silicon particles 31 from being destroyed during the laser drilling process of the negative electrode sheet.

[0042] In some embodiments, the probability of some silicon particles 31 overlapping in the same thickness direction is less than 10%.

[0043] In this embodiment, if there are too many overlapping silicon particles 31 in the same thickness direction, the battery cell will expand too much in the same thickness direction and exceed the specifications due to the expansion of the silicon particles 31 during the battery cell cycle. This application sets the probability of overlapping silicon particles 31 in the same thickness direction to less than 10%, which can increase the consistency of the electrode after cycling, reduce the thickness of the thickest point of the negative electrode after the expansion of the silicon particles 31, and thus reduce the thickness expansion of the battery cell.

[0044] In this embodiment, the particle size of the silicon particles is 4 μm to 7 μm; preferably, the particle size of the silicon particles is 5 μm; the silicon particles are selected from at least one of Si, SiC, and SiO; and / or the first graphite layer 20, the second graphite layer 30, and the third graphite layer 40 all contain graphite particles, and the particle size of the graphite particles is 7 μm to 14 μm.

[0045] The method for preparing the negative electrode sheet in the present invention includes:

[0046] Mixing graphite, a first conductive agent, a first binder, and a first co-solvent and performing a first stirring process, adding deionized water to obtain a graphite slurry;

[0047] The silicon material, graphite, the second conductive agent, the second binder, and the second co-solvent are mixed and subjected to a second stirring process, and deionized water is added to obtain a high-silicon slurry;

[0048] The graphite slurry and the high silicon slurry are coated on at least one surface of the current collector 10 by extrusion coating to obtain a negative electrode sheet.

[0049] Specifically, the first graphite layer 20 and the third graphite layer 40 in the present application are both made of silicon-free graphite slurry, and the second graphite layer 30 is made of high-concentration silicon slurry. The two slurries are then simultaneously coated onto at least one surface of the current collector 10 by extrusion coating. By adopting the extrusion coating method, a high-precision coating effect can be achieved, and the coating amount can be flexibly controlled by micro-adjustment, ensuring the accuracy and uniformity of the coating. It can also ensure that the coating slurry has strong adhesion to the surface of the current collector 10 while ensuring that the entire slurry flow channel is sealed, effectively preventing other pollutants from entering, and ensuring the coating quality.

[0050] In some embodiments, the solid content of the graphite slurry is greater than 95%; and / or the solid content of the high silicon slurry is greater than 90%.

[0051] In this embodiment, since the flow rate and viscosity of the slurry need to be controlled during the extrusion coating process to ensure the coating uniformity of the three graphite layers, the viscosity of the graphite slurry and the high-silicon slurry can be controlled by limiting the solid content of the graphite slurry and the high-silicon slurry, thereby further facilitating the control of the flow rate of the slurry, and setting the viscosity of the graphite slurry and the high-silicon slurry to a higher level can also facilitate enhancing the bonding force between the graphite layers; further, the viscosity of the obtained slurry can be adjusted to be between 3K-8K by regulating the solid content of the slurry.

[0052] In some embodiments, the graphite slurry is calculated by mass percentage as follows: graphite: first conductive agent: first binder: first solubility agent (95% to 99%): (0.1% to 3%): (0.1% to 2%): (0% to 1%); the high silicon slurry is calculated by mass percentage as follows: silicon material: graphite: second conductive agent: second binder: second solubility agent (50% to 99%): (0.1% to 50%): (0.1% to 3%): (0.1% to 2%): (0% to 1%); the silicon material is selected from at least one of Si, SiC (silicon carbide), and SiO (silicon oxide), preferably SiC, which has higher electron mobility and chemical stability.

[0053] In this embodiment, the first binder and the second binder are respectively selected from at least one of polyvinylidene fluoride, carboxymethyl cellulose, polyvinyl alcohol, and sodium alginate; the first conductive agent and the second conductive agent are respectively selected from at least one of conductive carbon black, carbon nanotubes, and graphene; the first co-solvent and the second co-solvent are both EC (ethylene carbonate).

[0054] In some embodiments, the first stirring process and the second stirring process are both dry stirring; and / or, the graphite slurry and the high silicon slurry are coated on at least one surface of the current collector 10 by extrusion coating to obtain the negative electrode sheet, which includes:

[0055] Input the graphite slurry into the pipes on both sides of the coating die of the extrusion coating machine;

[0056] Inputting the high silicon slurry into the middle pipe of the coating die of the extrusion coating machine;

[0057] The graphite slurry and the high silicon slurry are simultaneously extruded and coated onto at least one surface of the current collector 10 using an extrusion coater to obtain a negative electrode sheet.

[0058] In this embodiment, the dry stirring method is adopted to make the material distribution more uniform, improve the dispersibility of active substances and conductive agents, thereby reducing agglomeration and improving the electrochemical performance of the battery. In addition, the dry stirring method can improve the sedimentation stability and rheological properties of the slurry, which helps to achieve a coating with uniform thickness, reduce the porosity of the coated electrode, and is beneficial to increase the energy density of the battery, increase the bonding strength of the electrode, thereby improving the cycle performance and rate characteristics of the battery. Compared with the traditional wet method, the dry stirring process significantly shortens the stirring time and improves production efficiency; optionally, the duration of the first stirring treatment and the second stirring treatment is 30min to 60min.

[0059] Furthermore, in this embodiment, an extrusion coater is used to coat the first graphite layer 20, the second graphite layer 30 and the third graphite layer 40 simultaneously, so the following is specially designed: Figure 2 In the coating die shown in the figure, graphite slurry enters from the pipes on both sides of the coating die, and high silicon slurry enters from the middle pipe of the coating die, which can realize the simultaneous coating of three graphite layers and better control the coating uniformity and flatness of the negative electrode sheet. The distribution uniformity of silicon particles 31 can be controlled without adding additional slurry components, so that the expansion of silicon particles 31 after the battery cell undergoes multiple cycles will not cause the thickness expansion of the battery cell to exceed the specification.

[0060] In some embodiments, the coating speed of extrusion coating is less than 20 m / min.

[0061] Specifically, during the coating process, a speed that is too fast may cause the slurry to not be evenly coated on the current collector 10, thereby affecting the capacity, internal resistance and cycle life of the battery. If the coating speed is too fast, the surface coating of the electrode may crack, fall off, etc.; at the same time, too fast a coating speed will also increase the risk of coating defects such as thick head and thin tail, thick edges on both sides, and dark spots. These defects will seriously affect the performance and safety of the battery. Therefore, this application limits the coating speed of extrusion coating to less than 20m / min, which can ensure that the supply speed of the slurry can match the coating speed, and can also ensure the uniformity of the coating, thereby avoiding defects caused by excessively fast coating that affect the performance and safety of the battery.

[0062] The present invention further provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet is the negative electrode sheet as described above or the negative electrode sheet prepared by the preparation method as described above.

[0063] The utility model also provides an electrical device, comprising the above-mentioned secondary battery.

[0064] Electrical equipment can be cars, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and electric tools, etc. Cars can be fuel cars, gas cars, or new energy cars. New energy cars can be pure electric cars, hybrid cars, or extended-range cars, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Electric tools include metal cutting electric tools, grinding electric tools, assembly electric tools, and railway electric tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. The embodiments of the present application do not impose any special restrictions on the above-mentioned electrical equipment.

[0065] In order to make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be described in further detail below in conjunction with specific implementation methods, but the implementation methods of the present invention are not limited thereto.

[0066] Example 1

[0067] The negative electrode sheet prepared in this embodiment includes: a current collector; a first graphite layer, arranged on at least one surface of the current collector; a second graphite layer, arranged on the surface of the first graphite layer facing away from the current collector; and a third graphite layer, arranged on the surface of the second graphite layer facing away from the first graphite layer. A plurality of silicon particles are distributed in the second graphite layer, and the probability of silicon particles overlapping in the same thickness direction is 4.5%. The particle size of the silicon particles is 5 μm, the thickness of the first graphite layer is h1=10 μm, the thickness of the second graphite layer is h2=5.3 μm, and the thickness of the third graphite layer is h3=20 μm. At this time, h2=15%(h1+h2+h3), that is, the thickness of the second graphite layer accounts for 15% of the total thickness of the three graphite layers.

[0068] The preparation method of the negative electrode sheet includes:

[0069] Graphite, conductive carbon black, polyvinylidene fluoride, and ethylene carbonate are mixed in a mass percentage of 96%:2%:1%:1%, stirred for 30 minutes, deionized water is added, and stirred for 90 minutes to obtain a graphite slurry; SiC, graphite, conductive carbon black, polyvinylidene fluoride, and ethylene carbonate are mixed in a mass percentage of 70%:26%:2%:1%:1%, stirred for 30 minutes, deionized water is added, and stirred for 90 minutes to obtain a high-silicon slurry; the graphite slurry is input into the pipes on both sides of the coating die of an extrusion coater, and the high-silicon slurry is input into the middle pipe of the coating die of the extrusion coater. The graphite slurry and the high-silicon slurry are simultaneously extruded and coated on at least one surface of the current collector at a coating speed of 15m / min using the extrusion coater to obtain a negative electrode sheet.

[0070] Preparation of positive electrode sheet: lithium iron phosphate, conductive agent superconducting carbon (Super-P), and binder polyvinylidene fluoride (PVDF) are mixed evenly in a mass ratio of 97:1.5:1.5 to prepare a lithium-ion battery positive electrode slurry with a certain viscosity. The slurry is coated on the current collector aluminum foil, dried at 85°C, cold pressed, and cut into strips for use.

[0071] Diaphragm: PE-based film is coated with aluminum oxide / PVDF slurry on both sides, dried, and cut into strips for later use.

[0072] Electrolyte: Lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (the mass ratio of EC, DMC, and EMC was 1:2:1) to obtain an electrolyte with a concentration of 1 mol / L.

[0073] Preparation of secondary battery: Roll the above-mentioned positive electrode sheet, separator, negative electrode sheet and electrolyte into a soft-pack battery, let it stand at room temperature for 24 hours and at high temperature for 16 hours before use.

[0074] Example 2

[0075] The difference between this embodiment and embodiment 1 is that the probability of silicon particles overlapping in the same thickness direction is 6%, wherein the particle size of the silicon particles is 5 μm, the thickness of the first graphite layer h1 = 10 μm, the thickness of the second graphite layer h2 = 7.5 μm, and the thickness of the third graphite layer h3 = 20 μm. At this time, h2 = 20% (h1 + h2 + h3), that is, the thickness of the second graphite layer accounts for 20% of the total thickness of the three graphite layers.

[0076] Example 3

[0077] The difference between this embodiment and embodiment 1 is that the probability of silicon particles overlapping in the same thickness direction is 9%, wherein the particle size of the silicon particles is 5 μm, the thickness of the first graphite layer h1 = 10 μm, the thickness of the second graphite layer h2 = 13 μm, and the thickness of the third graphite layer h3 = 20 μm. At this time, h2 = 30% (h1 + h2 + h3), that is, the thickness of the second graphite layer accounts for 30% of the total thickness of the three graphite layers.

[0078] Comparative Example 1

[0079] A method for preparing a negative electrode sheet, comprising:

[0080] SiC, graphite, conductive carbon black, polyvinylidene fluoride, and ethylene carbonate were mixed in a mass percentage of 70%:26%:2%:1%:1%, stirred for 30 minutes, deionized water was added, and stirred for 90 minutes to obtain a mixed slurry, which was then coated on at least one surface of a current collector to obtain a negative electrode sheet.

[0081] like Figure 3 The negative electrode sheet prepared in this comparative example includes: a current collector 10 ; a graphite layer 50 disposed on at least one surface of the current collector; and silicon particles 31 dispersed in the graphite layer 50 .

[0082] A secondary battery was prepared using the same method as in Example 1.

[0083] Comparative Example 2

[0084] The difference between this embodiment and embodiment 1 is that the probability of silicon particles overlapping in the same thickness direction is 12%, wherein the particle size of the silicon particles is 5 μm, the thickness of the first graphite layer h1 = 10 μm, the thickness of the second graphite layer h2 = 20 μm, and the thickness of the third graphite layer h3 = 20 μm. At this time, h2 = 40% (h1 + h2 + h3), that is, the thickness of the second graphite layer accounts for 40% of the total thickness of the three graphite layers.

[0085] During the preparation of the negative electrode sheets of Examples 1 to 3 and Comparative Examples 1 to 2, the product of the area density and gram capacity of the negative electrode sheets was kept at the same value to ensure the accuracy of the test results.

[0086] The secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 2 were subjected to charge and discharge cycle tests, and the microstructure of the negative electrode sheet after the cycle test was observed using a CP cross-section polisher (argon ion polisher). The thickness of the thickest point of the negative electrode sheet in the Z-axis direction was taken as the thickness of the negative electrode sheet after the cycle, and compared with the initial thickness of the negative electrode sheet before the cycle. The thickness expansion rate of the negative electrode sheet after 400 cls of charge and discharge cycles was obtained. The results are shown in Table 1.

[0087] Table 1

[0088] sample The thickness expansion rate of the negative electrode sheet after 400cls charge and discharge cycles Example 1 11.58％ Example 2 12.24％ Example 3 12.57％ Comparative Example 1 13.97％ Comparative Example 2 13.53％

[0089] Figure 3 This is a schematic diagram of the structure of the negative electrode sheet in Comparative Example 1 before the charge-discharge cycle test. It can be seen that the surface of the electrode sheet is relatively flat before the charge-discharge cycle test. Figure 4 This is a schematic diagram of the structure of the negative electrode sheet in Comparative Example 1 after 400 cls of charge-discharge cycle testing. It can be seen that due to the large number of silicon particles along the same thickness direction and their disordered dispersion, the thickness of the negative electrode sheet also expands significantly with the expansion of the silicon particles. The thickness of the electrode sheet measured after the cycle test is the thickness corresponding to the maximum thickness point of the electrode sheet. Comparative Example 1 and Examples 1-3 show that the negative electrode sheet prepared using the method of the present application has a small thickness expansion rate after cycling, which can be reduced by more than 1.4% compared to the thickness expansion rate of negative electrode sheets in the prior art after cycling testing.

[0090] According to Comparative Example 2 and Examples 1 to 3, it can be seen that when the thickness of the second graphite layer is too large, the probability of silicon particles overlapping in the same thickness direction will increase. Therefore, during the battery cycle, the thickness of the thickest point of the electrode will be too large due to the superposition of silicon particle expansion, thereby causing the thickness of the electrode to expand beyond the specification.

[0091] The above are only some or preferred embodiments of the present invention. Neither the text nor the drawings can limit the scope of protection of the present invention. All equivalent structural transformations made by using the contents of the present invention specification and drawings under the overall concept of the present invention, or direct / indirect application in other related technical fields are included in the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that: include: current collector; a first graphite layer, the first graphite layer being disposed on at least one surface of the current collector; a second graphite layer, the second graphite layer being disposed on a surface of the first graphite layer facing away from the current collector; a third graphite layer, the third graphite layer being disposed on a surface of the second graphite layer facing away from the first graphite layer; Wherein, a plurality of silicon particles are distributed in the second graphite layer, and no silicon particles overlap or some silicon particles overlap in the same thickness direction.

2. The negative electrode sheet according to claim 1, characterized in that: The thickness of the second graphite layer is greater than the particle size of the silicon particles and less than twice the particle size of the silicon particles.

3. The negative electrode sheet according to claim 1, characterized in that: The thickness of the second graphite layer is less than or equal to 30% of the total thickness of the first graphite layer, the second graphite layer, and the third graphite layer.

4. The negative electrode sheet according to claim 1, characterized in that: The thickness of the third graphite layer is greater than the particle size of the silicon particles.

5. The negative electrode sheet according to claim 1, characterized in that: The probability of some silicon particles overlapping in the same thickness direction is less than 10%.

6. The negative electrode sheet according to claim 1, characterized in that: The particle size of the silicon particles is 4 μm to 7 μm; And / or, the first graphite layer, the second graphite layer, and the third graphite layer all contain graphite particles, and the particle size of the graphite particles is 7 μm to 14 μm.

7. The negative electrode sheet according to claim 6, characterized in that: The particle size of the silicon particles is 5 μm.

8. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator between the positive electrode sheet and the negative electrode sheet, characterized in that: The negative electrode sheet is the negative electrode sheet according to any one of claims 1 to 7.

9. An electrical device, characterized in that: Comprising the secondary battery as claimed in claim 8.

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