Negative electrode sheet, electrode assembly, battery and electronic device

Abstract

The present application provides a negative electrode sheet comprising a current collector and an active material layer disposed on the current collector. The negative electrode sheet further comprises a first coating layer disposed between the current collector and the active material layer, and a second coating layer disposed between the first coating layer and the active material layer. The active material layer and the second coating layer comprise carbon nanotubes, and the carbon nanotubes in the active material layer and the carbon nanotubes in the second coating layer are interconnected. The present application also provides an electrode assembly, a battery, and an electronic device.

Description

Technical Field

[0001] The present application relates to the field of batteries, and in particular to a negative electrode sheet, an electrode assembly having the negative electrode sheet, a battery having the electrode assembly, and an electronic device having the battery. Background Art

[0002] Lithium-ion batteries, with their advantages of high specific energy, high operating voltage, low self-discharge, compact size, and light weight, are widely used in consumer electronics. However, with the rapid development of electric vehicles and consumer electronics, people are increasingly concerned about and demanding higher battery energy density.

[0003] At present, graphite is a relatively common negative electrode material for lithium-ion batteries. It has excellent conductivity and high cycle stability, but its theoretical gram capacity of 372mAh / g can no longer meet the widespread demand for high-energy-density lithium-ion batteries. The theoretical gram capacity of silicon for lithium insertion is as high as 4200mAh / g, and its reserves on Earth are very abundant, so it has great application prospects. However, the volume of silicon particles expands significantly during the cycle process, which will reduce the adhesion between the active material layer and the current collector in the negative electrode sheet, causing the active material layer to fall off, and then causing the entire battery cell to deform. At the same time, the silicon particles themselves have poor conductivity, and the volume expansion of silicon particles will also reduce the contact force between them and the conductive agent or carbon material, thereby further deteriorating the conductive performance.

[0004] In order to improve the interfacial adhesion and conductivity of silicon negative electrode sheets, the existing technology usually adopts methods such as improving the negative electrode slurry preparation process, improving the conductive network, modifying the silicon material, or improving the negative electrode current collector structure. Among them, improving the negative electrode slurry preparation process mainly improves the adhesion performance of the negative electrode sheet by increasing the amount of adhesive used. However, increasing the amount of adhesive used will increase the sheet resistance, thereby worsening the dynamic performance of the battery cell. Improving the conductive network mainly increases the amount of conductive agent used. However, increasing the amount of conductive agent used will increase the processing difficulty, reduce the energy density of the battery cell, and cannot improve the interfacial adhesion of the negative electrode sheet. The modification of silicon materials is a long-term work with little effect in the short term. Improving the negative electrode current collector structure usually involves adding a primer on the surface of the copper foil, which is beneficial to improving electrical properties and improving interfacial adhesion. However, the primer layer requires a large amount of adhesive, which worsens the conductive performance.

[0005] Therefore, the problems of low interfacial adhesion and poor conductivity of silicon negative electrode sheets still need to be solved urgently. Summary of the Invention

[0006] In view of the above, it is necessary to propose a negative electrode sheet with high interface adhesion and conductivity to solve the above problems.

[0007] A preferred embodiment of the present application provides a negative electrode sheet comprising a current collector and an active material layer disposed on the current collector. The negative electrode sheet further comprises: a first coating layer disposed between the current collector and the active material layer; and a second coating layer disposed between the first coating layer and the active material layer. The active material layer and the second coating layer comprise carbon nanotubes, and the carbon nanotubes in the active material layer and the carbon nanotubes in the second coating layer are interconnected.

[0008] The carbon nanotubes in the active material layer and the carbon nanotubes in the second coating layer act as conductive agents, providing good electrical conductivity and improving the poor electrical conductivity of silicon materials. Furthermore, because the carbon nanotubes have a longer tube length and a smaller tube diameter, the carbon nanotubes in the active material layer and the carbon nanotubes in the second coating layer are interconnected, which can improve the interfacial adhesion of the negative electrode sheet and help suppress the volume expansion of the active particles in the active material layer. Furthermore, the problem of reduced electrical conductivity due to volume expansion can be avoided, thereby improving the energy density and cycle performance of the battery.

[0009] In some embodiments of the present application, the length of the carbon nanotube is not less than 3 μm and the diameter is not greater than 30 nm.

[0010] When the length of the carbon nanotubes is less than 3 μm, the effect of enhancing the interfacial adhesion cannot be achieved; when the diameter of the carbon nanotubes is greater than 30 nm, the interfacial adhesion between the active material layer and the second coating layer will also be affected.

[0011] In some embodiments of the present application, the thickness of the second coating layer is 200 nm-1000 nm.

[0012] When the thickness of the second coating layer is less than 200 nm, the interfacial adhesion between the active material layer and the second coating layer is reduced; and when the thickness of the second coating layer is greater than 1000 nm, the energy density of the battery is reduced.

[0013] In some embodiments of the present application, the mass percentage of the carbon nanotubes in the second coating layer is 75%-90%, and the second coating layer further includes a dispersant with a mass percentage of 5%-15% and a binder with a mass percentage of 1%-10%.

[0014] When the thickness of the negative electrode sheet is the same, the larger amount of carbon nanotube conductive agent in the second coating layer can not only improve the conductivity of the silicon negative electrode sheet, but also ensure the adhesion of the interface of the silicon negative electrode sheet.

[0015] In some embodiments of the present application, the thickness of the first coating layer is 1 μm-5 μm.

[0016] In some embodiments of the present application, the first coating layer includes 45%-70% by mass of a conductive agent, 1%-5% by mass of a thickener, and 25%-50% by mass of a binder.

[0017] Compared with the second coating layer, the content of the binder in the first coating layer is higher, which can improve the conductivity of the negative electrode sheet while ensuring better adhesion between the interfaces of the silicon negative electrode sheets.

[0018] In some embodiments of the present application, the mass percentage of the carbon nanotubes in the active material layer is 0.01%-5%, and the active material layer also includes a carbon material with a mass percentage of 75%-95%, a silicon material with a mass percentage of 1%-20%, a thickener with a mass percentage of 0%-5% and a binder with a mass percentage of 1%-5%.

[0019] The present application ensures the conductivity and adhesion of the active material layer, and at the same time, because the content of the binder in the active material layer is relatively low, the energy density of the battery can be improved.

[0020] The present application also provides an electrode assembly, comprising a positive electrode sheet and a separator. The electrode assembly further comprises the negative electrode sheet as described above, and the electrode assembly is formed by laminating or winding the positive electrode sheet, the separator, and the negative electrode sheet.

[0021] The present application also provides a battery, comprising the electrode assembly as described above.

[0022] The present application also provides an electronic device, comprising the battery as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic cross-sectional view of the negative electrode sheet provided in an embodiment of the present application.

[0024] Figure 2 A schematic structural diagram of a battery provided in an embodiment of the present application.

[0025] Figure 3 A schematic structural diagram of an electronic device provided in an embodiment of the present application.

[0026] Figure 4 This is a scanning electron microscope (SEM) image of the cross section of the negative electrode sheet prepared in this application.

[0027] Description of main component symbols

[0028] Negative electrode 10

[0029] Current collector 11

[0030] Active material layer 12

[0031] First coating 13

[0032] Second coating 14

[0033] Positive electrode 20

[0034] Diaphragm 30

[0035] Carbon nanotubes 120, 140

[0036] Electrode assembly 100

[0037] Battery 200

[0038] Electronic device 300

[0039] The following specific implementation methods will further illustrate the present application in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0040] See also Figure 1 , an embodiment of the present application provides a negative electrode sheet 10, the negative electrode sheet 10 including a current collector 11 and an active material layer 12 located on the current collector 11. The negative electrode sheet 10 also includes a first coating layer 13 and a second coating layer 14. The first coating layer 13 is located between the current collector 11 and the active material layer 12. The second coating layer 14 is located between the first coating layer 13 and the active material layer 12. The active material layer 12 includes carbon nanotubes (CNTs) 120, and the second coating layer 14 includes carbon nanotubes 140, and the carbon nanotubes 120 of the active material layer 12 and the carbon nanotubes 140 of the second coating layer 14 are interconnected.

[0041] The carbon nanotubes 120 of the active material layer 12 and the carbon nanotubes 140 of the second coating layer 14 described herein act as conductive agents, providing excellent electrical conductivity and improving the poor electrical conductivity of silicon materials. Furthermore, because the carbon nanotubes 120 and 140 have a long tube length and a small tube diameter, they can connect to each other, improving the interfacial adhesion of the negative electrode sheet 10 and helping to suppress the volume expansion of the active particles in the active material layer 12. Furthermore, the problem of reduced electrical conductivity due to volume expansion can be avoided, thereby improving the energy density and cycle performance of the battery.

[0042] Optionally, the carbon nanotubes 120 and the carbon nanotubes 140 may be connected to each other by being entangled and interwoven.

[0043] In this embodiment, the carbon nanotubes 120 of the active material layer 12 are also interconnected. That is, the carbon nanotubes 120 of the active material layer 12 can form a two-dimensional conductive network structure, which can further suppress the volume expansion of the active particles in the active material layer 12 and improve the conductive performance.

[0044] In some embodiments of the present application, the carbon nanotubes 140 of the second coating layer 14 have a length of not less than 3 μm and a diameter of not more than 30 nm. That is, the carbon nanotubes 140 have an elongated appearance, which facilitates the entanglement and interweaving of the carbon nanotubes 120 of the active material layer 12 and the carbon nanotubes 140 of the second coating layer 14. When the length of the carbon nanotubes 140 is less than 3 μm, an effective long-range connection cannot be formed between the active material layer 12 and the second coating layer 14, thereby failing to achieve the effect of enhancing the interfacial adhesion; when the diameter of the carbon nanotubes 140 is greater than 30 nm, the specific surface area of the second coating layer 14 decreases, and when the content of the carbon nanotubes 140 is constant, the number of the carbon nanotubes 140 will be reduced, thereby affecting the interfacial adhesion between the active material layer 12 and the second coating layer 14.

[0045] Furthermore, the carbon nanotubes 120 in the active material layer 12 have a length of no less than 3 μm and a diameter of no greater than 30 nm, thereby further facilitating the entanglement and interweaving of the carbon nanotubes 120 in the active material layer 12 with the carbon nanotubes 140 in the second coating layer 14, and also facilitating the mutual entanglement and interweaving of the carbon nanotubes 120 in the active material layer 12 to form a two-dimensional bonded network. The carbon nanotubes 120 and 140 comprise at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes, and may be surface-modified to enhance conductivity.

[0046] In some embodiments of the present application, the thickness of the second coating layer 14 is 200 nm to 1000 nm. When the thickness of the second coating layer 14 is less than 200 nm, the thickness uniformity of the second coating layer 14 is poor, which may result in partial exposure, thereby reducing the interfacial adhesion between the active material layer 12 and the second coating layer 14. When the thickness of the second coating layer 14 is greater than 1000 nm, the thickness of the active material layer 12 will be reduced while the thickness of the negative electrode sheet 10 remains the same, thereby reducing the energy density of the battery.

[0047] In some embodiments of the present application, the mass percentage of the carbon nanotubes 140 in the second coating layer 14 is 75%-90%, and the second coating layer 14 further includes a dispersant with a mass percentage of 5%-15% and a binder with a mass percentage of 1%-10%. Due to the high content of carbon nanotubes 140 as a conductive agent in the second coating layer 14, while ensuring the adhesion between the interfaces, the conductivity of the silicon negative electrode plate can be improved, thereby improving the overall performance of the battery. Among them, the dispersant can be selected from at least one of polyvinyl pyrrolidone (PVP), sodium carboxymethyl cellulose (CMC-Na) and lithium carboxymethyl cellulose (CMC-Li). The binder is selected from at least one of styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, epoxy resin, polyvinyl alcohol, polyimide, polyamide-imide, polyacrylamide, polyacrylic acid and their salts.

[0048] In some embodiments of the present application, the thickness of the first coating layer 13 is 1 μm-5 μm.

[0049] In some embodiments of the present application, the first coating layer 13 comprises 45%-70% by weight of a conductive agent, 1%-5% by weight of a thickener, and 25%-50% by weight of a binder. Compared to the second coating layer 14, the first coating layer 13 contains a higher content of the binder, thereby enhancing the conductivity of the negative electrode sheet 10 while ensuring better adhesion between the silicon negative electrode sheet interfaces. The conductive agent is selected from at least one of carbon nanotubes, amorphous carbon, conductive graphite, conductive carbon black (Super P, SP), acetylene black, and vapor-grown carbon fiber (VGCF). The thickener is selected from at least one of carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), and cellulose. The binder is selected from at least one of styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, epoxy resin, polyvinyl alcohol, polyimide, polyamide-imide, polyacrylamide, polyacrylic acid, and their salts.

[0050] In some embodiments of the present application, the carbon nanotubes 120 comprise 0.01% to 5% by mass of the active material layer 12. The active material layer 12 also comprises 75% to 95% by mass of a carbon material, 1% to 20% by mass of a silicon material, 0% to 5% by mass of a thickener, and 1% to 5% by mass of a binder. In other words, the negative electrode sheet 10 is a silicon-carbon composite negative electrode sheet. While ensuring the conductivity and adhesion of the active material layer 12, the low binder content in the active material layer 12 can improve the battery's energy density.

[0051] The mass ratio of the silicon material to the carbon material is (5% to 20%):(80% to 95%). The silicon material may be at least one of nano-silicon, silicon oxide, silicon monoxide, and a silicon-containing alloy. The carbon material may be hard carbon, soft carbon, natural graphite, artificial graphite, or mesophase carbon microbeads. The thickener is selected from at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and cellulose. The binder is selected from at least one of styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, epoxy resin, polyvinyl alcohol, polyimide, polyamide-imide, polyacrylamide, polyacrylic acid, and salts thereof.

[0052] like Figure 2 As shown, the embodiment of the present application further provides an electrode assembly 100, which includes a positive electrode sheet 20 and a negative electrode sheet 10. The electrode assembly 100 is formed by laminating or winding the positive electrode sheet 20 and the negative electrode sheet 10. Furthermore, the electrode assembly 100 also includes a separator 30 located between the positive electrode sheet 20 and the negative electrode sheet 10.

[0053] The embodiment of the present application further provides a battery 200 , wherein the battery 200 includes the electrode assembly 100 . The battery 200 can be obtained after the electrode assembly 100 is injected with liquid, packaged, and formed.

[0054] like Figure 3 As shown, an embodiment of the present application further provides an electronic device 300, which includes the battery 200. The electronic device 300 may be a consumer electronic product, such as a smartphone. It is understood that in other embodiments, the electronic device 300 is not limited to the smartphone shown in the figure, but may also be a power tool, energy storage device, power device, etc. For example, the electronic device 300 may also be an electric car.

[0055] The present application is described in detail below through examples and comparative examples.

[0056] Example 1

[0057] The first step is to prepare a thickener CMC glue solution: CMC powder is added to deionized water to adjust the solid content of the glue solution to 1.5%, and the mixture is slowly stirred at a speed of 500 rpm for 5 hours to obtain a CMC glue solution;

[0058] The second step is to prepare a conductive adhesive: add carbon nanotubes with a length greater than 3 μm into the CMC adhesive and stir rapidly at a speed of 2000 rpm for 1 hour to obtain a conductive adhesive;

[0059] The third step is to add silicon material and carbon material: artificial graphite and silicon oxide are added to the conductive glue at the same time, and after slowly stirring at a speed of 30 rpm for 30 minutes, deionized water is added to adjust the solid content of the conductive glue to 65%, and then kneading at a speed of 30 rpm for 1.5 hours;

[0060] The fourth step is to add polyacrylic acid binder: polyacrylic acid is continued to be added to the conductive adhesive containing silicon material and carbon material, and slowly stirred at a speed of 500 rpm for 0.5 h. Deionized water is added to adjust the solid content of the conductive adhesive to 45%, and then rapidly stirred at a speed of 2000 rpm for 2 h to obtain a slurry, wherein the mass ratio of carbon material, silicon material, conductive agent, binder and CMC is 86:9.5:0.5:3:1;

[0061] Step 5, sieving: filtering the slurry through a 150-mesh sieve to obtain an active material layer slurry;

[0062] Step 6: Preparation of the first coating slurry: Conductive agent Super P (SP), adhesive polyacrylic acid, and thickener CMC were mixed uniformly in a mass ratio of 60:35:5, and deionized water was added to adjust the solid content of the conductive adhesive to 15%. The mixture was then rapidly stirred at 1500 rpm for 1 hour to obtain the first coating slurry.

[0063] Step 7: Preparation of the second coating slurry: The conductive agent CNT, the dispersant CMC, and the adhesive polyacrylic acid were dispersed and stirred at 2000 rpm in a mass ratio of 85:12:3 for 2 hours, deionized water was added to adjust the solid content to 5%, and then rapidly stirred at 2000 rpm for 1 hour to obtain the second coating slurry;

[0064] Step 8: Applying the first coating layer: Apply the first coating layer slurry on the surface of the copper foil negative electrode current collector with a thickness of 10 μm at a coating speed of 2 m / min. During the coating process, the slurry is placed in an oven with a fan for heating and drying. The fan speed is 30 m / s and the oven temperature is 100°C. After heating and drying, a first coating layer with a thickness of 3.5 μm is obtained.

[0065] Step 9, applying the second coating layer: applying the second coating layer slurry on the surface of the first coating layer at a coating speed of 3 m / min. During coating, the coating layer is placed in an oven with a fan for heating and drying. The fan speed is 30 m / s and the oven temperature is 100°C. After heating and drying, a second coating layer with a thickness of 0.5 μm is obtained.

[0066] The tenth step is to coat the active material layer: the active material layer slurry obtained after filtration is coated on the surface of the second coating layer at a coating speed of 5 m / min. During coating, it is placed in an oven and blown with a fan to achieve heating and drying. The fan speed is 30 m / s and the oven temperature is 100°C. After heating and drying, the negative electrode sheet is obtained.

[0067] Example 2

[0068] The preparation method is substantially the same as that of Example 1, except that the conductive agent of the first coating layer is CNT.

[0069] Example 3

[0070] The preparation method is substantially the same as that of Example 1, except that the conductive agent of the first coating layer is VGCF.

[0071] Example 4

[0072] The preparation method is substantially the same as that of Example 1, except that the conductive agent of the first coating layer is conductive graphite.

[0073] Example 5

[0074] The preparation method is substantially the same as that of Example 1, except that the conductive agent of the first coating layer is acetylene black.

[0075] Example 6

[0076] The preparation method is substantially the same as that of Example 1, except that the content ratio of the conductive agent, the binder, and the thickener in the first coating layer is 45:50:5.

[0077] Example 7

[0078] The preparation method is substantially the same as that of Example 1, except that the content ratio of the conductive agent, the binder, and the thickener in the first coating layer is 55:40:5.

[0079] Example 8

[0080] The preparation method is substantially the same as that of Example 1, except that the content ratio of the conductive agent, the binder, and the thickener in the first coating layer is 70:25:5.

[0081] Example 9

[0082] The preparation method is substantially the same as that of Example 1, except that the content ratio of the conductive agent, the dispersant, and the binder in the second coating layer is 75:15:10.

[0083] Example 10

[0084] The preparation method is substantially the same as that of Example 1, except that the content ratio of the conductive agent, the dispersant, and the binder in the second coating layer is 80:12:8.

[0085] Example 11

[0086] The preparation method is substantially the same as that of Example 1, except that the content ratio of the conductive agent, the dispersant, and the binder in the second coating layer is 90:7:3.

[0087] Example 12

[0088] The preparation method is substantially the same as that of Example 1, except that the length of the carbon nanotubes in the second coating layer is 1 μm.

[0089] Example 13

[0090] The preparation method is substantially the same as that of Example 1, except that the length of the carbon nanotubes in the second coating layer is 5 μm.

[0091] Example 14

[0092] The preparation method is substantially the same as that of Example 1, except that the length of the carbon nanotubes in the second coating layer is 7 μm.

[0093] Example 15

[0094] The preparation method is substantially the same as that of Example 1, except that the length of the carbon nanotubes in the second coating layer is 10 μm.

[0095] Example 16

[0096] The preparation method is substantially the same as that of Example 1, except that the diameter of the carbon nanotubes in the second coating layer is 6 nm.

[0097] Example 17

[0098] The preparation method is substantially the same as that of Example 1, except that the diameter of the carbon nanotubes in the second coating layer is 15 nm.

[0099] Example 18

[0100] The preparation method is substantially the same as that of Example 1, except that the diameter of the carbon nanotubes in the second coating layer is 40 nm.

[0101] Example 19

[0102] The preparation method is substantially the same as that of Example 1, except that the length of the carbon nanotubes in the active material layer is 1 μm.

[0103] Example 20

[0104] The preparation method is substantially the same as that of Example 1, except that the length of the carbon nanotubes in the active material layer is 5 μm.

[0105] Example 21

[0106] The preparation method is substantially the same as that of Example 1, except that the length of the carbon nanotubes in the active material layer is 10 μm.

[0107] Example 22

[0108] The preparation method is substantially the same as that of Example 1, except that the thickness of the second coating layer is 150 nm.

[0109] Example 23

[0110] The preparation method is substantially the same as that of Example 1, except that the thickness of the second coating layer is 200 nm.

[0111] Example 24

[0112] The preparation method is substantially the same as that of Example 1, except that the thickness of the second coating layer is 800 nm.

[0113] Example 25

[0114] The preparation method is substantially the same as that of Example 1, except that the thickness of the second coating layer is 1000 nm.

[0115] Example 22

[0116] The preparation method is substantially the same as that of Example 1, except that the thickness of the second coating layer is 1200 nm.

[0117] Comparative Example 1

[0118] The preparation method is substantially the same as that of Example 1, except that the type of conductive agent in the second coating layer is SP.

[0119] Comparative Example 2

[0120] The preparation method is substantially the same as that of Example 1, except that the type of conductive agent in the second coating layer is VGCF.

[0121] Comparative Example 3

[0122] The preparation method is substantially the same as that of Example 1, except that the second coating layer is not included.

[0123] Comparative Example 4

[0124] The preparation method is substantially the same as that of Example 1, except that the type of the conductive agent in the active material layer is SP.

[0125] Comparative Example 5

[0126] The preparation method is substantially the same as that of Example 1, except that the type of conductive agent in the active material layer is VGCF.

[0127] The negative electrode sheet of Example 1 was tested by scanning electron microscopy at the cross section. The results are as follows: Figure 4 As shown. Figure 4 It can be seen from the figure that the carbon nanotubes are entangled with each other between the active material layer and the secondary coating of the negative electrode sheet.

[0128] The adhesive force and film resistance of the negative electrode sheets of Examples 1-26 and Comparative Examples 1-5 were tested. Lithium-ion batteries were prepared using the negative electrode sheets of Examples 1-26 and Comparative Examples 1-5, and the cycle performance of each lithium-ion battery was tested.

[0129] Among them, the testing method of the bonding force is as follows: the first step is to cut each negative electrode sheet with a width of 30mm and a length of 100-160mm as the negative electrode sheet to be tested, and stick the negative electrode sheet to the steel plate through a special double-sided tape (width 20mm, length 90-150mm), with the sample test surface facing down. Then, insert the paper tape under the negative electrode sheet and fix it with wrinkle glue. The width of the paper tape is equal to that of the negative electrode sheet, and the length is 80-200mm greater than the length of the negative electrode sheet. The second step is to use a high-speed rail tensile machine (force sensor parameter is 500N), fix the end of the steel plate without the negative electrode sheet with the lower clamp of the high-speed rail tensile machine, fold the paper tape upwards, fix it with the upper clamp of the high-speed rail tensile machine, and then stretch it. Stop stretching when the curve in the operation interface is flat and the displacement is greater than 70mm. Read the average value of the flat part of the curve, which is the bonding force of the negative electrode sheet.

[0130] The test method of the diaphragm resistance is as follows: the cross-sectional area is 42.5mm 2 Each negative electrode sheet is used as the negative electrode sheet to be tested, and the negative electrode sheet is placed between the two terminals of the diaphragm resistance tester. The controller handle is pressed down to make the terminals completely contact with the coating area of the negative electrode sheet. The pressure is 5.0±0.3Kgf. The test time is 15s, and the diaphragm resistance is read.

[0131] The test method for the cycling performance is as follows: at temperatures of 25°C and 45°C, each lithium-ion battery is charged to 4.45V at a rate of 0.7C and discharged to 3.0V at a rate of 0.7C, and a full charge and discharge cycle test is performed until the capacity of the lithium-ion battery is less than 80% of the initial capacity, and the number of cycles is recorded.

[0132] The test results are shown in Table 1.

[0133] Table 1 Main preparation parameters and properties of Examples and Comparative Examples

[0134]

[0135]

[0136] As shown in Table 1, compared to Comparative Examples 1-5, the negative electrode sheets of Examples 1-26 have a double coating, with carbon nanotubes used as the conductive agent in both the secondary coating and the active material layer. This facilitates entanglement between the carbon nanotubes in the active material layer and the secondary coating, resulting in a higher adhesion strength. Furthermore, the negative electrode sheets have lower sheet resistance and higher conductivity, resulting in better cycling performance for the corresponding lithium-ion batteries.

[0137] Among them, under the condition that the negative electrode sheet meets the requirements of double coating and the conductive agents of the second coating layer and the active material layer are both carbon nanotubes, the length of the carbon nanotubes in the second coating layer will affect the interfacial adhesion of the negative electrode sheet.

[0138] Compared with Example 12, the carbon nanotubes in the second coating layer of the negative electrode sheets of Examples 1 and 13-15 are longer (not less than 3 μm) and therefore have greater adhesion.

[0139] Secondly, the content of the binder in the first coating layer and the second coating layer will also affect the conductivity of the negative electrode sheet.

[0140] Compared with Examples 9 and 10, since the binder content of the second coating layer in the negative electrode sheets of Examples 1 and 11 is lower, the negative electrode sheets have lower sheet resistance and higher conductivity, and the corresponding lithium-ion battery has better cycle performance.

[0141] Furthermore, the thickness of the second coating layer also affects the conductivity of the negative electrode sheet. Compared to Examples 22 and 26, the thickness of the second coating layer in the negative electrode sheets of Examples 1, 23-25 is 200-100 nanometers, which can reduce the sheet resistance of the negative electrode sheet and improve conductivity and cycle performance.

[0142] Thirdly, the length of the carbon nanotubes in the active material layer affects the interfacial adhesion of the negative electrode sheet. Compared to Example 19, the carbon nanotubes in the active material layer of the negative electrode sheets of Examples 1, 20-21 are longer. This facilitates the interconnection between the carbon nanotubes in the active material layer and the carbon nanotubes in the secondary coating, resulting in higher interfacial adhesion of the negative electrode sheet.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, ordinary technicians in this field should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the scope of protection of the technical solutions of the present application.

Claims

1. A negative electrode sheet comprising a current collector and an active material layer located on the current collector; It is characterized in that The active material layer includes silicon material, and the negative electrode sheet further includes: a first coating layer located between the current collector and the active material layer; and a second coating layer disposed between the first coating layer and the active material layer; The active material layer and the second coating layer include carbon nanotubes, the carbon nanotubes of the active material layer and the carbon nanotubes of the second coating layer are interconnected by winding and interweaving, and the thickness of the second coating layer is 200nm-1000nm.

2. The negative electrode sheet according to claim 1, wherein: The carbon nanotubes have a length of no less than 3 μm and a diameter of no more than 30 nm.

3. The negative electrode sheet according to claim 1, wherein: The mass percentage of the carbon nanotubes in the second coating layer is 75%-90%. The second coating layer further comprises a dispersant with a mass percentage of 5%-15% and a binder with a mass percentage of 1%-10%.

4. The negative electrode sheet according to claim 1, wherein The thickness of the first coating layer is 1 μm-5 μm.

5. The negative electrode sheet according to claim 1, wherein: The first coating layer includes 45%-70% by mass of a conductive agent, 1%-5% by mass of a thickener, and 25%-50% by mass of a binder.

6. The negative electrode sheet according to claim 1, wherein: The mass percentage of the carbon nanotubes in the active material layer is 0.01%-5%, the mass percentage of the silicon material in the active material layer is 1%-20%, and the active material layer also includes a carbon material with a mass percentage of 75-95%, a thickener with a mass percentage of 0%-5%, and a binder with a mass percentage of 1-5%.

7. An electrode assembly comprising a positive electrode sheet and a separator, characterized in that: The electrode assembly further comprises a negative electrode sheet according to any one of claims 1 to 6, and the electrode assembly is formed by laminating or winding the positive electrode sheet, the separator, and the negative electrode sheet. A battery comprising the electrode assembly according to claim 7 .

9. An electronic device comprising the battery according to claim 8.

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