Dry negative electrode sheet, secondary battery and electric device

Abstract

The invention relates to a dry negative electrode sheet, a secondary battery and an electric device. The dry-method negative electrode plate comprises a current collector and a dry electrode film located on the surface of the current collector, the dry electrode film comprises a negative electrode active material and a binder, the negative electrode active material comprises a carbon-based material and a silicon-based material, the sphericity degree of the carbon-based material is 0.85-0.98, the sphericity degree of the silicon-based material is 0.65-0.9, the binder comprises a first binder and a second binder, and the first binder and the second binder are arranged on the surface of the current collector. The first binder comprises polytetrafluoroethylene, and the second binder comprises one or more of polyvinylidene fluoride, a polyvinylidene fluoride-hexafluoropropylene copolymer, polyolefin, polyethylene oxide and carboxymethyl cellulose; and the pole piece cohesion of the dry-method negative pole piece under the condition of 0% SOC is greater than or equal to 20N / m. According to the dry-method negative electrode plate, the cohesion of the electrode plate of a graphite silicon-doped system can be improved, and the cycle performance of a battery cell is improved.

Description

Technical Field

[0001] The present application relates to the field of batteries. Specifically, the present application relates to a dry-type negative electrode sheet, a secondary battery, and an electrical device. Background Art

[0002] Compared with traditional wet electrodes, dry electrodes eliminate the use of solvents in the electrode mixing and processing procedures, avoiding the influence of the floating, migration of the binder and solvent residues on the electrode and cell performance during the baking process. However, due to different processing methods, there are some significant differences in the electrode performance between dry electrodes and wet electrodes. For example, in wet electrodes, the binder is dissolved and stirred evenly and then mixed with the electrode material, so the distribution of the binder in each part of the electrode active layer is relatively uniform, and there is no orientation related to the processing direction in the mechanical properties during the coating process. In contrast, in the processing of dry electrodes, in order to avoid using organic solvents and obtain an electrode film that can be independently self-supporting without a current collector, a binder material with fibrillating ability needs to be used. The fibrillating and drawing of the binder wind around the main material and auxiliary material particles such as conductive agents, so that the negative electrode sheet can be independently self-supporting and formed. However, due to the fibrillating and drawing characteristics of the binder, during the processing of dry electrodes mainly by calendering, the main body of the negative electrode sheet undergoes slip between the main material particles due to the calendering force, resulting in further aggravation of the fibrillating and drawing of the binder. Therefore, when testing the mechanical properties of the prepared negative electrode sheet, there will be a significant stress orientation problem, that is, the mechanical properties of the negative electrode sheet along the processing orientation are much greater than those of the negative electrode sheet in other orientation directions. A typical test index for this problem is the cohesive force of the negative electrode sheet. However, existing literature and patent work basically focus on the degree of dispersion uniformity of dry electrode materials, and rarely focus on the improvement of the internal cohesive force of the electrode. Summary of the Invention

[0003] The purpose of the present application is to provide a dry-type negative electrode sheet, which has higher cohesive force compared with traditional pure graphite dry-type negative electrode sheets, can effectively resist the expansion of the thickness of the negative electrode sheet after full charge in a certain amount of silicon-doped system, and improve the problems of negative electrode sheet coating peeling and negative electrode sheet not being resistant to pressure caused by insufficient internal cohesive force of the negative electrode sheet during the cycling and use of the cell, thereby greatly improving the cycling performance of the cell using the dry-type negative electrode sheet.

[0004] The first aspect of the present application provides a dry-type negative electrode sheet, which includes a current collector and a dry electrode film located on the surface of the current collector. The dry electrode film includes a negative electrode active material and a binder. The negative electrode active material includes a carbon-based material and a silicon-based material. The sphericity of the carbon-based material is 0.85 to 0.98, and the sphericity of the silicon-based material is 0.65 to 0.9. The binder includes a first binder and a second binder. The first binder includes polytetrafluoroethylene, and the second binder includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyolefin, polyethylene oxide, and carboxymethyl cellulose; the cohesive force within the electrode sheet of the dry-type negative electrode sheet under 0% SOC conditions is ≥20 N / m.

[0005] The second aspect of the present application provides a secondary battery, which includes the dry-type negative electrode sheet described in the first aspect.

[0006] The third aspect of the present application provides an electrical device, which includes the secondary battery described in the second aspect.

[0007] The beneficial effects of the present application are as follows:

[0008] The dry-type negative electrode sheet of the present application can improve the cohesive force within the electrode sheet of the graphite-doped silicon system and improve the cycle performance of the battery cell. Description of the Drawings

[0009] Figure 1 It is a graph of the cycle capacity retention rate of the lithium-ion batteries in Examples 6, 9-10 and Comparative Examples 9-11 of the present application. Detailed Embodiments

[0010] For the sake of simplicity, the present application only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly recorded; and any lower limit can be combined with other lower limits to form a range not explicitly recorded. Similarly, any upper limit can be combined with any other upper limit to form a range not explicitly recorded. In addition, each individually disclosed point or single numerical value itself can be used as a lower limit or an upper limit and combined with any other point or single numerical value or combined with other lower limits or upper limits to form a range not explicitly recorded.

[0011] Unless otherwise specified, the terms used in the present application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in the present application can be measured by various commonly used measurement methods in the art (for example, they can be tested according to the methods given in the embodiments of the present application).

[0012] A list of items joined by the term "at least one of", "at least one kind of", or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C.

[0013] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and not to limit the scope of the present application.

[0014] I. Dry-type negative electrode sheet

[0015] The dry-type negative electrode sheet provided by the present application includes a current collector and a dry electrode film located on the surface of the current collector. The dry electrode film includes a negative electrode active material and a binder. The negative electrode active material includes a carbon-based material and a silicon-based material. The sphericity of the carbon-based material is 0.85 - 0.98, and the sphericity of the silicon-based material is 0.65 - 0.9. The binder includes a first binder and a second binder. The first binder includes polytetrafluoroethylene, and the second binder includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyolefin, polyethylene oxide, and carboxymethyl cellulose; the cohesive force of the dry-type negative electrode sheet within the electrode sheet under 0% SOC conditions is ≥20 N / m.

[0016] In some embodiments, the dry electrode film does not contain a solvent, specifically does not contain detectable processing solvents, processing solvent residues, and processing solvent impurities. These processing solvents include, but are not limited to, water, pyrrolidone, acetate, ketone, alcohol, diol, toluene, and xylene.

[0017] In some embodiments, the cohesive force of the dry-type negative electrode sheet under 0% SOC condition is ≥ 20 N / m, such as 20 N / m, 25 N / m, 30 N / m, 35 N / m, 40 N / m, 45 N / m, 50 N / m or any range therebetween. In some preferred embodiments, the cohesive force of the dry-type negative electrode sheet under 0% SOC condition is ≥ 30 N / m. In some silicon-doped negative electrode systems, when the cohesive force of the negative electrode sheet is insufficient, due to the large volume change generated during the lithium insertion and extraction of the negative electrode, the electrode sheet detaches from the current collector; or the negative electrode sheet is extruded by the positive electrode sheet with greater cohesive force during the cyclic use of the battery cell, resulting in a decrease in the porosity inside the negative electrode sheet, thereby causing a rapid decline in the cyclic capacity retention rate of the battery cell. The cohesive force of the dry-type negative electrode sheet under 0% SOC condition is generally ≤ 140 N / m, such as ≤ 140 N / m, ≤ 130 N / m, ≤ 120 N / m, ≤ 110 N / m, ≤ 100 N / m, ≤ 90 N / m, ≤ 80 N / m, ≤ 70 N / m, ≤ 60 N / m. In some embodiments, the cohesive force of the dry-type negative electrode sheet under 0% SOC condition is 20 N / m - 140 N / m.

[0018] In some embodiments, the sphericity of the carbon-based material is exemplarily 0.85, 0.88, 0.90, 0.92, 0.95, 0.98 or any range therebetween. In some preferred embodiments, the sphericity of the carbon-based material is 0.92 - 0.98.

[0019] In some embodiments, the sphericity of the silicon-based material is exemplarily 0.65, 0.70, 0.75, 0.80, 0.85, 0.90 or any range therebetween. In some embodiments, the silicon-based material is a silicon oxide compound, and the sphericity of the silicon oxide compound is 0.65 - 0.85, such as 0.65, 0.70, 0.75, 0.80, 0.85 or any range therebetween. In some embodiments, the silicon-based material is a silicon-carbon composite, and the sphericity of the silicon-carbon composite is 0.7 - 0.9, such as 0.70, 0.75, 0.80, 0.85, 0.90 or any range therebetween.

[0020] In some embodiments, the D50 of the carbon-based material is 7 μm - 15 μm, the D90 is 10 μm - 25 μm, the D50 is exemplarily 7 μm, 10 μm, 12 μm, 15 μm or any range therebetween, and the D90 is exemplarily 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm or any range therebetween.

[0021] In some embodiments, the D50 of the silicon-based material is 2 μm to 8 μm, the D90 is 6 μm to 15 μm, the D50 is exemplarily 2 μm, 5 μm, 8 μm or any range therebetween, and the D90 is exemplarily 6 μm, 8 μm, 10 μm, 12 μm, 15 μm or any range therebetween.

[0022] If the D50 is too small, the specific surface area of the active material particles is too large, which easily leads to insufficient coverage of the binder and reduces the internal cohesion of the electrode sheet. If the D50 is too large, the active material layer is relatively rough, the compaction effect becomes poor, and interface detachment may occur. If the D90 is too large, the active material particles are likely to form defects in the coating, affecting the conductivity and mechanical strength. If the D90 is too small, the possibility of particle agglomeration increases, resulting in uneven distribution of the active layer components and affecting the internal cohesion of the electrode sheet.

[0023] In some embodiments, the carbon-based material includes at least one of artificial graphite and natural graphite, and the silicon-based material includes at least one of silicon oxides and silicon-carbon composites. In some embodiments, based on the mass of the negative electrode active material, the mass content of the silicon-based material is 1% - 15%, exemplarily 1%, 3%, 5%, 8%, 10%, 12%, 15% or any range therebetween. In some embodiments, based on the mass of the negative electrode active material, the mass content of the silicon-based material is 8% - 15%.

[0024] In some embodiments, the weight-average molecular weight of the second binder is 300,000 - 1,600,000, the D50 particle size is 3 μm - 25 μm, the weight-average molecular weight is exemplarily 300,000, 500,000, 800,000, 1,000,000, 1,200,000, 1,400,000, 1,600,000 or any range therebetween, and the D50 particle size is exemplarily 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm or any range therebetween. In some embodiments, the polyolefin includes, but is not limited to, polyethylene.

[0025] In a preferred embodiment, the second binder is polyethylene, the weight-average molecular weight of the polyethylene is 300,000 - 800,000, and the D50 particle size is 15 μm - 25 μm. In another preferred embodiment, the second binder is polyvinylidene fluoride, the weight-average molecular weight of the polyvinylidene fluoride is 1,000,000 - 1,600,000, and the D50 particle size is 3 μm - 8 μm.

[0026] In some embodiments, the compaction density of the dry-process negative electrode sheet is 1.40 g / cm 3 ~1.65 g / cm 3 ,exemplarily 1.40 g / cm 3 、1.45 g / cm 3 、1.50 g / cm3 、 1.55 g / cm 3 、 1.60 g / cm 3 、 1.65 g / cm 3 or any range therebetween. The compaction density in this application refers to the double-sided compaction density of the dry-type negative electrode sheet.

[0027] In some embodiments, the method for preparing the dry electrode film includes the step of roll-pressing the electrode mixture into a film. The electrode mixture can be mixed by one or more of high-speed dispersion, twin-screw extrusion, and jet mill for the materials containing the active material and the binder, and preferably the twin-screw extrusion mixing method is adopted. Among them, the mixing of the twin-screw extruder mainly depends on the pin structure on the screw and the barrel wall of the barrel, and kneading is carried out while transporting the materials forward. The rotation speed of the screw is usually slow, generally between 10 rpm and 100 rpm, to provide appropriate shear force; high-speed dispersion mainly depends on the high-speed rotation of the motor to drive the rotation of the dispersion blade, and its typical rotation speed is between 1000 rpm and 10000 rpm. In some embodiments, the temperature of the roll-pressing into a film is 80°C - 110°C, such as 80°C, 90°C, 100°C, 110°C or any range therebetween.

[0028] In some embodiments, the method for preparing the dry-type negative electrode sheet includes the step of thermally compounding the dry electrode film with the current collector, and the temperature of the thermal compounding is 100°C - 120°C, such as 100°C, 105°C, 110°C, 115°C, 120°C or any range therebetween.

[0029] In some embodiments, the dry-type negative electrode sheet is prepared by a method including the following steps

[0030] (I) Mix the negative electrode active material, the conductive agent, the first binder, and the second binder to obtain mixture A;

[0031] (II) Roll-press mixture A into a film to form a self-supporting film;

[0032] (III) Thermally compound the self-supporting film with the current collector to obtain the dry-type negative electrode sheet.

[0033] In step (I), the mixing preferably adopts the twin-screw extrusion mixing method. In step (II), the temperature of the roll-pressing into a film is exemplarily 80°C - 110°C. In step (III), the temperature of the thermal compounding is preferably 100°C - 120°C.

[0034] II. Secondary battery

[0035] In some embodiments, the secondary battery includes a positive electrode sheet, a dry negative electrode sheet, and an electrolyte. In some embodiments, the positive electrode sheet is a wet positive electrode sheet, which is obtained by a conventional wet process. In some embodiments, the positive electrode sheet is a dry positive electrode sheet, which is obtained by a dry process.

[0036] In some embodiments, the secondary battery may include an outer package, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.; or it may be a soft package, such as a pouch soft package, and non-limiting examples of the material of the soft package include polypropylene, polybutylene terephthalate, and polybutylene succinate. In the present application, the shape of the secondary battery is not particularly limited, and it may be cylindrical, square, or any other shape.

[0037] In some embodiments, the present application also provides a battery module. The battery module includes the above-mentioned secondary battery. Since the battery module of the present application adopts the above-mentioned secondary battery, it has at least the same advantages as the secondary battery. The number of secondary batteries included in the battery module of the present application may be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0038] In some embodiments, the present application also provides a battery pack, which includes the above-mentioned battery module. The number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0039] III. Electrical device

[0040] The present application also provides an electrical device, which includes at least one of the above-mentioned secondary battery, battery module, or battery pack.

[0041] In some embodiments, the electrical device includes, but is not limited to: electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems, etc. In order to meet the high power and high energy density requirements of the device for the secondary battery, a battery pack or a battery module can be adopted.

[0042] In some other embodiments, the electrical device may be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires thinness and lightness, and a secondary battery can be used as the power source.

[0043] Examples and comparative examples

[0044] In the present application, unless otherwise specified, the materials or reagents used are all commercially available.

[0045] Testing methods

[0046] 1. Cohesion test

[0047] The lithium-ion batteries of each example and comparative example were discharged at a constant current of 0.1C to 2.5V, with a cut-off current of 0.05C. The lithium-ion batteries were disassembled in a glove box filled with argon to obtain dry-type negative electrode sheets. The obtained dry-type negative electrode sheets were cut into samples with dimensions of 200 ± 50 mm (length) × 15 ± 0.5 mm (width) using a slicing knife, and were soaked and cleaned with a low-boiling-point dimethyl carbonate (DMC) solvent for 1 hour, then air-dried for standby.

[0048] The active material layer side of the electrode sheet samples of each example and comparative example was adhered to one end of a stainless steel plate that had been cleaned and dried with alcohol using double-sided tape with a width of 18 mm. The size of the stainless steel plate was 100 mm × 40 mm × 2 mm, and the adhesion length of the electrode sheet on the stainless steel plate was 75 ± 5 mm. The width of the double-sided tape should be greater than the width of the electrode sheet sample. This adhesion area was denoted as the bottom adhesion layer. Then, a transparent tape with a width of 12.7 mm was pasted onto the active material surface of the electrode sheet sample that had not been adhered with double-sided tape, with an adhesion length of 75 ± 5 mm. This adhesion area was denoted as the surface adhesion layer.

[0049] A roller with a diameter of 120 mm and a mass of 3 kg was used to apply uniform pressure to the adhesion surface between the electrode sheet and the stainless steel plate, reciprocating 5 times to ensure firm adhesion. Then, the end of the electrode sheet was folded 180°, and about 1 cm of the transparent tape of the surface adhesion layer was manually removed with a blade to form a peeling end. Then, the peeling end of the electrode sheet was fixed to the upper fixture of the testing machine, and the stainless steel plate was fixed to the lower fixture. The testing machine should be equipped with a 10N sensor to ensure accurate measurement of the cohesive force. A peeling test was carried out at a tensile speed of 100 mm / min, and the effective peeling length should not be less than 60 mm. The force-displacement data was recorded during the test. The data of the stroke part where the electrode sheet was not stressed in the obtained force-displacement curve (≤ 70% of the average value of the total stroke force) was removed, and then the average value of the internal force fluctuations of the force load in the main body 50 mm displacement interval was taken as the average force of the electrode sheet. Then, this average value was divided by the width of the electrode sheet (at least 3 measurement points, with an accuracy of 0.1 mm) to obtain the cohesive force of the electrode sheet.

[0050] 2. Cycling performance test

[0051] The lithium-ion batteries of Examples 6, 9 - 10 and Comparative Examples 9 - 11 were first charged at a constant current of 1C to 4.25V in an environment of 25°C, rested for 5 min, then discharged at a constant current of 1C to 2.5V, rested for 30 min, and then charged and discharged repeatedly in this way. The capacity retention rate of the battery cycle was recorded.

[0052] Comparative Example 1

[0053] Manufacture of dry-type negative electrode sheet: Add the first binder polytetrafluoroethylene (PTFE) to the negative electrode active material graphite, mix evenly at low temperature for 10 min under the condition of below 10 °C, then add the conductive agent carbon black, mix at low speed for 10 min at a linear speed of 0.5 m / s, add the second binder polyethylene (PE), and continue to mix evenly at low temperature for 10 min under the condition of below 10 °C. Among them, the D50 of graphite is 12.7 μm, the D90 is 20.5 μm, and the sphericity is 0.85; the D50 of the second binder PE is 17.2 μm, and the weight average molecular weight is 57.3 w; the mass ratio of the active material graphite, the first binder PTFE, the second binder PE and the conductive agent carbon black is 96:2.4:0.6:1. After mixing evenly, raise the temperature of the barrel to 85 °C ± 3 °C, carry out high-speed dispersion at a linear speed of 18 m / s, stop after mixing evenly for 30 min, take samples, classify through a 10-mesh sieve to obtain a dry electrode mixture. Feed the obtained electrode mixture powder into a calender (roll diameter 88 mm, roll temperature 110 °C, roll speed 20 rpm) through a blanking hopper for film formation and thinning, and cut off the uneven areas at the edges of the obtained self-supporting film to obtain a self-supporting film with smooth edges. Place the obtained self-supporting film on the surface of the carbon-coated copper foil, and thermally compound the self-supporting film on both sides to the two sides of the carbon-coated current collector at 120 °C in a thermal compounding device to obtain a dry-type negative electrode sheet, where the double-sided compaction density of the electrode sheet is controlled to be 1.60 g / cm 3 .

[0054] Manufacture of lithium secondary battery: Cut the obtained dry-type negative electrode sheet into a size of 55 mm in width × 69 mm in length, stack the sheets in the order of dry-type negative electrode sheet, separator, and wet-type ternary positive electrode sheet, with the separator in the middle of the positive and negative electrode sheets, and carry out lamination at a temperature of 100 °C, wind to obtain a bare battery core, place the bare battery core in an aluminum-plastic film outer package, inject electrolyte after baking in a vacuum baking oven at 60 °C for 24 h, and then obtain a lithium-ion battery after formation, degassing, and secondary sealing. Among them, the wet-type ternary positive electrode sheet is prepared by the following method: Mix the binder polyvinylidene fluoride (PVDF) with water, stir to obtain a colloidal solution, and sequentially add the active material LiNi 0.9 Co 0.05 Mn 0.05 O2 and the conductive agent carbon black into the colloidal solution, mix and stir to obtain a positive electrode slurry, where the weight ratio of the active material, the binder, and the conductive agent is 96:1.5:2.5, evenly coat the positive electrode slurry on the aluminum foil, and obtain a wet-type positive electrode sheet after baking, rolling, and slitting. The electrolyte is prepared as follows: Mix the solvents ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a mass ratio of 1:1:1, add lithium hexafluorophosphate (LiPF6), and prepare a solution with a lithium salt concentration of 1 mol / L.

[0055] Comparative Examples 2-4 and Examples 1-5

[0056] Comparative Examples 2-4 and Examples 1-5 are achieved by adjusting the active material, the sphericity of graphite, and the mixing method on the basis of Comparative Example 1. The specific adjustment measures and detailed data are shown in Table 1.

[0057] Table 1

[0058]

[0059] a : The D50 of silicon oxide is 5.3 μm, the D90 is 10.1 μm, and the sphericity is 0.85.

[0060] b : The D50 of silicon carbide is 6.7 μm, the D90 is 12.7 μm, and the sphericity is 0.85.

[0061] As can be seen from the data in Table 1, when mixing by the twin-screw extrusion method, since there is a kneading zone on the twin-screw extrusion screw, the material is forcibly conveyed in the barrel of the extruder, generating frictional forces both with the barrel wall and with the kneading structure on the screw. This makes the fiber orientation of the electrode mixture in the extruder more random and sufficient, resulting in a greater cohesive force of the dry-process negative electrode sheet. On the contrary, when mixing by the high-speed dispersion method, the material moves along one orientation of the dispersion paddle, resulting in less random orientation and thus a smaller cohesive force of the dry-process negative electrode sheet. In addition, increasing the sphericity of the graphite material also helps to improve the powder fluidity, reduce agglomeration, and enhance the coating uniformity and calendering consistency. When the sphericity is too low, it may lead to uneven mixing.

[0062] Examples 6-8 and Comparative Examples 5-8

[0063] Examples 6-8 and Comparative Examples 5-8 are achieved by adjusting the film-forming temperature, the thermal composite temperature, and the compaction density on the basis of Example 5. The specific adjustment measures and detailed data are shown in Table 2.

[0064] Table 2

[0065]

[0066]

[0067] As can be seen from the data in Table 2, the higher the temperature during thermal lamination, the greater the cohesive force of the electrode sheet after lamination. However, when the temperature of thermal lamination is too high, problems such as the self-supporting film sticking to the roller during lamination may occur, resulting in electrode sheet breakage and affecting the product appearance. When the temperature of thermal lamination is too low, it may not ensure good lamination between the self-supporting film and the current collector, and the interfacial resistance increases. The greater the compaction density, the greater the cohesive force of the electrode sheet. However, too large a compaction density may cause the particles of the active material to break, and problems such as calendering wrinkles appear in the tab area of the electrode sheet. Too low a compaction density will lead to a too large thickness of the electrode sheet, an increase in resistivity, and an over-thick battery cell, not meeting the design performance requirements.

[0068] Examples 6, 9 - 10 and Comparative Examples 9 - 12

[0069] Example 10 was achieved by adjusting the lamination temperature and the second binder on the basis of Comparative Example 10. The second binder used was polyvinylidene fluoride (PVDF) with a D50 particle size of 5.5 μm and a weight average molecular weight of 137.6w. Comparative Examples 9 - 10 were achieved by adjusting the particle size, weight average molecular weight, and mixing method of the second binder PE on the basis of Example 6. Comparative Example 11 was achieved by adjusting the type of active material and the mixing method on the basis of Example 10. Comparative Example 12 was a graphite negative electrode sheet obtained by a conventional wet process. The specific adjustment measures and detailed data are shown in Table 3.

[0070] Table 3

[0071]

[0072]

[0073] As can be seen from the data in Table 3, when the molecular weight of the second binder PE is too large, its cohesive force shows a significant decrease, and the cycle capacity retention rate shows signs of rapid decay. This may be because the increase in the molecular weight of PE results in the failure to reach the melting point temperature of PE during the film-forming stage, making it impossible for PE to be well distributed between the active materials to construct a cohesive network with sufficient strength. When the molecular weight of the second binder PE is too small, due to its too low melting softening point temperature, it is easy to cause the adhesion of the dry electrode sheet to the roller surface under the given temperature conditions, and then a complete dry electrode sheet cannot be obtained, affecting the manufacturing yield of the electrode sheet. Considering Examples 9 and 10, when the type of the second binder changes, the cohesive force results of the electrode sheet will also change partially. It can be found that under the same dosage conditions, the improvement effect of the cohesive force of the electrode sheet is that PE is better than PVDF, but there is no significant difference in the capacity retention rate during the early stage of cycling between the two second binders. Considering Comparative Example 11, which is a dry-process negative electrode sheet of pure graphite prepared by a high-speed dispersion method, its cohesive force is small, but the capacity retention rate during cycling is equivalent to that of the wet-process negative electrode in Comparative Example 12. This shows that for a pure dry-process graphite negative electrode, it is not necessary to control the cohesive force of the electrode sheet to the same level as that of the wet-process electrode sheet.

[0074] The data of the above examples and comparative examples show that there are significant differences in the cohesive force of the dry-process negative electrode sheet and the wet-process electrode sheet. However, for a negative electrode system with a small silicon doping amount, it is not necessary to reach the same level of cohesive force as the wet-process negative electrode. By optimizing the mixing method of the dry-process negative electrode, the type, molecular weight and addition amount of the second binder, improving the processing performance of the negative electrode active material such as sphericity, or optimizing the temperature and compaction density during the thermal composite of the electrode sheet, the cohesive force level of the electrode sheet can be improved. The final evaluation index of the cohesive force level is carried out in two types. One is the measurement of the cohesive force of the electrode sheet after formation and discharging to 0% SOC state to ensure that the battery cell can still maintain sufficient cohesive force during the subsequent test process. On the other hand, it is to evaluate the cycle capacity retention rate of the battery cell to determine the cycle charge and discharge ability of the battery cell under the current cohesive force state. According to the solution provided by the present invention, it can be applied to the dry-process negative electrode of a silicon-doped system with a certain mass fraction, and can effectively improve the cohesive force level of the electrode sheet and the cycle life of the battery cell.

[0075] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those of ordinary skill in the art will recognize that some modifications and changes can be made to the described embodiments without departing from the scope of the present application as described in the appended claims.

Claims

1. A dry-process negative electrode sheet, comprising a current collector and a dry electrode film located on the surface of the current collector, characterized in that: The dry electrode film includes a negative electrode active material and a binder, the negative electrode active material includes a carbon-based material and a silicon-based material, the sphericity of the carbon-based material is 0.85-0.98, the sphericity of the silicon-based material is 0.65-0.9, the binder includes a first binder and a second binder, the first binder includes polytetrafluoroethylene, and the second binder includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyolefin, polyethylene oxide and carboxymethyl cellulose; the dry negative electrode sheet has a sheet cohesion of ≥20N / m under 0% SOC conditions.

2. The secondary battery according to claim 1, characterized in that: The D50 of the carbon-based material is 7 μm to 15 μm, and the D90 is 10 μm to 25 μm; the D50 of the silicon-based material is 2 μm to 8 μm, and the D90 is 6 μm to 15 μm.

3. The secondary battery according to claim 1 or 2, characterized in that: Based on the mass of the negative electrode active material, the mass content of the silicon-based material is 1%-15%, and the silicon-based material is selected from silicon-oxygen compounds and / or silicon-carbon composites.

4. The secondary battery according to claim 1 or 2, characterized in that: The weight average molecular weight of the second binder is 30w-160w, and the D50 particle size is 3μm-25μm.

5. The secondary battery according to claim 1 or 2, characterized in that: The preparation method of the dry electrode film comprises the step of rolling an electrode mixture into a film, wherein the electrode mixture is obtained by a mixing method of twin-screw extrusion.

6. The secondary battery according to claim 1 or 2, characterized in that: The dry-process negative electrode sheet has a cohesive force of ≥30 N / m under 0% SOC conditions.

7. The secondary battery according to claim 6, characterized in that: The second binder is polyethylene, the weight average molecular weight of the polyethylene is 30w-80w, and the D50 particle size is 15μm-25μm; or, The second binder is polyvinylidene fluoride, and the weight average molecular weight of the polyvinylidene fluoride is 100w-160w, and the D50 particle size is 3μm-8μm.

8. The secondary battery according to claim 6, characterized in that: The method for preparing the dry negative electrode sheet comprises the step of thermally combining the dry electrode film with the current collector, wherein the temperature of the thermal combination is 100° C.-120° C.

9. The secondary battery according to claim 1 or 2, characterized in that: The compaction density of the dry-process negative electrode sheet is 1.40 g / cm 3 ~1.65g / cm 3 .

10. A secondary battery comprising the dry-process negative electrode sheet according to any one of claims 1 to 9.

11. An electric device comprising the secondary battery according to claim 10.

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