Silicon-carbon negative electrode slurry, preparation method and application thereof

Abstract

The present application relates to lithium ion battery, disclose a kind of preparation method of silicon-carbon negative electrode slurry, comprising (1) silicon-based material is mixed with single-walled carbon nanotube, conductive carbon black is stirred in vacuum, carboxymethyl cellulose sodium is added and continue to be stirred evenly to obtain first slurry;(2) polyacrylic acid is added to first slurry and stirred in vacuum, and evenly stirred to obtain second slurry;(3) graphite material and polyacrylic acid are added to second slurry and stirred in vacuum, continue to stir, disperse after adding water to obtain third slurry;(4) when the viscosity of third slurry is 4000-8000mpa·s, SBR is added and stirred in vacuum, and then dispersed to obtain fourth slurry;(5) after defoaming treatment to fourth slurry, stirring, screening to obtain silicon-carbon negative electrode slurry.The beneficial effects of the present application are that the prepared silicon-carbon negative electrode slurry can effectively alleviate the volume expansion of silicon particles during the charging and discharging process of silicon-carbon negative electrode, avoid damaging the overall structure of silicon-carbon negative electrode, thereby improving the cycle performance of battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a silicon-carbon negative electrode slurry and a preparation method and application thereof. BACKGROUND

[0002] The capacity of the graphite negative electrode widely used in the current commercial lithium battery is about 360 mAh / g, which is very close to its theoretical specific capacity (372 mAh / g). It is difficult to improve the energy density of the battery on the graphite negative electrode, so people turn their attention to silicon materials. Silicon is the second most abundant element in the earth's crust, accounting for 25.7% of the total mass of the earth's crust. It is abundant in source and cheap in price, and is environmentally friendly, so the research on silicon negative electrode materials has attracted widespread attention. However, the commercial application of silicon negative electrode materials is still limited by some factors. The silicon particles in the electrode material will be broken and pulverized during the lithium intercalation / deintercalation process, and the separation between the silicon particles and the substrate will occur, which will increase the internal resistance of the battery, rapidly reduce the capacity and deteriorate the cycle performance.

[0003] The current conventional method is to mix silicon materials with graphite. Under the premise of improving the capacity, the advantages of large specific surface area of graphite are used to buffer the volume expansion and shrinkage of silicon materials during the charging and discharging process. For example, the negative electrode sheet is prepared by using a silicon-carbon negative electrode slurry in the Chinese invention patent document with the publication number CN107819105A. Although this technology can alleviate the material pulverization and shedding caused by the volume expansion of silicon-based materials, the long-term volume expansion and shrinkage of silicon-based materials will cause the disconnection of the internal conductive network of the electrode, thereby reducing the cycle life of the battery. SUMMARY

[0004] The technical problem to be solved by the present application is how to provide a preparation method of a silicon-carbon negative electrode slurry, which can effectively alleviate the volume expansion of silicon particles during the charging and discharging process of the silicon-carbon negative electrode, avoid damaging the overall structure of the silicon-carbon negative electrode, and thus improve the cycle performance of the battery.

[0005] The present application solves the above technical problems by the following technical means:

[0006] The present application provides a preparation method of a silicon-carbon negative electrode slurry, which comprises the following steps:

[0007] (1) mixing silicon-based materials, single-walled carbon nanotubes and conductive carbon black, and performing vacuum stirring, adding sodium carboxymethyl cellulose and continuing to stir uniformly to obtain a first slurry;

[0008] (2) adding polyacrylic acid to the first slurry and performing vacuum stirring to obtain a second slurry;

[0009] (3) adding graphite material and polyacrylic acid into the second slurry and stirring under vacuum, and then adding water and continuing to stir and disperse to obtain a third slurry;

[0010] (4) when the viscosity of the third slurry reaches a set viscosity, adding styrene-butadiene rubber and stirring under vacuum, and then dispersing to obtain a fourth slurry;

[0011] (5) after defoaming treatment, stirring and sieving the fourth slurry to obtain a silicon-carbon negative electrode slurry.

[0012] Beneficial effects: The single-walled carbon nanotubes, conductive carbon black and silicon-based material are mixed in the application, so that the conductive agent and silicon have more infiltration, which not only improves the conductivity of the negative electrode material, but also inhibits the expansion of silicon to a certain extent. The sodium carboxymethyl cellulose added as a dispersant makes the silicon-based material have a better dispersion effect in the negative electrode slurry. The polyacrylic acid added later as a binder is uniformly coated on the surface of the silicon negative electrode after stirring and kneading. The graphite material and polyacrylic acid are added and kneaded and dispersed, and then the styrene-butadiene rubber is added and dispersed to obtain a silicon-carbon negative electrode slurry. The pre-added polyacrylic acid is kneaded, and then the polyacrylic acid is added again to further supplement the amount of the binder, which effectively improves the adhesion and cohesion of the electrode sheet. The silicon-carbon negative electrode slurry obtained by this kind of mixing method is uniformly dispersed and has stable performance. In the subsequent preparation of the electrode sheet, the cross-linking reaction of the sodium carboxymethyl cellulose with excellent elasticity and the polyacrylic acid with strong rigidity occurs and forms a 3D conductive network during high-temperature baking, which further enhances the cohesion between the particles and inhibits the expansion of silicon, effectively solving the problem of particle breakage and pulverization of silicon during the volume expansion of the silicon negative electrode during the charging and discharging process, ensuring the overall structure of the silicon-carbon negative electrode and further improving the cycle performance of the lithium ion battery.

[0013] In addition, the styrene-butadiene rubber added after the dispersion and uniform slurry end has a good toughening effect on the negative electrode slurry, which not only effectively prevents the brittle electrode sheet from breaking, but also provides good cushioning for the slurry particles, further reducing the possibility of silicon particle breakage, and thus enhancing the effect of inhibiting the expansion of the electrode sheet.

[0014] Preferably, the stirring speed in step (1) is 10-30 rpm, and the stirring time is 28-32 min.

[0015] Preferably, the stirring speed in step (2) is 10-30 rpm, and the stirring time is 0.8-1.2 min.

[0016] Preferably, the stirring speed in step (3) is 10-30 rpm, the dispersion speed is 3000-5000 rpm, and the time is 2.8-3.2 h.

[0017] Preferably, the stirring speed after adding the styrene-butadiene rubber in the step (4) is 10-30 rpm, the dispersion speed is 1000-2000 rpm, and the time is 28-30 min.

[0018] Preferably, the amount of each material in the step (1) is as follows: 10-30 parts of the silicon-based material, 0.1-0.4 parts of the single-walled carbon nanotube, 1-3 parts of the conductive carbon black, and 1-2 parts of the sodium carboxymethyl cellulose.

[0019] Preferably, the amount of the polyacrylic acid added in the step (2) is 2-10 parts.

[0020] Preferably, the amount of the graphite material added in the step (3) is 68-90 parts, and the amount of the polyacrylic acid added is 0-8 parts.

[0021] Preferably, the amount of the styrene-butadiene rubber added in the step (4) is 1-4 parts.

[0022] Preferably, the silicon-based material in the step (1) includes at least one of silicon-oxygen particles and silicon-carbon particles.

[0023] Preferably, the order of adding each material in the step (1) is as follows: the silicon-based material is mixed with the conductive carbon black, then the single-walled carbon nanotube is added and mixed, and then the sodium carboxymethyl cellulose is added for dry mixing to obtain the first slurry.

[0024] Beneficial effects: In the present application, the silicon-based material is mixed with the conductive carbon black, and then the single-walled carbon nanotube and the sodium carboxymethyl cellulose are sequentially added and dry mixed. The polyacrylic acid is then added to the obtained first slurry, so that the materials are pre-mixed with the silicon-based material, which not only saves the mixing and stirring time, but also ensures the uniformity of the mixture, further improves the cycle performance of the lithium ion battery, and enhances the effect of inhibiting the expansion of the pole piece.

[0025] Preferably, the solid content of the mixture after adding water in the step (3) is 40-50 wt%.

[0026] Preferably, the viscosity in the step (4) is set to 4000-8000 mpa·s.

[0027] Preferably, the stirring speed in the step (5) is 5-10 rpm, and the mesh number of the sieving is 120-150.

[0028] The second aspect of the present application provides a silicon-carbon negative electrode slurry prepared by the above preparation method.

[0029] The third aspect of the present application provides a negative electrode sheet prepared from the above silicon-carbon negative electrode slurry.

[0030] The fourth aspect of the present application provides a method for preparing the negative electrode sheet, comprising the following steps: coating the silicon-carbon negative electrode slurry on the negative electrode current collector, drying to obtain an active material layer, rolling and die cutting, and then baking at 90-160 DEG C under vacuum for 1-2 hours to obtain the negative electrode sheet.

[0031] The present application has the advantages of:

[0032] 1. The present application mixes single-walled carbon nanotubes, conductive carbon black and silicon-based materials, so that the conductive agent and silicon have more infiltration, which not only improves the conductivity of the negative electrode material, but also inhibits the expansion of silicon to some extent; the added sodium carboxymethyl cellulose as a dispersant makes the silicon-based material have a better dispersion effect in the negative electrode slurry; the added polyacrylic acid as a binder is uniformly coated on the surface of the silicon negative electrode after stirring and kneading; the added graphite material and polyacrylic acid are kneaded and dispersed, and then the butadiene-styrene rubber is added and dispersed to obtain the silicon-carbon negative electrode slurry; the pre-added polyacrylic acid is kneaded, and then the polyacrylic acid is added again to further supplement the amount of the binder, which effectively improves the adhesion and cohesion of the electrode sheet; the silicon-carbon negative electrode slurry obtained by this kind of mixing method is uniformly dispersed and has stable performance; and in the subsequent preparation process of the electrode sheet, the cross-linking reaction of the sodium carboxymethyl cellulose with excellent elasticity and the polyacrylic acid with strong rigidity occurs and forms a 3D conductive network under high-temperature baking, which further enhances the cohesion between the particles and inhibits the expansion of silicon, effectively solves the problem of the rupture and pulverization of silicon particles caused by the volume expansion of silicon during the charging and discharging process of the silicon negative electrode, ensures the overall structure of the silicon-carbon negative electrode, and further improves the cycle performance of the lithium ion battery;

[0033] 2. The butadiene-styrene rubber added after the dispersion and uniform slurry of the present application also has a good toughening effect on the negative electrode slurry, which not only effectively prevents the brittle electrode sheet from breaking, but also provides good cushioning for the slurry particles, further reduces the possibility of the rupture of silicon particles, and thus enhances the effect of inhibiting the expansion of the electrode sheet;

[0034] 3. The present application mixes the silicon-based material with the conductive carbon black, and then sequentially adds the single-walled carbon nanotubes and the sodium carboxymethyl cellulose and performs dry mixing, so that the first slurry obtained by adding the polyacrylic acid realizes the pre-mixing of the materials with the silicon-based material, which not only saves the mixing and stirring time, but also ensures the uniformity of the mixing, further improves the cycle performance of the lithium ion battery and enhances the effect of inhibiting the expansion of the electrode sheet. DETAILED DESCRIPTION

[0035] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of the present application.

[0036] The test materials and reagents used in the following examples, and the like, can be obtained from commercial channels unless otherwise specified.

[0037] The specific techniques or conditions not specified in the examples can be carried out according to the techniques or conditions described in the literature in the art or according to the product instructions.

[0038] Embodiment 1

[0039] The first aspect of the present embodiment provides a preparation method of a silicon-carbon negative electrode slurry, comprising the following steps:

[0040] (1) Mix silicon-oxygen particles and silicon-carbon particles into a silicon-based material according to a mass ratio of 1:5, mix 20 parts of the silicon-based material with 2.2 parts of conductive carbon black, and perform vacuum stirring at a speed of 20 rpm under a condition of -20 kpa; then add 0.2 parts of single-walled carbon nanotubes and continue to stir and mix, and then add 1 part of sodium carboxymethyl cellulose and continue to stir for 30 min, to obtain a first slurry by dry mixing.

[0041] (2) Add 2 parts of polyacrylic acid to the first slurry, and perform vacuum stirring at a speed of 20 rpm under a condition of -20 kpa for 1 h to obtain a second slurry.

[0042] (3) Add 73 parts of graphite material and 4 parts of polyacrylic acid to the second slurry, and perform vacuum stirring at a speed of 20 rpm under a condition of -20 kpa, add water to make the solid content in the mixture 45 wt%, continue to stir and disperse uniformly at a speed of 3000 rpm, and obtain a third slurry after 3 h.

[0043] (4) When the viscosity of the third slurry is 6000 mpa·s, add 2.5 parts of styrene-butadiene rubber (SBR), and perform vacuum stirring at a speed of 10 rpm under a condition of -20 kpa, and then disperse uniformly at a speed of 1000 rpm, and obtain a fourth slurry after 30 min.

[0044] (5) Before discharging, the fourth slurry is placed in a double-planetary stirrer, the double-planetary stirrer is reversed to remove bubbles, then stirred uniformly at a speed of 5 rpm, and then passed through a 150-mesh sieve to obtain a silicon-carbon negative electrode slurry.

[0045] The second aspect of the embodiment provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry, comprising the following steps: coating the silicon-carbon negative electrode slurry on a negative electrode current collector at a surface density of 145 g / m 2 , drying to obtain an active material layer, and then rolling, die cutting, and baking under vacuum at 150°C for 2 h to obtain the negative electrode sheet.

[0046] Embodiment 2

[0047] The first aspect of the embodiment provides a method for preparing a silicon-carbon negative electrode slurry, comprising the following steps:

[0048] (1) Mixing silicon-oxygen particles and silicon-carbon particles into a silicon-based material at a mass ratio of 1:5, mixing 20 parts of the silicon-based material with 2.2 parts of conductive carbon black, and stirring under vacuum at a speed of 10 rpm and a pressure of -20 kpa; then adding 0.2 parts of single-walled carbon nanotubes and continuing to stir and mix, and then adding 1 part of sodium carboxymethyl cellulose and continuing to stir for 32 min to obtain a first slurry by dry mixing.

[0049] (2) Adding 2 parts of polyacrylic acid to the first slurry and stirring under vacuum at a speed of 10 rpm and a pressure of -20 kpa for 1.2 h to obtain a second slurry.

[0050] (3) Adding 73 parts of a graphite material and 4 parts of polyacrylic acid to the second slurry and stirring under vacuum at a speed of 10 rpm and a pressure of -20 kpa, adding water to make the solid content of the mixture 45 wt%, and continuing to stir and disperse uniformly at a speed of 4000 rpm, to obtain a third slurry after 3.2 h.

[0051] (4) When the viscosity of the third slurry is 6000 mpa·s, adding 2.5 parts of styrene-butadiene rubber (SBR) and stirring under vacuum at a speed of 20 rpm and a pressure of -20 kpa, and then dispersing uniformly at a speed of 2000 rpm, to obtain a fourth slurry after 30 min.

[0052] (5) Before discharging, the fourth slurry is placed in a double-planetary stirrer, the double-planetary stirrer is reversed to remove bubbles, and then the slurry is stirred uniformly at a speed of 8 rpm, and then sieved through a 150-mesh sieve to obtain a silicon-carbon negative electrode slurry.

[0053] The second aspect of the embodiment provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry, comprising the following steps: coating the silicon-carbon negative electrode slurry on a negative electrode current collector at a surface density of 145 g / m 2 , drying to obtain an active material layer, and then rolling, die cutting, and baking under vacuum at 150°C for 2 h to obtain the negative electrode sheet.

[0054] Embodiment 3

[0055] The first aspect of the embodiment provides a preparation method of a silicon-carbon negative electrode slurry, comprising the following steps:

[0056] (1) Mix silicon-oxygen particles and silicon-carbon particles into silicon-based material according to a mass ratio of 1:5, mix 20 parts of the silicon-based material with 2.2 parts of conductive carbon black, and perform vacuum stirring at a speed of 30 rpm under a condition of -20 kpa; then add 0.2 parts of single-walled carbon nanotubes and continue stirring and mixing, add 1 part of sodium carboxymethyl cellulose, and continue stirring for 28 min to obtain a first slurry by dry mixing.

[0057] (2) Add 2 parts of polyacrylic acid to the first slurry, and perform vacuum stirring at a speed of 30 rpm under a condition of -20 kpa for 0.8 h to obtain a second slurry.

[0058] (3) Add 73 parts of graphite material and 4 parts of polyacrylic acid to the second slurry, and perform vacuum stirring at a speed of 30 rpm under a condition of -20 kpa, add water to make the solid content of the mixture 45 wt%, continue stirring and disperse uniformly at a speed of 5000 rpm, and obtain a third slurry after 2.8 h.

[0059] (4) When the viscosity of the third slurry is 6000 mpa·s, add 2.5 parts of butadiene-styrene rubber (SBR), and perform vacuum stirring at a speed of 30 rpm under a condition of -20 kpa, and disperse uniformly at a speed of 1500 rpm, and obtain a fourth slurry after 28 min.

[0060] (5) Before discharging, the fourth slurry is placed in a double-planetary stirrer, the double-planetary stirrer is reversed to remove bubbles, then stirred uniformly at a speed of 10 rpm, and then sieved through a 150-mesh sieve to obtain a silicon-carbon negative electrode slurry.

[0061] The second aspect of the embodiment provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry, comprising the following steps: coating the silicon-carbon negative electrode slurry on a negative electrode current collector at a surface density of 145 g / m 2 , drying to obtain an active material layer, rolling, die cutting, and baking under a vacuum condition at 150°C for 2 h to obtain a negative electrode sheet.

[0062] Example 4

[0063] The first aspect of the embodiment provides a preparation method of a silicon-carbon negative electrode slurry, comprising the following steps:

[0064] (1) The silicon-oxygen particles and silicon-carbon particles are mixed into a silicon-based material at a mass ratio of 1:5. 10 parts of the silicon-based material are mixed with 3 parts of conductive carbon black, and vacuum stirring is performed at a speed of 20 rpm and under a condition of -20 kpa. Then, 0.4 parts of single-walled carbon nanotubes are added and stirring is continued. After 1.5 parts of sodium carboxymethyl cellulose are added, stirring is continued for 30 min. In this way, a first slurry is obtained by dry mixing.

[0065] (2) 6 parts of polyacrylic acid are added to the first slurry, and vacuum stirring is performed at a speed of 20 rpm and under a condition of -20 kpa for 1 h. A second slurry is obtained.

[0066] (3) 90 parts of a graphite material and 8 parts of polyacrylic acid are added to the second slurry, and vacuum stirring is performed at a speed of 20 rpm and under a condition of -20 kpa. Water is added to make the solid content of the mixture 40 wt%, and stirring is continued. Uniform dispersion is achieved at a speed of 3000 rpm. After 3 h, a third slurry is obtained.

[0067] (4) When the viscosity of the third slurry is 4000 mpa·s, 1 part of butadiene-styrene rubber (SBR) is added, and vacuum stirring is performed at a speed of 10 rpm and under a condition of -20 kpa. Uniform dispersion is achieved at a speed of 1000 rpm. After 30 min, a fourth slurry is obtained.

[0068] (5) Before discharging, the fourth slurry is placed in a double-planetary stirrer, and bubble removal is achieved by reversing the double-planetary stirrer. Then, uniform stirring is achieved at a speed of 5 rpm. Finally, a silicon-carbon negative electrode slurry is obtained by passing through a 150-mesh sieve.

[0069] The second aspect of the embodiment provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry. The method comprises the following steps: the silicon-carbon negative electrode slurry is coated on a negative electrode current collector at a surface density of 145 g / m 2 The active material layer is dried, rolled, and die-cut. Then, the negative electrode sheet is obtained by baking under a vacuum condition at 90°C for 1.5 h.

[0070] Example 5

[0071] The first aspect of the embodiment provides a method for preparing a silicon-carbon negative electrode slurry. The method comprises the following steps:

[0072] (1) The silicon-oxygen particles and silicon-carbon particles are mixed into a silicon-based material at a mass ratio of 1:5. 30 parts of the silicon-based material are mixed with 1 part of conductive carbon black, and vacuum stirring is performed at a speed of 20 rpm and under a condition of -20 kpa. Then, 0.1 parts of single-walled carbon nanotubes are added and stirring is continued. After 2 parts of sodium carboxymethyl cellulose are added, stirring is continued for 30 min. In this way, a first slurry is obtained by dry mixing.

[0073] (2) 10 parts of polyacrylic acid were added to the first slurry, and vacuum stirring was performed at a speed of 20 rpm and under a condition of -20 kPa for 1 h to obtain a second slurry.

[0074] (3) 68 parts of a graphite material and 0 parts of polyacrylic acid were added to the second slurry, and vacuum stirring was performed at a speed of 20 rpm and under a condition of -20 kPa, water was added to make the solid content in the mixture 50 wt%, and uniform dispersion was performed at a speed of 3000 rpm until 3 h to obtain a third slurry.

[0075] (4) When the viscosity of the third slurry was 8000 mPa·s, 4 parts of styrene-butadiene rubber (SBR) were added, vacuum stirring was performed at a speed of 10 rpm and under a condition of -20 kPa, and uniform dispersion was performed at a speed of 1000 rpm until 30 min to obtain a fourth slurry.

[0076] (5) Before discharging, the fourth slurry was placed in a double planetary mixer, the double planetary mixer was reversed to remove bubbles, uniform stirring was performed at a speed of 5 rpm, and then a 150-mesh sieve was passed to obtain a silicon-carbon negative electrode slurry.

[0077] The second aspect of the embodiment provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry, which comprises the following steps: the silicon-carbon negative electrode slurry is coated on a negative electrode current collector at a surface density of 145 g / m 2 , and is dried to obtain an active material layer, and after rolling and die cutting, the active material layer is baked under a vacuum condition at 160°C for 1.0 h to obtain a negative electrode sheet.

[0078] Comparative Example 1

[0079] The first aspect of the comparative example provides a method for preparing a silicon-carbon negative electrode slurry, which is different from the embodiment 1 in that the stirring speed of the slurry in step (4) is 100 rpm, and the dispersion speed is 10000 rpm; the stirring speed of the slurry in step (5) is 50 rpm; and other operations are the same as those in the embodiment 1, so as to prepare a silicon-carbon negative electrode slurry.

[0080] The second aspect of the comparative example provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry, which comprises the following steps: the silicon-carbon negative electrode slurry is coated on a negative electrode current collector at a surface density of 145 g / m 2 , and is dried to obtain an active material layer, and after rolling and die cutting, the active material layer is baked under a vacuum condition at 150°C for 2 h to obtain a negative electrode sheet.

[0081] Comparative Example 2

[0082] The first aspect of this comparative example provides a method for preparing silicon-carbon anode slurry, which differs from Example 1 in that: the amount of polyacrylic acid used in steps (2) and (3) is 1 part, and other operations are the same as in Example 1, thereby preparing silicon-carbon anode slurry.

[0083] The second aspect of this comparative example provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry, comprising the following steps: mixing the silicon-carbon negative electrode slurry at 145 g / m 2 The areal density is coated on the negative electrode current collector, dried to obtain the active material layer, and then rolled, die-cut, and baked under vacuum at 150°C for 2 hours to obtain the negative electrode sheet.

[0084] Comparative Example 3

[0085] This comparative example provides a method for preparing a negative electrode sheet using the silicon-carbon negative electrode slurry in Example 1. The difference between this method and Example 1 is that the baking temperature of the electrode sheet is 80°C, and the other operations are the same as in Example 1, thereby preparing the negative electrode sheet.

[0086] Comparative Example 4

[0087] This comparative example provides a method for preparing a negative electrode sheet using the silicon-carbon negative electrode slurry in Comparative Example 1. The difference between this method and Comparative Example 1 is that the baking temperature of the electrode sheet is 80°C, and other operations are the same as in Comparative Example 1, thereby preparing the negative electrode sheet.

[0088] Comparative Example 5

[0089] This comparative example provides a method for preparing a negative electrode sheet using the silicon-carbon negative electrode slurry in Comparative Example 2. The difference between this method and Comparative Example 2 is that the baking temperature of the electrode sheet is 80°C, and other operations are the same as in Comparative Example 2, thereby preparing the negative electrode sheet.

[0090] Comparative Example 6

[0091] The first aspect of this comparative example provides a method for preparing a silicon-carbon anode slurry, which differs from Example 1 in that: in step (3), the graphite material is first mixed with silicon-based material in step (1) to form a silicon-carbon composite material, and the single-walled carbon nanotubes in step (1) are mixed with conductive carbon black to form a conductive agent. Then, the silicon-carbon composite material, the conductive agent, and sodium carboxymethyl cellulose are mixed and dispersed evenly to obtain the first slurry. In step (3), only polyacrylic acid is added, and other operations are the same as in Example 1. In this way, a silicon-carbon anode slurry is prepared.

[0092] The second aspect of this comparative example provides a method for preparing a negative electrode sheet using the above-mentioned silicon-carbon negative electrode slurry, comprising the following steps: mixing the silicon-carbon negative electrode slurry at 145 g / m 2 The areal density is coated on the negative electrode current collector, dried to obtain the active material layer, and then rolled, die-cut, and baked under vacuum at 150°C for 2 hours to obtain the negative electrode sheet.

[0093] Test Example 1

[0094] The negative electrode sheets prepared in Examples 1-5 and Comparative Examples 1-6 were subjected to electrode peeling strength test according to the method 4 in the Test Method for Adhesive Tape Peeling Strength (GB / T 2792-2014): Test Method for Adhesive Tape and Release Material 180° Peeling Strength, and the test results are shown in Table 1.

[0095] The negative electrode sheets prepared in Examples 1-5 and Comparative Examples 1-6 were assembled into lithium ion batteries, and the capacity retention rate and the electrode expansion rate of the batteries were tested after 400 cycles at a current density of 1 C at 3-4.25 V and 25°C. The test results are shown in Table 1.

[0096] Table 1: Test results of lithium ion battery performance

[0097]

[0098] As can be seen from Table 1, the lithium ion battery in the examples of the present application has better mechanical properties, cycle capacity retention rate and lower electrode expansion rate than the lithium ion battery of the comparative examples, i.e. the lithium ion battery prepared in the examples of the present application has better battery performance. Specific analysis shows that: on the one hand, compared with Comparative Example 1, the lower stirring speed and dispersion speed in the mixing process of the examples make the binder well coated on the surface of the silicon-based material, which not only prevents the binder from not being in sufficient contact with the silicon-based material due to the too high stirring speed, but also prevents the problem of poor slurry stability caused by the breakage of the sodium carboxymethyl cellulose molecular chain and the low viscosity of the slurry due to the too high dispersion speed; on the other hand, compared with Comparative Examples 2 and 6, the mixing process of the examples, in which the conductive carbon black, single-walled carbon nanotube and sodium carboxymethyl cellulose are sequentially added to the silicon-based material and then polyacrylic acid is added, makes the single-walled carbon nanotube preferentially physically coated on the surface of the silicon-based material, at this time, more single-walled carbon nanotubes act on the silicon-based material, which not only enhances the electrical conductivity of the silicon-based material, but also better suppresses the expansion rate of the silicon-based material; and the more polyacrylic acid coated on the surface of the silicon-based material forms a better restraint on the silicon-based material, which significantly improves the cycle expansion rate; indicating that the mixing process of the silicon-carbon composite material plays an important role in improving the cycle performance of the battery.

[0099] In addition, compared with Comparative Example 3, after the electrode baking temperature of the examples of the present application reaches the temperature of the binder, the high-temperature crosslinking reaction occurs between the polyacrylic acid and the sodium carboxymethyl cellulose, which coats the silicon-based surface through the three-dimensional structure of the macromolecule and the small molecule, effectively improving the cycle performance of the battery and the expansion rate suppression effect.

[0100] The implementation principle of the present application is that the present application mixes single-walled carbon nanotubes, conductive carbon black and silicon-based materials, so that the conductive agent and silicon have more infiltration, not only improving the conductivity of the negative electrode material, but also to some extent inhibiting the expansion of silicon; the simultaneously added sodium carboxymethyl cellulose as a dispersing agent makes the silicon-based material have a more optimal dispersion effect in the negative electrode slurry; the subsequently added polyacrylic acid as a binder is uniformly coated on the surface of the silicon negative electrode after stirring and kneading; the graphite material and polyacrylic acid are further added and kneaded and dispersed, and then the butadiene-styrene rubber is added and uniformly dispersed to obtain a silicon-carbon negative electrode slurry; after pre-kneading with polyacrylic acid, the polyacrylic acid is added again to further supplement the amount of the binder, effectively improving the adhesion and cohesion of the pole piece, and the silicon-carbon negative electrode slurry obtained by this kind of slurry has uniform dispersion and stable performance; and in the subsequent process of preparing the pole piece, the cross-linking reaction of the carboxymethyl cellulose sodium with excellent elasticity and the polyacrylic acid with strong rigidity occurs and forms a 3D conductive network under high temperature baking, further enhancing the cohesion between the particles, playing a role in inhibiting the expansion of silicon, effectively solving the problem of the rupture and pulverization of silicon particles caused by the volume expansion of the silicon negative electrode during the charging and discharging process, ensuring the overall structure of the silicon-carbon negative electrode, and further improving the cycle performance of the lithium ion battery.

[0101] The butadiene-styrene rubber added after the dispersion and uniform slurry of the present application also has a good toughening effect on the negative electrode slurry, not only effectively preventing the brittle pole piece from breaking, but also providing good cushioning for the slurry particles, further reducing the possibility of the rupture of silicon particles, and thus enhancing the effect of inhibiting the expansion of the pole piece.

[0102] The present application mixes the silicon-based material with conductive carbon black, and then sequentially adds single-walled carbon nanotubes and sodium carboxymethyl cellulose and performs dry mixing, so that the addition of polyacrylic acid in the first slurry obtained in this way realizes the pre-mixing of each material with the silicon-based material, not only saving the mixing and stirring time, but also ensuring the uniformity of the mixing, further improving the cycle performance of the lithium ion battery and enhancing the effect of inhibiting the expansion of the pole piece.

[0103] The above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can modify the technical solutions described in the foregoing examples, or make equivalent substitutions for part of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a negative electrode, characterized in that: Includes the following steps: (1) The silicon-based material is mixed with single-walled carbon nanotubes and conductive carbon black and then stirred under vacuum. Sodium carboxymethyl cellulose is added and stirred evenly to obtain the first slurry. The order of adding each material is as follows: the silicon-based material is mixed with conductive carbon black, then single-walled carbon nanotubes are added and mixed, and then sodium carboxymethyl cellulose is added and dry-mixed to obtain the first slurry. (2) Add polyacrylic acid to the first slurry and stir under vacuum until uniform to obtain the second slurry; (3) Add graphite material and polyacrylic acid to the second slurry and stir under vacuum. Add water and continue stirring and dispersing to obtain the third slurry. (4) When the viscosity of the third slurry reaches the set viscosity, styrene-butadiene rubber is added and vacuum stirred, and then dispersed to obtain the fourth slurry; the stirring speed after adding styrene-butadiene rubber is 10-30 rpm, and the dispersion speed is 1000-2000 rpm. (5) After defoaming the fourth slurry, the silicon-carbon anode slurry is obtained by stirring and sieving. (6) The silicon-carbon negative electrode slurry is coated on the negative electrode current collector, dried to obtain the active material layer, and then rolled and die-cut. It is then baked in a vacuum at 90-160°C for 1-2 hours to obtain the negative electrode sheet.

2. The method for preparing the negative electrode sheet according to claim 1, characterized in that: In step (1), the vacuum stirring speed is 10-30 rpm and the time is 28-32 min; in step (2), the vacuum stirring speed is 10-30 rpm and the time is 0.8-1.2 min; in step (3), the stirring speed is 10-30 rpm and the dispersion speed is 3000-5000 rpm.

3. The method for preparing the negative electrode sheet according to claim 1, characterized in that: In step (1), the amounts of each material by weight are as follows: 10-30 parts of silicon-based material, 0.1-0.4 parts of single-walled carbon nanotubes, 1-3 parts of conductive carbon black, and 1-2 parts of sodium carboxymethyl cellulose; in step (2), the amount of polyacrylic acid added is 2-10 parts; in step (3), the amount of graphite material added is 68-90 parts, and the amount of polyacrylic acid added is 0-8 parts; in step (4), the amount of styrene-butadiene rubber added is 1-4 parts.

4. The method for preparing the negative electrode sheet according to claim 1, characterized in that: In step (1), the silicon-based material is at least one of silicon-oxygen particles and silicon-carbon particles.

5. The method for preparing the negative electrode sheet according to claim 1, characterized in that: The solid content of the mixture after adding water in step (3) is 40-50 wt%; the viscosity set in step (4) is 4000-8000 mPa·s.

6. The method for preparing the negative electrode sheet according to claim 1, characterized in that: The stirring speed in step (5) is 5-10 rpm, and the sieve mesh size is 120-150 mesh.

7. A negative electrode sheet prepared by the method of any one of claims 1-6.

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