Negative electrode sheet, preparation method thereof, and lithium-ion battery

Abstract

The present invention belongs to the field of lithium-ion battery technology, and specifically relates to a negative electrode sheet, a method for preparing the same, and a lithium-ion battery. The negative electrode sheet preparation method comprises the following steps: a slurrying step: mixing and dispersing a negative electrode active material, a binder, a conductive agent, and a solvent to obtain a first slurry, wherein the binder comprises a polyfluororesin binder, and the polyfluororesin binder is added to a mixed solution containing the negative electrode active material in the form of a dispersed emulsion; a drying step: drying the first slurry to obtain a first mixed material; a fiberizing step: fiberizing the first mixed material to obtain a second mixed material; and a sheeting step: laminating the second mixed material onto a negative electrode current collector after a film pressing step to obtain a negative electrode sheet. Compared to conventional dry-process preparation of negative electrode sheets, the present invention can evenly distribute the polyfluororesin binder between the active materials, thereby improving the stability of the negative electrode sheet peel strength.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a negative electrode plate and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Energy is the foundation and driving force of economic development. With the rapid development of the equipment economy, people's demand for energy is increasing, and the development of new energy sources has become a key research direction in the current energy field. As a green and environmentally friendly energy storage device, lithium-ion batteries have the advantages of high operating voltage, no memory effect, high power density, high energy density, and long cycle life. They have a wide range of applications and are widely used in various electronic products.

[0003] The existing wet coating process, through a series of steps such as mixing, homogenization, coating, drying and rolling, finally obtains a negative electrode sheet of suitable thickness. However, the technical defects of coating slurry to prepare the electrode sheet are: high requirements on the viscosity and solid content of the slurry, continuous feeding of the current collector, long drying tunnel length, large floor space and high energy loss.

[0004] The dry electrode preparation technology can overcome the above defects. The conventional dry electrode preparation technology requires mixing the active material and the binder and then hot rolling to complete the preparation of the electrode. The binder used is usually a polyfluororesin binder. The current dry electrode preparation technology cannot make the polyfluororesin binder evenly distributed between the active materials, resulting in large fluctuations in the peel strength of the prepared negative electrode. For example, Chinese patent CN113871561A discloses a dry electrode preparation method and electrode and battery cells. The main steps are to first fiberize the mixed material, then sieve it to obtain a mixture with a consistent degree of fiberization, and then press and bond it to prepare the electrode. However, it can be seen from its SEM that even if the fiberized material is screened, the binder under a high-power microscope is still in a clustered state and the distribution is not very uniform. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a negative electrode plate, a preparation method thereof, and a lithium-ion battery. The present invention prepares a slurry by mixing and dispersing a negative electrode active material, a binder, a conductive agent, and a solvent, and then dries the slurry to obtain a first mixture. After the first mixture is subjected to a fiberization treatment, a negative electrode plate is prepared. Compared with the conventional dry method for preparing negative electrode plates, the polyfluororesin binder can be evenly distributed between the active materials, thereby improving the stability of the peel strength of the negative electrode plate.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps:

[0008] Slurrying process: mixing and dispersing the negative electrode active material, a binder, a conductive agent and a solvent to obtain a first slurry, wherein the binder includes a polyfluororesin binder, and the polyfluororesin binder is added to the mixed solution including the negative electrode active material in the form of a dispersed emulsion;

[0009] Drying process: drying the first slurry to obtain a first mixed material;

[0010] Fiberization treatment step: performing fiberization treatment on the first mixed material to obtain a second mixed material;

[0011] Tablet making process: the second mixed material is pressed onto the negative electrode current collector after the film pressing process to obtain the negative electrode sheet.

[0012] The present invention prepares a slurry by mixing and dispersing a negative electrode active material, a binder, a conductive agent and a solvent, then dries the slurry to obtain a first mixed material, and performs a fiberization treatment on the first mixed material to prepare a negative electrode sheet. Compared with the conventional dry method for preparing negative electrode sheets, the polyfluorinated resin binder can be evenly distributed between the active materials, thereby improving the stability of the peeling strength of the negative electrode sheet.

[0013] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the negative electrode active material includes at least one of a graphite material and a silicon-based material.

[0014] Here, the graphite material includes at least one of natural graphite, artificial graphite, modified graphite, and composite graphite.

[0015] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the binder also includes an auxiliary binder, and the auxiliary binder includes at least one of sodium carboxymethyl cellulose (such as CMC2200, CMC2300, CMC2500 or MAC350, etc.), styrene-butadiene rubber (SBR), polyvinyl alcohol, and polyacrylic acid.

[0016] The present invention limits the binder to also include an auxiliary binder, thereby utilizing the supporting function of the polyfluororesin binder and improving the peel strength of the negative electrode sheet, reducing the binder content requirement, and thus improving the energy density of the battery.

[0017] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the mass ratio of the polyfluororesin binder to the auxiliary binder is 1:(0.7-4), for example, it can be 1:0.7, 1:2 or 1:4.

[0018] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the polyfluororesin binder includes polytetrafluoroethylene (PTFE).

[0019] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the conductive agent includes at least one of conductive carbon black (SP), carbon nanotubes, and carbon fibers.

[0020] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the solvent is water.

[0021] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the mass ratio of the negative electrode active material, the conductive agent and the binder is (80-99): (0.3-5): (0.5-15), and the sum of the respective amounts in the mass ratio of the three is 100; that is, the mass fraction of the negative electrode active material in the total amount of dry material is 80%-99%, the mass fraction of the conductive agent in the total amount of dry material is 0.3%-5%, and the mass fraction of the binder in the total amount of dry material is 0.5%-15%, and the total amount of dry material is the sum of the masses of the negative electrode active material, the conductive agent and the binder.

[0022] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the mass ratio of the negative electrode active material, the conductive agent and the binder is (90-98): (0.5-6): (0.5-8).

[0023] The negative electrode sheet provided by the present invention has high peeling strength and high content of negative electrode active material, and the lithium ion battery prepared from the negative electrode sheet has high energy density.

[0024] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the second mixture is pressed onto the negative electrode collector after the film pressing process, including: after the second mixture is made into an electrode film by a hot pressing roller device, the electrode film is pressed onto the negative electrode collector by a hot pressing roller.

[0025] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the second mixture is made into an electrode membrane through a hot pressing roller device, including: the second mixture is roll-formed through two vertical rollers to form an electrode membrane, wherein the rolling temperature is 60°C-80°C (for example, it can be 60°C, 70°C or 80°C, etc.). If the temperature is too high, the binder is easily deactivated.

[0026] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the electrode membrane is pressed onto the negative electrode collector by a hot pressing roller, including: rolling the electrode membrane and the negative electrode collector by two horizontal rollers to fit the electrode membrane and the negative electrode collector, wherein the rolling temperature of the horizontal roller close to the negative electrode collector (the contact roller on the negative electrode collector side) is 80-120°C (for example, it can be 80°C, 90°C, 100°C, 110°C or 120°C, etc.), and the rolling temperature of the horizontal roller close to the electrode membrane (the contact roller on the electrode membrane side) is 60-80°C.

[0027] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the negative electrode current collector is copper foil, and the copper foil is a plain foil and does not need to be coated with carbon or glue.

[0028] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, in the fiberization process step, the fiberization method is air flow milling or ultra-high-speed shearing.

[0029] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the air flow pressure of the air flow milling is 0.1 MPa-1 MPa (for example, it can be 0.1 MPa, 0.3 MPa, 0.5 MPa, 0.7 MPa or 1 MPa, etc.).

[0030] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the ultra-high speed shear speed is 1000-8000 rpm (for example, it can be 1000 rpm, 2000 rpm, 3000 rpm, 4000 rpm, 5000 rpm, 6000 rpm, 7000 rpm or 8000 rpm, etc.), and the time is 30-180 min (for example, it can be 30 min, 60 min, 90 min, 120 min, 150 min or 180 min, etc.).

[0031] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, in the drying process, the drying method includes one of normal pressure drying, reduced pressure drying, and freeze drying.

[0032] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the temperature of the normal pressure drying is 85-130°C, for example, the temperature can be 85°C, 100°C, 110°C, 120°C or 130°C.

[0033] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the pressure of the reduced pressure drying is -0.06 to -0.08 MPa (for example, it can be -0.06 MPa, -0.07 MPa or -0.08 MPa, etc.), and the temperature is 50-90°C (for example, the temperature can be 50°C, 60°C, 70°C, 80°C or 90°C, etc.).

[0034] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the freeze-drying temperature is -10°C to -50°C (for example, it can be -10°C, -20°C, -40°C, or -50°C, etc.), the freeze-drying pressure is less than 611Pa, and the time is 0.5-24h (for example, it can be 0.5h, 2h, 5h, 10h, 15h, 20h or 24h, etc.).

[0035] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the solid content of the first slurry is 45%-60% (for example, it can be 45%, 50%, 55% or 60%, etc.).

[0036] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, in the slurry mixing process, the negative electrode active material and the conductive agent are added to the mixer for a first stirring and dispersion, and then the solvent is added for a second stirring and dispersion, and finally the polyfluororesin binder dispersion emulsion is added for a third stirring and dispersion.

[0037] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, in the slurry mixing step, the negative electrode active material, the conductive agent and the powdered binder are added to a mixer and subjected to a first stirring and dispersion process.

[0038] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, in the slurry mixing process, after the third stirring and dispersing step, a binder emulsion other than the polyfluororesin binder dispersion emulsion is added to perform a fourth stirring and dispersing step.

[0039] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the revolution speed of the third stirring and dispersing is 20-40 rpm, the dispersion speed is 3-10 m / s (for example, it can be 3 m / s, 5 m / s, 7 m / s or 10 m / s, etc.), and the stirring time is 20-240 min (for example, it can be 20 min, 50 min, 80 min, 100 min, 150 min, 200 min or 240 min, etc.), and PTFE fiberization is prevented by controlling the dispersion speed.

[0040] In the above-mentioned method for preparing the negative electrode sheet, as a preferred embodiment, the revolution speed of the first stirring and dispersing is 30-50 rpm, the dispersion speed is 0, and the time is 30-60 min; the revolution speed of the second stirring and dispersing is 30-40 rpm, the dispersion speed is 10-18 m / s, and the time is 90-240 min (for example, it can be 90 min, 120 min, 150 min, 200 min or 240 min, etc.); the revolution speed of the fourth stirring and dispersing is 20~50 rpm, and the time is 20~40 min.

[0041] In the above-mentioned method for preparing the negative electrode plate, as a preferred embodiment, in the film making process, the negative electrode plate includes a negative electrode collector and an electrode membrane layer attached to the surface of the negative electrode collector, and the thickness of the electrode membrane layer is 20um-200um (for example, it can be 20um, 60um, 120um or 200um, etc.).

[0042] In a second aspect, the present invention provides a negative electrode plate, which is made by the preparation method of the negative electrode plate provided in the first aspect.

[0043] In a third aspect, the present invention provides a lithium-ion battery, comprising the negative electrode sheet described in the second aspect.

[0044] Compared with the prior art, the present invention has at least one of the following advantages:

[0045] (1) The present invention prepares a slurry by mixing and dispersing a negative electrode active material, a binder, a conductive agent and a solvent, then dries the slurry to obtain a first mixed material, and then performs a fiberization treatment on the first mixed material to prepare a negative electrode sheet. Compared with the conventional dry method for preparing negative electrode sheets, the polyfluororesin binder can be evenly distributed between the active materials, thereby improving the stability of the peeling strength of the negative electrode sheet.

[0046] (2) The uniform dispersion of the polyfluororesin binder emulsion in the slurry can achieve the purpose of uniform fiberization, avoiding the agglomeration after fiberization caused by the uneven dispersion of binders such as PTFE in the traditional dry electrode preparation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Flowchart of the method for preparing the negative electrode sheet provided in Example 1 of the present invention;

[0048] Figure 2 This is a SEM image of the negative electrode active material of Example 1 of the present invention;

[0049] Figure 3 This is a SEM image of the electrode membrane of Example 1 of the present invention;

[0050] Figure 4 This is a comparison chart of the peel strength of the negative electrode sheets provided by randomly selected Example 1 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0051] In order to further understand the present invention, the preferred embodiments of the present invention are described below in conjunction with examples. The scope of protection of this application includes but is not limited to the following examples. The following examples are only used to illustrate the advantages and effects of the technical solutions of this application and do not constitute a limitation on the scope of protection of this application. Equivalent substitutions made by those skilled in the art based on this application are all within the scope of protection of this application.

[0052] Unless otherwise defined, the technical terms used in the following examples have the same meanings as commonly understood by those skilled in the art to which this invention belongs. The experimental reagents used in the following examples, unless otherwise specified, are conventional biochemical reagents; the experimental reagent amounts, unless otherwise specified, are those used in routine experimental procedures; and the experimental methods, unless otherwise specified, are conventional methods.

[0053] In a first aspect, the method for preparing a negative electrode sheet provided by an embodiment of the present invention comprises the following steps:

[0054] S1. Slurrying process: adding the negative electrode active material, conductive agent and powdered binder into a blender, performing a first stirring and dispersion, then adding a solvent for a second stirring and dispersion, finally adding a polyfluororesin binder dispersion emulsion, performing a third stirring and dispersion, then adding a binder emulsion other than the polyfluororesin binder dispersion emulsion, performing a fourth stirring and dispersion, and obtaining a first slurry, wherein the binder comprises a polyfluororesin binder dispersion emulsion, the negative electrode active material comprises at least one of a graphite material and a silicon-based material, the binder further comprises an auxiliary binder, the auxiliary binder comprises at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber (SBR), polyvinyl alcohol, and polyacrylic acid, the mass ratio of the polyfluororesin binder to the auxiliary binder is 1:(0.7-4), the polyfluororesin binder comprises polytetrafluoroethylene (PTFE), and the conductive agent comprises The invention relates to a method for preparing a negative electrode active material comprising at least one of conductive carbon black (SP), carbon nanotubes, and carbon fibers, wherein the solvent is water, the mass ratio of the negative electrode active material, the conductive agent, and the binder is (80-99): (0.3-5): (0.5-15), and the sum of the respective amounts in the mass ratio of the three is 100, the solid content of the first slurry is 45%-60%, the revolution speed of the third stirring and dispersing is 20-40 rpm, the dispersion speed is 3-10 m / s, and the stirring time is 20-240 min, the revolution speed of the first stirring and dispersing is 30-50 rpm, the dispersion speed is 0, and the time is 30-60 min; the revolution speed of the second stirring and dispersing is 30-40 rpm, the dispersion speed is 10-18 m / s, and the time is 90-240 min; the revolution speed of the fourth stirring and dispersing is 20-50 rpm, and the time is 20-40 min.

[0055] S2, drying process: drying the first slurry to obtain a first mixed material, wherein the drying method includes one of normal pressure drying, reduced pressure drying, and freeze drying, wherein the temperature of the normal pressure drying is 85-130°C; the pressure of the reduced pressure drying is -0.06 to -0.08 MPa, and the temperature is 50-90°C; the temperature of the freeze drying is -10°C to -50°C, the freeze drying pressure is less than 611 Pa, and the time is 0.5-24h.

[0056] S3, fiberization process: the first mixture is subjected to fiberization treatment to obtain a second mixture, wherein the fiberization method is air flow milling or ultra-high-speed shearing; the air flow pressure of the air flow milling is 0.1 MPa-1 MPa; the speed of the ultra-high-speed shearing is 1000-8000 rpm, and the time is 30-180 min.

[0057] S4, sheet making process: after the second mixed material is made into an electrode membrane by a hot pressing roller device, the electrode membrane is pressed onto the negative electrode collector by a hot pressing roller to obtain a negative electrode sheet, wherein the process of making the second mixed material into an electrode membrane by a hot pressing roller device comprises: rolling the second mixed material through two vertical rollers to form an electrode membrane, the rolling temperature is 60°C-80°C; the process of pressing the electrode membrane onto the negative electrode collector by a hot pressing roller comprises: rolling the electrode membrane and the negative electrode collector by two horizontal rollers. The method comprises the steps of: pressing the electrode membrane and the negative electrode current collector to fit together, wherein the rolling temperature of the horizontal roller close to the negative electrode current collector (the contact roller on the negative electrode current collector side) is 80-120° C., and the rolling temperature of the horizontal roller close to the electrode membrane (the contact roller on the electrode membrane side) is 60-80° C.; the negative electrode current collector is copper foil, which is a plain foil and does not require carbon coating or glue coating; the negative electrode sheet includes a negative electrode current collector and an electrode membrane layer attached to the surface of the negative electrode current collector, and the thickness of the electrode membrane layer is 20 μm-200 μm.

[0058] At present, CMC in the negative electrode sheet of the battery has good thermal stability and conductivity. Its main functions in the battery are: 1) It helps to disperse the negative electrode active material and conductive agent; 2) The carboxyl and hydroxyl groups in CMC can form covalent bonds with lithium ions, which shortens the transmission distance between lithium ions and the negative electrode active material, improves the electrochemical performance and long cycle life of the battery; 3) There is a strong hydrogen bond between CMC and silicon-based materials, which helps to alleviate the expansion of silicon-based materials during the cycle.

[0059] The present invention uses a slurrying process to dissolve sodium carboxymethyl cellulose in a solution and evenly disperse it on the surface of the active material, which is beneficial for the reaction of sodium carboxymethyl cellulose with lithium to form a conductive path and shorten the electron and ion transmission path of the active material.

[0060] The manufacture of traditional dry-process electrodes requires the use of glue-coated or carbon-coated foil. The present invention perfectly maintains the functions of sodium carboxymethyl cellulose and SBR through the slurrying process. In the later film-making process, sodium carboxymethyl cellulose and SBR can well bond the electrode membrane and copper foil.

[0061] Through the slurry mixing process, the present invention can perfectly exert the bonding effect of CMC and SBR, improve the peel strength of the electrode membrane, and reduce the amount of polyfluororesin binder used in the preparation process, thereby not affecting the conductivity of the material and effectively utilizing the supporting effect of the polyfluororesin binder.

[0062] In a second aspect, an embodiment of the present invention provides a negative electrode plate, which is manufactured by the preparation method of the negative electrode plate provided in the first aspect.

[0063] In a third aspect, an embodiment of the present invention provides a lithium-ion battery, comprising the negative electrode sheet described in the second aspect.

[0064] In order to further understand the present invention, the negative electrode plate and its preparation method and the lithium ion battery provided by the present invention are described in detail below with reference to the embodiments. The protection scope of the present invention is not limited by the following embodiments.

[0065] In the following examples and comparative examples:

[0066] PTFE dispersion emulsion: solid content 60%, viscosity 10-100*10 -3 PaS.

[0067] Testing method for peel strength: Cut the electrode pieces prepared in the following embodiments and comparative examples into test samples with a size of 90x120mm, and stick the cut electrode pieces to the middle of a thin steel plate with double-sided tape, with the end faces flush. The thin steel plate should be wiped clean with dust-free paper in advance to leave no stains or dust. Bend one end of the sample 180° in the opposite direction and fix it on the tensile probe. Peel it 180° at a constant rate of 5cm / min to test the peel strength of the sample.

[0068] Example 1

[0069] Figure 1 A flow chart of a method for preparing a negative electrode sheet according to an embodiment of the present invention is shown in FIG. Figure 1 As shown, the method for preparing the negative electrode sheet provided by the embodiment of the present invention includes the following steps:

[0070] S1. Slurrying process: spherical graphite, conductive agent (SP), CMC2200, SBR and PTFE are mixed in a mass ratio of 96:0.5:1.2:1.6:0.7. Specifically, spherical graphite, SP and CMC2200 are transferred to a mixer for dry mixing (first stirring and dispersion), with a revolution speed of 40 rpm, a dispersion speed of 0, and a time of 30 min; then, water is added according to a solid content of 56%, and after sufficient wetting, high-speed stirring is turned on to fully disperse it (second stirring and dispersion), with a revolution speed of 30 rpm, a dispersion speed of 12 m / s, and a time of 120 min; then, PTFE dispersion emulsion and water are added according to a solid content of 53%, and a third stirring and dispersion is carried out, with a revolution speed of 30 rpm, a dispersion speed of 8 m / s, and a time of 30 min; finally, SBR glue is added and stirred at a low speed until uniformly dispersed, with a revolution speed of 30 rpm, a dispersion speed of 0, and a time of 30 min to obtain the first slurry.

[0071] S2. Drying process: freeze-dry the first slurry prepared in step S1 at a drying temperature of -30°C for 2 hours under a vacuum pressure of 500 Pa to obtain a first mixed material.

[0072] S3, fiberization treatment process: the first mixed material prepared in step S2 is fiberized by an air flow pulverization device at an air flow pressure of 0.1 MPa to obtain a second mixed material.

[0073] S4. Sheet making process: The second mixed material is sheeted twice (roller forming) through two vertical rollers, the temperature of the first sheeting is 60°C, and the temperature of the second sheeting is 80°C to obtain an electrode membrane; the electrode membrane is laminated on both sides of the copper foil through two horizontal rollers to obtain a negative electrode sheet, the temperature of the upper roller of the horizontal roller (the horizontal roller in contact with the electrode membrane) is 60°C, and the temperature of the lower roller of the horizontal roller (the horizontal roller in contact with the copper foil) is 90°C. The total thickness of the electrode membrane layer on the negative electrode sheet is 105um.

[0074] like Figure 2 The SEM image of spherical graphite shown in the figure shows that the spherical graphite particles are distinct, the contact area is small and it is not easy to be adhered. Figure 3 This is a SEM image of the electrode membrane of an embodiment of the present invention. It can be seen that the PTFE after fiberization is well dispersed without large-area agglomeration, and a grid structure is constructed to restrain the spherical graphite.

[0075] 17 negative electrode sheets prepared in this embodiment were randomly selected and tested for peel strength. The test results are as follows: Figure 4 As shown, the maximum peel strength is 18.6 N / m, the minimum is 18.3 N / m, and the average is 18.4 N / m. The peel strengths of different negative electrode sheets are not much different, indicating that the peel strength of the negative electrode sheet prepared in this embodiment is relatively stable, and further indicates that the preparation method provided in this embodiment can make the binder evenly distributed between the active materials.

[0076] Example 2

[0077] The method for preparing a negative electrode sheet provided in an embodiment of the present invention comprises the following steps:

[0078] S1. Slurrying process: Silicon carbon, conductive agent (SP), CMC2200, SBR and PTFE are mixed in a mass ratio of 90:1.5:1.5:2:5. Specifically, silicon carbon, SP and CMC2200 are transferred to a mixer for dry mixing (first stirring and dispersion), with a revolution speed of 40 rpm, a dispersion speed of 0, and a time of 60 min; then, water is added according to a solid content of 60%, and after sufficient wetting, high-speed stirring is turned on to fully disperse it (second stirring and dispersion), with a revolution speed of 30 rpm, a dispersion speed of 15 m / s, and a time of 90 min; then, PTFE dispersion emulsion and water are added according to a solid content of 50%, and a third stirring and dispersion is carried out, with a revolution speed of 40 rpm, a dispersion speed of 5 m / s, and a time of 90 min; finally, SBR glue is added and stirred at low speed until uniformly dispersed, with a revolution speed of 50 rpm, a dispersion speed of 0, and a time of 30 min to obtain the first slurry.

[0079] S2. Drying process: freeze-dry the first slurry prepared in step S1 at a drying temperature of -40°C for 3 hours under a vacuum pressure of 400 Pa to obtain a first mixed material.

[0080] S3, fiberization treatment process: the first mixture prepared in step S2 is subjected to fiberization by an ultra-high-speed shearing device at a rotation speed of 5600 rpm and a stirring time of 60 min to obtain a second mixture.

[0081] S4. Sheet making process: The second mixed material is sheeted twice (roller forming) through two vertical rollers, the temperature of the first sheeting is 70°C, and the temperature of the second sheeting is 80°C to obtain an electrode membrane; the electrode membrane is laminated on both sides of the copper foil through two horizontal rollers to obtain a negative electrode sheet, the temperature of the upper roller of the horizontal roller (the horizontal roller in contact with the electrode membrane) is 70°C, and the temperature of the lower roller of the horizontal roller (the horizontal roller in contact with the copper foil) is 90°C. The total thickness of the electrode membrane layer on the negative electrode sheet is 110um.

[0082] 17 negative electrode sheets prepared in this embodiment were randomly selected for peel strength testing. The maximum peel strength was 20.4 N / m, the minimum was 20 N / m, and the average was 20.1 N / m. The peel strengths of different negative electrode sheets were not much different, indicating that the peel strength of the negative electrode sheets prepared in this embodiment was relatively stable, and further indicating that the preparation method provided in this embodiment can make the binder evenly distributed between the active materials.

[0083] Example 3

[0084] The preparation method of the negative electrode sheet provided in this embodiment is basically the same as that in Example 1, except that in the slurry mixing process, CMC2200 and SBR are replaced with PTFE, which specifically includes the following steps:

[0085] S1. Slurrying process: Spherical graphite, conductive agent (SP) and PTFE are slurried in a mass ratio of 96:0.5:3.5. Specifically, the spherical graphite and SP are transferred to a mixer for dry mixing (first stirring and dispersion) at a revolution speed of 40 rpm, a dispersion speed of 0, and a time of 30 minutes. Then, water is added according to a solid content of 56%, and after sufficient wetting, high-speed stirring is turned on to fully disperse it (second stirring and dispersion) at a revolution speed of 30 rpm, a dispersion speed of 12 m / s, and a time of 120 minutes. Subsequently, PTFE dispersion emulsion and water are added according to a solid content of 53%, and a third stirring and dispersion is performed at a revolution speed of 30 rpm, a dispersion speed of 8 m / s, and a time of 30 minutes to obtain the first slurry.

[0086] S2 to S4 are exactly the same as those in Example 1.

[0087] 17 negative electrode sheets prepared in this embodiment were randomly selected for peel strength testing. The maximum peel strength was 10.8, the minimum was 10.2, and the average was 10.5. The peel strengths of different negative electrode sheets were not much different, indicating that the peel strength of the negative electrode sheet prepared in this embodiment was relatively stable, and the peel strength was significantly smaller than that of Example 1, indicating that the combined use of CMC, SBR and PTFE can significantly improve the peel strength of the sheet and reduce the binder content requirement.

[0088] Comparative Example 1

[0089] This comparative example adopts the traditional dry electrode preparation technology to prepare the negative electrode plate. The PTFE and SBR used in this comparative example are powders. The preparation method of the negative electrode plate provided in this comparative example includes the following steps:

[0090] The mass ratio of spherical graphite, conductive agent (SP), CMC2200, SBR and PTFE is 96:0.5:1.2:1.6:0.7. The spherical graphite, SP, SBR and CMC2200 are transferred to a blender for dry mixing (first stirring and dispersion) at an orbital speed of 40 rpm, a dispersion speed of 0, and a time of 30 min. Then, PTFE powder is added and mixed in a blender at an orbital speed of 800 rpm and a stirring time of 30 min. Then, the fiberization process and the sheeting process are performed according to steps S3-S4 of Example 1 to prepare the negative electrode sheet.

[0091] 17 negative electrode sheets prepared in this embodiment were randomly selected and tested for peel strength. The test results are as follows: Figure 4 As shown in the figure, the maximum peel strength is 8.7, the minimum is 5.2, and the average is 7.2. The peel strength of different negative electrode sheets fluctuates greatly. Figure 4It can be seen that the peel strength of the negative electrode sheet prepared in Example 1 is more stable, indicating that the binder is more evenly dispersed, while the peel strength of Comparative Example 1 fluctuates greatly, indicating that the adhesiveness is unevenly dispersed; the peel strength of this comparative example is significantly lower than that of Example 1, which is mainly attributed to the relatively low content of PTFE and the low degree of fiberization of CMC and SBR, which cannot fully exert their bonding effect.

[0092] Comparative Example 2

[0093] This comparative example adopts the traditional dry electrode preparation technology to prepare the negative electrode sheet. The PTFE and SBR used in this comparative example are powders. The preparation method of the negative electrode sheet provided in this comparative example includes the following steps: the mass ratio of spherical graphite, conductive agent (SP), CMC2200, SBR and PTFE is 94:0.5:1.2:1.6:2.7, the spherical graphite, SP and CMC2200, SBR are transferred to a blender for dry mixing (first stirring and dispersion), the revolution speed is 40 rpm, the dispersion speed is 0, and the time is 30 min; then, PTFE powder is added and mixed in the blender, the revolution speed is 800 rpm, and the stirring time is 30 min. Then, the fiberization process and the sheeting process are performed with reference to steps S3-S4 of Example 1 to prepare the negative electrode sheet.

[0094] 17 negative electrode sheets prepared in this embodiment were randomly selected and tested for peel strength. The maximum peel strength was 9.7, the minimum was 6.2, and the average was 8.2. Compared with Comparative Example 1, the peel strength was improved, indicating that increasing the proportion of PTFE is beneficial to improving the peel strength. Compared with Example 1, the total amount of the binder is increased, but the peel strength is not as high as that of Example 1, indicating that CMC and SBR cannot play a good bonding role using the traditional dry electrode preparation method.

[0095] Comparative Example 3

[0096] The preparation method of the negative electrode plate provided in this comparative example is basically the same as that of Example 3, except that the PTFE dispersed emulsion in Example 3 is replaced by PTFE powder.

[0097] The negative electrode was prepared by referring to the preparation method of Example 3.

[0098] Seventeen negative electrode plates prepared in this comparative example were randomly selected for peel strength testing. The maximum peel strength was 9.8, the minimum was 8.4, and the average was 9.0. Compared with Example 3, the peel strength was weaker and the difference was not obvious, but the fluctuation of the peel strength was significantly greater than that of Example 3, indicating that after replacing the PTFE dispersed emulsion with PTFE powder, the PTFE was unevenly distributed among the active materials.

[0099] Comparative Example 4

[0100] This comparative example adopts the traditional dry electrode preparation technology to prepare the negative electrode plate. The PTFE used in this comparative example is a powder. The preparation method of the negative electrode plate provided in this comparative example includes the following steps: the mass ratio of spherical graphite, conductive agent (SP) and PTFE is 96:0.5:3.5, the spherical graphite and SP are transferred to a blender for dry mixing (first stirring and dispersion), the revolution speed is 40 rpm, the dispersion speed is 0, and the time is 30 min; then, PTFE powder is added and mixed in a stirring tank, the revolution speed is 800 rpm, and the stirring time is 30 min. Then, the fiberization process and the sheeting process are performed with reference to steps S3-S4 of Example 3 to prepare the negative electrode plate.

[0101] The structural diagram of the negative electrode sheet prepared in this comparative example shows that the PTFE agglomeration phenomenon is obvious and the grid has a surface shape.

[0102] 17 negative electrode sheets prepared in this comparative example were randomly selected for peel strength testing. The maximum peel strength was 9.3, the minimum was 7.5, and the average was 8.5. The peel strength of different negative electrode sheets prepared by the traditional dry method was slightly lower than that of Example 3, but with greater fluctuations.

[0103] It can be seen from Comparative Example 4 and Comparative Example 1 that when part of the PTFE is replaced with CMC2200 and SBR, the peel strength decreases significantly, indicating that the degree of fiberization of CMC and SBR is low; it can be seen from Example 3 and Example 1 that when CMC, SBR and PTFE are used in combination and the preparation method of the negative electrode provided in Example 1 is adopted at the same time, the peel strength of the electrode can be significantly improved and the requirement for the binder content is reduced, indicating that the preparation method provided in Example 1 can significantly improve the adhesion of CMC and SBR.

[0104] Comparative Example 5

[0105] The preparation method of the negative electrode sheet provided in this comparative example is basically the same as that in Example 1, except that after the slurry mixing process, a coating process is used to prepare the negative electrode sheet, that is, the first slurry is coated on both sides of the copper foil, and then dried and rolled to prepare the negative electrode sheet.

[0106] 17 negative electrode sheets prepared in this embodiment were randomly selected and tested for peel strength, with an average value of 14.1 N / m.

[0107] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing a negative electrode sheet, characterized in that: The following steps are involved: Slurrying process: mixing and dispersing the negative electrode active material, a binder, a conductive agent and a solvent to obtain a first slurry, wherein the binder includes a polyfluororesin binder, and the polyfluororesin binder is added to the mixed solution including the negative electrode active material in the form of a dispersed emulsion; Drying process: drying the first slurry to obtain a first mixed material; Fiberization treatment step: performing fiberization treatment on the first mixed material to obtain a second mixed material; Sheet making step: pressing the second mixed material onto the negative electrode current collector after the film pressing step to obtain a negative electrode sheet; The polyfluororesin binder includes polytetrafluoroethylene; The negative electrode active material includes at least one of a graphite material and a silicon-based material; The binder further includes an auxiliary binder, and the auxiliary binder includes at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl alcohol, and polyacrylic acid; The mass ratio of the negative electrode active material, the conductive agent and the binder is (80-99): (0.3-5): (0.5-15); The mass ratio of the polyfluororesin binder to the auxiliary binder is 1:(0.7-4).

2. The method for preparing a negative electrode sheet according to claim 1, wherein: The conductive agent includes at least one of conductive carbon black, carbon nanotubes, and carbon fibers; And / or, the solvent is water.

3. The method for preparing a negative electrode sheet according to claim 1, wherein: The step of pressing the second mixture onto the negative electrode current collector after the film pressing process includes: forming the second mixture into an electrode film through a hot pressing roller device, and then pressing the electrode film onto the negative electrode current collector through a hot pressing roller.

4. The method for preparing a negative electrode sheet according to claim 3, wherein: The step of forming the electrode membrane by a hot pressing roller device comprises: forming the electrode membrane by rolling the second mixed material through two vertical rollers, wherein the rolling temperature is 60°C to 80°C; And / or, pressing the electrode membrane onto the negative electrode current collector by means of a hot pressing roller comprises: rolling the electrode membrane and the negative electrode current collector by means of two horizontal rollers so as to fit the electrode membrane and the negative electrode current collector, wherein the rolling temperature of the horizontal roller close to the negative electrode current collector is 80-120°C, and the rolling temperature of the horizontal roller close to the electrode membrane is 60-80°C.

5. The method for preparing a negative electrode sheet according to claim 1, wherein: In the fiberization process, the fiberization method is air flow milling or ultra-high-speed shearing; And / or, in the drying process, the drying method includes one of normal pressure drying, reduced pressure drying, and freeze drying; And / or, in the sheeting process, the negative electrode sheet includes a negative electrode current collector and an electrode membrane layer attached to the surface of the negative electrode current collector, and the thickness of the electrode membrane layer is 20 μm-200 μm; And / or, in the slurry mixing process, the negative electrode active material and the conductive agent are added to the mixer for a first stirring and dispersion, then the solvent is added for a second stirring and dispersion, and finally the polyfluororesin binder dispersion emulsion is added for a third stirring and dispersion.

6. The method for preparing a negative electrode sheet according to claim 5, characterized in that: In the slurry mixing process, after the third stirring and dispersing, a binder emulsion other than the polyfluororesin binder dispersion emulsion is added to perform a fourth stirring and dispersing, wherein the revolution speed of the fourth stirring and dispersing is 20 to 50 rpm and the time is 20 to 40 minutes; And / or, the revolution speed of the third stirring and dispersing is 20-40 rpm, the dispersion speed is 3-10 m / s, and the stirring time is 20-240 min; And / or, the first stirring and dispersing process has a revolution speed of 30-50 rpm, a dispersion speed of 0, and a time of 30-60 min; And / or, the second stirring and dispersing process has a revolution speed of 30-40 rpm, a dispersion speed of 10-18 m / s, and a time of 90-240 min.

7. A negative electrode plate, characterized in that: The negative electrode sheet is made by the preparation method of the negative electrode sheet according to any one of claims 1 to 6.

8. A lithium ion battery, characterized in that: The lithium-ion battery comprises the negative electrode sheet according to claim 7.

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