Graphite negative electrode sheet and manufacturing method therefor, and lithium-ion battery

Abstract

The present application provides a graphite negative electrode sheet and a manufacturing method therefor, and a lithium-ion battery. The manufacturing method comprises: step S1, material mixing; step S2, coating; and step S3, magnetization: periodically performing N magnetization treatments on a coated electrode sheet to obtain the graphite negative electrode sheet, wherein N is an integer greater than or equal to 1. Each magnetization treatment comprises a first magnetization performed in a first direction, a second magnetization performed in a second direction, and a third magnetization performed in a third direction; the first magnetization, the second magnetization, and the third magnetization are performed in any order; and the first direction is perpendicular to a plane direction of the coated electrode sheet, and the second direction and the third direction are perpendicular to each other and parallel to the plane direction of the coated electrode sheet. In the present application, a coated electrode sheet coated with a graphite active material slurry undergoes three magnetization treatments to obtain an oriented graphite negative electrode sheet, which shortens the migration path of lithium ions during deintercalation, thereby achieving a high charge-discharge rate and excellent cycle performance.

Description

Graphite negative electrode sheet, preparation method thereof and lithium ion battery

[0001] The present application claims priority to the Chinese patent application No. 202411363259.2, filed on September 27, 2024, to the Chinese Patent Office, the content of the above application being hereby incorporated by reference into the present application. TECHNICAL FIELD

[0002] The present application relates to the field of lithium ion batteries, in particular, to a graphite negative electrode sheet, a preparation method thereof and a lithium ion battery. BACKGROUND

[0003] With the rapid development of electric vehicles, people are eager for electric vehicles to have long endurance and fast charging and discharging performance, therefore, the demand for high-capacity and high-rate lithium ion batteries increases year by year. Lithium titanate has a high charge and discharge rate due to its unique crystal structure, however, its low specific capacity and high voltage platform seriously restrict its application in electric vehicles. Graphite has more than twice the specific capacity of lithium titanate, and has become the main negative electrode material for electric vehicles. However, as the demand for power of electric vehicle batteries continues to increase, higher requirements are placed on the charge and discharge rate performance of negative electrode materials.

[0004] Currently, to overcome the defect of low charge and discharge rate of graphite, many methods are to modify the graphite material. For example, Chinese patent CN201410787061.7 uses pitch to coat graphite and high-temperature graphitization to prepare a fast-charging graphite lithium ion battery negative electrode material. The pitch coated on the surface of the graphite is converted into graphite carbon through high-temperature graphitization, and the performance is close to that of the bulk graphite, so the rate performance is limited. Chinese patent CN201710186013.6 uses high-temperature graphitization to prepare artificial graphite, and then mixes it with pitch to carbonize and prepare a fast-charging graphite lithium ion battery negative electrode material. Although this method coats amorphous carbon on the surface of the graphite, it needs multiple high-temperature heat treatments, the process is complex, the energy consumption is high, and the surface-coated carbon is not subjected to high-temperature graphitization treatment, so the amorphous carbon has many structural defects, the first efficiency of the material is low, and the cycle performance is poor.

[0005] Therefore, how to provide a graphite negative electrode sheet and a corresponding preparation method, more significantly improve the electrochemical performance of the negative electrode sheet, and ultimately make the lithium ion battery in which it is located exhibit higher capacity, first efficiency and cycle stability, is one of the important technical problems to be solved in the field. SUMMARY

[0006] The main purpose of the present application is to provide a graphite negative electrode sheet, a preparation method thereof and a lithium ion battery, to solve the problem of poor rate performance of the battery caused by the difficulty of lithium ion insertion and extraction in the charge and discharge process of the negative electrode sheet of the lithium ion battery in the prior art.

[0007] To achieve the above object, the present application provides a method for preparing a graphite negative electrode sheet, comprising the following steps: Step S1, mixing: adding graphite, a conductive agent and a binder into a solvent to obtain a slurry by mixing; Step S2, coating: preparing a current collector and coating the slurry on at least one side surface of the current collector to obtain a coated electrode sheet, the coated electrode sheet having a length direction and a width direction perpendicular to each other; Step S3, magnetization: periodically performing N groups of magnetization treatment on the coated electrode sheet, wherein N is an integer greater than or equal to 1; each group of magnetization treatment comprises a first magnetization along a first direction, a second magnetization along a second direction and a third magnetization along a third direction, the first magnetization, the second magnetization and the third magnetization being performed in any order, and the first direction being perpendicular to the plane direction of the coated electrode sheet, the second direction and the third direction being perpendicular to each other and parallel to the plane direction of the coated electrode sheet.

[0008] Further, in Step S3, N = 1, the first direction is perpendicular to the plane direction of the coated electrode sheet; the second direction is parallel to the plane direction of the coated electrode sheet and perpendicular to the length direction of the coated electrode sheet; and the third direction is parallel to the plane direction of the coated electrode sheet and perpendicular to the width direction of the coated electrode sheet; preferably, the magnetic field strength used in the three magnetization treatments is independently 0.5-1.5T.

[0009] Further, the time of the first magnetization, the second magnetization and the third magnetization is independently 1-120s.

[0010] Further, the particle size D50 of the graphite is 3-20μm; preferably, the graphite is natural graphite and / or artificial graphite, more preferably natural graphite.

[0011] Further, the ratio of the sum of the weights of the conductive agent and the binder to the weight of the graphite is (10-2):(90-98), the solid content of the slurry is 30-60%, and the coating viscosity is 3000-7000pa·s.

[0012] Further, the conductive agent is selected from one or more of acetylene black, carbon black, conductive graphite, carbon fiber, carbon nanotube and graphene; and / or, the binder is selected from one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyaniline and citric acid; preferably, the solvent is water.

[0013] Further, the step S1 further comprises: adding the first binder into the solvent to perform first mixing to obtain an intermediate slurry; and adding the graphite, the conductive agent and the second binder into the intermediate slurry to perform second mixing to obtain the slurry; preferably, the time for the first mixing is 1-2 h, and the time for the second mixing is 3-9 h; preferably, the first binder and the second binder are each independently one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyaniline and citric acid; more preferably, the mixing is achieved by using a double-screw mixer and / or a double-planetary stirrer.

[0014] Another aspect of the present application provides a graphite negative electrode sheet prepared by the above method for preparing a graphite negative electrode sheet.

[0015] Further, the OI value of the graphite negative electrode sheet is 0.1-1.

[0016] Yet another aspect of the present application provides a lithium ion battery comprising a negative electrode sheet, a positive electrode sheet and an electrolyte, wherein the negative electrode sheet is the above graphite negative electrode sheet.

[0017] By applying the technical solution of the present application, the graphite negative electrode sheet is obtained by performing three times of magnetization treatment on the coated electrode sheet coated with the graphite active material slurry, which shortens the migration path of lithium ions in the deintercalation process, thereby ensuring high charge-discharge rate and excellent cycle performance of the material. BRIEF DESCRIPTION OF DRAWINGS

[0018] The drawings accompanying the specification of the present application form a part thereof, serve to provide further understanding of the present application, and together with the specification explain the present application, and do not constitute an improper limitation on the present application. In the drawings:

[0019] Fig. 1 is a SEM photo of the graphite negative electrode sheet obtained in Example 1 at a magnification of 5k;

[0020] Fig. 2 is a SEM photo of the graphite negative electrode sheet obtained in Example 1 at a magnification of 2k;

[0021] Fig. 3 is a SEM photo of the graphite negative electrode sheet obtained in Comparative Example 1 at a magnification of 5k;

[0022] Fig. 4 is a SEM photo of the graphite negative electrode sheet obtained in Comparative Example 1 at a magnification of 2k;

[0023] Fig. 5 is an XRD comparison chart of the graphite negative electrode sheets obtained in Example 1 and Comparative Example 1 (the upper part is Example 1, and the lower part is Comparative Example 1). DETAILED DESCRIPTION

[0024] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other in the case of no conflict. The present application will be described in detail below with reference to the embodiments.

[0025] As described in the background, the prior art has the problem of poor rate performance of the battery due to the difficulty of lithium ion insertion and extraction in the lithium ion battery negative electrode sheet during charging and discharging. In order to solve the above technical problems, the present application provides a preparation method of a graphite negative electrode sheet, comprising the following steps: step S1, mixing: adding graphite, conductive agent and binder into a solvent, and mixing to obtain a slurry; step S2, coating: preparing a current collector, and coating the slurry on at least one side surface of the current collector to obtain a coated electrode sheet, the coated electrode sheet having a length direction and a width direction perpendicular to each other; step S3, magnetization: periodically performing N groups of magnetization treatment on the coated electrode sheet, wherein N is an integer ≥1; each group of magnetization treatment includes first magnetization along a first direction, second magnetization along a second direction, and third magnetization along a third direction, the first magnetization, the second magnetization and the third magnetization being performed in any order, and the first direction being perpendicular to the plane direction of the coated electrode sheet, the second direction and the third direction being perpendicular to each other and parallel to the plane direction of the coated electrode sheet.

[0026] The present application obtains a special oriented graphite negative electrode sheet by magnetizing the coated electrode sheet coated with graphite active material slurry for a number of times which is a multiple of 3, which shortens the migration path of lithium ion during deintercalation, thereby ensuring high charge-discharge rate and excellent cycle performance of the material. Specifically, during the preparation of the electrode sheet, the electrode sheet coated with active slurry is placed in a magnetic field in a specific direction. Due to the diamagnetism of graphite, it will show directional arrangement under the action of the magnetic field, thereby significantly reducing the OI value of the graphite and the tortuosity of the graphite negative electrode sheet, and improving the charge-discharge rate performance and long cycle life of the graphite negative electrode sheet.

[0027] However, it should be noted that the preparation method provided in the present application is aimed at graphite as raw material, which can realize the effect of particle rearrangement by utilizing its own diamagnetism, compared with the one-dimensional negative active carbon material commonly used in the art, such as conductive carbon black. Compared with other two-dimensional negative active carbon materials commonly used in the art, especially graphene, it has the characteristics of easy dispersion and the material itself has a certain self-supporting ability and can act as a negative active main material in lithium ion batteries, so that under the action of a specific magnetic field, the final orientation effect can be achieved. In the preparation process, unlike the experimental ideas commonly used in the art for pretreating the graphite material itself to improve the intrinsic properties of the graphite material, the present application creatively limits the treatment object of multiple magnetization treatment to the coated electrode sheet coated with graphite active material. With such a setting, the graphite negative electrode sheet with highly oriented graphite active material, excellent rate performance and long cycle performance is finally obtained.

[0028] In several typical embodiments, in order to enable the cross section of the graphite active material to be more effectively vertically oriented on the surface of the negative electrode sheet, it is preferred that in step S3, N = 1, the first direction is perpendicular to the plane direction of the coated electrode sheet; the second direction is parallel to the plane direction of the coated electrode sheet and perpendicular to the length direction of the coated electrode sheet; and the third direction is parallel to the plane direction of the coated electrode sheet and perpendicular to the width direction of the coated electrode sheet. Further, the magnetic field strength used for the three magnetization treatments is independently 0.5-1.5T. Through a large number of experiments, the inventors designed the magnetic field direction used for the three magnetization treatments, and at the same time, the above specific strength range was optimized, and under this condition, a graphite negative electrode sheet with higher rate performance and better long cycle stability was obtained. In order to strengthen the magnetization effect, improve the orientation degree of the graphite material, and finally improve the rate performance of the electrode sheet, it is more preferred that the time of the first magnetization, the second magnetization and the third magnetization is independently 1-120s.

[0029] Further, in order to enable the three magnetization orientations to be more effectively performed, thereby more significantly improving the rate performance of the obtained graphite negative electrode sheet, it is preferred that the particle size D50 of the graphite is 3-20μm. In several typical embodiments, the graphite is natural graphite and / or artificial graphite. In several more preferred embodiments, the graphite is natural graphite, because the natural graphite is generally in the form of large flaky, and the diamagnetic characteristic is more obvious.

[0030] In the preparation of the slurry, in order to improve the dispersion uniformity and the spreadability on the surface of the electrode sheet, so that the graphite active material is better dispersed on the surface of the electrode sheet, and the rate performance of the obtained electrode sheet is more superior, in several typical embodiments, the ratio of the sum of the weights of the conductive agent and the binder to the weight of the graphite is (10-2):(90-98), the solid content of the slurry is 30-60%, and the coating viscosity is 3000-7000 pa·s.

[0031] In several typical embodiments, the conductive agent is selected from one or more of acetylene black, carbon black, conductive graphite, carbon fiber, carbon nanotube and graphene; and / or, the binder is selected from one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyaniline and citric acid. In theory, both the conductive agent and the binder can be selected from the types commonly used in the art, but the inventors have found through a large number of experiments that the above several specific types are more suitable for the graphite negative active system provided in the present application, and can more effectively promote the orientation of the graphite material on the surface of the electrode sheet, thereby more significantly improving the rate performance of the obtained graphite negative electrode sheet. In order to improve the uniformity and stability of the slurry system, optimize the uniform distribution on the surface of the electrode sheet, and improve the cycle stability, the solvent is preferably water.

[0032] Further, in order to improve the dispersion uniformity of each component during the preparation of the slurry and the stability of the slurry system, and ultimately strengthen the orientation of the graphite material and improve the rate performance of the electrode sheet, it is preferred that step S1 further comprises: adding the first binder to the solvent to perform first mixing to obtain an intermediate slurry; adding the graphite, the conductive agent and the second binder to the intermediate slurry to perform second mixing to obtain the slurry. Further, the inventors have found through a large number of experiments that the first mixing time is 1-2 h and the second mixing time is 3-9 h, and under these conditions, a slurry with better dispersion and uniformity is obtained, thereby more effectively optimizing the structure of the coating layer on the surface of the obtained graphite negative electrode sheet and improving the various performances of the electrode sheet. In several typical embodiments, the first binder and the second binder are each independently one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyaniline and citric acid. In several more preferred embodiments, mixing is achieved using a double-screw mixer and / or a double-planetary stirrer, and compared with other mixing methods, the slurry obtained by mixing using these two devices has higher dispersion uniformity and higher mixing efficiency.

[0033] Another aspect of the present application provides a graphite negative electrode sheet prepared by the above method for preparing a graphite negative electrode sheet. The obtained graphite negative electrode sheet has extremely high orientation of the graphite negative active material thereon, effectively shortening the migration path of lithium ions during the deintercalation process, thereby having high charge-discharge rate and cycle performance.

[0034] In particular, further, the OI value of the graphite negative electrode sheet is 0.1-1, i.e., the graphite negative active material thereon has extremely high orientation, and in the application process of the negative electrode sheet, Li + The deintercalation can be more efficient along the end surface of the graphite negative electrode, significantly reducing the tortuosity of the graphite negative electrode, thereby more effectively improving the electrical performance of the obtained negative electrode sheet, especially the long cycle performance.

[0035] Yet another aspect of the present application provides a lithium ion battery comprising a negative electrode sheet, a positive electrode sheet and an electrolyte, the negative electrode sheet being the above graphite negative electrode sheet.

[0036] Since the end surface of the graphite negative active material on the surface of the above graphite negative electrode sheet is perpendicular to the negative current collector, it has high orientation, high charge-discharge rate and excellent cycle performance, and therefore when it is used as a negative element in a lithium ion battery, the obtained lithium ion battery also has high capacity, high initial efficiency, high charge-discharge rate and long cycle performance.

[0037] The present application will be further described in detail below in conjunction with specific examples, which should not be construed as limiting the scope of the present application.

[0038] Unless otherwise defined, all professional terms used herein have the same meaning as generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific examples and are not intended to limit the scope of protection of the present application.

[0039] Example 1

[0040] Preparation of a graphite negative electrode sheet:

[0041] 0.2 kg of binder CMC and 10 kg of distilled water were added to a double-planet mixer and mixed and stirred for 2 h, then 0.1 kg of conductive carbon black, 9.5 kg of natural graphite (d50 of 12.0 μm) and 0.2 kg of binder PAA were added in sequence, mixed for 3 h, screened to remove bubbles and then transferred to a coating machine for coating. At this time, the viscosity of the slurry was 4100 mPa·s, and the coated electrode sheet was obtained after coating.

[0042] Before drying the coated electrode sheet, it is first magnetized in a magnetic field of 0.6 T for 5 s, then magnetized in a magnetic field of 0.8 T for 5 s, and finally magnetized in a magnetic field of 1.0 T for 10 s. The direction of the magnetic field in the first magnetization is perpendicular to the coated surface of the coated electrode sheet, the direction of the magnetic field in the second magnetization is parallel to the coated surface on the coated electrode sheet and perpendicular to the short side, and the direction of the magnetic field in the third magnetization is parallel to the coated surface on the coated electrode sheet and perpendicular to the long side.

[0043] After the three magnetization processes, the normal electrode sheet drying and winding and subsequent electrode sheet processing are performed to obtain a graphite negative electrode sheet. The SEM photo with a magnification of 5k is shown in FIG. 1, the SEM photo with a magnification of 2k is shown in FIG. 2, and the XRD pattern is shown in FIG. 5.

[0044] Example 2

[0045] Preparation of a graphite negative electrode sheet:

[0046] 0.2 kg of binder CMC and 10 kg of distilled water are added to a double-planet mixer and mixed and stirred for 2 h, then 0.1 kg of conductive carbon black, 9.5 kg of natural graphite (d50 of 12.0 μm), and 0.2 kg of binder PAA are sequentially added and mixed for 3 h, and then transferred to a coating machine for coating after defoaming by sieving. At this time, the viscosity of the slurry is 4100 mPa·s, and a coated electrode sheet is obtained after coating.

[0047] Before drying the coated electrode sheet, it is first magnetized in a magnetic field of 0.6 T for 10 s, then magnetized in a magnetic field of 0.8 T for 5 s, and finally magnetized in a magnetic field of 1.0 T for 10 s. The direction of the magnetic field in the first magnetization is perpendicular to the coated surface of the coated electrode sheet, the direction of the magnetic field in the second magnetization is parallel to the coated surface on the coated electrode sheet and perpendicular to the short side, and the direction of the magnetic field in the third magnetization is parallel to the coated surface on the coated electrode sheet and perpendicular to the long side.

[0048] After the three magnetization processes, the normal electrode sheet drying and winding and subsequent electrode sheet processing are performed to obtain a graphite negative electrode sheet.

[0049] Example 3

[0050] Preparation of a graphite negative electrode sheet:

[0051] 0.2 kg of binder CMC and 10 kg of distilled water were added into a double planetary mixer and mixed for 2 h, then 0.1 kg of conductive carbon black, 9.5 kg of natural graphite (d50 of 12.0 μm) and 0.2 kg of binder PAA were added in sequence, mixed for 3 h, and then transferred to a coating machine after defoaming by sieving. At this time, the viscosity of the slurry was 4100 m Pa·s, and the coated electrode sheet was obtained after coating.

[0052] Before drying the coated electrode sheet, it was first magnetized for 10 s in a magnetic field of 0.6 T, then magnetized for 10 s in a magnetic field of 0.8 T, and finally magnetized for 10 s in a magnetic field of 1.0 T. The direction of the magnetic field in the first magnetization was perpendicular to the coated surface of the coated electrode sheet, the direction of the magnetic field in the second magnetization was parallel to the coated surface on the coated electrode sheet and perpendicular to the short side, and the direction of the magnetic field in the third magnetization was parallel to the coated surface on the coated electrode sheet and perpendicular to the long side.

[0053] After the three magnetizations, the normal electrode sheet drying and winding and the subsequent electrode sheet processing technology were performed, and the graphite negative electrode sheet was obtained.

[0054] Example 4

[0055] Preparation of a graphite negative electrode sheet:

[0056] The difference between this example and Example 1 is only that the graphite used is artificial graphite instead of natural graphite.

[0057] Example 5

[0058] Preparation of a graphite negative electrode sheet:

[0059] The difference between this example and Example 1 is only that the magnetic field strength used in the three magnetizations is different, specifically: the magnetic field strength in the first magnetization is changed to 0.3 T, the magnetic field strength in the second magnetization is changed to 0.3 T, and the magnetic field strength in the third magnetization is changed to 2.0 T.

[0060] Example 6

[0061] Preparation of a graphite negative electrode sheet:

[0062] The difference between this example and Example 1 is only that the magnetic field strength used in the three magnetizations is different, specifically: the magnetic field strength in the first magnetization is changed to 2.0 T, the magnetic field strength in the second magnetization is changed to 2.0 T, and the magnetic field strength in the third magnetization is changed to 0.3 T.

[0063] Example 7

[0064] Preparation of a graphite negative electrode sheet:

[0065] The difference between this example and Example 1 is only that the d50 of the natural graphite used is 6.0 pm.

[0066] Example 8

[0067] Preparation of a graphite negative electrode sheet:

[0068] The difference between this example and Example 1 is only that the d50 of the natural graphite used is 18.0 pm.

[0069] Example 9

[0070] Preparation of a graphite negative electrode sheet:

[0071] The difference between this example and Example 1 is only that the amounts of the component materials in the slurry formulation are different, specifically: the binder CMC is 0.2 kg, the binder SBR is 0.2 kg, the conductive agent carbon black is 0.1 kg, the natural graphite is 9.5 kg, and the distilled water is 10 kg. At this time, the solid content of the obtained slurry is 50%, and the viscosity is 3600 m Pa-s.

[0072] Example 10

[0073] Preparation of a graphite negative electrode sheet:

[0074] The difference between this example and Example 1 is only that the amounts of the component materials in the slurry formulation are different, specifically: the binder CMC is 0.1 kg, the binder PAA is 0.3 kg, the conductive agent carbon black is 0.1 kg, the natural graphite is 9.5 kg, and the distilled water is 10 kg. At this time, the solid content of the obtained slurry is 50%, and the viscosity is 3600 m Pa-s.

[0075] Example 11

[0076] Preparation of a graphite negative electrode sheet:

[0077] The difference between this example and Example 1 is only that, in the preparation process of the slurry, the first mixing (i.e., the mixing of CMC and deionized water) time is 30 min, and the second mixing (i.e., the mixing to obtain the final slurry) time is 10 h.

[0078] Example 12

[0079] Preparation of a graphite negative electrode sheet:

[0080] The difference between this example and Example 1 is only that, in the preparation process of the slurry, the first mixing (i.e., the mixing of CMC and deionized water) time is 5 h, and the second mixing (i.e., the mixing to obtain the final slurry) time is 2 h.

[0081] Comparative Example 1

[0082] Preparation of a graphite negative electrode sheet:

[0083] The difference between this comparative example and Example 1 is only that the coated electrode sheet is not subjected to three magnetization treatments, but is directly dried to obtain a negative electrode sheet. The SEM photograph at a magnification of 5k is shown in Figure 3, the SEM photograph at a magnification of 2k is shown in Figure 4, and the XRD pattern is shown in Figure 5.

[0084] Comparative Example 2

[0085] Preparation of a graphite negative electrode sheet:

[0086] The difference between this comparative example and Example 4 is only that the coated electrode sheet is not subjected to three magnetization treatments, but is directly dried to obtain a negative electrode sheet.

[0087] Comparative Example 3

[0088] Preparation of a graphite negative electrode sheet:

[0089] The difference between this comparative example and Example 1 is only that the natural graphite used is subjected to three magnetization treatments with the same magnetic field strength as in Example 1 before mixing, and then the magnetized natural graphite is mixed with other components to prepare a slurry, and the coated electrode sheet is obtained after coating and drying. No magnetization treatment is performed after the coated electrode sheet is obtained.

[0090] Comparative Example 4

[0091] Preparation of a graphite negative electrode sheet:

[0092] The difference between this comparative example and Example 1 is only that the natural graphite is replaced by graphene of the same weight, which is Model GMC96 of Japan Adeka Company.

[0093] Comparative Example 5

[0094] Preparation of a graphite negative electrode sheet:

[0095] The difference between this comparative example and Example 1 is only that the coated electrode sheet is subjected to two magnetization treatments, i.e. a first magnetization treatment with a magnetic field strength of 0.6T and a time of 5s, and a second magnetization treatment with a magnetic field strength of 0.8T and a time of 10s. The magnetic field direction of the first magnetization treatment is perpendicular to the coating surface of the coated electrode sheet, and the magnetic field direction of the second magnetization treatment is parallel to the coating surface on the coated electrode sheet and perpendicular to the short side.

[0096] Comparative Example 6

[0097] Preparation of a graphite negative electrode sheet:

[0098] The comparative example differs from Example 1 only in that the coated pole piece is subjected to only one magnetization treatment, i.e. a magnetization treatment with a magnetic field strength of 0.6 T and a time of 5 s, the magnetic field direction being perpendicular to the coated surface of the coated pole piece.

[0099] Test method

[0100] Pole piece OI value test: XRD analysis method is used.

[0101] Battery assembly: LiNi 0.8 Co 0.1 Mn 0.1 O2 pole piece as the positive electrode, N / P design is 1.1, 1 mol / L LiPF6 solution of EC / EMC (volume ratio is 3:7) as electrolyte, celgard2400 microporous diaphragm as diaphragm, the graphite negative electrode pole piece obtained from each example and comparative example is assembled to obtain a battery cell, and then a 10 Ah battery sample is obtained.

[0102] Battery performance test: under room temperature conditions, the obtained battery sample is subjected to constant current constant voltage charge and discharge for 1 week at a current density of 0.1 C, then subjected to constant current constant voltage charge and discharge for the 2nd week at a current density of 1 C, and finally subjected to constant current constant voltage charge and discharge for 100 weeks at a current density of 0.5 C, the voltage range is 2.75-4.2 V, the first week charge capacity, the first efficiency, the ratio of the 2nd week charge capacity to the 1st week charge capacity (denoted as 1C / 0.1C), and the ratio of the 102nd week charge capacity to the 3rd week charge capacity (denoted as 100 week cycle retention rate) are recorded.

[0103] The test results obtained are shown in Table 1 and Table 2.

[0104] Table 1

[0105] Table 2

[0106] From the above description, it can be seen that the above-mentioned examples of the present application realize the improvement of the rate performance of the pole piece itself by subjecting the negative pole piece coated with graphite active material to three magnetization treatments, and finally exhibit the improvement of various electrical properties of lithium ion batteries, specifically: improve the fast charging performance of the battery cell, especially under high rate conditions, which can significantly improve the constant current ratio of the battery cell; at the same time, the cycle performance is also improved to a certain extent.

[0107] It should be noted that the terms "first", "second", and the like in the description and in the claims of the present application are used for distinguishing between similar objects and not necessarily for describing a specific sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the present application described herein are, for example, capable of orderly execution or performance.

[0108] The preferred embodiments of the present application have been described above with the understanding that these embodiments are intended to be illustrative and not restrictive of possible embodiments of the application. Changes and modifications can be made by those of ordinary skill in the art, which equate to the principles and spirit of the application, and it is therefore intended that this application include all such changes and modifications as fall within the scope of the appended claims.

Claims

1. A method for producing a graphite negative electrode sheet, characterized by, The method comprises the following steps: Step S1, mixing: adding graphite, conductive agent and binder into a solvent to obtain a slurry by mixing; Step S2, coating: preparing a current collector and coating the slurry on at least one side surface of the current collector to obtain a coated electrode sheet, the coated electrode sheet having a length direction and a width direction perpendicular to each other; Step S3, magnetization: periodically performing N groups of magnetization treatment on the coated electrode sheet to obtain the graphite negative electrode sheet, wherein N is an integer greater than or equal to 1; Each group of magnetization treatment comprises a first magnetization in a first direction, a second magnetization in a second direction and a third magnetization in a third direction, the first magnetization, the second magnetization and the third magnetization being performed in any order, and the first direction being perpendicular to the plane direction of the coated electrode sheet, the second direction and the third direction being perpendicular to each other and parallel to the plane direction of the coated electrode sheet.

2. The method for producing a graphite negative electrode sheet according to claim 1, characterized by, In step S3, N = 1, The first direction is perpendicular to the plane direction of the coated electrode sheet; The second direction is parallel to the plane direction of the coated electrode sheet and perpendicular to the length direction of the coated electrode sheet; The third direction is parallel to the plane direction of the coated electrode sheet and perpendicular to the width direction of the coated electrode sheet; Preferably, the magnetic field strength used in the three magnetization treatments is independently 0.5-1.5T.

3. The method for producing a graphite negative electrode sheet according to claim 2, characterized by, The time of the first magnetization, the second magnetization and the third magnetization is independently 1-120s.

4. The method for producing a graphite negative electrode sheet according to any one of claims 1 to 3, characterized by, The particle size D50 of the graphite is 3-20μm; Preferably, the graphite is natural graphite and / or artificial graphite, more preferably natural graphite.

5. The method for producing a graphite negative electrode sheet according to any one of claims 1 to 4, characterized by, The ratio of the sum of the weights of the conductive agent and the binder to the weight of the graphite is (10-2):(90-98), the solid content of the slurry is 30-60%, and the coating viscosity is 3000-7000pa·s.

6. The method for producing a graphite negative electrode sheet according to any one of claims 1 to 5, characterized by, The conductive agent is selected from one or more of acetylene black, carbon black, conductive graphite, carbon fiber, carbon nanotube and graphene; and / or, the binder is selected from one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyaniline and citric acid; Preferably, the solvent is water.

7. The method of producing a graphite negative electrode sheet according to any one of claims 1 to 6, characterized by, Step S1 further comprises: adding a first binder into the solvent to perform first mixing to obtain an intermediate slurry; adding the graphite, the conductive agent and a second binder into the intermediate slurry to perform second mixing to obtain the slurry; Preferably, the time of the first mixing is 1-2h, and the time of the second mixing is 3-9h; Preferably, the first binder and the second binder are independently one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyaniline and citric acid; More preferably, the mixing is realized by using a double screw mixer and / or a double planetary mixer.

8. A graphite negative electrode sheet prepared by the method for preparing a graphite negative electrode sheet according to any one of claims 1 to 7.

9. The graphite negative electrode sheet according to claim 8, characterized by The OI value of the graphite negative electrode sheet is 0.1-1.

10. A lithium-ion battery comprising a negative electrode sheet, a positive electrode sheet, and an electrolyte, characterized by The negative electrode sheet is the graphite negative electrode sheet described in claim 8 or 9. The negative electrode sheet is the graphite negative electrode sheet described in claim 8 or 9.

Images