A method for preparing a lithium-ion battery

Abstract

The present invention discloses a method for preparing a lithium-ion battery, comprising the following steps: S1, preparing a graphene-coated lithium titanate composite material; S2, preparing a positive electrode sheet using a material including high-voltage lithium cobalt oxide; S3, preparing a negative electrode sheet using the graphene-coated lithium titanate composite material in step S1; S4, cutting the prepared positive and negative electrode sheets using a die-cutting machine, and then laminating, injecting, and forming the battery to complete the battery preparation; S5, conducting rate discharge, ultra-low temperature discharge capacity, rate cycle performance, and needle puncture performance tests on the prepared battery. The lithium-ion battery prepared using this preparation method has excellent comprehensive performance, effectively achieving a balance between long battery life, high safety, ultra-low temperature discharge, and power performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery preparation, and in particular relates to a method for preparing a lithium ion battery. Background Art

[0002] As a new generation of green energy, lithium-ion batteries have been widely used in various fields such as electronic equipment, power energy, and energy storage due to their advantages in service life, safety, energy density, power performance, and ability to be used in extreme environments. However, with the continuous improvement of application demands in various fields, especially the improvement of comprehensive performance requirements of batteries in extreme environments, traditional lithium-ion batteries mainly focus on improving performance in a single direction of the battery, which may sacrifice other battery properties such as power performance, service life, and safety performance. The overall performance of the battery is poor, resulting in a relatively single application scenario for the battery, and it is unable to simultaneously take into account long life, high safety, excellent power performance, and meet the normal use requirements in extreme environments. For example, the patent with authorization (announcement) number CN106058245B discloses a low-temperature lithium-ion battery with excellent discharge performance under low-temperature conditions. The battery comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode active material used in the positive electrode is a ternary material of nickel-cobalt-manganese oxide doped with lithium iron manganese phosphate, wherein the mass percentage of lithium iron manganese phosphate doped in the positive electrode active material is 5% to 30%. The negative electrode active material used in the negative electrode is graphite or carbon-coated graphite material. The organic solvent used in the electrolyte is a mixed system of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, and propylene carbonate, and the concentration of lithium ions in the electrolyte is 0.9 to 1.3 mol / L. As shown in the accompanying drawings, the discharge capacity retention rate reaches more than 100% at 55°C, and the discharge capacity retention rate is more than 92% at 0°C. The discharge capacity is as high as 84.8% at -20°C and as low as about 60%. It can be seen that the discharge capacity retention rate of the lithium-ion battery gradually decreases with decreasing temperature. Therefore, under more extreme and lower temperature conditions, the battery cannot guarantee good discharge performance, which in turn affects the battery life. Summary of the Invention

[0003] In order to solve the above problems, the present invention provides a method for preparing a lithium-ion battery that can take into account safety performance, service life, power performance and the requirements for use in extremely low temperature environments.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for preparing a lithium-ion battery comprises the following steps:

[0006] S1. preparing a graphene-coated lithium titanate composite material;

[0007] S2. preparing a positive electrode sheet using a material including high-voltage lithium cobalt oxide;

[0008] S3, preparing a negative electrode sheet using the graphene-coated lithium titanate composite material in step S1;

[0009] S4. Cut the prepared positive and negative electrode sheets by a die-cutting machine, and then stack them to complete the cell preparation, liquid injection, and formation to complete the battery preparation;

[0010] S5. After the preparation is completed, the battery is subjected to tests including rate discharge, discharge capacity in ultra-low temperature environment, rate cycle performance, and needle puncture performance verification.

[0011] The preparation method of the graphene-coated lithium titanate composite material comprises the following steps:

[0012] (1) lithium titanate and graphene are mixed in a mass ratio of (80-90): (20-10), and ball milled until uniformly dispersed;

[0013] (2) sintering the lithium titanate and graphene mixed and ball-milled in step (1) to prepare a graphene-coated lithium titanate composite material;

[0014] (3) The graphene-coated lithium titanate composite material was taken out, ground into powder, and then sieved to obtain a graphene-coated lithium titanate composite material sample.

[0015] In step (1), the ball milling speed is 350 r / min, and the ball milling time ranges from 4 h to 6 h.

[0016] In step (2), lithium titanate and graphene are placed in a tube furnace for sintering at a sintering temperature of 240° C. to 280° C. and a sintering time ranging from 12 h to 20 h.

[0017] The preparation method of the positive electrode sheet includes the following steps:

[0018] 1) Polyvinylidene fluoride (PVDF) was added to N-methylpyrrolidone and stirred in a planetary mixer for 20-30 minutes;

[0019] 2) selecting high-voltage lithium cobalt oxide, superconducting carbon black (SP), and carbon nanotubes (CNTS) according to a mass ratio and adding them into a planetary mixer for mixing and stirring; when the slurry viscosity is tested and qualified, a positive electrode slurry is obtained;

[0020] 3) The positive electrode slurry is coated on the carbon-coated aluminum foil using an extrusion coating machine and dried to obtain a positive electrode sheet.

[0021] In step 2), the planetary mixer is used for mixing and stirring at 800 r / min-1200 r / min for 60 min-90 min; the viscosity of the slurry is tested in the range of 4000 mPa·s-6000 mPa·s.

[0022] In step 3), the coating surface density is 1.9 g / 100 cm 2 -2.3g / 100cm 2 .

[0023] The method for preparing the negative electrode sheet includes the following steps:

[0024] ① Polyvinylidene fluoride (PVDF) was added to N-methylpyrrolidone and stirred in a planetary mixer at a stirring speed ranging from 20 min to 30 min;

[0025] ② The graphene-coated lithium titanate composite material prepared in step S1 is selected, and superconductive carbon black (SP) and carbon nanotubes (CNTS) are weighed according to a mass ratio and added into a planetary mixer for mixing and stirring. When the slurry viscosity is tested and qualified, a negative electrode slurry is obtained;

[0026] ③ Use an extrusion coating machine to evenly coat the negative electrode slurry on the carbon-coated aluminum foil and dry it to obtain the negative electrode sheet.

[0027] In step ②, mixing and stirring are performed in a planetary mixer at 800 r / min-1200 r / min for 60 min-90 min; the viscosity of the slurry is tested in the range of 5000 mPa·s-7000 mPa·s.

[0028] In step ③, the coating surface density is 2.0g / 100cm 2 -2.5g / 100cm 2 .

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] The present invention provides a method for preparing a lithium-ion battery. The lithium-ion battery prepared by the method has good comprehensive performance and effectively achieves a balance between long battery life, high safety, extremely low temperature discharge and power performance.

[0031] The invention discloses a method for preparing a lithium-ion battery, which optimizes and improves the performance of lithium titanate materials by adopting a mixed sintering method. The lithium titanate is mainly coated with graphene materials with different contents, so that the conductive properties of the lithium titanate material are effectively improved, and the comprehensive performance such as battery rate and low temperature is effectively optimized from the material perspective.

[0032] From the perspective of battery design, high-voltage lithium cobalt oxide material with a wide temperature range and excellent rate performance is selected as the positive electrode material, matched with the optimized and improved lithium titanate negative electrode material, and a special electrolyte for wide-temperature range lithium titanate batteries is selected for full battery design and preparation. Among them, carbon-coated aluminum foil is preferably used as the current collector, and the electrode preparation parameters are explored and optimized to prepare a lithium-ion battery that can take into account power performance, ultra-low temperature discharge performance, long life and high safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0034] Figure 1 This is a SEM test image of the graphene-coated lithium titanate composite material in Example 1;

[0035] Figure 2 This is a 50C rate discharge curve of the lithium-ion battery prepared in Example 1;

[0036] Figure 3 The discharge curves of the lithium-ion battery prepared in Example 1 at -55°C and -60°C environments are shown;

[0037] Figure 4 This is a graph showing the 10C rate cycling performance of the lithium-ion battery prepared in Example 1;

[0038] Figure 5 This is a diagram of the state of the lithium-ion battery prepared in Example 1 after acupuncture. DETAILED DESCRIPTION

[0039] The present invention is further described below with reference to the accompanying drawings and specific embodiments. However, it should not be understood that the scope of the subject matter described in the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, various modifications, substitutions and changes made according to common technical knowledge and customary means in the field are included in the scope of the present invention.

[0040] A method for preparing a lithium-ion battery in the present invention comprises the following steps:

[0041] S1. preparing a graphene-coated lithium titanate composite material;

[0042] S2. preparing a positive electrode sheet using a material including high-voltage lithium cobalt oxide;

[0043] S3, preparing a negative electrode sheet using the graphene-coated lithium titanate composite material in step S1;

[0044] S4. Cut the prepared positive and negative electrode sheets by a die-cutting machine, and then stack them to complete the cell preparation, liquid injection, and formation to complete the battery preparation;

[0045] S5. After the preparation is completed, the battery is subjected to tests including rate discharge, discharge capacity in ultra-low temperature environment, rate cycle performance, and needle puncture performance verification.

[0046] The preparation method of the graphene-coated lithium titanate composite material comprises the following steps:

[0047] (1) lithium titanate and graphene are mixed in a mass ratio of (80-90): (20-10), and ball milled until uniformly dispersed;

[0048] (2) sintering the lithium titanate and graphene mixed and ball-milled in step (1) to prepare a graphene-coated lithium titanate composite material;

[0049] (3) The graphene-coated lithium titanate composite material was taken out, ground into powder, and then sieved to obtain a graphene-coated lithium titanate composite material sample.

[0050] In step (1), the ball milling speed is 350 r / min, and the ball milling time ranges from 4 h to 6 h.

[0051] In step (2), lithium titanate and graphene are placed in a tube furnace for sintering at a sintering temperature of 240° C. to 280° C. and a sintering time ranging from 12 h to 20 h.

[0052] The preparation method of the positive electrode sheet includes the following steps:

[0053] 1) Polyvinylidene fluoride (PVDF) was added to N-methylpyrrolidone and stirred in a planetary mixer for 20-30 minutes;

[0054] 2) selecting high-voltage lithium cobalt oxide, superconducting carbon black (SP), and carbon nanotubes (CNTS) according to a mass ratio and adding them into a planetary mixer for mixing and stirring; when the slurry viscosity is tested and qualified, a positive electrode slurry is obtained;

[0055] 3) The positive electrode slurry is coated on the carbon-coated aluminum foil using an extrusion coating machine and dried to obtain a positive electrode sheet.

[0056] In step 2), the planetary mixer is used for mixing and stirring at 800 r / min-1200 r / min for 60 min-90 min; the viscosity of the slurry is tested in the range of 4000 mPa·s-6000 mPa·s.

[0057] In step 3), the coating surface density is 1.9 g / 100 cm 2 -2.3g / 100cm 2 .

[0058] The method for preparing the negative electrode sheet includes the following steps:

[0059] ① Polyvinylidene fluoride (PVDF) was added to N-methylpyrrolidone and stirred in a planetary mixer for 20-30 minutes;

[0060] ② The graphene-coated lithium titanate composite material prepared in step S1 is selected, and superconductive carbon black (SP) and carbon nanotubes (CNTS) are weighed according to a mass ratio and added into a planetary mixer for mixing and stirring. When the slurry viscosity is tested and qualified, a negative electrode slurry is obtained;

[0061] ③ Use an extrusion coating machine to evenly coat the negative electrode slurry on the carbon-coated aluminum foil and dry it to obtain the negative electrode sheet.

[0062] In step ②, mixing and stirring are performed in a planetary mixer at 800 r / min-1200 r / min for 60 min-90 min; the viscosity of the slurry is tested in the range of 5000 mPa·s-7000 mPa·s.

[0063] In step ③, the coating surface density is 2.0g / 100cm 2 -2.5g / 100cm 2 .

[0064] Example 1

[0065] S1. Preparation method of graphene-coated lithium titanate composite material.

[0066] (1) Lithium titanate and graphene were mixed in a mass ratio of 85:15 and ball-milled until uniformly dispersed at a speed of 350 r / min for 5 h;

[0067] (2) sintering the mixed ball-milled lithium titanate and graphene in step (1) at 260° C. for 18 h in a tube furnace to prepare a graphene-coated lithium titanate composite material;

[0068] (3) The graphene-coated lithium titanate composite material was taken out, ground into powder, and then passed through a 200-mesh sieve to obtain a graphene-coated lithium titanate composite material sample.

[0069] S2. A method for preparing a positive electrode plate using a material including high-voltage lithium cobalt oxide.

[0070] 1) 3 parts by mass of polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone and stirred at 600 rpm in a planetary mixer for 20-30 minutes;

[0071] 2) Further, 92 parts of high-voltage lithium cobalt oxide material, 3 parts of superconducting carbon black (SP), and 2 parts of carbon nanotubes (CNTS) were added to a planetary mixer according to a mass ratio and mixed at a speed of 1200 r / min for 80 minutes. When the viscosity of the slurry was tested to be in the range of 4000 mPa·s-6000 mPa·s, a positive electrode slurry was obtained;

[0072] 3) The positive electrode slurry is coated on the carbon-coated aluminum foil using an extrusion coater with a coating surface density of 2.1 g / 100 cm 2 ; Use a vacuum oven to bake at 110 degrees Celsius for more than 48 hours to obtain the positive electrode.

[0073] S3. A method for preparing a negative electrode sheet using the graphene-coated lithium titanate composite material in step S1.

[0074] ① Select 4 parts of polyvinylidene fluoride (PVDF) by mass ratio and add N-methylpyrrolidone and stir at 600 r / min in a planetary mixer for 20 min-30 min;

[0075] ② Further, 92 parts of graphene-coated lithium titanate composite material, 2 parts of superconducting carbon black (SP), and 2 parts of carbon nanotubes (CNTS) were added to a planetary mixer according to the mass ratio and mixed at 1000 r / min for 80 minutes. When the viscosity of the slurry was tested to be in the range of 5000 mPa·s-7000 mPa·s, the negative electrode slurry was obtained;

[0076] ③ Use an extrusion coater to coat the negative electrode slurry on the carbon-coated aluminum foil with a coating surface density of 2.4g / 100cm 2 ; Use a vacuum oven to bake at 110 degrees Celsius for more than 48 hours to obtain the negative electrode.

[0077] S4. After the prepared positive electrode sheets and negative electrode sheets are cut by a die-cutting machine, the battery is prepared, injected, and formed by stacking.

[0078] S5. The prepared battery was subjected to 50C rate discharge, ultra-low temperature -55°C and -60°C discharge capacity, 10C rate cycle performance, and needle puncture performance tests. The performance test results of the TV prepared in Example 1 are as follows:

[0079] Figure 1 The SEM image shows that the graphene material is evenly coated on the surface of lithium titanate and the morphology of the composite material is regular.

[0080] Figure 2 The discharge results show that the prepared lithium-ion battery can sustain discharge at a rate of 50C for more than 6s at room temperature.

[0081] Figure 3 The discharge results show that the discharge capacity retention rate of the prepared lithium-ion battery can reach more than 81% at -55°C and more than 66% at -60°C.

[0082] Figure 4 The cycling results show that the capacity retention rate of the prepared lithium-ion battery is still greater than 80% after 10,000 cycles at a 10C rate.

[0083] Figure 5 The puncture results showed that the prepared lithium-ion battery did not catch fire or explode after puncture, indicating that the prepared lithium-ion battery has excellent safety performance.

[0084] Example 2

[0085] S1. Preparation method of graphene-coated lithium titanate composite material.

[0086] (1) Lithium titanate and graphene were mixed in a mass ratio of 80:20 and ball-milled until uniformly dispersed at a speed of 350 r / min for 5 h;

[0087] (2) sintering the mixed ball-milled lithium titanate and graphene in step (1) at 260° C. for 18 h in a tube furnace to prepare a graphene-coated lithium titanate composite material;

[0088] (3) The graphene-coated lithium titanate composite material was taken out, ground into powder, and then passed through a 200-mesh sieve to obtain a graphene-coated lithium titanate composite material sample.

[0089] S2. A method for preparing a positive electrode plate using a material including high-voltage lithium cobalt oxide.

[0090] 1) 3 parts by mass of polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone and stirred at 600 rpm in a planetary mixer for 20-30 minutes;

[0091] 2) Further, 92 parts of high-voltage lithium cobalt oxide material, 3 parts of superconducting carbon black (SP), and 2 parts of carbon nanotubes (CNTS) were added to a planetary mixer according to a mass ratio and mixed at a speed of 1200 r / min for 80 minutes. When the viscosity of the slurry was tested to be in the range of 4000 mPa·s-6000 mPa·s, a positive electrode slurry was obtained;

[0092] 3) The positive electrode slurry is coated on the carbon-coated aluminum foil using an extrusion coater with a coating surface density of 2.1 g / 100 cm 2 ; Use a vacuum oven to bake at 110 degrees Celsius for more than 48 hours to obtain the positive electrode.

[0093] S3. A method for preparing a negative electrode sheet using the graphene-coated lithium titanate composite material in step S1.

[0094] ① Select 4 parts of polyvinylidene fluoride (PVDF) by mass ratio and add N-methylpyrrolidone and stir at 600 r / min in a planetary mixer for 20 min-30 min;

[0095] ② Further, 92 parts of graphene-coated lithium titanate composite material, 2 parts of superconducting carbon black (SP), and 2 parts of carbon nanotubes (CNTS) were added to a planetary mixer according to the mass ratio and mixed at 1000 r / min for 80 minutes. When the viscosity of the slurry was tested to be in the range of 5000 mPa·s-7000 mPa·s, the negative electrode slurry was obtained;

[0096] ③ Use an extrusion coater to coat the negative electrode slurry on the carbon-coated aluminum foil with a coating surface density of 2.4g / 100cm 2 ; Use a vacuum oven to bake at 110 degrees Celsius for more than 48 hours to obtain the negative electrode.

[0097] S4. After the prepared positive electrode sheets and negative electrode sheets are cut by a die-cutting machine, the battery is prepared, injected, and formed by stacking.

[0098] S5. The prepared batteries are subjected to 50C rate discharge and other performance tests for verification.

[0099] Example 3

[0100] S1. Preparation method of graphene-coated lithium titanate composite material.

[0101] (1) Lithium titanate and graphene were mixed in a mass ratio of 90:10 and ball-milled until uniformly dispersed at a speed of 350 r / min for 5 h;

[0102] (2) sintering the mixed ball-milled lithium titanate and graphene in step (1) at 260° C. for 18 h in a tube furnace to prepare a graphene-coated lithium titanate composite material;

[0103] (3) The graphene-coated lithium titanate composite material was taken out, ground into powder, and then passed through a 200-mesh sieve to obtain a graphene-coated lithium titanate composite material sample.

[0104] S2. A method for preparing a positive electrode plate using a material including high-voltage lithium cobalt oxide.

[0105] 1) 3 parts by mass of polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone and stirred at 600 rpm in a planetary mixer for 20-30 minutes;

[0106] 2) Further, 92 parts of high-voltage lithium cobalt oxide material, 3 parts of superconducting carbon black (SP), and 2 parts of carbon nanotubes (CNTS) were added to a planetary mixer according to a mass ratio and mixed at 1200 r / min for 80 minutes. When the viscosity of the slurry was tested to be in the range of 4000 mPa·s-6000 mPa·s, a positive electrode slurry was obtained;

[0107] 3) The positive electrode slurry is coated on the carbon-coated aluminum foil using an extrusion coater with a coating surface density of 2.1 g / 100 cm 2 ; Use a vacuum oven to bake at 110 degrees Celsius for more than 48 hours to obtain the positive electrode.

[0108] S3. A method for preparing a negative electrode sheet using the graphene-coated lithium titanate composite material in step S1.

[0109] ① Select 4 parts of polyvinylidene fluoride (PVDF) by mass ratio and add N-methylpyrrolidone (NMP) and stir in a planetary mixer at a speed of 600 r / min for 20 min-30 min;

[0110] ② Further, 92 parts of graphene-coated lithium titanate composite material, 2 parts of superconducting carbon black (SP), and 2 parts of carbon nanotubes (CNTS) were added to a planetary mixer according to the mass ratio and mixed at 1000 r / min for 80 minutes. When the viscosity of the slurry was tested to be in the range of 5000 mPa·s-7000 mPa·s, the negative electrode slurry was obtained;

[0111] ③ Use an extrusion coater to coat the negative electrode slurry on the carbon-coated aluminum foil with a coating surface density of 2.4g / 100cm 2 ; Use a vacuum oven to bake at 110 degrees Celsius for more than 48 hours to obtain the negative electrode.

[0112] S4. After the prepared positive electrode sheets and negative electrode sheets are cut by a die-cutting machine, the battery is prepared, injected, and formed by stacking.

[0113] S5. The prepared battery is subjected to 50C rate discharge and other performance tests for verification.

[0114] The present invention provides a method for preparing a lithium-ion battery, which optimizes and improves the performance of lithium titanate materials by adopting a mixed sintering method. The lithium titanate is mainly coated with graphene materials with different contents, so that the conductive properties of the lithium titanate material are effectively improved, and the comprehensive performance of the battery, such as rate and low temperature, is effectively optimized from the material perspective.

[0115] From the perspective of battery design, high-voltage lithium cobalt oxide material with a wide temperature range and excellent rate performance is selected as the positive electrode material, matched with the optimized and improved lithium titanate negative electrode material, and a special electrolyte for wide-temperature range lithium titanate batteries is selected for full battery design and preparation. Among them, carbon-coated aluminum foil is preferably used as the current collector, and the electrode preparation parameters are explored and optimized to prepare a lithium-ion battery that can take into account power performance, ultra-low temperature discharge performance, long life and high safety.

[0116] The prepared lithium-ion battery can sustain a discharge rate of more than 6 seconds at 50C at room temperature, retains over 81% of its discharge capacity at -55°C, and over 66% at -60°C. After 10,000 cycles at 10C, the capacity retention rate remains above 80%. The fully charged battery can pass a needle penetration test without catching fire or exploding. This can further expand the battery's operational environment, meeting the performance requirements of a wider range of applications and even in more demanding environments.

[0117] The above describes in detail the method for preparing a lithium-ion battery provided by the present invention. Specific examples are used herein to illustrate the structure and operating principles of the present invention. The description of the above embodiments is intended only to facilitate understanding of the method and core concepts of the present invention. It should be noted that those skilled in the art may make various improvements and modifications to the present invention without departing from the principles of the present invention, and such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a lithium-ion battery, characterized in that: The steps include: S1. preparing a graphene-coated lithium titanate composite material; S2. preparing a positive electrode sheet using a material including high-voltage lithium cobalt oxide; S3, preparing a negative electrode sheet using the graphene-coated lithium titanate composite material in step S1; S4. Cut the prepared positive and negative electrode sheets by a die-cutting machine, and then stack them to complete the cell preparation, liquid injection, and formation to complete the battery preparation; S5. After the preparation is completed, the battery is subjected to rate discharge, discharge capacity under ultra-low temperature environment, rate cycle performance, and needle puncture performance test verification; The preparation method of the graphene-coated lithium titanate composite material comprises the following steps: (1) lithium titanate and graphene are mixed in a mass ratio of (80-90): (20-10), and ball milled until uniformly dispersed; (2) sintering the lithium titanate and graphene mixed and ball-milled in step (1) to prepare a graphene-coated lithium titanate composite material; (3) taking out the graphene-coated lithium titanate composite material, grinding it into powder, and then sieving it to obtain a graphene-coated lithium titanate composite material sample; In step (2), lithium titanate and graphene are placed in a tube furnace and sintered at a temperature of 240°C-280°C and a sintering time of 12h-20h; The preparation method of the positive electrode sheet includes the following steps: 1) Polyvinylidene fluoride (PVDF) was added to N-methylpyrrolidone and stirred in a planetary mixer for 20-30 minutes; 2) selecting high-voltage lithium cobalt oxide, superconducting carbon black (SP), and carbon nanotubes (CNTS) according to a mass ratio and adding them into a planetary mixer for mixing and stirring; when the slurry viscosity is tested and qualified, a positive electrode slurry is obtained; 3) Using an extrusion coater to coat the positive electrode slurry on the carbon-coated aluminum foil and drying it to obtain a positive electrode sheet; The method for preparing the negative electrode sheet includes the following steps: ① Polyvinylidene fluoride (PVDF) was added to N-methylpyrrolidone and stirred in a planetary mixer for 20-30 minutes; ② The graphene-coated lithium titanate composite material prepared in step S1 is selected, and superconductive carbon black (SP) and carbon nanotubes (CNTS) are weighed according to a mass ratio and added into a planetary mixer for mixing and stirring. When the slurry viscosity is tested and qualified, a negative electrode slurry is obtained; ③ Use an extrusion coating machine to evenly coat the negative electrode slurry on the carbon-coated aluminum foil and dry it to obtain the negative electrode sheet.

2. The method for preparing a lithium-ion battery according to claim 1, wherein: In step (1), the ball milling speed is 350 r / min, and the ball milling time ranges from 4 h to 6 h.

3. The method for preparing a lithium-ion battery according to claim 1, wherein: In step 2), the planetary mixer is used for mixing and stirring at 800 r / min-1200 r / min for 60 min-90 min; the viscosity of the slurry is tested in the range of 4000 mPa·s-6000 mPa·s.

4. The method for preparing a lithium-ion battery according to claim 1, wherein: In step 3), the coating surface density is 1.9 g / 100 cm 2 -2.3g / 100cm 2 .

5. The method for preparing a lithium-ion battery according to claim 1, wherein: In step ②, mixing and stirring are performed in a planetary mixer at 800 r / min-1200 r / min for 60 min-90 min; the viscosity of the slurry is tested in the range of 5000 mPa·s-7000 mPa·s.

6. The method for preparing a lithium-ion battery according to claim 1, wherein: In step ③, the coating surface density is 2.0g / 100cm 2 -2.5g / 100cm 2 .

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