Negative plate, preparation method thereof and lithium ion battery

Abstract

The invention discloses a negative electrode plate and a preparation method thereof and a lithium ion battery, the preparation method of the negative electrode plate comprises the following steps: step 1, adding a waste negative electrode plate and a stripping solvent into a stirring kettle, stirring until a negative electrode coating on a current collector completely falls off, filtering, and demagnetizing filtrate to obtain a negative electrode material liquid; 2, adding the negative electrode material liquid and a binder into a stirring kettle, dispersing, demagnetizing and sieving to obtain negative electrode slurry; the stripping solvent comprises deionized water and other solvents; and 3, coating a copper foil current collector with the negative electrode slurry, and drying to obtain the negative electrode plate. Compared with the prior art, the current collector and the negative electrode coating of the waste negative electrode plate are stripped by adopting a stripping solvent dipping method, and the obtained negative electrode material is good in material liquid dispersity and free of caking, can be directly prepared into negative electrode slurry, and is coated to form a new negative electrode plate.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in mobile phones, portable computers, camcorders, cameras, electric vehicles, energy storage and other fields due to their high energy density, long cycle life, environmental protection, low self-discharge and other advantages. Due to the widespread use of lithium-ion batteries, the production volume of lithium-ion batteries has also become very large, and some defective products will be produced during the preparation process, especially before the battery is filled with liquid, due to coating, rolling, winding and other processes, resulting in a large number of scrapped pole pieces. The negative electrode materials of these scrapped pole pieces or winding cores are basically not damaged, and the positive electrode materials themselves have a stable structure and a very high recycling rate, so there is an urgent need to find a simple and efficient recycling method to deal with the scrapped pole pieces or winding cores.

[0003] In the prior art, the main method for recycling negative electrode sheets is to obtain negative electrode material powder through crushing, calcination, stripping, primary demagnetization, secondary crushing, and secondary demagnetization, with a recovery rate of up to 85.7%. However, the process is complicated, the recovery rate is low, the carbon content is high, and the BET is large, which seriously affects the regeneration performance of the negative electrode material.

[0004] In view of the above-mentioned defects in the current recycling of waste negative electrode sheets, it is indeed necessary to provide a technical solution to the above-mentioned problems. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a negative electrode sheet, which has the advantages of simple operation and low cost, and can recover a highly active negative electrode sheet through simple immersion separation, stirring and coating.

[0006] To achieve this object, the present invention provides the following solutions:

[0007] A method for preparing a negative electrode sheet comprises the following steps:

[0008] Step 1: Add the waste negative electrode sheet and the stripping solvent into a stirring kettle, stir until the negative electrode coating on the current collector is completely removed, filter, and demagnetize the filtrate to obtain a negative electrode material solution;

[0009] Step 2: adding the negative electrode material liquid and the binder into a stirring kettle for dispersion, demagnetization, and sieving to obtain a negative electrode slurry; the stripping solvent includes deionized water and other solvents;

[0010] Step three: coating the negative electrode slurry on a copper foil current collector and drying the copper foil to obtain a negative electrode sheet.

[0011] Preferably, in step 1, the other solvent is at least one of ethylene carbonate, dimethyl methanol and N-methylpyrrolidone.

[0012] Preferably, in step one, the waste negative electrode sheet is a waste negative electrode sheet that has not been injected with liquid; the negative electrode coating material of the waste negative electrode sheet includes at least one of artificial graphite, natural graphite, silicon-carbon material and hard carbon.

[0013] Preferably, in the step 1, the mass ratio of the waste negative electrode sheet to the stripping solvent is (30-60): (40-70).

[0014] Preferably, in step 1 and step 2, the stirring linear speed of the stirring tank is 3 to 35 m / s.

[0015] Preferably, in step 1 and step 2, the magnetic field strength of the demagnetization is 6000-12000 Gs.

[0016] Preferably, in the step 2, the solid content of the negative electrode slurry is 30-60%.

[0017] Preferably, in the step 2, the binder comprises acrylic resin or styrene-butadiene rubber, and the mass percentage of the binder to the solid in the negative electrode material solution is 0.02-1%.

[0018] The present invention also provides a negative electrode sheet, which is prepared by the above-mentioned method for preparing the negative electrode sheet; the film resistance of the negative electrode sheet is ≤0.1Ω / cm2; and the peeling force is ≥5N / m.

[0019] The present invention also provides a lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet and a separator spaced between the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet mentioned above.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention adopts a stripping solvent immersion method to strip the current collector and negative electrode coating of the waste negative electrode sheet, the stripping solvent is conducive to better dispersion of the negative electrode material in the solvent, avoiding its precipitation in the solvent, and the obtained negative electrode material liquid has good dispersibility and no agglomeration, and can be directly prepared into negative electrode slurry and coated to form new negative electrode sheets; the method does not need to crush the negative electrode sheet, avoiding the problem of excessively high metal impurity content in the negative electrode material caused by the introduction of metal debris into the negative electrode material liquid; the method also does not require the calcination step, effectively avoiding the problems of excessively high carbon content, excessively large specific surface area, and poor conductivity of the negative electrode material caused by calcination in the traditional method. DETAILED DESCRIPTION

[0021] In order to make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] According to a first aspect of the present invention, a method for preparing a negative electrode sheet is provided, comprising the following steps:

[0023] Step 1: Add the waste negative electrode sheet and the stripping solvent into a stirring kettle, stir until the negative electrode coating on the current collector is completely removed, filter, and demagnetize the filtrate to obtain a negative electrode material solution;

[0024] Step 2: adding the negative electrode material liquid and the binder into a stirring kettle for dispersion, demagnetization, and sieving to obtain a negative electrode slurry; the stripping solvent includes deionized water and other solvents;

[0025] Step three: coating the negative electrode slurry on a copper foil current collector and drying the copper foil to obtain a negative electrode sheet.

[0026] In one embodiment of the present invention, in step 1, the other solvent is at least one of ethylene carbonate, dimethyl carbinol and N-methyl pyrrolidone. Other solvents are beneficial to improving the wettability of deionized water with graphite and conductive carbon. The improvement of wettability is beneficial to stripping the negative electrode coating from the current collector, and is also beneficial to improving the subsequent slurry dispersion uniformity. The fineness, stripping force and film resistance of the negative electrode slurry that can be obtained are greatly improved.

[0027] In one embodiment of the present invention, in step 1, the stripping solvent is deionized water and N-methylpyrrolidone, and the mass ratio of deionized water and N-methylpyrrolidone is (90-99.5): (0.5-10), for example, it can be 90:10, 91:9, 92:8, 93:7, 94:6, 95:5, 96:4, 97:3, 98:2, 99:1, 99.5:0.5. Further, the optimal ratio is 95:5.

[0028] In one embodiment of the present invention, in step 1, the stripping solvent is ethylene carbonate and deionized water, and the mass ratio of deionized water to ethylene carbonate is (90-99.5): (0.5-10), for example, it can be 90:10, 91:9, 92:8, 93:7, 94:6, 95:5, 96:4, 97:3, 98:2, 99:1, 99.5:0.5. Further, the optimal ratio is 97:3.

[0029] In one embodiment of the present invention, in step 1, the stripping solvent is deionized water and dimethyl carbinol, and the mass ratio of deionized water to dimethyl carbinol is (90-99.5): (0.5-10), for example, it can be 90:10, 91:9, 92:8, 93:7, 94:6, 95:5, 96:4, 97:3, 98:2, 99:1, 99.5:0.5. Further, the optimal ratio is 97:3.

[0030] According to one embodiment of the present invention, in step 1, the stripping solvent is deionized water, N-methylpyrrolidone and ethylene carbonate, and the mass ratio of deionized water, N-methylpyrrolidone and ethylene carbonate is (80-99): (0.5-10): (0.5-10), for example, it can be 80:10:10, 82:9:9, 84:8:8, 86:7:7, 88:6:6, 90:5:5, 92:4:4, 94:6:6, 96:3:3, 98:1:1, 99:0.5:0.5, 81 :9:10, 82:8:10, 83:7:10, 84:6:10, 85:5:10, 86:4:10, 87:3:10, 88:2:10, 89:1:10, 89.5:0.5:10, 81:10:9, 82:10:8, 83:10:7, 84:10:6, 85:10:5, 86:10:4, 87:10:3, 88:10:2, 89:10:1, 89.5:10:0.5, further, the optimal ratio is 92:5:3.

[0031] In one embodiment of the present invention, in step 1, the stripping solvent is deionized water, dimethyl carbinol, N-methyl pyrrolidone and ethylene carbonate, and the mass ratio of deionized water, dimethyl carbinol, N-methyl pyrrolidone and ethylene carbonate is (70-98.5): (0.5-10): (0.5-10): (0.5-10). Furthermore, the optimal ratio is 90:2:5:3.

[0032] In one embodiment of the present invention, in step one, the waste negative electrode sheet is a waste negative electrode sheet that has not been injected with liquid; the negative electrode coating material of the waste negative electrode sheet includes at least one of artificial graphite, natural graphite, silicon-carbon material and hard carbon.

[0033] According to one embodiment of the present invention, in step one, the mass ratio of the waste negative electrode sheet to the stripping solvent is (30-60):(40-70), for example, it can be 30:40, 30:50, 30:65, 35:40, 35:50, 35:65, 50:40, 50:55, 50:65, 60:40, 60:50, 60:65.

[0034] In one embodiment of the present invention, in step 1 and step 2, the stirring line speed of the stirring tank is 3 to 35 m / s, for example, 3 m / s, 5 m / s, 8 m / s, 9 m / s, 10 m / s, 15 m / s, 17 m / s, 20 m / s, 25 m / s, 30 m / s, 31 m / s, 32 m / s, 34 m / s, 35 m / s. When the stirring line speed is within this range, sufficient shear force and turbulence intensity can be generated to fully disperse the coating and the current collector in the stirring tank to avoid material agglomeration or precipitation.

[0035] According to one embodiment of the present invention, in step 2, the stirring time of the stirring tank is 5 to 7 hours, for example, it can be 5h, 5.1h, 5.2h, 5.3h, 5.4h, 5.5h, 5.6h, 5.7h, 5.8h, 5.9h, 6h, 6.1h, 6.2h, 6.3h, 6.4h, 6.5h, 6.6h, 6.7h, 6.8h, 6.9h, and 7h.

[0036] In one embodiment of the present invention, in step 1 and step 2, the magnetic field strength of demagnetization is 6000-12000Gs, for example, 6000Gs, 7000Gs, 8000Gs, 9000Gs, 10000Gs, 11000Gs, 12000Gs. A higher magnetic field strength can more effectively capture the magnetic particles in the positive electrode slurry, especially those weakly magnetic tiny particles, thereby improving the thoroughness of demagnetization and avoiding the adverse effects of impurities on battery performance, such as reducing battery capacity and increasing internal resistance; at the same time, under the action of a strong magnetic field, the magnetic particles can be adsorbed onto the demagnetization equipment more quickly, thereby shortening the time of the entire demagnetization process and improving production efficiency.

[0037] In one embodiment of the present invention, in step 2, the solid content of the negative electrode slurry is 30-60%, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%. Appropriate solid content helps to optimize the electrode structure and ensure uniform coating of the negative electrode material. This can not only improve the uniformity and consistency of the battery, but also reduce the performance degradation caused by material shedding or structural changes, thereby extending the cycle life of the battery.

[0038] In one embodiment of the present invention, in step 2, the binder includes acrylic resin or styrene-butadiene rubber, and the mass percentage of the binder to the solid in the negative electrode material solution is 0.02-1%, for example, it can be 0.02%, 0.03%, 0.04%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, or 1%.

[0039] According to the second aspect of the present invention, there is also provided a negative electrode sheet, which is prepared by the above-mentioned method for preparing the negative electrode sheet; the membrane resistance of the negative electrode sheet is ≤0.1Ω / cm2; lower membrane resistance means that the conduction of electrons in the negative electrode sheet is smoother, reducing resistance loss, thereby improving the charging and discharging efficiency of the battery, which helps the battery to complete the charging and discharging process in a shorter time, improves the overall performance of the battery, and can also reduce the heat generation of the battery during the charging and discharging process, which helps to improve the safety and reliability of the battery.

[0040] The peeling force is ≥5N / m. The peeling force is expressed as the adhesion particle size between the current collector and the negative electrode material coating. The greater the peeling force between the current collector and the negative electrode coating, the more it helps prevent interlayer separation caused by volume expansion or contraction during the charge and discharge process of the battery, thereby improving the stability of the battery structure.

[0041] According to a third aspect of the present invention, there is also provided a lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet and a separator between the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet mentioned above.

[0042] The positive electrode sheet includes a positive electrode collector and a positive electrode material layer coated on at least one surface of the positive electrode collector. The positive electrode active material layer may include but is not limited to layered oxide positive electrode materials, spinel structure positive electrode materials, olivine structure positive electrode materials, and polyanion positive electrode materials; the layered oxide positive electrode materials include lithium nickel oxide (such as lithium nickel oxide), lithium cobalt oxide (such as lithium cobalt oxide LiCoO2), nickel cobalt manganese oxide (such as NCM), nickel cobalt aluminum oxide (such as NCA), etc.; the spinel structure positive electrode material includes lithium manganese oxide (LiMn2O4); the olivine structure positive electrode material includes lithium iron phosphate (LiFePO4); the polyanion positive electrode material includes phosphates and sulfates.

[0043] The positive electrode current collector can be any material suitable for use as a positive electrode current collector for lithium-ion batteries in the art, for example, aluminum foil or composite aluminum foil.

[0044] The separator can be selected from various separators used in lithium-ion batteries known to those skilled in the art, such as polyethylene separators, polypropylene separators, various composite separators, ceramic separators, and the like.

[0045] The secondary battery also includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt and an additive. Among them, the electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in anti-overcharge electrolytes; it can also be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; it can also be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additives include but are not limited to at least one of film-forming additives, conductive additives, flame retardant additives, anti-overcharge additives, additives for controlling the content of H2O and HF in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0046] The present invention will be further described below by means of specific examples.

[0047] Example 1

[0048] Preparation of negative electrode:

[0049] Step 1: Add 10 kg of waste negative electrode sheets and 5 kg of stripping solvent into a stirring kettle, stir at a linear speed of 3 m / s until the negative electrode coating on the current collector is completely detached, filter, and demagnetize the filtrate with an 8000 Gs iron remover to obtain a negative electrode material liquid; wherein the stripping solvent is ethylene carbonate and deionized water;

[0050] Step 2: Add the negative electrode material liquid and 0.02 kg of SBR into a stirring tank for dispersion, with a stirring line speed of 13 m / s and a stirring time of 6 h. Demagnetize with an 8000 Gs iron remover and sieve to obtain a negative electrode slurry.

[0051] Step 3: coating the negative electrode slurry on the negative electrode current collector and drying at 85° C. to obtain a negative electrode sheet.

[0052] Preparation of lithium-ion batteries:

[0053] The negative electrode sheet, the positive electrode sheet and the above-mentioned separator are interlaced and wound and then packaged with aluminum-plastic film to prepare a battery cell, which is placed in a battery shell, and an electrolyte is added and then sealed to obtain a lithium-ion battery. Among them, the active material of the positive electrode sheet is lithium iron phosphate; the electrolyte salt of the electrolyte is lithium hexafluorophosphate, and the solvent is ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a mass ratio of 1:2:1, and the electrolyte concentration is 1 mol / L.

[0054] Table 1 shows the data of Examples 1 to 8 and Comparative Example 1. The rest is the same as Example 1 and will not be described again.

[0055]

[0056] The fineness test of the negative electrode slurries of Examples 1 to 8 and Comparative Example 1 was performed to observe the dispersibility of the negative electrode slurries. The test results are shown in Table 2;

[0057] The negative electrode sheets of Examples 1 to 8 and Comparative Example 1 were tested for electrode sheet peeling and membrane resistance using a tensile tester and a membrane resistance meter, respectively. The test results are shown in Table 2.

[0058] Table 2

[0059]

[0060] As shown in Table 1, the dispersion of Examples 1 to 8 is compared with that of Comparative Example 1. It can be seen that the fineness of the negative electrode slurry of Examples 1 to 8 is very small, indicating uniform dispersion, while the fineness of the negative electrode slurry of Comparative Example 1 is relatively large, indicating the presence of agglomeration. This shows that by using the stripping solvent of the present invention to impregnate the waste negative electrode sheets, the obtained negative electrode material liquid has good dispersibility and no agglomeration, and can be directly prepared into a negative electrode slurry, which is coated to form a new negative electrode sheet, thereby reducing the cost of recycling the waste negative electrode sheets; it also shows that other solvents (at least one of ethylene carbonate, dimethyl methanol and N-methylpyrrolidone) are beneficial to improving the wettability of deionized water with graphite and conductive carbon, and the improvement of wettability is beneficial to the stripping of the negative electrode coating from the current collector, and is also beneficial to the improvement of the subsequent slurry dispersion uniformity, so the obtained negative electrode slurry fineness, stripping force and membrane resistance are greatly improved.

[0061] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for preparing a negative electrode sheet, characterized in that: The following steps are involved: Step 1: Add the waste negative electrode sheet and the stripping solvent into a stirring kettle, stir until the negative electrode coating on the current collector is completely removed, filter, and demagnetize the filtrate to obtain a negative electrode material solution; Step 2: adding the negative electrode material liquid and the binder into a stirring kettle for dispersion, demagnetization, and sieving to obtain a negative electrode slurry; the stripping solvent includes deionized water and other solvents; Step three: coating the negative electrode slurry on a copper foil current collector and drying the copper foil to obtain a negative electrode sheet.

2. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step 1, the other solvent is at least one of ethylene carbonate, dimethyl methanol and N-methylpyrrolidone.

3. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step 1, the waste negative electrode sheet is a waste negative electrode sheet that has not been injected with liquid; the negative electrode coating material of the waste negative electrode sheet includes at least one of artificial graphite, natural graphite, silicon-carbon material and hard carbon.

4. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step 1, the mass ratio of the waste negative electrode sheet to the stripping solvent is (30-60): (40-70).

5. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step 1 and the step 2, the stirring linear speed of the stirring tank is 3 to 35 m / s.

6. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step 1 and the step 2, the magnetic field strength of the demagnetization is 6000-12000 Gs.

7. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step 2, the solid content of the negative electrode slurry is 30-60%.

8. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step 2, the binder includes acrylic resin or styrene-butadiene rubber, and the mass percentage of the binder to the solid in the negative electrode material solution is 0.02-1%.

9. A negative electrode sheet, characterized in that: Prepared by the preparation method of the negative electrode sheet according to claims 1 to 8; the film resistance of the negative electrode sheet is ≤ 0.1Ω / cm 2 ; Peel force ≥5N / m.

10. A lithium ion battery comprising a positive electrode sheet, a negative electrode sheet and a separator between the positive electrode sheet and the negative electrode sheet, characterized in that: The negative electrode sheet is a negative electrode sheet prepared by the method for preparing a negative electrode sheet according to any one of claims 1 to 7 or a negative electrode sheet according to claim 9.

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