Negative plate, preparation method thereof and battery

Abstract

The invention provides a negative plate, a preparation method thereof and a battery. The negative plate comprises a current collector, a first coating layer and a second coating layer, wherein the two sides of the current collector are coated with the first coating layer and the second coating layer; the first coating layer is formed by curing first negative electrode slurry, the second coating layer is formed by curing second negative electrode slurry, both the first negative electrode slurry and the second negative electrode slurry comprise binders, the content of the binders in the first negative electrode slurry is A, and the content of the binders in the second negative electrode slurry is B; the content of the binder in the first coating layer is A ', the content of the binder in the second negative electrode slurry is B, the content of the binder in the second coating layer is B', A 'is smaller than A, and B is smaller than B'; in the baking process of the negative electrode plate, the binder in the first negative electrode slurry floats upwards to the second negative electrode slurry, and the content of the binder in the first coating layer and the content of the binder in the second coating layer just meet the minimum demand quantity; the situation that the binder in the negative plate is excessive in a traditional method is reduced, and the energy density of the battery cell is improved while the requirement that the negative plate has good binding performance can be just met.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a negative electrode sheet, its preparation method, and a battery. Background Technology

[0002] During the manufacturing process of lithium-ion batteries, after the negative electrode sheet is coated, the binder in the negative electrode slurry floats to the surface during baking as the solvent evaporates. Therefore, in the negative electrode formulation design, a slightly excessive amount of binder is added to prevent poor adhesion of the lower layers of binder after they float to the surface. In single-layer coating, the binder content in the negative electrode slurry is uniform. After the binder floats to the surface, a slightly excessive or just enough amount of binder in the lower layers can achieve a good adhesion effect, but excessive binder in the upper layers will cause a loss of energy density. Therefore, overcoming the above-mentioned technical problems and defects has become a key issue that needs to be addressed. Summary of the Invention

[0003] To address the problem of energy density loss in batteries due to excessive use of binders, this invention provides a negative electrode sheet, its preparation method, and a battery.

[0004] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: The first aspect of the present invention provides a negative electrode sheet, including a current collector and a first coating layer coated on both sides of the current collector, wherein a second coating layer is coated on both sides of the first coating layer away from the current collector. The first coating layer is formed by baking and curing a first negative electrode slurry. The first negative electrode slurry includes a first binder. The first binder accounts for 100% of the total mass of solid matter in the first negative electrode slurry. The percentage of the first binder in the total mass of solid matter in the first negative electrode slurry is A, and the value of A ranges from 0.1% to A and from 0.1% to 2%. The first binder accounts for 100% of the total mass of the first coating layer. The percentage of the first binder in the total mass of the first coating layer is A', and the value of A' ranges from 0.1% to A and from A to 2%. A' and A satisfy the relationship: A' < A. The second coating layer is formed by baking and curing a second negative electrode slurry. The second negative electrode slurry includes a second binder. The binder constitutes 100% of the total solid matter in the second negative electrode slurry. The percentage of the second binder in the total solid matter of the second negative electrode slurry is B, and the value of B ranges from 0.1% to 2%. The binder in the second coating layer constitutes 100% of the total solid matter in the second coating layer. The mass percentage of B' in the second coating layer is B', and the value of B' ranges from 0.1% to 2%. B' and B satisfy the relationship: B < B'. The relationship between A' and B' is: 0.95≤A' / B'≤1.05.

[0005] Optionally, A and B satisfy the relationship: 1≤A / B≤10.

[0006] Optionally, the mass of the first binder in the first negative electrode slurry is a, the mass of the first binder in the first coating layer is a', the mass of the second binder in the second negative electrode slurry is b, and the mass of the binder in the second coating layer is b'. The relationship between a', a, b and b' is: b'=b+a-a'.

[0007] Optionally, the first negative electrode slurry further includes a first negative electrode active material and a first thickener, with the first negative electrode active material accounting for 97% to 99% of the total mass of the first negative electrode slurry and the first thickener accounting for 0.1% to 2% of the total mass of the first negative electrode slurry, based on the total mass of the first negative electrode slurry.

[0008] Optionally, the first binder includes one or more of styrene-butadiene rubber, acrylic resin, fluoropolymer, nitrile rubber, polyvinyl alcohol, and sodium alginate; the first negative electrode active material includes one or more of graphite and silicon carbide; and the first thickener includes one or more of sodium carboxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, sodium alginate, and polyvinyl alcohol.

[0009] Optionally, the viscosity of the first negative electrode slurry at discharge is 3000~5000 cP.

[0010] Optionally, the second negative electrode slurry further includes a second negative electrode active material and a second thickener, which together constitute 100% of the total mass of the second negative electrode slurry. The second negative electrode active material accounts for 97% to 99% of the total mass of the second negative electrode slurry, and the second thickener accounts for 0.1% to 2% of the total mass of the second negative electrode slurry.

[0011] Optionally, the second binder includes one or more of styrene-butadiene rubber, acrylic resin, fluoropolymer, nitrile rubber, polyvinyl alcohol, and sodium alginate; the second negative electrode active material includes one or more of graphite and silicon carbide; and the second thickener includes one or more of sodium carboxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, sodium alginate, and polyvinyl alcohol.

[0012] Optionally, the discharge viscosity of the second negative electrode slurry is 3000~5000 cP.

[0013] A second aspect of the present invention provides a method for preparing a negative electrode sheet, comprising the following steps: S1: The first negative electrode slurry and the second negative electrode slurry are sequentially coated on both sides of the current collector to form a negative electrode precursor; S2: Dry the negative electrode precursor at a temperature of 62-115℃ for 4.5-21 minutes.

[0014] Optionally, the step of "drying the negative electrode precursor" in S2 includes the following steps: S21: The negative electrode precursor is dried for the first time at a temperature of 65-75℃ for 2-5 minutes. S22: Dry the negative electrode sheet in S21 a second time. The temperature of the second drying is 82-88℃ and the time is 0.5-3min. S23: Dry the negative electrode sheet in S22 for the third time. The temperature of the third drying is 62-68℃ and the time is 1-5 minutes. S24: Dry the negative electrode sheet in S23 for the fourth time. The temperature of the fourth drying is 105-115℃ and the time is 1-8 minutes.

[0015] A third aspect of the present invention provides a battery comprising a negative electrode sheet as described above, or a negative electrode sheet prepared by the method described above.

[0016] According to the present invention, a negative electrode sheet, its preparation method, and a battery are provided. The present invention involves coating a first negative electrode slurry on both sides of a current collector, and coating a second negative electrode slurry on the side of the first negative electrode slurry facing away from the current collector. During the electrode baking process, a first binder in the first negative electrode slurry floats to the second negative electrode slurry. After baking, the mass percentage of the first binder in the first coating layer becomes A', and the mass percentage of the binder in the second negative electrode coating layer becomes B'. After the first binder in the first negative electrode slurry floats to the surface, the content of the first binder in the first coating layer is just sufficient to ensure the bonding effect. Low demand; the initial content of the second binder in the second negative electrode slurry cannot guarantee the bonding effect with the negative electrode sheet. During the electrode baking process, when the first binder in the first negative electrode slurry floats to the second negative electrode slurry, after drying, the content of binder in the second coating layer just reaches the minimum amount of binder required to guarantee the bonding effect, thereby minimizing the amount of binder used and reducing the situation of excessive binder in the negative electrode sheet in traditional methods. By designing to ensure that the binder content is consistent throughout the vertical direction, the energy density of the cell is improved while just meeting the requirement that the electrode sheet has good bonding performance. Detailed Implementation

[0017] To make the technical problems solved, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0018] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified.

[0019] In one embodiment, the first aspect of the present invention provides a negative electrode sheet, which includes a current collector and a first coating layer coated on both sides of the current collector, wherein a second coating layer is coated on the side of the first coating layer facing away from the current collector. The first coating layer is formed by baking and curing a first negative electrode slurry. The first negative electrode slurry includes a first binder. The first binder accounts for 100% of the total mass of solid matter in the first negative electrode slurry. The percentage of the first binder in the total mass of solid matter in the first negative electrode slurry is A, and the value of A ranges from 0.1% to A and from 0.1% to 2%. The first binder accounts for 100% of the total mass of the first coating layer. The percentage of the first binder in the total mass of the first coating layer is A', and the value of A' ranges from 0.1% to A and from A to 2%. A' and A satisfy the relationship: A' < A. The second coating layer is formed by baking and curing a second negative electrode slurry. The second negative electrode slurry includes a second binder. The binder constitutes 100% of the total solid matter in the second negative electrode slurry. The percentage of the second binder in the total solid matter of the second negative electrode slurry is B, and the value of B ranges from 0.1% to 2%. The binder in the second coating layer constitutes 100% of the total solid matter in the second coating layer. The mass percentage of B' in the second coating layer is B', and the value of B' ranges from 0.1% to 2%. B' and B satisfy the relationship: B < B'. The relationship between A' and B' is: 0.95≤A' / B'≤1.05.

[0020] Specifically, the percentage of the first binder in the total mass of solid matter in the first negative electrode slurry is any one of the following values ​​or a range of any two values: 0.11%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2%. In a preferred embodiment, the percentage of the first binder in the total mass of solid matter in the first negative electrode slurry is 0.8%-1.2%.

[0021] When the percentage of the first binder in the total mass of solid matter in the first negative electrode slurry meets the condition of 0.1% < A ≤ 2%, good adhesion between the first coating layer and the current collector, as well as between the first coating layer and the second coating layer, can be ensured. Furthermore, the first binder in the first negative electrode slurry has a surplus, allowing it to float to the second coating layer during the baking process. When the percentage of the first binder in the total mass of solid matter in the first negative electrode slurry is less than 0.1%, the adhesion between the first coating layer and the current collector will be weak, and the first coating layer may easily detach during cycling. When the percentage of the first binder in the total mass of solid matter in the first negative electrode slurry is greater than 2%, the content of the first binder in the first coating layer will be too high, affecting the dispersion of the first negative electrode slurry during mixing, reducing the content of the main material in the first negative electrode slurry, and affecting the battery energy density.

[0022] Specifically, the percentage of the second binder in the total mass of solid matter in the second negative electrode slurry is any one of the following values, or a range of any two values: 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 1.99%. In a preferred embodiment, the percentage of the second binder in the total mass of solid matter in the second negative electrode slurry is 0.8%-1.2%.

[0023] When the percentage of the second binder in the total mass of solid matter in the second negative electrode slurry satisfies the condition: 0.1% ≤ B < 2%, the adhesion between the second coating layer and the first coating layer is not strong. When the first binder in the first coating layer floats to the second coating layer, there is good adhesion between the second coating layer and the first coating layer. When the percentage of the second binder in the total mass of solid matter in the second negative electrode slurry is less than 0.1%, the adhesion between the second coating layer and the first coating layer will be weak, and the second coating layer is prone to falling off during cycling. When the percentage of the second binder in the total mass of solid matter in the second negative electrode slurry is greater than 2%, the content of the second binder in the second coating layer will be too high, which will affect the dispersion of the second negative electrode slurry during the mixing process, reduce the content of the main material in the second negative electrode slurry, and affect the energy density of the battery.

[0024] Specifically, the ratio of A' to B' is any one value or a range of any two values ​​selected from 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.01, 1.02, 1.03, 1.04, or 1.05; in a preferred embodiment, the ratio of A' to B' is 0.98-1.02.

[0025] When the ratio of A' to B' is 0.95-1.05, the amount of the first binder in the first coating layer that floats to the second coating layer is appropriate. The first coating layer will not have excess first binder and can ensure good adhesion between the main material particles in the first coating layer and the current collector. The binder content in the second coating layer can ensure good adhesion between the second coating layer and the first coating layer. When the ratio of A' to B' is less than 0.95, the amount of the first binder floating from the first coating layer to the second coating layer will be insufficient. The first binder in the coating layer is redundant, the binder content in the second coating layer is insufficient, the adhesion between the second coating layer and the first coating layer is insufficient, and the second coating layer is prone to peeling off and powdering. When the ratio of A' to B' is greater than 1.05, it will cause the first coating layer to float to the second coating layer. The first binder content in the second coating layer is too high, the first binder content in the first coating layer is insufficient, the adhesion between the first coating layer and the current collector is insufficient, and it is prone to peeling off and powdering. The redundant binder content in the second coating layer will cause a loss of battery energy density.

[0026] The present invention involves coating a first negative electrode slurry on both sides of the current collector, and coating a second negative electrode slurry on the side of the first negative electrode slurry away from the current collector. During the electrode baking process, the first binder in the first negative electrode slurry floats to the second negative electrode slurry. After baking, the mass percentage of the first binder in the first coating layer becomes A', and the mass percentage of the binder in the second negative electrode coating layer becomes B'. After the first binder in the first negative electrode slurry floats to the surface, the content of the first binder in the first coating layer just reaches the minimum required amount of binder to ensure the bonding effect. Initially, the content of the second binder in the second negative electrode slurry cannot guarantee the bonding effect with the negative electrode sheet. During the electrode baking process, when the first binder in the first negative electrode slurry floats to the second negative electrode slurry and is dried, the content of the binder in the second coating layer just reaches the minimum required amount of binder to ensure the bonding effect. This minimizes the amount of binder used and reduces the situation of excessive binder in the negative electrode sheet in traditional methods. By designing to keep the binder content consistent throughout the vertical direction, the energy density of the cell is improved while ensuring that the electrode sheet has good bonding performance.

[0027] In one embodiment, A and B satisfy the relationship: 1≤A / B≤10.

[0028] Specifically, the ratio of A to B is any one value or a range of any two values ​​from 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or 10; in a preferred embodiment, the ratio of A to B is 1-2.

[0029] When the ratio of A to B is 1-10, the amount of the first binder floating from the first coating layer to the second coating layer is appropriate. The first coating layer will not have redundant first binder, and the binder content in the second coating layer ensures good adhesion between the main material particles in the second coating layer and the first coating layer. When the ratio of A to B is less than 1, the amount of the first binder floating from the first coating layer to the second coating layer will be insufficient, resulting in redundant first binder in the first coating layer. The amount of binder in the second coating layer will also be insufficient, leading to weak adhesion between the second and first coating layers, and the second coating layer is prone to peeling and powdering. When the ratio of A to B is greater than 10, the amount of binder floating from the first coating layer to the second coating layer will be too high, resulting in insufficient first binder in the first coating layer. The adhesion between the first coating layer and the current collector will be weak, leading to peeling and powdering. Redundant binder in the second coating layer will cause a loss of battery energy density.

[0030] In one embodiment, the mass of the first binder in the first negative electrode slurry is a, the mass of the first binder in the first coating layer is a', the mass of the second binder in the second negative electrode slurry is b, and the mass of the binder in the second coating layer is b'. The relationship between a', a, b, and b' is: b' = b + a - a'.

[0031] The content of the first binder that actually plays a bonding role in the first coating layer is a'; the content of the binder that actually plays a bonding role in the second coating layer is b'; after baking, the content of the first binder in the first negative electrode slurry to float to the second negative electrode slurry is a-a'. Therefore, the content of the binder that actually plays a bonding role in the second coating layer is b'=b+a-a'.

[0032] After the first binder in the first negative electrode slurry of this application floats to the surface, the content of the first binder in the first coating layer is a', which is just enough to meet the minimum requirement of the first binder to ensure the bonding effect. The content of the second binder in the second negative electrode slurry, b, is initially insufficient to guarantee the bonding effect with the negative electrode sheet. During the electrode baking process, when the first binder in the first negative electrode slurry floats to the second negative electrode slurry and is dried, the content of the binder in the second coating layer, b', just meets the minimum requirement of the binder to ensure the bonding effect, thereby minimizing the amount of binder used and reducing the situation of excessive binder in the negative electrode sheet in traditional methods. By designing to ensure that the content of binder is consistent throughout the vertical direction, the energy density of the cell is improved while ensuring that the electrode sheet has good bonding performance.

[0033] In one embodiment, the first negative electrode slurry further includes a first negative electrode active material and a first thickener, with the first negative electrode active material accounting for 97% to 99% of the total mass of the first negative electrode slurry and the first thickener accounting for 0.1% to 2% of the total mass of the first negative electrode slurry, based on the total mass of the first negative electrode slurry.

[0034] Specifically, the percentage content of the first negative electrode active material in the total mass of the first negative electrode slurry is any one value or a range of any two values ​​from 97%, 97.1%, 97.2%, 97.3%, 97.4%, 97.5%, 97.6%, 97.7%, 97.8%, 97.9%, 98%, 98.1%, 98.2%, 98.3%, 98.4%, 98.5%, 98.6%, 98.7%, 98.8%, 98.9%, or 99%; in a preferred embodiment, the percentage content of the first negative electrode active material in the total mass of the first negative electrode slurry is 97.5%-98.5%.

[0035] When the percentage of the first negative electrode active material in the total mass of the first negative electrode slurry is 97% to 99%, the battery has a high energy density. When the percentage of the first negative electrode active material in the total mass of the first negative electrode slurry is less than 97%, the content of the first negative electrode active material is low, which affects the energy density of the battery. When the percentage of the first negative electrode active material in the total mass of the first negative electrode slurry is greater than 99%, it will result in too little content of the first binder and the first thickener. Too little first binder will cause the first coating layer to peel off or shed powder. Too little first thickener will affect the dispersion effect of each component when the first negative electrode slurry is prepared.

[0036] Specifically, the percentage of the first thickener in the total mass of the first negative electrode slurry is any one value or a range of any two values ​​from 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2%; in a preferred embodiment, the percentage of the first thickener in the total mass of the first negative electrode slurry is 0.8%-1.2%.

[0037] When the first thickener accounts for 0.1%-2% of the total mass of the first negative electrode slurry, the components in the first negative electrode slurry have a good dispersion effect and are relatively uniformly dispersed during formulation. When the first thickener accounts for less than 0.1% of the total mass of the first negative electrode slurry, the content of the first thickener is too low, which affects the dispersion effect of the first negative electrode slurry. When the first thickener accounts for more than 2% of the total mass of the first negative electrode slurry, the proportion of the first thickener is too high, the proportion of the first negative electrode active material is reduced, and the energy density of the battery is affected.

[0038] In one embodiment, the first binder includes one or more of styrene-butadiene rubber, acrylic resin, fluoropolymer, nitrile rubber, polyvinyl alcohol, and sodium alginate; the first negative electrode active material includes one or more of graphite and silicon carbide; and the first thickener includes one or more of sodium carboxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, sodium alginate, and polyvinyl alcohol.

[0039] In one embodiment, the viscosity of the first negative electrode slurry at discharge is 3000~5000 cP.

[0040] Specifically, the viscosity of the first negative electrode slurry at discharge is any one value or a range of any two values ​​from 3000 cP, 3100 cP, 3200 cP, 3300 cP, 3400 cP, 3500 cP, 3600 cP, 3700 cP, 3800 cP, 3900 cP, 4000 cP, 4100 cP, 4200 cP, 4300 cP, 4400 cP, 4500 cP, 4600 cP, 4700 cP, 4800 cP, 4900 cP, or 5000 cP; in a preferred embodiment, the viscosity of the first negative electrode slurry at discharge is 3500~4500 cP.

[0041] In one embodiment, the second negative electrode slurry further includes a second negative electrode active material and a second thickener, with the second negative electrode active material accounting for 97% to 99% of the total mass of the second negative electrode slurry and the second thickener accounting for 0.1% to 2% of the total mass of the second negative electrode slurry, based on the total mass of the second negative electrode slurry.

[0042] Specifically, the percentage content of the second negative electrode active material in the total mass of the second negative electrode slurry is any one value or a range of any two values ​​from 97%, 97.1%, 97.2%, 97.3%, 97.4%, 97.5%, 97.6%, 97.7%, 97.8%, 97.9%, 98%, 98.1%, 98.2%, 98.3%, 98.4%, 98.5%, 98.6%, 98.7%, 98.8%, 98.9%, or 99%; in a preferred embodiment, the percentage content of the second negative electrode active material in the total mass of the second negative electrode slurry is 97.5%-98.5%.

[0043] When the percentage of the second negative electrode active material in the total mass of the second negative electrode slurry is 97% to 99%, the battery has a high energy density. When the percentage of the second negative electrode active material in the total mass of the second negative electrode slurry is less than 97%, the content of the second negative electrode active material will be low, affecting the energy density of the battery. When the percentage of the second negative electrode active material in the total mass of the second negative electrode slurry is greater than 99%, the content of the second binder and the second thickener will be too low. Too little second binder will cause the second coating layer to peel off or shed powder. Too little second thickener will affect the dispersion effect of each component during the formulation of the second negative electrode slurry.

[0044] Specifically, the percentage content of the second thickener in the total mass of the second negative electrode slurry is any one value or a range of any two values ​​from 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2%; in a preferred embodiment, the percentage content of the second thickener in the total mass of the second negative electrode slurry is 0.8%-1.2%.

[0045] When the second thickener accounts for 0.1%-2% of the total mass of the second negative electrode slurry, the components of the second negative electrode slurry have a good dispersion effect and are relatively uniformly dispersed during formulation. When the second thickener accounts for less than 0.1% of the total mass of the second negative electrode slurry, the content of the second thickener will be too low, affecting the dispersion effect of the second negative electrode slurry. When the second thickener accounts for more than 2% of the total mass of the second negative electrode slurry, the proportion of the second thickener will be too high, the proportion of the second negative electrode active material will decrease, and the energy density of the battery will be affected.

[0046] In one embodiment, the second binder includes one or more of styrene-butadiene rubber, acrylic resin, fluoropolymer, nitrile rubber, polyvinyl alcohol, and sodium alginate; the second negative electrode active material includes one or more of graphite and silicon carbide; and the second thickener includes one or more of sodium carboxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, sodium alginate, and polyvinyl alcohol.

[0047] In one embodiment, the discharge viscosity of the second negative electrode slurry is 3000~5000 cP.

[0048] Specifically, the viscosity of the second negative electrode slurry at discharge is any two values ​​or a range of any two values ​​selected from 3000 cP, 3100 cP, 3200 cP, 3300 cP, 3400 cP, 3500 cP, 3600 cP, 3700 cP, 3800 cP, 3900 cP, 4000 cP, 4100 cP, 4200 cP, 4300 cP, 4400 cP, 4500 cP, 4600 cP, 4700 cP, 4800 cP, 4900 cP, or 5000 cP; in a preferred embodiment, the viscosity of the first negative electrode slurry at discharge is 3500~4500 cP.

[0049] A second aspect of the present invention provides a method for preparing a negative electrode sheet, comprising the following steps: S1: The first negative electrode slurry and the second negative electrode slurry are sequentially coated on both sides of the current collector to form a negative electrode precursor; S2: Dry the negative electrode precursor at a temperature of 62-115℃ for 4.5-21 minutes.

[0050] In step 1, the first negative electrode active material, the first binder and the first thickener are mixed in a ratio of (98~99):(0.1~2):(0.1~2) to prepare the first negative electrode slurry, and the solid content of the first negative electrode slurry is 40%~60%.

[0051] The second negative electrode active material, the second binder, and the second thickener are mixed in a ratio of (98~99):(0.1~2):(0.1~2) to prepare a second negative electrode slurry, and the solid content of the second negative electrode slurry is 40%~60%.

[0052] In step 2, by controlling the drying temperature and time, the floating content of the first binder in the first negative electrode slurry is precisely controlled, so that the content of the first binder in the first coating layer is at the minimum required amount; the content of the binder in the second coating layer is just at the minimum required amount of binder to ensure the bonding effect, thereby minimizing the amount of binder used and reducing the situation of excessive binder in the negative electrode sheet in the traditional method. By designing to make the binder content consistent throughout the vertical direction, the energy density of the cell is improved while just meeting the requirement that the electrode sheet has good bonding performance.

[0053] In one embodiment, the step of "drying the negative electrode precursor" in S2 includes the following steps: S21: The negative electrode precursor is dried for the first time at a temperature of 65-75℃ for 2-5 minutes. S22: Dry the negative electrode sheet in S21 a second time. The temperature of the second drying is 82-88℃ and the time is 0.5-3min. S23: Dry the negative electrode sheet in S22 for the third time. The temperature of the third drying is 62-68℃ and the time is 1-5 minutes. S24: Dry the negative electrode sheet in S23 for the fourth time. The temperature of the fourth drying is 105-115℃ and the time is 1-8 minutes.

[0054] In step S21, when the drying temperature is 65-75℃ and the time is 2-5 min, the solvent in the first negative electrode slurry can evaporate to form vertical pores, providing a path for the first binder to float. In step S22, when the drying temperature is 82-88℃ and the time is 0.5-3min, it can be ensured that the first binder migrates from the first negative electrode slurry to the second negative electrode slurry, so that the content of the first binder in the first coating layer is the minimum required amount; the content of the binder in the second coating layer just reaches the minimum required amount of binder to ensure the bonding effect, so that the amount of binder used in the negative electrode sheet is minimized.

[0055] In step S23, when the drying temperature is 62-68℃ and the time is 1-5 min, the first negative electrode slurry and the second negative electrode slurry undergo a sol-gel transition, thereby locking the content of the first binder in the first coating layer and the content of the binder in the second coating layer.

[0056] In step S24, when the drying temperature is 105-115℃ and the time is 1-8 minutes, the first negative electrode slurry is deeply cured, and the crystal water, residual solvent, etc. are completely removed to form the first coating layer; the second negative electrode slurry is deeply cured, and the crystal water, residual solvent, etc. are completely removed to form the second coating layer.

[0057] By controlling the drying temperature and time, the amount of the first binder floating in the first negative electrode slurry is precisely controlled, so that the amount of the first binder in the first coating layer is at the minimum required amount; the amount of binder in the second coating layer is just enough to ensure the minimum required amount of binder to guarantee the bonding effect, thereby minimizing the amount of binder used and reducing the situation of excessive binder in the negative electrode sheet in traditional methods. By designing to ensure that the binder content is consistent throughout the vertical direction, the energy density of the cell is improved while just meeting the requirement that the electrode sheet has good bonding performance.

[0058] A third aspect of the present invention provides a battery comprising a negative electrode sheet as described above, or a negative electrode sheet prepared by the method described above.

[0059] In some embodiments, the battery can be a secondary battery, which can be a lithium-ion battery. Typically, a battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. In the actual battery manufacturing process, firstly, the positive electrode, separator, negative electrode, and separator are sequentially stacked and assembled to obtain a bare cell. The assembly method can be, for example, stacking or winding. Then, electrolyte is injected into the dry bare cell, and after formation and aging, a secondary battery is obtained. The negative electrode is the negative electrode described in the above embodiments. The positive electrode includes a positive current collector and a positive active material layer coated on the positive current collector. Specifically, the positive active material layer can include a positive active material, a conductive agent, and a binder. The positive active material can include, for example, at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt manganese aluminum oxide; the conductive agent can include, for example, at least one of conductive carbon black, carbon nanotubes, and acetylene black; the binder can include, for example, polyvinylidene fluoride (PVDF). The separator can be, for example, a polyethylene membrane, a polypropylene membrane, a PVDF membrane, or a nonwoven fabric. Electrolytes may include, for example, lithium salts and solvents. The types of lithium salts and solvents are not specifically limited and can be selected according to actual needs.

[0060] The beneficial effects of the present invention will be further illustrated below with reference to the embodiments.

[0061] To make the inventive objectives, technical solutions, and beneficial effects of this invention clearer, the invention is further described in detail below with reference to embodiments. However, it should be understood that the embodiments of this invention are merely for illustrative purposes and not for limiting the invention, and the embodiments are not limited to those given in the specification. Materials not specified in the embodiments were prepared under conventional conditions or according to the conditions recommended by the material supplier.

[0062] Furthermore, it should be understood that the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, does not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. It should also be understood that the combined connection relationship between one or more devices / apparatus mentioned in this invention does not preclude the existence of other devices / apparatus before or after the combined devices / apparatus, or the insertion of other devices / apparatus between these explicitly mentioned devices / apparatus, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0063] In the following embodiments, the reagents, materials and instruments used, unless otherwise specified, are commercially available or can be obtained through synthesis methods known in the art.

[0064] Table 1. Design of negative electrode component parameters for Examples 1-9 and Comparative Examples 1-5; Table 2. Design of drying parameters for negative electrode sheets in Examples 3 and 10-17; Example 1 This embodiment illustrates the negative electrode sheet and battery disclosed in this invention; it includes the following operational steps: Preparation of negative electrode: The first negative electrode slurry is prepared by mixing the first negative electrode active material graphite, the first binder SBR and the first thickener CMC in a ratio of 98:0.2:1.8; The first negative electrode active material graphite, the first binder SBR and the first thickener CMC are mixed in a ratio of 98:0.1:1.9 to prepare the second negative electrode slurry; The first and second negative electrode slurries are sequentially coated onto the copper foil of the negative electrode current collector on a coating machine to form the negative electrode precursor. The negative electrode precursor is first dried at 70°C for 3 minutes; then dried a second time at 85°C for 2 minutes; then dried a third time at 65°C for 3 minutes; and finally dried a fourth time at 110°C for 5 minutes. After rolling, die-cutting, and other processes, the negative electrode is obtained. At this point, the content of the first binder in the first coating layer is 0.15%, and the content of the binder in the second coating layer is 0.15%.

[0065] Preparation of positive electrode: Lithium cobalt oxide, single-walled carbon nanotubes, conductive carbon black SP, and binder PVDF are mixed in a ratio of 97.3:0.5:1.0:1.2 to prepare a positive electrode slurry. The positive electrode slurry is then coated onto the surface of the positive electrode current collector on a coating machine. After drying, rolling, die-cutting, and other processes, a positive electrode sheet that meets the requirements is obtained.

[0066] Electrolyte preparation: Lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a mass ratio of 1:2:1 to obtain an electrolyte with a concentration of 1 mol / L.

[0067] Preparation of the diaphragm: PE porous polymer film is used as the membrane substrate; Battery manufacturing: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then stacked in this manner to form a battery cell. The battery cell is placed into a pre-formed battery casing, and the electrolyte is injected into the baked and dried battery cell. After vacuum sealing, settling, and formation processes, the battery is obtained.

[0068] Examples 2-9 Examples 2-9 illustrate the negative electrode and battery disclosed in this invention, including most of the operating steps in Example 1. The electrode drying process in Examples 2-9 is the same as in Example 3, except that: The composition parameters of the negative electrode shown in Table 1 are used.

[0069] Examples 10-17 Examples 10-17 illustrate the negative electrode and battery disclosed in this invention, including most of the operating steps in Example 3. The electrode composition parameters of Examples 10-17 are the same as those of Example 3, except that: The drying parameters for the negative electrode shown in Table 2 were used.

[0070] Comparative Examples 1-5 Comparative Examples 1-4 are used to illustrate the negative electrode sheet and battery disclosed in this invention, including most of the operating steps in Example 1. The electrode drying process of Comparative Examples 1-4 is the same as that of Example 3, except that: The composition parameters of the negative electrode shown in Table 1 are used.

[0071] Comparative Example 5 uses a single-layer coating method to form a coating layer on the surface of the current collector, and the content of the binder is 1.2%.

[0072] Performance testing The following performance tests were performed on the batteries prepared in Examples 1-17 and Comparative Examples 1-5: 1. Cell energy density test: Place the cell in a 25℃ room for 2 hours; discharge at a rate of 0.5C to the lower limit voltage (3.0V), let stand for 10 minutes; charge at a rate of 0.5C with constant current and constant voltage to the upper limit voltage (4.4V), and cut off the constant voltage step with a current of 0.05C; let stand for 10 minutes; discharge at a rate of 0.5C to the lower limit voltage (3.0V), and record the discharge capacity C and energy E.

[0073] This embodiment uses model 486579 as the model. That is, the thickness T of the battery cell is 4.8mm, the width W is 65mm, and the height H is 79mm.

[0074] The energy density of this battery cell is calculated as follows: ED = E / (T * W * H); 2. Peel Strength Test (Dedicated Peel Force Tester): Take a piece of double-sided tape approximately 100mm long and firmly attach one side to the center of a standard steel plate. Press it down by hand or with a small roller to ensure no air bubbles. Remove the release paper from the other side of the double-sided tape. Place the electrode sample with the coated side facing upwards and the current collector facing downwards, and smoothly attach it to the double-sided tape, ensuring the sample is perpendicular to the long side of the steel plate and has at least 75mm of effective adhesion length. Use a rubber roller of a specified weight (e.g., 2kg) to roll back and forth on the sample 3 times at a speed of approximately 10 mm / s, ensuring full contact between the electrode and the double-sided tape without air bubbles. The direction and number of rolls must be consistent. Fix the prepared assembly on the moving beam of the material testing machine. Clamp the free end (unattached part) of the electrode sample with the upper clamp. Check to ensure the peel angle is 180°; that is, the sample is perpendicular to the clamp and then bent 180° in the opposite direction to flatten it onto the steel plate. Test using a peel strength tester. The test results are shown in Table 3.

[0075] Table 3 Battery electrochemical performance Comparing Examples 1, 3, 5 and Comparative Example 1, it can be seen that when the content of the first binder is less than 0.1% and the content of the second binder is less than 0.1%, the peel strength of the electrode is poor.

[0076] Comparing Examples 1, 3, 5 and Comparative Example 2, it can be seen that when the content of the first binder is greater than 2% and the content of the second binder is greater than 2%, the energy density of the electrode is low.

[0077] Comparing Examples 2-4 with Comparative Examples 3-4, it can be seen that when the ratio of A' to B' is 0.95-1.05, the first coating layer has good adhesion to the current collector and high peel strength; the second coating layer has good adhesion to the first coating layer; when the ratio of A' to B' is less than 0.95, the peel strength of the second coating layer is poor; when the ratio of A' to B' is greater than 1.05, the peel strength of the first coating layer is poor.

[0078] Comparing Examples 5-9, it can be seen that when the ratio of A to B is 1-10, the first coating layer will not have redundant first adhesive, the peel strength between the second coating layer and the first coating layer is high, and there is good adhesion. When the ratio of A to B is less than 1, it will result in redundant first adhesive in the first coating layer, insufficient adhesion between the second coating layer and the first coating layer, poor peel strength of the second coating layer, and low battery energy density. When the ratio of A to B is greater than 10, it will result in insufficient first adhesive content in the first coating layer, poor peel strength between the first coating layer and the current collector, and redundant adhesive content in the second coating layer, which will cause a loss of battery energy density.

[0079] Comparing Example 3 and Comparative Example 5, it can be seen that the peel strength of Example 3 is the same as that of Comparative Example 5, but the energy density of Comparative Example 5 is relatively low.

[0080] Comparing Examples 3 and 10-17, it can be seen that when the drying process of the negative electrode precursor meets the following conditions: the temperature of the first drying is 65-75℃, and the time is 2-5 min; the temperature of the second drying is 82-88℃, and the time is 0.5-3 min; the temperature of the third drying is 62-68℃, and the time is 1-5 min; and the temperature of the fourth drying is 105-115℃, and the time is 1-8 min, the floating content of the first binder in the first negative electrode slurry can be precisely controlled, thereby ensuring that the content of the first binder in the first coating layer is at the minimum required amount; and the content of the binder in the second coating layer is just enough to ensure the bonding effect. The minimum required amount of binder is minimized, thus reducing the excessive binder content in the negative electrode sheet as in traditional methods. By designing the binder content to remain consistent throughout the vertical direction, the energy density of the battery cell is improved while ensuring good adhesion of the electrode sheet. If the drying process of the negative electrode sheet precursor does not meet any of the above-mentioned time or temperature requirements, the amount of the first binder floating in the first negative electrode slurry cannot be precisely controlled, which may result in too much or too little first binder in the first coating layer; or too much or too little binder in the second coating layer, resulting in poor adhesion; and thus failing to improve the energy density of the battery cell.

[0081] This invention involves coating a first negative electrode slurry on both sides of the current collector, and coating a second negative electrode slurry on the side of the first negative electrode slurry away from the current collector. During electrode baking, a first binder in the first negative electrode slurry floats to the second negative electrode slurry. After the first binder floats to the surface, the content of the first binder in the first coating layer is just enough to ensure the minimum required amount of binder for bonding effect. Initially, the content of the second binder in the second negative electrode slurry cannot guarantee the bonding effect with the negative electrode. However, during electrode baking, as the first binder in the first negative electrode slurry floats to the second negative electrode slurry, and after drying, the content of the binder in the second coating layer is just enough to guarantee the minimum required amount of binder for bonding effect. This minimizes the amount of binder used and reduces the excessive binder in the negative electrode in traditional methods. By designing the binder content to remain consistent throughout the vertical direction, the energy density of the battery cell is improved while ensuring good bonding performance of the electrode.

[0082] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized by: The first coating layer is formed by baking and curing a first negative electrode slurry, and the first negative electrode slurry comprises a first binder, wherein, taking the total mass of solid substances in the first negative electrode slurry as 100%, the percentage content of the first binder in the total mass of solid substances in the first negative electrode slurry is A, and the value range of A is 0.1%≤A≤2%; taking the total mass of the first coating layer as 100%, the percentage content of the first binder in the total mass of the first coating layer is A', and the value range of A' is 0.1%≤A'≤2%; A' and A satisfy the relationship A'≤A. The second coating layer is formed by baking and curing a second negative electrode slurry, and the second negative electrode slurry comprises a second binder, wherein, taking the total mass of solid substances in the second negative electrode slurry as 100%, the percentage content of the second binder in the total mass of solid substances in the second negative electrode slurry is B, and the value range of B is 0.1%≤B≤2%; taking the total mass of the second coating layer as 100%, the percentage content of the binder in the second coating layer is B', and the value range of B' is 0.1%≤B'≤2%; B' and B satisfy the relationship B'≥B. A' and B' satisfy the relationship 0.95≤A' / B'≤1.

05. A and B satisfy the relationship 1≤A / B≤10.

2. The negative electrode sheet according to claim 1, characterized by: The mass of the first binder in the first negative electrode slurry is a, the mass of the first binder in the first coating layer is a', the mass of the second binder in the second negative electrode slurry is b, and the mass of the binder in the second coating layer is b'; a', a, b and b' satisfy the relationship b'=b+a-a'.

3. The negative electrode sheet according to claim 1, characterized by: The first negative electrode slurry further comprises a first negative electrode active material and a first thickening agent, wherein, taking the total mass of the first negative electrode slurry as 100%, the percentage content of the first negative electrode active material in the total mass of the first negative electrode slurry is 97%-99%, and the percentage content of the first thickening agent in the total mass of the first negative electrode slurry is 0.1-2%.

4. The negative electrode sheet according to claim 1, characterized by: The first binder comprises one or more of styrene-butadiene rubber, acrylic resin, fluoropolymer, nitrile rubber, polyvinyl alcohol and sodium alginate; the first negative electrode active material comprises one or more of graphite and silicon-carbon material; and the first thickening agent comprises one or more of sodium carboxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, sodium alginate and polyvinyl alcohol.

5. The negative electrode sheet according to claim 4, characterized by: The viscosity of the first negative electrode slurry is 3000-5000 cP.

6. The negative electrode sheet according to claim 1, characterized by: The second negative electrode slurry further comprises a second negative electrode active material and a second thickening agent, wherein, taking the total mass of the second negative electrode slurry as 100%, the percentage content of the second negative electrode active material in the total mass of the second negative electrode slurry is 97%-99%, and the percentage content of the second thickening agent in the total mass of the second negative electrode slurry is 0.1-2%.

7. The negative electrode sheet according to claim 1, characterized by: ​ 8. The negative electrode sheet according to claim 7, characterized by: The second binder comprises one or more of styrene butadiene rubber, acrylic resin, fluoropolymer, nitrile rubber, polyvinyl alcohol, sodium alginate; the second negative electrode active material comprises one or more of graphite, silicon-carbon material; the second thickening agent comprises one or more of sodium carboxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, sodium alginate, polyvinyl alcohol.

9. The negative electrode sheet according to claim 1, characterized by: The viscosity of the second negative electrode slurry is 3000-5000 cP.

10. The method of producing a negative electrode sheet according to any one of claims 1 to 9, characterized by: The method comprises the following steps: S1: coating the first negative electrode slurry and the second negative electrode slurry on both sides of the current collector in sequence respectively; forming a negative electrode sheet precursor; S2: drying the negative electrode sheet precursor, the drying temperature is 62-115℃, and the time is 4.5-21 min.

11. The method of producing a negative electrode sheet according to claim 10, characterized by: The "drying the negative electrode sheet precursor" in S2 comprises the following steps: S21: first drying the negative electrode sheet precursor, the first drying temperature is 65-75℃, and the time is 2-5 min; S22: second drying the negative electrode sheet in S21, the second drying temperature is 82-88℃, and the time is 0.5-3 min; S23: third drying the negative electrode sheet in S22, the third drying temperature is 62-68℃, and the time is 1-5 min; S24: fourth drying the negative electrode sheet in S23, the fourth drying temperature is 105-115℃, and the time is 1-8 min.

12. A battery, characterized by: The negative electrode sheet comprises the negative electrode sheet as claimed in any one of claims 1 to 9, or the negative electrode sheet prepared by the preparation method as claimed in any one of claims 10-11.

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